911 GT2 compression ratio

On a forced induction motor (ie. turbo or supercharger) the compression is lowered so more boost can be run on pump gas or a certain type of gas. Im sure theres some one who cant explain it better then me but thats the basic idea.

 

The lower the compression the more boost you can run at 91 octane. A car that is NA and running a 11.5:1 compression would also need a higher octane.

On pump gas you could run about 20lbs of boost with a chip. If you get abigger turbo you would need bigger injectors but could run the same boost. The CFM's are a lot higher so more power.

I raced with NA cars that had 12.5:1 compression and used c-16 race gas. I also ran with 7.0:1 and 26 lbs of boost with c-16


Antony

 

A Prius has a 13:1 compression ratio, and runs extra lean for "more economy" (and less reliability) that doesn't mean the Prius has a performance engine.

A low compression modern engine would be like the 4.0 inline 6 in the old jeeps, which is only 9:1 as well.

but, when comparing a high performance engine, compression means additional heat, but a longer burn, and a higher velocity "spring" action from the compressed air pushing the piston back down.

The thing to worry about is something called "detonation" which happens when the heat of compression pre-ignites the fuel before the spark does, causing a loss of power, efficiency, and back fire etc.. This is why hi performance cars that are turbo, or supercharged often "pull timing" to prevent detonation when running on the track in 90 F temperatures.

Porsche have tremendous street, and racing technology that they use. In the past, porsche used twin spark plugs per cylinder to aid in reducing the potential of detonation in turbocharged cars while they were still OHC 2 valves per cylinder... These days, Porsche have a computer management system that measures the potential for detonation ( cylinder head temperatures, and spark etc.) thus the CGT runs at 12:1 compression ratio.

with a modern turbo/ supercharged car like what is in the 997 turbo, the car is actually running about 14:1 - 15:1 compression when "on boost" at full psi, that is why such cars are build as a low compression in the normal atmospheric conditions of 9:1.

fuel octane has an effect on detonation as well. A higher octane has a slower burn, and avoids "knocking" which is "detonation" through requiring more energy in order to ignite ... a lower octane has a more explosive burn, and is easier to ignite.
hope that helps

 

The fuel mapping and how well your car is able to keep the stoch 7.0:1 air/fuel ratio plays a part as well on the octane you use.
If you think about diesel cars that run 5.0:1 compression with 35lbs of boost they have octane ratings of around 45. Then again they thrive on detonation to work

Antony

edit: Isaac kinda took that one lol

 

No worries Anthony, I definitely don't know it perfect, and am always learning more .... I didn't even get into stoichiometry of a charged air car vs. normal aspiration...or thermal flow properties in 4 valve heads, or increasing exhaust velocity and intake velocity, or harmonic compression in the intake rams for tuning peak efficiency at a specific rpm range, piston design, not to mention cam duration and lift.... hehe, engines are wonderful things

 

I agree and was really happy I got to help build our engines. Learned a lot.

Antony