Thread: Compression Test Woes

I know this is a few months old, but misinformation needs to be pointed out.

this is partially correct, so it is in fact, wrong as well. the most basic explanation is cam overlap. it takes into account the intake and exhaust closing points, as well as duration. the intake closing point is the only factor if and only if the exhaust stays closed during the compression stroke. the tighter the LSA, and the more duration, the longer both valves will be open at the same time, hence affecting dynamic compression and compression tests.

Explain to me in what engine does the exhaust valve stay "open" during the compression stroke?

Walker

if the cams have a large overlap they will bleed off dynamic compression hence lowering your numbers by a bit but should still be close to the FSM. this is because a valve is open a slight time during the compression stroke to aid in exhaust velecoity (I belive thats why anyway).


FWIW I had a low compression thread too with basically the same setup and mine happened to be debris undera valve seat got that taken care of and my compression numbers are 150-145-145-145 with S4 cams and the almost same setup as OP.

I said it already. the cam with more overlap. its the time when the exhaust stays open during the intake stroke ~ cams with lots of duration and tight lsa will have the intake stroke become the compression stroke, at lower rpm, because there is plenty of time to fill the cylinder.

and yes something can be wrong with your seats or guides as well, which will affect compression numbers.

Edit: Thought guides were same as rocker arm shims..

I honestly appreciate your noble attempt to avoid misinformation on this subject. However I strongly disagree with your rebuttal.



Nissanboi and LE25,

I think you are both confused as to what dynamic compression is. This article, while I have not read past the first few paragraphs has a decent definition of "dynamic" compression that will help thinks make sense: Dynamic CR

As a rule, compression can only occur when all valves are both shut. Disregard anything else for this application period.



Inertial effects of inlet air are not capable of realistically packing the cylinder at cranking rpm in any way. Overlap does not pack the cylinder it aids in exhaust gas evacuation. Cylinder packing by way of dynamic pressure / inertia occurs after TDC and after the exhaust valve is closed. Again, at cranking speeds non of this applies. Exhaust pressure is assumed ambient for our purposes as is inlet pressure.

Overlap will not change your compression reading unless it occurred past BDC, which no cam ever made has done for that would be stupid. Think about it. If you have a completely empty cylinder at 0 gauge pressure BDC. You seal it off, and compress it to your specified compression ratio, then you take your pressure. Overlap occurs well before the cylinder is at BDC.

The most likely cause of the slightly lowered numbers, is due to the time that the intake valve closes (as I already explained). For you valve train nuts, this is the most critical event in valve timing period. This is where you can effectively lower your compression by closing the valve before or after tdc, generating negative pressure and then having to make that up again on the way back up or packing the cylinder if you have a badass design. In efficient racing engines they sometimes leave the valve open past bdc, however this does not apply to our situation at all. Reasons for doing this are for another thread.

I strongly suggest all who are confused look at a basic camshaft profile diagram with camshaft lift vs crank angle. Study it and invision the piston movement. This will help you to answer this question properly. Valve timing can be tricky and it is easy to get mixed up.

If this is still not making complete sense. Tell me. A critical skill of any automotive engineer is to be able to "garbage man" engine mechanics and dynamics as my engines professor used to always tell me.

Walker

Anything I can check and/or replace on the head without pulling it which would be causing lower compression? Thinking of running a leakdown test .

Run a leakdown test and if turbo remove the exhaust manifold. I didnt do that the first time and had air escaping between cylinders and thought hg. After replacing hg o did a leakdown test with the manifolds off and found out it was 2 exhaust valve seats. My theory is that the turbo was a restriction so it was easier for the escaped air to travel up the manifold in lieu of through the exhaust. But a leakdown test is the very best thing you can do.