How much horsepower will a 454 throttle body or a holley tb support. Holley says 275 but people on here say you can turn more fuel pressure to it and make more. I would like to know before I get started. I have a 91' rs and I have a 350 for it that I'm putting vortec heads on with a edelbrock intake and probably about a 268 or 272 cam. Will this even work with the tb injection or do I need to go with a carb. I would rather give up a little power and keep it. thanks

Holley says 275 because they don't include true 80 or 90 lb/hr injectors with their TB's.

You can turn the fuel pressure way up on TBI injectors (upwards of 30 lbs has been tried) so you theoretically feed over 500 hp. So far Fast355 has put up over 425 hp with 55 or 60 lb/hr injectors at 30 psi, so it's looking like it is entirely possible.

Lately there have been a few member playing with super chargers and getting close to maxing out the 90 lb injectors, it will be interesting to see what kind of numbers they put up.

I hope you realize that your motor will require lots and lots of DIY tuning, mail order won't cut it.

This has to be answered from both airflow and fuel flow standpoints.

From an air flow standpoint, 470+ hp is possible. This can be shown by scaling an LT1 2-bore throttle body up from 48mm to 50.8mm diameter, and by knowing that an LT1 with good heads (LT1 !), cam (GM LT4 HOT cam), and exhaust can make over 420+ hp at the flywheel.

From a fuel flow standpoint, TBI injectors were used by GM at factory 28-32 psi levels on the last production years of the 454SS truck, so copcar 9C1 injectors rated for 68 lb/hr fuel rate could move as much as 115 lb/hr using 32 psi. This works out to be:

2*115*0.9/.45 = 460 hp

using 2 injectors, 90% duty cycle, and a BSFC of 0.45 lb fuel per hp per hour. A more efficient engine with .42 BSFC could make 490+ hp. And a number of people have used higher fuel pressures than 32 psi, so higher outputs are possible.

None of the above deals with making the engine idle well, nor tuning, nor cam choice, nor headflow, nor exhaust etc. The above points on air- and fuel-flow are intended only to show what could be done all else being performance-wise equal.

Holley isn't going to quote any more than 275 hp because that's about what you avg person might obtain without really knowing what they're doing across modifications and tuning. So they're going to be very conservative. HTH.

I have a stock 7.4L GM TBI unit. My last WOT datalog showed 90 MAP. key on is 99. I will suggest I have a restriction. due to hood clearance I can run only a 14 inch by 2 inch round KN air filter. Do you think that filter can provide suffcient airflow?

I am investigating cold air but clearance and routing issues exist.

If I understand the units right, you have approximately a 1.47 psi (10.13 kPa) pressure drop across the air cleaner during WOT. I'll assume you have a 350 for what follows.

air_cleaner flow area = 14*pi*2 = 88 sq inches, or 0.0568 sq meters

pressure*area = force
10.13e3*0.0568 = 575.4 Newtons force

From personal experience I know that a 350 at 5000 rpm and 65% VE will demand around 388 cfm airflow. 388 cfm, or 6.466 cf/sec, flowing through 88 sq inches of filter area means that the average flow speed is 127 inches/sec (3.22 meters/sec).

Remember that force*distance = work, so if you divide that by elapsed time you get: work/time = power

force*velocity = drag_power
575.5*3.22 = 1853 N m/sec = 1853 Watts

745 Watts is equal to 1 hp, so 1853 Watts is a loss of 2.5 hp

Now I don't know if my calcs apply to your specific engine, but what I did in the above can easily be scaled to the actual engine size and actual engine rpm. So IMO you could benefit from using a bigger air filter, or ducting such that you use two remote air filters so that you can double the flow area. The avg flow through each filter will be half, but the pressure drop should be less than half of 1.5 psi because flow pressure loss goes as the square of the velocity, so if you half the velocity through each filter then you should observe a 1/4 reduction factor in drag force and hence power.

IIRC Fast355 has taken measurements on pressure drop across air cleaners & TBI units so he might have something to say about this. HTH.

Yes a 350 CID. Engine specs out for max HP at 5800 rpms.


Is your calcultion done at max HP or max TQ? I will guess max HP as the motor is an air pump. so I will redo your calc at 5800 and see what I come up with. But I will need a different calculator.

Different motor and all, BBC vs small block, but..

I have a stock 7.4l TBI on a non stock 454. Running higher fuel pressure helped the fuel flow situation, 27 psi WOT, vs. stock 12 psi or so.

Airflow is the real problem, i think. WOT at the end of the 1/8th mile shows about 90-92 MAP. Adjusting the IAC throttle follower to allow more IAC opening at WOT helps it to 94-96 MAP. The above is with the air cleaner and housing in place. Traps about 77 mph. Just for kicks I removed the Air cleaner and housing, no other changes. Trapped 80.45 mph. MAP about 97.

100 map, key off.

Not for sure if this helps, but does show the fuel problem can be solved with an external regulator, and maybe a different fuel pump.

My old 94 LT1 (also a 350) pulled 388 cfm @ 5000 rpm at approx 65% VE... so that's where those numbers came from.

Since you have a 350 also, and see max flow (and presumably max power) near 5800, then just scale everything by 5800/5000 = 1.16... or 16% higher loss. So you'd see 2.9 hp loss instead of 2.5, assuming everything else holds.

It's not a huge powerloss, but remember that the loss will get worse as the filter gets more clogged/dirty. That's why really large surface area filters are so good --- they still filter well but they have very little flow loss because the flow speed per-unit-area is so small, so the flow loss is quite small.

The analogy is breathing through a straw with a papertowel at the end, vs breathing with your mouth wide open with paper towel in front of it. You still need the same volume of breathing air, but when you use thr straw the flow speed has to be much higher, and so the flow losses are extreme. Using the big-area method, the flow speed is small and hence so are the flow losses.

So using big-area filters remotely located (hopefully in a cold-air-draw location), and then ducted (in your case, maybe 2 ducts) to the engine would be the optimum thing to do.