I'm just looking for some insight on the amount of compression ratio in a cylinder and what lowering it for FI does in terms of PSI. If stock compression is 10.3:1 for a 2003 engine - what would it have for a compression test rating in PSI? As we drop the compression ratio in a built motor for FI down to 8.8:1 - what is the expected PSI in this range for a healthy newly built engine. In between these ratios is there a consistent drop in PSI per engine? Thank you, appreciated.

 

the FSM for a 2005 g35 coupe (i assume it will be the same or very similar for your G) says standard compression pressure is 184 psi with variance no greater than 15 psi across cylinders.

 

Thank you - but that is still for the stock compression, so I'm wondering what is to be expected when you change out the rods and lower the compression for a F/I build. A built engine/TT turbo combo I'm interested in purchasing just came back with the follow results and I don't know how good it is:

8.8:1 Compression Ratio on a Stage 2 IPP Engine with 25k miles:
Cylinder/PSI/Leak %
1/130/13%
2/140/11%
3/135/20%
4/140/15%
5/135/17%
6/140/13%

They are well within the variance of 15%, coming in at 7.2% difference which is good - but does this fall within a good range for 8.8:1 ratio and are the leakdown numbers something I should worry about?

Thanks...

 

my motor is built to 9:1 and I'm at about 145psi a cyclinder, so those look pretty solid.

 

Well that's good news then, just what I'm looking for. Thank you.

 

nominal is 185, 185/10.3=17.96*8.8=158, minimum 142, 142/10.3=13.77*8.8= 121. so compression is between nominal and minimum.
leakdown is high though especially 3 and 5

for comparison i run 8.0:1 compression and get 130 psi compression pretty much across the board +-1, and 10% leakdown with ring gaps set for racing nitrous engines.(very very loose about .0028, for comparison to that your average turbo engine with a 3.78 inch bore would be .0021-.0023) so my compression should be lower and leakdown higher then most engines encountered here.

 

For you infiniti0606, the fact that the psi from all 6 cylinders is within 5 psi says that at least for the condition of the Rings and Valves the Compression Test is Good! Who knows what it was new, or what was actually done to the motor.

That being said, you can Not calculate Compression PSI based on the CR rating of an aftermarket piston, also a new motor will be more psi than a used one.

1. Compression PSI is based off of Dynamic compression, not Static Compression.
2. Static CR rating of a pistion can vary a lot based on,
a. How much the block has been decked.
b. How much the Heads have been machined.
c. Thickness of head gasket.
d. Combustion chamber modified.
3. Dynamic Compression will be less than Static because of the Camshaft and Valve timing. The bigger the Camshaft the bigger the difference between Static and Dynamic. Can be as much as -1.5 with just a Radical street cam. So, a 10 to 1 CR rated piston could be a low as 8.5 to 1 Dynamic CR.

 

Wow, lots of good information here. Thank you all, it really clears up some questions.

In terms of the leak down tho - since the numbers have good compression, is the leak down something i should be worried about, and when I am comparing the leak down numbers do I compare 1 bank vs the other, cylinders 1/2, 3/4, 5/6 etc.. if I go 1/2, 3/4 etc then the leak numbers are all within 5% of each other. It seems to me that if it is a valve or the piston, either way there is no quick fix.. it means something is coming apart....no? My understanding is as long as I'm under 30% I'm ok, and 20% is moderate at best...