My all-knowing, all-powerful snap-on scanner informs me that I'm getting some serious knock retard here on my new engine. Basically anything over 3000 rpm, even at 1/3 throttle and I'm getting a knock and knock retard. To sum up the engine, 10.5 compression ratio with aluminum vette heads, 91 octane gas, 6* timing (ok, more like 7), and a custom burned prom from fastchip. And yes I'm using a 350 knock sensor. At one point I saw 14* knock retard!! That was about 60% throttle at 3800 rpm. I don't even want to know what it's doing at WOT. I tried swapping out the custom fastchip for the stock 350 prom but it made no difference. So what's the deal here? Is it really detonating, and if so, why? Or is it somehow a false reading? What could be the problem? The injectors were all flowmatched so I shouldn't be having a problem with one cylinder going too lean, but you never know. Is this a legitimate possibility? Ideas here, please!
One more thing, I'm pretty sure the ESC module on a 91 is attached to the wiper motor, right? I swapped that for a 350 one too.

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91 Z28
Come see The Vicious...

 

do you have a AFPR? what is the FP? it may be going lean in upper rpm ranges. sorry i'm not more help, all this puter stuff makes my head spin.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

Yup, I've got an afpr. The pressure is set at 50. The integrator was reading in the range of 95-115 when I'm getting the knock, so that would be rich.

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91 Z28
Come see The Vicious...

 

I thought of something else it does that might be related: when I put it in park and let it idle it will idle fine for a few seconds, and then rough for a few seconds, then fine, then rough again. This idle coincides perfectly with the O2 sensor's readings of rich to lean. Every time it's rich it idles fine, but whenever it's lean it idles rough. Also, one other little problem. One of the valve seals popped up off where it is supposed to be seated above the guide (or else my machinist who assembled the heads never put it on all the way) and it's sitting way up the valve stem, doing absolutely no good. Anyway, it's burning a little bit of oil, you can faintly see it coming out the exhaust. I haven't fixed it yet because I don't have an air compressor; I'm waiting until I can beg, borrow, or steal someone elses. So maybe it's sucking extra air through there or something, possibly enough to lean it out and cause a knock at higher rpm? It's a longshot but I'm really running out of ideas here.

 

Well, first off I will agree that 10.5:1 CR is not so much compression that you should be having problems with ping with 91 octane. But going to a little higher octane wouldn't hurt. You can see if a little higher octane makes any difference but it sounds like it may not.......

Is any of this knock audible? I would get another scantool on it and make certain that the knock is really happening. It's a long shot, but sometimes scanners can need software updates or something and will display knock when it isn't there.... I doubt that it is happening with a Snap On scantool, but you don't want to end up chasing your tail.

This is the type of situation where logging data is nice. You could just drive it around under the conditions that the knock is occuring and then sit down and review the data afterwards. Then you could pinpoint how much timing you are running when the knock is occuring and take a few degrees out here and there.

I see no way that the valve guide would allow enough air into the cylinder to cause problems - it will allow oil in, though. And the burning of oil usually doesn't have much of an effect on the readings of the O2 sensor, in my experience.

With the BLMs reading rich it really throws another wrench into the mess...... Logging some data and reviewing it would be really nice at this point - along with examining the code in the PROM. Gotta cut this short.....

Good Luck!
Laterzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

could also be did you advance the cam when you put it in or install it straight up? did you degree it? or read the card? often times they will advance it from the factory and can cause detonation your talking about

 

I didn't advance the cam at all. I also didn't degree it since I don't have a degree wheel.
Matt, the scanner has a movie/capture mode that I put it in so I could review all the specs after making a run. That's how I got all the details of this mess. I'll try the scanner out on the firebird and see if it shows anything similar. And here in KS they don't sell anything higher than 91 octane.

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91 Z28
Come see The Vicious...

[This message has been edited by ViciousZ (edited December 09, 2001).]

 

also if you say those are vette heads are they 350 or 305 and what year? they could be the 58cc units, givin im sure you had them decked, also the block decked, if the pistons are flat tops that will have your compression in the 11:1 area. also 91 octain just isn't enough, most everywhere else is 93-94 and thats just enought. so in a way your only running mid grade gas. also check your timing curve, if its really agressive that can cause detonation also. but agian aluminum heads will resist detonation more then iron, for the hell of it i would throw in a gallon of 103 plus octain gas in it and see if that solves the problem, if the codes go away then you know you have to much compression for the 91 octain



[This message has been edited by TempesT68 (edited December 09, 2001).]

 

The vette heads are 350 originally with a 58cc combustion chamber, but they were ported and polished to make them 60cc. Then I had them milled a little, just barely skimmed. Before I built the engine I calculated everything and the c.r. was 10.5:1. The timing curve would be a function of the chip. I'm having this problem with both the fastchip and the stock 350 PROM. And as for octane, 91 is as good as it gets around here, that's the premium gold stuff. If I can't run 91 octane with a 10.5 aluminum head, that's f-ed up.

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91 Z28
Come see The Vicious...

 

agian just for the hell of it i would run your tank allmost empty and put a gallon or 2 of some 103 plus octain in it. if that solves the problem then you will have to run a race gas/91 octain mix.

 

Or install a thicker head gasket. I'm NOT running race gas in my daily driver. I don't have the car with me to try it, it's back home.

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91 Z28
Come see The Vicious...

[This message has been edited by ViciousZ (edited December 09, 2001).]

 

Ok, nobody think too hard on this. It seems that my firechickie has the same problem. Coincidence? I think not. Guess my scanner's gone nutty, it showed the bird getting up to 7* retard, around 4000. Not quite as bad as the camaro, but hard to believe on a completely stock motor that's timed correctly.

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91 Z28
Come see The Vicious...

 

that does seem a little goofy, might want to try someone else's to see if it reads diffrent. also running 103 or higher octain does NOT affect anything in a bad way its just the octain level. mixing 2 gallons of 103 with about 10 gallons of 91 should give you around 95 octain or so, raiseing the fuel enough to stop the detonation if that is the problem (i know alot of people that run the gas mix on higher compression car) and leaded fuels are basically impossible to find and thats the only thing that way cause problems fuel wise as it clogged up the cat. but seems to me the gauge may be goofed up

[This message has been edited by TempesT68 (edited December 09, 2001).]

 

Yeah, it all sounded a bit fishy to me..... And especially since you were showing a bunch of knock retard with the stock bin.... You really shouldn't be seeing much for knock with a stock bin aside for a few high-load, low-rpm, high-ish throttle input situations. But even then, it should be very few and far between with the higher octane fuel you are running.

And as for octane, 91 should be plenty of octane with aluminum heads and 10.5:1 CR. You could likely run 87 octane through an engine with that setup and it would be OK.... It might have a few problems here and there, but nothing serious. And don't forget that our cars have this wonderful knock sensor system that protects the engine for us (ask your local Mustang owner about how much he hates not having an ignition system on his/her car to protect the engine!!).

So I would get a hold of another scantool and see what it shows. A Tech 1 or Tech 1a would be the best bet as they are specifically made for GM cars and, if the dealership is smart, it will be updated with the latest software and should display accurate readings.

Good Luck!!!!
Laterzzzzzzzzzzz

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Seems that a point was skipped so I'll hit it.

Fix that valve seal, if not to take care of your knock, just to stop from burning oil, its a SEAL!

The reason I think it may relate to the knock though is that enough oil in the cylinder will reduce your octane rating thus increasing chances of knock, especially at 10.5:1. Its worth a shot, and nobody told you to run race gas, but they did tell you to boost your octane. A simple bottle of octane boost will tell you if its octane related or not.

good luck.

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1985 Iroc-z, 14 second 305, new engine in the works.

 

I think ur the first person to bring up just getting a bottle of octane booster instead of racing gas or higher octane gas. Thats what i suggest too.


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- David
88' GTA 5.7L TPI MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT, Flowmaster 80 Series Muffler added to Unkown CatBack, Ported Intake, and an Energy Suspension Poly trans mount that is worse then the broken rubber tranny mount

http://www.geocities.com/david_angel_16

 

octain boosers really dont do a whole lot, if you read them they bairly raise the octain, like from 91 to like 91.6 running higher octain gas is the only real way