prins@cornell.UUCP (Jan Prins) (11/23/85)
>BTW, the compression was between 170 and 180 psi on all cylinders; does >anyone know what the spec is? The shop manual doesn't seem to have it!! I feel factory spec compression figures are not very useful as an engine indicator, and I suspect that's why they're often not featured prominently in service manuals. Consider that these measurements are taken over a compression stroke that is of vastly longer duration than an operating stroke. Ring sealing depends on combustion chamber pressure, which simply doesn't build very quickly at cranking rpms. At the same time leakage past the rings takes place for a longer time. Therefore the pressures observed are not the same as those you would see during operation. Since the components involved are assembled to specified tolerances, manufacturers are often able to give ballpark compression figures for a new engine. As the engine wears the compression figures drop but are not a reliable indicator of power production ability. When the compression figures fall below the "specs" they invite the conclusion that the top end is "worn out", which is not necessarily true. Of course a serious problem like a stuck valve or a warped head can be diagnosed with a compression test, but in that case the magnitude of the figures involved is not important, just their difference. And these sorts of problems are pretty obvious to begin with. I once saw a Toyota 18RC engine that wouldn't start. The compression had been measured between 30 and 50 psi on all cylinders. The spec was, as above, 170 psi. Engine junk? Well, not entirely. The starting procedure had to be adapted to give maximum compression - i.e. throttles wide open. Once started in this fashion the engine made normal power and burned no oil. Since then that car has piled up another 20,000 miles. So it seems to me that a compression test only validates a claim that an engine top end is relatively new. I think their use in estimating the "life" of an engine is overrated. Jan Prins UUCP: {vax135, ihnp4, decvax}!cornell!prins Csnet: prins@cornell ARPA: prins@gvax.cs.cornell.edu BITNET: PRINS@CRNLCS
cottrell@sdcsla.UUCP (Gary Cottrell) (11/26/85)
In article <1083@cornell.UUCP> prins@cornell.UUCP (Jan Prins) writes: >I once saw a Toyota 18RC engine that wouldn't start. The compression had >been measured between 30 and 50 psi on all cylinders. The spec was, >as above, 170 psi. Engine junk? Well, not entirely. The starting >procedure had to be adapted to give maximum compression - i.e. throttles >wide open. Once started in this fashion the engine made normal power >and burned no oil. Since then that car has piled up another 20,000 miles. > One friend sold another friend his "junk" Toyota 18RC for 5 dollars. The thing ran terribly. Besides tuning it up, we checked the compression, which was 140 on all the cylinders. We adjusted the valves, and checked it again. 180 on all cylinders. I find it hard to believe your 18rc is getting normal power. Try adjusting the valves. You might get a nice surprise! gary cottrell cottrell@nprdc (ARPA) Institute for Cognitive Science sdcsvax!sdcsla!cottrell (USENET) UCSD