I've had my 385 (Scat cast crank, 6" forged rods, forged Probe pistons, stock oil pan) installed for a couple of weeks now and we've been tuning it and trying to shake out the last of the bugs. It seems to run fine...just changed the crappy break-in oil over to Mobil 1 15-50 after it passed 500 miles. Didn't see anything unusual in the oil...no shiny stuff at all, really. The engine holds 60 PSI of oil pressure at cold idle, 35-40 PSI at hot idle.

The problem I'm having is that there is a strange "knocking" that I can hear inside the car once the engine warms up. We've been over it a couple times trying to find where the noise is coming from...my buddy put a screwdriver to his ear and went over the entire block and oil pan with the engine running. He can't figure out where it's coming from. Also, it's not loud enough to hear when you're standing over the engine...I can only hear it when I'm inside the car with the windows up at idle. It's a really soft "knock knock knock knock" noise that goes away once I start accelerating.

Any ideas on what this could be? If the knocking isn't coming from the bottom end, what else could be causing it? Could it just be the forged pistons? I ran hypereutectics with my last setup...never had an engine with forged pistons before, so I'm not sure of what kind of noises I'm supposed to be hearing.

 

No ideas at all? What other info can I give to make it easier to diagnose?

 

Does it change with rpm?

Does it go away at higher rpm?

Are you using new lifters?

Rod bolt hitting the oil pan?

 

It either goes away or becomes inaudible at any RPM above idle (idle is typically 700-800 RPM).

Using new Comp R lifters...less than 1000 miles on them. It doesn't sound like a lifter though.

Rod bolt hitting the oil pan *could* be a possibility, but we couldn't hear or feel anything hitting when we were under the car while it was running. The knock is very quiet and it's only audible from inside the car. I'm using a stock oil pan, and the rods are Eagle I-Beams with ARP rod bolts.

 

Are you sure you dont have an exhuast leak coming from the header or the y-pipe. I know my car has a small leak at the y-pipe and at real low RPM it sounds kinda like a tapping or knocking sound. This is prolly not it but it doesnt seem like your getting too many new ideas so I thought I'd give ya one.

 

That could also be a possibility...I didn't have any "donut" gaskets for the header/y-pipe connection, so we used hi-temp RTV instead. I'll get a spray bottle and some soapy water to see if it's leaking there.

 

I wouldnt think that RTV would be able to hold the pressure of the exhuast so i would think there is some type of exhuast leak. My friend just put headers on his car and it has a leak and it sounds like a tapping or knocking also so i would look into it.

 

It could very well be piston slap. How much piston to wall clearance are you running? Have you heard new Chevy trucks with V-8's lately? Man, they are really bad for it.

 

Update on the situation:

The engine makes a "scrubbing" or "squeaking" sound when you stand over it. It almost sounds like a dying tensioner pulley, but it's not...the noise is still there when the serpentine belt is pulled. It's not coming from the valvetrain...it sounds like it's internal. The scrubbing noise doesn't sound like scrubbing from the inside of the car...it sounds more like a knock when you're inside with the windows up. Oil pressure when I first put the 15-50 in was 60 PSI cold idle, 45 PSI hot idle...it's now 40 PSI cold idle, 25-30 PSI hot idle. When cold, it will put out about 60 PSI while driving around normally. It refuses to go over 40-45 PSI once warmed up (haven't taken it further than 4500-5000 RPM though, so it might go a bit higher at WOT). We still can't hear anything from underneath...nothing seems to be hitting the pan, no exhaust leaks, etc.

Any ideas? This car is going to get pushed in the river if it doesn't start behaving.

 

There's a dry river in my backyard I would gladly let you push it in

Good luck, I guarantee it will end up being something stupid that is rubbing at certain frequencies.

 

Could it be that your water pump is doing this? Mine was on its way out and making noise because it was loosing ball bearings. It wasn't leaking out of the weep hole though.

 

I have a CSI electric pump in there...it's not making any strange noises at all.

 

I wonder if since you hear a squeak outside and a knock inside if maybe you have headers or exhaust plumbing rubbing the underbody. Maybe something funny with the engine mounts?

 

Engine mounts are new Prothane mounts, tranny mount is a new Prothane mount. The headers and exhaust aren't touching anything.

After looking over some old posts and doing some digging on the internet, I'm wondering if I could have screwed up a cam bearing (or multiple bearings). Symptoms I'm having:

1. Reduced oil pressure...not as high as it was originally. As stated before, the pressure refuses to go above 40-45 PSI once the engine gets warm.

2. "Squeaking" noise coming from inside the engine...someone mentioned this on another forum. I would think that a spun cam bearing would squeak or squeal for a short period of time and then quit though...not continue indefinitely like mine has.

3. Vibrations down low in the RPM range...typically 1500-2500, worst around 1800-2100. Someone else had mentioned this in LT1 Tech. I plan on unbolting the torque converter from the flexplate (hopefully tonight) to see if I can isolate the vibration to the motor. I originally thought the vibration was coming from the converter, but maybe not.

If I'm right (and hopefully I'm not)...could the knocking be from the cam banging around due to worn bearings, or would a knocking sound only come from a worn rod bearing?

 

Cam bearing could be the culprit. I can tell you this, with the water pump drive and everything up front, that front cam bearing gets worn clear through the babbit and into the copper/brass whatever it is really quick. Mine was bad enough that I changed the front bearing when I did the cam swap. Increased valvespring pressure also puts additional load on them.