Did a compression check on a 440 I just bought. It supposedly has 10.1 compression pistons 906 heads that have been milled 30 and the block has been zero decked. The cylinders are showing about 250 psi average.Not complaining but seems high to me. I think it has higher compression than 10.1. What do you all think? Thanks.

Your cam might have (in fact if the 250 number is real it DOES HAVE) an early intake closing event.

What cam is in it?

Lots of variables here. 250 is pretty high

mopar 484 purple shaft

pull a head off and measure. while youre in there check the degree of the cam.

agreed, that is very high. It'd be nice if you had a borescope to see whats in there. My HF unit was not cheap but it has came in very handy/saved my bacon several times.

Was the motor run just prior to the compression test?

No I havnt ran it yet.

check the gauge, or try another gauge.

I checked the gauge and it is ok.

Have you cranked it over with plugs in it? I'm thinking it might not even crank over very well without a great starter and battery.

If it has been sitting for a while, it's very possible the lifters are no longer pumped up, which makes the duration at the valve much shorter....... And the compression test will read high.

If the 250psi had been observed just after the motor was run and warmed up, and you knew the gauge was accurate....... Then I'd say the cam was like a tooth off(advanced), or the motor is way over 10:1cr(as in waaaaaaaaaaay over).

Yeah, I agree. I've seen them be in error before.
Has anyone ever seen an engine with these cylinder numbers run on even 93 octane unleaded? It seems to me an engine with 250psi would need over 100 octane to avoid knocking like 20 bill collectors at the door.

agreed, sumpin ain't right (& if it is, it is race gas city). Keep after it & keep us posted.

That sounds like almost 100 PSI too much for that CR and cam, if you told me 150-175 I'd say it sounds right. That sounds like a 13.5:1 engine

I don't know what kinds of parts the op has laying around, but if the motor has an adjustable valvetrain, or the op has some rockers kicking around, and some solid lifters..... You could pull two hyd lifters out, swap in two solids, adjust lash to about .003 lash, retest.

If it still tests stupid high(and you're absolutely sure the gauge is accurate.....then IMO, something needs to come apart to see what's what.

I agree with Fast68.

You are doing the compression check right, with all the plugs out.

You say average. What is the spread? Have you gotten the same result on more than one cylinder?

R.

I did the compression check one plug at a time on all 8 cylinders. All cylinders were in between 248 and 265. The motor did set for a while on a stand before I put it in the car.I havnt ran it yet I am waiting on HP Exhaust manifolds to be ceramicoted. fast68plymouth sounds like you are on to something. Once I run motor I will check again and post results. Thanks for everyones input.

Exhaust manifolds on an engine that cranks that high?

Well, I didn't think it would crank that high. It shouldn't . I am trying to figure out why it is. I am hoping my exhaust manifolds will be alright with the motor I thought I bought.

It will like it.

If I was bored (& had plenty of time) which is the story of my life I would get a guestimate on the CC's of 906's milled 30 & an average HG thickness & the supposed zero deck & plug it in the the KB pistons calculator site (its a fun tool) & there there should be a site or chart somewhere on the SCR that would produce those kind of numbers then see what piston dome it would have to have to achieve that. that'd keep ya busy till you confirm with another gauge and or open it up (first I'd borrow a borescope). Keep us updated. EDIT At least I would see what dome/CC's would be needed to get 10:1 which is what they came up with & they did mention zero deck so likely they did measure everything.

Just for comparison, some of the "better" FAST type builds I've been involved with pump around 240-250 and more.
Those are 14:1+ compression with very fast rate solid roller cams.

Russ, did you ever check the cranking pressure on your 505?

My friends stock eliminator motor is 205-210, and my 383 was around 200 the way it was in the car.
Lowered the cr from just under 11:1 to 9.7:1, and went from a 106lsa cam to a wide lsa cam with a later closing point and the cranking pressure dropped to 155.

I played around with the Wallace cranking compression calculator.

It seems "highly unlikely" the motor is 10:1cr with the 484 cam, cranking 240+ psi...... Or the gauge is wrong(or maybe a little of both).

A 10:1 446 with the 484 cam installed at 104 should pump 156psi at zero feet elevation.
Assuming the duration is actually "284", and a 104 c/l, the intake closing point is 66abtc.

Changing that number to "0", only moves the needle to 212psi.
Changing the cr to 11:1 with the closing point still at "0" shows 240psi.

Assuming the cam is installed correctly;
14:1cr shows 237psi
14.5:1cr shows 248psi

I also plugged in the numbers from one of the FAST motors that pumped high, and it showed 240psi.

At this point, the gauge is still suspect in my mind.

Lash @ 0.025 = 190 - 195 psi
Lash @ 0.014 = 175 - 180 psi.

10.8:1 CR.

But probably have an actual intake closing point of 78° to 80° ABDC.

I think there is a lot of merit in the bled down lifter theory. Always kinda questioned getting an accurate compression test with today's hyd. lifters in general.

Using the Wallace calculator, even assuming the valve is closed for the full 180deg of the compression stroke(which is saying the lifter bleed down makes the cam 132deg shorter), it still only showed 212psi.
Even with the lifters fully collapsed, I'm not sure the intake closing point would change by 66deg.

I'm pretty confident the lifters are at least partially collapsed, but not enough to add 100psi to the compression test.

I think there is a good chance the gauge is reading a little happy.

Just got engine running. I ran it for a few minutes then checked compression. It was at 195psi average on all cylinders.Turns out fast68plymouth was right on lifter theory. Thanks again.

So you think that everyone else was an idiot ?

No of coarse not. Why would you ask that? Everyone that replied had valid points. fast69plymouth point just happened to be correct.

I missed something, If you dont mind, what made the psi reading come down from 250 to 195? I did not grasp the lifter post.

1st page 11th post.

Got it & I learned something

I've actually been right about a few engine related issues once or twice before

Glad it seems like it's worked out for you.

We noticed (& are appreciative).

Fast68 - please check for a PM from me.