Hi folks. Saw this on a local-ish Facbook page: Facebook
They’re claiming that some new LED light-equipped green coils are allowing the engine to reach 6400 rpm. I’ve sent an email to B&S with my concern, but wanted to see if anyone has seen this first hand?
People put too much faith in the ignition pulse for the RPM limiter. Really need to get an actual hall sensor or similar onto the crank/flywheel to see what it’s turning.
Surprised it’s not done more in tech. Mychron wheel speed sensor works well, I used one for an EFI conversion with the flywheel magnet as a trigger.
I don’t see why tech doesn’t involve running one of these on the clutch drum since I agree - the MyChron max rpm is just not going to work well with the intermittent spark signal during cut
So you guys think it’s just the inherent lack of accuracy with the Mychron RPM pickup? I can see that being an issue.
I’ve seen coils read 6400 on the MyChron before on track. One, the MyChron RPM sensor doesn’t particularly enjoy rev limiters so it will read the RPM a little weird, but also on track if you’re at the limiter and get a bump draft you will push the engine over the rev limit as well. If you’re geared a tooth or two shorter than the kart pushing they can easily push your kart a couple hundred RPM over.
The only way to actually test it is to put the kart on the stand and hit the limiter with an external tach, there are too many variables on track that could look like a coil not limiting correctly.
You can’t use the tach max rpm to police rev limiters, but you could use the rpm trace. For example, if the rpm trace is just as smooth at 6,200 as it is at 5,800, it’s not limiting correctly. If the signal goes nuts past 6,050 or so, it is probably fine. When it goes nuts, the peaks can spike above the limit, but that’s more in how the timing is calculated in the MyChron and isn’t inherently physical, so the “max” in the summary readout is not the end-all be-all answer
Simple math will tell you if its correct or not. tire circumference, gear ratio, and speed, solve for RPM
Circumference is not a constant.
that is true, but if i was really going to do it, i would get a reading of speed at 6000 rpm, do the calculation to figure out tire circumference. From there, i would use that to calculate my max rpm at my top speed. tire growth from 6000 to 6400 rpm will be pretty minimal
It not just variable on track though. You’ll even find differences between the two tires on your rear axle.. which one do you take for your calc?
Depends how accurate you want to be of course..
That’s basically what I do, but you don’t need to go all the way down in the weeds to the circumference. The ratio of rpm/mph is pretty consistent for a given gearing in my experience. The fourth digit fluctuates a bit with pressure and things, but, in a straight line, it is plenty accurate for these purposes.
Still won’t tell you if the kart was shoved through the limiter by another kart with longer gearing though. The only way I’d ever be okay with the limiter being teched is with an external tool for directly measuring rpm on the stand or GPS-embedded video showing the racing conditions when it over-revved. Seems like the stand method would be easier.