CP Forged Pistons

[ashley]

Does compression directly relate to power? I.e if you have the exact same spec engine one with 8:1 forged pistons and one with 9:1 would there be a power difference?

Yep- higher compression = more power

....but as Bob is saying, as compression goes up, so does susceptibility to knock. That's why the classic turbo charged engines would drop compression significantly in order to turn the boost up, with the draw back that off-boost performance suffers.

You can run more boost without dropping the compression, but that typically involves less advance, so combustion takes place later in the cycle with risks of damage to the cylinder walls as they are exposed to high temperature/ pressure spikes.

Differing opinions on which way to go in modern engines....a lot of folk use 8.5:1 pistons, which seems a reasonable real world compromise...

[MR2 Rich]

Does compression directly relate to power? I.e if you have the exact same spec engine one with 8:1 forged pistons and one with 9:1 would there be a power difference?

Yep- higher compression = more power

But it will be on the knock sensor 99.987% of the time unless I put super duper uber rocket fuel in it?......

[bobhatton]

Does compression directly relate to power? I.e if you have the exact same spec engine one with 8:1 forged pistons and one with 9:1 would there be a power difference?

It does not work like that.

You design the engine and come up with the right compression for the design.

If the design is changed the compression may change.

[Peter Gidden]

but as Bob is saying, as compression goes up, so does susceptibility to knock. That's why the classic turbo charged engines would drop compression significantly in order to turn the boost up, with the draw back that off-boost performance suffers.

You can run more boost without dropping the compression, but that typically involves less advance, so combustion takes place later in the cycle with risks of damage to the cylinder walls as they are exposed to high temperature/ pressure spikes.

The custom pistons designed and manufactured by Cosworth for my Noble have a 7.9:1 CR.

As a point of interest, the Audi Quattro GrpB rally cars ran a CR of 7.4:1.

[bobhatton]

Yep- higher compression = more power

....but as Bob is saying, as compression goes up, so does susceptibility to knock. That's why the classic turbo charged engines would drop compression significantly in order to turn the boost up, with the draw back that off-boost performance suffers.

By droping C/R from say 9.0:1 to 8.0:1 off boost your power drop by 3%, yes only 3%

But on boost you will gain may be 25% or even more by being able to run more boost.

[Odin_S]

Hi Bob,

Like I said before I've bought Wiseco pistons with CR of 9.0:1 I've been researching I need a Apexi HG of 1.8mm but I've seen 2.1mm what would happen if a 2.1mm was put on?

[bobhatton]

Hi Bob,

Like I said before I've bought Wiseco pistons with CR of 9.0:1 I've been researching I need a Apexi HG of 1.8mm but I've seen 2.1mm what would happen if a 2.1mm was put on?

You should never fit a thick head gasket to lower compression, all that will do is trap a larger amount of charge that can detonate even more.
You use the head gasket thickness to give the correct piston to head clearance together with block and piston height.

Your pistons are wrong for any more than 0.5 bar boost.

Design the engine and get the correct compression pistons

[Peter Gidden]

Like I said before I've bought Wiseco pistons with CR of 9.0:1 I've been researching I need a Apexi HG of 1.8mm but I've seen 2.1mm what would happen if a 2.1mm was put on?

Your CR will reduce. How much by? Do the maths, it's not difficult.

Not sure i'd want to fit a HG that thick. Potentially introduces all sorts of other issues.

[Adam86]

What do the three numbers given for the compression ratio represent?

Such as the 8.5:1 for standard rev3 compression?

[Marf]

http://en.wikipedia.org/wiki/Compression_ratio

The compression ratio of an ...... engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.

So basically the ratio of the volume of the combustion chamber with the piston at TDC vs BDC.

[Odin_S]

Yea I did things backwards. I've already bought the pistons. Don't know if any company would do an exchange with my pistons

[Adam86]

http://en.wikipedia.org/wiki/Compression_ratio

The compression ratio of an ...... engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.

So basically the ratio of the volume of the combustion chamber with the piston at TDC vs BDC.

Cheers I now understand the compression ratio of 8:1

But what's the 5 representing?

I ask because if I change from 8.5.1 to 8.0:1 I want to understand what im doing.

[ashley]

8.5 means eight and a half, or eight and a half times the volume at bottom dead centre compared to top dead centre (or have I totally misunderstood the question?)

[bobhatton]

http://en.wikipedia.org/wiki/Compression_ratio

The compression ratio of an ...... engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.

So basically the ratio of the volume of the combustion chamber with the piston at TDC vs BDC.

Cheers I now understand the compression ratio of 8:1

But what's the 5 representing?

I ask because if I change from 8.5.1 to 8.0:1 I want to understand what im doing.

8.5 is eight and a half, 0.5 is a half of 1

What boost is your engine going to be designed to run on, with what fuel and what RPM?

It may be you will need 7.5:1 pistons

[MR2 Rich]

Who does 7.5.1 forged pistons for these engines? If it is such a bad thing to do why are there so many forged pistons for these engines that are 9.0.1?

[Adam86]

Ah right got it lol. I was thinking it was 3 individual numbers.

Well I think my head gasket might be going so if the engines being openend up I'll do a bottom end rebuild as well since its on 95k.

When my turbo failed I bought a hybrid CT20b previously run at 1.4 bar ( 340bhp apparently)

My aim would be to achieve a safe setup at 1.3 bar. I'm currently at 1.2 running the 205cc so I hope the cc will be enough to keep my inlet temps safe.

Would be using v-power and standard rev limit.

[Peter Gidden]

If it is such a bad thing to do why are there so many forged pistons for these engines that are 9.0.1?

US fuel.

[MR2 Rich]

They have stuff better than V-Power?

[Peter Gidden]

They have stuff better than V-Power?

No, but in terms of mass production, they're biggest market has low octane fuel, and will only want to boost to modest levels. So average performance for average customers, but there are lots of them.

Anyone wanting to boost higher than average would buy Wiseco's 8:1 pistions.

[MR2 Rich]

Interesting, so with a 9.1 compression ratio and 99 V-Power what is a safe power?