dialing in an adjustable zener?

[Tony Hanna]

Here's how I went about it. I ran a line from a little 12v air compressor to the map sensor and T'd a bleed valve in between. Starting with the bleed valve all the way open, I turned the compressor on and slowly closed the bleed until the pressure gauge on the compressor read 25 psi. With 25 psi to the map and the key on, I then read the voltage between the map signal and ground and adjusted the zener until I was getting 4.20 volts.

Does this method sound like it will work?:confused: I don't have a helper to read the dvom while I'm driving, so this was the best way I could think of to do it.:)
Thanks,

[butchsuppe]

I did basicly the same thing but adjusted mine to 4.7 v at which it hit cutout. Than I adjusted it a half turn at a time until it didn,t hit cutout. Don,t forget lower voltage means less fuel.

[Tony Hanna]

I did basicly the same thing but adjusted mine to 4.7 v at which it hit cutout. Than I adjusted it a half turn at a time until it didn,t hit cutout. Don,t forget lower voltage means less fuel.

What computer are you running? Mine's an '89 TI so cutout happens at 4.3v right? I figured 4.20 would be close enough since I'm adding extra fuel with a pressure switch/solenoid/nozzle setup. I suppose I could run it up to 4.28 or 4.29, but I'm a bit worried that it might hit cutout with altitude changes. I think there was a post about that awhile back.
Thanks,

[butchsuppe]

At the time I had a T2 Smec with extra inj. but since then I switched to a 3bar cal ( in a smec I socketed myself ) with +20,s. Problem with the extra inj. is it,s only good for an extra 2 PSI boost. I did get it up to 18 PSI without detonation tought. I believe cutout on your smec is 4.7v. try setting it there and see what happens, just back it off a little at a time if it hits cut out.

[iTurbo]

I have installed two of the adjustable zener 'MAP clamp' from MM Racing into my Omni GLH Turbo ('89 TI electronics) and '87 Shelby Lancer ('87 TII electronics).

I installed the circuit and initially set the the trim pot at maximum position so that, when installed, it basically did not limit voltage at all and was essentially transparent to the operation of the MAP sensor. I have it installed with longer wires going into the glove box so that it can be adjusted inside the car while driving. The circuit itself is housed in a gutted Oldsmobile keyless entry fob.

To dial it in, I used a multimeter and probed the two wires going to the adjustable zener. I set my boost controller (grainger valve) so that it was making 14-15 psi at WOT; just enough to hit overboost in the higher RPM. This way I could easily zero in on exactly what voltage would trigger the overboost condition. With me driving, I had my brother monitor the multimeter and take note of the MAP output voltage just as overboost cutout was triggered. I then made repeated WOT runs, slowly turning down the trim pot until MAP ouput voltage was limited to just under the overboost threshold. This took about 4-5 WOT runs before it was dialed in sucessfully.

On both of my cars, I found the overboost threshold to be about 4.2v. It is important to note that this will vary from car to car, dependent on elevation. I live at 5000ft elevation and a car at sea level would very likely trigger overboost at a higher MAP ouput voltage. I have never had to readjust the MAP clamp since installing it and it has been very reliable.

[Tony Hanna]

At the time I had a T2 Smec with extra inj. but since then I switched to a 3bar cal ( in a smec I socketed myself ) with +20,s. Problem with the extra inj. is it,s only good for an extra 2 PSI boost. I did get it up to 18 PSI without detonation tought. I believe cutout on your smec is 4.7v. try setting it there and see what happens, just back it off a little at a time if it hits cut out.

That's interesting. I always thought it was 4.3 for the stock computers and 4.7 for the MP computers. I'm running an extra injector setup made from a fuel solenoid and a nozzle that can be tuned using different size jets, so I should be good there. I've got a .014 jet in it now which should be plenty of fuel for 18-19 psi.:)


I have installed two of the adjustable zener 'MAP clamp' from MM Racing into my Omni GLH Turbo ('89 TI electronics) and '87 Shelby Lancer ('87 TII electronics).

I installed the circuit and initially set the the trim pot at maximum position so that, when installed, it basically did not limit voltage at all and was essentially transparent to the operation of the MAP sensor. I have it installed with longer wires going into the glove box so that it can be adjusted inside the car while driving. The circuit itself is housed in a gutted Oldsmobile keyless entry fob.

To dial it in, I used a multimeter and probed the two wires going to the adjustable zener. I set my boost controller (grainger valve) so that it was making 14-15 psi at WOT; just enough to hit overboost in the higher RPM. This way I could easily zero in on exactly what voltage would trigger the overboost condition. With me driving, I had my brother monitor the multimeter and take note of the MAP output voltage just as overboost cutout was triggered. I then made repeated WOT runs, slowly turning down the trim pot until MAP ouput voltage was limited to just under the overboost threshold. This took about 4-5 WOT runs before it was dialed in sucessfully.

On both of my cars, I found the overboost threshold to be about 4.2v. It is important to note that this will vary from car to car, dependent on elevation. I live at 5000ft elevation and a car at sea level would very likely trigger overboost at a higher MAP ouput voltage. I have never had to readjust the MAP clamp since installing it and it has been very reliable.

That's interesting. I'm at about 600 feet here, so maybe I can get away with a little more.
I wonder if the computer will set an overboost code with the key on and the engine off?
If it will, then I should be able to air up the map, raise the map volts until it sets a code, and drop the voltage by like .01 at a shot reseting the codes each time until I'm just below the point where it sets an overboost code. Sound reasonable?

The reason I'm trying to do it this way is that I don't have a helper to read the dvom while I drive and trying to read a dvom in traffic while hitting cutout doesn't sound like much fun.:)
If I can figure out a way to get it dead on using regulated air pressure to simulate boost, that's the way I'd like to do it.
Thanks,

[iTurbo]

Your idea sounds like it should work, although I have never tried it that way. Around here I-90 is practically void of any traffic at any time of day so that part was easy for me.

[Tony Hanna]

Your idea sounds like it should work, although I have never tried it that way. Around here I-90 is practically void of any traffic at any time of day so that part was easy for me.

Well, it turns out the smec is too smart for it's own good.
It won't set a code 45 with the key on and engine off. Won't even do it with the engine at idle and 4.9 volts at the map.:mad:
Best I could get out of it is a code 14 (map voltage above 4.7). It sets the code 14 at 4.61v at the map but not at 4.60v. I guess I'll start with it at 4.60 and if it still hits cutout, I'll start dropping it .01v at a time.
When I built this adjustable zener, I used a 1.5k pot instead of a 10k like I did the last time.
It's so touchy with the 1.5k that you can practically breathe on it and change the voltage by .05 or more. I really need to fix that first since the vibration could be adjusting it for me.:o

[iTurbo]

Here is what overboost looked like on my multimeter (Shelby Lancer):

http://www.turbosedan.com/shelby/MOV01971.MPG

Let us know how it goes!

[Tony Hanna]

Here is what overboost looked like on my multimeter (Shelby Lancer):

http://www.turbosedan.com/shelby/MOV01971.MPG

Let us know how it goes!

Nice. That's really what I need to do. Maybe I can temporarily tape my dvom to my radar detector bracket so I can see the thing while I'm driving. :thumb:

[Tony Hanna]

This is wierd. I've finally got my dvom installed in the car where I can see it while driving and it seems like cutout happens at a different voltage every time. One time it might be 4.51 then the next might be 4.56 and the following 4.54. I've brought it down to 4.50 now, so we'll see what happens...