How much psi should I see when cranking - 20v 270 kelfords.

[Grant #2]

Hi folks,

When I first got my blacktop 20v (11:1 compression ratio) I was seeing between 200-205psi on all pistons.

2 years later I thought I blew a head gasket, but the head checked out straight. I had the machine shop flycut the head just to remove any nicks or scratches from me handling it. At the same time I did bit of port work on the intake side only and removed a moderate amount of material to make the intake port smooth.

At the same time I took out maybe 1.5cc of material from the head de-shrouding and fun stuff like that.

After installing the head I have compression numbers of around 178-184psi across the board. I also have quite a bit less torque below 5000 rpm than I did before, but it pulls really strong above that. I'm supposed to go to the dyno pretty soon to get some real mapping done.

My question is though, with 270 degree cams and 180psi pf cranking compression, would you suspect I need more compression? The kelfords have always had less torque below 4500 rpm than stock and my tuner feels that 270 degrees is a lot for a street car and thinks I should have higher than 11:1 compression ratio (which is now probably less since I did some grinding in the CC. I wish I took some measurements at the time but I was in a bit of a rush to put it back together).
I'm debating if I should put in a trd metal headgasket (.8mm) before I pay for a bunch of tuning to be done.

thoughts?

[FoldKing86]

Personally the numbers look ok, if the cr was around 150ish id be questioning things.

Yes do the head gasket if you have the means and opportunity. Just watch your cam timing vs valves hitting pistons, i cant see any troubles you might have but check.

yes id agree, the more duration of the cams the higher the cr you need to aid low rpm torque. Im lead to believe upping the valve lift helps with torque aswell.

Best you could do is triple check cam timing and maybe get the dyno guy to have a fiddle with cams as to what works best.

What Kelfords timing card says and what the engine whats can and will be 2 different things sometimes, as people are finding out.

[Grant #2]

Any other opinions?

[ke_70]

whats tox getting? aren't you guys running the same/similar setups?

[Grant #2]

Not really, he has a 7ag, I have a 4ag blacktop, he's got 284 duration cams, I have 270, and I removed some material from my combustion chamber.

I'm just hoping people with engines + cams could chime in with what kind of psi they see during compression check.

[Panda BVB700]

dont mean to dig up an older post but what are the specs on your cams as this will all depend on the amount of overlap the cams have, while cranking to check the comp in each cylinder if the cams have a large over-lap you will be loosing some of the comp..

by the sound of it this sounds like a pretty decent amount of comp pending on the cam specs (if they are all around the same comp)

[Grant #2]

the specs are in the first post. Also it's my understanding that overlap doesn't have anything to do with compression.

[assassin10000]

Wrong, overlap will drop compression test #'s.

Same idea behind running low overlap cams when forced inducted.


180 lbs is plenty for a compression test, your just checking to see if there is a massive loss compared to the other cylinders.

Andrew

[Grant #2]

Care to explain how? Politely please :cool:

[assassin10000]

LOL. I was thinking in terms of a running motor... that could loose part of it's intake charge out the exhaust. I'll put the crack pipe down now.


A large duration cam that stays open long enough (ends up being partly open during the compression stroke) can cause a loss of compression. Not the overlap between intake/exhaust, but staying open while the piston is moving up.

Andrew

[Grant #2]

haha I was wondering how you were going to explain that one!
another thing I realized is, if you have the intake cam too far advanced, the dynamic compression become much better because the intake valve closes earlier, but at low rpm I think a lot of exhaust wants to run out the open intake.

Anyways that's what I'm noticing recently driving around with a really advanced intake cam.