I'm almost done with my turbo setup, just need a carb and a few other items. I'm looking at either the holley 2 barrel 500cfm or edelbrock 4 barrel 500cfm. What would be the better choice for the street on a 250c.i. and 6 psi blow through? I have a clifford intake and custom exhaust manifold.
Thanks.

The 2 bbl Holley will work good with low boost and will start being a cork over 325hp. A 4bbl 500 will be better, not sure about the Edelbrocks for a blow thru setup. Don't know what mods need to be done to them to work.
A small Holley double pumper with mech secondaries in the 500 range would be great, they do not make one. You can get a 600 though.

I think 600cfm may be too much for my lower rpm street motor. If I can find good instructions on modifying the edelbrock for blow through then go with 4 barrels? My power goal is right at 325 hp.
Thanks for the help

All of my research so far is steering me towards Holley for blow through set ups.

It too bad Holley does not make a 450-500 CFM double pumper. It would be a great fit. Have thought about modifying a 450 to do it. Tom

A 450 Holley? I'll check into it. Apparently the holleys design makes it the absolute best for blow through other than C&S type. The 390cfm is probably much too small for boost?

You can get the 450 as a mech. Or vaccum secondarys.You can also change the back half from a mertering plate to a metering block by useing the meter block off a 390 this is what had told me a few yrs back. And if you change over to center hung float bowls you can then have a dual feed instead of the rod the feeds from one bowl to the other.

If it's blow-through, it can be any size that works with the engine N/A - the extra flow comes from the increased density. Roughly RPM × displacement ÷ 3456.
If it's draw-through, it has to be large enough to supply the higher power level. RPM × displacement ÷ 3456, then × by the pressure ratio.
Assuming 14.7 as ambient:
5 psi = (14.7 + 5) ÷ 14.7 = 1.34
10 psi = (14.7 + 10) ÷ 14.7 = 1.68
15 psi = (14.7 + 15) ÷ 14.7 = 2.02

http://www.theturboforums.com/smf/index.php?topic=136075.0

http://www.theturboforums.com/smf/index.php?topic=22107.0


MBHD

OK. It's a blow through setup.

(4000rpm)(254c.i.)/3456 = 293.98 CFM??

So I think I'll just run the Holley 390cfm four barrel. I've used on on this motor naturally aspirated and it worked great.
Thanks again for all of the help!
I'll post my progress in a few weeks.

OK guys, Just talked to Randy from the Carb Shop aka customcarbs.com

I'm going to supply them with an old used mechanical secondary holley 390cfm and they will send it back tested and ready to run under boost!

So, I'm in the market for a mechanical secondary Holley 390cfm if anyone has one laying around.

You don't need a bigger carb to blow through than you would put on the motor naturally aspirated. Sometimes a smaller one actually works better. If you have the 390, I would definitely use it, as the price is right. I'm an edelbrock guy, and they can be used, but the lack of solid floats become a liability after a certain point.

I am using a Holley 2300 350 cfm 2 bbl. It works really good and easy to mod.

Thanks for the info guys!