Compression Figures

[jimnielsen]

I have so far avoided doing a compression test on the race engine that I built for my Q4, however, seeing as I had the whole thing in a million pieces and a mate over to help reassemble the new clutch into it, I decided to do a compression test too. The results were:

Cylinder #1 207 psi
Cylinder #2 212 psi
Cylinder #3 210 psi
Cylinder #4 210 psi

These results seem suitably consistent, but pretty high..

what causes high static compression figures besides rings and valves that seal? Can this be a problem?

we conducted the test with all spark plugs out (the pressure gauge in one) and the throttle wide open.

jim.

[1NRO]

A sypmtom of the higher CR Jim. The oe cams will push it up too.

They are very consistant, good ring seal, just goes to show if build nicely a long gentle run-in isn't needed.

[jimnielsen]

It sure didn't get one of those!

[#84]

Compression test with all plugs out? I suppose you fit a pressure sensor to one cyl through spark plug hpole and then crank the engine. Right?

/A

[#84]

14,5 bar from August the first, 1793...

[SteveNZ]

Wow that's a lot of compression, but that's what you expected with those pistons though right? Do you think its too high?

[jimnielsen]

well, i am pleased that its acceptably even, though the numbers are a bit higher than I expected, and of course it is a function of higher comp pistons and standard duration cams. The engine is working very well indeed, and I am planning on making another that has the same pistons / rods - But it would be very instructive to take the head off the engine at this point, remove the pistons and look at the condition of the skirts and the ring lands etc. After all I am running the boost up to 1.5 bar with a stock head gasket and stock head bolts and so it would be interesting to see how things look inside...

having said that, I am not going to pull it apart!

jim.

[1NRO]

It'd be interesting though

Maybe the crowns don't show burn pattern too well though, the E85 will keep them clean still?

[jimnielsen]

yep, it still looks shiny down the spark plug holes! I really don't think that you could run the car the way I am without using E85. It seemed weird at first but once you get used to using it you just forget it!