4 cylinder engine + megajolt and a 6 cylinder tach

xellex · Post by **xellex** » Wed Apr 28, 2010 10:47 am

Hi guys, I'm new here, just installed Edis+Megajolt on a 1600cc aircooled VW engine. Everything went great, and I'm happy with it. Right now I'm playing with the maps.
I have a cheap ass rev counter that came from the car's previous owner, and it never worked good, it shows much less RPM than the real engine RPM. But still, I conected it to the MJLJ tach output, and it works just like before, when I had the distributor ignition.
Now, a few days ago, I got my hands on a full set of Porsche 911 gauges, which I want to use in my dashboard, but it just came to my mind that the Porsche tach is setup for a 6 cylinder engine (no 4-6 switch or anything). Is there any setting in the Megajolt unit that I could do, so that the tach signal would corespond the Porsche tach, but on my 4cylinder engine?

Thanks,
Alex.

Post by **brentp** » Wed Apr 28, 2010 3:48 pm

Is the 6 cylinder Porsche tach modifiable? you might want to check Pelican forums on that. Otherwise, you could look for a 914/924/944 tach to use in it's place.

xellex · Post by **xellex** » Wed Apr 28, 2010 9:37 pm

yes, the porsche 914 tach is the same dimensions and looks almost the same as the 911 one, and works for 4cylinder engines, but I thought I could manage with what I already have. No, I don't think I could modify the 911 tach. I'm probably better off selling this one and getting a 914 one.

xellex · Post by **xellex** » Wed Nov 23, 2011 11:04 pm

Hi guys, I'm back with more questions, I hope someone will have the time to go through this with me:
So, this is the current situation:
4 cylinder engine.
6 cylinder tach. Not modifiable. (I kept the 911 tacho and I don't want to open or mess with it in any way, and I don't want to change it to another unit).
COP wasted spark setup (4 individual GSX-r coils and 2 edis-4 units linked in parallel).
The tacho shows 2/3 of the real RPM, connected to the tach-out pin on the MJLJ unit.

I was thinking of the following solution (sorry if it sounds stupid):
Get a 3-input tacho driver (like the one sold on the triggerwheels site), and connect it to 3 of the 4 coils. Run the signal to the tacho from it. Would it show the real RPM, fooling the tacho into believing it's a 6cylinder engine? Am I totally of track here?
Thanks in advance!

Post by **brentp** » Fri Nov 25, 2011 4:49 pm

Probably not, as the parallel EDIS units will be firing simultaneously, and would appear as a single pulse on the other side of the tach adapter.

xellex · Post by **xellex** » Wed Nov 30, 2011 12:32 am

yeah, that's what I was afraid of...
So, I'm back to square one.
Anyone got some other idea? I'm sure there has to be an easy fix, but I can't see it

A buddy of mine was thinking of making me a sort of "piggy-back" unit, but he never has enough free time to design it.

Post by **brentp** » Wed Nov 30, 2011 6:18 am

Look around the porsche forums to see if the tach can be modified to show RPMs correctly. Otherwise, you might be able to find a different tach to replace the original unit!

NITROPIXIE · Post by **NITROPIXIE** » Sat Dec 17, 2011 8:15 pm

Having got a bit dirty with a MK1 Golf tacho a while ago I found that the chip that drives it was a PWM chip (can't remember the identification code on it). It basically worked by the circuitry counting the number of ignition pulses it received and outputting a PWM signal. The PWM signal then drove the clock/tacho which was basically an ammeter but calibrated with rpm on the dial.

The circuit board was very simple but I did not remove it from the tacho to get a circuit diagram. Maybe something on the huge to do list I now have.

So I'm thinking it may be possible to change some of the values of components on the circuit board in order to alter the PWM to the number of ignition pulses it recieves. Possibly utilising variable components and the megajolts tacho to calibrate.

It would be the long winded way of doing something but then it would be a lot of fun and torment.

Ryan