Hi all, did a compression test on a few motors, thought i'd post the results for comment and the benefit of others. All are factory stock, two are turbo's and one is na. the guage used is a brand new ABW screw-in type. each cylinder was tested at cranking speed for about 10 compression strokes with the throttle held wide open. i also tested the motors first cold then hot. turbo 1 cyl.....cold...........hot 1.......135...........130 2.......137...........130 3.......135...........130 4.......145...........137 5.......145...........135 6.......140...........135 turbo 2 had a similar spread to turbo 1 but with all pressures about 5 - 10 psi higher and again cold pressures were higher than hot. the na motor measured 190 - 195 psi cold and 185 - 190 hot [very happy with that motor!! ] So, the na motor does have another head on it which must have been milled a bit to account for the higher than factory pressure spec [173 psi] the two turbo's seem to be down a bit on the factory spec which is 165 psi, but i can't make the numbers in the book add up for the turbo motor. If 165 psi is correct, then the cr has got to be higher than 7.8 : 1, more like 8.6 : 1 If however, we take the factory quoted cr of 7.8 : 1 to be correct, then this should yeild a pressure of about 150 psi, which makes turbo 2 look to be in pretty good shape, and turbo 1 down a little but still acceptable. Some original photos have not been recovered yet.

you think too much. if there all roughly the same and not a great deal down then I would not worry about it. the rings and valve seating faces will wear with age(more so the rings) and bring it down a bit.

Interesting thread and results. We don't get enough threads like these anymore. Threads that provoke some thought, show some results and can be used in future for comparison. Good Work. Now i want to check mine and compare Some original photos have not been recovered yet.

Quote: you think too much. if there all roughly the same and not a great deal down then I would not worry about it. the rings and valve seating faces will wear with age(more so the rings) and bring it down a bit. no worries here, just a list of results. my only concern is the factory figures quoted for the turbo motor - they don't add up for me. 165 psi is more than 7.8 : 1 im inclined to believe the 7.8 : 1 figure, and the pressure will be about 150 psi at best. rb30 power, will be interested to see what numbers you come up with. imo the screw-in type guage is the only way to go, especially with deep seated plug holes and manifolds that get in the way like on these motors. No. 4 is the trickiest one to get at, but is no problem once you get the hang of how to flex the hose to get it to drop in to the thread

what a coincidence, i did a compression test on mine today. Screw in gauge, RB30ET- engine was warm, lowest reading i got was 132psi, and highest was 140psi. There is no simple way to work out what your compression ratio is off cylinder pressure. there are too many variables.

Quote: There is no simple way to work out what your compression ratio is off cylinder pressure. there are too many variables. thats true, though i was using the na figures as a comparison with the only significant difference to be considered between the na and turbo motor is the inlet closing degrees. If we assume the factory figures for the na motor to be correct, i.e. 9.0 : 1 @ 300 rpm cranking speed @ wot will yeild 173 psi, then this equals about 19.2 psi per ratio. If we then translate that to the turbo cr of 7.8 : 1, then we get 149.7 psi [7.8 x 19.2] The difference in camshaft inlet closing times can be safely ignored here, as the turbo cam closes the inlet later than the na cam, and if factored in would only reduce the expected reading even further. We could also work the figures back the other way, by assuming the factory quoted turbo figures to be correct. This would then mean the na motor should give about 195 psi instead of the quoted 173 psi. There's two other problems though with working it this way. One is that we are now talking over 21 psi per ratio which is off scale high to the generally accepted range of 17 - 20 psi per ratio, and the other is that those of us that have cylinders down around the 130 - 135 psi range need to do some homework to find out why, as a loss of 30 psi down from 165 psi is getting out of spec too far. At this stage I'm still standing by my argument that the quoted 165 psi for the rb30et can't be right, as compared to the figures for the na motor. There is actually a reason for all this fuss, as turbo 1 doesn't have any where near the off-the-mark grunt that turbo 2 has. both are stock 3.45 one wheeler auto's, but turbo 2 will break traction very quickly when nailed from a standing start [much faster spool-up], turbo 1 is nowhere near it even when overboosted to 10 psi! I tried swapping the turbo assemblies - no difference. There are some other variables to check yet, such as making sure turbo 2's diff ratio is in fact still 3.45

compression tested mine warm a few weeks ago, factory turbo motor, ranging from 151 - 155 psi. I was quite happy with that. it was a screw in ABW compression tested and i let it crank 5 times for each cyl. vlhdt 86 there are lot of factors that could affect the take off, i.e. mixture, timing, tyres, even the weight of the car, which would be hard to eliminate. having said that i have driven a friends vlt which compression tested quite low ( think it was around the 130psi mark) and it was quite different to mine when they were in the same state.

how did they drive differently natho, interested to know. not as smooth?

his just felt slower lol.

http://www.calaisturbo.com.au/showth...ht=compression (http://www.calaisturbo.com.au/showthread.php?t=41967&highlight=compression) this is what i came up with when i did a similar test. i dont really know the history on this motor which was the original reason i did the test. However, 40,000k's after i dropped her in still not using oil except when it had a sticky valve tho.

interesting, need to do one on mine, see whats going on..

Quote: vlhdt 86 there are lot of factors that could affect the take off Thats 4 sure natho, and my method of comparison, whilst not totally without merit, is somewhat un-scientific and difficult Your figures and the compression/driving comparison is interesting. I can see how low compression could strike on two fronts in a turbo motor. The one common to all motors is lower combustion efficiency. This I expect would lead to lower exhaust gas flows and slower spool-up of a turbo. imo 130 psi is too low, 150 seems more like what they should be pushing on average. Maybe the 165 psi quoted in the gm book is correct, but if it is, I'm sure the c.r. will calculate out to more than 7.8 : 1 Time to break some more slide rules and calculate the cr myself!! Some original photos have not been recovered yet.

Quote: Originally Posted by VLHDT 86 Thats 4 sure natho, and my method of comparison, whilst not totally without merit, is somewhat un-scientific and difficult Your figures and the compression/driving comparison is interesting. I can see how low compression could strike on two fronts in a turbo motor. The one common to all motors is lower combustion efficiency. This I expect would lead to lower exhaust gas flows and slower spool-up of a turbo. imo 130 psi is too low, 150 seems more like what they should be pushing on average. Maybe the 165 psi quoted in the gm book is correct, but if it is, I'm sure the c.r. will calculate out to more than 7.8 : 1 Time to break some more slide rules and calculate the cr myself!! are your figures for a dry test? if so do a wet test and you'll get higher compression reading... Some original photos have not been recovered yet.

tried this on no. 1, pressure jumped from 130 to 150 psi. it doesn't surprise me that the rings and bores have got problems, the compressor wheel in the turbo had been severely 'sand blasted' like the car had been driven for some time with no aircleaner or something.

whats dry test and wet test?

wet testing means squirting a bit of oil through the plug hole to wet up the top ring. just helps determine if rings/bores are causing a low pressure reading.

There are a lot of factors that can effect the actual measured cranking pressure, probably the most important thing is that they all end up fairly close to each other. Here are some things that can effect the readings: Temperature of the engine Cranking speed Throttle open, or throttle closed ? Inlet valve timing The biggest unknown is heat. Suppose you take a fixed volume of gas and squash it down to something much smaller. Two things happen. First obviously the pressure rises, but less obvious the temperature also rises too. But that hot compressed gas will rapidly cool back down, and the pressure will fall away, even if no gas can actually leak out. So you can either compress it very fast, and measure the hot pressure. Or you can compress it very slowly and allow it to cool right back to ambient before measuring the pressure. That would give a significantly lower pressure reading than the first pressure. So there is a lot going on inside the engine when you crank it over, the air compresses and heats rapidly, but it also cools very rapidly, (and some air leaks out). What you end up with is what you see on the gauge. It is very difficult to relate the actual gauge reading to anything else, because of the unknown influence of very rapid heat transfer during cranking. So don't underestimate the important effects of heat on the reading your compression gauge gives.