Gear Ratio and RPM deciding on gears

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Rule of thumb for street performance is multiply the tach by two. 3000 @ 60 is what many cars were for the street .. My 70 RR with a 354 Dana 4spd and 275 60's was 3000 @ 60 It was a good driver and still quite quick. I can remember the tach and speedo were both straight up at 60 in 4th



I would say tire size should be your first choice. You can by and sell gears easier then you can sell used tires when their hanging on the quarters. LOL
 
What do you not have any power under 3000? Seriously, that makes no sense.

You may need to work on your tune up or something.
yeah, I mentioned that in post 11. To the OP, if you have an issue with your engine or tune, do NOT try to compensate the problem by throwing gears, or anything else at it. IF you just want to move the car out of the hole, put a 8" 4800 converter in it. That should help it out under 3000 rpms.
 
yeah, I mentioned that in post 11. To the OP, if you have an issue with your engine or tune, do NOT try to compensate the problem by throwing gears, or anything else at it. IF you just want to move the car out of the hole, put a 8" 4800 converter in it. That should help it out under 3000 rpms.
Gotta wonder about the 904. Is it there because it was available, or is there because someone wanted a lighter transmission and did the work needed for it to survive racing and hard use?

Lots of good suggestions in this thread.
I'm with A-bodyJoe - install the diff already owned, and run it.
And with A/J et al to then adjust the timing and/or get better fuel.
Drive it and see how it is.

Honestly, if there was no time crunch, and there obviously isn't, I'd start with a simple compression check and then measure cam lift with a dial indicator and preferably a degree wheel.
 
Gotta wonder about the 904. Is it there because it was available, or is there because someone wanted a lighter transmission and did the work needed for it to survive racing and hard use?

Lots of good suggestions in this thread.
I'm with A-bodyJoe - install the diff already owned, and run it.
And with A/J et al to then adjust the timing and/or get better fuel.
Drive it and see how it is.

Honestly, if there was no time crunch, and there obviously isn't, I'd start with a simple compression check and then measure cam lift with a dial indicator and preferably a degree wheel.
yeah, and really all we have is "it's a 340 that's lazy under 3000 rpm's". We don't know what that means. Was the expectation to pull the front wheels up 2 ft and can only pull them 4" ??? Or did he get beat in a stop light war by a '82 dodge truck with a slant six? Who knows..... LOL
 
I was just thinking of my post above and it took me back to the 70's. Those were the days you could take used car test drives on the new car lot's. They would hand you the keys to anything but a HEMI, LS6 or 429CJ. You could choose what ever gear ratio you wanted to test out, They were all over the lots. I myself would always look for the low geared 4spd cars to practice pulling gears.

Today I'm 65 years old. 456's 29-31 inch tires and unlimited RPM's are what brings back the old day's. LOL

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I don't understand this pictorial, as to why the rpm changes at the top; what is your torque convertor doing?
The speed to rpm ratio should be a linear relationship, and can be written as xxxmph per 1000rpm. So then if your gears bring 2000rpm @50 mph, then 100mph should be 4000 rpm. It's a straight line.

A) I don't care about your combo. You say your 340 is lazy below 3000 and I believe it. There are a couple of cures for that; #1) is more cylinder pressure and #2) is a higher stall TC so your engine never has to pull in that soft zone.
As to #1) there are a couple of ways to achieve that. Obviously the first is a higher compression ratio, and the second is an earlier closing Intake valve to trap more pressure.
As to Ica, there are several ways to get an earlier closing Ica; one is to retime your current cam and 2) is a smaller cam, and 3) is a faster cam, and 4) is a different type of cam
Hold that thought.
B) The engine is already built, and it already has a detonation issue.
From this one can surmise that 1) the cylinder pressure may already be excessive, or 2) the timing curve may be too aggressive, or 3) you need to get a higher octane gas, or 4) the engine is too heavily loaded. Hold that thought
C) powerband; for a performance streeter, you are only concerned with first gear, the bottom of second gear and the 1-2 powerloss at the shift. The speed limit is ~65mph so if second gear takes you to 107 mph @5500 with 2.71s that is, as you already know, a crummy gear.
Here's what's happening to make your bottom end soft;
The low stall forces your engine to work from a too low point on the power curve, and the 2.71s keep it there for what seems forever. If the tires don't spin, it's a friggen long wait to where the power starts. I get it. I've been there. Hang on to that.
D) where do you want the power? You gotta make a choice, cuz without an overdrive, you just can't have it all. Furthermore, with an automatic you already have a gear handicap. So is it ; 1) off the line, or 2) at ~32mph, or 3) for passing at say 55mph? The set-ups are each different
E) cruise rpm. This can make or break your combo. And cruise rpm is a highly personal decision.
For you;
with a 340 that has a performance cam; forget about selecting a cruise rpm for economy, especially if the cylinder pressure is down.
the 340, in street trim, is torque-handicapped compared to a longer stroke engine. So it's gonna need more stall, and more starter gear, and this is a problem with the A904 because the low gear is just 2.45. aaaaand this drives the cruise rpm ever higher. So, you gotta make a decision; do you want a city bomber . Or do you need that combo to cruise long distance in comfort. If the latter then only you can define comfort. Some guys are OK at 3500. Some guys get cranky by 2500. I like 2200. This is where the highly personal comes in.
I have full length duals the really sing at 2800. That sounds great around town with a 4-speed. But I can't cruise at 2800. I cruised 4 years at 56=2600 and it drove me crazy. I finally sprung for an overdrive and now 65=2240, and I am a happy guy. But here's the thing, I live 20 miles from the nearest urban center, and I used to drive this car 35 minutes one way, every day to work and back. So that again points to the highly personal cruise rpm. If you only plan one roadtrip per summer, and it's just an hour away, then that is a different situation.
Also, if you don't have dual 3inchers, your exhaust may be quieter. NVH is the deal here; Noise,Vibration, and Harshness. It's not just rpm. Moving stuff vibrates. and shakes and oscillates. My Barracuda has a very annoying natural body oscillation at 55 mph. It doesn't matter what rpm or what gear I install, or how many times I balance the driveshaft or adjust the driveline angles... it just has a natural hiway rhythm at 55 mph. Just 3 mph either way and it sorta disappears.
F) ok lets pull it together.
#1) you gotta fix your detonation issue. Rear gear and stall may help you, but you gotta eliminate it.
#2) you gotta rate your cruise rpm as to importance. Every next decision depends on this
#3) you select the rear gear to get the cruise rpm
#4) this establishes your starter gear
#5) if the take-off is boring, you increase the stall-rpm,until it's not.
#6), if kick-down into low at 32mph is boring, you increase the stall, until it's not
#7) if KD at 55mph gets you a screaming 340 but no rapid transit, your rear gears are wrong;start over.
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on another note;
since you have 2.71 gears, you have a rolling dyno. A dyno measure torque over time and pukes out a convenient picture of horsepower. With 2.71 gears, your mph at 6000 will be close to 70mph in first gear. The only other piece of information you need is your on-the-start-line weight. Then you just measure the time required to accelerate between any two mph points, and you can assign a horsepower to that, which was required to do that. Ten you repeat as often as you want. Then plot the points on a graph and voila, there is your dyno-chart. It doesn't have to be dead-nuts accurate. What you want is the shape of the graph.and a rough idea of the power at various mphs, which you are gonna convert to rpm. Now when you go to select a stall-speed, you are gonna look on your chart to be able to compare the power at the current stall, to help select that higher stall TC, to achieve the take-off power with the new stall.

As to powerband; your transmission, in conjunction with the engine-combo, determines the powerband. The 904 ratios are 2.45-1.45-1.00
So if your engine combo peaks at 5500rpm, and the power plateau is 400rpm wide, and you then arbitrarily choose a 6000 rpm shift, take a breath, then at the 1-2 shift, your Rs will fall to 1.45/2.45x6000=3550. Ergo; your powerband has to be from 3550 to 6000rpm. That is a flippin' huge 2450rpm powerband.
In comparison, The regular 4-speed has ratios of 2.66-1.92-1.40-1.00. At the same 6000 shift,the rpm-drop is 1.92/2.66x6000=4330, which is just 1670rpm.
If you look on a typical dyno curve, you are gonna see that at 4330rpm the power could be easily 50Plus horsepower higher than at 3550.
So for a guy with a 904 and a 6000 shift,at the track; he's not concerned with any power numbers below 3550rpm.
But you, as a streeter, I'm gonna bet, that 95% of your 340's life is gonna be spent below 3550, so for you this rpm band is of prime importance. If you get this wrong you will not be a happy camper.

My opinion;
My hi-compression 367 like a minimum starter gear of 10/1 with a 4-speed. It likes 11/1 better. I have a clutch so I can rev the engine up to whatever I want , to find whatever power I need, to accelerate from zero mpg, at whatever mood I'm in. I like to dump the clutch and go. So I blip the throttle, and let the flywheel do the work, then accelerate with whatever throttle setting I need. So, I could be "stalling" at 1600 or 3000
But you, with a 904, don't have that luxury. However you do have something, as good or better. Your TC is a Torque Multiplier, as well as a fluid coupling. The internal TM in that TC varies with input and load and speed, so it's kindof an automatic trans all by itself. At zero mph, the torque differential thru it is the greatest, and may be as high as 1.8.. That is to say, for every 1.0 ftlb that goes into the TC 1.8 may come out. But this ratio rapidly diminishes as the car starts moving, and still more in each subsequent gear, and eventually might stabilize at somewhere between 1.08 and 1.05.
Ok so how do you make that work for you?
Well lets say my engine makes 140 ftlbs at 1800rpm. and it like the 11/1 starter gear, so at zero mph, I slip the clutch out. the rear axkes are gonna be seeing;
140x11=1540 ftlbs and away I go. 1800 rpm with this combo is 13mph, so by 13 mph my engine is putting down 1540ftlbs to the rear tires. There is no TC to multiply so that is all I got.
Lets say your engine also puts out 140@1800rpm. So at zero mph, with the same 11/1 starter gear,and an 1800TC, you would expect the same 1540ftlbs. But with the internal TC ratio of 1.8, this is increased to 2770.... for just an instant. Then as your car begins to move, the TC ratio begins ramp downwards, but by 13mph the ratio might still be 1.4 (I'm guessing), and your rpm may have increased to 2400 so at this time your engine may be putting out 180 ftlbs so now;
180x1.4x11=2770 ftlbs. From this point on, the TM in the TC is gonna continue to drop, and the engine torque is gonna continue to increase... upto maybe the high 3000s RPM, depending on your combo, where the torque will plateau for a bit, then begin to decrease.
What's the point? Well 2770 ftlbs is more than enough to break traction of any sized street tire you can fit in your stock tubs. Lets say your 340 peaks at 340 ftlbs and the TC gets down to 1.1TM at that rpm and speed, still in 11/1 starter gear, this maths out to
340x1.1x11=4110 ftlbs. This might be 28 mph, and obviously, at WOT, the tires are still spinning.......... ergo it follows that the 11/1 starter gear, for your combo, is overkill.
So what is 11/1 for you? ------ 11/2.45=4.49..

Ok so if 4.49 is overkill, then what is a good gear?
Well, the target has to include the internal TM of the TC. So if 1540 is enough for me to power away briskly then we can do the math backwards and get a real-world number for you. Suppose you currently have a 2400stall TC and your engine puts out 180ftlbs there; just suppose. Then 1540/(180x1.8x2.45)=1.94 rear gear...... to power away briskly.
But 1540 is not enough for me to break traction. I need about 2200. So
2200/(180x1.8x2.45)=2.77 now. But your TC is gonna lose TM as soon as the tires spin, because the torque differential is lost. So we have to swap out the 1.8 for something closser to1.4 say, and
2200/(180x1.4x2.45)=3.56s. Hmmmmmmm were have we seen this number before? lol. Sure everybody loves the 3.55s.... because with them, your 340 can bust the tires loose with the 1.8TM ratio, and as the rpm rises the TM in the TC is falling, but the torque output of the engine is rising, so the spin can be maintained, and your engine does not need to be at WOT to make it happen.
Bottom-line;
So while I need 4.10s to make this happen with a regular 4-speed, your 904 equipped engine might only need 3.55s. I say might, because IDK if your engine can make 180ftlbs at 2400rpm. I pulled those numbers out my arse.
But here's the trick; If your engine pulls 10% less, then you'll need either 10% more rear gear, or 'SOME" more stall, or some combination.
Opinion coming
I've had hi-compression 340s for most of my driving life since 1969. I know they are soft below 3000. This is particularly noticeable with a 4-speed. And that is why they usually got 3.55s or better. Whereas the auto cars usually got 3.23s and 2400 to 2600 TCs. When you stick a performance cam in 'em, the autos feel it right away. I don't have a clue how you ended up with 2.71s lol, but 3.55s will make a 3.55/2.71=31% improvement. But a hi-stall can easily surpass that. Consider the numbers I used 180/140=plus 28.6% more available engine torque. Even if the numbers are not dead-nuts accurate to your combo; the percentage should be close. If you add the two, which you can, then by swapping to both 3.55s AND a hi-stall, you could see a 60% improvement in footpounds to the tires. That will really wake up your otherwise lazy bottom end. Don't forget, the TC will make little to no difference once it has reached "coupling". After that, the TM is strictly in the gears. And finally with 3.55s and 27" tires 65=2870, which only you can decide if it is acceptable. My cure was to drive slower. And eventually, I went to an overdrive.

Oh, I gotta mention this;(more opinion)
My 367 combo is "fierce". First gear at WOT is just a tireburner. So I like to nail it at 30 to 35 mph, in second gear, to see what happens. my rpm here is just 2700. But it still lights up the BFG 295/50-15s
For you with 3.55s; 32 mph in second will be ~2150, so you'll want the KD to work, and jump it up to 3650 and POW! yur gone. 5500 will get you 48 with the 3.55s. Maybe your cam will get you 60@6800,lol, but I think it's better to shift earlier. Then 60mph in second will get you about 4000.
As you can see; this is no where near your power peak. Remember in the very beginning of this post, I said that the autos were a performance limitation for a streeter? This is why I said that. Your auto only goes thru the power peak once, on the way to 60mph, and on the street, the tires will be spinning most,if not all, of the way thru first gear, and you end up near peak torque, instead of peak power. This will kill your average power from zero to 60mph. And the ET will suffer, because of it. About the best cure for this is to bring the suspension up to speed to try and reduce your tire slip; spinning ain't winning..... Sometimes a softstart ain't a bad thing.........
Maybe you can imagine, that with 3.55s, if you could build a combo to make peak power at 4000, then you could go thru the power-peak twice on the way to 60mph. And if the average power was the same between this low-rpm combo, and your 340 combo....... you both might hit 60 at the same time. That would probably require more cubes tho.

Conversly; it might be better to gear for peak power at 60mph in first gear, and jump up the stall to get off the line better.
Suppose your engine falls off the cam at 6200, then you would need 3.23s to top out at 60=6200, and you could cruise at 65=2610 @zero-slip.
Off the line it would be a lil lazy..... unless you jumped up the stall. I like a 3500 for this. But remember this is a pretty big cam to peak at around 5900. That is a solid two sizes, maybe three, over the factory cam, say a 292*. Hyup, I had one of those in my 367 at 11.3Scr and with 3.55s it was ferocious at the top. But I got rid of it PDQ, on account of it was unacceptable on the bottom,for me, with a regular 4 speed.
I hope you don't have a 292 in a 10/1 iron headed 340,lol. 3.55s will never cut it.
How do you know? Well firstly tune the heck out of it with a properly synced up transfer slot. then crank the timing back to get about 750 rpm in neutral. Then measure the manifold vacuum. If it's down at 8 inches or less, and out the tailpipes it is singing a beautiful rumpiddy-rumpiddy rump, that's the one.
But I doubt you have this on account of your idle timing was 19* right? And you said the power-timing was around 31 IIRC, and delayed to prevent detonation; so the 292 I don't think would behave that way, even with 2.71 gears.
That timing curve is also killing your low-rpm power

Hi AJ thanks for taking the time to do this I really appreciate it! Not sure why the curves are not linear either. I used the Spicer online calculator which seemed very simplistic. I do have a lot of critical unknowns (engine specs and converter). I do know I need to get the Sure Grip in with something other than the 2.71's. I want to have fun around town and prefer to drive more lower end torque with this more than cruise comfortably. I don't care about noise, in fact I like the roar of the 340 and would rather have it loud than quiet. My VW is quiet if I want that. I will replace the converter after I get the rear gears done and re-tune the car against the new gearing.
I think the issues are I have a aggressive four speed motor with mechanical advance, steel heads and a high rise manifold that is not a great set up for an automatic. This set up looks like more of a drag set up with poor low end response due to what I mentioned. At some point I will change out the cam but in the mean time the 4.10's I think will help with a tight high stall converter behind it.
Thanks again for the post I still have to read it over a few times to understand it as I am still learning about this.....
 
Going from 2.71's to 4.10's will 100% help the car move from a dead stop. Put the 4.10's in and smile and be done. :)
 
if you have a 3.55 ready to go in then throw it in and see how you like it. can do all the computer crap in the world and its still not gonna tell what it feels like..

Or necessarily be correct. Much like all the static and dynamic compression calculators. A computer can only do "so much".
 
I guess we all just need to shut the hell up. AJ's the only one who gets thanks around here.
 
Who determines what is reasonable? Opinions are all over the map, just like asking if a cam is too big or too small. You mentioned it’s not a huge difference between a 3.55 and a 4.10, so I say go with the 4.10. Why do I say that? Because I run a 4.10 w/27” tires and run down the interstate every now and then on 30+ mile jaunts to work during rush hour, above the speed limit to keep up with traffic or in most cases just blowing by everyone for extended periods. These posts saying it gets old running low gears at higher speeds for any length of time just sound like old softies to me. Is it a hot rod or what?! Some of these guys should just get a loaded Chrysler Newport or similar if low rpm sailing is what one wants! LOL:p
Cool! Is this with a 3 speed? I was running 3.55s and 28" tire on my dart sport 360 and seemed a little wound up on the interstate. Also was a little tough keeping up with the other cars! Now have 3.91's (which I haven't run yet.) Just curious.
 
Cool! Is this with a 3 speed? I was running 3.55s and 28" tire on my dart sport 360 and seemed a little wound up on the interstate. Also was a little tough keeping up with the other cars! Now have 3.91's (which I haven't run yet.) Just curious.
Auto no overdrive. What I have seems reasonable to me is all I can say. It’s possible I may seem unreasonable to others though:eek:
 
Here is a great SBM street combo;
3.23s with a Commando 3.09 low and a .78 od splitter.
60Mph is 5800 in First-over, which can be powershifted into. On the shift, the rpm drops to 0.78x5800=4500, so that is a 1300rpm powerband; which you might split 35/40% over the peak and 65/60% under (just guessing) to get low-ET. The overall gear to do this with is 7.78; remember this number. and the starter gear is 9.98. Then;30 mph is 3720rpm in first.
BUT: the only way to get to 7.78 with a 1.45 second gear is; 7.78/1.45=5.38s..........
The engine doesn't care how you get the 7.78.
With an automatic;
Could be 7.78/1.45 =5.38.
Could be 7.78/2.45=3.23s (first gear now),
Could be 7.78/1.54=5.05. (A999Second) suggest 4.88s
Could be 7.78/2.74=2.87s,( A999 low) suggest 2.94s
With an A833
Could be 7.78/1.92=4.10s; badaBoom!
Could be 7.78/1.77=4.40s, (suggest 4.30s) I really liked those.
Could be 7.78/1.67=4.56s The overdrive box. Sounds good but low is then 4.56x3.09=14.09 which is ludicrously low for a streeter, and then you get the 1-2 rpm drop to 54%, which is just a killer; .54x5800=3130. Yur hot 340 got torque down there? Not hardly,lol.
The more gears, the better, but sooner or later first gear becomes useless(see above); and by experience I can tell you that with a hot 360, three gears is too many. Yes I tried it,lol.
Now all you gotta do is build the engine to make peak power at about 5800 less perhaps 300= 5500 ; lessee I see that as about a 235@.050 and yur done.
Will it be quick?
Not particularly , until you figure out how to keep it from spinning the tires the whole way.
Will it be fun?
Are you kidding!
I'd say 11, on a 1-10 scale. 15 if in a corner. 20 if you can control it at full lock, and don't wipe it out;Yahoo!
 
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Cool! Is this with a 3 speed? I was running 3.55s and 28" tire on my dart sport 360 and seemed a little wound up on the interstate. Also was a little tough keeping up with the other cars! Now have 3.91's (which I haven't run yet.) Just curious.
yup 3 speed auto 904A when you say wound up I assume you mean too high revving right? Do you recall any speed/RPM's?
 
I don't understand this pictorial, as to why the rpm changes at the top; what is your torque convertor doing?
The speed to rpm ratio should be a linear relationship, and can be written as xxxmph per 1000rpm. So then if your gears bring 2000rpm @50 mph, then 100mph should be 4000 rpm. It's a straight line.

A) I don't care about your combo. You say your 340 is lazy below 3000 and I believe it. There are a couple of cures for that; #1) is more cylinder pressure and #2) is a higher stall TC so your engine never has to pull in that soft zone.
As to #1) there are a couple of ways to achieve that. Obviously the first is a higher compression ratio, and the second is an earlier closing Intake valve to trap more pressure.
As to Ica, there are several ways to get an earlier closing Ica; one is to retime your current cam and 2) is a smaller cam, and 3) is a faster cam, and 4) is a different type of cam
Hold that thought.
B) The engine is already built, and it already has a detonation issue.
From this one can surmise that 1) the cylinder pressure may already be excessive, or 2) the timing curve may be too aggressive, or 3) you need to get a higher octane gas, or 4) the engine is too heavily loaded. Hold that thought
C) powerband; for a performance streeter, you are only concerned with first gear, the bottom of second gear and the 1-2 powerloss at the shift. The speed limit is ~65mph so if second gear takes you to 107 mph @5500 with 2.71s that is, as you already know, a crummy gear.
Here's what's happening to make your bottom end soft;
The low stall forces your engine to work from a too low point on the power curve, and the 2.71s keep it there for what seems forever. If the tires don't spin, it's a friggen long wait to where the power starts. I get it. I've been there. Hang on to that.
D) where do you want the power? You gotta make a choice, cuz without an overdrive, you just can't have it all. Furthermore, with an automatic you already have a gear handicap. So is it ; 1) off the line, or 2) at ~32mph, or 3) for passing at say 55mph? The set-ups are each different
E) cruise rpm. This can make or break your combo. And cruise rpm is a highly personal decision.
For you;
with a 340 that has a performance cam; forget about selecting a cruise rpm for economy, especially if the cylinder pressure is down.
the 340, in street trim, is torque-handicapped compared to a longer stroke engine. So it's gonna need more stall, and more starter gear, and this is a problem with the A904 because the low gear is just 2.45. aaaaand this drives the cruise rpm ever higher. So, you gotta make a decision; do you want a city bomber . Or do you need that combo to cruise long distance in comfort. If the latter then only you can define comfort. Some guys are OK at 3500. Some guys get cranky by 2500. I like 2200. This is where the highly personal comes in.
I have full length duals the really sing at 2800. That sounds great around town with a 4-speed. But I can't cruise at 2800. I cruised 4 years at 56=2600 and it drove me crazy. I finally sprung for an overdrive and now 65=2240, and I am a happy guy. But here's the thing, I live 20 miles from the nearest urban center, and I used to drive this car 35 minutes one way, every day to work and back. So that again points to the highly personal cruise rpm. If you only plan one roadtrip per summer, and it's just an hour away, then that is a different situation.
Also, if you don't have dual 3inchers, your exhaust may be quieter. NVH is the deal here; Noise,Vibration, and Harshness. It's not just rpm. Moving stuff vibrates. and shakes and oscillates. My Barracuda has a very annoying natural body oscillation at 55 mph. It doesn't matter what rpm or what gear I install, or how many times I balance the driveshaft or adjust the driveline angles... it just has a natural hiway rhythm at 55 mph. Just 3 mph either way and it sorta disappears.
F) ok lets pull it together.
#1) you gotta fix your detonation issue. Rear gear and stall may help you, but you gotta eliminate it.
#2) you gotta rate your cruise rpm as to importance. Every next decision depends on this
#3) you select the rear gear to get the cruise rpm
#4) this establishes your starter gear
#5) if the take-off is boring, you increase the stall-rpm,until it's not.
#6), if kick-down into low at 32mph is boring, you increase the stall, until it's not
#7) if KD at 55mph gets you a screaming 340 but no rapid transit, your rear gears are wrong;start over.
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on another note;
since you have 2.71 gears, you have a rolling dyno. A dyno measure torque over time and pukes out a convenient picture of horsepower. With 2.71 gears, your mph at 6000 will be close to 70mph in first gear. The only other piece of information you need is your on-the-start-line weight. Then you just measure the time required to accelerate between any two mph points, and you can assign a horsepower to that, which was required to do that. Ten you repeat as often as you want. Then plot the points on a graph and voila, there is your dyno-chart. It doesn't have to be dead-nuts accurate. What you want is the shape of the graph.and a rough idea of the power at various mphs, which you are gonna convert to rpm. Now when you go to select a stall-speed, you are gonna look on your chart to be able to compare the power at the current stall, to help select that higher stall TC, to achieve the take-off power with the new stall.

As to powerband; your transmission, in conjunction with the engine-combo, determines the powerband. The 904 ratios are 2.45-1.45-1.00
So if your engine combo peaks at 5500rpm, and the power plateau is 400rpm wide, and you then arbitrarily choose a 6000 rpm shift, take a breath, then at the 1-2 shift, your Rs will fall to 1.45/2.45x6000=3550. Ergo; your powerband has to be from 3550 to 6000rpm. That is a flippin' huge 2450rpm powerband.
In comparison, The regular 4-speed has ratios of 2.66-1.92-1.40-1.00. At the same 6000 shift,the rpm-drop is 1.92/2.66x6000=4330, which is just 1670rpm.
If you look on a typical dyno curve, you are gonna see that at 4330rpm the power could be easily 50Plus horsepower higher than at 3550.
So for a guy with a 904 and a 6000 shift,at the track; he's not concerned with any power numbers below 3550rpm.
But you, as a streeter, I'm gonna bet, that 95% of your 340's life is gonna be spent below 3550, so for you this rpm band is of prime importance. If you get this wrong you will not be a happy camper.

My opinion;
My hi-compression 367 like a minimum starter gear of 10/1 with a 4-speed. It likes 11/1 better. I have a clutch so I can rev the engine up to whatever I want , to find whatever power I need, to accelerate from zero mpg, at whatever mood I'm in. I like to dump the clutch and go. So I blip the throttle, and let the flywheel do the work, then accelerate with whatever throttle setting I need. So, I could be "stalling" at 1600 or 3000
But you, with a 904, don't have that luxury. However you do have something, as good or better. Your TC is a Torque Multiplier, as well as a fluid coupling. The internal TM in that TC varies with input and load and speed, so it's kindof an automatic trans all by itself. At zero mph, the torque differential thru it is the greatest, and may be as high as 1.8.. That is to say, for every 1.0 ftlb that goes into the TC 1.8 may come out. But this ratio rapidly diminishes as the car starts moving, and still more in each subsequent gear, and eventually might stabilize at somewhere between 1.08 and 1.05.
Ok so how do you make that work for you?
Well lets say my engine makes 140 ftlbs at 1800rpm. and it like the 11/1 starter gear, so at zero mph, I slip the clutch out. the rear axkes are gonna be seeing;
140x11=1540 ftlbs and away I go. 1800 rpm with this combo is 13mph, so by 13 mph my engine is putting down 1540ftlbs to the rear tires. There is no TC to multiply so that is all I got.
Lets say your engine also puts out 140@1800rpm. So at zero mph, with the same 11/1 starter gear,and an 1800TC, you would expect the same 1540ftlbs. But with the internal TC ratio of 1.8, this is increased to 2770.... for just an instant. Then as your car begins to move, the TC ratio begins ramp downwards, but by 13mph the ratio might still be 1.4 (I'm guessing), and your rpm may have increased to 2400 so at this time your engine may be putting out 180 ftlbs so now;
180x1.4x11=2770 ftlbs. From this point on, the TM in the TC is gonna continue to drop, and the engine torque is gonna continue to increase... upto maybe the high 3000s RPM, depending on your combo, where the torque will plateau for a bit, then begin to decrease.
What's the point? Well 2770 ftlbs is more than enough to break traction of any sized street tire you can fit in your stock tubs. Lets say your 340 peaks at 340 ftlbs and the TC gets down to 1.1TM at that rpm and speed, still in 11/1 starter gear, this maths out to
340x1.1x11=4110 ftlbs. This might be 28 mph, and obviously, at WOT, the tires are still spinning.......... ergo it follows that the 11/1 starter gear, for your combo, is overkill.
So what is 11/1 for you? ------ 11/2.45=4.49..

Ok so if 4.49 is overkill, then what is a good gear?
Well, the target has to include the internal TM of the TC. So if 1540 is enough for me to power away briskly then we can do the math backwards and get a real-world number for you. Suppose you currently have a 2400stall TC and your engine puts out 180ftlbs there; just suppose. Then 1540/(180x1.8x2.45)=1.94 rear gear...... to power away briskly.
But 1540 is not enough for me to break traction. I need about 2200. So
2200/(180x1.8x2.45)=2.77 now. But your TC is gonna lose TM as soon as the tires spin, because the torque differential is lost. So we have to swap out the 1.8 for something closser to1.4 say, and
2200/(180x1.4x2.45)=3.56s. Hmmmmmmm were have we seen this number before? lol. Sure everybody loves the 3.55s.... because with them, your 340 can bust the tires loose with the 1.8TM ratio, and as the rpm rises the TM in the TC is falling, but the torque output of the engine is rising, so the spin can be maintained, and your engine does not need to be at WOT to make it happen.
Bottom-line;
So while I need 4.10s to make this happen with a regular 4-speed, your 904 equipped engine might only need 3.55s. I say might, because IDK if your engine can make 180ftlbs at 2400rpm. I pulled those numbers out my arse.
But here's the trick; If your engine pulls 10% less, then you'll need either 10% more rear gear, or 'SOME" more stall, or some combination.
Opinion coming
I've had hi-compression 340s for most of my driving life since 1969. I know they are soft below 3000. This is particularly noticeable with a 4-speed. And that is why they usually got 3.55s or better. Whereas the auto cars usually got 3.23s and 2400 to 2600 TCs. When you stick a performance cam in 'em, the autos feel it right away. I don't have a clue how you ended up with 2.71s lol, but 3.55s will make a 3.55/2.71=31% improvement. But a hi-stall can easily surpass that. Consider the numbers I used 180/140=plus 28.6% more available engine torque. Even if the numbers are not dead-nuts accurate to your combo; the percentage should be close. If you add the two, which you can, then by swapping to both 3.55s AND a hi-stall, you could see a 60% improvement in footpounds to the tires. That will really wake up your otherwise lazy bottom end. Don't forget, the TC will make little to no difference once it has reached "coupling". After that, the TM is strictly in the gears. And finally with 3.55s and 27" tires 65=2870, which only you can decide if it is acceptable. My cure was to drive slower. And eventually, I went to an overdrive.

Oh, I gotta mention this;(more opinion)
My 367 combo is "fierce". First gear at WOT is just a tireburner. So I like to nail it at 30 to 35 mph, in second gear, to see what happens. my rpm here is just 2700. But it still lights up the BFG 295/50-15s
For you with 3.55s; 32 mph in second will be ~2150, so you'll want the KD to work, and jump it up to 3650 and POW! yur gone. 5500 will get you 48 with the 3.55s. Maybe your cam will get you 60@6800,lol, but I think it's better to shift earlier. Then 60mph in second will get you about 4000.
As you can see; this is no where near your power peak. Remember in the very beginning of this post, I said that the autos were a performance limitation for a streeter? This is why I said that. Your auto only goes thru the power peak once, on the way to 60mph, and on the street, the tires will be spinning most,if not all, of the way thru first gear, and you end up near peak torque, instead of peak power. This will kill your average power from zero to 60mph. And the ET will suffer, because of it. About the best cure for this is to bring the suspension up to speed to try and reduce your tire slip; spinning ain't winning..... Sometimes a softstart ain't a bad thing.........
Maybe you can imagine, that with 3.55s, if you could build a combo to make peak power at 4000, then you could go thru the power-peak twice on the way to 60mph. And if the average power was the same between this low-rpm combo, and your 340 combo....... you both might hit 60 at the same time. That would probably require more cubes tho.

Conversly; it might be better to gear for peak power at 60mph in first gear, and jump up the stall to get off the line better.
Suppose your engine falls off the cam at 6200, then you would need 3.23s to top out at 60=6200, and you could cruise at 65=2610 @zero-slip.
Off the line it would be a lil lazy..... unless you jumped up the stall. I like a 3500 for this. But remember this is a pretty big cam to peak at around 5900. That is a solid two sizes, maybe three, over the factory cam, say a 292*. Hyup, I had one of those in my 367 at 11.3Scr and with 3.55s it was ferocious at the top. But I got rid of it PDQ, on account of it was unacceptable on the bottom,for me, with a regular 4 speed.
I hope you don't have a 292 in a 10/1 iron headed 340,lol. 3.55s will never cut it.
How do you know? Well firstly tune the heck out of it with a properly synced up transfer slot. then crank the timing back to get about 750 rpm in neutral. Then measure the manifold vacuum. If it's down at 8 inches or less, and out the tailpipes it is singing a beautiful rumpiddy-rumpiddy rump, that's the one.
But I doubt you have this on account of your idle timing was 19* right? And you said the power-timing was around 31 IIRC, and delayed to prevent detonation; so the 292 I don't think would behave that way, even with 2.71 gears.
That timing curve is also killing your low-rpm power
Wow this was like reading a book. But I agree 3500 converter and a set of 323's and she should turn the tires for 1/2 block in low. By a converter is first on my list
 
yup 3 speed auto 904A when you say wound up I assume you mean too high revving right? Do you recall any speed/RPM's?
Well I don't have a tach to know rpm's but by ear engine seems to be up there in rpm's. But I don't drive too much on interstate. I could say it's livable but my exhaust also plays a role in how it feels/sounds.
 
If you are running an 8 3/4" rear and the 4.10-geared 3rd member is ready to go I'd just put it in and see how it runs, doesn't take more than an hour or 2 if you know what you're doing. If you don't like it then get some 3.55s.

Just a ramble about gearing... Around here the freeway speed limit is 75 and I like to cruise 75-80 which puts my Duster's 360 just under 3000 RPM with 2.94 gears and 28" tall tires. I plan to put in 3.55 gears because that's what my cam was custom-ground for but then I won't feel so eager to cruise at 80 mph. I also go to events at a track about 160 miles from me so my solution? I got a 1972 Dodge D200 to tow it. 17 MPG on the freeway is what I get currently with the Duster, it'll only get worse with 3.55s. No I don't think I'll save any money on gas in the long run (I doubt the truck will do better than 15 MPG when towing) but I really really don't like driving a car for 2+ hours with the engine at 3500 RPM. The biggest reason though is so I don't have to worry about breaking my Duster at the track and not having a way to get home unless I pay out the @$$ for a 160-mile tow.

Not sure if that helps at all lol just what I'm currently dealing with regarding gearing on my car.
 
If you are running an 8 3/4" rear and the 4.10-geared 3rd member is ready to go I'd just put it in and see how it runs, doesn't take more than an hour or 2 if you know what you're doing. If you don't like it then get some 3.55s.

Just a ramble about gearing... Around here the freeway speed limit is 75 and I like to cruise 75-80 which puts my Duster's 360 just under 3000 RPM with 2.94 gears and 28" tall tires. I plan to put in 3.55 gears because that's what my cam was custom-ground for but then I won't feel so eager to cruise at 80 mph. I also go to events at a track about 160 miles from me so my solution? I got a 1972 Dodge D200 to tow it. 17 MPG on the freeway is what I get currently with the Duster, it'll only get worse with 3.55s. No I don't think I'll save any money on gas in the long run (I doubt the truck will do better than 15 MPG when towing) but I really really don't like driving a car for 2+ hours with the engine at 3500 RPM. The biggest reason though is so I don't have to worry about breaking my Duster at the track and not having a way to get home unless I pay out the @$$ for a 160-mile tow.

Not sure if that helps at all lol just what I'm currently dealing with regarding gearing on my car.
I hear ya man. So your car is turning into a race car having a trailer for it. Unfortunately I am running with an 8-1/4 and having a shop install it. This is why I am making a big deal about it.
 
I hear ya man. So your car is turning into a race car having a trailer for it. Unfortunately I am running with an 8-1/4 and having a shop install it. This is why I am making a big deal about it.

Ohh, yeah that's a bit more of a pickle I get it now. Much harder to change your mind with an 8 1/4" even if you can do it yourself. I know for damn sure though, even if 3.55s aren't quite enough they will make a MASSIVE difference going from 2.71s. I bet even 3.23s would make it run a lot better.

I tried 4.10s in my Duster once, I didn't like it. All the car could do was launch hard in a straight line and cruise around town. I didn't even have the balls to rev it up high enough to keep up with 65 mph traffic because it was already screaming at 4000 RPM; I just held the RPMs there and let everyone pass me. It sucked. My engine didn't seem to need them either it already had massive low-end torque and I shifted at only 5500 RPM. BUT that is a very different combo than it sounds like with your 340 so take that with a grain of salt. Still even if the engine can handle it cruising at 4000 RPM sucks!
 
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the math says 4000rpm at zero slip =78.58 mph with 27" tires.
65 shoulddabin 3320@zeroslip.
With an automatic and 65=4000 that would be ~21% slip. That's a lotta slip

With a manual trans. I'd be looking to verify stuff.
My 4.88s cruised at 65=4000 with 27s. Yur right, it sucked so bad I had to get a GVod, to drag it down to 3120
 
I hear ya man. So your car is turning into a race car having a trailer for it. Unfortunately I am running with an 8-1/4 and having a shop install it. This is why I am making a big deal about it.

Might want to stick with the 3.55s for a while if you have them. If that engine is making any decent power and you get it to hook, that 8 1/4" diff may not like it and you'll be in the market for an 8 3/4". You should be able to get an idea on your converter stall by standing on the brakes in drive and loading the engine, it will stop revving somewhere around your stall speed and give you an idea if it's 2500, 3500, or whatever. Don't do that more than a couple seconds or the trans may not like it, and check your driver side motor mount afterwards.

FWIW...I have a 340/727 with 3.55s on a 29" tire and I don't think that I would be comfortable at 75mph+ for more than half an hour on the highway with it. It's right smack in the power band, but it's starting to get loud and such. On the back roads, no issues and I could go 3.91 if I know I'm not going cross country on the highway.
 
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