I rebuilt my 1988 with Trickflow heads and total seal rings, with everythings else stay stock. Now the engine knock at near WOT. I had check it with a scan tool and it showed the computer does retard the timing but the engine still knock. The highest Octane I can get around here is 91 Octane.

Knock sensor and EST are new also.

What do I need to do to correct this problem?

Would an adjustable fuel pressure regulator help?

Thanks in Advance

 

Piston shape? compression ratio?

 

Did you get the knock sensors torqued in properly? :confused:

 

Flat piston if that's what you're asking. I don't really know the compression ratio. But I think its might be a little more than 9.5:1. I check with my brother who help me put this thing together

 

How can I torque the knock sensor in properly? The sensor can only be tighten by an open end wrench?

 

VQT's engine specs :
STOCK L98 Cam, lifters, intake
Trick Flow 64cc 87+ non LT1 aluminum heads non CNC part # TFS-30400005 (I believe.)
Total seal gapless rings, street chromemoly
Speed Pro 9.6:1 ? Flat top forged piston 0.030 over.

VQT's brother.
Nick

 

Could it be a tuning issue, due to the different heads? Compression ratio with 64cc chambers and flat top pistons should be less than 10:1, probably closer to 9.5:1, which shouldn't be causing your trouble. Just guessing.

Jason

 

Jason,
What type of tuning would you suggest? It's a L98 and I believe could be running a bit lean. No Wideband 02 to check. I have suggested to my brother putting in an adjustable fuel pressure regulator and upping 2 PSI until the knock goes away. Any help would be appreciated.

Thanks
Nick

 

Nick,

I have TFS 23d heads on my 84. CR is around 10.2:1 with the stock pistons. Was 9:1 with stock heads. What gasket did you use? I used the FelPro 1010 which has a .039 crush thickness. 9.5 sounds a bit low for aluminum heads.
What is your total advance at WOT. I have found that the TFS heads don't like more than 34d total and maybe a bit less. You may also want to check out the upgrade for ESC module. Are you sure its real knock and not false knock?
:skep:

 

Not so. Check your Chevy service manual. I believe it's 15 foot pounds torque. If you get 'em too tight, ( or too loose) they won't work properly.

Also, DO NOT use threading tape or thread sealer on the sensor(s) when you install. Doing so can also affect your sensors ability to hear. The orange coating that already on the sensor is all you need. :thumbs:

 

I'll let my brother answer the rest of the questions but I know for sure its a real knock. When I accelerate the car up to 2500-3000 rpm that's when the engine start knocking. I had a garage with a snap on scan tool attach to the ALDL with the mechanic siting in the pass. seat monitoring the scan tool and I was driving to re-create the knock. The scan tool showed that the ECM did retarted the timing but the engine still knock. He suggested I try octane boster to see if it eliminate the knock but I like to tune the engine to the point that it will take 91 octane or less.
How do I check the total advance at WOT?
The timing is at 6 degree. I did retard it to 4 degree and it still knock...

 

I saw that torque number in my service manual but could got the torque wrench in to tighten it to spec. I did use a little thread sealer to make sure that it won't leaks. From what happening, the sensor hear the knock.

 

It could be a combination of things causing the knock.

You can get some idea of compression ratio from a pressure check on a cylinder. My '88 has dished pistons with 58cc chambers and around 9.5:1 CR; the '90 & '91 L98 uses the same heads with flat-top pistons for around 10:1 (i believe) CR. I don't think CR is the problem but check cylinder compresion; FYI the '90 & '91 L98s gave around 190-200psi I think.

Install a 160deg stat & fan switch to drop engine temps.

Verify base timing is at 6 deg BTDC. You could back off a little to see if knock stops.

Install an AFPR and tune for best WOT performance of use your friends scanner to get abut 920-940mV at WOT and high RPM.

BTW, what O2 reading did the scan tool show when you were getting knock & knock retard?

 

Without the help of a scantool, it could be very difficult to diagnose. You need to see what's happening at the O2 sensor. Do you have a prom burner and tuning software? If not, try bumping FP by 1 lb at a time and see what happens. This is using a meat cleaver to cut cake method but without the right tools, its tough. If it gets better, then its a lean mixture problem. I am still wondering if the quench area is correct. You never said what head gasket you used. What plugs also? 2500-3000 should be getting close to Max VE on a stock L98.

 

Disclaimer, I'm not a professional mechanic, but I did stay at a Holiday Inn Express and I'd say if you have real knock, then its either timing related or you have a lean condition. You need to determine what fuel pressure is at @WOT; you also need to know what the O2 sensor is reading when knock occurs. If you have access to a scanner also look at interagtor and BLM those two reading can help determine if the computer is seeing a lean condition or rich condition. A flase rich reading will result in the computer leaning out the mixture.

 

Base timing is at 6 and I did tried to retarded it to 4 but the engine still knock.

What's an AFPR? I'll try to use the scan tool again when I can get to his garage this weekend since it is in Los Angeles and I work in Vegas. I did not ask for all the data from the scan tool last weekend :rolleyes:

 

Dominic Sorresso,

No I don't have the Prom burner and tuner software. My bother did suggest trying the adjustable FP to see if the knock go away. It seem to be the least expensive alternative.

 

Rich86,

What's a Holiday Inn Express? The smog guy already told me that the car runs a little lean when I change the registration to NV from PA last month. The car did pass smog. I need to use that scan tool again and record all the data down this time. :rolleyes:

 

Dominic Sorresso,
We used Fel Pro head gaskets, can't say which one. They are the ones that come with the full gasket set for 88 L98 aluminum heads. I'm pretty sure the installed CR is 9.6:1 It was a little more then the factory 9.5:1 but definately less then 10:1.
How do you set the timing for 34 BTDC at WOT ? Do you need a new prom?
It looks like bumping the fuel regulator PSI might solve the problem? Even if it's a meat cleaver to cut a cake, I'm sure my brother would see it as a good fix. We are not out for maximum performance, just daily driveable reliability.
Unfortunately I'm in Virginia and he's in Vegas so I can't give you any more info about the car.

Rick86,
I take it mechanics stay at Holiday Inn Express?

65Z01,
Will putting in a lower thermostat and fan switch allow the engine to run cooler? The car is in Vegas which I believe has an average temp of 90-100degrees. Any other things need to be done ? Larger radiator? More/Change cooling fans?

Viet,
Looks like you definately need to do the test again and save all the data.

 

Nick,

Chevy Hi-Performance had a good article on this very subject in the Feb.2001 edition. Something you can do without scantool is check compression. It should be at 180-195lbs in all cylinders. If its higher than that, it could explain detonation.
"We used Fel Pro head gaskets, can't say which one. They are the ones that come with the full gasket set for 88 L98 aluminum heads. I'm pretty sure the installed CR is 9.6:1 It was a little more then the factory 9.5:1 but definately less then 10:1."

If stock CR was 9:1, then CR with aluminum heads needs to be a point higher in order to make up for lower thermal efficiency. I would use te gaskets recommended by TFS. Its too much work to put heads on and I wouldn't leave anything to chance with mismatch of gaskets.


"How do you set the timing for 34 BTDC at WOT ? Do you need a new prom?"

Yes. You can check using timing light and degreed balancer if you don't have scantoool. Lowering initial is one way to effect change.


"It looks like bumping the fuel regulator PSI might solve the problem?"

Best fix right now if lack of fuel is the real issue.

You may want to re-read this regarding head gaskets. The FelPro 1010 doesn't indent aluminum heads. http://www.trickflow.com/product/prh...ure_index2.htm


[Modified by Dominic Sorresso, 4:24 PM 6/18/2003]