Road Race Carbs: Why 2 PV's?

[StanJ]

Hi Randy, thanks for the reply and the discussion. You’re right; all of my recommendations in this forum thus-far have assumed separate, isolated curb idle circuitry on the secondary side. To me, that’s kind of like American Express…don’t leave home without it. Even on the required #4777 650-cfm carburetors that we prepare for competitors in the New England D.I.R.T. Modified Series where “4-corner idle” conversions aren’t allowed, we still do this utilizing a hidden screw-in restrictor in place of the needle valve and route the required additional idle air correction from the vent area so that the bleed size doesn’t tip off the inspector.

Personally, I’ve never found any sort of power/throttle response/emissions/mileage liability from running a properly tailored secondary idle circuit…in fact, in my experience a “2-corner idle” carb on the street is very often a mileage and/or emissions liability due to the smaller idle air corrector size and its resultant effect on circuit overlap. Even our vac. secondary “poor man’s traction control” stuff utilizes a 4-corner system.

You’re exactly right about 1-to-1 secondary linkage on larger double-pumpers making the car a handful to drive smoothly up off the corner when available traction (or just the correct line) isn’t optimal, and also right about the fact that the “cure” required in to have the best of both worlds -- progression built in to the accelerator actuation system (pedal control) itself – requires a bit more engineering that a lot of “Joe-Track-Day” guys want to invest in, but we’ve found that there is a significant “serious amateur” segment of that market that isn’t willing to compromise.

Regarding accelerator pump timing, we’ve found a chassis dyno to be invaluable here, as it allows us to accurately graph pump shot demand of a customer’s engine under a variety of load conditions and then machine custom accelerator pump cam profiles to match. These custom profiles, especially when combined with a neat little manifold pressure-sensitive shuttle valve that allows us to bypass un-needed accelerator pump fuel back to the bowl during high manifold vacuum conditions (as in compression braking through a turn in a lower gear with RPM’s up, then immediately back to WOT) without the circuit delay problems associated with the “intentional leaks” that so many carb builders are fond of these days, let us dial in accelerator pump function to a point pretty close to perfect…excluding Mother Nature’s variables.

Doubtless, I’m reading your post wrong with regard to manifold vacuum at part-throttle cruise and decell situations. Under these conditions, manifold vacuum is relatively high. Remember the “shift now” idiot lights from the early 80’s? They were triggered by manifold vacuum sensors, usually set at 10-12” Hg. Also, notice the number of cars (hopefully none of them with our carburetors, mind you…but occasionally some good running cars nonetheless) that belch huge orange flames out of the exhaust under hard decell; testament to the excessive amount of raw fuel passing through the cylinders and on out into the headers. Fuel flows through any carburetor circuit (with the exception of the accelerator pump) in response to pressure differential between the float bowl (usually ambient unless purposely engineered to be otherwise) and the discharge point for that circuit. If you’re speaking about the main circuits then yes, the pressure differential between the float bowls and the boosters is closer to balanced at part-throttle cruise than it is at higher engine demand conditions….but the same cannot be said for the idle circuit since the discharge points are well down the throttle bores, with both possibly beneath the leading edge of the throttle blades. These circuits are subjected to much higher pressure differential under part throttle, and while you won’t see it with a lambda sensor these circuits can be real fuel wasters unless they are properly calibrated.

[Big-Foot]

Hello Stan,

RE: Carb size - Certainly the smaller carbs such as the 600-650's will lend themselves to 1:1 configuration and tuning than will the 800-850's.
D.I.R.T. - I've done a bit of work with DIRT South (a number of years ago now) and lately with Wissota / IMCA / STARS cars. Good series - tough rules!

Chassis Dyno: Yes an exceptionally valuable tuning tool. For years we tried to do "track tuning" on Shop Dynos and were only marginally successful, but we did have a higher ratio of repeatability than on the chassis dyno. Still the Chassis Dyno helps us make the car more driveable.

Vacuum signal: I guess it depends on your vantage point and what engine / carb combo you are using. Certainly closed throttle/high RPM will yield exceptionally high vacuum which will pull a lot of un-needed fuel from the idle circuits. I've been able to tune with air-bleeds to come up with happy mediums, but regardless of what we do, we'll never come up with something that works as well as EFI in part throttle operations.

Funny you should mention the fire balls belching from the exhaust. Not that many years ago when Tommy Kendall was winning everything in sight in Trans-Am, Jack Roush was playing with a device that was actually taking advantage of fuel in the exhaust systems. Imagine a 5" Venturi in a 6" exhaust pipe. Now , just downstream of that venturi, you inject a little fuel and a little air when the throttle is closed. The resulting fireball creates a negative pressure in the exhaust system in the middle of and on the "other" side of the venturi! This would actually help scavenge the exhaust and would make a pronounced improvement in off-corner throttle response.
If you were watching this effect on Tommy's car, you also noticed that about mid-season, it quit belching quite so much flame. Seems that Pro-Racing had a little talk with them about it. I can't help but imagine that it also had a negative impact on fuel economy.

Now if we could only solve the cold starting characteristics of of methanol - we could kick OPEC's butts and get off of gasoline altogether...

P.S. I started to mock-up a PV extension yesterday.