Hyup
the Commando and the regular A833 have very similar ratios, the principle differences being First gear, namely;
2.66-1.92-1.40-1.00 for the regular, and
3.09-1.92-1.40-1.00 for the Commando.
This makes the First gear to be 3.09/2.66=plus 16%
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This allows a 16% change in final drive ratio for no loss in performance of first gear. For example; going from a
2.66 x 3.23=8.59, is about same as
3.09 x 2.76=8.53
But if your 360 is a bit lazy off the line, with 2.66 x 3.55=9.44 ( which is the usual combination), then you could try 3.09 x 3.23=9.98, or like I have done
3.09 x 3.55=10.97, which takes off like 2.66 x 4.10=10.91 I find this gear very versatile in my combo.
>>The biggest deal with all this fooling around is what happens to the rpm at cruising. with 27" tires; 65 =
2872 with 3.55s, 2613 with 3.23s, 2379 with 2.94s, and
2233 with 2.76s.
The 2.76s will give the best fuel economy,
if you can optimize the timing and the fueling. 2233 is about the lower limit for a 360 and a stock-type distributor, and a modest street cam. Any lower cruise rpm and you probably will not be able to cruise-time it and therefore will not get any additional fuel economy.
Another problem is that as the cruise ratio goes down, so does the starter gear. By the time you get to 3.09 x 2.76=8.44, take-off is getting sluggish. At this point, you only have three options to mitigate that, namely;
1) rev the engine and slip the clutch out.
2) make the car lighter, or
3) make the engine produce more low-rpm torque
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Option#1: is too Mustang5.0 for me. I hate that. Maybe you're different
Option#2: is pretty limited; the first hundred pounds is fairly easy, but after that it gets hard, then harder, then nearly impossible. Every 100pounds is worth about 10 ftlbs of engine torque
Option#3: was my choice; and the only way to do that with a given engine size is with cylinder pressure.
Lets talk about #1. With a starter gear of 3.09 x 2.76=8.53, and 27" tires
1) your slowest roadspeed is about 7.0 mph @750 rpm, down to 5.18@550rpm. So parading is Not an option. And getting down to 550 in gear and crawling is gonna take a really good tune, especially as the Static Compression ratio begins to fall.There is a big big difference in the tune between 550 in Neutral, and 550 in gear and crawling.
I have tried this 8.53 combo, and I don't recommend it. and here's why; Second gear is 1.92 x 2.76=5.30Roadgear which is a really big drop from 8.53, namely 60%.
>>In Normal driving, I like to shift at 2800. My combo is just getting on the cam there and is sounding pretty healthy thru dual 3" full-length pipes. But the Commando splits are 60% -73%-71%, so shifting at 2800, the drops are to 1680 into Second, 2044 into Third, and 1988 into fourth. So then the ol' 360 needs some balls down there. Forget 8/1 compression and a cam, that's a really bad idea.
>>Now this is made worse by the fact that the speed will vary as the rear gear varies. For example; In Second gear,
1680 is 25.5 with 2.76s/42.5@2800, =17 mph
but only 19.8 with 3.55s/33@2800 =13.2mph; ..... So the 360 is forced to not only pull ~4more mph in 2nd gear with the 2.76s; but ending nearly 10 mph later..
>> In performance driving, those 2.76s will get you;
52mph in First at 5500, 57@6000, 61.5@6500 ; if you shift because the Rpm is getting too high, then the Rs will fall to 60% and yur engine is off the cam, so you might as well leave it in first.
But you say, yur only gonna install a small cam, peaking at ~4800. Ok but for low ET yur gonna want to wring that up a lil higher to wherever the outgoing power in First matches the incoming power in Second. With a 60% rpm drop, yur gonna have to wring it out. My guess is 5500 is the starting point.
Like I said; I tried this combo, and didn't like it.
>>But I did learn something.
Firstly; I am a streeter and don't give a hoot about bragging rights from track-results.
I learned that Second gear is where the fun is. If you match Second gear to your wants, needs, and driving style, this will produce the most amount of personal satisfaction.
So then, I had to decide where I wanted the Power in Second gear to be.
Firstly, no matter what street-cam you put into your 360, it will have at least, a good Second gear. But the Powerband gets more narrow with each bigger cam and each smaller cam, starts just a lil sooner. About the best powerband that yur gonna get is from the torque peak to the power peak, plus the overspeed allowance. This usually comes in around 1500 plus about 600 on the overspeed, so say 2100.
Forget the 2.76s.
So lets attach a speed value to this to get an idea of the Second gear ratio required. I like to be outta overspeed at 60 mph or so.
So the Formula is this
Mph= (rpm x Tc)/ 1056 x R1 x R2)
where Tc is Tire circumference, and the Rs are the gears involved.
Ok so we know; Mph =60, TC with 27s is 84.8, R1(2nd) is 1.92, and rpm is around 5800 with a really small cam
What we want to know is R2.
So we rearrange the Formula like this;
1/R2= (Mph/rpmTc)x 1056 R1 =4.05+ the rear gear. the proof
(5800 x 84.8)/ (1056 x 1.92 x 4.05)m = 60
ok so lets round DOWN to 3.91s. and I get 5800=62 mph.
The cam thus can be 600 rpm overspeeded at 5800 so power peaking at 5200; that's gonna be a 232* @.050 cam ( I bought a
[email protected], lol)
The torque peak of this cam should be ~3700 and with 3.91s this puts it at 39.6 mph, while 5200=55.6.
So then your powerband is from 39.6 to 55.6mph, and your rear gear for this exercise is written in stone at 3.91.
Now, hang on, just cuz the torque peak occurs at 3700, doesn't mean the bottom end below that is junk; it only means that the peak is at 3700.
Lets say you matched all yur parts really well and your engine is making an easy 400ftlbs at 3700. That translates to 282 hp. Think about that.
282hp with a roadgear of 3.91 x 1.92=7.81, is enough to produce wheelspin with nearly any street tire. So this is a really fun combo...... until it comes to fuel-economy and cruising;
With 3.91s and 27s, 65=3165............... OOps
I solved that with a GVod, that would turn 3.91s into .78 x 3.91=3.05s
Now, I don't run this combo, I mean I tried it, but found out that my 367 was powerfull enough to do this with 3.55s no problem. Even 3.23s could be used.
And finally, lets get back to the starter gear, with 3.91s your choices are
3.91 x 3.09=12.08, and 3.91 x 2.66=10.40 12.08 will get you 550rpm @3.66 mph, now you can parade.
If I had to chose between the two, I would prefer the 10.40. It is enough for good drive-away, but not so much that I'm looking to shift right away because the engine is taching up so quick. And the tighter 1-2 shift is very welcome.
>> Furthermore, this is just an exercise in math. A 232* cam will suck gas pretty bad on the hiway no matter what the cruse rpm is. And the reason is the short power duration, which sacrifices the last few degrees of energy in the expanding gasses in favor of rpm; that's just the way it is.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If fuel economy was of first importance, and I could not afford an overdrive; here's what I would do;
I would design that 360 to cruise with the 2.76s for 65 =2233 with 27s. This is the lowest rpm that you can just barely supply adequate cruise timing for with factory parts. This engine is likely to want about 56* of cruise timing. You can modify the factory Vcan for 24*, leaving a minimum of 32* to the combination of mechanical plus initial.
The mechanical can be set up to deliver about 12* by 2250, leaving an initial of 20*. IMO, that's awful high for a stick-car, but it's doable. Like I said 2.76s is about as low as you can comfortably cruise at, with factory parts. FWIW, I cruise mine at 2240,lol.
So now your rear gear has been selected for max fuel economy.
This will make the Commando 3.09 low, mandatory for 3.09 x 2.76=8.53 starter gear, about as low as you dare go. So now that just leaves the engine.
To pull this off, the 360 is gonna need a ton of low-rpm power. As said earlier, this takes cylinder pressure. So the first place you are gonna have to look at, is what heads to use.
The problem is not making enough power at 2233 rpm.
The problem is doing it efficiently enough to get the economy that you are looking for. To do this, there is no substitute for cylinder pressure. The more you have, the more heat will be created in the chamber, and that translates directly to power-production.
With iron heads, you are limited to about 160 psi at WOT on best gas with closed-chambers.
With alloy heads, I have run up to 195psi at WOT on 85E10.
The limiting factor is detonation which breaks parts and destroys engines.
So now you may have to make a budgetary decision.
But before you do.
At this point, the above scenario is based on 2.76s.
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I gotta tell ya, unless you have 3.55s , You can retune this combo to run on any rear gear that you already have, that is no bigger than 3.23s, so not having to swap gears could get you nearly half the price of a set of alloy heads, which, at the new possible pressure, will more than make up for the cruise rpm difference. In case that's clear as Mud, let me say it a different way; Running alloy heads at say 185 psi with 3.23s, will make better fuel economy than open chambered iron heads will with 2.76s
And it will be easier to get the cruise timing maxed out with higher than 2.76s, and not have the initial timing set so high. Each gear size bigger than 2.76s is worth about 3 to 4* initial. So 3.23s may get you down to 14/12* initial, which is where the stick-car needs to run.
Ok then, lets say we have the rear gear set at 3.23. Now we don't need the Commando anymore. We saved anuther pile of cash.
So now lets put that saved money into making pressure.
>>>The next decision is how much pressure do we actually need. For max fuel economy, the answer is, as much as possible, because only a fraction of the pressure that the engine is capable of making, is gonna get past semi-closed throttle blades.
Lets say you build an engine capable of making 200psi at WOT; What is it capable of at idle? Well just do a compression test on your engine with the throttles closed, and register just the first two readings and average them. My guess is that you would be lucky to get that 200psi engine to make even 120psi. So this would be 60%; but the thing is, it will run detonation free at 200! so why are we asking it to run on 120? Well, if we had set the Wot to 150, the cruising pressure might only be 90psi. Or, if we want 200 at cruising, the Wot would have to be set to 333, which would quickly destroy the engine.
So we have to set the WOT as high as we have fuel for, to get the cruise pressure up.
So your choices are; a minimum of 150psi with iron, open chambers running on 87, to 190psi with closed-chamber alloys running on 87E10.
The "point-to-point" mpg range on your modest cammed long power-stroke duration combo is gonna be maybe 18/20@150, to maybe 26/28@195
How do I know that? Well IDK, maybe cuz I already tried it? lol. I mean your results may vary, but statistically speaking, not by much.
So once you have chosen your heads, you have to look at the combination of total chamber volume versus the Ica of the cam, to get the pressure you need, and whatever the power ends up being, well that's what you get.
Suppose you go with the alloys and suppose they are 63cc. And
yur gonna have to run the .039 Fell-pros for reliability, which are 8.6cc
Yur gonna have to install flat top pistons no matter what. And they usually will have a 5cc allowance for eyebrows. and
I'm gonna tell you that the Q with those .039s is exactly that, but the engine will do better with less all the way down to .028 which yes, I have run, With flat tops then, your pistons will have to come up to .039 less .028=.011 . I have been running my KB107s at plus .007 for IDK, 80,000 miles with no failure so that's what I can recommend, namely a pop-up of .007.. My pistons are at 4.040 and my cylinders at 4.0450
If you run that, your total chamber will be 63 +8.6 +5 -1.5 pop-up=77.1, and your swept will be 753.9cc , for an Scr of 10.78, not too shabby; and your Q would be .039 less .007=.032; close enough.
So from here we go to Ica selection to make max pressure. Using the Wallace, I get 61* for 185 psi; 59* for 189psi
From experience, I can see that this will be easy, Lets go for the one with 59*.
Firstly; Ima gonna use the,
@David Vizard rule of 128, to find your best LCa and with a 2.02 valve I get 107*
Then working backwards I get this;
266/274/107 advertised with 121 compression = an Ica of 59* and a power duration of 114* for plenty of potential economy, and in at 105= plus 2* . The overlap is a strong 56*
The .050 on this could be in the range of 222 to 218 on the intake, so the power will peak maybe 4900, and shift-rpm around 5400. This cam will have plenty of bottom end and together with the pressure would make a real street powerhouse, easily pulling those 3.23s. And since 65mph=2614, will make pretty darn good fuel economy as well, together with that "long" 114* of power extraction.
So how much did we spend?
Well, we didn't need to buy a Commando trans, and we didn't need to buy a rear gear. So near as I can tell we mighta saved an easy 1200 before shipping, but spent that on the heads instead. Pistons we needed anyhow, but we decked the block So all in all, pretty darn cheap.
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I know you didn't ask for any of this and sure as heck somebody is gonna call me on it, but
was it helpful to you?
If you get the idea, just work out a combo for yourself, using the concepts I provided. Need more? Just ask.
BTW,
A cam with 56* of overlap is usually considered a pretty gnarly street cam. But in this case, on a 107Lsa, it leaves a lot of compression/power cycles to work with, so is not as big as it would seem , nor as small. For guys who are used to working with 110* LSAs, this 266/274/107 might seem like a bit of an enigma. The fact is that this, in your combo, with 3.23s is about as good as it can ever be.
Yes I think you could run a lil less gear later on, just don't try and run much lower than 2400rpm cuz the economy is gonna start to go away.
Yes I think you could run a lil less of a cam, but I don't think less will net much better economy and, a smaller 107* cam. will just end up with more than you need bottom-end torque.
Yes I think you could run less cylinder pressure. You could forgo the decking entirely, in which case the KB-107s might fall in at .012 below deck. Just run a .028 gasket with a known good reputation. The Q will be back to .040, and performance will hardly affected.
Yes, a small spreadbore carb will be all that you need, and for economy, forget a double-pumper.
As for intakes, I'd run any aluminum spreadbore street intake, it hardly matters. I might even consider a street tunnel-ram with tiny 4bbls and staged progressive. That could be a real hoot.
At this power level, you could even run Magnum alloy heads and Magnum-sized runners, with 1.92 intakes and that would allow an LSA of 106* and you can put the extra degrees into the Power-extraction cycle, grabbing a tiny bit of economy there. Yeah I think that's what I might do.
Yes Magnum heads can be run on LAs.
ok breakfast time
Oh wait; The Commando trans was designed for smaller engines or engines with less powerful bottom ends, or combos running very low-number rear gears. The whole idea behind it is get some take-off power outta the trade. The downside is that Second was not changed, so it's sortof way out there. The combos Here detailed won't need it.
In fact, this combo argues very well for the super wide but more progressive ratios of the A833OD. The ratios are
3.09-1.67-1.00-.73od, the splits are .54-.60-.73
With this combo, you can run just about any rear gear you want to.
To get 65=2400, you would run 3.91 and the proof is 65=2310, perfect.
The starter is 3.09 x 3.91=12.08 good for parading. Second is 6.53 which is about the perfect Second gear there is. The only down side to this trans is the 54% 1-2 split which your hi-pressure 360 will not really have a problem with, but it will take some getting used to. So
Yes I would, try the A833od. It's actually a very good compromise, for this hi-pressure combo. Plus you can buy them for cheap. I would just recommend one thing; to stay away from the slanty ones which have a smaller mainshaft. I don't know how long one of those would last behind your stump puller 360, lol. Once you get Second gear working for you, from 30 to 50/55 you are gonna have so soooo much fun.
Ok now it's breakfast time and I'll edit later........... done
Ok one more thing; Here's a comparo of the different transmissions/rear ends, required to get, IMO, a really great Second gear: I am looking for a Second Roadgear of ~6.80ish
1) *9.44-
6.82-4.97-3.55-- this is the standard 2.66Low A833 with 3.55s
2) 10.97-
6.82-4.97-3.55-- this is the Commando and 3.55s
3) 12.67-
6.84-4.10-2.99-- this is the Mopar od box with 4.10s
4) 12.08-
6.53-3.97-2.85-- This is the Mopar od with 3.91s
5) 10.97-
6.82-4.97-3.55--2.77-- My Commando with 3.55s and a GVod
As to #5, I shift this; 10.97-6.82-4.97-3.88(3rd over)-2.77(4th-over)
the splits are
.62-.72-.78-.71od, tight and progressive, and a nice drop into od. I have run this combo with as much as 4.89!s for like a weekend, to parade with; 550rpm was a tic under 3mph. I also ran it with 4.30s for about a summer. and another time with 3.91s; but Second gear was still done too soon. I just keep coming back to that 6.82 Roadgear. I even ran it with 3.23s, for a Second Roadgear of 6.20, which actually wasn't that bad.....
