I just found the Hanger 18 site about modding Holleys for blow-thru turbo use. I'm left a bit confused. It was my understanding from other sources that in a blow-thru setup, the power valve doesn't work correctly. The Hanger site doesn't touch on this, but just suggests how to enrichen the valve. Further, nothing is mentioned on sealing the throttle shafts.

I want to use a Holley 390 4bbl. What should I do differently from the suggestions on that site? Thanks!
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The PV may or may not work correctly once the mods in hangar18 are complete. Mine worked at WOT however the engine was lean at part throttle. Every carb seems to be a little different here. You can try a boost referenced PV circuit or vent tube extensions to correct the situation.

My carb seems to be OK with the throttle shafts. There are several ways of doing this. Search around on turbomustangs.com for threads and pics.

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11.02 @ 120.56
http://www.youtube.com/watch?v=EAxRmoDgsdY
Chassis dyno
http://www.youtube.com/watch?v=wd6hFGzLJMc

it should work properly. It is the manifold vacuum that holds the power valve closed, keeping the enrichment circuit closed. When you drop below 6.5 (eg) the spring overcomes the vacuum and opens the power valve, uncovering the circuit. At boost there is a positive pressure and that still allows the PV to stay open. Newer Holley carbs (1994) and HP carbs have a "PV protection check valve" that closes when manifold vacuum becomes positive, ie boost or backfire. Itll close and you should still have your PVR's open. I dont know what the limit on boost pressure that the PV diaphragm can take before it ruptures but it should be safe with anything less than a backfire (BIG psi). Draw through carbs will have to have the PVR referenced from below the turbo, like the supercharged Holley models. Holley sells throttle shaft bushings, little white teflon sleeves that you install during a major rebuild. But a Holley in good condition should not need these.

My question is does the carb have to be a double pumper style or can u modify a. Mech secondary carb to work properly in a blow through setup
thanks Aaron [/code]

DP and mechanical secondary is the same style. Vaccum secondary uses a vacuum source that is basically a tap into the primary venturi, the faster the air moves across the tap (down the primary bores), the more vacuum is created in the pot and that pulls the secondaries open. There is no relation between intake vacuum and secondary actuation vacuum. Most blow throgh carbs are mechanical (DP) secondary style. The only linkage between the vacuum secondary and the primary is a rod that prevents the secondaries from opening faster that the primeries, no actuation, just a retarding link. Carter/Edelbrocks are velocity sensing, no problem there as are Rochesters.

Pishta I miss spoke on my first post what I meant to say can a vacuum sec carb be modified for a blow through setup

The reason Im asking I'm in the process of a turbo build and I have a 650 DP already setup with the mods already done and I just got a lighty used 500 holley vac secondary and I was kind of hoping I could mod the 500 and have a spare in case the 650 DP is to big
Thanks Aaron

never heard of a 500 vac Holley, but if its a 4150/4160, then you CAN use a vacuum carb but you have to set it up as a blow thru. I just posted a quote from a carb guru and it says in a nutshell that you take the spring from the top of the diaphragm of the vacuum pot and put it on the bottom, manipulate the drag linkage from the pot to the secondaries to make it push the secondaries open when the pot gets pressurized (rod goes down from the pot) from the boost and then you got a carb that is jetted normal for primary running and power valve jetted when boosted. This carb comes on like an afterburner though, not as seemless as a DP. Its in here somewhere, just look my name up in a search.

Yah printed that post u did and was just reading it.
Aaron

It has been my understanding that the power valve references vacuum from below the throttle butterfly. So it seems to me that in a blow through system, the power valve would work normally....no? But what choice of power valve (the vacuum rating) would be best in blow through use? And how would primary/secondary jet choices be made?
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A BRPV is referenced to the carb hat. Same as the regulator.

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11.02 @ 120.56
http://www.youtube.com/watch?v=EAxRmoDgsdY
Chassis dyno
http://www.youtube.com/watch?v=wd6hFGzLJMc

Please understand that I'm learning here and am not suggesting that anyone is wrong.

Well...this power valve stuff is not fully making sense to me. I may not be the brightest bulb in the house, but I know that when the engine idles, or the throttle is closed from cruzing or high revving, is when (in a naturally resperiated engine anyway) the vacuum is highest. Holley's are designed to keep the PV closed under vacuum, just as you'd want.

When you close the throttle on a turbo engine after a boost condition, you will get at least some vacuum under the throttle plate, but for a short time the turbo will still make boost above the throttle plate, and if the PV is referenced to the hat, or above the throttle plate, it will see that boost/lack of vacuum and cause a rich condition when you don't want it.

In fact, with the hat used as a reference for the PV, there will always be at least some boost/lack of vacuum (even if miniscule) except at the lowest idle. Seems to me that a reference under the throttle would act more like the stock naturally asperiated setup (if, of course, somewhat exaggerated), and shut down the PV when it's not needed. Even the Hanger 18 stuff, while it does address PV mods for more fuel, doesn't even mention referencing the PV, which leaves me to believe they leave it stock.

Since others are agreeing with the hat reference, can someone tell me where my thinking is going wrong here?

I cannot see the benefit of referencing the PV to the carb inlet under any circumstances. The power valve always needs to 'sense' engine load.

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Joshua

Okay Joshua, that sounds like you're in agreement with me.....yes/no?

Yes!

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Joshua

The BRPV does not work in the traditional sense. A BRPV os opened by boost and not vacuum. There is usually a pressure difference between the carb hat and below the carb's butterflies. The rest of the carb sees carb hat pressures as well.

If you rely on traditional PV function with boost, it can become confused and flutter or not open at all.

Connecting to the carb hat works for me and is the generally accepted place for best results as found by the guys on turbomustngs.com.

_________________
11.02 @ 120.56
http://www.youtube.com/watch?v=EAxRmoDgsdY
Chassis dyno
http://www.youtube.com/watch?v=wd6hFGzLJMc