Hello,
Two weeks now we are trying to built my 2.0lt Evo 9 motor with cosworth rods and pistons.
The issue was when we did a volumetric measure to find the CR.
The suprise was that the CR is 8.1:1 !!!!!
My mechanic measure the CR with the following method.
He was putting liquid through the spark hole and checked the volume.It was 70ml.
70ml on a 2.0lt is 8.1:1 CR
For 9.0:1 measurement must be 63ml.
The measure in the ported Cosworrth Big Head was 48ml
the OEM head gasket was 6ml
the piston itself was 6.5ml and the cylinder gap!!!!!!!! was 9.5ml

The main issue is the gap between the top of the block and the top of the piston.
My mechanic told me that piston top end must be in the same height with the top block end but look at the photos !!!!
That is the problem with the CR.

Can anyone help me with that?
What is wrong?

 

Buschur race pistons have zero deck height

 

Shot in the dark but rod bearings?

 

Wow, that should help him out a lot...

To the OP, are you using cosworth crank/rods, anything unusual about your rotating assembly?

Scorke

 

Way to help him. Have you contacted Cosworth?

 

So Al what is going on?
Is this ok or pistons have a problem?
I spoke with Cosworth and they gave me the specs... but the piston is as Cosworth said!!!!!!

 

What about them?
It is from Cosworth (standard size) and we haven't any issue with them.

 

I use OEM Mitsubishi crank
Cosworth Rods
Cosworth pistons
Total Seal piston Rings
All cosworth bearings.
Everythink is ok with the assembly.

 

Yes I already email Jeremy from Cosworth UK.
I am waiting for his answer and some photos of cosworth ready 2.0lt Evo block.

 

I have no clue what I'm talking about in terms of cc'ing an engine but.. you mentioned putting liquid through the spark plug hole? Aren't you supposed to have the spark plug installed when measuring though?

 

He measured the volume.
He told me that 70ml (70cc) on the TDC is 8.1:1 CR.
The issue is why the cosworth has this gap?
Are your piston have any gap???

 

That isn't necessarily true. If aluminum rods are used, the additional rod stretch should be factored into the piston deck height to minimize chances of the piston hitting the head. In other cases, the piston may actually be brought slightly above the deck. But if the piston and head are designed to have quench area, excessive recess in the deck height will kill it.

Also, IIRC, the volume of the cc of the Cosworth head is about 3 ml greater than what I measured with a stock head (someone may want to verify that), so the actual CR with a stock head may be slightly higher, and may be more representative of what CR is advertised with the pistons.

 

What kind of headgasket did you buy?Do not get a 1.5mm headgasket for the european market evo ix. The stock headgasket in our cars is 1.2mm. Perhaps they use a 1.5mm in the US to meet their own emmisions tests. This should rise your compression a bit more.

 

In your case if you want additional compression ratio - perhaps you can conisder to deck the head and the block slightly

I am not sure about the specs on the cosworth Evo pistons, but on the Subaru side they are really very good pistons

Low compression is not that big a deal IMHO with turbo cars although something closer to 9 / 1 may be better

AL

 

Cosworth makes only one compression ratio piston, according to their catalog. It is 8.8:1 compression ratio.

https://mzmperformance.com/CW-PA2689.htm as an example.

They also make the same piston, but for a 2.4 stroker crank, so make sure you got the right part number.

As long as everything was correctly installed and with the right parts, you have what you have, and it's like Cosworth intended.

I'd not consider decking the block. If you need to change compression ratio, it should be done by changing the pistons, if that's a huge concern to you.

Decking the block changes the distance between the crank centerline and the cams, which alters the timing.