Compression question

**Steve Webb** · 15-12-12, 12:33

This one has popped up as I've had a couple of questions come through on email.
For you guys who have rebuilt, or are rebuilding your engines, when it comes to pistons and compression, have you stuck with the standard 8:1 comp ratio or have you tried something a little higher.

I remember back about 10 years and the late Barry Waterhouse put together a high compression engine for his Integrale. The thinking behind this was to give better off boost performance.
So has anyone on here gone in that direction?

**abarth 22** · 15-12-12, 13:30

i let std comp ratio...

**mpampis_** · 17-12-12, 06:12

It depents how much boost you want to use. With 8:1 CR you can use a lot of boost. After my rebuild i increase the CR to 8.4:1 .
Think about that newer engines (giuluetta) are using 9.4:1 and are working with 1.5 bar overboost.

**Steve Webb** · 17-12-12, 07:05

Does the 8.4:1 make much of a difference to the way the engine behaves off boost? Is it very noticeable?

**mpampis_** · 17-12-12, 07:19

Well the engine generally respond much quicker with higher CR but i can't feel the difference because at the same time i change the flywheel with a much lighter one and i take out the balance shafts so i had a final result that i didn't know how the engine is working with only CR increasment. For me i thing that the CR could be felt if it will increased about 1 point for example 9.0:1 but you have to be sure that you have to limit the boost for no more than 1.5 bar and use a high octane gas (98 or 100 ron) a good one fmic would help to keep the air cold (so same air with less mass) and afr about 11. All these will help the high CR engine to work great without knocks or detonation

**corriedw** · 17-12-12, 08:36

Compression ratios are directly tied to the fuel octane value. We can not compare the new Guilietta's engine with the Q4.

First a direct injection engin compress air and not a air/fuel mixture. The fuel is added later. Therefor it do not detonate with high compression ratios.

Don't be fooled by the high pressures the G's turbo boost. The Guilietta's cam only open 6.5mm max against the 9+mm of the Q4. This means at the high pressure, the G's turbo deliver the same amount of air in the combustion chamber as a slightly worked Q4's. The pressure is manifold air pressure and do not mean more air in the combustion chamber.

The reason for the high pressure and small valve opening is the air enters the valve at high speed, scavange the combustion chamber much better and the result is better combustion and CO levels.

**mpampis_** · 17-12-12, 11:24

I agree, i mention giulietta as a newer technology engine. That what i say before is that even you increase the CR with high octane gas, cold air and logical boost preasure the engine will not detonate

**corriedw** · 17-12-12, 22:33

I agree with you.

I am just afraid some people might think they can run these high compression and boost levels on our engines.

**1NRO** · 18-12-12, 21:19

Jim ran 9.3:1 on his car, admittedly he used E85 which helped I'm sure but for some time he ran standard camshafts with the resulting high dynamic compression and very high cranking compression tests. This was combined at one stage with a mishap with boost that went very high, a split cylinder I think was the result but no damage to pistons which were refitted in the next engine (along with camshafts more suited to the Cr).
A number of the Fiat Coupe guys are running Cr at a similar level, all seem to have no problems with pump fuel, at least I've never heard of anyone having problems.

**jimnielsen** · 20-12-12, 08:49

My 'old' car is still happily running 9.3:1 compression with 1.5 bar boost. It all depends on the total engine setup including the fuel and the ecu. The actual compression number is just that 'a number' you have to configure the whole system to suit whatever compression you want to use.