Dual quad tbi setup? Start from mid 80's cross-fire ecu?

[Keithnh]

New to fuel injection performance modifications and just trying to get my bearings really. Printed out the "TBI Conversion" article on homepage and was glad to have clear pictures and explanations of basic idea of parts. I would like my first application to be a dual quad (2tbi unit) intake on a custom Pontiac 444ci/400 block with a flat tappet cam. Looking for a very strong high compression 6500rpm maximum operating range in a street driven car. Im not sure this is possible with tbi, have read that tbi is very strong in lower and midrange with 300hp attainable easily with a single on a 350 small block. The rpm limiting factor not being electrical or injector capabilities but flow of tbi unit, heads and intake. I will have ported heads and a dual quad manifold with very straight runners "Offy Equa-Flow" and was thinking 2 sbc 500cfm? tbi's might be just the ticket to have an abundant fuel supply with a self-regulating mechanism(computer) to keep from wasting fuel. Never knew there was such a deal from the factory but recently heard in the mid 80's there was a "Cross-fire" tbi setup used with little success, maybe too much for small block?? I would be running an electric fuel pump and tunable fuel pressure adjuster as well as possibly high performance manifold plates. Think there has been enough aftermarket advancement this may be possible by now and work pretty nice. Just ordered a book on Chevrolet Fuel injection so have some reading to do.

[Byron]

CFM Technologies has the 4bbl - TBI adapters for spread bore and square bore.
Crossfire was 2 single barrel throttle bodies.

[Byron]

Big block TBI's are also bigger with 2" throats with small block TBI's being 1 11/16 bore

[lionelhutz]

The crossfire is too old to bother with. It was only 2 injectors and any newer TBI computer is also capable of firing 2 injectors.

You could do the http://www.dynamicefi.com/ conversion as well as the adder option to run 2 TBI's with the computer. You'd also get the extra features their system offers.

But then, one of the computers from factory that was used for both TBI and multi-port might have V8 TBI code and be capable of driving 4 TBI injectors. Possibly it will require a little modification. Hopefully, someone else who knows more will chime in.

[1project2many]

Hello, and welcome to the forum.

It's neat to see non-Chevy engines getting EFI. I have a friend with an early Poncho fullsize convertible and 455 that's done an EFI swap using a 4bbl manifold with PFI. It's a very cool project.

I'd like to meet the person or people who convinced you the Crossfire system was no good. Usually a poorly working system wasn't the system's fault. But it's not built for what you want to do. It's two single throttle bodies that flow as much as the stock smallblock Chevy TBI.



You can see some pictures of what folks do to the stock manifolds here. It's hard pressed to support over 300hp.

Dual TBI as a first TBI swap project is a big step. There are some challenges in making sure the fuel system will supply both TB's properly and in building a proper throttle linkage which can be synchronized for matching throttle angle. And there's distribution issues in the manifold but those may be secondary if you have a proven manifold. There's wiring but it's not that much worse than a single TBI retrofit. PCM choice could be critical to making this work. Electrically the ecm / pcm must be modified to work with four TBI injectors and of course there's the actual work of tuning this combination so I'd choose the 16197427 or contact Dynamic EFI for an EBL ecm. It's not impossible but it is going to be more work than a single TBI swap.

2bbl TB air flow is measured at higher vacuum than a 4bbl carburetor so the flow numbers you find are not directly comparable. As mentioned, the large bore 454 TB's are the ones to get. Tuning for good street operation will take some time so if you get frustrated quickly with an engine that doesn't run right you might want to practice on some stock or mildly modified vehicles before tackling this one.

[Byron]

I wonder about progressive dual TBI's - could it be done with 2 separate ECM's - separate TPS and maps for each?

[1project2many]

I've seen some very interesting stuff over the years and I think a determined person could make the dual ecm solution work. But I'd prefer to stick with a single ecm to reduce the number of variables. Yes, pun intended.

[Byron]

Both ecm's would share the distributor, coolant temp sensor, ???

[Six_Shooter]

No modifications are necessary to run 4 injectors from a TBI PCM. I have a friend with two cars running stock (unmodified) '7060s, running home made 4 bbl throttle bodies made from pairs of GM throttle bodies. He simply wired the injectors in series pairs and has had ZERO trouble for years this way.

I agrre a single ECM solution is best.

Also progressive is not easily do-able with a stock GM ECM, since it has no provisions for progressive secondary injectors. If that's a must or highly desired a Megasquirt is what I'd recommend. The problem with trying to run progressive throttles without also running progressive injectors is that you will get a large amount of fuel puddling above the secondary throttle plates and then when they do open will cause a rich condition. This will not always happen at the same load or RPM, so it would be difficult to tune around, also if you stab the throttle let off and then right back into the throttle, you'd likely not get that excess fuel on the second stab, so that would cause a lean condition. Best to just keep the throttle linkage 1:1 for all throttle plates if you go with an ECM that's not capable of progressive injector control.

As a side note, I like the crossfire system. I have on eon my grandfather's '71 truck, running off a '7427 PCM, that is also controlling a 4L60E. :)

[1project2many]

Yes, shared sensors.

GM ran staged injectors on the ZR1 with an RPM triggered external driver module. It'd be interesting to scope the injector pattern if the VE is set to "0" for a range of cells as that approach to staging the injectors would work with multiple ecm's. And of course, if the code is acessible then it's possible to use custom code to zero injector output below a specific TPS.

[Six_Shooter]

The LT5 didn't use staged injectors, it used a set that could be switched on and off. And to switch them on and off the engine had to be off, and the switch turned with the ignition key, then restarted. All 16 injectors were AFAIK always on when the high power mode was engaged, just using a different VE table than the 8 injector mode.

[Fast355]

The LT5 didn't use staged injectors, it used a set that could be switched on and off. And to switch them on and off the engine had to be off, and the switch turned with the ignition key, then restarted. All 16 injectors were AFAIK always on when the high power mode was engaged, just using a different VE table than the 8 injector mode.

Actually I am fairly certain 1 project is correct. The switch disabled the secondary throttle plate (s) on the ZR1 but the secondary set of injectors and throttle only opened at higher rpms.

[1project2many]

The LT5 didn't use staged injectors, it used a set that could be switched on and off. And to switch them on and off the engine had to be off, and the switch turned with the ignition key, then restarted. All 16 injectors were AFAIK always on when the high power mode was engaged, just using a different VE table than the 8 injector mode.

This seems like a mix n match of system info. LT5 TB had tiny primary blade and larger secondary blades. Also had primary / secondary runner combination. When secondaries were active, second set of 8 injectors came online. Activation was RPM and TPS based (I said RPM only). LT5 also came with "valet" key which would disable secondary operation. That system would prevent secondaries from opening and would prevent secondary injectors from operating. There are some descriptions, pictures, and maps for OEM TPS v RPM curve to activate secondaries on this page:
http://www.corvetteactioncenter.com/...diagnosis.html

[Byron]

Damn Mercruiser engines. LT5 built by Mercury marine. total POS to work on. DIS under intake manifold made changing wires a bitch.
I work on Vettes all day and cringe when a ZR-1 drives up.

[1project2many]

Yep. I went from GM dealer land in '91 to Corvette specialty dealer in '98 and lived through ZR1 then LT1 days. I fix school buses now so maybe my Vette experiences did more damage than I realized.