HP loss for running lower cp ratio...??

[mike gerlach]

I'm trying to decide on an engine combination for a pro-street car. Would like to go with a big ci small block Chevy (450ci), solid roller, good heads (GM sb2.2)etc, but I'm having trouble with my decision on compression ratio. I guess my question is does it make sense to use a lower cp ratio (10.5-11.) to be able to squeak by on pump gas for street use, or go with 13:1+ and just run race gas.

What kind of horsepower hit will I be taking to run the lower more street friendly compression ratio....


mike

[mike gerlach]

guess no one wants to touch this question.......

[Marv D]

Well,, just some thoughts to let you know we arent ignoring you. Was just waiting for the more experienced to chime in,, but...
Race gas may carry more BTU's when compressed to optimum, but I don't see race gas vs pump gas as really the issue. Thing is lower compression to tolerate pump fuel will not support as much cam as a motor built for higher compression. The cam is the guts of things (maybe Jay will offer a better explaination) but heres an old engine builders general 'rule of thumb'

For cam 0.050" duration of 250° you need a minimum of 10:1 compression
For 260° @ 0.050" you need a minimum of 11:1,
270° requires 12:1.........

That is the mininum compression to support the duration. Otherwise you typically don't build enough cylinder pressure to support the cam or burn the fuel.

You certianally wouldn't put pump fuel in a 13:1 motor, and likewise you don't put 117 octane leaded race fuel in a 10:1 motor. So you have to start with a fuel requirement in mind before you start your engine plan.

Armed with those (very general) rules,,,, fuel dictates the compression you can run, compression dictates the cam duration, and the cam dictates how much a/f your going to get through the heads and into the cylinders. You make power by burning fuel,, the more that get's into the cylinders, the more power you make.

SO in a nut shell, I don't think anyone can really answer the question you asked in how much power would you be giving up in pump fuel vs race fuel. Too many other components are going to have to change that will make or break the torque curve to simplify it that much.

[ROB]

A good rule of thumb is a 1% gain in engine power for every 4% increase in compression ratio.

In your case, you are potentially looking at around 7.5% power from 10.5 to 13.25 to 1. Example: 700 HP @ 10.5 to 1 vs. 752 HP @ 13.25 to 1.

Since you spend far more time driving at part throttle on the street, and the cost differential on race fuel vs. pump premium, and the probability that you'll get less than 15 MPG, I'd lean towards the 10.5 to 1 ratio.

[mike gerlach]

thanks guys....... these are exactly the intelligent responses I was looking for....

I knew there was a strong relationship with cam timing vs. static compression, but it starts getting really confusing. Your explanation Marv for the cp ration basically enabling the engine to use a "larger" cam clears up the question very well....

mike