Is this Holley a good choice?

[tjnavyblue]

Bort, could you expound a little on why a 2v would be preferable to a similar sized 4v carb, given both had mechanical secondaries?

I am aware Does10's runs a 4v on his falcon... what are the pros and cons to these?

 

[Bort62]

Bort, could you expound a little on why a 2v would be preferable to a similar sized 4v carb, given both had mechanical secondaries?

I am aware Does10's runs a 4v on his falcon... what are the pros and cons to these?

I am hardly a carb expert. However -

Any time you list a flow rate, it has to be in reference to a pressure drop. Without something pushing or pulling - there is no flow.

So, different carbs are rated differently. What you don't want, with any motor (turbo or otherwise) is a carb that is too large.

When the carb is too large, the pressure drop through the venturi is low. This also means that the velocity though the venturi is low.

The lower the velocity, the lower the venturi effect - and this is what sucks fuel out of the carb and into the engine.

If you go to big w/ the carb, and hence get too low of velocity, you can have problems with your fuel metering.

Obviously if you go too small, you can have problems w/ too much pressure drop (restriction) too... so it's a happy medium.

However, on a turbo engine - you have a little pressure drop to burn. Typically the limiting factor to the amount of boost you can run is the point at which you start detonating. What affects that is the pressure in the intake manifold (for simplicity's sake).

So, let's say you are targeting 15 psi in the manifold. If your carb gave you a pressure drop of 2 PSI @ WOT (full volume flow), then you would rise to about 17 psi in the intake tract (pre-carb) and your wastegate would open. You would have a certain velocity, X, through the venturi(s).

Now, put a slightly smaller carb on... Now your pressure drop is 4 psi. Now when your wastegate opens @ 15psi manifold, your intake tract is at 19 psi. Same manifold pressure, same detonation limit, same volume flow, but higher pressure drop through the carb because of the increased restriction.

What that means is higher velocity (X+Y) through the venturi. This will, in general, make the carb work better. Think about it: To flow the same amount of stuff through a smaller hole in the same amount of time - it needs to be moving faster.

And all it costs you is a little extra restriction of passing some % more exhaust gas through the turbine as opposed to out the wastegate. So you gain venturi velocity and lose really nothing.. win-win.

Venturi velocity = stronger differntial pressure signal in the carb, which will make it work better overall.

Make sense?

 

[Anonymous]

I'm going to ask a silly question here, please feel free to correct me if I'm wrong...

But, when referring to a "vacuum secondary", isn't it really a matter of the difference in pressure between the area above the throttle plate, and the area below?
Ergo, if you have more pressure above the plate, but less below because it is restricted by the plate, and being evacuating by the cylinder action, wouldn't it still work the same as pulling a vacuum?
Isn't there a relative pressure differential between the 2 areas that really makes the difference?
Seriously, I'm not trying to correct, just asking out of ignorance and looking for education here...

 

[Bort62]

I'm going to ask a silly question here, please feel free to correct me if I'm wrong...

But, when referring to a "vacuum secondary", isn't it really a matter of the difference in pressure between the area above the throttle plate, and the area below?
Ergo, if you have more pressure above the plate, but less below because it is restricted by the plate, and being evacuating by the cylinder action, wouldn't it still work the same as pulling a vacuum?
Isn't there a relative pressure differential between the 2 areas that really makes the difference?
Seriously, I'm not trying to correct, just asking out of ignorance and looking for education here...

You are generally correct. It's "Vacuum" is a misleading word. It actually works on differential pressure.

But, it depends where the reference is. If it's compared above/below the throttle plate inside the carb, then it will work fine.

If it references vac to the outside world, then it wont.

Vacuum is an absolute thing, and in the sense of forced induction it doesn't make sense. Any device that is "vacuum" operated is really just differential pressure operated.

 

[350kmileford]

Just verifying, we are going to be restricted to Holley carbs on the aluminum head, right?

 

[wallaka]

You can put whatever you want on there. It just needs a simple flat adapter plate drilled to the right bolt pattern. The intakes are intentionally made to require an adapter for ANY carb, that way other carbs can be made up. It comes with either a Holley 2V or 4V style adapter, true. It takes only simple fabrication to run a spreadbore carb like the Edelbrocks.

This is with the "4V" style intake. There is a Weber style intake for Webers or individual throttly body injection that is different. Heck, you could run those with a plenum box. There is already at least one of those on a turbo engine. Also, the Oz-2V intakes will fit with some modification.

There is a thread on the intakes in the Aluminium head forum.

 

[BENJENBROOKS]

I don't know if you want to hear about this, but I have a friend using a holley 390 with vac secondaries for boost. this is on a dodge slant six in his 66 dart.

He has referenced the secondaries to boost. He removed the mechanical linkage to shut down the secondaries and replaced with a limit switch that will vent the secondary diaphram to external air, this shuts off the secondaries when not wide open.

The power valve stays the same and you can control the WOT mixture by simply changing the jets in the secondaries.

Just a different way to do it. Controlling the secondaries took a lot of experimentation with the diaphram and springs. if not right, the secondaries pop open before boost climbs to need extra fuel creating a rich codition.

 

[Bort62]

Sure, nothing wrong with that.

Just might be a little more work than mech secondaries.

I personally would just do EFI

 

[XFlow_Fairlane]

if I were doing it over I would prob get a barry grant carb. same design but not much more for a little bit better quality. 350 or 500 cfm. since you are not going to need a big cam with the turbo the 350 woul dbe a better choice for a 200.