Road Race Carbs: Why 2 PV's?

[Curt C.]

Just curious. How come some carbs (most notably road race applications) have two PV's? More fuel requirements? Seems like drag cars would need two too.

[Big-Foot]

HI Curt, 2 PVs help in part throttle transition as the PV will enrichen the mixture earlier than if just running all the fuel through the jets.
Drag cars really don't spend much time at part throttle. The additional PV is just another point of failure as well.

Hope this helps!

[Jack21]

Dual PV's aren't just for race cars. Street carbs use them too. Spread bores (for you GM guys) had front and rear PV's on DP carbs.

PV on the secondaries help part throttle fuel metering depending on engine demand as determined by engine vacuum.

[StanJ]

"Power valve" (or enrichment valve) circuitry can be used to tailor the shape of a carburetor's fuel delivery curve in several ways, including mid-to-high load enrichment (thus allowing "cleaner" idle and low-load running) and also as a high-load "lean out" for situations where class rules require the use of a carburetor with venturi sizes substantially smaller than what would normally be required to satisfy the engine's airflow demand (I'll be happy to explain the "how and why" details of this to anyone who wants to know, but otherwise won't bore all of you with them here).

In most road-racing and oval-track applications, sharp part-throttle response and fuel mileage are very important, so we utilize power valves (along with a couple of key additions that I'll cover in a moment) in the carburetors that we build for these applications in order to keep un-needed fuel out of the cylinders (and off the sparkplugs) until throttle demand requires it.

As for drag racing carburetors utilizing a power valve only in the "front" (or primary) metering block, the main reason has to do with fuel movement inside the rear float bowl when these cars launch. To understand what happens here, picture in your mind's eye a Holley (or Demon) metering block and float bowl on the secondary (rear) side of a drag carb. With the vehicle "staged", the level of fuel in the bowl would be --depending on float adjustment -- just to the bottom of the sight plug opening and essentially parallel with the ground; covering the jets...and the power valve fuel inlet, if one were installed. However, when the car leaves the starting line, the combination of acceleration forces and chassis roll (think wheel stand here) cause the fuel in the rear bowl to "slosh" violently to the back of the bowl and away from the metering block...uncovering first the power valve fuel inlet (which is less that .625" below the "static" level") and then the jets themselves. Under those conditions of course, fuel delivery into the engine is cut more-or-less in half, resulting in a huge "bog" that drops the nose of car back down until the jets and PV are again covered by fuel...at which time the whole process often repeats itself until the driver aborts the pass or until the car finally reaches sufficient speed so that it's decreased rate of acceleration no longer causes the jets to be uncovered. "Jet extensions" -- which have become practically universal in drag racing carbs -- solve part of the problem by allowing the jets to feed from the rear of the float bowl...but keeping the power valve fuel inlet covered is much more difficult due to the design of the float mechanism. Since drag racing is generally not a "part throttle" sport, most competitors solve the remaining issue by simply not utilizing the power valve circuitry in the rear of the carburetor (installing a plug if necessary to block it off) and installing larger jets in the rear metering block to compensate for the lost fuel flow.

As I mentioned earlier, road racing and oval-track competitors need the part-throttle fuel curve benefits that power valves make possible, and for them the "fuel slosh" issue mentioned above wasn't really a serious problem until about 15 years ago when advances in chassis/tire technology drastically increased forward bite out of the corners. Addressing the problem here wasn't quite so simple, since these cars can't afford the total disruption of fuel delivery under deceleration that drag racing-style jet extensions cause either. Our solution involves a baffle that we fabricate and install over the power valve fuel inlet which keeps it covered with fuel during high lateral loads, along with a modified float and a reshaped float bowl interior that is much more resistant to fuel movement away from the jets. Together, they work extremely well...and we have even used this technology successfully on some quicker (sub 10 sec.) drag cars.

[Curt C.]

StanJ, Excellent dissertation. I would be intersted to learn more about your baffles and float bowl mods. Got a website? Give yourself a plug

[Excaliber]

I recently replaced the 660 cfm "center squirter" Holleys (yeah, two of them) on my FE. These are high dollar "drag race" carbs with no PV's and mechanical secondaries.

They were a "two speed" carb. Go like hell, and a real mean idle. Anything in between just wasn't worth much for the street.

Twin 600 vaccum secondaries with PV's were MUCH better for the street and I suspect will be for road racing also. Pretty good idle to, considering my radical cam and all. IF I had my choice I'd go with center pivot floats also, for road race application.

"Bog", off the line WAS an issue that could be overcome with the correct "launch". But it was tricky to get it right.

[StanJ]

Thanks, Curt C. Even though this is our 18th year in business, you’ve caught me “between” website hosting services. My company, Stallion Racing Components, maintained a small template-based site through Quickbooks for several years but they recently sold off their web-hosting service to Verizon…who immediately tripled the monthly rate with no product or service improvements that I could tell. The folks at GraphX are putting together a custom site for us now; I’ll post a link when it’s up…but in the meantime I’ll try to post some pics here tomorrow of the parts I mentioned.

Excalibur, you’re definitely headed in the right direction with a change to the vac. secondary 600’s. The #4224 660 cfm center-squirter carbs are some of the most un-streetable carburetors Holley ever produced (second only to their #4543 850-cfm brethren in that department) due in equal parts to their 1-to-1 secondary linkage and their use of a single 50 cc accelerator pump which – as the name implies – discharges from the carb center into all four venturi at once. Even in the exact application for which they were originally designed (high RPM drag racing small blocks using dual carbs and a tunnel ram) the 660’s never really ended up being the optimum set-up, since the “lowly” #4777 650 cfm double-pumper was functionally identical with regard to airflow (same venturi diameter/shape, same booster design, same throttle body size) once the choke tower was milled away, and was much more flexible with regard to tuning. Granted, the 650 double pumpers are a bit longer due to the presence of a secondary metering block rather than the 660’s in-bowl metering “plate” and because of that you can’t mount a pair of 650’s inline on most low-profile dual-quad manifolds, but the drag guys had been mounting their carbs sideways since long before the 660’s were introduced anyway.

If I could offer a couple of suggestions though, consider plumbing a small (-3 is fine) line connecting the vacuum pods together in your 600’s, and also consider installing metering plates that utilize regular Holley jets and changeable idle feed orifices. These plates are available from several companies (including us) and offer improved main circuit fuel emulsification and – more importantly to you -- the ability to tune for quicker idle circuit response in the secondary…which allows you to use springs of the proper rate (lighter) in the vacuum pods, which you need for dual carb applications if you want to achieve WOT in the secondary venturis. These two upgrades will reward you with much smoother and more consistent throttle response, and likely with a few more top-end horses as well.

[Excaliber]

Yup, got the vaccum pods connected, got the nice meter plates, found the right springs (trial and error) for the secondaries and was just about to get the jet's dialed in when the cam went away and I had to stop and over haul the modor!

I have to pull the front carb to get at the distributor cap, Mallory BIG dist head.

[StanJ]

Curt, here as a couple of pics showing the PV baffle and how it is installed. In oval-track applications such as this one, we orient the baffle with the fuel entry hole slightly toward the passenger side (the metering block pictured below is a primary)...which is where most of the fuel in the float bowl will be as the car exits the turn (for road racing applications we install them with the fuel entry hole facing straight down). We sell a pair of these (including new bowl and metering block gaskets) for $18.00 U.S. As you can see though, this is not a complicated piece to make...well within the capabilities of almost any machinist. If you have a lathe and some .625" diameter 6061 bar stock lying around, feel free to build your own with my blessing.

When I took these pictures, I didn't realize that we were out of stock on the modified float bowls; should have some finished up by the end of the current week.
Stan Justus
Stallion Racing Components
1704 Dallas Lake Rd.
Soddy Daisy, TN 37379
(423) 847-0441
(423) 847-0461 fax

[StanJ]

...and installed.

[Mike Simard]

Curt, for road racing you would want your primaries and secondaries to move at the same rate so that your throttle travel is even with no sudden point where secondaries start opening. It would be nice to have the secondaries mirror the primaries, power valve and all.

StanJ, thanks for joining in! That's a great idea to have a power valve baffle.

[Big-Foot]

StanJ - While I disagree about the primary and secondary throttles needing to be actuated in a linear fashion, I am intrigued by your baffle for the power-valve.
It is something I have given a little thought to in some of our cars since we properly install jet extensions, but leave the PV to suck air under hard acceleration. Of course this is only relative to those carbs that have a secondary PV, but very much a potential problem..

[StanJ]

Originally posted by Big-Foot


StanJ - While I disagree about the primary and secondary throttles needing to be actuated in a linear fashion..

Hmmm... I’m not exactly sure what my previous post might have implied regarding secondary throttle bore actuation other than the fact that street carbs with “1-to-1” ratio of primary/secondary opening tend to be difficult to manage in traffic, and likewise problematic on long-cammed, low/no flywheel inertia racing vehicles when negotiating the infield and pit lanes. The question does serve well as a springboard for the subject though… and actually, a 1-to-1 primary/secondary opening ratio does have a couple of potential advantages; it just requires a bit of engineering to utilize those advantages without liability.

Looking at the fuel metering circuitry of practically all Holley 4150 series (double-pumper 4brl) and 2300 series (2brl) carburetors, you will see two separate fuel delivery circuits for each venturi: the “main” circuit which discharges fuel from the venturi discharge nozzle (or “booster”), and the “idle” circuit, which discharges fuel alternately through the curb-idle orifice below the throttle plates (the rate of discharge here is controlled by the position of the mixture screws on the sides of the metering blocks) or through the transfer slot depending on throttle opening. Proper function and optimization of the idle circuits is a book unto itself and in the interest of verbiage probably a subject better saved for another post, but for now the point is that while fuel entering the air stream by way of the main circuit has the advantage of having been pre-atomized to an extent, fuel entering the air stream through the idle circuits does not. Certainly, both circuits utilize air bleeds…but - given its design and routing – the idle circuit’s air bleed does little more than act as a signal corrector with regard to circuit timing. Where allowed by class rules, we have some throttle bore modifications that help this situation, but you can’t completely cure it.

From one of my posts on another board:

The real issue is the actual Air/vaporized Fuel ratio present in the cylinders at the time of ignition. For a moment, let’s assume that a theoretically “perfect” stoichiometric air/fuel ratio of 14.7 to 1 (on gasoline) is optimum for an engine under all conditions (this isn’t the case under all conditions, but bear with me here). Fuel that arrives in the cylinder as a liquid (either film or droplet) cannot participate in the combustion process until it is vaporized…usually by the heat of compression (which is too late to have any bearing on detonation), or even later by the heat released by combustion itself (which it too late to do much of anything other than raise exhaust temperatures). It’s a matter of hydrocarbon molecules having physical access to oxygen molecules; and hydrocarbon molecules beneath the surface of a droplet or film have no such access. So, to achieve an effective 14.7 to 1 A/F ratio in the vaporized charge, we’ve got to start out with something a bit richer than that at the carburetor in order to allow for the fuel that puddles on the manifold floor, gets centrifuged out of the air stream at all of the bends in the intake runner, falls out of suspension due to local pressure increases in the intake runner, etc. Just how much richer depends on how well -- or not -- we can keep each of the previously mentioned things from happening. Therefore, it stands to reason that a carburetor which atomizes a given amount of fuel into the greatest number of droplets of the smallest size – thereby minimizing the centrifuge effect while simultaneously providing a greater amount of droplet surface area from which those droplets can evaporate on their way to the cylinder – is the most desirable. The main circuit discharge droplet size of an “out of the box” Holley 750HP 4br. averages about 60 angstroms in diameter at 1.5” Hg, while our fully prepped Trans-Am pieces produce droplet sizes that average around 20 angstroms and can safely run a little over 1 full point leaner in A/F due to a greater percentage of it’s total fuel discharge arriving in the cylinder in a combustible state. This works out to 15-18 lbs of fuel per hour at 750 horsepower, or about 1.5 mpg advantage at Elkhart Lake.

Now to tie this all together, let’s consider tooling down the highway at part throttle (assuming that any of us here actually do that). With a conventional “progressive” secondary linkage on our carburetor, the primary throttles are considerable farther open than the secondary throttles under these conditions, with considerably more air passing through them. The main circuits of those venturi are online, and the airspeed down the primary throttle bore walls is sufficient to begin “curtaining off” the idle circuit discharge points. But over on the secondary side, the throttle blades are still almost closed; likely open just enough to expose the curb-idle discharge and transfer slots to full manifold vacuum. This results in a lot of wasted fuel as well as well as a host of other potential problems (reduced spark plug life, cylinder wall washing, high header temps, etc.) toward the rear of the engine; single plane intake manifold-equipped Fords being particularly susceptible due to their runner departure angles.

How do we best address the problem? By going back to the 1-to-1 linkage that I damned in the beginning…but with a twist: progression built into the throttle actuation itself. This is fairly easy to do with “cable” systems, easier still with “throttle-by-wire” servo actuation, and one of our customers (Lozano Bros. Racing Engines of Cibolo, TX) builds a very nice “adjustable” mechanical set up for Trans-Am that could be adapted to road cars. 1-to-1 linkage does sometimes require some circuit timing modifications to the carburetor in order to optimize things, but it’s all “win/win” stuff…no real downside performance-wise.

A lot of our customers don’t want to go to the effort and expense of re-engineering their throttle systems to take advantage of it, but yes…1-to-1 primary/secondary throttle actuation at the carburetor can be made to work well.

[Rick Parker]

Stan:

Who is the perveyor of those particular blue "whistles"?

Rick

[Mike Simard]

Stan, thanks for the 1:1 info. I assumed that everyone else ran 1:1 and I believe Bigfoot was commenting on my assumption a few posts ago. While you're in a 1:1 discussion mood I have some other thoughts. I recently converted my BG850 to 1:1. First impressions were that it works better than expected on the street. I wonder if it even promotes better cyl to cyl distribution on a single plane at part throttle. It does feel overcarbed below 2k now, it will have some circuitry mods now but I'm also tempted to come up with a modified 1:1 link arm to allow a very slight progression for the first wee little bit of throttle opening. The idea was to mount a stud on the primary linkage that could be moved to allow progression changes, it would also have to have a better method of changing the linkage arm length than bending a silly bar. Is there something like that out there already?

[StanJ]

Originally posted by Rick Parker


Stan:

Who is the perveyor of those particular blue "whistles"?

Rick

The blue vent whistles are available through:

Braswell Carburetion
7671 N. Business Park Dr.
Phoenix, AZ 85743-9177
(520) 579-9177

Part number 590301

[Curt C.]

The reason I asked the question was that I recently purchased a BG General Competition Race Demon and partly converted it over to a Road Race model. I went with a General Competition model because I wanted the vacuum ports and did not want a 1:1 linkage that comes on the RR models. I installed road race floats and spring loaded needle & seats. Before I pulled the plug out of the secondary and installed a PV I wanted to understand exactly why I was doing so. Sounds like running a secondary PV makes a lot of sense - better part throttle and benefits from less fuel usage. I installed the second PV today and took a little curise around the neighborhood. No noticeable change is street manners which is good because I had the carb set up to my liking (No bog and a 12.7 : 1 A/F ratio under full load in 4th gear). Will be going to the track (road course) on 4/30 and will report any changes. I also plan to contact Stan and get a pair of those PV baffles.

[StanJ]

Curt, check the size of the orifice in those spring-loaded needle & seat assemblies before heading to the track. We’ve never used them so I don’t claim to be up to date on everything thats available now, but the ones I’m familiar with (Holley parts) have a .097” diameter orifice, which might be dangerously small (or not) for your engine. In general numbers, here’s how it breaks down…

The standard “performance” needle & seat assemblies that Holley installed for decades in their double-pumpers have an orifice size of .110” diameter. At .400” float drop (any more than that and the fuel delivery curve starts going to hell anyway) the net flow area (total orifice area less that taken up by the portion of the needle still remaining within the orifice) is .000854 sq. in. At a consistent supply pressure of 8 PSI, a pair of these will flow a bit over 46 gallons of gasoline/hour…enough to feed a 680-700 horsepower engine assuming a BSFC of .44

Using the same parameters for one of the previously described Holley .097” “off-road” spring-loaded needle & seat assemblies, the effective area drops to .00643” sq. in., reducing the capacity of a pair of them to safely feeding only around 520 horsepower. Keep in mind that these numbers are for steady-state demand; on a tight road course you’d probably have a safety factor of 10%.

A current 355” non-restrictor plate Nextel Cup engine with “front row” potential produces around 840 horsepower now, and in those applications we use a .130” needle & seat assembly with our own unique needle design (completely clears the orifice)…but even this one is pushed to its limits on some 2brl applications…500+ horsepower and only one needle & seat assembly to get the fuel through. When needed and when/where rules allow, we utilize the PCI dual needle & seat Superbowls. These are a practical necessity for high HP alcohol-fueled engines, and even some gasoline-fueled single carb drag racing and SCTA “Bonneville” LSR big-blocks.

[Big-Foot]

Hi Stan - Thanks for your considerate response.
I too have been building these carbs as well as gas and alcohol FI systems for many years - you're right on in many aspects although your post seems to be slanted to carbs that have a secondary idle circuit.
As I alluded to earlier in my response about the 1:1 throttle actuation - We have found that it has a tendancy to even out fuel distribution (good) under steady throttle settings, but introduces too many other variables that are exceptionally difficult to manage efficiently or cost effectively - a couple are;
1) Driveability - 1:1 actuation is much faster given the same amount of pedal travel and the flow of air is too great at too low of an RPM for most roadrace applications. This makes the engine less controllable off the corners in my opinion. Some people call them "Light-Switch" carbs.

2) Accelerator pump circuitry in a 1:1 has always been a royal pain in regard to getting them to be responsive - but not overly - in all the ranges. Unless you are willing to change final drive ratios to optimize your setup for a specific RPM band at each track, you will find yourself re-working the carb tune-up for each track. While that's fine for you and I - your average Joe-Track-Day guy is going to be lost.

In regard to fuel being introduced by a secondary idle circuit which the carb is at part throttle (on it's mains) and the secondaries are closed - I have not seen this to be the case. When considering that the vacuum signal is relatively low at part throttle (depending upon the size of the carb vs. displacement), the balance of pressure between the plenum and inlet side of the carb will not typically allow the flow of fuel through the secondary idle transfer slots - or at least not an appreciable flow to upset the fuel mixture towards/away from Stoich. I would find it a stretch to think that the minimal amount of fuel that may be drawn from this secondary idle circuit, during part throttle operation, would be substantial enough to cause cylinder washdown or elevated exhaust temps.

In conclusion:
In the grand scheme of things, the absolute best carb is one that has a true single variable venturi system that works correctly (Kendig / Predator were close). The second best (in my opinion) is one that actuates the secondaries via vacuum control where the engine will not see the additional air until it can use it.
That being said - the Double-Pumper carbs were originally built because people could not "FEEL" the secondaries coming in or were having problems with bad diaphragms / actuation so they would put "The Screw" in the secondary safety link to FORCE the secondaries open (we know what that does.. I will always maintain that if you can FEEL the secondaries coming in, they are setup wrong...)... Still I, and a number of other builder / tuners rely on the mechanical secondaries as the mainstay of competition carburetors because they are typically easier to tune and the removal of the secondary diaphragm eliminates another point of potential failure.

Again - Thanks for the conversation, it's good to get the neurons working!

[What'saCobra?]

Seems true to me, also. Once it is set up close enough, mechanical secondaries allow you to learn how to drive the car with that engine and carb combo and not also have to learn where and if the secondaries come in at the same time in the same way. Mechanical secondaries are for sure repeatable and if you are insisting on running a little lean (not for me), you can learn how much it will take by feel. If you are on the rich side a little, no problemski.

Particularly the new Holley HP series is such a high "Q" circuit, even WOT at loRPM will not screw up the stoich metering unless you are seriously below 1600 rpm or so. Carbs certainly have come a long way, baby. These old 3310's will have to go in someone else's motor for historical and hysterical reasons. Give me the new stuff every time.

Though i live in Florida, haven't had a choke in a Cobra for over 10 years...simply not ever needed. Learn how to make it work with your foot and all is cool. So, it pops a little sometimes on start, but doesn't quit running and 45 seconds later it's ready to idle at 700 -
800.

As written elsewhere not so long ago, with the new HP and a proper full MSD package with the mag trigger, so many problems simply melted into te garage floor. Talk about easy to set up and run. Out of the box...would you believe?

OK, then a little tweek or two down in jet size to eliminate Holleys insurance policy practice of rich first...

Talk about nice. Life does get better. Better engineering replaces dodgey engineering.