Casey_SS

Been getting audible detonation on the new 396 and having trouble finding the cause. Race gas and/or reduced timing makes it no longer audible but there's still KR in the logs under the same conditions I would normally hear it. Reduced timing to 15 degrees total across the board in Tunercats and still get max KR in Datamaster. Here's a list of what I've tried so far to no avail:
- Went down 2 heat ranges on the plugs,
- verified NO vacuum leaks
- verified all sensor readings and behavior appears normal in Datamaster
- no DTCs
- replaced MAP sensor for giggles
- cleaned MAF...twice
- swapped knock modules (I have an LT4 and a standard OBDII - I'm running an OBDI PCM converted to work with my OBDII KS)
- installed an air/water separator in the PCV line to the TB to ensure no oil is being sucked into the cylinders - hasn't caught a drop yet
- monitored temps at each header primary under load on the dyno with a pyrometer - the two rear cylinders run about 100 degrees cooler than the others, which are all consistent with each other and in the expected range. I believe the rears run cooler because they're a little richer due to airflow characteristics of the intake
- verified head gasket integrity using the SnapOn tester that detects combustion gases in the radiator
- AFR is 13:1 under load throughout the powerband (dyno wideband)

I'm stumped, the builder is stumped, the dyno tuner is stumped....looking for new ideas to consider.

Here are the motor specs for reference:
Fully forged 396
11.5:1 static compression
236/244 .580 lift hyd. cam (112 LSA)
LE ported trickflow heads
home ported intake (basic gasket match & blended runners - nothing extreme)
Racetronix 37lb injectors
58mm TB
Pacesetter longtubes

Guest47904

I doubt you have detonation since it is so devastating to an engine. More likely you have preignition. However, it needs to be known which you have. Easy to do with the right equipment.

Second, you must confirm your KS is functioning properly. Obviously not as important as finding the source of the firing problem.

One troubling issue is your AFR. You report the AFR is 13. And that the rear 2 cylinders are richer than the other 6. That would mean 6 are leaner than 13, doesn't it? Why don't you try richening the mixture a little more since a lean mixture is more prone to preignition? For that matter, what happens when the system is in open loop?

What's the water temp during all this?

What happens when you force the system into speed density mode?

Casey_SS

I can't disagree on your preignition vs. detonation comment...what equipment would you recommend to verify this?
The KS ohms with the wire removed test in the normal range....is there another way to verify proper operation?
We did play around with adding fuel on the dyno...didn't change the KR behavior. Open loop operation is the same in regards to knock. Water temps are consistently in the 180s and oil temps are 190-220 measured at the pan. Haven't done any speed density tuning yet...might play around with this when I go back to the dyno in a couple weeks.

Injuneer

As a general guideline, if you want to page someone specific, send them an e-mail or a PM. Paging in the forums is discouraged.

WS6T3RROR

What plugs are in it? What do they look like? flecked with pepper, stark white, oily? What race gas are you running, and from where? Straight race gas or some mix with pump gas?

Is it making the power you expect it to or not at all?

I also agree with speedy, keep er in the 12.5-12.8 range for a/f. Thats usually where I find max power, that i feel safe with.

Its really confusing that you backed the timing all the way down and were still unable to make it stop. Usually anything in the mid 20* range will not knock even with 12.5 compression. Make sure nothing is bumping on the floor and no a/c lines are rubbing against the hood, fuel lines etc. What I mean to say is make sure its actually detonation. You could have had a reduction in sound just because you arent making as much power with no timing and the engine isnt torquing over as hard bringing something into contact.

Casey_SS

Injuneer - apologies for the faux paux...haven't spent much time here in a while and wasn't aware that was an issue these days. Seemed to be common practice a few years ago but I can see how it may have gotten out of hand with all the traffic here. Thanks for the heads-up.

Plugs are AC Delco FR1LS. The porcelains look a little flecky and get slightly darker from front to back but are all pretty close. Each plug is white on one side of the ground strap. The race gas I used for testing was 101 unleaded from Austin Performance (local race shop). The last time I ran it I mixed 5 gallons in with about an 1/8 tank of 93.

It is making the expected power. It put down 410/390 at 6k with no drop off or breakups. The dyno graph looks stellar except for a couple of mid-range dips where KR kicked in. It should keep making power to around 6600 or so but we weren't going for a max pull and didn't take it that high. I'm expecting 430ish on a full run and it doesn't look like it'll have any problem doing that once I get this sorted out. We'll be playing with the fueling more to see where it's happiest once this issue is resolved but it doesn't seem to be the culprit at this point. Most of the KR kicks in around 3k at 80kpa or higher, at which point the O2s are generally in the low-mid 900s.

I have been considering the possibility that I'm chasing both real and false knock at the same time. Simply upping the max KR values from 5 degrees to 10 or more has taken care of the audible knock but I still get KR in the logs under the same conditions as I would hear it before. Ignoring this for the time being, I've gone on a witch hunt for header clearance problems and the like but can't find anything to fix. Both headers have roughly 3/4" of clearance to the motor mounts and more than that everywhere else. Motor mounts are poly and I've stared at the gap above the motor mounts while revving the engine hard and it just doesn't torque over much at all. I'm still looking for other things it could be though and I did just find a few bolts on the pass. side header flange that were loose by about half a turn...I'll run another log tomorrow and see if anything improves.

Thanks for all the ideas guys....please keep 'em coming. It's very helpful to bat this around and think about it from different angles.

Guest47904

Item 1)Item 2)

Item 3)For item 1, you would use an oscilloscope attached to the plug wires. As cylinder pressures rise, the plug requires more energy to spark. Since the only difference between preignition and detonation is one occurs before the spark and one after the spark. At least one can be seen on the scope. I saw a recent story from Snap-on how 2 of their gurus figured out an exhaust manifold was made wrong by looking at the spark wave forms. The cylinder pressure caused the firing wave form on the O-scope to look much different than the rest of the cylinders.

Item 2, With the KS in the block and KOEO, tap the block with something like a screw driver and look at the response given by the PCM.

With regard to item 3, I am convinced you are chasing false knock. If the same thing occurs in open loop (with regard to KR) as in closed loop, I don't think your KS circuit is working properly. What I wanted you to do was force the computer into speed density mode to see how it responded, NOT program it in speed density mode.

You should stick with the correct OBDI KS rated from 3300 to 4500 ohms. It should be verified from the KS terminal to engine block. You could also replace the KS for a short time with something like a 3700 ohm resistor to see if the computer stops pulling timing.

hosspwr94

just to play a little devils advocate here... the drivers side header will be the one to worry about with clearance issues. I had a similar problem with my new header install. I had about 3/4" to an 1" clearance at the rear of the Hooker longtube, had solid motor mounts on and then switched to poly's and started having problems. Did the same thing you did on the checking while reving but the problem there is that while only reving there is no load on the motor. Come to find out that after we put a load on the motor the header would raise up against the crossmember and low and behold knock! This is just my experience. So my suggestion would be to check your clearances again and if somehow you can check it with some load on the engine. With the way you say your dyno looked it really sounds like false knock.

Casey_SS

The header clearance under load is a likely enough theory and I'm running low on alternatives at the moment. Still doesn't account for the audible knock I guess I'll just take it one step at a time. Taking a break right now from pulling the driver side header out... I'll grind the motor mount housing down and dimple the header in that area and see if I can get another inch or so of clearance. I'll update with results soon...thanks for the suggestion and your experience with the issue.

Speedy - any particular reason you recommend going to an OBDI KS or is that just something to try? Reason I ask is that my OBDI pcm is tuned to work with this KS and I have to swap the OBDII pcm back in every year to pass emissions. Would rather not have swap sensors every year as well if it's not necessary. Everything I've read indicates that as long as the pcm is tuned for the different resistance, there's no issue. Am I misinformed?

SnakeSkinner28

Not sure if you tried this or not and no one has seemed to mention it but if you think you are getting false knock check and make sure you havent over tightened the knock sensor on the block. Happens a lot and people dont know it and on a motor like yours Im sure it would cause issues. Who knows if thats your problem but it sure would be an easy fix if it was.

Antz97ZNJ

Is it? Oboy..