Honda Prelude Forum

I always thought that the higher the compression ratio of an engine the better mpg it should get. However I noticed the RSX gets significantly higher mpg with a 9.6:1 then the RSX Type-S and its 11:1 compression ratio. I'm sure the type S has more aggressive cam timing which would account for the difference.

So is my original theory correct, as long as all things are equal, that a higher compression ratio equals better mpg? I am of course referring to realistic CR's.

if anything, a higher CR = moarrr powaa, and usuallyyyyyyyyyyyyy (operative word here) more power = worse mpg. but there are exceptions, of course.

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its hard to compare the two also because the type s came with a 6 spd transmission

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Apparently the final gear of both have the same ratio.

if you raise the compression ratio and make no changes to the injectors your mpg should go up some.

I noticed a boost in my mpg when I put in type s pistons

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As far as the rsx goes the type s has larger injectors

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How much? Would a freer flowing exhaust theoretically give a bump in mpg after raising the cr?

Theoretically yes. What we call breathing mods (intake, exhaust, etc) make the engine run more efficiently, which is where the extra power comes from when you do those mods. A bumped up comp ratio would also help the air/fuel mix burn more efficiently, hence making more power while using less fuel. As for how effective it would be on it's own, I have no idea.

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I thought a greater increase in air coming in would hurt mileage, more air requires more fuel right?

Yes (provided you want to run the same A/F ratio - you could lean the mixture out by adding air without adding fuel) but the more fuel you burn and the more power you'll make. Burn that fuel efficiently and yo'll improve your economy.

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