MSD 6C and detonation

[canyncarvr]

I've run a 6C for a very long time. Never had a spark problem.

BUT...I've always had detonation problems. I figured it was due to the crap gas of today and settled with running Trick or VP fuel. When running at the track, I didn't care. I always used some sort of real 'fuel'. I have the car on the street now more than the track..and the detonation issue is a problem.

My 6C quit. Car idling in the driveway...died. Crank, no start.

All MSD suggested tests pointed to the 6C. Wired in a new 4-pin stock box..started right up.

I had the engine tuned to something that sort'a kind'a worked with the 6C..no detonation unless I put my foot in it. That setup (4ºBTDC initial) with the stock box was a pig BUT I had no detonation whatsoever! I started bumping up the initial advance. I ended up with 10ºBTDC initial, 44º total (DC distributor with Mallory's YH mech advance set to 22º, and an 11º vacuum can), runs fine.

No detonation. None.

I don't understand.

I ran a question of 'Why?' by the MSD forum. Didn't get anywhere.

Anyone with a conjecture as to why an MSD box causes detonation where a stock style box does not?

General idea from my understanding is that a 'better' spark may well lead to LESS likelihood of detonation, not more...possibly less time for pocket gasses to heat up, pressurize into ignition.

This is a normally aspirated, carbureted engine. There is no rotor phase problem coming from an electrical retard with a full mechanical advance.

Any input would be appreciated.

Thanks!

Oh...I can provide all the specs on this engine, but that's not the point. The MSD detonated, and OEM style box does not..even with a considerably MORE advanced (initial AND total) tune. Yes, everything else the same: Fuel, gearing, carb setup...all the same.

 

[Abodybomber]

More efficient spark,more flame front,more cylinder pressure. If you curve that distributor,to 12 to 16 initial, 34-36 total at 2500-3000 ,sure would run better.

 

[canyncarvr]

More efficient spark,more flame front,more cylinder pressure. If you curve that distributor,to 12 to 16 initial, 34-36 total at 2500-3000 ,sure would run better.

OK. Would you equate those characteristics to an increased burn 'rate'?


Your curve figures I assume EXclude any vacuum advance. Correct? With only 22º advance in the distributor, I think that's a safe assumption.

If so, why do you want to negate a load sensitive timing advance in a street application?

Yes, 12ºBTDC initial, 34º total @ 2400RPM with NO vacuum advance is what I ran at the track. My cam got me a pressurized intake manifold at idle, launch RPM was around 3500-4000RPM with shift points around 6500RPM....vacuum advance was useless.

 

[Abodybomber]

Correct,both counts.

 

[canyncarvr]

Other factors being equal, an increased burn rate is less likely to detonate, not more likely.

From 'Engine Basics: Detonation and Pre-Ignition, ways to reduce detonation:

Another thing you can do is increase the burn rate of the combustion chamber. That is why with modem engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate.

Written by Allen W. Cline, CONTACT! magazine, January 2000

I don't pretend to say I understand all the physics of the air pump known as the internal combustion engine, but some things make sense. If the MSD is 'better' in regard to combustion and flame propagation, why would it not tend LESS toward detonation?

I don't know Mr. Cline, I'm not vouching for his statement, but offer it simply for consideration. What he says does make sense, no?