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Just curious how many folks run externally referanced power valves on a roots blown engine.
Whats your version of a blower carb atop a Roots?What mods did you make to what ckt and how did it respond?

George,
I have "blower calibrated" carbs on my Buick, they are not boost referenced. They were advertised as blower carbs. Two 750 DP's. The engine I just finished for my Nova has boost referenced carbs. It's a fairly easy modification. I have instructions if you are interested.

I have not heard of that before guys. Does that mean that with the blower boost a regular power valve allows too much fuel so you somehow use a circuit outside the carb to richen up the mixture when needed?

ya, educate me on this too.

I've heard of such things with blow thru's, turbos and centrificals and such, but not with a roots blower sucking thru a carb. I'd think a regular power valve would work just fine.

From Hot Rod magazine Tech section:

Now, as for your original question on carburetors, we have to look at how a Roots 6-71 blower operates. This blower was designed to move air, not build pressure. This means it demands an unrestricted inlet. That's why you always see a pair of carburetors on Roots blowers. Running one small carburetor is too much of a restriction on the inlet side. For this application, you would be best served with a pair of 750- or 800-cfm carburetors. Both Holley and Barry Grant (among many others) sell specific draw-through blower carburetors that are modified to block off the undercarb manifold inlet for the power valve, since a supercharged engine may see relatively high vacuum levels such as 4 to 5 inches of vacuum (just below the carbs), even at wide-open throttle (WOT). For this reason, most draw-through blower carbs are modified for an external power-valve source below the supercharger. This ensures the power valve will open under boost when additional fuel supplied by it is really needed. Previous to this modification, many tuners resorted to blocking off the power valve and increasing jet sizes on the primary side of the carburetor by six to eight jet sizes. This made the part-throttle air/fuel ratio excessively rich, killing throttle response and mileage.

Thanks

WOW! The tech article from Hot Rod is actually good info and correct. Usually you get a bunch of sales pitch and no real info.

As the article mentioned boost referencing is typically done on the primary side. You just don't run a secondary PV and jet accordingly.

The thing with a roots blower is, there is always going to be vacuum at the base of the carbs. In a N/A application vacuum in the manifold drops under hard acceleration and this opens the power valve/s. With a boost reference carb, the boost will open the PV. In most cases roots blown cars dont "need" boost referencing, it is a safety measure, to make sure you dont go lean. Lean can mean destruction with a blower.

Royce..I boost referanced my 850 dp,s so they would sence real engine load vs vacuum above the rotors.With both carbs on the bench..this mod only took about an hour of my time.
I used the ported vacuum nipple in the primary metering block..plugged the baseplate source and drilled on an angle thru the main body into the pv chamber.Voila..externally referanced pv.s at no cost.
I like the above hot rod article..but can i condense it.
Externally referancing the power valves atop a roots allows them to operate as intended.
I drive my car with a boost/vacuum gauge and a wideband 02 sensor.At cruise around town and with only 1/4 throttle input,the manifold vacuum snaps from 13"hg to 5 lbs of boost immediatley.I need the pv enrichment right now and i feel the boost in the pv chamber speeds up the pv opening point.Not change the opening point dictated by the spring..more like shove it open.
I myself..would never run a street car without a primary pv.Anything less would be flushing mpg down the drain and washing cyl walls.I actually jetted down on the primary main jets and simply opened up the pvcr to target my wot 11.8/1 af ratio.With this change..I cruise 55 mph @ 14/1 af.Tap into boost and the quick opening pv and increased channel restriction size has me in my targeted safe zone.

George that's the same modification I made to my 850's. Very simple and I will be pulling the 750's off of my Buick to do the same. It's cheap and easy. The only thing that costs is the AN fittings I am plumbing the carb to manifold with.

There are more complex ways of doing the same job, but I don't know why anyone would bother doing it any other way.

My 750's do have primary PV's and I agree there is no reason to ever run a car on the street without and actually most race cars can benefit from PV's

Let's take this one step further. Do you guys run a PCV on your blown engines? There are two schools of thought on it. some people say you should never EVER run a PCV on a blown engine and others say it's no problem. Thoughts?