How much does camshaft overlap affect cranking compression?

[arknotts]

I just finished rebuilding the 7M. It runs pretty good, and after 50 miles I did a compression test to make sure everything was actually good . I did it on a fully warmed up engine, and all of the cylinders were within 5 psi of each other although the readings were a little lower than I had hoped (average of 125 psi). This tells me that things are sealing well, but either:

1) I used too thick of a headgasket (2.0 mm). Yes, I did do all of the measurements and calculations but I could have made a mistake somewhere. I was actually expecting it to be a little higher than stock compression because I have the Titan Street pistons and they had to take quite a bit off the head and block to clean them up.
2) My cam timing is affecting the cranking compression.

Concerning #2, I had to set my exhaust at +3 and intake at -2 just to get it to idle decent on the stock ECU. I only get about 14-15 in/hg of vacuum at idle so I'm thinking they are still overlapping a bit. My question is: would this cause cranking compression to read as low as it is?

 

[Dan_Gyoba]

Unless your intake cam is late, so that it is still open at the start of the compression stroke, your cams should have zero effect on compression.

Cam overlap is all at the exhaust/intake strokes. Compression testing is all the compression stroke. The cams should NEVER be open during the compression stroke regardless of what cam timing you choose to use.

 

[arknotts]

Unless your intake cam is late, so that it is still open at the start of the compression stroke, your cams should have zero effect on compression.

Cam overlap is all at the exhaust/intake strokes. Compression testing is all the compression stroke. The cams should NEVER be open during the compression stroke regardless of what cam timing you choose to use.

Makes sense in theory, but I've read things to the contrary. See the last four paragraphs of this article:

http://www.badasscars.com/index.cfm/page/ptype=product/product_id=68/prd68.htm

Since my intake cam is set at -2, it opens a little later, and hence closes a little later too. Would this be enough for it to be open for a split second during the compression stroke?

EDIT: I see what you're saying about literal overlap not causing this. I agree with you there, and I probably shouldn't have called it overlap. So I guess intake timing is really the only thing in discussion here.

 

[Dan_Gyoba]

You still should never have your intake cam open during any part of the compression stroke, as this introduces the possibility for LOTS of problems. Even if it is for a split second, it would be right at the edge of the cam lobe, so it's not even full flow.

If your cam timing is off enough to affect compression testing numbers, I would expect that the car would not run at all, or would be spectacularly crappy.

It's far more likely that your throttle body isn't all the way open, or is just being restrictive somehow, or some other environmental factor.

 

[arknotts]

According to the BC cam card, the intake valve closes (@ .050" lift) 40 degrees ABDC. I just verified this on the car itself with a dial gauge. Are you sure this wouldn't affect compression? I guess they best way to make sure is to advance it more and redo the compression test. I just want to be sure before I think about swapping head gaskets.

 

[Nick M]

You still should never have your intake cam open during any part of the compression stroke, as this introduces the possibility for LOTS of problems.

I know this is nitpicky, but a custom grind from Comp or Crane might if it is a blower cam. The postive pressure will still go in. But it likely won't be open for very long.

 

[supra13's]

I had this problem after installing new cams and had a couple valve shimmed to tight. But if your readings are within 5psi of eachother that probably isn't what's goin on. Just a thought though.

 

[arknotts]

I had this problem after installing new cams and had a couple valve shimmed to tight. But if your readings are within 5psi of eachother that probably isn't what's goin on. Just a thought though.

Yeah all of my valves are to stock spec bordering on the loose side.