[Mk2] [Turbo] Compression Ratio

[darkwolf]

Hi I am starting a new engine build and most of the forged pistons I look at have a cr of 9:0-1 where as far as I can see the standard cr is 8:5-1 so my question is can any one tell me if this will make a big difference in the way my enging will run or should I be looking at going lower with the cr Any help will be appreciated

[bobhatton]

Off the shelf pistons are no good if you want to run more boost.

If you send me a PM I can give you more info on what to use

[greeny]

Off the shelf pistons are no good if you want to run more boost.

If you send me a PM I can give you more info on what to use

Bit of a silly statement...

Why not go higher c/r?

Need to know what your plans/goals are first to help out.

[darkwolf]

I'm not 100% sure on what my long term plans are for the engine so I was planing on making the block as strong as possible so I can increase bhp in the future


My short term plans was to get about 350bhp out of the engine and the only reason I am looking to forge the block now is I found one of the pistons had cracked and a bit had broken away

[greeny]

I'm not 100% sure on what my long term plans are for the engine so I was planing on making the block as strong as possible so I can increase bhp in the future


My short term plans was to get about 350bhp out of the engine and the only reason I am looking to forge the block now is I found one of the pistons had cracked and a bit had broken away
Image Replaced With URL For Quote http://i275.photobucket.com/albums/jj29 ... eb9063.jpg

Was it number 3? What boost was you running and on what setup?

What thickness head gasket are the pistons you're looking at rated with?

It's been a while but iirc most the piston manufacturers tend to quote a c/r with a different thickness from stock.

[darkwolf]

It was piston 4 that failed on a standard head gasket and I don't know what boost it was run at as I bought the engine as a non runner


As for the pistons I have bin looking at non of the adds I look at state what thickness of head gasket to use with them

[Al-sw20]

It depends on what piston and headgasket thickness as to what compression ratio you end up with.

Info courtesy of MR2OC.com

Here are the headgaskets we worked with that are available for the 3sgte:

TRD: 0.8mm
Cometic: 1mm
HKS: 1mm & 1.6mm
Apexi: 1.1mm & 1.5mm & 1.8mm & 2.1mm

(CP/JE/SRP/Ross/Arias Pistons)+(TRD 0.8mm Headgasket)=(9.2:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Cometic/HKS 1mm Headgasket)=(9.0:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 1.1mm Headgasket)=(8.8:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Factory 1.4224mm or Apexi 1.5mm or HKS 1.6mm Headgasket)=(~8.5:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 1.8mm Headgasket)=(8.375:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 2.1mm Headgasket)=(8.1:1)


Wiseco's 9.0:1 piston however is 9.0:1 with a factory .056" (1.4224mm) headgasket, so it truely is a 9.0:1 at the same headgasket thickness where the rest of them are 8.5:1

If you want to get close to an 8.5:1 compression ratio with a Wiseco Piston you will need to use the Apexi 1.8mm Headgasket. That combo will put you right around 8.6:1 compression. or you can use the Apexi 2.1mm headgasket to get to 8.4:1 ... more clearly illustrated below:

(Wiseco Pistons)+(TRD .8mm Headgasket)=(9.4:1)
(Wiseco Pistons)+(Cometic/HKS 1mm Headgasket)=(9.2:1)
(Wiseco Pistons)+(Apexi 1.1mm Headgasket)=(9.1:1)
(Wiseco Pistons)+(Factory 1.4224mm or Apexi 1.5mm or HKS 1.6mm Headgasket)=(~9.0:1)
note: i put the "~" because all of my calculations come up saying the wiseco piston is 8.8:1 with a factory headgasket thickness, but they advertise 9.0:1 so its about 9.0:1
(Wiseco Pistons)+(Apexi 1.8mm Headgasket)=(8.6:1)
(Wiseco Pistons)+(Apexi 2.1mm Headgasket)=(8.4:1)

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[Super_red]

Wow what a mine field. Seams strange they all need a thick HG to work, I would have tohught the purpose of changing pistons was to lower the CR with the stock HG.

What about different rod lengths? Do these exist for the 3S?

[Al-sw20]

The stock headgasket will give you a 8.5:1 compression as shown here:

(CP/JE/SRP/Ross/Arias Pistons)+(Factory 1.4224mm=(~8.5:1)

Why do you want a lower CR? It isn't 1980 anymore.

The days of all or nothing turbocharged cars are thankfully over. With the correct tuning, fuel and intercooling you can have a reasonably high compression ratio, and still run decent amounts of boost.

Dropping the CR lower than 8:1 is robbing Peter to pay Paul.

[bobhatton]

The correct way to increase boost on any engine is to lower the compression ratio for the same fuel and intercooling.
It makes no difference if it is 1980 or 2012 the physics are still the same, that will never change.
If the compression is not lowered then the ignition will have to be retarded to stop detonation and that will lose far more power than dropping the compression.

The correct head gasket to use to give the correct piston to head clearance is 1mm.
All off the shelve pistons are to fit stock engines with road fuel, or raised boost and race fuel. If you are changing anything and want more power the engine design will determine what custom pistons will need to me made.

[Super_red]

So is the cometic/HKS 1mm gasket the one to use? I had heard that the stock rev3+ item was the one to go for? Are there as many option with rods or are these all the same length?

[bobhatton]

That will work ok.
So what is the design compression ratio you need?
You can get rods made in any length you like but all the stock ones are the same length.

[Super_red]

Its perhaps a different thread for me but I am currently looking at the GTX2871r which is supposed to be good at 2 bar. I am trying to build an engine that makes its power at as low an RPM as possible hopefully 400bhp by 7k.

I may be dreaming but for me its more about the delivery than the headline power, I want it to drive like my Astra VXR did but with more power. Almost instant spool and ECU controlled boost so it rises with RPM rather than all at once.

[darkwolf]

so just to clarify any of the CP/JE/SRP/Ross/Arias Pistons with a Factory head gasket will give me a c/r of 8.5:1

and can any one tell me what my c/r would be if i used the wossner pistons
and what would be needed to hit 400/450 whp

[Al-sw20]

Oh yah make sure you keep your squish area. That will do more for det suppression than lowering compression slightly.