This is from the Endyn site. This makes some sense as I forgot " mixture lag time." I still want to know why the factory doesn't cut spark. I am sure this is done currently as an emissions strategy (HC's) as no ignition with a rev limiter is legal for use on an OBDII vehicle. But this does not explain why older vehicles cut fuel rather than spark.
"I look at the BTM and think that it's essential for many applications, but as the enthusiast moves on into better electronics on down the road, the boost retard feature may be phased out. You'll still need the ignitions power and the built-in "Soft Touch" rev-limiter is mandatory.
The factory ECU cuts off fuel to limit RPM on the engine. While it's an "effective" way to do it, you might notice the harsh "bang-bang-bang" that results from accidently finding the pre-set rev limit.
Most everyone knows that shutting off fuel when engine loads are high is a sure invitation for destructive detonation. Well, guess what. That's basically what the engine experiences when it hits the magic fuel cut off point.
Unless detonation is "designed-in" to the combustion characteristics of an engine, it's effects are always destructive and the damage is cumulative. A little bit here adds to a little bit from there, and so on. You finally end up needing new rings and bearings....if you're fortunate.
The "Soft Touch" feature of the MSD-6AL and BTM permits one to insert a plug with desired RPM limit built into it and when the engine hits it, the ignition causes alternating cylinders to misfire, holding the engine at the prescribed point with ease and NO damage. We typically use a plug-in that sets the rev limit 200 RPM under the factory cut-off point. The "Soft Touch" is mandatory, if you care about your engine.
I've thrown several stones, but you guys will have to let me know if I wounded any birds with this."
.....The Old one....