What compression ratio do I need
 

I have contacted crawford performance about purchasing one of their 2.5 short blocks. I inquired about compression and they said they could make it, to what ever CR I wanted, So my car is UK 97 GC8, I will be fitting the standard heads back on the new block. What is the CR of my standard car and do I have to match this ratio with the 2.5 engine?

Andy

i could be wrong but im sure the cp ratio is more about keeping the engine safe when adding more boost, higher/lower cp ratio is more down to tuning ????

Not sure if turbos work the same, but on a n/a car fuel octane rating is a consideration aswell. The higher the octane rating is, the higher the CR can be raised before det. Pressume the theory is the same when running a turbo.

Whoever is building the engine should have a good knowledge of suitable CR's to match the components used (especially pistons) and what the engine is getting used for. I.E, competition or road. High compression ratios put a lot more stress on the engine and are really only suitable for competition engines that are regularly stripped down and rebuilt.

Hope this is of some help.:)

Doug.

between 8 and 8.5:1 should be about right. If you are pushing for power with a big turbo, then go at the lower end to enable more boost to be run (and still get sensible ignition timing) or if you only want moderate power, but better off boost reponse and fuel consumption, then go with the higher CR. There are a number of examples of people using over 8.5:1 and most of them result in reduced tuning potential.

Paul

Hi Paul, I am aiming for ~370bhp with a 18g turbo. I want low down instant torque. If I ask crawford to make the block with say 8.5 CR and its a little too high for my requirements, I suppose I could then use a slightly thicker head gasket to drop the CR down a little. As you can tell I am a novice with this theory and would appreciate any comments before I commit to purchase.

Andy

with a descent fueling and timing you can run 2.0 bar with a CR of 8.5:1
For your power goal is 8.5:1 perfect with 1.4 bar of boost on a TD05-18G and will give you the estimated power your after

One of the reasons that we build our motors to suite your compression ratio needs is to avoid the negative impact of using the wrong head gasket.
The thickness of the head gasket is an integral part of the design for the internal compnenets of your motor.

If you run your motor without making a change to the cylinder heads and the compression ratio seems to high, Pull the heads and open up the combustion chamber to match the larger diameter of the 2.5L bore. This will increase your chamber volume and lower your compression ratio with no ill effects. :)

Quirt Crawford