Technical SBC Weird Compression Reading, RESOLVED!! And Updated Yet Again

I wonder, don’t really know, if it would be the same as running a smaller CC head on a stock cammed engine? Heard and read of several folks that had high cranking pressure when doing so. Again, I don’t know, but offering a thought

 

Johnny mentioned earlier, there is less overlap on that cylinder, so it will build more pressure

 

Again! My understanding is a leak down is to find LOW compression problems. How in the world will that prove or diagnose a HIGH COMPRESSION condition.

Ben

 

It may prove nothing here (or will it ?). It’s just the next step of testing without pulling things apart. Let’s say all check’s out with a leak down. Then it proves a guy is only going to look at all moving parts on #4 cly. as mentioned many times prior.

 

Where’s another question that’s been on my mind. Would a mix up of rocker ratio be that stupid of a cause?

 

it could be. I doubt the difference would be noticeable, though.

 

Or may make you look harder at the other seven. Put them up top, air them up and listen.

 

If the leakage percentage was the same for all the cylinders and didn't drop on #4, that would tend to indicate that the sealing on all cylinders was the same. I think a wet compression test would provide some valuable information. If the compression numbers on the 150# cylinders rose and the 180# cylinder didn't (or rise as much) I would think about a bad or stuck oil ring letting a little oil pass therefor giving a better seal on that cylinder.

One other thing to consider is that the #4 combustion chamber may be carboned up enough to raise the compression numbers. The OP says that it has been a few years since the engine was "overhauled". He said the plugs were black. It would have been worth the investment for the seller to replace the plugs to mask a carboned up plug. After letting the car sit it might be worth while to watch the tail pipe for blue/gray smoke on start up and during deceleration.

 

This whole time was to do all 8 and not just #4.

On the used 283 I dropped into my girl friends 56 I had 3 holes holes border line leakage but I could hear air coming from the exhaust ports. A quick smack on the valve stem made the leakage go away and well, the numbers shot up for some reason.

 

The valves are open quite a bit more than just the 180 degrees the exhaust (or intake) stroke lasts, racing cams can be open >300 degrees, making it very clear that the exhaust valves start opening long before the piston has reached bottom dead center on the combustion stroke, the intake valves begin opening long before the exhaust stroke has ended, and the intake valve closes long after the compression stroke has begun.

On a running engine, especially high rpm, the "extra" time the valves are open improve engine breathing - even if you may not get any significant flow you get extra time to open the valves higher, meaning you can have more flow once the piston is moving in the right direction.
The downside is that on low rpm you get flow in the wrong direction. The inlet valve closes long after compression has started, air is pushed back out into the intake, the exhaust valve opens early and lets cylinder pressure out even though it could have spent more time pushing the piston down.

This is why modern engines have some version of variable cam timing, it gives some adaptation to suit the rpm better.

 

Ok, I'm back in the shop...here's a couple pictures of the 8 equally sooty black, fouled looking plugs.

And, the set up I used to measure the cam lobe lift.


The results: exhaust lift range .295-.306", intake range .285-.304". I'm not sure of the normal acceptable manufacturing tolerance (I would have expected close to zero??), or the tolerance of my measuring set-up for thst matter. I may re-do the measurement exercise to make sure my results are repeatable. Here's the actual measurements I recorded.


And lastly, a photo of the bottom of #3 intake (lowest lift measured) lifter. It doesn't look remarkable to me.

 

Rockers/rocker ratio all appear to be stock 1.5:1

 

I'm just looking at the photo of the bottom of the lifter I posted above on my computer (instead of my phone) and it looks like it's concave...I think that's a function of the light, my phone/camera and quality of the photo ...in real view it looks at least flat (I would need to check more closely to see if it's convex. The markings on it (and the oil that was on it) also appeared to show the lifter was rotating.

I'm heading to the shop again shortly to re-do the cam lobe measurements to check for repeatability and determine a range of error in my measurement equipment/technique.

Bottom line, I don't think there is a smoking gun that explains the high compression #4 cylinder in the camshaft. Once I'm done with the camshaft measurement re-do, I'll re-set the valves and maybe do a wet compression test across all cylinders as my next test.

 

I don't know if it's just the picture and lighting but that lifter bottom looks concave and it looks like pitting on the lifter's sides. If the lifter bottom is concave you definitely have cam wear.

 

I just checked a few lifter bottoms against a straight edge...they are slightly convex.

 

I don't know if it's already been suggested, but if the intake gasket was leaking on the valley side is it possible it was also sucking in oil. That by itself could be cause of a higher compression reading, as in that one cylinder would possibly show the results of a wet test and the others not.

 

Considering the condition of the spark plugs, would a badly carboned-up cylinder possibly raise the cranking compression that much?

 

While I'd bet money against it, stay tuned cause it looks to be that's where this is headed.

 

Test #2 cam lobe measurement is much more consistent. I found the rockers just being pushed aside (as I had done for the first test), were causing some deflection to the pushrod and measurement set up/dial gauge. On the second test, I fully removed the rockers and got more accurate and consistent measurements of .295" +/- .003" across all lobes. I think the remaining variance can be partially explained by the quality of measurement equipment and possibly some wear and mfg. variance.

I'm going to try a wet compression test next.

Thanks all for your thoughtful input, insights and suggestions.

 

Wet compression test results....ding, ding, ding we have a winner!!

#1, 5 & 7 = 175lbs
#3 = 180lbs
#2 & 8 = 185 lbs
#4 & 6 = 190 lbs

Not perfect, but probably indicative of the "thrifty" rebuild...so, I think those that suggested oil on #4 rings from the leaking intake gasket had it figured out. Again thanks so much for all that weighed in. I'm pleased to say the heads are staying on...(for now anyway) (woohoo!!).

 

Glad you found it and helped others while doing it. We all are still learning.

 

Anothercarguy , just a suggestion to you & anyone else working on an open or partially nopen engine , masking tape over open ports , rags stuffed in open holes can save a lot of heartaches when something slips !

 

Thanks for your follow-up to provide us with a reply and insight into the mystery. All too often we never
hear back from the poster. Happy New Year and Motoring!

 

We normally do a "poor mans" leak down test by squirting some oil down the plug holes to seal the rings.
If all the cylinder compression is now higher and possibly equal, then the leakage was past the ring's (from the 1st test)
If the same cylinder is still down then it could be valve leakage.

 

Thanks for the post/answer to your problem, I learned something from it, will I remember it ? hopefully. LOL

 

As a thread update...I drove the car last summer, putting about 3500 miles on the engine. It ran fine, but used a touch of oil (couldn't see it out the exhaust). It didn't seem to have the power I expected nor did it obtain the economy I've managed from other similar sbc's. My original thinking was perhaps the smogger era heads were holding it back.

So, I planned to swap the heads for a set of camel humps to increase the compression and improve breathing. To prep for the swap, I freshened a set of heads I had with a mild port and polish, treated them to studs and guide plates, new springs and valves, guides and seals and gave them a coat of paint.

This would also give me a chance to replace the billet era valve covers with something a bit more traditional. I purchased these through the HAMB thanks to @GlassThamesDoug . I'll probably give them a polish and add some paint detail.

So, all I had to do is get at it. My cabriolet project was at a good resting point, the daily drivers all had a round of their annual maintenance, so the '32 made it's way into the shop. The engine top half came apart in a couple of hours. Once the heads were off, I could see the PO had bored the block .030" and had added flat top pistons. But, the cylinder walls had very little visible cross hatching left, and a few of the cylinders had minor vertical scuffing on the thrust side. I didn't photograph the cylinders (not sure my phone would adequately show it). I suspect the cylinders had been fuel washed very early after the rebuild. The PO couldn't get the tri-power to run properly and after much effort and frustration finally settled on blocking the outboard carbs (i should note that I've since corrected that issue, and the tri-power works great now). Also, as reported earlier, there were significant vacuum leaks from the intake valley, so to get it to run at all, the center carb was adjusted way rich to compensate. Anyway, long story short...the short block also had to come out.

On the plus side, this gave me a chance to do a better job of measuring the cam.

With a more appropriate measurement set-up, the cam measured .298" lift on both the intake and exhaust lobes (.447" at the valve with a 1.5 ratio rocker) and 222 degrees @.050" on both the intake and exhaust. It's a hydraulic version of the GM L79 327-350hp cam. And, all lobes were within .001", so no appreciable sign of cam lobe wear.

I now have another block at the machine shop. The new rotating assembly has already been balanced and is ready to go. I'll also take the opportunity to install a roller cam and lifters. I hope to pick everything up in a couple weeks to begin assembly of the fresh short block. I'll probably set the first engine aside and eventually treat it to a fresh flex hone and new rings as an economical small block.

 

Looking good....

327 350hp was hyd from factory.. called the 151 cam. The most copied OEM cam in GM history. Even J.C. Whitney offered them for $50 in the 1980s.. I degreed one of the JC cams, it was spot on.