How to tell what your compression ratio is

[n0de]

Ok so I've ran into a bit of a problem here. I picked up a fresh built 2.3L stroker engine for an extremely good price (probably less than most spend on groceries and gas for one month), it came with eagle rods and wiseco pistons (all brand new and installed), the only issue is the person I purchased it from had been building it for more than 5 years and cannot remember the compression ratio of the pistons.

Is there any way for me to find the compression ratio, this is obviously something I have to know.

The engine is on a stand atm with the oil pan off. Are there any serial numbers on the top of the pistons themselves? I could just pull the head if that is the case.

 

[jayson427]

thats kind of weird you just dont forget what cp your pistons are on a build at least i wouldnt. depending on the brand of piston some do have numbers on the top (i think wesico's do have numbers on the top) and some dont. you could jump the starter relay and rent a compression tester from autozone. as long as it has oil in it i think you can still get a reading without the motor in the car.

 

[n0de]

thats kind of weird you just dont forget what cp your pistons are on a build at least i wouldnt. depending on the brand of piston some do have numbers on the top (i think wesico's do have numbers on the top) and some dont. you could jump the starter relay and rent a compression tester from autozone. as long as it has oil in it i think you can still get a reading without the motor in the car.

I don't think that compression has any direct correlation to compression ratio.

I know it was kind of weird that he wouldn't know, I was a little skeptical over the phone about the whole deal but when i saw it in person I knew they were forged pistons/rods, just didn't know what to make of it when he couldn't tell me the CR.

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[jayson427]

from the compression you can get a good guess at the compression ratio. higher compression pistons are around 200-220psi lower compression is like 165-180 i want to say. im sure you can look it up and find out.

 

[n0de]

from the compression you can get a good guess at the compression ratio. higher compression pistons are around 200-220psi lower compression is like 165-180 i want to say. im sure you can look it up and find out.

I believe that to be correct as far as other threads that ive read, i guess the difference is that since this is a newly built engine it should have near perfect compression.

 

[BogusSVO]

V1+V2+V3+V4+V5/ V1+V2+V3+V4

V1= Combustion chamber cc
V2= head gasket volume
V3= Piston dish cc
V4= Piston to deck clearance @TDC
V5= Cylinder volume

Milling the head changes V1
Decking the block changes V4

V5= Pi x R2 X stroke

 

[pauleyman]

Anybody wanna guess how you measure v1 and v3? If you know the definition of meniscus you know the answer.

 

[n0de]

V1+V2+V3+V4+V5/ V1+V2+V3+V4

V1= Combustion chamber cc
V2= head gasket volume
V3= Piston dish cc
V4= Piston to deck clearance @TDC
V5= Cylinder volume

Milling the head changes V1
Decking the block changes V4

V5= Pi x R2 X stroke

shit.

 

[boost97gst]

from the compression you can get a good guess at the compression ratio. higher compression pistons are around 200-220psi lower compression is like 165-180 i want to say. im sure you can look it up and find out.

Bingo: Dont get too fancy or make this too complicated. Lets say theyr 9:1, are your plans going to drastically change if its actually 8.5:1? I dont think so. Check compression by jumping starter(with oil in it of course). IIRC on a 2.0 with 8.5:1's its right at 180. You may want to PM some guys with a 2.3L ask some details after you get your numbers.
Also remember youre doing it on a new engine that hasnt even been broken in. Just put it in the car and break it in, im sure youll be happy

 

[DSMboostin15]

Anybody wanna guess how you measure v1 and v3? If you know the definition of meniscus you know the answer.

You can either use modeling clay with a known density to calculate the volume or a liquid and measure, at the meniscus, in a graduated cylinder. What's my prize?

 

[BogusSVO]

V1 and V3

http://www.dsmtuners.com/forums/articles-engine-fuel/341621-how-cc-combustion-chamber.html

If using aftermarket pistons, the volume should be listed on the spec sheet.

 

[n0de]

Bingo: Dont get too fancy or make this too complicated. Lets say theyr 9:1, are your plans going to drastically change if its actually 8.5:1? I dont think so. Check compression by jumping starter(with oil in it of course). IIRC on a 2.0 with 8.5:1's its right at 180. You may want to PM some guys with a 2.3L ask some details after you get your numbers.
Also remember youre doing it on a new engine that hasnt even been broken in. Just put it in the car and break it in, im sure youll be happy

Thanks, I more than likely will just compression test it after its in the car before i give it it's first startup.

V1 and V3

http://www.dsmtuners.com/forums/articles-engine-fuel/341621-how-cc-combustion-chamber.html

If using aftermarket pistons, the volume should be listed on the spec sheet.

If only I had a spec sheet for these pistons. I wish I did.

 

[BogusSVO]

Have you looked down the spark plug hole, with the piston at BDC and tried to see a part number?

 

[n0de]

Have you looked down the spark plug hole, with the piston at BDC and tried to see a part number?

Ok, so i removed a spark plug and fortunately I was able to read the numbers off of the piston (so shinny).

0811L
6571M855

That is exactly how/what it reads.

So this is 21cc 1.130 X 85.5

 

[n0de]

Guessing its 8.5:1?

 

[donniekak]

Should be 8.3:1.

 

[weebz1]

From what I've found those are 8.8:1

Wiseco Pistons - Pro Tru Sport Compact Series - Ohe's Performance Parts - Performance Parts

 

[NHerron]

Jezzzuss 16 posts and still not resolved. There's only so many pistons you can get off the shelf for a stroker.

They're .020" over, 8.8:1CR, standard Wiseco strokers.
EXTREME PSI : Your #1 Source for In Stock Performance Parts

Compression test is not the best comparision unless you and the other guy have the similar setups, mostly cams being the influence. For example Calan read 187psi whereas I read 150psi. We both have the same setups but different cams.

 

[n0de]

Jezzzuss 16 posts and still not resolved. There's only so many pistons you can get off the shelf for a stroker.

They're .020" over, 8.8:1CR, standard Wiseco strokers.
EXTREME PSI : Your #1 Source for In Stock Performance Parts

Compression test is not the best comparision unless you and the other guy have the similar setups, mostly cams being the influence. For example Calan read 187psi whereas I read 150psi. We both have the same setups but different cams.

Thank you. The thread that I found while searching for the model number suggested they were 8.3 CR for some reason. Thanks for the links that you've all provided.