GM TBI unit size, airflow, and fuel pressure showdown

[PlasticBoob]

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

Haha you're right, I completely agree, that's why I built my Jeep with a single exhaust and low/mid range cam. But HP is more connected to airflow/high rpm than torque, and I do drive it a lot on the street and highway, so drivable horsepower was a factor for me. I wanted lots of low-end torque, but I also didn't want the engine to be a dog over 3,000rpm.

Oh and I've always heard from the old timers on here that dogleg AMC heads were some of the best flowing ever put out on "normal" engines.

[PlasticBoob]

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.

I'm in Long Beach, so if you want to hang out some time just let me know! I can come up there or you can come visit if you want to take a look at the rat's nest under my hood. I'm going to be re-wiring it soon, it's a mess.

[PlasticBoob]

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.

Ethan, I'm not an expert either, and hate doing math, but the restricted flow of the 350 TBI unit may another factor in addition to elevation that's hurting your acceleration times. Depends on your engine mods, etc.

I see 350 TBI ratings from 480-500cfm. A stock 401 in '74 put out 215hp with a carburetor, and a TBI 350 makes around 210hp. So if you're modded slightly you may well be losing a bit of power in the high end, like having a restrictor plate. Though your elevation might make up for running a smaller throttle body. Someone would have to bust out the math/Desktop Dyno to know for sure.

[DAHoyle]

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

I'd like to make a few points. First, just because you have never heard something, doesn't mean it isn't true. I specifically looked into just that question, when I started considering modifications, so I just might have a little more insight on the subject. Since you apparently aren't aware of it, I can assume you are somewhat less knowledgable than you would like to believe.

Many(relative term) people adjust their fuel pressure, and it does, or can have emmense benefits, when you start tuning the injection for higher horsepower, bigger cams, different heads, or just different fuel requirements in general. A fuel pressure adjustment is generally the first place that a tuner starts. Almost everyone with a tuner skips straight to 12-13 PSI, and many go well beyond that. Generally speaking the factory pump(pre 95 tahoe) starts falling on it's face at 16-18PSI, so to get higher, the pump must be replaced.

Lots of folks run "extreme' systems, as you put it, with bigger cams, free flowing heads, larger displacements, and even a couple of twin setups running on tunnel rams, as well as cross rams. Not everyone finds the factory computer as limiting as you do, and have found ways around it. Even a few small airflow imrovements will quickly overwhelm the stock pressure of 9-13 PSI, at the top of the power band. Guess why so many folks had problems with lean running trucks when these things first came out. Hmmm, think I'll toss some headers on this baby, and lose the catalytic converter. Yep, you guessed it. Not enough fuel from the factory system, to even manage that tiny little mod. Over time, some folks figured out how to adjust the FPR. You're just about 20 years behind the times. Of course, everyone blamed those darned computers, but the fact of the matter, is that the program kicked out of open loop operation at about 75% throttle opening, and had a fixxed fuel curve programmed in. Change the fuel requirements, and the computer didn't know it. It was left out of the loop, so to speak. You could reprogram the chip, and in fact there was an entire market that opened up, offering reprogrammed chips, that essentially did nothing anywhere in the power band, except in that top 25% of throttle. Guess what they did. They fed it more fuel. They also sucked the big one, but hey, they sold like hotcakes. The smarter folks started tuning, and in most cases, they started with fuel pressure.


Second, It is a very simple matter to change the fuel pressure, by modifying the spring pressure eithe by shimming the spring, installing stiffer springs, converting the factory regulator housing to adjustable, or even vacumme referencing it. GMPP even sells a vacume refferenced pressure regulator. When you start climbing even higher, then bypassing the factory regulator and installing an external, becomes the fix.

Seriously, if you are limiting your thinking to the factory programming, then you are probably right, but there is a huge world out there that you apparently are unaware of. There are more than a few TBI fire breathers out there, well over twice the factory power ratings.

Lastly, I don't feel that I owe you any proof, just because you don't know what you are talking about, butr since you asked so nice(that's sarcasm, by the way)

http://www.edelbrock.com/automotive_new/misc/tech_center/install/3000/3501.pdf

You will note the very first caution statement, right under where it says:
STOP!
BEFORE BEGINNING INSTALLATION,
READ THESE IMPORTANT NOTES…

While that isn't really proof, it does tend to make one believe that the '95 tahoe has the required pump for the kit.

How about an actual installation in the vehicle with the mythical pump. Would that convince you? Yep, a '95 Tahoe, with a TBI, and a high pressure pump.

http://www.sporttruck.com/techarticles/0412st_chevy_truck_tbi_to_efi_conversion/index.html

Since the Edelbrock system requires 45PSI, and you don't have to upgrade the pump in a Tahoe, well, guess what. That means the Tahoe has a pump that puts out at least 45PSI. Does it need it, nope, probably not. Can the factory TBI setup work with that, well, going to have to take a shot in the dark, and say that if it comes with a TBI LO5 engine, and a 45 PSI pump, (both equipped by good ol Chevrolet) I'm going to have to guess that it will work just ducky.

On top of that, it is somewhat curious that AC Delco lists the 95 Tahoe fuel pressure regulator, with only one vehicle that it is compatible with. Care to guess what it is? Oh, here's a clue. It is a '95 Tahoe. I have read the regulated pressure was 30#, but I have been unable to find that reference, so I didn't include it in my previous posts. I will find it tho.

Seriously, you are not the expert that you seem to think, and quite honestly don't know squat on the system you claim to know so well, other than it's stock applications. There are plenty of folks that go way farther than that, and have increased both power, and economy of the factory TBI setup, with some innovative tuning.

What do you think Howell does, to get away with running the system on non-compatible engines. Oh yeah, they change the program.... They aren't the only ones who know how, and there are plenty of semi intelligent shade tree mechanics who do exactly the same thing. It doesn't take a rocket scientist, and pretty much the tools of the trade are an ALDL cable, a laptop, and some Data logging software. From there, you can either burn your own chips, or send your log to another hacker who can. If you really want to play, then you can skip the whole chip burning thing, and go straight to an emulator. http://www.dynamicefi.com/

[FSJ Guy]

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.

Poor Rocket Dog. We're gotten SOO far off topic, it's not even funny!

OK, yes it is.

Rocket Dog, aside from grabbing ALL the parts from the donor vehicle, what else do you need help with?

[srobertsfsj]

Poor Rocket Dog. We're gotten SOO far off topic, it's not even funny!

OK, yes it is.

Rocket Dog, aside from grabbing ALL the parts from the donor vehicle, what else do you need help with?

....and no one answered my question either

What would be the best year/make/model donor vehicle to look for at the junkyard for the 350 TBI setup?

[DAHoyle]

....and no one answered my question either

What would be the best year/make/model donor vehicle to look for at the junkyard for the 350 TBI setup?

Why would you assume that there is a "best one"?

They are almost all functionally identical. Some minor programming differences, among other things(different injector flow ratings), but pretty much all the same hardware.

the obvious exception would be the aforementioned BBC which has a different throttle body, and the 95 Tahoe.

[PlasticBoob]

DAHoyle, I just woke up and didn't read your post thoroughly, but here are my thoughts:

Does it need it, nope, probably not.

I see there's really no argument here then.

* The 1995 Tahoe put out 200hp. Stock, it clearly doesn't need 45psi. Chevy knew they were switching to the Vortec the next year, so maybe for some reason (ordering, parts stocking) they equipped '95 Tahoes with the '96+ SFI pump. I'm not doubting the pump is capable of 45psi, it's very reasonable to assume that it can put out that or more, since we have a regulator that is regulating it down.

* The instructions you listed were for Edelbrock's MPFI - I still don't see any TBI system that says it requires 45psi, though I guess it could certainly be possible on the aftermarket units that run FOUR injectors. 45psi just seems really high for twin-injector TBI. Again, going by the flow-rate calculators, the injectors would have to be REALLY tiny for 45psi, because from what I've found in my research, 45psi on stock 305, 350, or even 454 injectors should make them stick open. They simply can't close under 45psi.

Here is an interesting discussion I found on another forum:

http://www.gm-trucks.com/forums/index.php?showtopic=8229

My book had a misprint. through 95, it is 9-13 psi. 96 and up is 60psi for SFI systems. Mines a 95. It should have said 96 and up, not 95 and up so it messed me up.

Have a good one, guys.

[FSJ Guy]

....and no one answered my question either

What would be the best year/make/model donor vehicle to look for at the junkyard for the 350 TBI setup?

Then go start your own thread.

GM Astrovans often have the 1227747 ECM that is so popular. But they have the V6, too.

You'll want a TBI off a 350 so that you get the larger injectors (V6's have smaller (lower lb/hr) rated injectors, IIRC).

If you CAN fine it, the ASDU broadcast code is a good starting point. If you are burning chips, then of course it doesn't matter what chip is in there. But you DO want an ECM off a 350 so it has the correct "limp home" chip. The V6 limp home chip won't get you very far in the event of a failure. But failure is rare, so if you HAVE to get a v^ limp home chip, it's probably not a big deal.

As always, grab EVERY sensor you can get. Even if it's no good, now you know what to ask for @ the parts store.

Or you could just buy Michael's TBI. He's had it for years and hasn't installed it yet. Once his daughter's out of college, he might get to it. But by then he'll want the hover conversion kit instead.

[PlasticBoob]

....and no one answered my question either

What would be the best year/make/model donor vehicle to look for at the junkyard for the 350 TBI setup?

Sorry about that, there's a lot going on in this thread. Like DAHoyle said, one 350 system is just as good as the next. HOWEVER, there is a certain model # ECU that is much more desirable because it can be modded six ways from Sunday. I think it's the Boeing 747? 7747? Something like that.

[DAHoyle]

* The instructions you listed were for Edelbrock's MPFI - I still don't see any TBI system that says it requires 45psi, though I guess it could certainly be possible on the aftermarket units that run FOUR injectors. 45psi just seems really high for twin-injector TBI. Again, going by the flow-rate calculators, the injectors would have to be REALLY tiny for 45psi, because from what I've found in my MegaSquirt research, 45psi on stock 305, 350, or even 454 injectors should make them stick open. They simply can't close under 45psi.

Here is an interesting discussion I found on another forum:

http://www.gm-trucks.com/forums/index.php?showtopic=8229

Have a good one, guys.

Yes, the instructions were in fact for an Edelbrock MPFI kit, with the specific purpose of converting a TBI setup to MPFI. They even use the stock throttle body on the new manifold. I was not using it to state that all TBI system needs that pressure. I only brought it up because AlsChopShop decided to call me out on the pump, and the vehicle it was in. I would guess that you are on the right track about why gm did it. It is irrelevant. My only point is that they DID do it, and just because AlsChopShop didn't know it, doesn't mean that it isn't fact. His post was both condescending, and totally wrong. That's a poor combination.

On the other hand, I have seen references of 30PSI for that vehicle, altho I can't find it right now, so I'm going to guess that gm did some innovative programming in there.

The idea that fuel pressure is not done to tune the engines, is also totally wrong, and we have seen many examples mentioned just in this post, not only by myself. One was a very long example linked to by none other than you.

http://www.megamanual.com/v22manual/minj.htm

Simply put, tuning the fuel pressure, in combination with reprogramming, is and has been, a valuable tool for those who want to more with the TBI system than GM originally designed it for, and to say that nobody needs more than 13PSI is the same as saying that nobody needs more than 200 HP, literally as well as figuratively. I guarantee I can find dozens of threads that describe TBI Builds that go from mild to wild with pressures from stock, to well over double that.

[PlasticBoob]

Ah hah! Found more info in this site: http://www.dynamicefi.com/TBI_Fueling.php


First, more CFM numbers"

In the 2-bbl TBI series the popular units are found in three sizes:

• 4.3l, 5.0l, and 5.7l engines:

• • have 1-11/16" bores and flow about 490 CFM

• 7.4l engines:

• • have 2" bores and flow about 645 CFM

Just watch for the '94 - '95 BBC truck units as the injectors only flow 46 #/hr at 13 psi. Which is 74 #/hr at the 30 psi these TBI units use. On the up side they come with a 30 psi FPR. The down side is that the injectors are small.

Notice how he only mentions this for the Big Block engines. If this is true for the 350, then there you have it. Just like I thought - if Chevy was running TBI at 30+psi, they'd have to run smaller injectors, and apparently they did, on the 454 anyway.
The injectors I'm running flow ~80lb/hour at ~12psi.

[PlasticBoob]

My only point is that they DID do it, and just because AlsChopShop didn't know it, doesn't mean that it isn't fact. His post was both condescending, and totally wrong. That's a poor combination.

Hey, I can't speak for him, but at least we're all debating and getting more EFI info out there for people to read up on! I'd love to see IFSJA have an active fuel injection sub-forum.

[FSJ Guy]

I'd love to see IFSJA have an active fuel injection sub-forum.

But then we'd need a Megasquirt section, a TPI section, a re-programmed PROM section and a section for those who aren't programming.

[Joe Guilbeau]

Hey guys/gals

If you really want to know the details and not waste a lot of time quoting he said/she said references on the WWW, try a U.S. Patent Search to read the patents on the items of question.

You can learn the GM Throttle Body Injection ECU functions and theory of operation along with specifiec pulse width modification signal fed to their injectors. It is all there, and it is from the folks (engineers) who actually designed and wrung out the prototypes and when they got a working prototype they slapped a patent on it.

In any case it has been mildly entertaining to read the posts so far, and so I offer some additional information that may be of some possible use to some of the readers.

The subject matter below is in reference to flow rates, and injectors and programming mainly on Holley Pro Jection offerings (not as a thread hijack, but as a snapshot of what will work on an AMC-360).


For reference:

The Holley Pro-Jection 2-Injector Throttle Body Injection flowed up to 670 cfm at 3.0 inches of manifold vacuum (i.e. about the same performance as 4-barrel carbs).

The injectors were high-flow, low pressure capable of 80 pounds of fuel per hour which their manuals say is enough to supply a 275 horsepower engine.

U.S. Patent Number 4,708,117 will give you all the theory you need to know concerning Fuel Injectors (Holley injectors used)

The pressure regulator used was factory pre-set for 15 P.S.I. and adjustable between 12 P.S.I. and 18 P.S.I.

Not all of the fuel was utilized by the fuel injectors, so a return line is required for the excess fuel. (This means that you match the fuel pump to the injectors flow rates)

Holley also advertised Fuel Injectors P.N. 522-9508 for up to 1,520 Horse Power, these units flow 95 P.S.I. and there are eight of them, Summit Racing charges $800 for the injectors.

For you GM TBI guys/gals this site seems to have some rational suggestions
http://www.tbichips.com/truckmods.htm

WINALSL
http://www.tbichips.com/winaldl.htm

Some examples and write ups about project trucks
http://www.tbichips.com/jaredproject.htm

Play nice...