I'll keep this direct and tech based, but it's still an off the wall question.

We are building a Vortec headed 350 with an LT4 hot cam for my dad's 91 RS that was originally an L03 T5 car. Obviously this engine will want more fuel than 2 injectors can offer so it's time to do something different for the injection. Instead of running the EBL with an aftermarket port injection manifold, we got a really nice GMPP Vortec dual plane intake manifold that we are planning running a Quadrajet on. We were going to stab in a vacuum advance dizzy and get rid of the TBI all together. Dad is an old school guy and knows Q-jets so this is comfortable choice for him that he can diagnose and repair himself.

Tonight we began the discussion about how to handle wiring issues such as how to control his cooling fan, speedometer, tach, etc. As we talked I lamented the fact that we were loosing cooling fan control, knock protection, ignition timing control, and the little bit fo fuel monitoring that the stock narrow band O2 will provide. Then I thought if we could adapt a throttle position sensor to the Q-jet, we could just leave everything intact and use the TBI computer to control everything but fuel.

The benefit is that we get to keep the knock retard, fan control, and ignition timing control, as well as the O2 sensor as a gut check for fuel.

The draw back is that this may not work or that it remains a complicated setup that Dad won't want to deal with.

So can anyone tell me why this won't work? Didn't the later Q-jets have throttle position sensors on them? If so are they a 0-4.5V range like the TPI and TBI units? Will I need to run this in open loop only or can I run it in closed loop and just open up the BLMs so the ECM doesn't get unhappy and throw the MIL? If this wasn't already evident I have the equipment to burn chips for TBI and TPI ECMs, and Dads car is currently running my tune on the L03. Even if a major sensor like the CTS or the MAP sensor failed, the engine should still run half decent because the ECM only controls timing at this point. It should still be reliable and easy for Dad to fix, yet I can still use the benefits of the ECM to control timing. The best part is that is basically free to try this as far as I can tell at this point.

So tell me guys, has anyone else tried this? Certainly I'm not the only one who has tried this. Let me know if I'm a total failure for thinking of doing this or if I'm a mad genius that has stumbled on a great compromise for higher HP engines to still use stock TBI stuff.

 

to me it is going to try to control fuel at all times. it will also try to control timing along with fuel if it is not seeing the correct correlkation between the 2. i think you are going to be dealing with a timing issue as well as an 02 sensor code constantly. you might as well go carb. i have yet to see a tbi system work with just timing or fuel. it usually wants both. and i think with the correct tbi maybe a 350 or at worst a 454 tbi and injectors and the correct fuel injectors and pressure it could be tuned to run the engine you are wanting to run.

 

I do not know all the specifics but if you hook up a tps to your carb you can still use the computer for spark timing etc as far as I can tell. A few guys on some of the truck forums have done this not just for the spark but because they still need to control the transmission. I may not be 100% right but this is what I gather from some stuff I have read.

They sell tps kits for holleys and stuff for this purpose.

 

I agree

 

tps on a carb was for the transmission only. the ecm is still going to be looking for fuel.

 

Ok thanks for the clarification. I personally like FI better and suggest you stick with it if you got it regardless.

 

You've not given anything specific here. The ECM uses coolant temp and MAP to determine timing. Timing does not change based on O2 readings or fuel trims. Gotta give specifics here or it's no help.

I like FI too, but even with a 454 TB only 2 injectors will have trouble feeding a 425hp engine while yet still being able to control fuel at idle properly. It takes 2 BIG injectors to do that, then they are too big to idle and respond crisply at low rpm. That is why people switch to the EBL and run 8 injectors. This way I get a good fuel metering device and a good intake manifold yet I keep all my timing and spark control of the EFI.

Still don't have a solid reason not to try this yet....

 

You can't beat the roar of the secondaries opening on the q-jet.

Look at the CCC-qjets. The computer controlled the primary metering only, using the carb's TPS and VAC sensor. The 85 and later units used the KS for knock retard.

You can connect an external TPS to a mechanical qjet but you can't use the CCC-qjet without hooking everything up. The mixture solenoid will go full rich on the primaries if the proper computer is not controlling it.

Not sure what part of the trans the TPS had anything to do with on the ccc qjet, other than lockup which also relied on the VSS and TPS. The fuel and timing tables were, like the TBI, driven by TPS and vacuum readings (with O2 for fuel trim).

 

that is exactly what I plan to do. We've already got 2 older Q-jets to use. Dad is going to build one and all I'll need to do is put a TPS on it.

I'll re-ask some of my main concerns again for clarity.

Will the ECM throw the MIL if it figures out it can't effect fuel trims? Can I avert this by opening the BLM windows in the chip? Or should I run it in open loop mode all the time so O2 feed and fuel trims are ignored?

Will the ECM be unhappy if it runs with the injectors unplugged? (i'm not sure if the drivers will be happy with no injectors or if it even matters.) I know it will be unhappy if the IAC is unplugged, I don't really have a solution for that yet and am open to ideas. If I knew the resistance or voltage the ECM wanted to see I could just make a dummy plug or solder a resister across the ECM terminals.

 

you can buy a fan controller cheap. i wouldnt run a quadrajet. i would run a holley double pumper. HEI vacuum advance distributor. and reqire the TCC lockup with the tci kit or buy a controller from B&M.

 

There is a write up on the DIY_PROM that I did when I ran a carb with a '7747 running the spark advance. It is in the Tuning Guide Book sticky, chapter 5, '8746/'8063/'7747 ECMs: SA Logic.

It works, and as I was using a stick trans didn't use a TPS. If running an auto with a TCC then will need it for the ECM to control lockup.

Note that a 454 TBI with the 80.5#/hr injectors at 30 psi will handle 462 HP, at 85% DC and .45 BSFC. Running a vacuum referenced FPR (VRFPR) helps in keeping the PW larger at low loads. This enhances drive-ability and makes for easier tuning.

But for someone that knows how to set up a q-jet they are a good carb.

RBob.

 

Thanks Rbob I'll dig into that.

From the reading I had done on here I thought people had trouble around 400hp getting idle and WOT fueling to co-exist. I'll discuss this with Dad and see what he wants to do. Other than getting a good deal on the intake manifold, we've not spent lots of money to commit one way or another.

The Q-jet is something Dad can work with without my support which was the main reason for switching to it. I want him to be able to be self-sufficient, but I don't want to give up knock protection and timing control unless I have to. The combination of carb and computer timing control could be the best of both worlds for us. He can work with the carb, and the rest should be set it and forget it. Even if there is a malfunction it will still run to get him home. MAP sensor or CTS failure won't cause the dizzy to stop firing so he is not any less reliable this way than he would be with a regular vacuum operated HEI.

 

wow i just got edumacated. this is nice to know. so i was wrong. sorry bud. i am going to read this also. plus i do know they offer a tps kit for the holley. but i bet someone or you could retrofit on to the carb.

 

I have done this with my car, and while not the best idea around, it works. I used a spring loaded pot off an electric forklift, bolted it to the manifold and twisted some wire to the carb to pull it. as for the mil light... im pretty much used to it being on and ignore it, i can tell on my own when something is up. the reason i attempted this was one, the wiring solution, two converter control, three because i hate carbs and plan on returning to injection. the biggest short fall of this is the lack of idle control. with an auto trans getting a reasonable park/neutral idle speed that wont stall in gear is a pain. this may be aggravated by my cam but i havent looked into it. the advantages of the computer controlled timing are clear.

 

I was in the high 360 RWHP range through a 700r4, 9.5" corporate 14-bolt rear, 2,800 stall torque converter, clutch fan, and long heavy 2 piece driveshaft. Well over 400 Flywheel HP and it idled perfectly and had plenty of WOT fuel left.

 

You're asking for a lot of work for little benefit. Having the ECM control timing is nice and knock retard is nice, but guys have gone very fast with just simple mchanical advance + vacuum advance distributors. Knock retard is not going to happen with that setup but if you know how to tune you can find the "limits" pretty easily.

I run QJets and HEIs all the time. One time I took my old 400ci Qjet/HEI small block, untouched, and dropped it into my brother's EFI Trans Am (TPIS Miniram intake). Similar weight vehicles but different transmissions and rear end ratios. He ran 1 tenth quicker and 2 MPH faster than I did on the same motor. But his 60' time was 2 tenths quicker than mine (traction limited, not a lack of torque) and his T/A was MUCH more aerodynamic than my old Malibu. That's not a big difference between a tweaked-out EFI system and an old carb + mechanical distributor.

Clearly his installation had more top end charge than mine to overcome a slight weight disadvantage for the T/A, but if the two cars could have run head-to-head he would have out-lauched me, I would have pulled even around the bottom of 2nd gear and the two cars would have sat fender-to-fender until almost 100 MPH. And I seriously doubt my old Weiand dual plane intake was the equal of his TPIS Miniram, airflow-wise.

I only mention this to illustrate that having all the flexibility of EFI doesn't necessarily mean you're going to run RADICALLY faster than the "old school" stuff. And building your own "hybrid" system could easily negate all the benefits of what you are trying to achieve unless you really know what you're doing.

FYI- early 80s GM light truck engines had a non-computer controlled Qjet carb with a computer controlled HEI distributor. I believe they called it the "electronic spark control" system.

 

If you are running open loop then it shouldn't throw any fuel related codes, because it will be ignoring the O2 input anyways. If it does trip the CEL then just see what those codes are and if they don't matter un-check them and reburn a chip.

Or you could put 2 TBIs on it like I did.

Either way it shouldn't be that hard.

 

I will say this....With enough engine, cylinder heads, and cam the dual TBI setup absolutely LOVES a dual tunnel ram setup. I know of a certain '39 Chevrolet running around with two big block TBIs spraying E85 into a tunnel rammed 12.5:1 406 with Brodix track one heads pushing power back to a 4L80E and 4.10 geared rear-end. Cruises effortlessly around town and absolutely FLIES with the throttle planted to the firewall.

 

Fan control as in the electric fan on the back of the radiator?

Isn't that started/stopped based on temperature at the little switch located in the head on the passenger side of the engine? All mine are. The fans also come on when you turn on the AC but the 86 uses an AC system pressure switch (you have to have freon and the compressor building pressure). On the 83 it doesn't matter whether the AC is actually working or not, the fan still comes on.

I know where there's a 91 RS with the computer removed and a built 327 in it and the fan still runs on it (when the temperature gets up there).

Or did I read the stuff above wrong about needing the computer to control the cooling fan?