Hello. I have a turbo kit for a 12a that is set up for a holley carb. For those of you who know a lot about forced induction I have two questions. . .

1. Since the system has the carb before the turbo can I still make a custom flange for a blow off valve after the turbo? My main worry is about blowing a fuel/air mixture underneath the hood of a hot motor, although the wastegate will be doing the same thing in a way.

2. Also I think common sense dictates on this one, but I'll ask anyway. Can I route an intercooler on the car. I am hesitant because that is a long way to blow a fuel air mixture and I am worried about the mixture not staying uniform between air/fuel molecules (i.e. fuel attracting to the intercooler veins, etc.)

Thanks

 

Well A BOV will not vent air fuel mixture, it will vent compressed air after the turbo, if the seals are less than optimum then it will vent some of the engine oil etc...

 

The system goes like this though:

intake>carb>turbo>bov>motor

since fuel is added to air in the carb stage, you will be blowing out a fuel/air mixture through the bov.

 

So its a draw through system??

If its a blow through or even a FI system, they go like this:

Intake -> Compressor Wheel -> IC -> BOV -> Carb -> Exhaust -> Exhaust Wheel -> Spins the Compressor Wheel

The BOV or Compressor By Pass Valve is there to save your compressor wheel when the throttle plates slam closed, thus it HAS to be between the compressor wheel and the carb. Damage occurs when forced air is streaming twords the carb, but the plates are closed. The air hits the plates and goes back where it came, causing turbulence and shock to the compressor wheel blades. Thats why its really called a CBV, it by passes the compressor wheel when the plates are closed

 

Okay, since it is a "draw through" type I really have no reason to install a blow-off valve because the throttle plates will be closing before the turbo, rather than being located after the turbo on "blow through" systems. So the whole problem of back pressure against the plates is avoided with this system.

My only questions now are about the intercooler and also about the effects of closing the throttle plate on the "draw through" system. When letting off the gas at high rpms, the throttle plate will close but the compressor wheel will still be spinning fast. Since the plates are before the compressor in the "draw through" system this will most likely cause a low pressure area inside the chamber. Any ideas as to what this will do?
(suck fuel from the idle jet; increase the speed of the intake air when the plate is opened; cause stress on the turbine since it can't suck any more air)

P.S. This turbo kit was already installed on a car and it hauled ***, but had a lot of turbo lag and eventually caused the motor to die. I am just trying to make it perform as efficiently and fast as possible while understanding the whole system in the process.

 

Correct, no BOV is need in a draw-through application. With the throttle closed the compressor will continue to spin away happily in vacuum.

You shouldn't use an intercooler either due to the possibility of fuel condensing in the core. It could make for some some pretty fireworks if you have an engine backfire.

 

That's one of the advantages of a draw-through... the turbo doesn't spin down when you shift and you don't have to worry about surge

There's another problem with intercooling a draw through system - the longer and less direct you make the distance between the carb and the engine the more like fuel will fall out of suspension. Bad, bad bad.

Also you need a special kind of turbo seal - turbos aren't generally designed to put up with high vacuum.

 

Draw-through systems are good coz they're so easy to set up. For years the fastest untubbed rotor in Australia was an 808 running a draw-through system with a DCOE Weber and methanol injection. The whole car owed the owner about 5 or 6 grand.

 

You also don't have to worry about referencing the fuel pressure to boost... Man if it weren't for the sticky wicket with the intercooling it'd be wonderful! That and the carb has to be sized much larger than for a blowthru, and the turbo has to be a bit larger as well (since there is always a pressure drop through a carb). And then there's the issue with referencing the power circuit to manifold pressure...

 

Ok disreguard what I said, I had the wrong type of turbo