Hypothetical engine question

[neilca]

Lets say I have racing engine that produces 200 hp at 10:1 compression on pump gas. How much of an increase in hp would be seen if I went to 12.5:1 and race gas. All other variables are constant.

[mreid]

I'll play. About 5%, so 210hp, but you need a twin plug set up. So, machine your heads while you are replacing the p&c's.

[chris_seven]

Compression Ratio HP Determination Program

[neilca]

Thanks Chris,

That calculator is awesome. It says 6%. Mark was dang close.

I also found this on anther site

" You might get 7% - 10% by going from 9:1 to 10:1, but it'll be less from 10:1 to 11:1, less yet from 11:1 to 12:1 and so on."

In my case that little bit is not worth chasing.

[HawgRyder]

Remember...speed costs money....how fast do you want to spend?
LOL.
Bob

[Henry Schmidt]

Originally Posted by neilca

" You might get 7% - 10% by going from 9:1 to 10:1, but it'll be less from 10:1 to 11:1, less yet from 11:1 to 12:1 and so on."

In my case that little bit is not worth chasing.

These numbers were very well understood by Porsche race engineers back in the 60s-70s. That is one of the reasons why factory twin plug NA racing engines were all built with 10.4:1 as apposed to choosing higher ratios.

[3literpwr]

Nice calculator.... My question is how is the CR change effecting torque? I'm trying to decide between 10.5 and 11.6:1 wossners for my 95mm SC rally build.

[PFM]

3literpwr,

You are correct to ask about TQ. The change in compression should be coupled with a cam change to optimize the power curve. If all things remain unchanged the TQ increases a bit more at peak TQ than other areas but in general go up in the same 5% range. A cam change can take more advantage of more compression. A larger cam benefits more from increased compression than a small cam does in the HP chase.

[chris_seven]

Complex issue but increasing static compression does generally increase torque but will also increase peak cylinder temperature which can have an impact on detonation.

There is also a compromise with exhaust sizes and smaller exhausts tend to maximise the improvement in torque with increasing compression but can have a negative effect on the engines ability to rev and hence a negative impact on horsepower.

I would always choose Horsepower rather than Torque and select gear ratios accordingly.

It seems important to consider the entire power train before deciding on compression and looking at speeds in gears, maximum speed, tractive effort and max engine revs would all seem to have a part to play in the decision.