How to increase C/R?

[911TT33]

Excuse the dumb question, but how do you increase the C/R from 7.0:1 to 7.5:1?

Is it simply dropping in some new higher compression pistons, or is some machining required to the heads?

[copbait73]

There are several ways to do this but what are you after in engine/vehicle performance?

All things being equal higher C.R. needs reduced boost to reduce the resultant higher firing pressure. This nets you less HP and slower top speed. The positive is quicker acceleration to that reduced top speed.

[Alan L]

No machining - this removes material and CR. You are trying to fill the cylinder void space with more material (piston = more CR (squeeze)).
But you have to know why you are doing this. porsche designed this configuration for a reason. If you want more oomph, the simpler way is via more boost (with required changes elsewhere).
Alan

[911TT33]

I'm after better pre-boost response and drivability.

I know I can increase boost for more top end HP and speed, but my car is purely street driven.

[cole930]

TT33,

It would help if you added, to your signature block, the year 930 you have as well as any mods you have. Allows us to better help with questions.

The 7:1 compression ratio of a stock 930 goes to as much as 12:1 under heavy boost. Compression ratios of that magnitude can destroy engines pretty fast if appropriate safety measures are not taken. Raising compression ratios on a street car is not necessary to gain lower end performance.

Planning performance enhancements on a 930 should always be preceeded by some standard upgrades that help prepare a suitable platform for further modifications. These would include a larger long neck Intercooler with a late style BOV. A minimum of Euro exhaust system with free flow muffler, headers of smaller primary dia, would be preferred for a street car, and a late style quick spool turbo.

This would get you a noticeable performance difference and put you on the right track to fast. From this point I would suggest SC cams set at around 1.6 mm, a good programmable ignition system to allow dialing in a good timing curve, 930 bottom end performance suffers greatly from the stock factory timing curve. Then add a dwur or adjustable wur to allow fuel tuning. The single most significant change that can be made to a street 930 is a shorter ring and pinion ratio, but this is expensive.

Hope this helps.

Cole

[Alan L]

You can also get better low end by making sure your fuel and ignition are perfectly set and functioning right - it has a big effect on the low end - been there, done that.
My dizzy advance was frozen on the collar. That slowed pre boost performance like you wouldnt believe. Then there is the fuel.
Alan

[full quack]

Maybe a simpler & cheaper approach to better low end performance would be to change your ring & pinion ratio?

Just a thought.

Mark