compression test on brand new build

[olidaviesuk]

Just a quick question, has anybody done a compression test on a brand new build with standard pistons and liners with a mild cam in it?

Just finished off the build on the new race engine today, have had a few teething problems, fuel and valve timing related, but resolved all those today..

Did a compression test on the engine (hot) after running the cam in (so engine has about 2hrs on it now) and is 105psi across all 4, this strikes me as a bit low, but assuming this is due to the rings still not being bedded in yet... Also assuming the cam will make a difference to the compression test as well..? Have road tested and doesn't seem to be down on power, obviously haven't run on boost yet.

Anyone care to set my mind at rest..

[Ashy]

Throttle fully open during test?

[olidaviesuk]

yup fully wide open, compression builds very quickly as well. only takes 1-1.5 cranks to get to max..

starting to suspect the way i've done the valve timing is wrong though (again) :( can't really get my head round pushrod engines

have used the valve lift at TDC method (with dti gauge) to set it, however, I measured the valve lift from the top of the pushrod/tappet... Whereas thinking about it now, the rocker arm acts as a lever as one side is longer than the other, hence, the valves are probably opening/closing before or after they need to.. which would really muck the compressions up..

should I be measuring the valve lift from the top of the valve cap like you do with ohc engines....?

[Ashy]

you're right to use the pushrod to measure the lift... Can't remember what I used to get on my gtt, think it was about 120psi on each pot? 105 might not be too far off pluss its a new build!!

Why not put a drop of oil dow each plug hole and re-test, that would give you an idea as to whether the rings are still bedding in (which they probably are)

[Andrew Cooke]

105 is very low, which would suggest retarded timing. You should set it using the valve cap, if you're timing it using an inlet pushrod you would be advanced. Something doesn't quite add up.

When you set the timing it's worth checking an inlet and exhaust to double check

[olidaviesuk]

Andrew, you've hit it spot on, just spoken to cat... lift at tdc is on the valve cap as you said, but also haven't taken the clearance into account, which is still .35 with the tappets adjusted up (according to cat) so I need a figure of 0.90mm at TDC at the valve cap.

Just measured it and its way, way out.. Timing cover off again and hopefully should be purring like a kitten shortly...

Still not sure about the compressions though, as you said about the retarded timing..., I'll get the valve timing correct then remeasure then go from there...

[olidaviesuk]

just finished rejigging the cam timing, 140psi across all four now on a cold engine...

hope the silicon on the cam cover goes off okay in the snow... brrrrrrr :coffee: