Supervisor42

A few people have asked about this so I thought I would post it.
The problem this solves is how to kill the coil without killing the tach signal which is needed to monitor rpm.
This little setup uses the MSD RPM Activated Switch (P/N 8950), rpm module, a relay, and a resistor to add a rev-limit to the non-CC HEI distributors. Parts are less than $100. It also can be unplugged and the system put back to original in seconds without tools if a problem should arise. Simply plug the two terminals back together reconnecting the wire. You can even connect up that useless SES light to flash when limit occurs. The cycle speed of the limiter is such that it isn't subtle when it hits though.
You can see it work in this VIDEO where it is set at 6700 rpm.

Apeiron

All it needs to be really complete is a coffee ring in the lower right corner.

It would be kind of a harsh limiter, do you have problems with unburned fuel when you hit the limit?

Supervisor42

Definately harsh. But cheap and effective. I'm sure if you stood on it in neutral unburned fuel would be a problem however; not as big of a problem as rods hanging out the bottom of a previously expensive engine...
I'll work on the coffee ring and my penmanship.

84z28350

Hehe, nice drawing

Please update when you have the coffee ring problem sorted out

I might have to give something like this a try next summer, Though i rarely rev the **** outta my motor it would be nice to have something to at least shut down the spark when i blow up something rather than have it rev all the way to the moon!




So about its harshness. I presume thats because of the time it takes the relay to reset...? What about trying to wire up several relays in paralell somehow so that the cycle speed could be much faster...?

Supervisor42

I take it that nobody clicked on the video.
About the harshness: Think of the rev-limit like a fire extinguisher. It's there in case of emergency to prevent disaster. You shouldn't be using it 3 times a day.
I still have short pipes on my car and it hasn't backfired the many times I've hit it. I'm sure it would overheat a cat if it was tripping constantly but then again the engine wouldn't be very happy either.
Let's get back to breaking something: If the valvetrain is any good, the engine will rev past 8K in less than .5 second if something breaks.
I don't know about you, but I ain't that fast and I'm not ready to bet the farm that nothing will ever break .
Might be interesting to see what would happen if a solid state relay were used. Hmmm...

84z28350

I know the engine would spool up pretty fast when something goes, but at least it would ust be free wheeling past your limiter point and just accelerating from inertia rather than being powered. It may or may not save the motor, but at least you have a chance at saving it!

Sonix

iTrader: (1)

Interesting. I'll have to check this out when I get home so I can watch the video.

So it just kills your ignition (spark) as the RPMS go over what the MSD module is pinned for...So how is this different then the complete MSD rev-limiter? (ie, the store bought no fabrication required sorta deal?)
Don't they do the same thing?

Nifty, I might try this, to make me feel a little more comfortable speed shifting my car... Everytime I miss a shift and over-rev I wince...

Apeiron

"Real" rev-limiters cut off every other spark, or cut sparks off randomly to keep unburned fuel from loading up.

Supervisor42

About $50 and theirs works better.

Sonix

iTrader: (1)

Unburned fuel? So you dump a few cc's of gasoline out the tailpipe, that can't be all that bad right? You're only missing a few sparks, maybe a fraction of a second...
It'll hit the limiter before it shifts, with your auto tranny? Or were you holding it in first? It didn't look like you were so...

Interesting, I never knew you could use MSD's rpm activated switches without the very pricey MSD setup... Boy, RPM activated switches, I could think of GOBS of uses for those...

Gumby

Could build a real fancy NOS or turbo system.

forget the WOT switch, have a multy stage system on the cheap able to adjust rpms for traction or fuel needs at a set rpm, insted of WOT or whem some computer thinks ya need it with sensor lag n all.

Supervisor42

I was changing the WOT shift points above the anemic stock 5000 rpm.
1-2 too late, 2-3 too early but that's the subject of another thread...
still looking for the $100 one of those...

Apeiron

You know, an LM2907 costs about $1 in small quantities and with a handful of external components it would replace the RPM activated switch.

dimented24x7

iTrader: (2)

This is good stuff. I was going to try something like this with my PCM. I had written some software for it to proportionally decay out all the advance as it approached the rev limit and then cut the spark once the limit is passed and not allow any advance until the spark had been restored. Never got around to it, but I think Ill try using it again.

Sonix

iTrader: (1)

oh yea, the LM2907's of course... I probably have some still in my toolbox...

I went out on a limb and checked digikey, you're talking about something like this?
http://www.digikey.com/scripts/DkSea...699164&Site=US

Not sure if that's a common enough part to be normal lingo for most people
That what you meant though?

hmm, interesting, with some electrical skills you could make up a nifty little circuit board....

I was always wondering about rigging up a circuit board with a USB plug on it. Having input from the tach signal, and from the O2 sensor, then dumping onto the USB port and onto the laptop. Then you can isolate it, and essentially log the fueling curve vs your RPM. Pretty cool way to tune your carb inexpensively.

Supervisor42

This is by far the best way to do it. It's the way the more expensive rev-limits as well as GM's "top speed limit" works. If you retard the timing enough you don't have to kill sparks and no problem with unburned fuel.
Where were you when I needed you??
Just look how much fun I had with a LM339... Dual fan and TCC control:

Apeiron

At least you used a dedicated comparator instead of a bunch of op amps.

Supervisor42

I could have cut the component count in half by using darlington transistors driven directly but I wanted to use parts I had on hand. It works though. Nice to know that the fans turn off when the air-dam starts working above 30 mph. Helps with gas mileage. (something my setup needs as much as possible)

Apeiron

Optical relays would have worked well, too.