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

Holes # 1, 2, 3, 5, 6, 7, and 8 = 150lbs on the money!
Hole #4 = 180lbs...took the reading 3 times, same results.

Some of the back story:

By the numbers, the 350 engine is out of a '77 Chev 1/2 ton, currently in my '32 Ford Coupe...supposed to have been rebuilt by the PO a number of years ago with next to zero miles on it. The previous owner is known to be a bit thrifty with a dollar and short (or evasive?) on specific memory, invoices, details, specs. etc... so what is meant by "rebuilt" could be anything.

More back ground. I had engine running yesterday trying to set timing, adjust carbs (tri-power with 3 Rochesters, now have the proper base plates and rebuilds) etc.. The engine idle was erratic and showed signs of the intake leaking oil at the heads and I suspected a vaccum leak in the valley (I had already replaced the valve cover gaskets and oil pan gasket as it leaked like the Exon Valdez, so more gasket leakage was not a surprise). So the intake came off today and the intake gaskets support the vacuum leak hypothesis. I figure while I'm in there, I'll put a dial gauge to the lifters and measure the cam lobes for a possible flattened lobe and to try and figure out what cam it might have. But before backing off the rockers, I gave it a quick compression check and got the results above. I tried sticking my borescope in the spark plug holes, but it's difficult to get a real good look at what's going on within.

I have no miles on the car or engine yet...there were too many "findings" in every corner I looked. I'm now thinking the heads(s) may need to come off to see why the weird compression reading...any other thoughts or words of wisdom? I think I know the answer, but...

 

Possibly repaired on that one cylinder and it got a new(er) piston with tighter clearance on it or the rings.

 

Good theory...very possible.

 

losing a cam lobe on the exhaust....

 

Going to measure the cam lobes tomorrow...will report back. To be honest, after getting the weird compression reading I decided ti take a break for the evening.

 

Leak down test will point where to look.

 

If it was me I wouldn't worry about it. My 65 Olds 425 has one cylinder that is 30 psi lower and it will stay that way as far as I'm concerned. The car runs fine for the amount I drive it.

Pat

 

For true. If the leakdown is even, suspect the cam lobe going flat.

jack vines

 

As in just the one lobe Jim? So that would mean less over lap right?

 

I may need to look into what I can do to assemble a "backyard" leakdown tester as I don't have a leakdown tool kit/set.

 

It could be as simple as that cylinder having a lot more oil on the rings than the other cylinders. I usually set the piston in a coffee can of oil for a few minutes to have some oil on the rings and work some into the pin. It's a bit messy but has worked for me for 60 years.
I'd go along with one hole having a new piston or even a new piston in a sleeved cylinder that had higher compression. I did that on the 350 that I had in my 48 because it came with a bad cylinder wall.

 

What he said. Very common on that vintage 350. Low budget rebuild using a "good" used cam. That era Chevy engines were noted for bad cams.

 

the thrifty PO needed one piston and the one he used happened to be for "higher" compression?

Ben

 

At the Dodge dealership we had a customer with a van front motorhome. It had a bad miss. We found it had swallowed a valve and holed a piston. I suggested a used engine as we had a few. I also had a new short block. The frugal customer wanted to just fix that one cylinder. I told him I couldn't guarantee the repair because I didn't know what other damage might have been done. However, the one hole repair actually worked fine.

Fast forward about 4 or 5 years and he is in complaining that the motorhome had started using oil and we had overhauled the engine a year or so ago (hadn't seen him since the frugal repair). He didn't understand that replacing one piston and rod is not an overhaul. Might be what you are experiencing.

 

Just to shoot air in cylinder at TDC, bust out core of spark plug, braze 3" long. 1/4" pipe nipple , add male air line fitting. Or, take compression tester, unscrew Schrader, remove gauge, add male fit with adapter, pinch air line to slowly bleed in air. Here's 2 cases I ran into with cheapskate remans, both SBC , one had 1 bank at worn std. bore, other right side all .030" over! Next gem , when filled with coolant , filled up pan also, they just bored a cylinder with a crack. These were customers trying to save money, it didn't.

 

I've replaced 1 piston, they all got new rings and a hone. Also turned one rod journal.
If you pull a sbc out of a shitbox yellow chev*lle it's full of all kinds of BS, guy had no money.

 

If you have an inspection camera available, going through the spark plug holes and looking if the pistons seem to look the same could be interesting.

 

I'm with leaking it down. Lippy

 

If you lost a lobe, was there a lifter making noise on that cylinder? Excessive oil is possible, is that plug showing signs of oil?

 

There was no lifter noise, and on preliminary inspection all the rockers look to be adjusted about the same. The plugs looked pretty much the same across all 8 (black maybe fuel or oil fouled).

 

I'd bet the vacuum leak is more responsible for the rough idle, than the compression difference on #4.

 

If an exhaust lobe is wiped enough it will rap back through the carb, especially when you goose it.

 

I agree...I figured I would just check things out overall while I had things partially apart to make sure all is well...and be dammed if I didn't find something odd.

Excuse my ignorance on this...but I thought a leakdown test would point to a potential source for a loss of compression. How would/does it help when a cylinder has an unexplained higher compression? Not trying to challenge the suggestion, just hoping to learn/understand.

 

I guess looking for “a” bad hole is more common than finding “a” good hole.

 

But really, leak down is just the next step after a compression test. If you find no air leaking from the carb or tail pipe or breather (@ an exceptable amount) then you move on else where.

 

If you have an infrared heat gun . . .

After the engine has run for a couple of minutes, check the temperature of the exhaust manifold near the cylinder head at each exhaust port. If an exhaust valve is not opening properly you'll likely see a suspiciously low temperature reading at that port. On a vehicle with headers I've seen seven individual tubes with the paint burned off a few inches from the flange, but with the paint on one tube still mostly intact. That one tube was on a cylinder with a flat exhaust lobe.

 

I have an infrared heat gun...the engine has ram horn manifolds so would be difficult to get a cylinder by cylinder reading (like you would with headers). And the motor is not running at this point because the intake is off.

At this point, after confirming cam lobe lift tomorrow, I'm debating wherher I should remove the head to see what's what (my borescope view was inconclusive) or whether to just live with it.

 

100% where I would start.

 

Color me confused! How does a wiped cam lobe cause HIGHER compression. Highest compression should be with both valves closed, which means both valves are on the BACK of the lobe. Or am I missing something?

Fix the vacuum leak and drive!

Ben

 

Do a leak down test..... compression tests can be notoriously unreliable.