Hardware modifications GM style...

[Six_Shooter]

Just thought you guys might like to see this.


This is a TBI that I have helped with (welded it), for a friend of mine, to change some things from previous versions. IIRC this is #6.

This will be replacing an existing 4 bbl TBI, that is very similarly made, just the fuel system was quite different. We hope the new fuel system layout will help reduce some pressure drop that happens when AE kicks in.


http://a4.sphotos.ak.fbcdn.net/hphotos-ak-ash4/418536_10151225157205051_651865050_22752625_586785 410_n.jpg


http://a8.sphotos.ak.fbcdn.net/hphotos-ak-ash4/398583_10151225157490051_492619499_n.jpg


http://a7.sphotos.ak.fbcdn.net/hphotos-ak-ash4/423023_10151225164535051_651865050_22752656_163210 3117_n.jpg

[EagleMark]

Man that looks like a lot of work! Was it just the old style holly injector issue?

Very cool work though...

[Six_Shooter]

No, the old 4 bbl is also made from GM TBIs. We think it was just too much injector for fuel system layout that was used.

[93V8S10]

Cool throttle body!

Glad to see someone else's work bench looks as bad as mine.

[1project2many]

Interesting.

For years I've wanted to build a two or three TB intake using the smaller 2bbl units found on 2.8 truck engines but I never thought about a dual bore unit like that. The smaller bore should be better for response and would be less likely to allow fuel to drop out of suspension provided you can tailor the AE curves correctly. I wonder if there's any advantage to swapping to late / big block injector pods and using pfi injectors with fuel pressure as high as possible to keep droplet size small?

[JeepsAndGuns]

Very interesting indeed. I'm intrested in how you connected the throttle linkage. Do you have it set up to where both shafts open at the same rate, or do you have it rigged up like a old 4v carb with mechanical seconairys.

[Six_Shooter]

Right now the throttle linkage just opens both throttle shafts together. Eventually, we might set it up to be progressive, but I will have to build a progressive injector controller to do that, to reduce possible flooding of the later opening throttle plates.

We plan to run it about 20 PSIG or so, at WOT, and use the vac ref FPR to bring fuel pressure down for idle. With the current TB, we could get it to either run like a scalded cat with high fuel pressure, or idle along, and have good manners at part throttle with lower pressure, but couldn't seem to get both.

BTW, it flows over 1100 CFM, dry flow flow. :yikes:

[gregs78cam]

That's similar to what I had originally planned for my Camaro, but I decided to keep the linkage simple, and mounted them the same direction and real close.

[EagleMark]

Conjoined twins! :laugh:

[gregs78cam]

Ya know you wouldn't have to make a injector controller if you set the main linkage to be progressive for both shafts. Know what I mean? Make the cable attach to the throttle lever only slightly ahead of the shaft centerline, instead of centering the arc of travel over the centerline. That way you have a slower opening at the beginning for better part throttle control, and faster opening at the end ...... progressive.

[Six_Shooter]

Ya know you wouldn't have to make a injector controller if you set the main linkage to be progressive for both shafts. Know what I mean? Make the cable attach to the throttle lever only slightly ahead of the shaft centerline, instead of centering the arc of travel over the centerline. That way you have a slower opening at the beginning for better part throttle control, and faster opening at the end ...... progressive.

Not that kind of progressive, the kind where the larger throttle plates stay closed until the smaller throttle plates are opened a certain amount at which point the larger throttle shafts open quickly. It does require the injectors to be progressive for the larger throttle plates, other wise the fuel puddles above those throttle plates and can cause stumble issues. It's been tried. ;)

[gregs78cam]

Yea, somehow I missed that they were different size bores. You are right if you wanted the "secondaries" to be progressive. I was just suggesting you make them progressive together.

[EagleMark]

Not that kind of progressive, the kind where the larger throttle plates stay closed until the smaller throttle plates are opened a certain amount at which point the larger throttle shafts open quickly. It does require the injectors to be progressive for the larger throttle plates, other wise the fuel puddles above those throttle plates and can cause stumble issues. It's been tried. ;)That would be the first TBI Quadrajet! That is exactly how the Quadrajet carburator works.

You could open the throttle blades more on the big ones to prevent puddling? But how would you make injectors progressive? Holly did it! There old progressive four barrel had rear injectors turn on later... I found this out using a progresive Holly four barel on a GM system, when the secondaries opened the motor was flooded. Then I looked in at idle and they were filling up! We changed out the progresive linkage and it worked fine. The injector delay must be in the Holly ECM...

[1project2many]

You make the injectors progressive the same way they were on the LT5/ZR1 engine. When the TPS indicates the secondaries are about to open, bring a 2nd injector driver on line which fires the secondary injectors in parallel with the primaries.

[Six_Shooter]

That would be the first TBI Quadrajet! That is exactly how the Quadrajet carburator works.

You could open the throttle blades more on the big ones to prevent puddling? But how would you make injectors progressive? Holly did it! There old progressive four barrel had rear injectors turn on later... I found this out using a progresive Holly four barel on a GM system, when the secondaries opened the motor was flooded. Then I looked in at idle and they were filling up! We changed out the progresive linkage and it worked fine. The injector delay must be in the Holly ECM...

That is the exact reason the linkage is 1:1 between each pair of throttle plates. ;)


You make the injectors progressive the same way they were on the LT5/ZR1 engine. When the TPS indicates the secondaries are about to open, bring a 2nd injector driver on line which fires the secondary injectors in parallel with the primaries.

That's pretty much what I have in mind, though it would be an external controller, until I can learn how to write code for ECMs.

[1project2many]

The ZR5 box was external. It was triggered with a digital output from the ecm... it turned on above a specific TPS and turned off below a specific TPS.

[EagleMark]

The ZR5 box was external. It was triggered with a digital output from the ecm... it turned on above a specific TPS and turned off below a specific TPS.Interesting! So it was just like a module wired into ecm?

[1project2many]

Found diagrams to double check. ECM engaged relays on power side of injectors to bring them online. Secondary injectors shared driver with primary injectors. 1228331 /1990 LT5 wiring diagrams thanks to "Tyler Townsley" of corvetteforum.com:
http://www.sirgalahad.org/tyler/misc/90ecm1.jpg
http://www.sirgalahad.org/tyler/misc/90ecm2.jpg
http://www.sirgalahad.org/tyler/misc/90ecm157.jpg
http://www.sirgalahad.org/tyler/misc/90ecm158.jpg
http://www.sirgalahad.org/tyler/misc/90ecm159.jpg
http://www.sirgalahad.org/tyler/misc/90ecm160.jpg
http://www.sirgalahad.org/tyler/misc/90ecm161.jpg

[dave w]

An electronics idea I have to get the secondary injectors to work progressively is a 555 timer IC ( http://www.ti.com/lit/ds/symlink/lm555.pdf ) to turn on the secondary injectors based on the pulse with of the primary injectors. Something similar to a watch dog timer?

dave w

[1project2many]

If you can distinguish between AE (fuel delivered due to air density change in plenum) and load based fuel (fuel delivered due to air mass delivered to cylinder) you might have a workable approach. I found a copy of LT1 / 8051 code hanging around thirdgen. If the fueling routines could be worked out (no apparent AE tables) and duplicated in the P4 then you might be able to use a very simple circuit to follow the primary injector.

[gregs78cam]

you could build or buy one of these, wired to the secondarys, and just use a relay to put power to it when the secondary blades open.

http://www.jbperf.com/p&h_board/

[EagleMark]

I found a copy of LT1 / 8051 code hanging around thirdgen.I'd like a copy of that please!

[1project2many]

Haha! I think you posted in the thread it was in! I need to find the link as I forgot to save it on my pc last night.

[EagleMark]

I've got one a guy did but it is in several pieces and not completed. One of these days I'd like to learn this but the time it would take is probably not going to happen unless there's a dissesembler and commenter button?