Today going to talk about Blow thru Holley carbs and a small bit of history on how to enrich the Holley Carb's air/fuel mixture when the manifold goes into boost mode.
Over on the Turbo Forums there was a specific Forum on Boosted Carburetors, modifications that people tried, results from those efforts, and
if the efforts amounted to anything. Thousand and Thousands of Carb Mod posts over the years.
The issue that always came up is how do you add fuel to the carb when under boost mode.
With a Centrifugal Supercharger you could have a different boost level over the rpm range because the supercharger was tied to the engine rpm by the drive belt.
With a Turbo, the Engine goes into boost fairly quickly and then a Wastegate holds the boost level at that Boost pressure thru out the acceleration curve. So if you want 15 psi of boost you sent the wastegate control pressure (using a boost gage) and dial in the wastegate control rod to give you that boost level in the intake manifold.
So a Turbo is a bit easier to dial in for a constant boost pressure over a rpm range.
But back on the Holley Carb.
Power Valves ADD EXTRA FUEL to the Main Wells of the carb thru the Power Valve opening and distribute that extra flow thru either a "Two Window" or "Four Window" opening in the valve.
Then the fuel goes thru Power Valve Channel Restrictions to the Main Wells. A stock Holley Metering Block has TWO POWER VALVE RESTRICTIONS. A Blow Thru Holley Carb might have as many as SIX POWER VALVE RESTRICTIONS IN EACH METERING BLOCK to add the extra fuel needed to go with the air being supplied by the boosting device.
I have attached some pics of modified Power Valves and Metering Blocks.
Using this link:
https://www.theturboforums.com/threa....358671/page-2
you can go to Post #30 on the 2nd page of the thread and you can click on a video made by Steve Morris (about a C&S Blow thru Dyno Carb Test). C&S does a fine job on building Blow Thru Carbs. NOT CHEAP but they work very well and I have tried several other guys carb mods in the past. The C&S Carb Guy (ROGER) seems to have the Power Valve issues that puzzled others for years figured out. In this case better to just buy the carb vs spend months and months trying the same stuff the other guys tried with the same poor results.
I could show you lots of pics on Jet Extensions, Power valve Cup Extensions, modded Power Valve Metering Blocks [with 2 Holes (stock), with 4 Holes (performance and street), and 6 Holes (Race) drillings.
And that would be only a small part of the puzzle.
I will keep posting the typical mods done in the past on carbs but at the end of the day if you want to be successful talking to Roger at C&S might be the best path for a Boosted Holley carb.
Tom V.