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what are the advantages and disadvantages to each. I would like to go with a 4 barrel since the only reliable performance center in my town deals mainly with v8s w/4 barrels and i figure they would have the parts already in stock for one, plus it wouldn't take as long for them to tune one. Thanks for your help

I was at Pismo recently and rode in a turbo rail with a 650 double pumper. It was setup as a drawthru and ran pretty good. 2liter type 1 is what he had.

Robert

I am of the opinion that a 4 barrel is the way to go if space is not an issue. They are cheap (got mine in brand new condition from the neighbors for free!!! Try that with some IDA's!) and are VERY tunable. The tuning parts for them can be purchased anywhere which is a plus compared to European carbs. Also, if you use a 4 barrel which is equipped with vacuam secondaries, you can tune the opening mechanism (change spring rates)so the second two barrels open slowly or quickly as needed. Best of all, if your carb is a bit too large for the application, the vacuam secondaries will only open as much as needed so as to not over carburate the engine. Just my thoughts...

Did you modify the carb to open the secondaries under boost? How was that done?

Did you modify the carb to open the secondaries under boost? How was that done?

The secondaries are NOT modified to open under boost. There is no point. Vac secondaries open as a function of air velocity thru the primaries. Once air velocity thru the the primaries reaches a point where the primary size is restrictive, a circuit in the carb sends a vac signal to the opening mechanism for the secondaries. If everything is dialed in right, the secondaries will only open as much as needed to "assist" the primaries. This is a perfect situation for keeping optimum air velocity and fuel delivery thru all 4 barrels. We (locashracing.org) are still very much in the learning stages of turbos and Holley carbs. We are using a 750 spread bore (model 4011) which is equipped with very small primaries (GREAT DRIVEABILITY!!) and HUGE vacuam secondaries. When this thing is all done and dialed in, it may not even open the secondaries all the way, but that's the point...it only opens as much as needed!
Check out the SSG (Super poop Ghia) at www.locashracing.org Good luck!!

John, in Turbochargers by Hugh MacInnes, he states " that partially closing the throttle when under boost will produce vacum between the turbo and the carb. The increased vacum will close the power valve while the engine is under boost. When this happens, the engine will lean out and probably detonate." I know that RLR sells modified Holleys to get around this problem, they reference the power valve off of boost pressure, just not sure exactly how its done. Good Luck with the SSG, looking forward to see what it will do!

John, along with those jets I have a complete set of the Holley springs for the vacuum secondaries. All the colors. Remind me too bring them when I come over.

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Rice, it's what's for dinner!!!

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by RickS:
John, in Turbochargers by Hugh MacInnes, he states " that partially closing the throttle when under boost will produce vacum between the turbo and the carb. The increased vacum will close the power valve while the engine is under boost. When this happens, the engine will lean out and probably detonate." I know that RLR sells modified Holleys to get around this problem, they reference the power valve off of boost pressure, just not sure exactly how its done. Good Luck with the SSG, looking forward to see what it will do!<HR></BLOCKQUOTE>

Two things to understand about "Power Valves": 1. The normal/resting state (like you are holding it in your hand) for a power valve is in fact OPEN. A vacuum is required to CLOSE the power valve.
2. Because a power valve is held OPEN by a spring, the stiffer the spring, the greater the vacuum required to close the valve and the lighter the spring the less vacuum required to close the valve. I.E. Stiff=quick activation: Light=slower activation

With this being said, let's consider the draw-thru turbo motor. You have a carb (4 barrel I hope) mounted upstream of the turbocharger, and a motor (VW I hope!)mounted downstream. Now, at idle the vacuum throughout the intake system will be the same(as measured in inches-of-Mercury or in Hg)whereever you measure it. Take this same motor and apply a load to it and give it some throttle and see what happens...Between the carb and turbo you'll get one level of vacuum, yet between the turbo and the motor you may (will) get a TOTALLY DIFFERENT READING, perhaps even a positive reading (BOOST). This is where it's important to externally reference a power valve. Remember, the power valve is closed by vacuum, so if it's "looking" at the vacuum between the carb and turbo, it may not have opened. In fact (depending on the valve used) it may never open. This is because the vacuum under the carb could have been enough to overcome the powervalve spring through this whole "load" test. Now, if the valve is setup to look at the manifold BETWEEN the turbo and the motor, things would be different. As the motor starts to work harder, it pumps out more exhaust and spins up the turbo ( you should understand the basics here). The turbo sucks harder on the carb (VACUUM under carb) and forces that air/fuel into the motor (decreasing intake vacuum as measured between the turbo and motor). If this process goes far enough along, the vacuum level in the intake (between the turbo and motor) will decrease enough to release the powervalve (this is still going to occur BEFORE the motor goes on BOOST). If everything is jetted right, the additional fuel will be needed by now, as the motor is headed for the BOOST (FUN!!!!) stage. Once you let your foot off the gas, the vacuum levels return to a strong signal and the valve closes again...BTW, HOLLEY POWER VALVES DO NOT METER FUEL. They simply open or close at a given vacuum. The part numbers work like this...125-25=powervalve that opens at 2.5 inches Mercury(vacuum) 125-35=3.5 inches Mercury
125-45=4.5 inches Mercury
In the case of the SSG, it's carb (Holley 4011 spread bore) had a 125-65 powervalve. This means it would drop open at 6.5 inches of Mercury which was WAY TO SOON for this small motor. I repalced it with a 125-25 which opens at 2.5 in Hg. At this point there is much better air flow thru the motor and the extra fuel is more needed.

I must say I'm no expert on Holleys, but I'm learning as quick as I can. Spend some time in the Transportaion section at Barnes & Noble...It don't cost nuthin!!!!!

Thanks John, I didn't realize a Holley didn't meter fuel thru the power valve. I'll stick with my dellorto sidedraft and pressure sensitve regulator, for now at least.

That's hogwash. The power valve DOES METER FUEL, though it's activated by manifold vacuum (it sits in the bottom of the fuel well for cryin' out loud).

the powervalve system is critical to proper operation of the Holley carb, and on draw thru turbo use, it must be externally referenced to the pressure side of the turbo (I know, I ran one for 15 years on a turbo car).

The powervalve is basically another main jet, but who's operation is metered by a valve that's actuated by manifold vacuum (unless you reference it elsewhere). If you remove the powervalve, you need to step down on the main jets since the valve is effectively on all the time. The powervalve turns on under load, but it's main purpose is to allow lean running when NOT under a heavy load. For turbos, we can alter it's purpose to increase the fuel flow under boost, so it's perfect for our application.

This operation is covered in any decent Holley book if you don't believe me.


John
Aircooled.Net Inc.


<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Ricks:
Thanks John, I didn't realize a Holley didn't meter fuel thru the power valve. I'll stick with my dellorto sidedraft and pressure sensitve regulator, for now at least.<HR></BLOCKQUOTE>



[This message has been edited by JohnConnolly (edited 09-22-2001).]

John C. You missed my point. The power valve is simply the on/off switch for the power valve system. It DOES add fuel, however the valve itself does NOT determine how much. All 125-XXX power valves will flow the same rate of fuel, they just have different vacuum requirements for open/close condition. Fuel delivery is a function of the orifice (jet) the fuel must travel thru. On Holley carbs this is drilled into the carb body, so to change it you would have to drill a larger hole. Not recomended by most carb guru types.

I am very aware of how the powervalve works. My point was that your explaination was confusing, since even Ricks got confused!

the powervalve does in fact govern fuel flow when the valve is opened, the valve's # dictates the manifold vacuum that it opens at (which can by relocated to a pressure reference instead of manifold vacuum, which is very misleading to true engine load in draw-thru turbo use). so, the valve is open or closed (jets on or off). You change WHEN they come on by changing valve ratings. Of course, this doesn't apply to two-stage power valves, but that's another story entirely.


<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Ricks:
Thanks John, I didn't realize a Holley didn't meter fuel thru the power valve. I'll stick with my dellorto sidedraft and pressure sensitve regulator, for now at least.<HR></BLOCKQUOTE>



[This message has been edited by JohnConnolly (edited 09-22-2001).]

Can't we all just get along?

John C, is a vacuum secondary 4bbl the way to go for a draw thru application or would you recommend another way?(for those of us who can't afford EFI)

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Locash Mike
www.locashracing.org

I try to get along with everyone!

Here's my take on it.

The vac secondary 4bbl is VERY tunable, once you know what you are doing (I blew up 3 engines learning how to turbo a VW engine before I got it right). The Weber or Dellorto sidedraft works well too, but I don't have a clue how to do it (call Kawell)! The Holley is inexpensive (carb is $150 or so, the spring kit is $20, the jets are $30 for a LOT of jets), and the carb adapter is cheap too.

Getting Dells/Webers is/was difficult for a while, I have no idea if that's still the case. Even when you can get them, plan on $450 for a carb that works (Kawell) with the mods.

Some guys like to run a double pumper, and it may make more HP on the dyno, but a vac secondary carb will NEVER bog once it's set up, since the secondaries are an "open on deman" device (bog it, and they close).

I have seen sidedrafts (weber and dellorto), vac sec 4bbls, and double pumpers ALL make a LOT of HP, and same with injection.

The problem with any draw thru carb setup is you can forget a decklid, unless you have a ghia and are willing to do a LOT of custom plumbing to make it fit. Also, any carb application means the carb ANGLE, and float level, are CRUCIAL.

This is one application where FI is a natural (I sold my 4bbl, and am putting FI on it over the winter), since it doesn't take up barely any room, and there's no "level" to worry about, though you do have fuel supply and fuel pressure issues to worry about. The biggest thing on my new one is the turbo and the intercooler.

In short, you can make it work no matter which way you go, the secret lies in the execution and details.

John
Aircooled.Net Inc.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Locash Mike:
<B>Can't we all just get along?

John C, is a vacuum secondary 4bbl the way to go for a draw thru application or would you recommend another way?(for those of us who can't afford EFI)

</B><HR></BLOCKQUOTE>