Here you go. I have a '69 318 with fresh from the machine shop 302 heads milled .006, .028 head gasket, flat top pistons and a comp XE284 cam. Here's the thing that has me puzzled. When the engine is cold, my cylinder pressure is 135 lbs. When I run it for a few minutes and get it up to operating temp and run a compression test, it drops to 85 psi. I'm stumped. I've changed rocker assemblies, used adjustable rockers and just put a set of anti pump up lifters in it thinking that maybe the lifters were pumping up and not allowing the valves to close. Nothing has fixed the problem. Oh, I used molly rings too. Anyone have any ideas as to what is going on here?
Low compression when hot
- Thread starter Watchthis
- Start date
-
Usually a drop in compression when hot indicates a problem with rings.
magnummopar
Well-Known Member
if you used moly rings, did you do the proper break in procedure for moly rings?
thats an awfully large cam for an open chambered teener too, thus the low cold readings. Cam is probably lowering your comp down to 8-1 at best. That is not the source of the problem but it sure aint helping.
thats an awfully large cam for an open chambered teener too, thus the low cold readings. Cam is probably lowering your comp down to 8-1 at best. That is not the source of the problem but it sure aint helping.
The camshaft has zero effect on the compression ratio. None whatsoever.
DARTH V8R
It's a TRAP!
Static, no. Dynamic, absolutely has an effect.
Well sure. When someone says "compression ratio" I think static.
DARTH V8R
It's a TRAP!
Kinda figured you knew that
To the OP: you check all the cylinders? Even steven across the board?
Kinda figured you knew that
To the OP: you check all the cylinders? Even steven across the board?
Not to mention he said the compression was probably down to as low as 8:1 so I assumed he meant static, because the DCR of that engine with that cam is probably around 7 or maybe even less.
DARTH V8R
It's a TRAP!
Yeah the OP said flat tops, but didn't say how far down the hole they sit. .040? .080? 0? He needs to give more info.
Pistons are .040 down in the hole. It's bored .040 and the 302 heads are closed chambers. The comp. calculator I used showed a comp. ratio of 9.75:1. I plan to do a leakdown test soon. It just didn't make sense that it would lose and not gain comp. when hot .
DARTH V8R
It's a TRAP!
Your static CR is actually 9.47:1. Your Dynamic CR is 7.32:1. Did you test ALL cylinders?
Pluggin all that into the United Engine calculator, I got 7.521:1 dynamic compression, which is fine for pump gas all day long.
I still think there's something up with the rings if compression goes away hot.
I still think there's something up with the rings if compression goes away hot.
DARTH V8R
It's a TRAP!
oil the cylinder and try again. it'll tell yah for sure if it's the rings
yellow rose
Overnight Sensation
magnummopar
Well-Known Member
I would think that statement was pretty clear I was talking about dynamic as I dont see how a cam would lower static.
sorry I dont have time to run calculator on every posters build, I guessed it would be 8 or less and its 7.5....I was more concerned with solving OP's issues than splitting hairs, or you chastising me because I made a comment about cam causing a lowly 135# cylinder pressure.
Was my first sentence asking about how OP broke in moly rings? I will ask again,
OP how did you break in rings?
how many miles are on build?
What kind of oil do you run?
Whats your jetting like, looking at plug?
Did it ever get hot or lean?
now does the engine run worse hot then cold ,do you have excess crankcase blow by? if not you might need a new compression gauge I've had that problem once when the hot air hit the valve core in the end of the line it wouldn't hold right ,just a thought
nm9stheham
Well-Known Member
I have the same questions on break in. Has this actually be run on the street and some mild acceleration put on it? Also, what do you know about the hone finish that was put on the cylinders?
And if the engine shows 85 psi hot, and then cools down, does it go back to 135 psi?
The other issues, since the OP appears to have eliminated the rockers and lifters :
- the head gasket is not being sealed well. What type of head fasteners were used?
- were the heads and valves reworked? Just thinking of a hanging valve.
And if the engine shows 85 psi hot, and then cools down, does it go back to 135 psi?
The other issues, since the OP appears to have eliminated the rockers and lifters :
- the head gasket is not being sealed well. What type of head fasteners were used?
- were the heads and valves reworked? Just thinking of a hanging valve.
Forgive me if you've answered this, but have you actually checked the pushrod length/rocker to valve interface/lifter preload? You say you have adjustable rockers but they may not be that helpful if the pushrods are way off.
Darthomas
Ashamed to be seen in foreign cars
Thin oil, HOT thin oil and your anti-pump up lifters are conspiring against you.
They are leaking down so fast, that you aren't getting enough oil at cranking speed to keep the lifters full enough to open the valves fully. Go-fast parts are for go racing.
Factory had their homework done to sell a dependable all-around engine.
What's wrong with pumping up lifters at high rpm, it's cheap blow up insurance, and you should probably be grabbing the next gear at that point anyhow.
How much time do you spend over 5000 rpm?
They are leaking down so fast, that you aren't getting enough oil at cranking speed to keep the lifters full enough to open the valves fully. Go-fast parts are for go racing.
Factory had their homework done to sell a dependable all-around engine.
What's wrong with pumping up lifters at high rpm, it's cheap blow up insurance, and you should probably be grabbing the next gear at that point anyhow.
How much time do you spend over 5000 rpm?
T56MaxTorq
Well-Known Member
This sounds like a test question for an aircraft powerplant test. Which would indicate that your valve lash clearance is too tight and you're hanging the valves open when hot. When cold, less thermal expansion keeps the valves fully closed. You did mention messing around with adjustable rockers...
nm9stheham
Well-Known Member
That is an interesting idea. But lifters like the Rhoads lifters advertise lowering effective duration by about 10 degrees. I am not seeing that causing such a larger drop in compression due to the small loss of lift....in fact, it ought to raise cranking compression at the low cranking speeds by making the intake closure earlier. (Which is part of the way they work when running...) And the OP says it has not changed the issue to change to anti-pump-up lifters. So I assume from that statement the problem existed with regular ones.
mguner
How many is too many?
Fresh from the machine shop indicates rings not seated yet to me...
nm9stheham
Well-Known Member
I assume that is why the OP went to the adjustables. And he mentions anti-pump-up lifters ===> hydraulic. But indeed we don't know where the preload is being set....
A related thought is that there is a lifter pumping out a bit on that cylinder due to a really weak or broken spring.
It seems like doing a static pressure test with compressed air and with the cylinder hot is the next best step for the OP, to see where the air is escaping.
-
