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  #11  
Old 09-05-2012, 06:01 PM
Brit01 Brit01 is offline
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lean mixture would not cause detonation like this.

It would be accompanied by hesitation/stumbling, popping on the overun and white plugs.
Had this before in the past.

Unfortunately my AFR lambda/gauge is not working properly at the moment
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  #12  
Old 09-05-2012, 06:12 PM
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msdtech1955 msdtech1955 is offline
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Detonation - Causes and Effects

This article was taken from a series of postings on the Supra Club of New Zealand mailing list. The contributors were Stuart Woolford and Tony Bryant. The article was edited together by Cully Paterson.


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Knock, detonation, pinging, pre-ignition, etc are basically two things:

1 - Compression ignition, usually from over-advance of ignition and/or excessive compression (e.g. having added a turbo to an NA engine without lowering the compression) - this only happens after ignition, and is where the last of the combustible mix ignites from over-pressure during the last of the burn, giving a sudden detonation. A little is OK, and keeps the cylinder clean but causes larger stresses in an engine, this is what knock sensors monitor, as the detonation produces quite specific, and audible, noises.

2 - Pre-ignition/thermal ignition, this is often caused by too 'hot' spark plugs or hot-spots in the carbon crud that builds up in cylinders. It causes the combustible mix to start burning from a point/cause other than the spark plug ignition spark. This can be VERY bad if it happens very early in the compression stroke, where it can destroy an engine in one cycle! This is, fortunately, less common, however it can be caused by runaway compression ignition. This causes additional heat to build up and the can cause pre-ignition to start, and to then happen progressively earlier in the cycle (until BOOM!).

The causes of knock can be:

* Excessive compression - increases pressure / temperature

* Not enough octane rating - fuel more likely to explode

* Wrong mixture - excessive heating / lowering effective octane

* Ignition timing - too much advance increases peak cylinder pressures a LOT

* Poor cooling - leading to high cylinder temps and pre-ignition

* Dirty cylinders - more candidate points for pre-ignition

* ECU/sensor fault - lots of options
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  #13  
Old 09-06-2012, 07:58 AM
Brit01 Brit01 is offline
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Quote:
Originally Posted by msdtech1955 View Post
Detonation - Causes and Effects

This article was taken from a series of postings on the Supra Club of New Zealand mailing list. The contributors were Stuart Woolford and Tony Bryant. The article was edited together by Cully Paterson.


--------------------------------------------------------------------------------

Knock, detonation, pinging, pre-ignition, etc are basically two things:

1 - Compression ignition, usually from over-advance of ignition and/or excessive compression (e.g. having added a turbo to an NA engine without lowering the compression) - this only happens after ignition, and is where the last of the combustible mix ignites from over-pressure during the last of the burn, giving a sudden detonation. A little is OK, and keeps the cylinder clean but causes larger stresses in an engine, this is what knock sensors monitor, as the detonation produces quite specific, and audible, noises.

2 - Pre-ignition/thermal ignition, this is often caused by too 'hot' spark plugs or hot-spots in the carbon crud that builds up in cylinders. It causes the combustible mix to start burning from a point/cause other than the spark plug ignition spark. This can be VERY bad if it happens very early in the compression stroke, where it can destroy an engine in one cycle! This is, fortunately, less common, however it can be caused by runaway compression ignition. This causes additional heat to build up and the can cause pre-ignition to start, and to then happen progressively earlier in the cycle (until BOOM!).

The causes of knock can be:

* Excessive compression - increases pressure / temperature

* Not enough octane rating - fuel more likely to explode

* Wrong mixture - excessive heating / lowering effective octane

* Ignition timing - too much advance increases peak cylinder pressures a LOT

* Poor cooling - leading to high cylinder temps and pre-ignition

* Dirty cylinders - more candidate points for pre-ignition

* ECU/sensor fault - lots of options
I would rule out most of the above knowing the set up of my engine.
(recently rebuilt - twin dellorto carbs with 145 mains and 55 idles - factory set are 142 and 52 respectively always on the lean side for emission tests).
If the mixture was wrong you can feel the symptoms, not just hear it.
Also the plugs are a bit on the black side also. Just a touch rich. '8' NGK copper plugs. (7 is factory rating)
Timing is more retarded now than the manual's specification.
Octane booster added to fuel also.

Dirty cylinders could be a possibility as I rejetted and was running too rich on 60 idles, changed back to 55 a week ago.
Maybe it needs a good run on a motorway at high revs to clean it out a bit more.


I need to check the springs/weights in the dizzy also. These can become weak or break in a 25 year old device.
Before the MSD was installed I could have a good advance without knock and cool idling.
So first I will compare with the stock ignition again and see the difference.

Last edited by Brit01; 09-06-2012 at 08:12 AM.
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  #14  
Old 09-06-2012, 03:31 PM
Brit01 Brit01 is offline
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Would you recommend a colder plug? Maybe a 9 NGK?
2 heat ratings colder.
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  #15  
Old 09-07-2012, 01:03 PM
Brit01 Brit01 is offline
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Well I've got B9ES plugs to try this weekend. (2 heat settings colder than factory specs).

Dellorto informed me I don't want to go bigger on those main jets. 145 is plenty big enough even with an MSD.

If I still get detonation I will try a cold air intake (original).
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  #16  
Old 09-07-2012, 06:22 PM
Gaz64 Gaz64 is offline
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If you have gone to these extremes, I'd bet you have crossfiring happening inside your distributor cap.
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  #17  
Old 09-08-2012, 12:08 PM
Brit01 Brit01 is offline
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Quote:
Originally Posted by Gaz64 View Post
If you have gone to these extremes, I'd bet you have crossfiring happening inside your distributor cap.
cap looks like new inside. Can't see any signs off bad sparking.
timing is steady, idle smooth, 4 cylinders only so large gaps between the rotor contacts.
I will try to separate the Ht cables more.
Also it's a detonation issue under load- crossfiring tends to lead to missfiring as far as I believe.
I have Magnecor red cables - 8.5mm

Thinking the dizzy weights may need cleaning or the vacuum advance is failing.

Needs to do some checking this weekend.

Last edited by Brit01; 09-08-2012 at 12:18 PM.
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  #18  
Old 09-08-2012, 02:35 PM
Brit01 Brit01 is offline
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I gave a good suirt of wd40 and some oil around the shaft to drip down and some down the sides to reach the weights(hopefully).
Rotor springs feel ok.
Sucked on the vacuum while idling and it did move. Not much but some.

Plugs were a light brown. changed to 9 plugs.
also noted the colour change on the grounding nose but each one was at a different part of the nose! Not an accurate way of looking at whether it's retarded or advanced.
Advanced it to almost 8 degrees. tested it at full revs and it didn't exceed the 34 degrees- just spot on.

had a spin around and did't hear ant pinging.
idled cooler.

but it needs a good hard drive to test fully
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  #19  
Old 09-10-2012, 07:19 AM
Brit01 Brit01 is offline
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Nope still getting detonation!!!

Plugs showed no signs of lean mixtures.

I will disconnect the unit and compare it with stock ignition next.

Maybe the timing is out on the MSD or it's not working well with the stock coil or something.

Could of course be the dizzy weights/springs but this is a process of elimination so one thing at a time.
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  #20  
Old 09-10-2012, 12:43 PM
Brit01 Brit01 is offline
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Forgot to say timing was spot on. Idled at 8 degrees static very cool without the engine temp rising like it did before.
max advance was 34 degrees. Perfect.

But....

Tonight I'm disconnecting the 6A unit to test it on stock.
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