Trying to make sense of CR

[downforce22]

I bought this motor from a guy who had rebuilt it after snapping the timing belt, but he must have installed the piston rings wrongs as the compression is between 50-75 for all six cylinders. I need to do a rebuild and am trying to make sure my CR will be correct.

It is a m20B25 with 885 head and forged pistons and rods from Ross Racing. Supposedly the CR is calculated to be 9.75:1 for this application (was a euro m20 with the longer piston length) but I am looking at the specs and it seems something is off, perhaps I am misunderstanding this.

I am getting my specs from the piston manufacturer (Ross Racing) and this website:

http://web.archive.org/web/200505261...Mahle_m20.html

The Euro i motor should have a stroke of 75mm and a rod length of 135 mm, but the specs emailed to me from ross are as follows:

Stroke 3.528”
Rod 5.315”
Block height 8.110”

effective crown volume of -5 cubic centimeters.
The combustion chamber figured was 42 cc’s, and the crankshaft stroke was 3.528”. the conversion to MM requires that you divide by .03937.


The 206 mm, or 8.1" height is right. I guess the stroke is throwing me off because it should be 75 mm, or maybe 81 mm (3.1"), not 3.5" or 89.6 mm.

Was the engine calculated for another crankshaft? I did some research and it looks like the M54B30 has that crank and that is forged and would make sense to match the forged pistons and rods.

Thanks I may have answered my own question but somewhat doubt the previous owner would have the knowledge to find, buy and install the correct 89.6mm crank, hell, he had an eta head on it to begin with!

[jlevie]

I'm confused. Are you saying that the engine is supposed to have those rods & pistons? If so, pull the head and measure the stroke to see what crank was used. And check the piston deck height to see if the rods & pistons are the correct length for the crank.

[nando]

get an 89.6mm crank (M54 and S52), if it doesn't have one already, and build yourself a 3 liter using those pistons. :D

[whodwho]

You have too many unknown variables to even start to calculate CR.

You need to confirm at least these to start with:

Crank stroke
Piston compression height
Rod length
You should CC the head

And here is a CR calculator to help when you have numbers

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.0,m84.00,40.00,-22.00,m26.6,m140.00,m1.75,m0.000,7000,0,0

[downforce22]

Thanks for the responses guys!

I'm confused. Are you saying that the engine is supposed to have those rods & pistons? If so, pull the head and measure the stroke to see what crank was used. And check the piston deck height to see if the rods & pistons are the correct length for the crank.

I was trying to compare the stock pistons and CR (9.7:1) to what the PO bought when he rebuilt the motor. The information on that link does not match the information Ross gave me for piston stroke and that difference is what is throwing me off.

Either the information given by Ross Racing fort his order is that the crank is 89.6mm or we have a difference in measurements. The only way the stroke could be that long (3.5") is with the M54 or S52 crank. The crank is supposedly stock, so I guess I will have to pull it apart and check.

Here is a picture of the Ross pistons when I was replacing the head:

get an 89.6mm crank (M54 and S52), if it doesn't have one already, and build yourself a 3 liter using those pistons. :D

Not a bad idea, ill look into it. The CR would stay the same, right?

You have too many unknown variables to even start to calculate CR.

You need to confirm at least these to start with:

Crank stroke
Piston compression height
Rod length
You should CC the head

And here is a CR calculator to help when you have numbers

http://www.zealautowerks.com/index.h...0.000,7000,0,0

Thanks for the calculator, I got a CR of 8.88:1 for a stock us M20 but am not sure the difference in the euro m20. I will have to get more accurate measurements

[nando]

if it's got an 89.6mm crank, and custom pistons, it's no longer a "euro" M20 anyway.

[supernaughtIX]

Are those pics taken at TDC? if so, there's your answer to low compression numbers. Like Chris said, find a 89.6mm crank!

[downforce22]

if it's got an 89.6mm crank, and custom pistons, it's no longer a "euro" M20 anyway.

Thanks I know haha, but if it is a stock crank it would be essentially a stock euro motor with forged pistons and a 9.75:1 CR, I was just trying to figure it right on the CR calculator above.

Are those pics taken at TDC? if so, there's your answer to low compression numbers. Like Chris said, find a 89.6mm crank!

Yes it was very close to TDC if not exactly, but that would make sense, that looks about 15mm below the deck. Is it supposed to be flush with the deck of the block at TDC?

With stock crank that should be the difference if I am understanding this correctly. (75 vs 89.6) I found a few more pictures from a nice camera. That copper colored ring in the cylinders above the pistons looks like the difference.



big picture below

http://farm8.staticflickr.com/7102/7217386834_7f47ebee93_k.jpg

[LJ851]

Not a bad idea, ill look into it. The CR would stay the same, right?

No it would not.

Only 3 things affect your static compression ratio:
1) Total combustion chamber volume with the piston at TDC

2) Bore diameter

3) Stroke length

If you do not know all 3 of these, you are guessing in the dark what your CR is.



Also, the oil and carbon on the cylinder walls ( copper rings as you call them) are where the top piston ring stops at its highest point, not the top of the piston at its highest point.

[digger]

the "wear" marks in the bore from the top ring suggest that it may not be at TDC? The difference in throw between the 89.6mm and 75mm cranks is 7.3mm. so using a 89.6mm crank would lift the piston 7.3mm at TDC and lower it 7.3mm at BDC

[LJ851]

Edit, nvr mind i misunderstood when you said throw.

[jlevie]

Yes the pistons should be flush with the surface of the block at TDC. If not the CR is going to be really low.

[nando]

Thanks I know haha, but if it is a stock crank it would be essentially a stock euro motor with forged pistons and a 9.75:1 CR, I was just trying to figure it right on the CR calculator above.

but it's not, because the pin height is wrong for a 75mm crank. that 9.75:1 is only if you have the longer stroke crank.

[downforce22]

but it's not, because the pin height is wrong for a 75mm crank. that 9.75:1 is only if you have the longer stroke crank.

I understand what you are saying, but in a completely stock euro m20b25 isn't the crankshaft the 75mm crank?

That is what this website suggests stock crank for said motor is 75mm:

http://web.archive.org/web/200505261...Mahle_m20.html

And that is where I was getting confused as to where the extra compression came from in a 'stock' motor. Either that link is wrong as well (possible) and the crank in the car is not 75mm. IF that is the case then yes the CR is higher.

Either way I will have to check for markings on the crank that is in the car and see what size and what I should go from there. Are there any specific markings for the 75mm crank vs other?

the "wear" marks in the bore from the top ring suggest that it may not be at TDC? The difference in throw between the 89.6mm and 75mm cranks is 7.3mm. so using a 89.6mm crank would lift the piston 7.3mm at TDC and lower it 7.3mm at BDC

Thanks for clearing that up for me, that makes sense. The pictures were taken so long ago it may not have been at TDC. I guess upon disassembly I will see! Perhaps the crank is smaller or the rods are shorter if that picture was at TDC. It looks like pistons 2 3 4 and 5 are at the same height. Correct me if i am wrong but isnt 1 and 6 at tdc when 2 and 5 are going down and 3/4 are going up?(not quite BDC)

[supernaughtIX]

I understand what you are saying, but in a completely stock euro m20b25 isn't the crankshaft the 75mm crank?

That is what this website suggests stock crank for said motor is 75mm:

http://web.archive.org/web/200505261...Mahle_m20.html

And that is where I was getting confused as to where the extra compression came from in a 'stock' motor. Either that link is wrong as well (possible) and the crank in the car is not 75mm. IF that is the case then yes the CR is higher.

Either way I will have to check for markings on the crank that is in the car and see what size and what I should go from there. Are there any specific markings for the 75mm crank vs other?



Thanks for clearing that up for me, that makes sense. The pictures were taken so long ago it may not have been at TDC. I guess upon disassembly I will see! Perhaps the crank is smaller or the rods are shorter if that picture was at TDC. It looks like pistons 2 3 4 and 5 are at the same height. Correct me if i am wrong but isnt 1 and 6 at tdc when 2 and 5 are going down and 3/4 are going up?(not quite BDC)

A stock euro m20b25 has a higher CR due to the shape of the pistons. From that link, it says they are a little taller and have a different profile. This reduces the compressed chamber volume, and increases the CR. 84mm bore, 135mm rods and 75mm stroke crank.

In some 3.0L strokers, you use a 89.6mm stroke crank, 135mm rods and custom pistons(Like we presume the Ross setup uses). If you used the same pistons from the euro, the pistons would crash into the head. No go. Custom pistons are used that allow the piston top to be close to the deck height. They are shorter from the pin to the crown, and the height and shape(dome/dish/reliefs) dictate the final compressed chamber volume. If the combustion chamber was figured at 42cc for a 89.6mm crank, and you have a 75mm crank, then the resulting calc:
(pi*r^2*stroke+CV)/CV
uses CV= pi*r^2*(8.96-7.5)+42=~80.9cc
And you still have 2490cc displacement
Therefore you end up with:
(55.418*7.5+80.9)/80.9= 6.14:1 CR

That's all pretty rough, but should be a close estimate to what you actually have. 6.14ish CR with stroker parts, but same displacement. Its kind of a sad story really.