Carb questions for blow thru turbo

I'm trying to get together everything i need to start this turbo build and i'm having trouble with carburetor selection. I don't want to spend $700 on a carburetor when the whole turbo system itself won't cast that much. I have a summit racing 600 cfm carburetor and it works really good. I was wondering if I could make this one work (http://www.summitracing.com/parts/sum-m ... /overview/) since I already have it. If that one can't be made to work are there any other cheap carbs that are ready or just a couple of modifications away for a blow through system?

Thanks, John

I am not familiar with the carburetor you have there, but I can tell you that if you know what is required to make a blow through carburetor work, you can modify ANY carb to do so.

You really only need to address a few things.

First, if the floats are hollow brass, they must be replaced with nitrophyl units as the brass floats would crush when the bowls are pressurized.

Second, any check valve that may be in the power circuit must be removed or the the circuit will not operate under boost. The majority of your fuel should come from this circuit and it will be your job to determine how it works and to modify this circuit accordingly. Richening the mixture under boost is usually done by enlarging the pressed in power valve channel restrictions or PVCRs.

Third, the float bowls will have to be referenced to hat pressure or the carb will not function under boost at all. This may already be designed into your carb, but I have never had my hands on one of those and don't know for sure.

Most people will also tell you that you will have to remove the choke and mill the airhorn or else the engine may turn orange and implode, but I have the choke entirely functional on mine and it works well. I guess it would depend again on the design and how much pressure would be lost through the linkage etc.

You should probably consider a wideband mandatory to sort out your tune. They are key in making these build survive in my opinion.

Isn't a vacuum secondary also going to be a problem?

Oh man, i didnt even notice that. Mechanical secondaries would certainly be easier to work with, but holley vaccum secondary carbs have been made to work by actually reworking and utizing the diaphragm to open the secondaries under boost. A valve needed to be utilized to bleed boost pressure in those cases otherwise the engine would run wide open on the boosted secondaries even after the throttle was lifted. Mustn't that have sucked for the first guy to discover it, eh?

If you want to run about 12 lbs of boost, how do you set up the fuel pressure system?

Okay well are there any cheap carburetors that come ready for a blow thru setup or that are nearly there that I can finish by following instructions from the internet or a book or is this $700 dollar 650 cfm Demon carburetor a must have for blow thru turboing?

Thanks, John

Oh sure there are my friend. If you are comfortable with the idea of rebuilding a carb and making a few modifications and can follow some directions, you are in luck. 4150 series Holley carbs have been plentiful and dirt cheap in this country for a million years, and a ton of guys have made them work well for blow through so the leg work has already been done for you. Unlike blowing though a one barrel carburetor that has been out of production for 30 years. Then you are on your own and have to figure it out for yourself. Ask me how I know. 8)

You just need to pick up a new 4150 for a few hundred bucks or a used one for a hundred or so and perform the required modifications which are laid out very well for a beginner at the Hangar18 site. If you are reasonably handy you can do these mods yourself and they are basically free. See what you think:


http://www.hangar18fabrication.com/blowthru.html