OK, If a stock engine that has 170 hp @ 3800rpm with 8:1 compression while getting 13 mpg average. Why would increasing hp to 300 @ 4200rpm with 9:1 compression yield 10 mpg.
I know I don't have dyno #'s and all the other info pertaining to this question. But in general when taking a stock motor that yields roughly the above mention #'s and building it up with cam, intake, head work, headers etc. The end result is lower mpg's.
How would one go about getting the happy medium with building a more powerful motor yet keeping or yielding better mpg's?

Robert P.

 

What happens when you raise the compression the motor becomes more efficient, so on two otherwise identical motors, one with a lower compression ratio and one with higher, the higher, theoretically, should make more hp and tq and provide somewhat better fuel economy under normal driving. Talk with local engine builders and/ or owners of performance vehicles, see what compresson ratio they are able to obtain using local pump gas.

Performance and mpg doesn't usually mix, in a naturally aspirated set up, the only way you can get there is with someone who has built such a combination and/or has the ability to tune the snot out of your vehicle. Tuning is key in gaining performacne and decent mpg readings.

Here is an example, there is a fella in my town with a 3/4 ton chevy dually, with huge tires and a massive lift, towers over vehicles, it also has a 454. This fella went to an automotive school and learned the ins and outs of his motor, having acess to certain equipment he tuned his motor, it gets I believe he said 24-25 mpg highway and makes a lot more power. Thats great, my ford 2wd inline six gets 17-18 on a good day, and its pushing a half ton truck. All about the tune, you begin adding aftermarket parts then its a whole new ball game, you have to make sure that what you add complements your goals, good luck

 

Thanks jwtaylor,
I appreciate the thought.

Robert P.

 

The only problem with running high compression on pump gas is that you need a big cam to bleed off cylinder pressure or you will not be able to advance your timing enough to make it run correctly. Economy cams and high compression don't mix. That is why you don't see too many 10.5:1 engines running around from the factory.

 

Ratsmoker, I don't mean to differ, but back in my day, 10.5:1 was not a high compression engine. The chevy (gulp) 300hp 327 had 10.5:1 compression and that was the beginning for the high HP engines. Several had 11.5:1 ratios. Awww back in the good old days.....

 

The other thing they had back then was hi-octane leaded gasoline.
My brothers Galaxy500 called for 100+ octane gas. I don't recall the exact figure, but it was well above the highest you can buy around here.

 

There is a point at which performance and mileage meet....

compression among other factors like camshaft profile, cam type, valvetrain and cylinder head design as well as intake type can determine the kind of power and torque you will get.....

the challenge is to match the torque curve to the mass of the vehicle....and its proposed use.

High comp engines of today are runnng on lower octane engines and smog constraints....

 

Actually, you are reinforcing his point
Those high compression Chevy motors had cams with lots of overlap to bleed off the compression.
In other words, they had high static compression ratios, but low dynamic (the one that matters) compression ratios..

 

OK, you've peaked my interest.....could you possibly define what you mean by static and dynamic compression ratio? I'd like to see you explain this a little farther.......You are correct that the Octane was higher, but it was also calculated using a different formula, and also used lead as an anti knock compound. HP rating also changed....so....what would 300 (1969) hp be using todays (2004)hp calculation?

 

bump

 

Here is a good link to help you understand the difference between the two and how the two interact....

http://www.e30m3performance.com/myth...omp_ratio2.htm

http://www.e30m3performance.com/myth...comp_ratio.htm

cheers
R

 

there is a fella in my town with a 3/4 ton chevy dually, with huge tires and a massive lift, towers over vehicles, it also has a 454. This fella went to an automotive school and learned the ins and outs of his motor, having acess to certain equipment he tuned his motor, it gets I believe he said 24-25 mpg !!!! 454 Big block !

That's His tale or tail , I'll sit on mine.

Don

 

Well that is what was said, your probably right it seems hard to believe, but I have actually heard stories to confirm this. Not uncommon to fine tune a motor that otherwise gets poor mpg, to achieve better, but you make up your own mind. Your probably saying but, if they could get better mpg wouldn't they have done it from the factory? You tell me, an old example, people with carbed 300's switch to a 500 cfm 4 bbl and intake, from the stock setup, and report improved mpg and power. Something to think about

 

Well switching ti the 4bbl setup with new intake also helps by #1 reducing the cfm requirement in the primaries (as opposed to running two large 2bbl carb all the time) needed to run the engine during normal loads

#2 increasing overall cfm when needed...this is when you stomp on it and the
secondaries open up....

the added airflow of the newer intake should also help things...not to mention the usual weight reduction at the front of the car.

I would say that in order to have the best mileage you must take into consideration the power to weight ratio, as well as the driving habits.

good point JW

 

Concerning the dually getting 24-25, another factor may be the way the truck is geared, etc. With the Tq the 454 is making, and the big tires, w/ a 3:50 gear set he may only be turning 2000RPM or so at highway speed. Lower RPMS = Better MPG, provided the vehicle is not lugging and has the power to operate at that RPM

Case in point. 1979 Ford Bronco, 460 w/ mild cam, Holley carb and dual exhaust. (No other engine mods) C6, 3:50 gears, and 40" Gumbos. 20-22MPG on the highway driving from Indiana to Pennsylvania every other week in the mid 90's