maximum fuel pressure

[novadude]

How much can you safely run with a M4M 1975-up carb and a 0.130" - 0.135" needle and seat?  I'm finding a lot of conflicting info on the 'net.

[Cliff Ruggles]

Good question.  I've seen all sorts of different opinions on this topic, most have no testing to go with them.

I've tested all diameter seats, and different floats as well.

With the .135" seat and small float in the later carbs, they are fine to at least 8psi.  I use a larger .145" seat in my own carb (1977 Pontiac) at 7.5 psi and it is fine under all conditions.

I have at least one customer running a Super Stock q-jet that prefers the .125" seat and small float at 12psi, but most racers use a larger seat and a little less pressure.

The q-jets with the early style hinge pin location and large floats are limited for fuel pressure.  About 6.5 psi with a .135" seat is about the limit.  This is why they make a poor choice for big HP, but are still fine to at least 500hp with the right fuel delivery system.....Cliff

[novadude]

That's great info.  Thanks.

The reason I am asking is that I'd like to upgrade my mechanical pump and I'd like to avoid using a regulator.  Many of the higher volume pumps also have > 6 psi pressure.  Good example would be the RobbMc "550 hp" mechanical pump.  This is mostly a street car, and I don't think I need a "killer" electric pump and return system, as it is a mild SBC application that only runs street tires. 

I'd like to keep the plumbing a bit more "stealth" if possible, which is why I don't want a regulator if I can avoid it.  My carb is a Edelbrock 1904 style, so it would have the later float set-up.

[Cliff Ruggles]

I use a 1977 Pontiac carb, so same basic set-up as the 1904.  With a .145" inlet seat, and small float, I run 7.5 psi fuel pressure with zero issues anyplace.....Cliff