I would use a high input impedence (MOSFET) 555 timer chip in monostable mode with an ouput pulse width that was equal to, or less, than the shortest low-res pulse from the Opti-Spark. I would use the low-res pulse to drive the 555, and the output of the 555 to drive the tach.
I would do this for two reasons:

1) The tach would be "isolated" and wouldn't potentially "load down" the low-res signal -- the low res signal doesn't have much current capability and may not be able to drive the ECM and your tach together. This could become a problem if you were to drive a ECM, Delteq/LTCC, and a tach from the Opti-Spark.

2)The 555 fixed duration output would be consistent -- no varying pulse widths. Although, I don't think most modern tachs would have a problem with varying pulse widths.

http://www.williamson-labs.com/480_555.htm

http://www.doctronics.co.uk/555.htm

Tom Piper

 

Here is a person on this site i read about this is what he did .


My car is a 93. I had the intake drilled for a distributor and if you hadn't noticed the original ecm including all wiring was completely removed. that's an Accel DFi Gen-7 and the ditributor is a dual sync . i wanted to spin the motor very high and the opti wouldn't do that.

The basics of the motor are LT4 heads/intake/block (no difference really).4.060 x 3.75 Titanium rods,fully forged / lightweight everything. Hollow stem intake and exhaust valves, shaft rockers, arp studded everything. Pretty much every bolt on possible.. EWP, ATi damper, Custom fuel lines... Solid roller cam that's mild, i mainly wanted it for the rpms. Built it to handle 8k rpm


the complete suspension back to front has been redone, all poly bushings, ball joints, wheel bearings, tie rods (springs swaybars and shocks were done before i started this project), rear VBP toe bars, VBP camber bars, z-51 sport ratio rack (LOVE THIS), turn-one ps pump mod

Entire drivetrain was redone front to back.. motor. mcleod twin disc with alum flywheel (having master issues atm ), hardened steel halfshafts on driveshafts and halfshafts, custom stainless high speed balanced driveshaft, lightened and polished halfshafts, new d44 w/3.73 (wow 4.09s sucked !!) with everything new inside it (thanks Dave ) Brand new Blue Tag ZF-6 sent to ZFDOC who did a rebuilt/strengthen job, welding 2nd gear synchros, changing bearings and his short shifter (wow his short shifter is so > than the Ripper i used to have !)

Exhaust is Tpis longtubes random tech metallic cats, b&b exhaust fully mirror polished front to back (resonance is killing me but i LOVE it !!! I'll put up some sound clips for you guys since i know how to do them now. After i tune it I want everyone to hear a v-8 at 7800 rpm in a tunnel

Brakes are Baer 2 piece Rotors 13" front 12" rear , GS calipers, Goodridge stainless braided brake lines, DRM brake bias spring, hawk racing pads (street compund), wilwood Dot 3 high temp fluid. Wanted to go with the complete Movit setup but 2 things stopped me.. 1 was my fiancee and her need for an overpriced ornament on her finger and 2.. i have a hard time believing those massive brakes are worth all that extra cost.

Wheels are Iforged brushed aluminum w/clearcoat 19X11.5 rear and 19X10 front. 305/30/19 rears and 255/35/19 front Pirelli P-zero Rossos front. Offset is the GS 50mm rear and 56 front. I had initially purchased HREs for a great deal (540 R same sizes) but got an offer on them locally that i couldn't refuse. When i found that Iforged was looking to steal HRES market share by underpricing them, and after being told their lips and forgings are from the same supplier i jumped at the opportunity. I got a great deal and i got them in the sizes and finish that i always wanted and LOVE the look of the car. I'll try to get some night shots of the car, i think it really brings out the color.

 

Thats Diel's setup and damn! it is a helluva C4! One of my favorites on the board.

MSR, I'd shoot him a pm to this thread.

 

great, thanks to all. and thanks, tom, that's a fantastic idea. if i used 2 of those in parallel, with different circuits, could one be made to only recognize 2 pulses per revolution, therefore sending a cam position signal to do sequential injector firing?

not that i'd go to the trouble, but it's an interesting thought.

-michael

 

I installed a distributor in my 93 for many reasons. The main reason for doing it was i wanted to spin my motor to 8k. the opti wouldn't do that and the stock ecm i believe has no fueling or timing tables beyond 7100 or 7200. Another reason was apparently the opti has issues with its rotor breaking off with continuous higher rpm use. When my car was just bolt ons i broke 2 this way.

I should mention 4 years ago when i pulled the motor out of my car there was no proven dynaspark, or any mention of these aftermarket distributors. I still have to agree that they are very very accurate and do work well and the dynaspark and others really eliminate most of the stockers drawbacks.

If you are looking to build a motor it is very easy to get timing from the front of the motor. I bought the gm opti replacement timing mark. Basically its a plate with a timing mark that bolts to the block. I went with an ATI damper and the crank had to be keyed for it and it comes with timing marks on it already so that took care of that.

I had my Intake drilled by LT1intake.com I have heard he doesn't do these anymore. basically any machine shop should be able to drill you a hole in the back of an LTx manifold and the surface of the distributor needs to be at a slight 5 degree angle (believe its 5' i forget :/ ). It doesn't look very hard to do and infact my builder has done it himself for a supercharged motor he's building for himself.

The problems you will run into are these. Im pretty sure the EGR setup can NOT be used. The base of the distributor will be in the way of the egr valve. I used a block off plate and some of the intake had to be ground near the plate to make room for the distributor. Second the Fuel rail will not fit. If you think the fuel rail can work if it was slightly bent than i think you have solved that problem. On my motor when the distributor was dropped in, it needed to sit 1-2 more mm to be flat and the rail line coming out from the rail was in the way. I purchased an F-body rail off of ebay and had the lines cut off and AN fittings welded on. From there the options were endless and we used a heat resistant fuel line to an aeromotive regulator mounted below the coolant reservoir.

Again the main reasons i went with a distributor was for high rpms, and because we wanted to use a Gen 7 dfi ecu and run sequential. My 93 was batch fire and i wasn't too happy with what i knew about batch fire at the time.

The downsides? Im not sure It sure looks neat back there. I've popped my hood many times already and the few comments I've heard were " see its a L98 converted to new body panels and LTx intake" Most others didn't even notice the distributor at the back. Maybe the coil wire? I went with a MSD coil with the MSD ignition (mounted in the front area below the headlight) and the coil is mounted using the stock bracket to the front of the block. I liked it because it was convenient and there was no need to fabricate anywhere to mount it. The builders also said keeping it further away from the ecu was better for EMI protection. The first wire i used was 32 inches, and i cut it down to 27 inches with another single wire cut for me from Magnecor. I spoke to MSD and they said that anything below 36 inches was fine. I belive Icevette mounted his to the firewall which is very close. I may do the same in the near future.


there's probably alot I'm forgetting but I'll dig up a few pics for you guys.

 

Here's some pics.

First the distributor plate for timing.


Here's a pic of the intake and rails when i got them back from coating.


Here's a pic of the fuel rails.


Finished product. And yes I've already spinned her to 7850 at least 10x last night while data logging !

 

Beautiful.

 

I never said it wounldn't work... I just don't think it's a good use of technology. Besides, it violates more emissions laws than I can count (well, three biggies that I can come up with)!!

1) all the computer mods.

2) the removal of EGR

3) fails visual - big time.