Hey all...

I have noticed a couple times recently that I appear to be getting a light detonation(Spark knock) when lugging the TT down in 4th or 5th gear. I am aware of what detonation sounds like in my car under high boost conditions as I have gotten it a couple times when boosting over 18.5 lbs. It is not currently detonating under high boost conditions, but again when accelerating slowly out of like 30 mph in 4th. Timing is dead on 15 DBTDC, and no codes, 93 ocatane fuel on ash's 92 octane tune. Anything to worry about?

Eric

Nothing to worry about unless it is happening excessively. It is normal to get some detonation under heavy load in high gear at low RPM. The key cause is the fact that the throttle is usually opened fully, causing the full ambient air pressure to be present in the intake tract. When the throttle is partially closed, there is a large pressure drop across the throttles and the pressure is nearer to vacuum in the plenum. With a higher pressure in the plenum, you have greater cylinder filling and since the fuel maps and spark advance are not programmed at these low RPM/high load settings for that much air, you are running lean. The worst case scenario to achieve this type of detonation is to run the car in high gear going very slow up a hill with the pedal to the metal. I would advise against running the car like this very often. Simply run the lower gears out to a higher speed and you'll be fine.

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-- I've had a few Zs --



Edited 1 time(s). Last edit was august 19, 11:48am.

I would go one step further and say that NO pre-ignition is a good thing. If you're lugging the car like that then it's operator error. Either reprogramming the current owner or installing a new one usually corrects this condition.;)

Seriously though, if you can hear pre-ignition then there is a high risk of damage. You may also be seeing an increase in this because of a possible hot spot within the cylinder. What plugs are you running? Did you ever have a long stretch of running rich? If so you may have carbon build up. I’m not as concerned with issues when lugging the car since that is predictable, but if you’re hearing pinging during high boost, then that’s an issue of concern.

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*-- Just some stuffy advice from an old fart who's too set in his ways. --*

Does your car still have it's EGR?

I know some my dis-agree with this question but I would like to know due to a similar problem I am having with my Auto TT.

-Jason

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90 300ZX N/A - For sale
90 300ZX TT
- POP Charger
- Greddy Cat back Exhaust
- Blitz SPC-ID boost controller

Niel...

thats kinda what I was thinking. I don't know a great deal about fuel mapping in these cars, but I am sure that 15% throttle at 1500 rpm at 25 mph in 4th gear is not something typically compensated for.

JT...

have you seen gas prices? Hence the low RPM driving.

AO...

no EGR.

Thaks for the input guys, I didn't think it was a big deal, but better to find out from someone smarter than me! ;)

Eric

From the research I have done and my experiences the EGR has much more of an effect on the smooth operation of the engine than most have thought. Currently on my TT the EGR is removed(that way when I bought it) and on days where the amb temp is over 95 degrees I get major detonation under low RPM(between 1700 and 2200) load. I have backed the timing down to 13 BTDC with little to no improvement.

The base intent of the EGR is to lower combustion temps(which in turn lowers NOX) by slowing the combustion and by displacing some of the mixture in the chamber. With the ECU expecting the EGR it will run a little leaner under cruise conditions, since it is expecting there to be Exhaust gases in the chamber. I understand you are running a ASH ECU but from what I have read aftermarket programs don't compensate for EGR (correct me if I am wrong).

I am currently trying to locate all the tubing and valve to replace EGR and determine if it will correct the detonation problem. I will keep everyone informed if there really is a change.

Here is a good writeup which explains everything very well - Auto Inc.

-Jason

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90 300ZX N/A - For sale
90 300ZX TT
- POP Charger
- Greddy Cat back Exhaust
- Blitz SPC-ID boost controller