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Compression Test

morto86

By morto86,
in General Maintenance

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Ben C34 Grand Master

Ben C34

Posted
Posted

The comp ratio is swept vs unswept volume. Very simple. It gets much more difficult once the engine is spinng, as now the cam timing, condition of the valves and ring seals and head gasket play a part, along with volumetric efficiency, temperature etc. To figure out an unknown engines comp ratio from a comp test is never going to be accurate. Cam timing plays a huge role. If you are buying a built engine ask for the actual figures. If you are happy to assume its a high comp ratio motor based on that test, (which changed a fair bit once you asked for confirmation) by all means buy it.

StevenCJR31 Veteran

StevenCJR31

Posted
Posted

^^ this! Some very very early combustion engines had low compression figures from the factory due to the low CR of around 6 to 1

Id say rob is running 8 to 1 CR as hi boost turbos ADD a lot of compression pressure to the engine therefore lowering the CR he can run (part governed by fuel octane)

AngryRB Grand Master

AngryRB

Posted
Posted (edited)

Settle down. explain to me how an engine when built has a certain compression test result, then when worn had a lesser compression test result? Did it's comp ratio drop? can you run more boost cos it has less comp ratio? NO. explain how comp ratio effects the comp test results please :)

Ive compared 2 identical engines before, one with 150psi ( supposed to be 7.6-1) each cylinder and one with 180psi, and the one with 180psi cleary had more balls, the 180psi one had the head shaved which would of increased the comp ratio..

also a previous rb20det that was in nice condition, showed up as 150psi on average, showed 100rwkw without intercooler so about right I think, just for reference

Edited by AngryRB
KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

^^ this! Some very very early combustion engines had low compression figures from the factory due to the low CR of around 6 to 1

Id say rob is running 8 to 1 CR as hi boost turbos ADD a lot of compression pressure to the engine therefore lowering the CR he can run (part governed by fuel octane)

Ive compared 2 identical engines before, one with 150psi ( supposed to be 7.6-1) each cylinder and one with 180psi, and the one with 180psi cleary had more balls, the 180psi one had the head shaved which would of increased the comp ratio..

also a previous rb20det that was in nice condition, showed up as 150psi on average, showed 100rwkw without intercooler so about right I think, just for reference

I have low compression pistons as they were lying around the shop at the time of my low budget build. Consequently I have to run 25 psi to make the same power as others do with 18psi and high compression pistons! It was a very slow build and over time I added more expensive components to the top of the engine (what you might call the wrong way round)! If and when my engine goes bang I will rebore and fit 9.5:1 forged pistons and properly balance everything.
AngryRB Grand Master

AngryRB

Posted
Posted

must be a pain to drive off boost, im keen to do a rb30det with 9.5-10-1 with e85 and a 3076, nothing worse than a sluggish motor on the streets, but sometimes you just have to do the best with what you've got..

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

must be a pain to drive off boost, im keen to do a rb30det with 9.5-10-1 with e85 and a 3076, nothing worse than a sluggish motor on the streets, but sometimes you just have to do the best with what you've got..

Not at all a pain to drive off boost - accelerates quickly from 1000 rev/min in any gear. However your specs will make for a quick car!
Traah221 Contributor

Traah221

Posted
Posted

Anyone thought about how the one doing the test could be straight up doing it wrong? Sounds stupid, I know, but hey, it happens. If he keeps cranking it until the gauge needle stops moving, it would read around the 200 - 210 mark. Some people just plain don't know how to properly do a compression test.

To OP, yes, 200+ is way, way too high. Look for a number around the 150-170 mark.

Source: Cranking my RB25DET comp gauge pressure up to 205 across the board to perform an (admittedly ghetto) homemade leakdown test just by bumping it until the needle stopped.

Missileman Veteran

Missileman

Posted
Posted

I take my reading when the need stops moving as per the book...

When I did my RB20DET I had two comp gauges to use, the lowest set of figures were 149 to 154 psi, the gauge that read higher also achieved a 5psi spread.

post-89296-0-01969300-1384556275_thumb.jpg

post-89296-0-65841800-1384556293_thumb.jpg

AngryRB Grand Master

AngryRB

Posted
Posted

Anyone thought about how the one doing the test could be straight up doing it wrong? Sounds stupid, I know, but hey, it happens. If he keeps cranking it until the gauge needle stops moving, it would read around the 200 - 210 mark. Some people just plain don't know how to properly do a compression test.

To OP, yes, 200+ is way, way too high. Look for a number around the 150-170 mark.

Source: Cranking my RB25DET comp gauge pressure up to 205 across the board to perform an (admittedly ghetto) homemade leakdown test just by bumping it until the needle stopped.

Ive never seen the guage keep getting higher with longer cranking, in the first few cranks it hits the number and after that just bounces on that number, the engine just has what it has I don't get how you can make it higher than that.
KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

I just got the reply 210. Cold. Does this mean the head has been shaved previously

There is not necessarily anything wrong with this motor. It may be purpose built as a high compression engine designed to run on race fuel or E85. Back in my youth as a Ford mechanic we used to get cars with high compressions caused by excessive deposits and they were routinely "decoked"... not a term I hear these days. If the engine is in a car you should test drive it with some form of det detection to see if it is detonating. Ask the seller why the compression is so high - did he build it himself or know who did?

Ben C34 Grand Master

Ben C34

Posted
Posted

Nah its from a half cut car import mob offering start up warranty only. I dont really care about how to do a comp test coz I do know how to do it. Im wondering what are the pros and cons of having a high compression motor

Google will give you a much faster answer.
FBRacing Rising Star

FBRacing

Posted
Posted

Nah its from a half cut car import mob offering start up warranty only. I dont really care about how to do a comp test coz I do know how to do it. Im wondering what are the pros and cons of having a high compression motor

High compression is usually associated with a n/a. Low comp with forced induction. You trying to achieving any thing with 9.5ish.1 comp and above has a higher risk of detonation for your RB20det. Ideally for a force induction motor supercharged or turbo would be in the 8-8.5.1 range for reliable safe worry free function. This topic asking the pros and cons goes into a rabbit hole of controversy and opinion. Every engine builder believes what the do and how they build an engine equals to personal beliefs of religion aka everyone is wrong but themselves. I agree with Superben somewhat that no one will give you a correct answer unless you research for yourself. I hate telling someone to go "read" but in this case Google and the search function here on SAU is your best friend.

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