What could cause wholesale compression drop?

[wsutard]

Hey engine gurus, I recently purchased a 69 Coronet with a built 340. In diagnosing a bucking/surging issue my mechanic found that all my cylinders were low on the assumed compression. My question is, what could be causing lack of compression for every cylinder?

More detail:
340
Keith Black Pistons
H Beam Rods
Forged Steel Crank
Crowler Cam (285 dur, 480 lift, 110 deg)
Titanium Valves
Hooker Headers
Edelbrock Performance Carb and Intake (1406 Carb, 650 CFM)

Assumed compression for this engine is 10.5 to 1 (155ish PSI) based on the original 340 specs as well as the new internal components.

Right now the compression is reading between 7.5 and 8 to 1 (110-120 PSI).

Another tidbit, the vacuum is supposedly low. Not sure what this means yet as I am learning still.

What could cause this wholesale compression issue?

Here are my guesses based on a conversation with my mechanic:
1. Valve lash. We are going to pull the covers today to check the valve lash. I'm hoping that the valves are not opening all the way and thus not letting a full intake or exhaust. This would be the easiest to fix but I don't see how all 8 sets could be set incorrectly.

2. Mechanic has a theory that the Keith Black pistons are incorrectly assembled. Supposedly these pistons have a specific way in which the piston seals need to be installed and a lot of engine builders mess this up. My mechanic is not advocating looking into this yet as it would be a total tear down.

3. The parts that were advertised as in the engine, are not actually in the engine. While this could be a possibility the original 69 era engine was originally spec'ed at 10.5 to 1. So, even with stock parts the compression should still be in that range.

Any other ideas on what to check? I would like to go into my mechanic with ideas to discuss.

 

[bm02tj]

Have a leak down test to rule out rings
also cam timing can cause your trouble

 

[wsutard]

I get the leak down test. Read a few things on that.

What is the cam timing? Is that different from the overall timing?

Sorry for the newb question.

 

[bm02tj]

look at this
http://www.hotrod.com/how-to/engine/mopp-1211-degreeing-a-camshaft/

That will explain it

 

[moper]

You'd be amazed what some people can do with decent parts...
If it was me, the first thing I'd do is a leakdown test. Next - pull the intake and a head and verify what's in there. Next would be pull the timing cover to check the cam installation.
I'll add - there is NO correlation between the static compression ratio of any engine, and the accompanying cylinder pressures. So you can't tell if it's really 7.5, or 11.5:1 based only on what it pumps. You have to measure it. My impressions are the cam's not in right, and the ring seal may not be great.

 

[mmissile]

I'd yank it, and go thru the whole thing. No tellin' what's going on in there.

 

[PurpleBeeper]

Your engine might be just fine. Is it running OK? As Moper mentioned, you cannot "calculate" static compression based upon a cylinder pressure test.

Bm02tj is referring to a cams "overlap" which is the number of degrees where BOTH the intake & exhaust valves are open at the same time. A lot of overlap bleeds off some compression. Big overlap cams are good for the track, but not-so-good for your vacuum in street applications.

IF you had a carburetor that was set up really rich, you might "wash out" the rings (excess fuel washes the lubricating oil off the cylinder walls causing premature piston ring failure). You would likely be getting black smoke out of the tailpipe and/or a fuel smell at idle if this were the case.

Also, unless somebody added adjustable rocker arms (non-adjustable are stock) OR if they put in pushrods that were too long, you shouldn't be having a valve lash issue.

Lastly, you could "take a peak" at the top of the pistons to see if they are what you think they are using a boroscope ($25-$30) through the spark plug hole. I disagree with your mechanic on turning the ring gaps to different locations when installing pistons....that's pretty common knowledge.

 

[dvw]

Was it truly 10.5-1 or estimated? With stock deck height and combustion chambers with that cam 120 psi doesn't surprise me. Either it's less than 10.5, or the cam could be retarded. Loosening the valve lash would lessen duration and increase cylinder pressure, not reduce it.
Doug

 

[bm02tj]

The info I referenced was about over lap as well as advance and retard so I thought it would give him a well rounded explanation

 

[moper]

Overlap has precious little to do with pressure building in the cylinder. Even more so at starter cranking speeds. The only thing that will matter is the intake valve closing point in terms of the cam in relation to piston location. Overlap is the "time" in degrees between the intake valve opening and the exhaust valve closing. Neither of those affect the resulting pressure of the trapped volume. Where overlap is important is cylinder filling but only as it relates to cylinder emptying.

 

[wsutard]

Couple things:
Not sure why I left this out in the beginning. I felt like the engine was running great when running it spiritedly . When you mash on it it feels very healthy, it goes quick. It didn't make any noises out of the ordinary. However, what started this was a "reverse surge" that happens in all 4 gears at any low to cruising throttle position. I'm describing as "reverse surge" because you can feel the power cut and come back, causing the car to surge. In first gear, low RPM, it basically feels like the car is bucking and will do so violently if I let it. The initial thought would be that it is running lean, but when you smell the car it seems like it is running rich.

I say throttle position because the RPM doesn't really matter. I can be at 3000 in 3rd with very little throttle and feel the "surge". If I mash it from 1500 to 300 in third, the car goes and I cant feel the surge. (I'm guessing it might still be there but the vibration of the engine dampens) If I am in first at about 1500 and it starts to surge/buck you either put the clutch in, which stops the bucking or give it gas, which also stops the bucking.

I thought it was a carb or timing issue but the mechanic has looked at the timing and carb settings. He didn't see anything out of the ordinary with these.

The reason I bring up compression is because he pressure tested the two sides and found the range to be 110-120 PSI. Their assumption is that it should be 155 based on the equation atmospheric pressure x compression = PSI. (14.7 x 10.5 = 154.35)

Factory specs on a 340 from 69 state 10.5-1. no reason it should be any different now.

Reverse that equation for the known 110-120 PSA and you get 7.5-8ish compression.

I really have no reason to think that the parts stated on the purchase are not really there.

Looking at the exhaust output, there would be no concerns raised. Nothing black or blue or ever overly white. Not a lot of "smoke" to begin with.

I mentioned "lash" as I thought it was the correct term for when the rod is not tight against the rocker arms, causing the valves to not open correctly. My understanding is if the rods and arms are not properly seated then the valves will only open to something less than 100%. m

I am going in this afternoon to discuss so I will bring up:
Retesting the timing
preforming a leak down test
oscoping to take a look at the pistons

 

[Mike67]

Every cylinder was in that range??? 110-120.......

 

[wsutard]

Every cylinder was in that range??? 110-120.......

Yes

 

[IQ52]

Well then, is your mechanic going by the factory ignition timing specifications? What timing do you have at idle and what is the total timing? That cam is going to want way more advance than the factory recommended zero degrees for a manual transmission car.

 

[lewtot184]

the low speed surge is probably the "economy" calibration of the 1406 carb. there was a low compression 340 piston.

 

[AR67GTX]

I assume the compression test was done on a warm engine? It's not unusual for a cold engine with forged pistons (and accompanying larger piston-bore tolerance) to have significantly lower compression than when hot and expanded. Did you mechanic try putting some oil in a couple bores to see it the compression came up? Could be your rings never seated or you have had a carb flooding problem that washed the bores dry.

 

[wsutard]

Well he found the problem. He noticed that the distributor was wobbly and the gear had started to wear out causing issues and missed timing. Replaced the distributor and all is good.

Still doesn't explain the compression readings but he wasn't concerned about them. I drove it and wow, so happy now.

Now on to my sure grip issue.

Thanks for all the help!

 

[moper]

I was going to say - if it was a surging and all else is new/rebuilt, look at the distributor. Glad you found the issue.

Just for future needs should you have problems... Because the engine was rebuilt, and rebuilt using non-factory parts to make more than factory output - the "factory specs" mean nothing. Calculations based on atmospheric and cylinder presssure are not even close to accurate. As your results corroborate.

 

[miller]

Fully agree! Is this a fresh motor?
Still doesn't answer the compression thing. One thing to keep in mind...if all the 'bad' numbers (yeah, bad...not up to snuff) are only on one bank...common denominator there is only one thing. Head gasket. Lol, or head bolts not torqued right. Might want to at least keep an eye on it.

 

[Jcharger]

Couple things:


The reason I bring up compression is because he pressure tested the two sides and found the range to be 110-120 PSI. Their assumption is that it should be 155 based on the equation atmospheric pressure x compression = PSI. (14.7 x 10.5 = 154.35)

.....

Reverse that equation for the known 110-120 PSA and you get 7.5-8ish compression.

A quick clarification. The 14.7 x compression would work if the gases in the in cylinder did not heat up and if all of the gas came in at the bottom of the stroke and you are at sea level. A number of the posts here have referred to cam timing and intake closing. To calculate dynamic compression, which is what you are measuring, you should use something like: http://www.wallaceracing.com/dynamic-cr.php

Other things can affect the results of a compression test (like the tester itself, leaving the carb closed, engine cranking speed). I agree that 120 psi sounds a little low, but it is not crazy low.