Compression Ratio Question.

BHaner

BHaner

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Aug 1, 2006
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Having trouble figuring out what exactly my compression ratio SHOULD be. I don't know enough of the factory numbers to figure it out.

Here is what I have:

Removed .005 from the head.
Removed .003 from the block.
Using “339” marine .020 over head gasket.
Stock pistons, stock rings, stock bores.


Can someone help me figure it out?
 
GRotman said:
Can you measure your piston protrusion?
Click to expand...

Evertually... When I take the head off again. I don't have those numbers at this time.

Being mostly stock, shouldn't it be able to be figured in theory?
 
The 0.20 over is really about 0.18 crush size if that helps.With the weaker rods 3rd's have i would say lower is gooder.
 
I've found about 12 thou proud has been the average for what I've had apart, stock for the 04.5 and up engines. So add the .003 to that, or 15 thou proud would be ballpark.

The gasket drops you a point if I remember right, + a shunt hair for the bit off the deck.
 
BHaner said:
True... But how low is too low?
Click to expand...

I have a head thats never been shaved.And a 0.20 over hg.When it is 10* or lower and pluged in for 6 hrs it smokes hard.Runs fine, just smokes like someone is burning leaves.It dont bug me.Just letting you know.When it is 25 or so no smoke...ok maybe just a trace.I am guessing 15-1 or whatever is ok.If not someone will correct me.I did say guess.Some say to low on a dd the bottom end will suffer a little.
 
So what is the factory compression ration for an 07?

What about an 03 SO?
 
I have .065 shaved off the pistons, .020 over hg, and mine fires up at 0 degrees painlessly without smoke.
 
Hey dude, got your PM... looks like you're getting your question answer here.

I don't know why my compression was as low as it was. I did pull the head to check for problems but, didn't find a thing. What I did find was a bent valve which would have shown low compression in that one cylinder but all mine were low.

For comparison in what you used, I used the Snap-on CR compression test kit so I figure it was a good one to use. It's possible other brand may not seal as well or read the same ???? don't know.

My guess is either you have some weak rings, possibly some valves not seating all the way or they're normal readings.

Stock compression on our trucks is 18:1. I have mine down to about 16 or 17:1 from the shaved pistons.
 
smokingoat04 said:
I have .065 shaved off the pistons, .020 over hg, and mine fires up at 0 degrees painlessly without smoke.
Click to expand...

Maybe because yours is a cr and the injection process is different? Plus i run 2 stroke oil.Maybe when mine is real cold it is just not happy?Some winter fuel has kerosene in it i know ours does.When you cut .065 and used 0.20 over hg what did you figure you have for compression?
 
smokingoat04 said:
I have .065 shaved off the pistons, .020 over hg, and mine fires up at 0 degrees painlessly without smoke.
Click to expand...

Yours is a 305/555 right?

I have been hearing that the 03-04's do better with lower compression due to having only two injection events.

Guess with the lower compression on the 3 squirters, everything gets stacked up and out of time.
 
triton said:
Hey dude, got your PM... looks like you're getting your question answer here.

I don't know why my compression was as low as it was. I did pull the head to check for problems but, didn't find a thing. What I did find was a bent valve which would have shown low compression in that one cylinder but all mine were low.

For comparison in what you used, I used the Snap-on CR compression test kit so I figure it was a good one to use. It's possible other brand may not seal as well or read the same ???? don't know.

My guess is either you have some weak rings, possibly some valves not seating all the way or they're normal readings.

Stock compression on our trucks is 18:1. I have mine down to about 16 or 17:1 from the shaved pistons.
Click to expand...

Thanks for the reply. Yeah, my compression numbers are almost exactly the same as yours. However I didn't have anything cut off the pistons.

I do have new rings, and the head was just checked out. Everything seemed good there. No blowby, so I would guess the rings are seated.
 
I think the stock bore is 102.1 mm and stock stroke is 119.9 mm, and results in a stock CR of 17.2:1.

Assuming you have a net increase of 0.010" (0.003 + 0.005 and an 0.020 gasket squashed to 0.018), this would drop your compression ratio from 17.2 to about 16.59.

--Eric
 
triton said:
Stock compression on our trucks is 18:1. I have mine down to about 16 or 17:1 from the shaved pistons.
Click to expand...


Ty, I'm pretty sure stock compression is 17.2 : 1

With your pistons being cut but you don't know how much Scheid cut them and the fact that you are using firerings with a special gasket...you would need to find out those numbers to get an accurate comp. ratio...

I'm betting that Scheid cut more than you think and your gasket is thicker than the standard MLS gasket at full crush ( approx. .044)....

I say your in the low 15's at Least!!!



My new motor is as follows...

-Head was decked about .002 - .003 when I had it done over a year ago
-Block was decked .007 to get flat.
-I use a Jeff Mumau std. thinkness 12v gasket that is CNC'd to match the CR cooling holes........Approx .056 -.057 at full crush.....(So thicker than the MLS by about .010)
-My piston's were cut .020......The ceramic coating adds back a few thou...maybe .002...

The calculator came up with this on my build....Had I used a MLS (.044 crush) my Comp Ratio would have been right at 16.0:1 - 16.1:1......

But since I use a 12v with o-ring's....It's right around 15.7-15.8 : 1.....or so
 
So I have been getting various reports on gasket thickness.

Does anyone know for sure what the crush thickness of the factory MLS gasket, and the factory 12 valve gaskets are?
 
So if I use the numbers that Biggie just posted:

Stock MLS gasket: .044
.020+ 12 valve gasket: .076

Minus the amount taken off my block: .003

Gives me a net gain of .029 over stock.

If .020 gives 1 point of compression, then I lost 1.45 points.

If factory compression is 17.2:1, then I am at 15.75:1 in theory.
 
BHaner said:
So if I use the numbers that Biggie just posted:

Stock MLS gasket: .044
.020+ 12 valve gasket: .076

Minus the amount taken off my block: .003

Gives me a net gain of .029 over stock.

If .020 gives 1 point of compression, then I lost 1.45 points.

If factory compression is 17.2:1, then I am at 15.75:1 in theory.
Click to expand...


Not sure what temps are like where you live but does it white smoke at all when you crank it when completely cold.....Mine does just slightly now...

My OLD build had a Full .100 cut off the pistons...plus the extra thickness of the 24v gasket I used....We figured mine was as low as 14.1 - 14.5 :1.....It would white smoke bad when cold...but still crank pretty well...


The .044 was just what I was taught....Wished I could tell ya that I measured it for myself but I can't.....From reliable source though....
 
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