GM TBI unit size, airflow, and fuel pressure showdown

[PlasticBoob]

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?

[DAHoyle]

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.

[PlasticBoob]

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.

[jaber]

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...

[DAHoyle]

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.

[jaber]

Wow, that's impressive. So the later TBI Tahoe pumps are capable of 45psi, but does the system actually use that?

I just mounted this pump to the frame of the J-20 for the caddy TBI.

http://www.autozone.com/autozone/parts/Airtex-Master-Fuel-Pump/_/N-8vcz2?counter=0&filterByKeyWord=E2000&fromString=search&isSearchByPartNumber=true&itemIdentifier=386516_0_0_

This is the specs on the site...

Maximum Free Flow Rate (gph): 40
Maximum Pressure Range (psi): 95
Minimum Free Flow Rate (gph): 30
Minimum Pressure Range (psi): 70

Is this pump rated for 70 PSI?

[FSJ Guy]

The E2000 pump is popular for TBI conversions if you don't go with the in-tank pump. The E200 pump works fine for the usual 10-13 psi that most GM TBI systems run at.

There may be some TBI systems that run at 45 psi, but most of the junkyard systems that people use do not run that high.

[jaber]

I was told this pump would also feed an MPFI set up.

[PlasticBoob]

I just mounted this pump to the frame of the J-20 for the caddy TBI.

http://www.autozone.com/autozone/parts/Airtex-Master-Fuel-Pump/_/N-8vcz2?counter=0&filterByKeyWord=E2000&fromString=search&isSearchByPartNumber=true&itemIdentifier=386516_0_0_

This is the specs on the site...


Is this pump rated for 70 PSI?

Yes, that's an MPFI pump. I don't know what they mean by minimum, as most high-pressure pumps will happily put out a regulated 13-15psi all day long and not complain. I'm running an external Bosch turbine #69100, and the specs are 90-100 psi, 50 gal/hr. It's for a late-eighties Ford E-150, 5.8L.

There may be some TBI systems that run at 45 psi, but most of the junkyard systems that people use do not run that high.

Yes, and that late-model TBI Tahoe must be running reeeeally tiny injectors and fat pulses if it's pushing 45psi!

[content deleted - I no longer support MS]

There are also some GREAT injector/throttle body flow rate calculators to be found there. They should answer any specific questions you have about your setup.

[Rocket Dog]

wow huge discussion on this.

I just rebuild A 401 .20 over and about 9:1 cr. small RV cam.

I just wanted a more reliable ride since I think that I'm going to tow and DD the waggy more than putting it off camber.


any how. I don't have much experiace and guess the first step is to get the harness built agian.

[AlsChopShop]

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.

i've seen them on craigslist for less than $100, and even some on this site for free or swap for 350 units.

are you sure your 401 can flow to what the 454 unit can put out stock? the amc heads aren't known to flow much air, so you might be stuck with trying to dial down a pretty rich mixture, defeating the point of the larger injectors and larger throttle body.

Al

[AlsChopShop]

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.

feed me literature that proves this.

i'm a diehard chevy guy with the 88-98 bodystyle and have NEVER heard of any TBI unit asking for more than the required 9-13psi. i highly doubt this is true. you are probably mistaking it for the vortec motors that do require high pressures. the fuel pressure regulator can't even be dialed in that high, nor does it hardly ever need to be adjusted. even in 'extreme' (note: extreme in the tbi world is considered very mild to the rest of the world because of the ECM's limit) modifications to the engine and throttle body you don't need to mess with the regulator much if any. the tbi unit can't do anything with 45psi.

Al

[PlasticBoob]

i've seen them on craigslist for less than $100, and even some on this site for free or swap for 350 units.

are you sure your 401 can flow to what the 454 unit can put out stock? the amc heads aren't known to flow much air, so you might be stuck with trying to dial down a pretty rich mixture, defeating the point of the larger injectors and larger throttle body.

Al

She runs beautifully - lean where she should be and rich when it's needed. I am close to the limit on my idle pulsewidths but still within the acceptable range for PWM. No other hiccups, I'm one happy camper. Remember, the 454s these came off of only made about 230hp. I'm making at least 280. According to this article, the 454 heads were pretty bad:

On the heads, the stock TBI peanut port heads are just plain awful but some porting and larger valves can at least make them tolerable. There is a reason the stock TBI 454 only made 250hp at its best from GM and this is the key reason. You can make about 320hp with all the other upgrades but to get real 454 power your going to have to change the heads.

[AlsChopShop]

She runs beautifully - lean where she should be and rich when it's needed. I am close to the limit on my idle pulsewidths but still within the acceptable range for PWM. No other hiccups, I'm one happy camper. Remember, the 454s these came off of only made about 230hp. I'm making at least 280. According to this article, the 454 heads were pretty bad:

not that many 454's came with peanut port heads from what i've heard, and those that did weren't all that bad unless you were trying to modify them. they had more low end torque with that design. and do we really look at Hp numbers anyway? i thought that was only for the hon-duh guys.

Al

[FSJ Guy]

I want one of those $100 454 throttle bodies and injectors, too!

Although I think I'm doing OK with my "regular" 350 TB housing and whatever injectors Howell sent me back in 2001.