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Engine Compression

Guilt-Toy

By Guilt-Toy,
in Engines & Forced Induction

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Guilt-Toy Grand Master

Guilt-Toy

Posted
Posted

I got my new engine built and I was told it was going to have 8.8:1 compression but when i compresson tested it i got a reading of 120 in each cyl.

Now correct me if i am wrong but that is pretty low as my old stuffed engine had 150 in each cyl and that was around 8.3:1 or abouts (stock rb30e pistons)

So my question is can you convert that psi reading in the cyl to a compression ratio ? (was done warm with throttle open)

I would like to know what compression ratio it is.. looks to me that its in the 7's.. would that be right ?

I noticed the cometic head gasket looked like a shim plate. around 2mm so maybe thats why its ssooo low...

so how do i know the compression ratio of this engine? the engine builder said 8.8:1 but i dont think its there.

I dont really mind that its low because im going to add another 10psi boost and it will be fine on pump gas.. might try 25psi

cheers

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GTR1993 Mentor

GTR1993

Posted
Posted (edited)

you know the stroke, the CC of pistons, CC combustion chamber, headgasket thickness and how far under/over the piston sits compared to the block deck. From this you can work out an accurate compression ratio.

Just by going from the guage u have a few problems, different guages and cam overlap. You have static compression ratio, and dynamic comp ratio, so when you use a comp tester the cams if they are larger or set to a different position that can bleed off more/less air, therefore changing your readings.

also, you need to go thicker head gasket when you go oversized pistons (if you want to stick to the same comp ratio), unless they dish in the piston is larger as the comp ratio will rise.

Edited by GTR1993
Adriano Grand Master

Adriano

Posted
Posted

Getting oversize pistons makes stuff all difference to the comp, if tis the same cams as the previous motor, and has lost 30 psi, its a fair bet its running lower comp. How many k's has it done? Do you know if the engine builder cc'd the chambers, checked piston heights/squish etc. Is it an r33 head, if so i cant see any reason for a 2mm head gasket, usually you are trying to get as much comp out of them as you can, even with n/a pistons, do you know what brand of piston was used?

al_r33 Rising Star

al_r33

Posted
Posted

I had the same problem in my rb25 when it was rebuilt as i found out a year later that the comp tests shouw up as 120psi across all pistons. In the end i pulled the motor apart again and got it tested and the comp ratio was 8.1:1 with a std head gasket.

  • 1 year later...
Kranker Collaborator

Kranker

Posted
Posted

I really dont think you can accuratley compare cylinder compression with compression ratio.

The compression ratio is just the ratio of the volume of the combustion chamber at BDC to the volume of comb chamber at TDC, its just maths, not anything to do with psi...correct me if im wrong

Doesnt take into account loosely toleranced pistons, rings etc,blow by, or long duration cams with big overlap which will all reduce the actual compression of the engine

But i guess that if engine A and engine B, were built to the same tolerances, and were up to the same temperature, with the cams cams etc, but A was 9:1 and B was 8:1 then cylinder compression should be higher in engine A

So there can be a relation but its not a guarantee

take a look at http://www.type2.com/library/engineg/comrat.htm

they have a table from the bosch automotive handbook, that relates CR to compression, going by that you could have anything from 6.8-7.8:1, thats a pretty big range, but still supports ur guess that your compression ration is in the high 7's

hope you work it out mate

juggernaut1 Proficient

juggernaut1

Posted
Posted

Couple of calculators for you:

http://www.csgnetwork.com/compcalc.html

http://golenengineservice.com/calc/calccr.htm

Did you have the combustion chambers modified in any way - if you have then chances are the chambers will be bigger which will reduce your comp too.

I've just had an SR built an we initially installed 8.5 CP's but as the head had been modified (by JHH Engineering) we then had to install 9.0 CP's to get the static comp up to the target 8.8:1. We didn't have the head at the time the bottom end was built and had relied on general comments from JHH that the volume wouldn't vary much. Not having a go at JHH - I'm stoked with the job they did.

Just saying it pays to CC's everything and calculate the comp ratio before buttoning up the engine.

Personally I would rather run more comp, less timing and less boost. With E85 fuel coming on song I can then retune for more timing and boost.

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted
Couple of calculators for you:

http://www.csgnetwork.com/compcalc.html

http://golenengineservice.com/calc/calccr.htm

Did you have the combustion chambers modified in any way - if you have then chances are the chambers will be bigger which will reduce your comp too.

I've just had an SR built an we initially installed 8.5 CP's but as the head had been modified (by JHH Engineering) we then had to install 9.0 CP's to get the static comp up to the target 8.8:1. We didn't have the head at the time the bottom end was built and had relied on general comments from JHH that the volume wouldn't vary much. Not having a go at JHH - I'm stoked with the job they did.

Just saying it pays to CC's everything and calculate the comp ratio before buttoning up the engine.

Personally I would rather run more comp, less timing and less boost. With E85 fuel coming on song I can then retune for more timing and boost.

If you really want to know the precise ratio and do not have the information to calculate it (cc of head, exact bore and stroke) then perhaps you can measure it by pouring oil down the spark plug hole at TDC and then filling it again at BDC - potentially messy (bit of a mission to get the oil out again) but dead accurate (and no, I haven't done it myself).

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