PaulD

I've read where people have swapped in a complete 454 TBI to get better fuel flow and either bored the manifold or used an adapter on the stock manifold.

Has anyone simply used the 454 injectors in the small block TBI and reproggrammed for the higher fuel flow?

I am working with the vafpr to operate at higher pressure but I was wondering why I've never seen this combo discussed.

brodyscamaro

those 74 pph injectors are a lil too much for most SBC engines

PaulD

I know they're big for a stock engine but I was wondering if any of the unscrupulous modifiers who used the BB TBI on a modified SB engine considered using just the injectors to support 300+ hp instead of swapping the whole TBI with it's 2" bores.
This way additional fuel could be achieved without being concerned about the difference in bore size of the SB vs BB TBI. Naturally, the ECM would have to be modded, but that would have to happen anyway.
THis just seems to me to be a possible way to raise the HP potential of the SB TBI without the headaches of the larger throttle bores to be concerned about.

TBIWorks

iTrader: (2)

I was toying around with that same idea. Also, I might even try the whole 454 TBI with the injectors, instead of just the injectors. I'm building a hot 350 for my RS, and I am considering all my fuel delivery options. One thing I figured was that the Big Blocks were making 245hp. Surely a 350 making 100-125 more hp than that could use those injectors. But also, what about just trying stock 350 injectors with the fule pressure cranked up. You could acheive 70pph esily I'd think. What do you guys think?

brodyscamaro

you would also need more air not just more fuel

kdrolt

He knows.

The point was that the 305/350 throttle body itself (with 1 and 11/16" bores) has a cfm rating (maximum) that will flow enough air to conservatively support over 310 fwhp. The math is in the archives. So air (from the throttle body) is not the problem, so long as the everything else is suited to the task (cam, timing, ECM cal, heads, exhaust, intake).

The main problem is flowing enough fuel. No matter what the fuel system is, nor the engine size, you need approx 0.6 to 0.7 lbs_fuel/hr per fwhp.

Port fuel injected examples:
Bcar LT1, 260 fwhp * 0.734 = 192 lb/hr or 24 lb/hr per injector

Fcar L98, 245 fwhp * 0.734 = 180 lb/hr, or 22.5 lb/hr per injector (stock are 22)

TBI examples:
Bcar L05, 205 fwhp * 0.634 = 130 lb/hr, or 65 lb/hr per injector

Fcar LO3, 170 fwhp * 0.634 = 108 lb/hr, or 54 lb/hr per injector.

(Note that the conversion factor is different: 0.634 for TBI, and 0.734 for port f.i. I suspect that the duty cycles are different so the conversions are different.)

If someone is only shooting for 300 hp, or less, then the smaller throttle body will probably run better at idle/cruise than the larger one will (because the faster airflow through the smaller venturi will promote air-fuel mixing better, at least until the smaller TBI body starts to become severely flow limited). And the cost is less because you are only swapping injectors, so you don't have to hunt for a hard-to-find throttle body, or have your own machined to the larger size.

I suppose another way to state this, is that if you are shooting for well over 300 fwhp, you should use a throttle body with 2" bore diameters. If you aren't shooting for well over 300 hp, you should stick with the stock size throttle body. In either case, you need a combination of fuel injector size, and fuel pressure setting, to provide enough fuel to make the power you are aiming for.

What Paul was asking, and what no one has tried (to his/my knowledge) is if anyone has taken the big injectors and stuck them onto the small TBI body -- and how well did it work?

And FWIW:

74 lb/hr * 2 = 148 lb/hr
148 lb/hr / 0.634 lb/hr/hp = 233 fwhp

which is low for what most people want from a 350, but that's with the stock fuel pressure. If you used the vFPR and set it for 20 psi at WOT (and 11 psi for idle/cruise), then the fuel increase would be sqrt(20/11) = 1.34 or 34% more fuel and 34% more max power at WOT. That means 1.34*233 = 315 fwhp. I know tuning this well would be a chore, but at least there is enough fuel provided to keep up with the airflow of the 305/350 throttle body. FYI, FWIW. - Ken

TBIWorks

iTrader: (2)

what a great response. my only question now is(I posted on this), so if you use the stock throttle body with stock injectors(with higher fuel pressure)on a warmer motor, will the result be just less power, or will it be running lean?

PaulD

TBIworks;
My experience (with mods in sig) is that I run out of top end and start to run lean.

Yes, you can use higher pressure but then you are too rich at lower RPM. Not a big deal if you're always racing, but for most driving the engine is below 2500 which makes it too rich most of the time. You can usually bump it a little by shimming the spring ( the "quarter mod"). But don't expect a big difference.

What we're trying to do up here is use the vacuum assisted pressure regulator which raises the fuel pressure at low vacuum situations and lowers the pressure at idle and low revs. There are strings regarding this subject but, essentially, what we've found is the spring that comes with the vafpr is too stiff and keeps the pressure too high. THere are lighter springs about 15#/in compression rate that work better all around.
I haven't tried adding the vacuum delay yet but plan to because a sudden stomp of the gas pedal from stand still (But who need to do that????? ) results in a very rich condition and you can feel the loss of power.

If you decide to go this way, I would gladly send you a spring in exchange for you sharing your accomplishments. I bought a pack of 5. Sounds like we're both trying to get more fuel to feed a modified 350. Make sure you at least have cop car injectors, at 65pph, they are GM's biggest 350 injectors.

I don't happen to think more air is a problem yet ( thanks Ken, I don't have the patience for entering all that