TBI Throttle Position V.S. Power Output

I have a quick question... Since I'm putting the TBI setup into my Caprice it's resurrected the question I've had for quite some time now but never asked anyone about. I've noticed in all GM TBI equipped vehicles I've driven, and so have a few others that owned older TBI equipped vehicles, that the engine has more power at about 3/4 throttle than it does at actual WOT. Literally there is less power output at WOT than there is at a lesser throttle position like 3/4 to about 7/8. Why is that? My guess is that the injectors can't put out as much fuel as there is air coming in, but that's just a guess. But I have yet to drive a vehicle with TBI that doesn't do this. My truck does it, even with a brand new fuel pump, injectors and TB rebuild. My '88 Van did it, as did my grandfather's '93 Caprice as did my buddy's '89 Cadillac and his 1990 Corsica (that had a 1bbl TBI). Is there anything that can be done to change this? This is more of a curiosity thing than it is a problem.

I have a quick question... Since I'm putting the TBI setup into my Caprice it's resurrected the question I've had for quite some time now but never asked anyone about. I've noticed in all GM TBI equipped vehicles I've driven, and so have a few others that owned older TBI equipped vehicles, that the engine has more power at about 3/4 throttle than it does at actual WOT. Literally there is less power output at WOT than there is at a lesser throttle position like 3/4 to about 7/8. Why is that? My guess is that the injectors can't put out as much fuel as there is air coming in, but that's just a guess. But I have yet to drive a vehicle with TBI that doesn't do this. My truck does it, even with a brand new fuel pump, injectors and TB rebuild. My '88 Van did it, as did my grandfather's '93 Caprice as did my buddy's '89 Cadillac and his 1990 Corsica (that had a 1bbl TBI). Is there anything that can be done to change this? This is more of a curiosity thing than it is a problem.

So you are saying that you get less power when you press the pedal all the way to the floor than if you only press it 3/4 to the floor? If so, my 95 9c1 does it... I thought it was something I needed tuned or something, but I have definitely noticed that it responds better at 3/4 than if you put the pedal to the metal lol

Agreed and I think it is a matter of air flow vs. fuel flow. I think the TBI injectors simply do not flow enough fuel at WOT in most cases based on the 9-13psi of fuel they get. Once you up the fuel pressure, or compensate with ECM programming, this will disappear.

Speaking of ECM programming, what would it take to make programming adjustable via laptop in an OBD1 ECM. Obvoiusly a new PROM would be required right?

An adjustable fuel pressure regulator will solve the problem. Set it around 12 to 14 psi. The stock nonadjustable setting is usually around 8 to 10 psi.

Or just go tunnel ram with dual dominators :grinyes:

The pressure regulator is the part in the center rear of the throttle body right?

Correct

The aftermarket adjustable ones for the truck throttle bodys will work as well for the caprice. Same as the throttle body spacers although the stock aircleaner assembly may not clear the hood with the spacer.

What would I achieve with a spacer? It doesn't seem like it'd do anything.

Would something like this work? Click on the picture.
http://static.summitracing.com/global/images/prod/norm/jet-61500_m.jpg (http://store.summitracing.com/partdetail.asp?autofilter=1&part=JET%2D61500&N=700+4294880914+4294908216+4294840140+115&autoview=sku)

that looks like a limiter tho.
not something that will raise the pressure.

That is it exactly Pete.

that looks like a limiter tho.
not something that will raise the pressure.
Limiter? What do you mean?

That is it exactly Pete.
I thought so but I figured I'd get a second opinion.

Check out what I did years ago to my 89.

http://notdadsw41.home.att.net/tbi.htm

Cool. How'd it work out?

Much better than the static adjustment of the JET unit.

So wait a minute, let me get this straight... you put that TBI in place of an E4ME carburetor? How did the computer control the injectors?

Greygoose006, it is a limiter in a way. Your stock fuel pump for a TBI motor is putting out about 20 psi maybe a little more. Your stock fuel pressure regulator in the TBI unit is non adjustable and keeps it anywhere from 8 to 10 psi on average. The aftermarket adjustable units allow you to adjust the output. The motor responds a lot better with 12 to 14 psi. I can't speak from personal experience since I have not done this mod yet, but Hotrod magazine had an article on this using a TBI 350 in a truck.

Noooooo Nooooo Noooo.... The 89 was already TBI. I just modified it.

Ohhh, Okay I kinda figured. Did it make any difference in how the engine performed? I could nab a junkyard TBI and experiment with it, but I don't want to waste my time if it doesn't make a difference.

With all of the TBI mods done it was much free revving. It pulls harder clear through to the shift points. It used to fall on its face up close to the shift points at WOT.

A replacement fuel pump will help quite a bit. I used a pump for 94-96 LT1 power b-body to be able to get the pressure up and keep it there. It was too much strain on a TBI pump. I did have to adjust the pressure down quite a bit though!!