Power Valve Selection

Power Valve Selection

Cut and paste from myself in Power Valve Selection thread.

Fuel mixture should be lean at part throttle. It should get leaner with more throttle. Somehow all the magazine experts somehow missed this and misled most of us for years.

The power valve, or step up, is needed when the engine gets close to putting out maximum power for the rpm.

In this engine, they found it needed relatively early enrichment. That would be something like a 10.5 Power valve on an engine that cruised at 17 or 18"Hg. An engine that didn't need to go rich until 80 % load might be more like a 8.5 PV, and so forth. Wideband

A somewhat practical method on a modified engine is to note the vacuum when increasing throttle doesn't bring a gain in acceleration. Then try a PV that opens at or before that vacuum.

Mike Urich, at the time a VP at Holley, discusses something like that method in Holley Carburetors and Manifolds. Back then, it had to be done on a track with a stop watch while "crowding" a vacuum. I've tried it using the vacuum gage and datalogger with accelrometers and its hard to do. A flat spot will show up though. (as @Carnut68 and @mopowers wrote earlier)

A more technically precise method requires equipment not usually available to the hobbiest.
But if you happened to be so lucky as to know a place with NOx gas measurement and a load dyno then it can be done by observing the combustion.
"The purpose of the power valve selection is to enrichen the mixture prior to the combustion temperatures going too high. To set a powervalve correctly requires a NOx sensor. Different engine combinations need fuel enrichment at different power levels."
Bruce "Shrinker" Robertson. post #4 "Another Power Valve Question" archived Innovate Motorsports forum

Mattax said
In addition to the engine's natural needs, the enrichment point can be thrown off by timing that doesn't match the engine's needs and the fueling.

When the enrichment begins, vacuum advance goes away. Vacuum advance is for leaner, less dense, fuel mixtures that take longer to burn - just like Chrysler explained in its Master Tech Conference years ago.

Mattax said
the low number power valve myth I mentioned earlier.

Mattax said
Posted by Tuner, 12-15-2006, 01:00 AM

The history of the power valve hysteria has a few different influences behind it, corporate, media spin and international events that affected Big Oil.

The corporate influence came with Colt Industries when they bought control of the Holley Corporation in the late 60’s. When the genuine "old-school" boys were out of the way, the new wave of MBA’s started telling the ME’s how the business would be run instead of the other way around. Read that as profit became more important than quality. I was employed in speed equipment sales at the time and the declining quality was evident from that point on. One cost cutting measure was a reduction of maintenance of the injection molds and machines that made the parts. Die makers are highly skilled craftsmen, almost able to name their own price if they’re really good, especially in the days before CNC. We started noticing main bodies with more core shift, etc.

Every time a part is made in an injection mold, the molten zinc or aluminum dissolves some steel from the die cavity and carries it off in the part. As more aluminum is alloyed into the zinc, this process is accelerated. Zinc alloy higher in aluminum content is less expensive initially, but it increases maintenance cost. After a few 1000’s of parts are made the die cavities and the parts they make are no longer the original size but they look the same because the cavity erosion is uniform. I trust you see where this is headed.

The metal part of the power valves wandered off the blueprint but the diaphragm didn’t. The crimp process that assembles the casting, the diaphragm and the retainer washer wasn’t maintained well either, so some were loose and some were too tight, cutting the diaphragm. You can see how a backfire can cause a leak with a valve compromised like that. They pretty well have a handle on it now. A normal healthy valve can stand backfires, for that matter, nitrous backfires.

The low number valve for radical cams thing was influenced by this situation too. Smog regulations began dictating how engines were tuned in 1966 nationally (a few years before that in California) and by 1968 they were getting pretty lean and retarded. Before that almost all high output factory engines used #85 or higher power valves. The `65 425HP 396 Chevy used a #105 primary and #85 secondary. The purpose of that is to get it rich before it has much load on it so it won’t knock. Because the HP carbs had high number valves, that was what people wanted and what we sold. The low number valves were for dump trucks and economy cars. However, when the debacle with the poorly manufactured valves began, the valves we (and speed shops and warehouses across the country) had in stock that didn’t leak were the old pre-Colt Industries low numbered ones that didn’t sell before because they weren’t romantic enough. People didn’t want a Rambler power valve in their "Fueler 327" with the 950 3bbl. How this got tangled in the valve timing is it was in the same era when cams started getting much more radical. In 1965, most people thought a big cam was 250º at .050". By `70 or `71 durations had grown to pretty much what’s available today. When a low numbered power valve fixed a radical cam engine that wouldn’t idle folks fixated on the number, they might not catch that the valve they replaced was leaking. Brand new carbs came with valves in them that leaked. Once somebody jumped to that conclusion and the enthusiast media picked it up it spread by plagiarism and became lore.

OK, so there’s the corporate influence and media spin, now on to international events and Big Oil. Starting in `71, the highly leaded fuel was phased out down to "low-lead" (.1gm per gal.) by `72. That EPA mandate and the 1973 oil crisis, during which OPEC cut off the supply of middle-eastern oil to the US, brought about a change in the chemistry of the gasoline. American industry had agreed on standards describing materials compatibility for fuel and fuel systems that could not be met with the crude available and the amount of gasoline necessary to be made from it. This affected more than just power valves. The soft parts in fuel systems, hose, pump diaphragms, gaskets, o-rings, tips of inlet needles, floats, you name it, if it was soft the gasoline dissolved it or soaked into it and made it swell up. Inlet needle tips would swell and push the float to the bottom of the bowl. The floats soaked up with gas and sunk. (That was a big part of the bad rap the media played on the QJet.) The rubber would just sort of rinse off the diaphragms. If you handled a wet accelerator pump diaphragm it would smear off on your fingers. Needless to say, power valves made with the early materials leaked. The metering block gaskets expanded into the air and idle channels on the face of the block and plugged them. All the carb components had to be changed. Like a flammable game of musical chairs, the new parts had to push the old parts through the system. It took a couple of years for stuff for the less common carbs. Still, once in a while someone shows up with an old carb kit and it’s like déjà vu all over again.

originally snipped from: deadlink:innovatemotorsports.com/forums/showpost.php?p=29053&postcount=9
Archived here: Trouble with vacuum signal to power valve in Holley