Hello guys, This is my first post on this board and I need some turbo help.
I have a Garrett turbo off an older Diesel truck that is set-up on a '74 350. There is an oil supply to the turbo and a pipe to the bonnet.
Here's my problem the boost from the turbo will blow through the carb (Holley 750 cfm manuel sec.) and push the needles closed, and the carb runs out of gas in about 3 sec.s. So I was told all I have to do is build an air tight box around the carb to equalize pressure and that should fix the problem. Is that true? That's all I have to do?
Also the turbo generates like 20psi of boost and I was told that a stock small block chevy should run no more than 8 - 10 psi tops. So All I need to do is install a blow-off valve after the turbo and before the carb. to fix this right??
Any help is greatly appreciated, Thanks Garrett

yes on the blow off valve (BOV), the fuel system needs a boost referenced regulator to add the extra PSI and a pump capable of delivering it. the highest PSI should be the boost pressure + the carbed fuel pressure with out boost = what it needs to be at full boost. 20 psi boots + 8 psi fuel without boost = 28 psi fuel pressure at full boost. the boost is closing the needlesas you said and/or actually pushing the fuel back out of the carb. the box method can help as well but the fuel system is still needed.

Thanks Tom,
So if I run an 8 psi BOV and the carb. needs 4 psi I need at least a 12 psi fuel pump. And I also need a Fuel pressure regulator? where can I get one of those and how difficult are they to install, how much, and is there a right and wrong one???
Again Thanks, Garrett

Garrett,
Try the Mallory model 4309. It has a vacuum port to hook up the boost reference line.

4309 I believe is currently under recall

Oh great. Do you know what for?

Hey Josh, I'm running the same setup as you, except I'm not ported yet. What kind of times are you running, and what porting did you do to the head.

BTW I have the Mallory 3 port too, so what's wrong with it that it needed a recall?

Woa, blowoff valves are generally not that adjustable and are not intended to control boost. They also do nothing for protecting the turbo (think: you’ve go the gas pedal mashed, BOV opens and the turbo will just keep spinning up till it sends impeller shrapnel through the engine). What you need is a wastegate installed in the exhaust between the turbine inlet and the exhaust down pipe. You set that for a specific pressure and it opens, bypassing some of the exhaust around the turbine preventing it from spinning up any higher.

Depending on what fuel pump you’re using, you might be able to boost reference it (basically install a fitting to introduce boost pressure behind the diaphragm so that as the turbo builds boost the pump pumps more fuel pressure. The alternative is what has been suggested, to run a fuel pump that can pump in excess of the needed pressure and then using a boost referenced regulator. You’ll want to run the line to boost reference it from either a point in between the throttle plates and the turbo or if it is routed to a normal vacuum port on the intake you’ll need to run a check valve so that it does not see vacuum. Enclosing the carb in a box will not solve the problem since it is you’ll still see more air pressure inside the box then in the fuel line going to it preventing it from filling the carb.

I'm not even tottaly sure it is that model. itmight have been the diaphram and I do know that some models suffer from a return orafice that is too small. it causes a surging fuel pressure especially with really nice pumps.

ATOMonkey,
I don't have times yet. I have beat a 2001 Z-28, that beat a 10 sec. Mustang, who said he could run 9's if the wind would blow. LOL, just kidding. The Z-28 part is true though. As far as porting, I just did some pocket work on the intake, and the exhaust side a little. In fact, the exhaust isn't even complete yet. I have been busy getting ready for twins to arrive, and just bought a house. Here are some bad pics of the heads.

And 2:

Last one:

I stated that wrong. The car doesn't have exhaust completed yet. There, thats better.

That's encouraging since you haven't touched the CCs or cut the valve bosses all the way down. Thanks!

No, I didn't do either. I was a little worried about messing up the flow of the heads if I went real far on the intake side. I knew they were touchy do to that little vane running up to the guide boss. Since the I/E flow % isn't great with these heads, I went ahead and did the whole exhaust side. I didn't go overboard, b/c I didn't have access to a flow bench to see what I was gaining. I did make up a cardboard template to try and get all the exhaust shape as close to each other as possible. The teplates extended to the throat of the port, so it was the best I could do without a CNC machine.