SBC random high RPM knock

[Banjobend]

Hey guys, I have a c20 with a 350 that is mostly stock. 4brl carb, performer intake, headers, and dual exhaust. Been tuning the trans today and I ran it faster than I probably should have. No tach so I'm not sure of the rpms. But a couple times before the 2-3 shift, I heard a couple loud knocks. Sounded like a rod. When it happened I noticed a puff of blue smoke out of the exhaust. No noise at idle or when I'm just cruising.

Wondering if I'm about to lose one out the side of the block?

[VileZambonie]

Tuning the trans meaning what? What is the transmission and what did you do?

[JohnnyPopper]

If the noise is not present during idle or cruising, it's not likely a rod.

Could have been a lifter reaction.

Somewhat of a Mystery about the blue smoke though.

Theorizing that if a intake valve/lifter stuck closed, excess vacuum could draw up oil past the rings into the combustion chamber...and then BLU-POOF when it became unstuck.

[Banjobend]

Tuning the trans meaning what? What is the transmission and what did you do?

Th400. Put in a shift kit. First time I've reved it that high.

[Banjobend]

Could have been a lifter reaction.

This is possible. When I did the intake, the valley was full of carbon buildup so maybe something made it's way into a lifter?

I did notice the pvc hose has oil in it. Maybe an indication of something. Not sure how that could cause a knock, but might have something to do with the smoke cloud.

[JohnnyPopper]

First, the noise of sticky lifters can be loud enough to be confused with a rod knock. I have been tricked...

Second, if your rod bearing clearance is so loose to cause a knock, it will not stop until remedied. Well, you can run 75-140w gear oil as a substitute for regular oil I guess, it will be quieter.

In your case, the noise goes away.

So picture this scenario: one of your intake valve lifters gets gunked up with carbon to the point where the internal plunger sticks into a collapsed position.

It greatly increased the gap between the camshaft lobe, and the two begin to knock.

The camshaft lobe is smacking the lifter, which is now unable to open the intake valve completely, choking off supply to the piston, which is trying to pull air/fuel into the cylinder.

Being starved of air/fuel from a restricted valve opening, what would be normal vacuum drawn into the combustion chamber rises higher than normal.

The only source available to relieve the higher vacuum is from below, the crank case.

In the process, air and oil are being  drawn up into the combustion chamber by the higher vacuum.

When the lifter regains its proper function of supplying air/fuel, the oil in the combustion chamber burns off in a puff of BLU SMOKE.

That's all I got Spanky!