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Old 12-19-2010, 10:35 PM
Rocket Dog Rocket Dog is offline
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GM TBI unit size, airflow, and fuel pressure showdown

So I'm looking to get the TBI back on track and would like to get as much help from you guys as possible.

Here is what I have, TBI off a 94 suburban, 454. took the computer and harness, O2 sensor. Nothing else...... didn't know what else to take vs. buy.

any help you guys can give would be appreciated.

Also if anyone is in the LA, SoCal area, that can help..... it would be bonus!

Rocket Dog

Last edited by PlasticBoob : 03-01-2011 at 03:09 PM.
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Old 12-19-2010, 10:44 PM
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Hey, I can help you out, but what exactly do you need to know? I had a nice little website going on off-road.com, but they deleted it randomly and without warning. I have a backup I will be putting online but it's almost a year old.

If you want to save a few bucks, take as much as you can. Air and coolant temperature sensors (cheap brand new), TBI air intake if you're not going to be putting a custom one on, HEI module if you plan on using timing control. Sounds like you're well on your way?
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Old 12-19-2010, 10:47 PM
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This might also help you....

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Old 12-19-2010, 11:10 PM
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What ECM do you have? Either Binder Planet or Third Gen forums should be able to get you started.
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Old 12-19-2010, 11:54 PM
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http://www.viciouscustoms.com/TBI/junkyardtbi.asp.htm

they are doing a 6 cylinder but everything should be the same for a v8
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Old 12-20-2010, 07:19 AM
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What engine are you putting this on?
The 454 TBI, IMHO, is to big for our engines. I put one on a 425 Caddy and it was border line too much. If fuel economy is part of your goal, you should find the smaller SB TBI.
Just my .02.
The computer and harness is the same, its just the TB that is bigger.
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Old 12-20-2010, 08:48 AM
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Why do you say that Jaber? I understand that the injectors will be a bit larger and the ecm calibration is set up for the larger engine, but shouldn't the ECM should trim the additional fuel?
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Old 12-20-2010, 09:38 AM
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There are two general directions one can go on JY TBI installs.

Option #1: Install a system from a JY vehicle that had a motor similar to yours and adjust things such as fuel pressure, injectors and idle speed to get it to run decently on your engine.

Option #2: Install a system from a JY vehicle and reprogram the chip for your particular vehicle.

They are both valid ways of getting to the same destination. But the tips and tricks required are often different. Thus, using a 454 (2" bore) throttle body may work for one person, but not another, depending on their methods.
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Old 12-20-2010, 04:43 PM
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It will also depend on how much power you plan on making. I want to say the 5.7 TBI is good to about 400HP, and the 454 to about 500-550. More of a problem getting enough fuel flow than air from what I've read on them.

Jim
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  #10  
Old 12-20-2010, 04:47 PM
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Quote:
Originally Posted by blazer3664
It will also depend on how much power you plan on making. I want to say the 5.7 TBI is good to about 400HP, and the 454 to about 500-550. More of a problem getting enough fuel flow than air from what I've read on them.

Jim

I hear the stock 454 TBI unit flows around 600-650cfm, but I could not find concrete numbers anywhere. There are also places that will bore out your 350 and 454 throttle bodies to support even more horsepower. I am personally going to switch to custom MPFI for my next 401, but boring is something else for people to consider.
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Old 12-20-2010, 05:32 PM
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Quote:
Originally Posted by Jlamb
Why do you say that Jaber? I understand that the injectors will be a bit larger and the ecm calibration is set up for the larger engine, but shouldn't the ECM should trim the additional fuel?

I shoot lower on the fuel scale. From what I've been told, it is hard to dial the 454 in to the stock 360 block.
Plus, I already have an awesome tune for the smaller one. Burn a chip and go...
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  #12  
Old 12-20-2010, 05:50 PM
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Quote:
Originally Posted by jaber
I shoot lower on the fuel scale. From what I've been told, it is hard to dial the 454 in to the stock 360 block.
Plus, I already have an awesome tune for the smaller one. Burn a chip and go...
i've heard the same, especially from the chevy websites. that's usually why they are so cheap to come by.

Al
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  #13  
Old 12-20-2010, 06:36 PM
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Quote:
Originally Posted by AlsChopShop
i've heard the same, especially from the chevy websites. that's usually why they are so cheap to come by.

Al

Where do you find them so cheap? I looked high and low and ended up paying $125 for mine (complete and working). This was before Craigslist, though. I saw 305 TBIs all over So. Cal. junkyards for like $20. They're the same as the 350 unit, but with smaller injectors.

Dunno about the 360, but the 454 unit is perfectly matched to my 401.
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  #14  
Old 12-20-2010, 07:34 PM
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DAHoyle DAHoyle is offline
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Going to throw in my 2 cents, and that is about all it's worth.

By all means, toss in 454 throttle body. There is nothing wrong with it, and fuel flow is adjustable. I probably wouldn't run the injectors that came with it, and would step down to lower flow units. Chances are, you won't need all the fuel they provide, and there is no reason to try to tune around it. On the other hand, it can't hurt anything to have the airflow capability to upgrade, if you decide later to build a fire breather.

I drive an 89 burb, with the 5.7, and I am seriously considering stepping down to 5.0 injectors, and then upping the fuel pressure to get the flow back. I think it might help with atomization, but I don't have anything to back that up other than intuition. I'm not looking for higher than stock flow rates, just a better mixture. GM was very conservative in their fuel pressures, early on, but by the end of the TBI run, they were all the way up to 65PSI on the 5.7 in the Tahoe.

Simply put, you are pretty much going to have to make the thing run, and experiment. That means that there are no rules, other than common sense and some basic concepts. The thing to remember, is that it is not a carburetor, and you can not give it too much air. You can, on the other hand, give it so much fuel that the computer has a tough time compensating.

Give it a shot, and see what happens. Worst case, is that you have to replace it with a 50 dollar salvage smalblock unit.
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  #15  
Old 12-20-2010, 07:38 PM
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So after reading the thread and not knowing much about what vehicles had TBI. If one were to go to the junkyard to pull a TBI setup. Obviously not one with a 454 but what vehicles should I look for?
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  #16  
Old 12-20-2010, 07:45 PM
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Quote:
Originally Posted by DAHoyle
GM was very conservative in their fuel pressures, early on, but by the end of the TBI run, they were all the way up to 65PSI on the 5.7 in the Tahoe.

65psi on TBI? Are you sure about that? Are you sure you're not thinking of the SFI Vortecs used in the '96+ Tahoes?
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Old 12-20-2010, 08:53 PM
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Quote:
Originally Posted by PlasticBoob
65psi on TBI? Are you sure about that? Are you sure you're not thinking of the SFI Vortecs used in the '96+ Tahoes?

Sorry, typo. 45PSI. Not sure what the actual setting is on the regulator, but I do know the pumps put out at least 45 lbs. It was the only TBI vehicle that could be retrofitted to Edelbrock's PERFORMER MULTI-POINT EFI SYSTEM.

The system requires 43-45 lbs, and the in tank pump in the Tahoe is the only one capable of providing the pressure needed. As I said, I don't know what the regulator was set at, but there had to be a reason they were equipped with a high pressure pump. the earliest TBI pumps were marginal at 15 PSI.

In any case, any given injectors flow rating, is at a specific pressure. Alter the pressure, and you alter the flow rating. Obviously, you can reduce the pressure to the 454 injectors to flow less, but IMHO, a smaller injector at a higher pressure, is better than the opposite. I will be verifying that fact very soon.
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Old 12-20-2010, 09:02 PM
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Quote:
Originally Posted by DAHoyle
Sorry, typo. 45PSI. Not sure what the actual setting is on the regulator, but I do know the pumps put out at least 45 lbs. It was the only TBI vehicle that could be retrofitted to Edelbrock's PERFORMER MULTI-POINT EFI SYSTEM.

The system requires 43-45 lbs, and the in tank pump in the Tahoe is the only one capable of providing the pressure needed. As I said, I don't know what the regulator was set at, but there had to be a reason they were equipped with a high pressure pump.

Wow, that's impressive. So the later TBI Tahoe pumps are capable of 45psi, but does the system actually use that? I'm just a little confused... I know that MPFI/SFI pumps can run at TBI pressures (I have an MPFI pump feeding my TBI), but I didn't think GM would actually run ~45psi on a 200hp TBI V8. My 401 seems pretty happy at WOT running on 13-15psi and I'm making around 285hp. The TBI Tahoe injectors must be flowing at 100lbs/hour with that kinda pressure? Seems like a lot of gas for 200hp? Very interesting, I'd like to know what they were up to with that.
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Old 12-20-2010, 09:04 PM
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Quote:
Originally Posted by DAHoyle
Sorry, typo. 45PSI. Not sure what the actual setting is on the regulator, but I do know the pumps put out at least 45 lbs. It was the only TBI vehicle that could be retrofitted to Edelbrock's PERFORMER MULTI-POINT EFI SYSTEM.

The system requires 43-45 lbs, and the in tank pump in the Tahoe is the only one capable of providing the pressure needed. As I said, I don't know what the regulator was set at, but there had to be a reason they were equipped with a high pressure pump. the earliest TBI pumps were marginal at 15 PSI.

In any case, any given injectors flow rating, is at a specific pressure. Alter the pressure, and you alter the flow rating. Obviously, you can reduce the pressure to the 454 injectors to flow less, but IMHO, a smaller injector at a higher pressure, is better than the opposite. I will be verifying that fact very soon.

I will be eagerly awaiting your results...
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Old 12-20-2010, 10:01 PM
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Quote:
Originally Posted by PlasticBoob
Wow, that's impressive. So the later TBI Tahoe pumps are capable of 45psi, but does the system actually use that? I'm just a little confused... I know that MPFI/SFI pumps can run at TBI pressures (I have an MPFI pump feeding my TBI), but I didn't think GM would actually run ~45psi on a 200hp TBI V8. My 401 seems pretty happy at WOT running on 13-15psi and I'm making around 285hp. The TBI Tahoe injectors must be flowing at 100lbs/hour with that kinda pressure? Seems like a lot of gas for 200hp? Very interesting, I'd like to know what they were up to with that.

I wish I knew what the regulators were set at, but have been unable to find that information.

There is a great deal to be gained by playing with the pressure, especially in the areas of drivability. As I said, by running a lower rated injector, at higher pressures, there are two advantages that are seemingly obvious to me.

The first, is that, as I mentioned, it would seem that higher pressure, would create a more homogenized mixture. While I can't state that as a fact, it just seems like it should be the case.

The other advantage, is where things become really abstract, and has to do with the pulse width of the injection, and the effects on drivability.

In the case of a high performance engine, too small an injector, coupled with two low a pressure, means that the injectors can not physically supply enough fuel for the engine at the upper rpm ranges. That means the injector is continuously flowing, and yet the engine needs more. In short, you have a lean condition, at WOT, where it not only hurts performance, but can have devastating effects on the engine.

The next example, would be a high flow injector, at lower pressures. The injector is sized to have an excess reserve at the upper end of the rpm range, so there is no problem there. The problem occurs at low demand, where the computer has to really limit the pulse width of the injector, and at lower ranges, it may not be able to open and close the injectors quickly enough to effectively control the mixture, which means that it may run overly rich, effecting economy, and to a degree, performance. Obviously, a slight rich mixture at low rpm, is better than lean at high, but it still is not optimum, and you are throwing money away, and sacrificing drivability. more likely, it would continously hunt for the ideal AFR, first too lean, then too rich, but never be able to nail it down.

The last example, the one which I believe to be optimum, is to run a smaller injector at higher pressure. At the upper end of the power band, IMHO, the optimum duty cycle(ratio of open versus closed in the injector) would be in the neighborhood of 80-85%. That would leave a reserve at the top, and would allow the computer greater control of the duty cycle at lower rpm, thereby allowing it to more accurately influence the AFR.

The thing to remember, is that, while I have not found a specific reference, and I'm sure it varries from manufacturer to manufacturer, it takes a finite amount of time to open and close the injectors. There is some tuning to find out to what extent it effects the drivability, but there is no question that it is there. It is a simple matter to tell the injectors to open, but if it has to happen too quickly, the injector may well not close quickly enough, and thereby cause the engine to search to a small degree, with a small cycle of the AFR, first too rich, then too lean, as the computer tries to compensate. The degree of the cycle is likely very small, but it would be there, and is easily seen when looking at the datalogs.
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Last edited by DAHoyle : 12-20-2010 at 10:03 PM.
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