Why won't it start?

[grocery getter]

that's a lot of compression loss for rings not being broke in, on a bored and properly honed engine.

think about it, they're not equal....

something else is not quite right......

Again, there's likely more than one problem here.

I disagree. On a cold engine with unseated rings, as long as you have compression I wouldn't worry. (Heck, I've seen an engine run with barely over 40 psi average.) And you don't want them all to be equal, it's an indication of a leak. Get it running, get the rings seated, then check the compression when warmed up. Anything before that isn't going to tell you very much. IMHO.

What i'm trying to get across is with these static compression numbers you'd see alot of blowby. on the 65 psi cylinder you'd be looking at 50% of the A/F going to the crankase.
I get the weak numbers on break in, but these are a little extreme.

like most of the posts so far I agree timing is an issue. i'll stand by my thoughts that 65 psi on a new bored and honed cylinder is very weak
(and there's more than one problem).

Three things We can't dispute....Air/fuel,compression and ignition

 

[wallaka]

that's a lot of compression loss for rings not being broke in, on a bored and properly honed engine.

think about it, they're not equal....

something else is not quite right......

Again, there's likely more than one problem here.

I disagree. On a cold engine with unseated rings, as long as you have compression I wouldn't worry. (Heck, I've seen an engine run with barely over 40 psi average.) And you don't want them all to be equal, it's an indication of a leak. Get it running, get the rings seated, then check the compression when warmed up. Anything before that isn't going to tell you very much. IMHO.

What i'm trying to get across is with these static compression numbers you'd see alot of blowby. on the 65 psi cylinder you'd be looking at 50% of the A/F going to the crankase.
I get the weak numbers on break in, but these are a little extreme.

like most of the posts so far I agree timing is an issue. i'll stand by my thoughts that 65 psi on a new bored and honed cylinder is very weak
(and there's more than one problem).

Three things We can't dispute....Air/fuel,compression and ignition

Yeah, might have washed the cylinders down with gas. That will cause low numbers for sure.

 

[grocery getter]

that's a lot of compression loss for rings not being broke in, on a bored and properly honed engine.

think about it, they're not equal....

something else is not quite right......

Again, there's likely more than one problem here.

I disagree. On a cold engine with unseated rings, as long as you have compression I wouldn't worry. (Heck, I've seen an engine run with barely over 40 psi average.) And you don't want them all to be equal, it's an indication of a leak. Get it running, get the rings seated, then check the compression when warmed up. Anything before that isn't going to tell you very much. IMHO.

What i'm trying to get across is with these static compression numbers you'd see alot of blowby. on the 65 psi cylinder you'd be looking at 50% of the A/F going to the crankase.
I get the weak numbers on break in, but these are a little extreme.

like most of the posts so far I agree timing is an issue. i'll stand by my thoughts that 65 psi on a new bored and honed cylinder is very weak
(and there's more than one problem).

Three things We can't dispute....Air/fuel,compression and ignition

Yeah, might have washed the cylinders down with gas. That will cause low numbers for sure.

Maybe we can get silly back to shoot some oil down the cylinders before the next test.

 

[addo]

I'd ask if all rings are on the right way up, and indexed. Also, are the pistons matched to their bores?

The compression feels too low for my liking, unless the gauge or method is suspect.

 

[mustang6]

with these static compression numbers you'd see alot of blowby

Every new engine I've ever started did have a lot of blowby for the first minute or so until the rings started seating. I remember the days before hypereutectic pistons and moly rings, when the high performance standard was chrome rings with loose tolerance forged pistons- the sure sign that the rings were not seating was poor compression and continued blowby.

So I'm kind of leaning with the crowd that thinks these numbers will double when the rings seat, but I find this very interesting because I've never really done a compression test on an engine before it fired and then again after it was broken in. This will be interesting.

 

[mustangsilly]

Thanks, Jamyers.. You're right the timing was off by 180. The engine started right up and actually ran pretty good once I put the timing light on it and advanced the spark to about 16 deg. BT. And thanks for the info on the compression. How many miles to seat the rings? Should I be concerned about the disparity among the readings high to low?

 

[grocery getter]

Good job...wallaka called it.
along with the rings not seated,those weird numbers might also be from the lifters (if they're hydraulic, not fully pumped up) if you can,get another reading. i'll be real interested in the numbers even after a short first run. also, have the throttle wide open for the test.

mustang6, yeah the moly rings can be a problem, especially if the cylinders were not honed correctly.

 

[Mustang Boy]

well the thing with rings seating is it can take anywhere from 2-3 seconds to several minutes for the rings to seat but you should be able to tell a difference once they do cause itll "perk-up" cause of the added compression

 

[jamyers]

A side note on starting a new engine: With a new cam and lifters, be sure to rev the engine up to 2000-2400 rpm for the first 20 minutes of running, so assure plenty of oil on the cam/lifters.

After that, most new rings (especially plasma-moly) should be seated. If you don't think they are, go out and do 2 or 3 full-throttle runs, they'll be seated for sure then.

To check compression, start with a warmed-up engine, pull all the sparkplugs out and hold the throttle wide open. More important than the psi being high, you're looking for 2 things:
1. Adjacent cylinders shouldn't have identical numbers (might indicate a leak between them), and
2. The highest cylinder shouldn't be more than 15% higher than the lowest cylinder.

After that, as long as your lowest wasn't below say 160 or so I don't think I'd worry unless there are other problems or the engine just doesn't live up to expectations.

 

[fordconvert]

The last motor I built with molly rings seemed to take a couple of hundred miles to really brake in and smooth out. But I agree that on initial startup while you are checking oil pressure and making sure the timing is close the engine runs like poo. Without changing anything after that 20 min cam break in it should be 90% there.

 

[mustangsilly]

I just ran the engine for about 1/2 hour. I could tell by the way it smoothed out after a few minutes that the rings were seated (or at least partially seated). It ran very strong and pretty smooth. I haven't done a static compression test since then, but I will when it cools down a bit.

I did put a vacuum gauge on the manifold and only found about 8 in. of vacuum (needle was steady). That seems pretty low to me. I checked the carb gasket and made sure the mount was tight. Hoses all new. I couldn't find any leaks. Will it get better with the break-in? I'm using a CS 210 cam but I don't think that should reduce vacuum by that much.

 

[ludwig]

The vacuum will go down, depending on your cam overlap. Stock is 18"+; one guy lately got 14" or so with a special cam.

 

[BIGREDRASA]

See what you get with the PCV valve line plugged. I think someone needs to do some serious research on what to do about PCV Valves based on other engine mods. I can't imagine that one size/rating fits all.

 

[FALCONAROUND]

You have a vacuum leak somewhere. There is not a cam made that you'd want to run in your 6 that would drop you to 8 inch lbs of vacuum at idle...

Something is amiss...

The PCV comment is a thinker for a minute, but in reality all a PCV valve does is let combustion gasses that slip past the rings and into the crank case back out into the atmosphere (back into the intake really but you get the idea) size isn't really an issue with them for the most part, but I've seen dual PCV valves on engines help things out a bit.

 

[fordconvert]

The larger concern on PCV valves for most of us is the amount of controlled vac leak they provide which like all vac leaks (which are any air that does not go down the carb throat) makes things lean. If you have a 'stock' system you want to stick to the stock valve. If you have things like a 2bbl or have ripped the emissions off it is not going to matter much because you are so far from stock you are dang lucky it runs or you have been able to tune the system to work with what ever valve or lack of valve you have.

 

[fordconvert]

Mess with the timing?