SBC break in question

[68isgreat]

Originally Posted by 57taskforce

You can check for glazing with a borescope. Compression alone isn’t always a good way to diagnose it. Glazing does happen but it’s not as common as the internet makes it seem in my opinion. It is true that a hone is the only real way to break it. Don’t assume the worst is my advice here. Run it hard and get a full break in of 1500-2000 miles. If it runs good and isn’t using oil run it. Dont worry about the compression at that point, unless you built it to be a high performance engine which I haven’t seen mention of here.

Re-reading all these replies, and this one stands out. Great advice here. I'm automatically assuming the worst when it's not burning oil and it runs strong enough. I'm going to re-do the valve lash adjustment later this week and then just run it a while. The factory new old stock tbi throttle body I found on ebay really smoothed it out. It does ping just a little bit under load, so I'm going to try some higher octane fuel. The only other issue I'm having is my factory VSS not putting out a strong enough signal through the DRAC module to get the digital speedo to work, so I ordered a gps sending unit for the speedo. Everything else works. Strong 80psi oil pressure. It runs cool, around 175-180 according to the temp gauge, probably due to the 4 row radiator I put in. I just want a nice cruiser that I can take on the freeway for long bird hunting trips. I even put a cat on the exhaust to quiet it down and create factory back pressure for the ecm. It sounds good. Here's a pic of the engine:

[MySons68C20]

Beautiful work!! I like that you are going to run it as for now.
Did you ever cut open the oil filter and look for metal?
I'm thinking with great oil pressure and a quiet engine you are good just curious.
Thanks for keeping us up to date!

[68isgreat]

Originally Posted by MySons68C20

Beautiful work!! I like that you are going to run it as for now.
Did you ever cut open the oil filter and look for metal?
I'm thinking with great oil pressure and a quiet engine you are good just curious.
Thanks for keeping us up to date!

No but I did pull the pan to change the break in oil once a few weeks back due to a leaking pan gasket, and there was a little bit of metal sludge at the drain plug, but nothing you wouldn't expect from a new engine starting up. I'm going to change it again this week after running the seafoam, which dilutes the oil a bit, and I'll cut the filter open. How do you cut it? Hacksaw? Cut off wheel? Sounds messy. Good idea though. I'm also going to test the knock sensor and egr to see why it's pinging. If the egr orifice washer is too small or if it's not opening that would cause it. I had the opposite problem initially, and the wrong egr was choking it with carbon at idle after warm up and making it stall. Can't seem to find a happy medium. Man, this is really a learning experience.

[Accelo]

Compression Dropped from 190 to 120 PSI — What’s Going On?

A 70 PSI drop across all cylinders in just 10 hours of run time is significant. The fact that it's uniform across all cylinders points more toward a camshaft or valvetrain change — not ring sealing or mechanical damage, which would usually affect cylinders unevenly.

✅ 1. Camshaft Duration Could Be the Culprit

Yes, increased duration and overlap absolutely drops cranking compression. Here's how:

Stock camshaft: Shorter duration, earlier intake valve closure = higher cylinder pressure.

Performance camshaft: Longer duration, later intake valve closing (especially with more overlap) = less time for pressure to build before the piston reaches TDC on compression stroke.

If the new cam has significantly more duration or overlap than stock, 120 PSI cranking compression is expected and not necessarily a problem — especially if it's still running well and making power at higher RPMs.

2. Did You Use the Same Heads? — If the heads were swapped to ones with larger chambers, that would also lower compression. But again, that would be a known part of the rebuild. If chamber volume increased even slightly, it would show in the numbers.

🔧 Leak Down Testing — Yes, the Engine Will Try to Rotate

Even a couple of degrees off TDC and the compressed air will try to force the piston down, rotating the engine. Here’s how to handle that:

🛠️ Tips to Keep Engine from Rotating During Leak Down Test:

Use a breaker bar on the crank bolt with a long handle to hold the engine in place. You can even wedge the bar against the frame.

Put the car in gear (manual transmission) with the parking brake on — this can help resist rotation.

Use a piston stop tool (or fab one) if you really need to hold it dead at TDC.

Lower air pressure initially — start the test at 30-40 PSI and increase gradually once you’ve verified the piston is perfectly at TDC.

Aftermarket pistons typically have a lowered compression height to compensate for people that have blocks that have been decked. This is typically .020 or more. So even with a bore increase, that mathematically would raise compression this lower compression height makes a bigger difference and actually lowers compression. Head gasket selection can also drop compression by .1 point. It all adds up in the end.

[68isgreat]

Wow. So many factors to consider! Thanks for all the info. After talking with the builder today, I got home and readjusted the valves another half turn. What a difference! No noise, runs so much smoother and stronger and doesn't stall when put in gear now, and no pinging either. Amazing. Even sounds stronger.
I took it for a test drive and I was chirping the tires (32" BFG all terrains), so it's producing some power now. Im just going to run it for a while and see if compression improves. If not, then I'll go to plan B and take it to a pro for testing.

Thanks everyone, I really appreciate the advice!

[68isgreat]

Update: took it on the freeway yesterday after changing the oil (which was black from all the carbon the Seafoam treatment removed), and it ran great, and I opened it up to about 60-70mph in the left lane, and all was good for a mile or two then it started pinging really bad and losing power. Got home and checked the EGR valve and it doesn't hold vacuum at all, so it must not be opening and allowing exhaust gas in to stop the pinging. Engine has 20in vacuum at the vacuum lines from the throttle body, so that's good. No vacuum from the egr solenoid at idle, which is correct, so that's good. Tested my knock sensor with my multimeter set to AC and it wasn't responsive. So, I have to replace the EGR and knock sensor. Good grief. There goes another hundred bucks. Sigh. Hopefully that fixes the drivability issues. Fingers crossed.

[70STOVEBOLT]

Please keep us updated.

[68isgreat]

Originally Posted by 70STOVEBOLT

Please keep us updated.

Update: Turns out the knock sensor is fine. I just didn't test it correctly with my multimeter. Replaced the defective EGR with one I forgot I had on hand, and it erased the pinging completely, and it feels great under hard acceleration, which is great, but it's surging badly under light throttle. I drove it a few miles to see if the computer would figure it out, and it got better, but it's still doing it to a lesser degree. I think it's one or two things, or maybe both:

1. My engine runs too cool, around 175-180. I may have the wrong thermostat, or it's stuck open, so it never gets up to operating temp of 190+, which confuses the computer and its regulation of the EGR air/fuel mixture. I'm going to replace the thermostat with an OEM 195 degree one.

2. The EGR nipple opening that the vacuum hose attaches to is fairly large. My defective EGR I just replaced and other EGR's I've seen have a pinhole opening (such as the Dorman 911-428 or the Delphi EG10099 which are commonly used for this motor), and the large opening on my current one (not sure what part number it is) is likely letting too much air in, acting as a defacto vacuum leak that the computer can't control, causing the surging. I'm going to replace the EGR with a new pinhole one and test it at the store to make sure it holds vacuum correctly. I'm also going to use my smoke machine and re-test for any vacuum leaks at the EGR base gasket or wherever. Even the EGR gasket itself is problematic, as some have two tiny holes next to the main pintle orifice, and some do not. The ones that come in the box with many aftermarket EGR's do not. Apparently those small holes help the computer regulate vacuum better, so I'm using the gasket with the holes. This whole EGR thing has been really difficult to figure out, and is why I had to run it so long at idle initially after startup. It is what it is. Running it without an EGR isn't an option either, since that will make it run hot and cause pinging and possible burnt valves eventually, and throw codes. Once it's dialed in, it should run excellent.

There are some steep hills near my house, so I'm going to run it up and down those a few more times and then do another compression test and see if that has improved also. I've been doing compression tests with the throttle closed, not knowing any better, which lowers the numbers from what I've read due to lack of air in the chambers, so I'll re-test with the throttle open and see how much difference that makes. I'll do a wet test too by squirting oil in the cylinders to see if it's the cylinders or the valves causing the lower compression. I'll keep everyone posted. This is a great learning experience, I have to say. I'm no mechanic, just a guy trying to build his truck in his driveway for the first time, but I've learned a ton. I'm hoping this thread will be instructive for anyone facing similar tbi issues in the future.

[68isgreat]

Update: installed new thermostat, and engine is running at correct 195 degrees. Ran smoke test for vacuum leaks. Just a bit of smoke coming out of air cleaner gasket. Added a thicker gasket. Installed new egr with pinhole inlet and test drove, , and no more surging, but it did ping again under load uphill. Was thinking about enlarging the egr inlet with tiny drill bit to find a happy medium. But after researching some more, I learned that the egr gasket with two small holes is only for later tbi models. My 1990 burb engine uses the gasket without the holes. The holes actually cause unmetered air and act as a vacuum leak, preventing correct egr regulation on these early versions. Swapped to gasket that egr came with without the two small holes and test drove on freeway. Runs great with no pinging and no surging. Idles perfect at 700 rpm and 500 at idle in gear. Problem solved! Whew. Step by step problem solving did the trick. Will stretch it out on a longer freeway drive soon. So far so good!