Here's one for you SBC wizards.....

Fresh 350 .030 over stock 2 bolt main, crank, rods with speed pro hypertectuectic pictons 4 valve relief, 906 heads, edelbrock perfromer intake, headers, k1103 summit cam....

Noticed this noise the other night after a few 5000 rpm runs....
I slight but distinctive knock between 2500 and 3500 rpms both on accell and decell. This does not happen when its cold....
I think this is classic rod knock however there has been no change in oil pressure, 25 pounds at 650 rpm hot. No knock at idle. only at the rpm described. Timing is all in at 32 deg at 2500 rpm? running 94 sunoco.

I'm going to diagnose this over the next few days removing plug wire to cyl ect.... Also checking PS pump and water pump for noises...we'll see.
What do you guys think?

 

It's a mechanical knock correct? Not pinging? Does the issue go away above 3500rpm?

 

It does sound kinda like pre detonation and I may change my springs in the mechanical advance of the dist, but it does sound like a knock/ slap at the same time. I do have an agressive timing curve being its all in at 32 deg. Also it only does it once the thermostat opens, and engine temps are aroound 180 deg. I havent tried it up around 4000 to 5000 rpm to see if it does it, I will tonight.
If it is a rod knock it should do it at idle and at cold engine temps........ right?

 

Piston slap will be with a cold motor until things are really worn out. You'd probably be seeing burned oil is this were the case.

If it was a rod knock, it might be loud when first started because of lack of oil on the bearing but would probably turn quiet after a few seconds as the oil pressure rises. Then the rod knock would get more noticeable as the engine heats to due to the oil thinning out.

32 degrees total is fine, and you want to be all in around 2800-3000rpm, I'd change out the springs in the dizzy.

Also, do you have EGR still? Partial throttle pinging is common for EGR failure.

 

OK here's the latest and it kinda puts my mind at ease....I got the car to operating temp, and sure enough I get the knock rattle at 2200 to 2500 rpms not above or below.
I disconnected the vacuum advance ( crane cams Adj) and with the car in park in the driveway and there was NO KNOCK at all. My adjustable vacuum can was turned out 9 turns. I backed it out 2 turns at a time, and around 14 turns out the knock/rattle has quieted down, and moved to the lower RPM range approx. 1500 to 1800 range but very slight.
I backed the can all the way out and it is still a very small rattle/knock around there. Do you think going to heavier springs on the dizzy will help? I'm at 32 degrees all in by 2500 rpms ( Vortec heads) I can try I guess.

I have also been playing with my Rochester Q jet. I built one based off Cliff Ruggles book, and I've been dialling it in. I have a AEM Uego air fuel monitor which helps. Will a lean condition put the engine in a pre-detonation state like I'm experiencing? I never hear this engine knock noise with the other Rochester I had on the car.

The EGR is long gone......

Let me know what ya think.

 

Change the springs, you want to be all in at 2800-3000rpm. Check your initial timing again. And of course a lean condition can cause detonation, especially on a hot motor with timing all in too early. :P Sounds like we know your problem.

 

I changed the springs last night, I now have two blue springs, according to Crane it should be full advance at 3200 rpm, I also moved the lock out tab on the vacuum can from full lock out back two positions. Retimed the motor base timing with vacuum can disconnected ( no ESC non computer car Canadian ) My base timing is at 10 degrees, 22 mechanical advance all in at 3000 rpm, no knock. Connect the vacuum can adjust to 9 turns out and it seems to have fixed the problem. I was kinda worried I had a rod knock. Just goes to show you that you have to investigate the problem. I had figured my comp ratio is 9.81 to 1, with the aggressive timing curve and 94 fuel I guess it was too much for the vortec head.

Thanks for your input much appreciated !!!!

 

No problem, it's what we're here for! I'm glad to hear you got it fixed!