Would a bigger pump, say Welbro 400 (instead of a 255)cause problems with a lower HP engine? All engine now with carburator(850 Proform DP) but probably FI and power adders later so I will need the bigger pump eventually. System will be -6 pressure and -8 return, metal lines, with Aeromotive regulator in engine bay. Pump will be in fuel tank. Looking at at least 600 HP to start. The 400 would be too much pump until further mods are done. I have been doing some research and some people have reported some problems with idle quality and fuel heat soak. If you are running a bigger in tank pump than needed are you having any problems?

No in tank pump exp but if you are SURE you will be needing a much higher capacity pump I would just reg it down to get the psi manageable. a constant bypass system works good for a regular carbed system but your FI plans may change that. Nothing earthshattering but here is a BTT for ya for people who know more about this

I had a similar situation and what worked for me was a pump controller with a micro switch on the secondaries of the carb and 2 pressure regulators. One by the tank that knocked the pressure down to 10lbs or 12lbs which allowed the one by the engine to handle the volume better and took some strain off the pump.

I have been using aeromotive A1000 external pump for the last 15 years. I have a return style regulator near the carb. I run 10am lines from the tank to the regulator and 10an return lines. 8an lines to the carb.
this pump runs at 50psi+ fuel pressure to the regulator. the regulator is a aeromotive one for carbs. I run 7psi to the carbs. I also have a barry gant fuel pump relay box with a high / low setting controlled by a switch under the dash.
I have yet to have a problem with to much fuel.

the biggest problem I had is venting the tank. the stock vents wouldn't allow enough air into the tank, I probably didn't have then routed properly. I has to put a 8an bung on the filler neck and use a braided line looped around the filler neck and a check valve at the end of the line mounted above the top of the filler neck.

Thanks for all the info. On further discussion with Walbro and Aeromotive engineers I have decided to go smaller and change out the pump in the future. Both parties agreed, though for different reasons, that too much pump (especially in Arizona where I live) would not be good. Both agreed that to much bypass is not a good thing. Fuel would get too hot.

Hey Frank,

What size pump did they recommend for 600hp? I am also looking for something to support 620hp with EFI.

The Welbro 255 I think would support that just fine. I'm looking closer to 700 HP and I'm going to go with the 255 but I'm using a carb. There is an inverse relationship between output and pressure. The less pressure (carb 5-10 PSI) the pump will supply more fuel and support more power. The more pressure (FI 40-60 PSI) the pump will supply less fuel. I think the 255 would support 625 EFI. I'm no expert. You can go on Tanks Inc web site and they have the data. I think I remember someone saying it does not take a lot of pump to support high HP. Its when you start to get into power adders is when I think you need to go big. Most everyone I've talked to recommends not to go big until you need it. I went big in my other car (Aeromotive Stealth 340) and so far have had no issues whatsoever but I don't drive that car very much and I have a large return line.

A Walbro 255 worked just fine on my twin turbo 440. It had harder fuel pump demand than what you're talking about.