zacharyhorn

iTrader: (1)

Hey all. I have a 91 switched from tbi to carb and I'm running a pretty stout 355 combo. I am using the Mallory FPR to my holley carb. How much power will this stock pump produce? Will it work for my combo? Combo is in signature. I really don't feel like dropping the axle.

Zach

five7kid

iTrader: (14)

From the TBI forum stickies:

How much power can my stock fuel pump support?

The answer to this is not much. A stock pump can support a full exhaust, intake mods (manifold and air cleaner) and a very mild cam. Once you have all of these mods and you go with a nice set of heads or decently sized cam the stock pump will not be able to deliver the necessary PSI at wide open engine load. A high flow unit will supply enough power to your TBI system no matter what mods you go with. Popular choices include, stock GM TPI pumps, Walbro 190lph and 255lph, and the Holley 255lph. Each of these pumps can support gobs of power but will work safely with your TBI set-up because your fuel pressure regulator will keep the fuel entering into your TBI unit where it needs to be.

And that's for a 305.

I'd say prepare yourself for dropping the axle.

zacharyhorn

iTrader: (1)

Considering the stock tbi pump runs considerably higher than a mechanical pump, and I have the FPR turning the pressure down to about 6 psi, wont this work out ok? I have a couple buddies at the local shop that said it should be fine for a while. They told me to also watch after running that pump for a while to see if I would be able to notice a drop in power in the higher RPM range. I already have the regulator, I hope this will work for my combo.

camaronewbie

I've argued this point with some, and gotten no where really, but....

I would think there is quite a bit of difference between "pressure" and volume. And maybe it's a question of volume pumped at a given pressure, instead of pressure at a given volume pumped. For instance - I can turn my garden hose on where it just slowly runs out, say a 3/8 inch stream. My hose end (the hole) is 3/4 inch - so we aren't even at much volume. But, if I put my finger over the end, and apply enough pressure, thus decreasing the hole that the water has to run through, then I create pressure - no more is coming out (in fact I'd bet there's less "volume"), but there is more pressure.

For the carb swap, we are in need of reducing the pressure to the carb. This is accomplished in the pressure regulator by allowing more fuel to bypass back out the return line, thus reducing pressure on the feed line to the carb. On the Mallory unit, the adjustment screw increasingly closes the gap to the return line - you back off the adjustment screw, to decrease pressure to the carb, and screw in the adjustment to increase pressure to the carb.

However, aren't we decreasing volume to the carb as well, since by adjusting out, we allow more fuel to bypass out the return line, and thus less fuel is getting to the carb? We may be getting less pressure to the carb, but also aren't we getting less volume?

I have a 350, mild cam, headers, RPM intake, Edelbrock 600 carb, and TBI pump intank. My car more times than not falls flat on it's face when I punch it from a redlight - like it's out of gas. However, granny driving it does great, and even when I'm slightly into it - but it doesn't like pedal-to-floor from a stop, and never has since the swap.

So, experts, chime in - is this a volume problem I'm (we're) having? Is it a matter of the TBI pump not pumping enough volume, regardless of the pressure that volume is later regulated to?

five7kid

iTrader: (14)

TBI doesn't run all that much higher of pressure than carb (when compared to TPI, anyway).

There is some trade of volume for pressure. But, the volume needed is not a function of pressure. All the return-style reg does is maintain outlet pressure by regulating return flow - when outlet pressure drops (with higher demand), the return flow is reduced to make up for it. The return flow will be cut to zero with a TBI pump before it will with a TPI pump.

Think of it this way: The "power" for the fuel pump itself is a combination of volume and pressure. So, a TPI pump has to be more powerful to provide the pressure and volume for that system than a TBI pump has to be. If you lower the pressure for either pump, it will be able to provide more volume up to its power limit. Therefore, at carb pressure, the TPI pump, being the more powerful, will be able to supply more volume than the TBI pump will be able to.

Will your TBI pump supply sufficient volume to feed your stout 355? Maybe. Would a stock GM TPI, Walbro 190lph or 255lph, or Holley 255lph pump do the job? Without a doubt.

zacharyhorn

iTrader: (1)

Thanks for the info. I guess once I get her on the road, I'll have to see how it runs. Camaronewbie, could your issue be a carburator tuning issue?

five7kid

iTrader: (14)

I would say it is. Punching it WOT from a stop is not a condition where the carb would be starved for fuel - it should be full up, in fact.

How would you fix the problem? I'm not sure, because I don't deal with those carbs much because I basically just don't like their design. Most likely it's an accelerator pump issue, or a secondary air flap problem. Can you tune it to get rid of the problem? Probably - but the basic design of the carb still stinks for other reasons.