Compression Ratio vs MPH

[Don Whitmore]

Two super-stock motors make similar horsepower. They are similar in displacement. One motor has a higher compression ratio, but is limited somehow so that it makes similar horsepower as the other motor, which has a lower compression ratio.

Install both motors in the same super-stock car. Will they MPH the same?

D. Whitmore

[Joe Abbazia]

The mph should be the same if the horsepower is the.However when the air gets bad i would like to have the comp. motor.

[Don Whitmore]

Wow...392 views and only 1 reply? Or, is the answer that easy and I'm missing it?

[Jeff Lee]

I would say there are no more responses because Joe is correct. HP is a function of MPH. If nothing else changes (like aerodynamics or weight) then the MPH will be the same. I also agree with Joe that I would rather have the compression with bad air.

[Mike Taylor 3601]

I agree only thing I may add is higher comp. engine usaully makes a little more torque also if that was only change. Mike Taylor3601

[tgriffith]

I asked a motor guy that question,,,,,,,he claimed impossible to answer,,,,,,,for the motor that has higher compression to have same HP,,,your going to have differences in other places that reduces HP,,such as cam,,,that alone may bring down rpm curve so that it wouldnt MPH faster

he tends to think that the higher compression,,,the easier to get it up in rpm...on the other hand,,,once the motor is turning higher rpm,,lower compression will have less resistence,,,

he tends to think that your answer is YES more compression would make higher mph,,,,,reason for this is that most motors like a reduction of timing at higher RPM,,,if Im not mistaken,,,once your retard your timing , doesnt that fire the cylinder closer to the top, thus having more compression???,,,,I think cylinder pressure would be higher at 28 degrees VS 36 degrees...or am I thinking about this backwards??

[Jeff Lee]

The original question dealt with a smaller carb on a higer compression engine vs a bigger carb on a lower compression engine, all else equal.

[bill dedman]

If they both made the exact same maximum horsepower, you'd have to look at the areas under the curve to determine which engine would build more speed, because the MPH is going to be a reflection of the total amount of horsepower expended during the entire run, not just the maximum an engine makes at its power peak. They could be both putting out 500 HP max, but one might have a torque advantage at lower rpms that would affect acceleration, but wouldn't affect the maximum horsepower at its power-peak rpm.

Maximum horsepower shows what an engine can do at its power peak, but drop the rpms down 1500 and see which engine is pulling harder; that's the engine that will haul the weight up to a higher speed, given equal maximum horsepowers for the two. The total horsepower expended over the course of the run will be greater, and so will the speed in spite of the max HP being the same.

Now, as to which engine, the low-compression or the high compression engine would have more area under the curve, I am afraid there are too many variables for me to even attempt to answer that one. It would depend on a multitude of factors. That's my 2-cents...

Bill