How to boost reference your holley style carb
 

OK, so you think its complicated? Well, it can be. Power valves operate on vacuum, when a carb is installed on top of a roots style supercharger the power valve will see nothing but vacuum being that air is getting sucked thru it. This will keep the power valve closed and essentially starve your engine of that much needed fuel at high RPM.

Holley has for a long time offered a seemingly simple solution when you use their base plate and their main body.
When you first look at the diagram you may be confused. Here is what you really need to do.

This is looking down at the base plate on the main body side, NOT the manifold side.
1. Cut a channel from the spark port to the power valve vacuum feed hole, you can but do not have to take out the check valve/ball bearing, just channel thru the side of it.
2. Not only does the spark port have a hole in the base plate where the arrow is, but it also has one next to the butterfly in the bore, the one in the bore needs to be blocked off.
3. Reinstall and boom you are finished.
Attachment 387948



OK, so you have a holley base plate and upgraded to a Proform main body, if you look at the main body upside down you will notice that there is no spark port at all, the gasket has the hole for it but there is no hole in the main body itself. This is where it gets a bit more complicated. What you have to do.

1. drill a hole in the side of the main body INTO the power valve vacuum chamber, I used a 3/16 drill bit, remember to use oil when drilling! This will make your life so much easier.
Attachment 387949
Attachment 387950
Attachment 387951


Clean out the hole, and the surrounding areas, I used a compressor to blow all the aluminum out.
Attachment 387952


Now you need to plug the vacuum hole from the base plate all together, cause this vacuum hole will still be giving it vacuum. I used a two part epoxy (JB Weld)
Attachment 387953


Now you need to make a vacuum line that comes into the chamber, I used some spare 3/16 nitrous line I had laying around, you can also use brake line or what have you.
Again, I used a two part epoxy, rolled it around the shaft of the vacuum line and pressed it into the hole I drilled. It fits so snug that it will keep all the epoxy on the outside of the carb main body making an air tight seal.
Attachment 387954


Once everything cures and you reassemble the carb you are going to take your new vacuum line and attach it to your manifold. This will allow your power valve to see both boost and vacuum and is also where you will "T" off to connect your boost gauge.
Attachment 387955


Next step is power valve tuning. You can still use the same operation to tune your power valve as you normally would.


As an added precaution, I would suggest purchasing a notched float and jet extensions for the secondary side, this way then the fuel gets thrown to the back side of the bowel on take off, they can reach the fuel and not starve out the engine. You can find them for $20 shipped on ebay.

thanks for reading, any comments, questions, concerns are more than welcome.

Re: How to boost reference your holley style carb
 

Here's a simular way to refrence the PV - I posted this a long time ago:
http://www.fordmuscleforums.com/indu...d-engines.html

EDIT: AND - If you have a Speed Demon, there's a baseplate available you can just buy that bolts right up to the stock main body and comes with a PV referenced vacuum port. Fits 650's up to 750 CFM carbs.

Re: How to boost reference your holley style carb
 

I did see that but I don't have buck shot laying around, lol. Plus there is no modification to the base plate with the way I showed. Also this is completely reversible. Drill out the epoxy and plug the vacuum port and you are back to normal.

Re: How to boost reference your holley style carb
 

Obviously, you don't need to use buckshot. You could use epoxy instead.

I had a Holley and I did simular to what you described, by drilling through the main body. I chose to just use a dab of epoxy to fill the port on the bottom of the base plate instead. As you mentioned, I used a piece of brake line from NAPA and used it for the vacuum tap off from the main body. Worked fine. Yeah, it's reversible, but after refilling holes etc. - I would only run it on my car. I couldn't sell it to anyone for any decent price with a clear conscience. I didn't care for the way Holley recommends to do it.

Re: How to boost reference your holley style carb
 

Actually, when you purchase a boost referenced carb from holley, it is done the way I showed, not using the base plate as holley suggests you do it.

Food for thought.

Re: How to boost reference your holley style carb
 

sorry about bumping an old thread like this, but am I correct in assuming that these modifications apply to turbocharged applictations as well(blow through)?

Re: How to boost reference your holley style carb
 

Here's a good link for blow-through carbs:
http://www.hangar18fabrication.com/blowthru.html
Never done it myself...maybe someone else has and knows the in-and-outs around here. Good Luck!

Re: How to boost reference your holley style carb
 

Thanks, I'm familiar with that link, and plan to use it as a guide. I was curious specifically about the boost reference power valve mod outlined in the first post...

Re: How to boost reference your holley style carb
 

Do you have a part number on this?