One of the many features incorporated into the Holley "modular" carb series, is the use of a component referred to as a "power valve". In some variations of other Holley models, this type device/circuit is also referred to as an "economizer" valve/system. However, the economizer circuit is not usually serviceable in the sense that it can be replaced or "tuned" using variations of valve opening setpoints.
And to further complicate this issue, the replaceable-type Holley power valve component is also used in some other model carb variations and referred to as a "spark control" valve and is installed externally.
Typical Holley aftermarket carbs come spec'd out of the box with a "single stage" power valve. Genuine Holley power valves will always be marked with an identification number that is used to denote a particular point (based on manifold vacuum level) at which the valve opens. This opening is not a gradual event, but is basically like a toggle switch, either "off" or "on".
Only Holley carb service kits include a replacement power valve, and that is a generic item which fits "most" driving conditions. Aftermarket kits sometimes will include a replacement power valve (sometimes not), the power valve is an item that must be replaced with a new correct part at the time the carb is overhauled. This valve will wear out over time, can become damaged due to varnish/fuel deterioration, and is affected over time from exposure to the alcohol content which is found in all current gasoline blends in the u.s. Newer production power valves are much more resistant to alcohol blends than parts produced several years ago.
One tuning trick that can be of major benefit on Holley modular carbs as applied to any IH sv application, is the use of the so-called "two stage" power valve. This is nothing new...and in fact, many Holley 4150/4160 carbs which were oem on IH apps back in the day used these devices. They are most effective on vehicles that are pigs (such as all IH rigs), used in towing applications, and see use at varying altitudes during a typical day's operation under a heavy load (such as climbing a pass towing a trailer).
Identification of the operational characteristics of the two stage power valve has always been kinda voodoo since Holley did not really publish an easy-to-understand guide to their identification/numbering system.
Now though, thanks to my buddy dave wimsett's contacts within the Holley performance products organization (dave is a former nascar r&d dude for earnhart's ops), we have access to the info we need.
This is a spreadsheet which I.d.'s two stage power valve setpoints by the I.d. Number stamped on the valve. Simply find the number on the valve in question, then compare to the chart for the first and second stage setpoints.
In the future, we'll discuss how this circuit/component can be used to possibly improve fuel economy and performance if applied correctly. But...this is not the magic bullet, and May actually make no difference in mileage, simply is used to do a fine-tune of the modular Holley for your engine/application! In fact...I'm playing with this right now on my trailerdragger/beater. And my focus is fuel economy when pulling a trailer with a gross total vehicle combined weight of around 12,000lbs up to around 7000ft. In altitude.
The I.d. Number on the two stage power valve is not indicative of it's actual operational characteristics as is the number stamped on the single stage power valve. The two stage number is a reference number only and must be used with the attached spreadsheet for I.d.!!!
update:
After making a 250 mile roadride last Saturday pulling a utility trailer with a gross of around 1500lbs., the beater truck is now luxuriating in an 11mpg bath! That's up three mpg from it's prior state after my last round of carb-tweeking back last November. Keep in mind, the beater has a normal rolling weight of 6200lbs., again, IH vehicles (any breed) are major pigs! The crispness has returned, hot re-start is much improved (wasn't bad to begin with), and tip-in is perfect once again. The next test is an upcoming 250 mile pull with the 12,000lb. Gross load for comparison. I have one more main jet/two-stage power valve combo to try once I get some hard numbers for comparison.
Last night I ran it again for confirmation of the a/f ratio numbers on the exhaust gas analyzer. Throughout a complete test sequence, the a/f ratio stayed exactly where I'd hope to see it. But since that's a run in the shop with no engine load, the action of the power valve can't be exhibited since manifold vacuum cannot be varied in order to kick the power valve. Once I get some full load mileage data, then I'll do a road test with the gas analyzer installed. It's portable and can be placed in the seat for observation during an actual road run so that numbers can be recorded at any engine rpm/road speed/manifold vacuum point. Of course, that takes two folks to accomplish!
If I can end up with 8mpg average pulling the max load, then I'll be able to sometimes afford to actually use the beater/trailerdragger again on occasion! Prior to this round of tweeking, I was seeing 3mpg if lucky.
