1991 Z28 350 tpi with 59K miles. Ive been working with a member here tuning on the car all week and TunerPro is showing upwards of 5 degrees of knock retard at WOT. Im barely pushing 30 degrees of total timing

Car has Dyno Don headers/ypipe (no cats), TPIS intake ans SLP runners. The member helping me says that it seems to be occurring at areas of peak volumetric efficiency on TPI’s (3000-4000 rpm) and at or near WOT (which is where your dynamic compression…i.e., cylinder pressures, will be the highest and most sensitive to knock). So, it means its most likely real knock.

I had the heads off last summer to have them freshened up. Valve seals were bad. Stock rockers were put back on and tightened just past them making noise. They are not noisey at all.

So, what could be causing this?

Headers creating false knock signals at WOT?
Faulty knock sensor??
Or real knock detected from something going on in this low mileage basically stock engine?

 

What octane fuel are you using? Are you running an aftermarket harmonic balancer? I had an aftermarket balancer on my car that had the timing marks in the wrong spot, so my timing was off by 8 degrees. Also if your running the original one make sure the rubber didnt slip inside the outer pulley. That will also give you a wrong reading, and Ive seen that a few times as well.

 

Your slipped Balancer was causing knock retard ?
Are you running a wb gauge ? If so are you lean during wot ?

 

This was years back before i went ls. I had the opposite problem where the car had horrible performance. The balancer i got from summit had the marks printed 8 degrees off where they were suppose to be. Eventually used a piston stop to find true tdc and discovered the problem with the balancer. If his balancer isnt accurate he could be having the opposite problem where it has more timing than he thinks.

 

Knock Sensor torqued too much on install? I did that once and my engine would find knock all the time as soon as I started to get into throttle. Didn't matter what the timing was retarded too.

 

What's your knock count?

 

Probably somebody overtightened the knock sensor at some point in its life, and now it's hypersensitive.

Replace it.

Don't forget, those extremely smart and forward-looking GM engineers knew EFI was coming and were already planning for it back in the 50s, but they wanted to be stealthy about it and not tip their hand to the Frod and Xler engineers; so they cleverly disguised the knock sensor port as a coolant drain. The entire contents of the cooling system will probably come out when you remove it except for a few drops on the driver's side of the block so be prepared for a flood.

 

Yep, you get a good bath.

 

Truth.

And the knock sensor is some ridiculously weak torque spec. You could probably achieve the same with a 1/4" drive mini-ratchet.

Its like 14ft-lbs for the Gen2 LT1's, which is basically the same as SBC.

 

i think it was 10 after I was hammering on it for about 15 minutes around town and stuff. I’ll have to go back and check the log.

 

I took the car to the dyno today and did two back to back runs. I recorded data for both.

First run:
There was 3 knock counts while the car was idling. Then it picked up 2 more at 4600 RPMS. That was also the same time the knock retard was 2.8 degrees. AFR was 12.69 at that RPM as well.

Second run:
Knock retard hit 2.8 at 3800 RPMs and knock count went up 3 more between 3500 and 4000 rpms.

 

That's messed up, with a capital F.

After you changed out the sensor, did it rejoin the real world and the known universe instead of .... that other? How much did you pick up on the dyno after you changed it out?

 

I didn’t change the knock sensor out yet.

 

Oh, so all that dyno time went to waste when you already knew it was going to be bogus??? That's a shame. Dyno time is $$$$. I always make sure that anything the least bit questionable is taken care of before laying out that much $$$$$$ on it and then take the risk of coming back with nothing but a bunch of uselessness.

Oh well, maybe the butt dyno will read higher after you fix it.

 

Quit being such an a$$hole all the time when you make comments here. Every time I see a comment from you it’s either rude or condescending!! The Dyno time was $40 dollars!! I had a great time there and don’t regret doing it.

 

Rude? Condescending? All I'm doing is stating the facts. Facts are neither of those things.

You took a broken car to the dyno, and got some #s, that are in no way representative of the car's true potential. I can't, and won't, candy-coat it. That's just not in my DNA. It is what it is.

$40 is $40. Gone without getting what it could have produced. it's always a shame to spend $$$$ and not get the benefit from it. Which in this case, sounds like it happened to you. Oh well, we live and learn, or we don't live long, eh?? Lesson learned here (we can only hope), next time you'll do better.

Change out the KS, check your data logs, see if they make more real-world sense.

 

I ruined a good thing when I changed my knock sensor out. It thought it had to be in contact with the coolant so when I removed the old one and no coolant came out I cleaned it out. Then the bath and I didn't even need to. Lol.

 

Knock counts can happen in completely stock trim for numerous reasons. The total advance can be programmed substantially higher than any engine would run, and the ESC usually tests this on startup at least once.

Jump over to the ECM Programming forum and discover why. You may be worrying about nothing.

 

Would a head gasket with a compressed thickness of .028 cause the knock?

this is a copy and paste from a member who has been helping me with datalogging :

An iron headed 350 with the dynamic compression of a TPI in that rpm range is probably going to be very sensitive to any increase in static compression ratio with today's lousy gas.


I had a similar issue when I was trying to get away with 10.1:1 with aluminum heads using those same gaskets and trying to use with the LT4 Corvette spark map... tons of knock counts (which is prompted me to ask about your head gaskets).


I ended up downsizing to the Felpro .041 gaskets and the problem immediately went away.

 

I guess the question could be rephrased in that if the gasket thickness decreases by .010, it's effectively a 1/4 point increase in static compression ratio for standard 350 bore/stroke. I.e. if stock is 9.0:1 then you get 9.25:1. So then the question for everyone here is that 1/4 point increse sufficient to start producing knocking on an engine that didn' have knocking before?

What is the factory head gasket thickness?