hey guys, i got this question and if someone could explain it to me, i would appreciate it very much.

1. how important is the Compression Ratio? I know it is the higher the better, but what is the outcome of a higher ration? just makes the car faster?

2. whats the difference between ration 12.5 and 10.5? can u turn up the ratio by yourself? is it easy to achieve so???

thanks

 

Compression ratio is a function of the engine's cylinder bore and stroke.

It's not something that can be adjusted without major engine work, I'm afraid.

 

high requires premium gas....i guess you lower the ration if you take the engine apart and put a real thick gasket between the cylinder head and block, change the retaining screws, reprogram the computer and redesign the variable timing on the cams...in the end it would probably cost more than all the gas you would ever use in the car.

 

simple way to explain ...

Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
.... Wikipedia

 

Iimportantly, the more you compress it without it igniting the gas mixture, the more power (and probably mileage). A 12 to 1 compression would likely pre-ignite the air gas mixture if Porsche was mixing the gas and air before it went into the cylinder. To overcome this, they use the computer to time the insertion of gas at the last moment before ignition. In some cases, 3 times per ignition stroke. This, combined with computerized timing of the spark allows optimum ignition after compression and permits use of less than 93 octane gas if you don't demand full power. The 991 includes a new gas injector that sprays the pattern differently than the 997. I'm not sure, but I would bet that this is a prime reason the engine is so smooth at idle and stable at constant RPMs. This is a major engineering accomplishment that I believe has only just been optimized recently. Something we should all be proud to be a part of (on the user side).

ChuckJ
PS: I'm assuming they will have to lower that a little to put up with the combustion pressures associated with the turbo.

 

DFI in the 997.2 and 991 allowed Porsche to increase ratio to around 12:1

 

Yes, sorry if I wasn't clear about that. The DFI high pressure pump injects it right into the cylinder.

ChuckJ

 

... ok = = thanks for the tech part of the story, but would 12:1 vs 10:1 really make a huge difference in power output? and a lowed ratio car may partly due to fuel quality supplied?

 

Yes, it makes a lot of difference... More power on less fuel. Can run on lower octane fuel, but only because of it senses that it's not high enough octane fuel and reduces optimal fuel and ignition timing; but it's not good to do that to the engine long term.

ChuckJ