Loud knock before redline...

[Mark B]

I used to think that my car pulled all the way to redline with out any problems. If I bring the tach up to 5000rpm, I have no problems. A couple times a went a hair over that and got a loud knocking sound from the engine. However, after getting an ALDL cable and checking the rpm right from the engines computer, I found out that when my tach shows 5000rpm, the engine is actually at 4200-4300 rpm. That means that the knock that I heard was actually below the 5000rpm redline! The car has 142000 miles on it so I am guessing that my valvesprings are weak and they need to be replaced. Would everyone aggree with me?

Now if thats the case... And my valve springs need to be replaced... What does everyone think about upgrading the cam on a 142000 mile engine? I will be dropping in a new motor sometime in 2006 anyway, but I want the old 305 to run till then. I have my eyes on a used LT1 cam to replace the peanut cam that I have.

Thanks
Mark Balistreri

[ryan34]

I can't really help you, but when my mom took her camaro through state inspection, their machine read 2200 rpm. Her tach read 7000 . That was well passed the redline. Maybe there's a problem with the 3rd gen f-bodys' tach.

[Injuneer]

Does it just make noise, or does it feel like it loses power as well? Weak valve springs wouldn't cause "knock" (detonation). But if they were floating at such a low RPM, you would notice a power loss, and it just wouldn't rev any farther.

What exactly does the "knock" sound like?

[Trash man]

if u gonna due the LT1 cam, u better learn to do prom buring frist. otherwise its gonna be worse. i would do the L98 cam or LB9 cam if u dont wanna do the prom burning

[Mark B]

Third gen tachs are notorious for being bad. This is actually my 2nd tach. The first one was reading just like your moms. After 2-3000 rpm... The tach would be pinned past 8000rpm!!!
The sound that I hear is just a really loud knock/bang sound. I dont really get any loss of power. It only happens when the engine is at high RPM.
As far as the prom burning, I have that covered. I am no pro... but I have the gist of it. I will probably get a chip from TBIchips.com just for a baseline. And then I will work from that.

[Disifer]

Does it make the noise when you let off of the accelerator or while you're still on the gas? It could be backfiring a little bit. Mine will pop some when I get on it a little bit and teh engine's not all of the way up to operating temperature. I do have a gutted cat so that might explain my little bit of extra noise.

[Mark B]

While im on the gas.

What does detonation sound like?

[ryan34]

What about a bad motor or tranny mount?

[Injuneer]

The classic reference for knock/detontation is "like ball bearings rattling in a can".

[Mark B]

Its definitly doesnt sound like ball bearings in a can & I dont think it could be caused by a bad engine/trans mount because it doesnt happen when I apply/remove power from the car.

Now that I think about it Injuneer, sometimes (usually not too long after startup) if I get on the gas, the car wont want to rev past 3-4000 RPM. I thought it was a fuel problem, but mabey it has to do with my springs being tired.

Is it possible that the valves arent closing intime for the piston to come back up? I would imaging if they were hitting hard I would really know about it, but what if the valve just didnt close all the way and when the piston comes up, it hits the valve a little bit? Is this possible? If so, what is it called?

I am thinking of putting the new valve springs on the car and seeing if this solves the problem. But the springs I want to put on aren't matched to my peanut cam, they are matched to the LT1 cam. Will it hurt anything to put the heavy duty springs on with the stock baby cam?

What else could this loud knock sound be?

[Kevin - Blown 95 TA]

What spark plugs are in there?

[Mark B]

They are AC Delco Rapid Fires. #2's I think. They probably have 10K on them

[Kevin - Blown 95 TA]

They are AC Delco Rapid Fires. #2's I think. They probably have 10K on them

Are you sure you shouldn't have the RF #5 instead? Found this chart on the web...

Model Year Engine Type Rapidfire Plug Number

1969-1970 All small-block RF#7
1970 All big-block RF#10
1971-1985 All RF#2
1986 All Iron Head RF#2
1986-1991 Aluminum Head (VIN 8) RF#5
1990-1995 LT5 (VINJ) RF#92
1992-1996 Gen II small-block LT1/LT4 (VIN P) Not Available !
1997-1999 Gen III LS1 Not Available !

General notes:
Whenever the RF#2 is specified, if a slightly colder plug is desired for racing or very aggressive street driving, the RF#1 can be substituted. Whenever the RF#5 is specified, if a slightly colder plug is desired for racing or very aggressive street driving, the RF#9 can be substituted.