Quote:
Originally Posted by comptiger5000
Unfortunately, I can't find any stock dyno charts for my engine, but it certainly has enough low end torque to pull 60mph at 1400 - 1500 rpm. I would expect pretty much any V8 truck / SUV can.
Cleanspeed - Running below peak torque doesn't hurt efficiency. By that logic, I'd need to turn 3200 rpm on the highway, which would be absolutely ridiculous for this engine and vehicle. The goal is to spin the engine as slow as possible without lugging it, or throwing it into fuel enrichment due to too much throttle at cruise.
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Alright, let's make a comparison then; since the variables that affect fuel economy are much greater than engine speed alone, and that your engine is optimized for greatest force generation and lowest BSFC at 3200 rpms, then it would be fair to say that before that point and after that point, unless the cam and induction are optimized for it, the engine is not running at max efficiency. Unless there are devices like variable cam timing, variable induction, variable exhaust, that help to broaden the torque curve and make it more flat, you are inefficient before and after peak torque and hp. That's just the way it is with reciprocating engines.
Your engine has a long runner intake and mild cam timing to make more torque for a truck application. The sheer size of the engine ( 360ci ) means that below that point there will be torque to pull the gear, but with the current tuning and cam timing, do you think that it will be most efficient at 1600 rpms? You can cut the fuel, but you are still not at maximum cylinder pressure (ie peak torque), and that has a greater impact on mileage than engine speed. The motor is still on the way there, going through the rpm range.
Now let's take a Cummins diesel, same size. Peak torque is like 1600-1700 rpms. Compression, cam timing optimized to run at the speed you desire to run at cruise. Difference is at peak torque, stock it has 400+ ft/lbs and a lower BSFC (.33-.35) than your gasser (.50-.55), and because of the long stroke, more torque under the curve so gearing it taller does not affect it so much.
It was built this way to do it. But even as good as this engine is, go too far with the gearing one way or the other will cut the mpg.
Your engine was not designed from the factory to act like a diesel, which is what you want it to do with the stock set up. Will not happen.
To make a vehicle like yours, which was meant to be a performance vehicle for Jeep, to mpg better, it will take a cam swap to put peak torque way lower than 3200 rpms and some judicious calibrating of the electronics.
That and slowing down.