compression increase on 930
 

i was thinking about a mild compression increase for more low end power whilst rebuilding my 930 engine. what would be a safe increase?

Andy UK

7.5:1 is often used number by tuners...up from factory 7.0:1. It all depends on fuel and head design I guess as some 4-valve engines go up to 9.2:1 with 1 bar of boost...

i wasn't going to change the head design at all, just increase the comp for the better bottom end pull. do you think its worth it?

Yepp...7.0:1 is quite conservative, at least in Europe. If you are planing to use EFI then more so.

7.5:1 and some SC Cams would be nice :)

If going EFI using Carrera manifolds, go to GT2 EVO CAMs!

Make sure you have access to high octane gas...

-Wayne

wayne, i can get hold of 97ron in the UK, is that a good enough fuel?
and whats the theory behind having to use high octane due to the increase in comp?

I suggest you read A. Graham Bell's book on Forced Induction.

Wayne, I suggest you carry his books ;)

I think if you measure the compression you have now you'll find it is significantly lower than the factory number - like 6.5 instead of 7.0 to 1.
-Chris

I'm rebuilding my 87 930 with 7.5/1 RUF pistons. They are also 98mm (stock 97mm). Sould be together in a couple of weeks. I'll let you know.

Using the formula ECR= SCR * sq rt [1+ (boost/1)]
boost in bars (ie 930 full boost .8 to 1)

7.0 to 1 stock @ 1 bar = 9.9 to 1 effective comp ratio
7.5 to 1 RUF @ 1 bar = 10.6 to 1 effective comp ratio

with 10.6 ECR you better run high octane or back off the boost.

7.5 to 1 RUF @ .8 bar = 10.0 to 1 effective comp ratio, very close to stock.

Victor, You need to calculate intake valve closing point in your effective compression formula. The later you close the valve on the compression stroke the less swept volume you will have for your formula.
Changing from a stock turbo cam to an SC cam closes the valve later thus reducing the effective compression.

WydRyd, 7.5-1 works nicely with SC cams.

You need to run higher octane fuel to prevent detonation that can occur with higher boost pressures...

-Wayne

I agree that changing the cam will affect compression ratio. For my rebuild, I'm staying with the stock cam and I'm using the formula to calculate effective compression ratios at different boost pressures and different compression pistons. My conclusion is with the 7.5 to 1 pistons I need to back off of the boost from 1 bar to .8 .