MSD 6AL rev limiting

CowtownCobra · #1 (**permalink**) 10-31-2003, 11:05 AM

I have a MSD 6AL ignition and a Mallory unilite distributor and coil. I seem to be able to exceed the rpm for any chip I put in, but it does miss. Is this just the way these work, or could I have something miswired?

scottj · #2 (**permalink**) 10-31-2003, 11:33 AM

On my racecar I use a digital Tel Tac II tach which is very accurate and is pretty much the standard tach for that kind of car. Everyone I've ever talked to shows 100-200 rpm LESS than the chip with that tach.
When I built the Cobra I moved the 6AL from the racecar to it. The S-W tach in the Cobra shows that it now revs 100rpm HIGHER than the chip.

Scott

wilf leek · #3 (**permalink**) 10-31-2003, 01:55 PM

IF, as I suspect, the MSD limiter chip is actually just a resistor (different resistance for different rpm limits) then I would expect it at best to be +/- 1%. Tolerance bands on resistors can be +/-10% or better, only high-precision items approach the 1% mark.

That being the case (any elctronics experts to say yay or nay?), then the actual rpms that the thing cuts in (out) at would also vary within that tolerance band.

Plus of course, I view most tachos with the same degree of scepticism as to it's accuracy as I do all other automotive guages.

CJ428CJ · #4 (**permalink**) 10-31-2003, 02:09 PM

Dave,

Mine seems to work like yours. It starts to miss so I look down and discover that I'm up to 6200-6300 rpms with a 6000 chip in it.

Chris

scottj · #5 (**permalink**) 10-31-2003, 02:18 PM

Quote:

Originally posted by wilf leek

IF, as I suspect, the MSD limiter chip is actually just a resistor (different resistance for different rpm limits) then I would expect it at best to be +/- 1%. Tolerance bands on resistors can be +/-10% or better, only high-precision items approach the 1% mark.

That being the case (any elctronics experts to say yay or nay?), then the actual rpms that the thing cuts in (out) at would also vary within that tolerance band.

Plus of course, I view most tachos with the same degree of scepticism as to it's accuracy as I do all other automotive guages.

Not an electronics expert but, when I checked with MSD a few years ago, they said the chip has a +/-1% tolerance.

Mr.Fixit · #6 (**permalink**) 10-31-2003, 03:56 PM

So swap chips till the motor limits out at whatever RPM you think it should be. The accuracy of both the tach and chip might be lacking, but as long as they are repeatable and semi-precise, all should be fine.

Cracker · #7 (**permalink**) 10-31-2003, 07:31 PM

MSD systems use what they use to call Soft touch controls which allows the engine to rev past the chip but starts dropping off ignition to cylinders one a time. That is why you feel it missing or breaking up. This alternative is deemed better than an instant ignition overide. Think about it this way, if your system didn't continue to burn, albeit slower, you would have a quite abit of fuel continuing to be injected into the engine. I like it and have found myself pulling 2-3 hundred revs prior to the chip limit.