What exactly are the limitations of the 7749? I understand it has the peak and hold injector drivers but let's get down and dirty with this post. From what I gather the 7749 must have the ability to see higher than 6375rpm. I only assume this from the quad 4 application, or is this mistaken?
Next question, how to wire the 7749 to fire the TBI REALLY low impedance injectors. How about the firing strategy?
Now the last question. What code to start with when running TBI HEI with 1bar map sensor. I can only think off the top of my head that there is the sy/ty code, then the high reving 4 banger and then a turbo 4 banger.
I thought I could find this data that I know was posted not to long ago but the search was turning up nothing . Thanks guys.

 

JP, I wish I had seen your thread before I started one of my own. Here. As slow as I type I might have started it before you did.

Steve

 

Just wire it up as used in the Sunbird application, the Sunbird uses 4, TPI P+H injectors, that are 2.4 ohms each, the TBIs use 2 injectors of 1.2, bingo, cone.

The 749 will fire the injectors less often, ie less atomization, at lower throttle openings it's the shearing of fuel at the butterflies edge that does most of it anyways.

Just use the Sy code but select the 1 bar MAP option.

I first ran a TBI with a 749 about 6-7 years ago, but didn't know about the injector constant not working. Did well, with it, but ran out of time to go too far with it. I've mentioned to folks for years on and off to try it again.

 

The RPM limits often bandied about are not the hardware but the software. Notice even though the $8D mask (TPI) tables only go to 6375 RPM max, the rev limiter goes much higher.

It is just a simple matter of calculating a different RPM term and using that. A friend is running a '747 as spark only and routinely takes the engine to 7,400 RPM. This is a SBC w/small cap distributor and external coil, stock GM

As for the injector firing scheme, I'll bet that there is a NETRES strap or resistor that can be used to provide the same injector firing scheme as the '8746 (alternate on each DRP).

GM used the '730 in single TBI applications and I have to believe that the injector fired more often then once per revolution.

RBob.

 

0x038A KFRPMLOW Hi RPM Fuel Cutoff Thresh. Lo Limit 4535 RPM
0x038C KFRPMHI Hi RPM Fuel Cutoff Thresh. Hi Limit 4535 RPM

<G>
Lotsa room.

 

Doc's got two shiny pennies on it not mattering.

We're only talking about 1/2 as many firings. Plus with them firing less often effectively get you about 5% more injector. Since you 1/2 the *off* time periods at WOT to worry about the injectors going erratic.

At a glance things line up pretty well when you look at each cyl bank on a SBC you have 4-~20 PPH injectors, and with a TBI that'd be a single 80.

It ***might*** wind up being easier to run the big injectors with a 749 then 747.

 

Doc's gotta' keep an eye on his shiny pennies. He lost 14 of them to me once before

If the '749 is bank firing opposed to alternate on every DRP, that means that injection is happening 1/4 of the time. Which could have a large effect during idle. The difference is in port the fuel is in that runner for whenever that cylinder needs it. If it is all sitting in a plenum I think distribution would get bad.

OTOH, can have a lot of fun with the code and injector timing. Can have the code run async injector scheme at lower speeds then switch to banked sync once the RPM & load increases.

This is something that once the weather gets a little better I may try out myself. GM runs some of the '7747 cals (TBI) as async all of the time.

RBob.

 

749 looks to be batch fire.

The 747 injectors fire a total of 4 times per revolution.
The 749 fires them both once, for a total of 2 injector firings.

It's not like it's all sitting on the plenum floor, the atomization of dripping off the butterflies is what does more then at the nozzle at low rpm.

Not to mention, I've run it this way, and it did work.
I was looking at my notes, and the other thing that was off in what I was doing is that there was no MAT, and I was using a fixed resistor. I'll still bet that a 749, and thus a functional MAT would be a great thing. Exspecially to the oem TBI setup that doesn't, well it's there, but not really used in the primary fuel and spark corrections like in the 749.

 

Thanks for the commentary all knowing ones.:hail: :hail: No really, you guys are the best in my book, along with Traxion, Moates, Mangus, JoBy and all the others who have helped all diy-ers.

Steve

 

Interesting. Now off to the junkyard to pickup an ecm.

 

i'm also very interested in this, i may be hitting the junkyards sometime this week

(please stop raining!!!!)

knowing how long it takes me to figure anything out (if anything i try actualy works) you guys will probably have done this, and all got it running. please let me know how everythinng goes.

 

Anyone know what injectors (flow rate) a 2.0 TBI, or a 2.5 uses? Also, a 2.8 TBI. I'm going to be working on my S-10 TBI setup, and I was already thinking about running a 730 then a 749 code to see which I like better to decide what to put in the car.

I need to get going on this before I pull the 2.8 and replace it with the OBD2 stuff!

Thanks.