RPM's

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straightlinespeed

straightlinespeed

Sometimes I pretend to be normal
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Quick run down of my specs.

360, xe268 cam, magnum heads, air gap, Holley 670, TTI's, 727, 2800 stall converter, sure grip with 3.73 gears.

I've noticed that driving 60 mph Im close to 3k RPM going down the road. At 70 around 3500 RPM.

If I kick my car down to WOT I can literally almost watch the gas gauge drop. I know my fiance has made comments about it moving when I've punched it. I also get horse **** gas mileage. I know the car was not made or engine built for fuel economy but seriously I can only get about 80 miles for 3/4 of a tank of gas if Im heavy on the throttle. I've only put about 150 miles on it and 2 tanks of gas so far this summer.

I've been debating about picking up another center section or just putting in 3.23 or 3.55 gears. Not sure what affects this will have on my particular set up, so Im looking for your opinions. Thanks
 
What size wheels and tires ? I get better mileage than that from my stroker with 3:23 s and 245/60/14s , I am upgrading to 275/60/15s which are almost 3" bigger in diameter and travel about 10% further per revolution , If your running 3:73s with a 14 inch tire your killing yourself on the highway .
 
Mighty short tire. Your effective gear ratio is probably around 4.30.
 
2004R all the way!!! My cruising RPM dropped dramatically. It's not that simple as that but it worked for me.

Treblig
 
The tire size table shows them to be a 26" diameter tire which is a little bigger than my 245/60/14 . I would suggest playing around with the tremec calculator that uses gear ratio and tire size to compare RPM , your mileage still should be better than 80 miles a tank thats like 4.8 miles to the gallon , how black are your plugs ? Are your neighbours sucking on your tank at night ?
 
RustyRatRod said:
Mighty short tire. Your effective gear ratio is probably around 4.30.
Click to expand...

Ouch!! I dont like the sounds of that..

Treblig said:
2004R all the way!!! My cruising RPM dropped dramatically. It's not that simple as that but it worked for me.

Treblig
Click to expand...

That isnt in the cards for me right now, but it does intrigue me. Perhaps in the future.

cannucky said:
The tire size table shows them to be a 26" diameter tire which is a little bigger than my 245/60/14 . I would suggest playing around with the tremec calculator that uses gear ratio and tire size to compare RPM , your mileage still should be better than 80 miles a tank thats like 4.8 miles to the gallon , how black are your plugs ? Are your neighbours sucking on your tank at night ?
Click to expand...

Im open to suggestions on tire size. I actually had the plugs out this past weekend and they appear to be burning very well, and had good color to them. My timing might be a bit out of adjustment, but I doubt that would make a world of difference. Oh and I wish that was the case for the gas tank, but it stays parked in the garage.
 
Put a 2.94 suregrip in it. And put a stock stall rpm converter in there.
It will be a great highway power machine.
Long stroking it all the way to 145 or so. Cruisin at 70 (at 2650 rpms) will be like butter as your cam just came on torque. Cruising at 90 will also be like effortless velocity while your turning 3400 rpm.

At 2500 - 3000 rpms is likely your engine's peak sweet spot for mileage.

So your target driving speed and your gears should match with the cam curve and torque converters rpm stall area. A loose torque converter is just stirring oatmeal unless it is always operated well above the stall rpm so any time you are below that, you are loosing gas mileage and power there too. If you have a 2800 rpm stall, and you accelerate from 0 - 60 at light throttle where your rpms are from 1500 - 3000, the TC is just eating a pile of your energy as it hardly locked up during that time.


where you can have the best of both worlds is the 200R4 with lockup TC or a mopar unit of similar design. Then you can launch in an exaggerated (by old mopar standards) low gear and lock the TC when ever you want and have overdrive high top gear. Totally haul *** in every direction.
I cant afford an OD myself so I am going to try and compromise with 3.23 or 2.94 based on tire diameter and a low stall converter.
 
Those short tires are a mileage killer since like Rob said the final drive ratio is so low. Either put 3.23's in it or much taller tires on it. The 3.23 gears will help the mileage more than the tires cause taller tires weigh more so the drag from them is greater. Taller tires have the advantage of getting better traction but if that's not a problem I'd go with the 3.23's. Naturally any taller gears (or tires) will cost you some take off umph so keep that in mind. The converter, if it's a decent one doesn't really hurt the mileage as bad as some think as long as it's not slipping bad. If you go with a stock converter you may have trouble getting it to idle decent in gear at stop signs with that cam plus it'll lose a ton of take off.

Like Cannucky, I get far better mileage from my stroker but I also run 3.55's with 28" tall tires. When I keep my foot out of the 750 double pumper it can knock down 16 mpg. Even gear heading it I rarely see less than 12. Last fall I drove it 40 miles to the track, ran twice, drove home and checked the mileage and it got 14.5 mpg. Ran a 12.24 in the 1/4... As you can see fair mileage is achievable even with a hot car.

Even though your plugs might look ok on color the air/fuel ratio could still be off enough to affect MPG quite a bit. Install a wide band O2 sensor and you can watch what's happening. I put one on my Cuda and tuned the carb and it now gets 4-5 mpg more than it did and it's faster. Timing can make a huge difference in fuel mileage so if it isn't dialed in do so. If your distributor doesn't have vacuum advance get one that does. Shoot for 20-22 degrees of initial timing with 34 total and about 15 degrees of vacuum advance at part throttle. Tune the timing first then the carb cause timing affects air/fuel ratio

The suggestion of installing a OD trans sounds great until you look at the cost. If you drive a lot (at least 8000 miles a year) you'll recover the cost of installation in a few yrs. but if it's a weekend cruiser/hot rod it could take 15+ yrs. to recover the cost. I'm not knocking it, just looking at it realistically.
 
205-70-15 is just a tick over 26" tall so it's taller than the original tires were.
I would move to a vacuum secondary carb (if you don't already have one) and make sure everything is tuned correctly.

I think you have more MPG in what you have. The big problem is this is a new build and your right foot is working very hard to keep a smile on your face.
 
A different trire would be my first move. A GVOD would be my mech fix of choice.

I'm not seeing an issue if you are under 3k at 60 mph though.
 
727, 2800 stall converter
3.73 gears

the two mileage killers.
you could switch it to manual shift.
you could switch to 2004R and all that rigamarole to get a lock up converter
you could swap the rear to a 3.23

if it was my money i'd do a passon 5 speed (has overdrive), keep the 3.72 rear
[ame]http://www.passonperformance.com/images/stories/passon_documents/Passon_5-Speed_Flyer_2010-11-18.pdf[/ame]

but you will need pedal setup from a salvage yard, bell housing, clutch, flywheel (external balanced 360 is different than internal balanced) etc etc etc

so the summary answer is $$$ or drive it like it is
 
I will take those gears when you are ready to swap! 4.56 gears with a 4000 stall here, talk about bad 'mileage'... LOL
Having lots of fun for now but want to tame it down for sure.
 
Keep in mind that unless you drive the car quite a bit it is going to take a very long time to recoup the $$ it takes to make any sort of major changes...especially the one that was mentioned about a Passion 5 gear. How much does he want for the box, 4k or so is what I seem to remember seeing some where. By the time you buy all the necessary parts you will be over 5k.....how long is it going to take to "save" that money in fuel costs? Sure, there are a few folks here that will tell you that you can find a complete pedal assembly/flywheel/clutch assembly for $300....but I have never been that lucky....
 
3.23 isn't that much of a difference from what you have now, still vote spare 2.94, and big tires as the best option here. :angel5:

[ame]http://www.ebay.com/itm/Mopar-8-8-75-Dana-2-94-Ratio-OEM-12-Bolt-Rear-End-Ring-Pinion-Gear-Set-/171342779239?pt=Motors_Car_Truck_Parts_Accessories&hash=item27e4d35367&vxp=mtr[/ame]

If you have a tight converter you should have no issue with slip cruising a few hundred RPM below stall. Try it out, use a tach and see what your RPM vs speed is at compared to what your cruise would be with new gears, and compare with this tool. Rev it a little while your there to see how tight everything is.

Play with this to get an idea, and measure your tire from center of hub, to ground to get accurate size.


http://vexer.com/automotive-tools/speed-rpm-calculator
 
fishy68 said:
Those short tires are a mileage killer since like Rob said the final drive ratio is so low. Either put 3.23's in it or much taller tires on it. The 3.23 gears will help the mileage more than the tires cause taller tires weigh more so the drag from them is greater. Taller tires have the advantage of getting better traction but if that's not a problem I'd go with the 3.23's. Naturally any taller gears (or tires) will cost you some take off umph so keep that in mind. The converter, if it's a decent one doesn't really hurt the mileage as bad as some think as long as it's not slipping bad. If you go with a stock converter you may have trouble getting it to idle decent in gear at stop signs with that cam plus it'll lose a ton of take off.

I spent good money for a converter and had it custom built for my car. Now if I switch to different gears or tire size how is that going to affect the converter? I understand that I will loose some performance by swapping to different gears or taller tires, I just dont want it to be sluggish. As for a stock converter I had one in my car prior to buying the custom converter and you are correct I couldnt get it to idle very well and take off was embarrassing. Im pretty sure the kid on the tricycle whooped my butt at the stop light.

Like Cannucky, I get far better mileage from my stroker but I also run 3.55's with 28" tall tires. When I keep my foot out of the 750 double pumper it can knock down 16 mpg. Even gear heading it I rarely see less than 12. Last fall I drove it 40 miles to the track, ran twice, drove home and checked the mileage and it got 14.5 mpg. Ran a 12.24 in the 1/4... As you can see fair mileage is achievable even with a hot car.

So are you turning about 2600rpms at 60mph? I was debating about switching to a 235/70/15 tire and maybe 3.23 gears.

Even though your plugs might look ok on color the air/fuel ratio could still be off enough to affect MPG quite a bit. Install a wide band O2 sensor and you can watch what's happening. I put one on my Cuda and tuned the carb and it now gets 4-5 mpg more than it did and it's faster. Timing can make a huge difference in fuel mileage so if it isn't dialed in do so. If your distributor doesn't have vacuum advance get one that does. Shoot for 20-22 degrees of initial timing with 34 total and about 15 degrees of vacuum advance at part throttle. Tune the timing first then the carb cause timing affects air/fuel ratio

Im sure that I need a good tune, and it was already determined that I need to switch back to my stock jets to give it a try about my hesitation problems, which is another thread. I cant go that far advance with my initial timing. I need to have my dizzy curved, the person I got it from used it in his drag car so it has the light weight springs. I started at about 18* initial and when I reved it I went up to about 42*. So I backed it down to 15* and now Im at about 38* total. Again, totally different problem for sure but not contributing to my gas mileage.

The suggestion of installing a OD trans sounds great until you look at the cost. If you drive a lot (at least 8000 miles a year) you'll recover the cost of installation in a few yrs. but if it's a weekend cruiser/hot rod it could take 15+ yrs. to recover the cost. I'm not knocking it, just looking at it realistically.

Ya, its just not a possibility for me. Tires and gears are though.
Click to expand...

1968FormulaS340 said:
205-70-15 is just a tick over 26" tall so it's taller than the original tires were.
I would move to a vacuum secondary carb (if you don't already have one) and make sure everything is tuned correctly.

The 670 is a vacuum secondary carb. But I need to tune it all properly, I just suck at tuning and not sure Im doing things correctly.

I think you have more MPG in what you have. The big problem is this is a new build and your right foot is working very hard to keep a smile on your face.

Isnt this the truth!!!

Click to expand...

roccodart440 said:
A different trire would be my first move. A GVOD would be my mech fix of choice.

What does GVOD mean?
Click to expand...

moparlover said:
727, 2800 stall converter
3.73 gears

the two mileage killers.
you could switch it to manual shift. Not a option
you could switch to 2004R and all that rigamarole to get a lock up converter Not a Option
you could swap the rear to a 3.23 Is a Option

if it was my money i'd do a passon 5 speed (has overdrive), keep the 3.72 rear
http://www.passonperformance.com/images/stories/passon_documents/Passon_5-Speed_Flyer_2010-11-18.pdf

but you will need pedal setup from a salvage yard, bell housing, clutch, flywheel (external balanced 360 is different than internal balanced) etc etc etc

so the summary answer is $$$ or drive it like it is
Click to expand...

MobileCustoms said:
I will take those gears when you are ready to swap! 4.56 gears with a 4000 stall here, talk about bad 'mileage'... LOL
Having lots of fun for now but want to tame it down for sure.

For sure! We can work something out...Since you are close and all! Hell if you know how to tune a engine better than I do, come over, have some beer, and we can chit chat etc...
Click to expand...

inkjunkie said:
Keep in mind that unless you drive the car quite a bit it is going to take a very long time to recoup the $$ it takes to make any sort of major changes...especially the one that was mentioned about a Passion 5 gear. How much does he want for the box, 4k or so is what I seem to remember seeing some where. By the time you buy all the necessary parts you will be over 5k.....how long is it going to take to "save" that money in fuel costs? Sure, there are a few folks here that will tell you that you can find a complete pedal assembly/flywheel/clutch assembly for $300....but I have never been that lucky....

This is so true and the real reason I will not spend the time or money on swapping to a OD trans. I only drive this on the weekend because of collector plates. I would love to be able to go on a day cruise and not worry about how much Im spending on gas, considering I fill it with premium, or how far the next station is.
Click to expand...

PoisonDart74 said:
3.23 isn't that much of a difference from what you have now, still vote spare 2.94, and big tires as the best option here. :angel5:

http://www.ebay.com/itm/Mopar-8-8-7...Parts_Accessories&hash=item27e4d35367&vxp=mtr

Thats a good deal on a R/P. Im still curious as to the largest size tire that can fit in the opening? I have about equal space from the tire to the front rocker area and the inner lip to the sidewall of the tire.


If you have a tight converter you should have no issue with slip cruising a few hundred RPM below stall. Try it out, use a tach and see what your RPM vs speed is at compared to what your cruise would be with new gears, and compare with this tool. Rev it a little while your there to see how tight everything is.

One thing the builder of the converter mentioned was. "I set your converter at 2800 rpm of true stall, and str is set at 2.40. You will have perfect street manners, and a great wide open throttle launch." So Im curious if I switch gears or tire size what happens to my converter since it was built for this application?

Play with this to get an idea, and measure your tire from center of hub, to ground to get accurate size.


http://vexer.com/automotive-tools/speed-rpm-calculator

That is cool! Thanks
Click to expand...

I responded to everything in Red
 
Well since you love your converter and I dont blame you. You will want to maintain your engine rpm above the converter stall so that you are properly connected to the ground in order to get decent mileage. So, I guess if you either move 3" taller rear tire or change gears to taller and still maintain cruise speeds above stall speed, this would mean you are traveling further for the same revs.

Seems like you would want your target cruise rpm to be 3k to 3.1k.

I agree with everyone here, just throwing out my two cents.

I also agree that you may have better mileage left on the table with your existing build. Maybe a careful timing scheme with some vacuum advance, jetting changes too.

There is a very well thought out and nicely explained timing tuning plan posted in this thread by jbc426 which entails using manifold vacuum which is supposed to enhance cruise mileage and part throttle torque. He explains that there is a symbiosis to initial, total, curve rate, vac can choice and settings which would likely require a lot of patience, bit of fab and a good level of skill. I am giving it a solid effort to duplicate his recommendations in the near future.

http://www.forabodiesonly.com/mopar/showthread.php?t=278252
 
straightlinespeed said:
I spent good money for a converter and had it custom built for my car. Now if I switch to different gears or tire size how is that going to affect the converter? I understand that I will loose some performance by swapping to different gears or taller tires, I just dont want it to be sluggish. As for a stock converter I had one in my car prior to buying the custom converter and you are correct I couldnt get it to idle very well and take off was embarrassing. Im pretty sure the kid on the tricycle whooped my butt at the stop light.

I hear ya man. A good converter sure makes a difference.
Click to expand...
So are you turning about 2600rpms at 60mph? I was debating about switching to a 235/70/15 tire and maybe 3.23 gears.

2800 rpm at 60 with 3.55's and 28" tall tires. I just figured yours with 235/70/15 and 3.23 gears using 8-9% converter slip factor (that's a little on the high side but your converter was built for 3.73 gears so it'll slip a little more with 3.23's) for the converter and come up with 2550 rpm at 60 MPH
Click to expand...
Im sure that I need a good tune, and it was already determined that I need to switch back to my stock jets to give it a try about my hesitation problems, which is another thread. I cant go that far advance with my initial timing. I need to have my dizzy curved, the person I got it from used it in his drag car so it has the light weight springs. I started at about 18* initial and when I reved it I went up to about 42*. So I backed it down to 15* and now Im at about 38* total. Again, totally different problem for sure but not contributing to my gas mileage.
Click to expand...


Yeah you need it re-curved cause even 38* is quite a bit for a LA. Too bad you ain't close. I've done numerous distributors. Doesn't take long at all and really makes a big difference.
 
Do you smell any gas when you nail it and see your gauge moving?

Could someone have drilled in the trunk and gone through the top of the gas tank? I've heard of that before.


Pull your plugs? Are you running rich? Are the plugs wet fouled or black?


My 68 340 Formula S with a 3.91 gear screams at 4000 k at 70 MPH with a 650 Double Pumper. MP .484/284 cam with Rhoades lifters, stock converter and I get better mileage than that. (not sure of the tire size, but taller than stock).


You'd better shut that thing off when you fill up with gas, you'll gain on the gas pump and never reach full.... LOL!


I would check for gas leaks to make sure that it's not leaking out. Second see if you're running too rich. Then chase "more efficient" components...


Could a neighbor be siphoning your gas when you're not looking???
 
My 02 cents: 360 ported heads/2800 9.5 GER converter /3:55 TracLoc./" those 275/60/15's " actually measure under 27"...(750 4779 d/p) : Was getting 21-22 MPG@62 MPH,with vac advance hooked up & a vacuum gauge. Your can is smaller,pay attention to the small details.
 
Rice Nuker said:
Well since you love your converter and I dont blame you. You will want to maintain your engine rpm above the converter stall so that you are properly connected to the ground in order to get decent mileage. So, I guess if you either move 3" taller rear tire or change gears to taller and still maintain cruise speeds above stall speed, this would mean you are traveling further for the same revs.

Seems like you would want your target cruise rpm to be 3k to 3.1k.

I agree with everyone here, just throwing out my two cents.

I also agree that you may have better mileage left on the table with your existing build. Maybe a careful timing scheme with some vacuum advance, jetting changes too.

There is a very well thought out and nicely explained timing tuning plan posted in this thread by jbc426 which entails using manifold vacuum which is supposed to enhance cruise mileage and part throttle torque. He explains that there is a symbiosis to initial, total, curve rate, vac can choice and settings which would likely require a lot of patience, bit of fab and a good level of skill. I am giving it a solid effort to duplicate his recommendations in the near future.

http://www.forabodiesonly.com/mopar/showthread.php?t=278252
Click to expand...

Right now I am cruising in that 3k range at 60mph.

I'll take a read thru the thread you posted and try to find a place that will curve my dizzy for me locally and go from there. I agree with everyone that I have a lot left in my engine so I will look into this first!
 
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