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Cranking compression: A vs. M (cam)

shoestring

shoestring

Well-known member
Joined
Jun 20, 2009
Location
Swampscott, 01907
So because I have never seen it and because I like to (sometimes) share, thought I'd put up the comparative cranking compressions of the stock M cam and the substitute A cam I put in my n/a '91 245:

Cyl M cam A cam

1 180 177
2 185 185
3 190 185
4 185 190

Motor had about 240k on it. Draw what conclusions you will. I'm not trying to say anything, just trying to add some measured values to the sketchy data that's sometimes posted.
 
Edit to include before & after cam timing values. Indicate whether hot/cold and throttle plate held wide open (or not), to complete your picture :-)
 
Cold engine. Throttle plate at WOT as is correct. 5 revolutions of the engine per cylinder checked. There's a cam chart off the home page. Cams installed straight up.
 
What really would matter here is the effect of the cam overlap and duration on the achieved engine vacuum at idle. This would be a better comparison, between the different cams dynamic compression at idle.
My test A cam vs M cam in my 91 245 3.1 n/a was a drop of engine vacuum of 4-5 in Hg at idle, from 18 down to 13-14 in Hg for the A cam vs M, (A cam being lower vacuum).

This test was done after a valve adjust using a 3 value average per valve, hand lapping all shims to 0.0005 in variation across all valves, set at 0.019 in cold valve lash, hushers removed.

I should also note that the A cam vacuum fluctuated as the IAC struggled to maintain smooth consistent idle at the lower vacuum, hence the 1-2 in Hg variation I saw. If you run the A cam at higher idle like 900 rpm it sees in cars they were equipped with, the idle would be smooth and steady, but in the LH 2.4/3.1 cars this is how it behaves at 770 rpm.
 
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While cranking pressure is very useful in diagnosing issues, it's not very useful for performance metrics. If we were able to cheaply chart combustion pressure with regards to crank angle, that would be one thing. But that is easily $7k for a basic setup. Cams can change static/cranking pressure a lot, but that pressure won't tell you what happens at any sort of running rpm. I've built high compression engines with stock cams that ended up with pressures 40psi over the race cam. The race cam came alive over 3k and absolutely destroyed the stock cam at higher revs.
So while important for diagnosing running issues, I think comparing cranking pressures will get you no where fast... And probably going in the wrong direction.
 
travisminchew said:
its weird everyone has these low vacumm readings with a A cam, I have an A cam an my car idles at 18-19
Click to expand...

I know right?
I remember a sold stage 0 after my +T made my engine so much more responsive while not in boost. I didn't feel a miss at all, but a new cap and rotor gained me 2 more in Hg

Even better was the chassis bracing! Can't wait to build coilovers!!
 
It wouldn't have been accurate if I didn't shim them both to the same spec first. I think I went 0.014" cold. All the above statements about performance engines I pretty much agree with. If I was trying to allude to something, it's that because these figures are so similar, you should be able to expect similar low speed performance with the A as to the M with a gain in the upper register.

I can hear your eyes rolling.
 
shoestring said:
It wouldn't have been accurate if I didn't shim them both to the same spec first. I think I went 0.014" cold. All the above statements about performance engines I pretty much agree with. If I was trying to allude to something, it's that because these figures are so similar, you should be able to expect similar low speed performance with the A as to the M with a gain in the upper register.

I can hear your eyes rolling.
Click to expand...

Cranking pressure isn't necessarily a measure of low speed performance, not enough data to compare with a simple compression test.

:roll:
 
It's an indication of the combination of compression ratio, displacement, and intake closing timing.

Increase compression ratio, cranking compression should increase.
Increase displacement, pressure increase per revolution can increase.
Close the intake valve sooner, and cranking compression should increase.

I crank it 3 jumps of the needle, i.e. 6 crankshaft revolutions.
 
Thanks for sharing! It's fun to see sometimes.
shoestring said:
It wouldn't have been accurate if I didn't shim them both to the same spec first. I think I went 0.014" cold. All the above statements about performance engines I pretty much agree with. If I was trying to allude to something, it's that because these figures are so similar, you should be able to expect similar low speed performance with the A as to the M with a gain in the upper register.
Click to expand...
I agree, it does give you SOME idea of how it will behave, and it agrees with what I know/have experienced. The A cam is the best daily driver cam after the T cam for low end power and makes power at higher rpm better than it does, obviously, too.
badvlvo said:
Cranking pressure isn't necessarily a measure of low speed performance, not enough data to compare with a simple compression test.
Click to expand...
But it does tell you that the dynamic compression at low rpm is relatively unchanged, which does help you in some ways conclude exactly what he says, decent low rpm performance. Of course it doesn't tell you what the powerband of the cam will look like at higher rpm, so what, that's not what he said it'll tell you. He's just pointing out what he saw and coming to an accurate conclusion of how that camshaft should act in the described situation.
 
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