compression with 8:1 pistons?

[wild_maxx]

I am currently running 9:1 comp. supertech pistons made for the DE but when put in my E they are 8:1 comp pistons. I am did my first compression test and got about 130 across all 4 cylinders.... i do have a PDM re-ground cam.

question is... should i stay with the 8:1 comp pistons for the next motor? and should i expect the same type of compression numbers? Would the 8:1 pistons drop the compression that low?

also, with the PDM cam... would that affect my VAC. reading at idle? since the motor has been running the vac is always about 14... i have searched for leaks numerous times and didnt find any.

[Craving4Boost]

does anyone know what the 8:1 9:1 10:1 compression is actually measuring? sorry if its a noob question but i really have no IDEA. all i know is.....the lower the compression is..the safer because its honed in and has more room for play.

[TravBear2]

Its measing how many times more compressed the air is becoming... 10:1 means that the air is compressed down to 1/10 the pressure it is a BDC. I dont know if this came out right, but you get the idea.

[Chezedik]

Essentially you made the point. But let me put it this way: Let's suppose that the stroke of the motor is 10 inches, if the compression ratio were 10:1 then of course, the result of that stroke would be compressed to 1 inch. This is not quite as self explainatory as possible. This is referred to as dynamic compression. But the reality is called static compression. Static compression is the pressure inside of the cylinder. This means that although an engine may be at 8:1 @ 160lbs, but boring over the engine will result in higher static compression. The important part is to get in the ballpark and tune the rest out. Also, the room for play idea is retarded. The only reasons for running lower comps is to boost, reduce knock, and to make emissions. Otherwise, you sacrifice torque, HP, and FUN!

[nissanfanatic]

Lower compression ratio "*TYPICALLY*" means less pressure. This in turn means less heat. When you apply boost, heat definatly goes up. But 15psi and 9:1 CR is no where near as much pressure as 15psi and 8:1 CR. The thing is, horsepower is made by airflow. Torque is made by cylinder pressure. So, you lose some cylinder pressure, but instead of filling the combustion chamber with metal, you are filling it with air.

So, as you have lower compression chamber temps, you can run higher boost on lower octane fuel thus making more horsepower on lower octane fuel.

Compression ratio is a ratio of BDC combustion chamber volume-to-TDC combustion chamber volume.

http://e30m3performance.com/my...o.htm

Note this is "STATIC COMPRESSION RATIO".

Dynamic compression ratio takes air pressure into account when calculating compression ratio.

[Chezedik]

Sorry, I always get them backwards.

[hannibal]

Lower compression ratio "*TYPICALLY*" means less pressure. This in turn means less heat. When you apply boost, heat definatly goes up. But 15psi and 9:1 CR is no where near as much pressure as 15psi and 8:1 CR. The thing is, horsepower is made by airflow. Torque is made by cylinder pressure. So, you lose some cylinder pressure, but instead of filling the combustion chamber with metal, you are filling it with air.

So, as you have lower compression chamber temps, you can run higher boost on lower octane fuel thus making more horsepower on lower octane fuel.

Compression ratio is a ratio of BDC combustion chamber volume-to-TDC combustion chamber volume.

http://e30m3performance.com/my...o.htm

Note this is "STATIC COMPRESSION RATIO".

Dynamic compression ratio takes air pressure into account when calculating compression ratio.

Isnt that first statement backwards? In two motors running NA or with the same amount of boost, the one with higher CR would have higher cylinder pressure.

That second part is a great definition of CR.

I think 8:1 CR is kinda low. All of the 'featured' guys on ka-t.org are using 8.5, 8.8, or 9.1 CR.

[Chezedik]

That is what he was saying. 9:1 w/ 15psi would be considerably higher.

[mikesloud]

i THINK THAT IS ALL GOOD ! i UNDERSTAND HOW THAT WORKS, BUT TO THE ORIGINAL QUESTION , WILL DIFFERENT CAMS AND LOWER COMPRESSION EQUAL LOWER COMPRESSION NUMBERS WHEN CHECKED AT THE STARTER SPEED OF RPM? i HAVE SAME ISSUE , 130 ACROSS BOARD AND VAC OF 14-15 WITH JWT CAMS?

[hannibal]

Yes, lower compression pistons would create lower compression readings during a compression test.

I dont know what the exact value should be. The FSM lists the DOHC stock compression reading as 179psi. I dont know what the SOHC is, but 130psi doesnt sound so bad, especially if its across all cylinders

[Chezedik]

From what I have read, you can expect about 20lbs per compression point. This depends on the relative strength of the motor, and the aggressiveness of the camshafts. So you could expect the compression numbers to reflect that. About 120-130 running and about 85 cranking at WOT.