speed vs mileage vs peak torque

[blazinzuk]

Everything I have read and learned about engines say that your most efficient cruising speed is around your peak torque.

My dually has 4.56 gears and 235/85s on it, according to my estimations with my set up, 454 with upgraded cam intake manifold and headers, but stock otherwise, should hit peak torque right around 2500 rpm.

Now with the 235s and a 465 trans at 55 mph I should be right around 2600 rpm, probably a little more cause gear calculators rarely take into account actual static loaded radius of tires just overall height.

I realize a 454 isn't going to be getting decent mileage but I am hoping to get around 8 towing and 10 driving around.

Are my rpms going to be too high to expect these kinds of numbers?

 

[lectric80]

I know my buddy has a 79 Blazer with a 454 33's and 3.83 gears and he gets a max of 7 with the 454. He says it could probably be higher, but he hasn't found a downhill slope long enough to prove it. His cruising RPM is around 2400 at 60.

 

[76zimmer]

I think your right in the thought that the engine is most efficient right around peak torque, but best fuel mileage will be at an rpm less than peak torque. And don't forget that aerodynamics plays a huge part in that above 50mph.
I'm guessing here, but with your engine, I think best mileage would be about 1500rpm @ 55mph, if that is attainable in your combination.

 

[Hogback Fabrication]

Realistically, best MPG is going to be at the lowest RPM you can turn and not bog down the motor. Turning the power steering pump, water pump etc in addition to the motor pumping more air through it at higher RPM all contribute to lower MPG. There's a limit of course, but generally lower RPM is better for fuel economy.

 

[blazinzuk]

I do realize that the lower the rpm the better mileage I will get.

This is a tow rig though, its overall efficiency is important. I don't have alot of leeway in the building of this truck. I could probably justify new tires that are bigger. I am currently looking for some used gears to change from 4.56s to probably 3.73s. Looking at a bunch of different options as far as lowering the rpm.

This truck will haul Horton around, not sure how much he weighs probably around 5k or so plus the weight of the trailer plus all the gear I need. Like Chris said at some point the lower rpms become a problem because you are not making enough power to pull the entire load, and by power I mean the combination of gear and torque

Those with 454s where do you feel comfortable with your rpms at cruise speed?

 

[4X4HIGH]

The best ecomony IS when the engine is cruising at it's peak torque. The reason is the engine doesn't have to work as hard to do the work being asked of it. Putting a numerically lower gear will cause the RPM's to go down but the engine will have to work harder to do the same work and that is going to kill your MPG.

 

[blazinzuk]

So I, assuming my motor is going to make its peak torque at 2500 rpms or thereabouts, should go for a 2500 rpm cruising speed?

This was my initial thought but I always second guess myself. Hence I seek the help of the wise

 

[4X4HIGH]

So I assuming my motor is going to make its peak torque at 2500 rpms or thereabouts, I should go for a 2500 rpm cruising speed?

This was my initial thought but I always second guess myself. Hence I seek the help of the wise

The only way to know for sure where your peak torque is going to be is to run the rig on a chassis dyno and find out 100% for sure where it is. You can get a good idea using Desktop Dyno IF you have real true accurate info to input into the program.

Most typically though peak torque is usually between 2300-2500 RPM

 

[blazinzuk]

currently trying to find head flow numbers for my crappy current heads, I know my cam specs and intake manifold stuff. Know I am running a quadrajet, know what headers, stock rotating assembly. Zim has been running them for me. As soon as I get him some good numbers I should have a better idea of how this engine will perform.

Of course it will be different next summer after I get my 781 heads done

 

[4X4HIGH]

You will have to know what the piston deck height is to correctly compute CR along with compressed head gasket thickness and CC's of the combustion chambers (this varies ALOT even given a particular casting number).

 

[76zimmer]

I know there is a program out there that shows fuel consumption throughout the rpm range too. This might shed some light on how torque affects fuel mileage.
With all due respect Scott, I feel that many of the torque peaks I see are in the 3500 range. maybe thats because they are modified profiles, not stock, but still if an engine makes peak torque at even 3000 rpm, it would seem to take more fuel running an engine that fast, as opposed to 1500, IF the engine can get the job done at 1500. I realize that you might only be making 300tq @ 1500, as opposed to 500 @ 3500, but if you only need the 300tq. and your gearing is right to run 55mph @ that rpm, I would think the engine would be more fuel efficient at the lower rpm. Isn't that one reason why diesels are more fuel efficient, is they run at lower rpms and can make their torque peaks down low, so power isn't an issue at lower engine speeds with them?
Just my thoughts. I may be thinking in a wrong realm here.

 

[4X4HIGH]

I know there is a program out there that shows fuel consumption throughout the rpm range too. This might shed some light on how torque affects fuel mileage.
With all due respect Scott, I feel that many of the torque peaks I see are in the 3500 range. maybe thats because they are modified profiles, not stock, but still if an engine makes peak torque at even 3000 rpm, it would seem to take more fuel running an engine that fast, as opposed to 1500, IF the engine can get the job done at 1500. I realize that you might only be making 300tq @ 1500, as opposed to 500 @ 3500, but if you only need the 300tq. and your gearing is right to run 55mph @ that rpm, I would think the engine would be more fuel efficient at the lower rpm. Isn't that one reason why diesels are more fuel efficient, is they run at lower rpms and can make their torque peaks down low, so power isn't an issue at lower engine speeds with them?
Just my thoughts. I may be thinking in a wrong realm here.

If you look in any Chiltons, or Motors book you will see that peak torque is way down low in the RPM's, so the peak torque you're seeing is someone who has built an engine with HP in mind or just threw a bunch of parts together and actually hurt themselves without knowing it by moving the torque numbers up in the RPM range.

 

[76zimmer]

If you look in any Chiltons, or Motors book you will see that peak torque is way down low in the RPM's, so the peak torque you're seeing is someone who has built an engine with HP in mind or just threw a bunch of parts together and actually hurt themselves without knowing it by moving the torque numbers up in the RPM range.

That would be me...

Actually I figured I got 500 tq @ 2000, so lets go for some rpm too.