CP Forged Pistons

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ashley
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Re: CP Forged Pistons

Post by ashley »

MR2 Rich wrote: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...
Last edited by ashley on Thu Apr 17, 2014 6:40 pm, edited 1 time in total.
MR2 Rich
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Re: CP Forged Pistons

Post by MR2 Rich »

ashley wrote:
MR2 Rich wrote: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

:clown:
:mrgreen:

But it will be on the knock sensor 99.987% of the time unless I put super duper uber rocket fuel in it?......
No 2 :(
bobhatton
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Re: CP Forged Pistons

Post by bobhatton »

MR2 Rich wrote: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.
Designer for turbo set ups on F1 cars, and Nitrous Oxide Systems of the USA in the 80s
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

ashley wrote: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
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Re: CP Forged Pistons

Post by bobhatton »

ashley wrote:

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.
Designer for turbo set ups on F1 cars, and Nitrous Oxide Systems of the USA in the 80s
Odin_S
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Re: CP Forged Pistons

Post by 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
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Re: CP Forged Pistons

Post by bobhatton »

Odin_S wrote: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
Designer for turbo set ups on F1 cars, and Nitrous Oxide Systems of the USA in the 80s
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

Odin_S wrote: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
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Re: CP Forged Pistons

Post by Adam86 »

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

Such as the 8.5:1 for standard rev3 compression?
Marf
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Re: CP Forged Pistons

Post by 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
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Re: CP Forged Pistons

Post by 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 :-k
Adam86
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Re: CP Forged Pistons

Post by Adam86 »

Marf wrote: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
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Re: CP Forged Pistons

Post by 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?) :thumleft: :thumleft:
bobhatton
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Re: CP Forged Pistons

Post by bobhatton »

Adam86 wrote:
Marf wrote: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
Designer for turbo set ups on F1 cars, and Nitrous Oxide Systems of the USA in the 80s
MR2 Rich
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Re: CP Forged Pistons

Post by 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?
No 2 :(
Adam86
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Re: CP Forged Pistons

Post by 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
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Re: CP Forged Pistons

Post by Peter Gidden »

MR2 Rich wrote: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
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Re: CP Forged Pistons

Post by MR2 Rich »

They have stuff better than V-Power?
No 2 :(
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

MR2 Rich wrote: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.
Last edited by Peter Gidden on Sun Apr 20, 2014 8:14 pm, edited 1 time in total.
MR2 Rich
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Re: CP Forged Pistons

Post by MR2 Rich »

Interesting, so with a 9.1 compression ratio and 99 V-Power what is a safe power?
No 2 :(
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