I have'nt even gotten the car on the road yet and I am already thinking of a winter project. I have access to 2 garret t4 turbos, and blow off valves. I am not clear on turbo setups and am wondering if it is difficult or if it even makes sense to run these with a carb? Lots of fabrication I know but it would be fun.

Any info would be great.

 

Yes, you can run a blow-through configuration, the carb will have to be modified for it though.

If your good at fabricating I am sure it will be a fun project, be sure to post pictures!

 

Well, you can do a "draw through" setup, where the carb is above the turbo in the airstream. However this takes a lot of room and has a lot of lag. A "blow through" puts the carb between the turbo and the intake, but then you have to seal up the entire carb in a pressure box and also set up a fuel enrichment system to compensate for boost.

Easiest is to do a turbo with fuel injection. Or better yet, they make supercharger kits that have a sealed box for carburetors. You get everything in one kit. I have seen them in the Summit Racing catalog.

 

I could be wrong here, but I am almost positive I have seen many setups with out using a carb box. The carb will have to be sealed up, and the fuel pressure will have to be boost referrenced, but I think you just need a carb hat, and the proper carb to do the job...

 

That's what I was considering, is fabbing all the pipeing, and carb hat. The carb itself is my big concern, I am thinking that a holley or a speed demon would be a better choice over my edelbrock, does anyone know what needs to be done to any of them.

vetdrms: when you say the fuel pressure would have to be boost referenced, do you mean that the fuel pressure would need to increase under boost?

 

all the info you need is in the article but now you can buy the carb right from JEGS and just bolt it on. this article goes into great detail and even list all the parts and prices that he used. twin turbo would be the same just the carb hat would be different. and you would have no room in your engine bay anymore. good luck

http://www.highperformancepontiac.co...0309pon_turbo/

 

I'm not the carb expert, but increasing the fuel pressure in a carb doesn't do too much unless you have the proper jets. The floats shut off the fuel flow into the bowl when it's full. Having higher fuel pressure won't help if the jets won't pull it. The problem with boost and carbs, is that the carb doesn't have a way to compensate for the denser air under boost, so it runs lean. By enclosing the whole carb, the increased pressure is the same inside the carb, so more fuel flows through the same size jet.

EDIT: I read that article, and I didn't think about the fuel being pushed back into the tank. However, they still don't address enrichment under boost. Simply changing the jets makes the engine run rich over the whole range. If you build a system that builds boost quickly, maybe this isn't a big deal. Otherwise, you will be running rich under no-light boost.

I would also look at fuel injection. That is my next big project. I already have the Megasquirt computer, and a throttle body from Air Sensors that bolts right up in place of the Quadrajet. The system I am using also using distributorless ignition. The system uses a MAP sensor, so boost is no problem.

 

There was a commercially available set-up from Martin that included the manifolds, brackets, turbo, etc. for a suck-through system that fits the stock hood. I occasionally see these for between $500-$1,000. I think one is currently available for $800 in the Recycler (available on-line). The air plumbing is relatively short for better response. The turbo sits on the exhaust manifold and draws air in from and pushes air out under the carb.

One of our members has one on his '78 and said it provided about a 50% boost in power. Unforetunately, he can no longer smog it in CA and it mainly sits. (I think he's going to register it out of state and put it back on the road.)

 

www.turbomustangs.com

there is a sbc forum in there with tons of info and pictures....

good luck

Joe

 

You're mixing technologies (injection, which works on fuel pressure and carburetion, which works on air pressure).

Carbs always flow fuel based upon airflow through the venturi. As long as the air pressure in the venturi is lower than the air pressure in the bowl, it will flow fuel.

The enrichment comes from referencing the fuel pump pressure relief to a different atmosphere. Six psi of fuel pressure out of the pump at one atmosphere is different than 6 psi out of the pump at two atmospheres.

An enclosed chamber takes care of a lot of the setup difficulties with a blow-through system since both the carb throat and bowls see the same atmosphere and all that has to be referenced is the fuel pressure.

 

Gerry,
The pump doesn't actually pump fuel through the jets. Correct me if I'm wrong, but the pump puts fuel in the bowl. If the floats are high enough, no fuel flows into the bowl, yet you can still get fuel through the jets right? That means the pressure in the bowl is not coming from the fuel pump. Carbs are used to vacuum, so atmosperic pressure is enough to promote fuel flow through the jets. The turbo increases the density of the air, not necessarily the velocity through the carb. If the velocity doesn't change, the carb won't pull more fuel through the jets. This is why carbs run rich or lean depending on altitude. The fuel pressure has to be raised so fuel still flows into the bowls, but that doesn't increase the amount that flows through the jets.

 

Wheres Lars when you need him?

 

http://www.hangar18fabrication.com/blowthru

 

With the carb in a pressurized box you need more pump pressure to get fuel into the float bowl. The carb still functions the same only under more than ambient pressure. If it goes lean under WOT go a couple jet sizes larger. If this makes the cruise metering fat go one or two bigger on the metering rods. Simple!

BigBlockk

Later.....

 

HO-RACING use to sell kits to turbocharge late 70 Trans Ams with Pontiac 350-455 engines as well as Chevy 350's. It was a blow thru system but the whole carb was not enclosed only the air horn. It was very clean looking. You might be able to find an old kit, or do some research and fabricate your own.
Also the 80-81 Turbo Trans Am's had a draw thru system that was a very clean installation,after you got around all the emmissions stuff. I had an 80 and you could easily adapt that to a Chevy 350. The pipes would be the most difficult thing to fabricate.
You don't need a blow off valve at low boost, I'm assuming you want about 7 pounds. I did run my T/A at 9 pounds.
I have run high boost systems in my Buick Grand Nationals, up to 25 pounds with the stock turbo but it is not very efficient at such a high level. The fuel injection and computers make a high boost system easier to deal with.
Just checked on a recent issue of high performance pontiac and the original HO-RACING guys are thinking about retooling their turbo kits!