SB chevy timing retarding

[jl222]

 
 What MSD coil are you using, it has to be designed for a longer race than drag racing.

  But any coil should be good for a dyno test

                  JL222

[JimL]

 C12 should be sweet enough.  It must be something wearing out, because it looks like you've had enough fresh fuel to eliminate my idea.  At any rate, file that gasoline info away in the memory banks.  It can bite you in the strangest of circumstances, and its just hard to believe what it can do to a healthy engine.

Good work on your testing and reporting, and we all appreciate reading about it first hand.
Regards, JimL

[manta22]

I don't know about MSD, but the Crane ignition changes from multiple sparks to a single powerful spark above a certain RPM.

Regards, Neil  Tucson, AZ

[SPARKY]

most do above 2000-2500

[jlmccuan]

Jack Gifford pointed out that he was able to do adjustable cam timing of 16 degrees with as little as .001 change in gear mesh on a gear drive cam.  It stands to reason that that kind of change on a helical gear could create similar changes in timing at the distributor.

Do you have a link to the Jack Gifford thread?

[Stainless1]

I think that was in the Milwaukie Midget Thread

[wobblywalrus]

My experience with gasoline not burning fast enough for the rpm and spark advance curve is simply the lack of power at high rpm.  The torque and hp curves appeared to be smooth.  There was no noticeable transition from good to poor running like "the engine goes flat..."

[Koncretekid]

From an outside perspective, I do not understand why the motor would just drop off so suddenly when the timing retards to about 16* BTDC.  I have found my motor works best at full throttle, WOT with the timing retarded to about 20* BTDC (my timing disc is homemade and could be wrong).  Is it possible that the MSD is designed to retard at high RPM?  And, that the drop off in power could really be due to other causes, such as insufficient fuel, or insufficient air?  How about bad fuel, even though you said it was a new can?  Just saying, keep a wide open mind.  Quite often (speaking for myself) it seems we jump to conclusions.

[JimL]

Bo, the heavy fuel issue (when it has really high 90% like VP mr12) kicks in suddenly (depending on engine temp) when you reach an rpm that does not allow enough time span for about 90% vaporization. 

At that point, a cascade effect occurs which drives exhaust temps high, but poisons the next intake charge causing soot on the combustion chamber.  It really falls on its face, and the weird clue is that the spark plug porcelains are pretty clean and white, but the end of the threaded plug barrel is totally black soot.  I think its caused by the lean partial burn, that doesnt make enough heat in the combustion chamber to:
A- make power
and B- put enough heat on the surrounding aluminum to prevent the soot build.

All the fuel burns, but not necessarily in the top of the cylinder.  Thats why so many folks have scored pistons running this way, while not making any power.  It is a really ugly thing when it happens to you, because all your diagnostic methods go right down the drain. :-P

[wobblywalrus]

Thanks for the info Jim.  I wish I knew then what I am learning now.  Any advice on interpreting the distillation specs would be nice.  All that is a mystery to me.  Maybe a new "Fuel Spec" topic can be started.

[JimL]

Bo, long and rambling PM sent.  Sorryyyyyy!     :lol:

[wobblywalrus]

A few weeks ago I was looking at all sorts of advance curves for my Triumph.  They were developed for different sets of engine modifications.  Several of them reduced the spark lead at high rpm. 

[JimL]

When we mapped our turbo car, years ago, I learned about reduced cylinder fill as the rpm got up high.  You just dont have enough time to actually fill the cylinder, like you would at mid-range, and what you get is later.  That means you are going to get a smaller bang, so to speak.  If you picture the time event, over the range of piston position where "pushing" on it does any good, you see that the smaller bang needs to back up in time, a little bit, to get the largest proportion of "push" on the "sweet spot" of the power stroke.  It really is milliseconds of difference, but that is quite a few degrees at high rpm.

That "less air" business meant that my injector duration pretty well flattened out above 6000-6500 rpm....and I may have left it a little rich.  Unfortunately, my crank trigger mount broke, the engine banged the turbo (hard enough to break bearings) and the exhaust blew some chunks out of the dyno bay concrete floor.  I wasnt invited back to play with their big toy.

Variable valve timing race engines sometimes have two places where the intake cam is fully retarded....idle, and anything from somewhere past the torque peak all the way to rev limit.  Anywhere else, you are about as well off to just go to full camshaft advance and tune with fuel and spark.  Trying to fish around with interval hold positions just makes stuff wear out, anyway...except maybe Suzuki's ramped lobe system.  Thats pretty cool but I dont know if it is used, yet.  Probably all kinds of stuff I never heard of, going on by now.


Anyway, knowing that variable intake cam timing gets retarded for high rpm, kinda explains why spark needs to back up as well, dont you think?

[Crackerman]

Intake cam retarding increases volumetric efficiency at that rpm with more intake/exhaust overlap.
Any time you have less cylinder fill due to rpm increase, ve is falling and will plateau power or fuel needs. Better head of camshaft to get ve back up at those rpms is now necessary. Turbos and blowers help, buthere are still flow limitations in head.

[Hooley]

I was wondering if you had a chance to try the MAG in the motor and what results it had?

Hooley