PVCR jetting Vs no PVCR jetting ??????

[455-4+1]

I was running some numbers comparing the recomended jetting of various carbs with and without power valves

I have a spreadsheet written to ease up the math and couldnt find a correlation between the factory settings for carbs for primary Vs secondary calibration

Is this to account for dynamic distribution in the plenum affected by acceleration ??. I see the post on the 660's when mounted inline and that could make sense ??. Obviously at WOT the vehicle should be accelerating harder so the effects would be greater, at lesser throttle opening the effect of acceleration would be less

ie:

850 mighty Demaon annular

Primary jet = 0.080" (0.093” DIA)
Primary PVCR = 0.059"
Total area = 0.19054 sq/in

Secondary jet = 0.088" (0.104” DIA)
Primary PVCR = 0.000" blocked
Total area = 0.016990 sq/in

The non PV side (secondary) has around 12% less area than the PV side (primary)

Highspeed air bleeds are the same for primary and secondary at 0.033", idle bleeds the same at 0.070"

This was the way my Holley bible showed me how to calculate this, is there more to it ???
Discharge coefficients, booster draw, throttle angle at cruise ???

Going by my calcs the PVCR to equal out an 80/88 jet split should be around 0.046" or 0.047" with no other factors considered. Thats along way from 0.059" ???

Unless of course its done on purpose ..............................

[bigjoe1]

I can read fuel flow on the front and rear sides of any carburetor oon my dyno. I have learned alot about the jetting of carburetors from looking at it this way. Many carbs that are the same size front and rear do not flow the same amounut of fuel through each bowl. It is close, sometimes very close, some time pretty far apart.The Demon carbs use the 8 jet sizes up for no power valve. This is very close to perfect. Years agoi, I always went up 5 jet sizes for no power valve, but I had no way to verify what I was doing, It does have to do with the size of the holes under the power valves

JOE SHERMAN RACING ENGINES

[MadBill]

I was similarly confused. I decided the problem was not just the PVCR size (which of course varies, thus affecting the jet size split) or the presence in some carbs of a choke horn or different venturi sizes, which may affect airflow and signal strength, but also that the PVCR is higher up in the fuel than the main jet, thus seeing less pressure head and requiring more area for equivalent flow.
If the carb venturiis, etc. are equal, the best solution, as joe says, is to have separate fuel meters for the primary and secondary and make sure the flow is similar. If they aren't, I don't know what to suggest, apart from EGTs and plug reading.

We had a related problem a month or so ago. All sorts of weird issues with a brand new 'matched set' of 850's from a well-respected carb shop. After wasting better than half a day on the dyno, we took them apart and measured everything. Not only did they have different PVCRs (by 0.009"), one had all screw-in emulsion, PCVR, etc. jets and the other was all drilled orifices. Seems one was obsolete stock. They sent us a new carb, but no one stepped up for the dyno tab.
Anyone using four fuel turbines for 2x4 set ups?

[455-4+1]

Good point on the height of the PVCR where it enters the main well Vs the jet location

I guess the same could be said for the height in relation to the first E hole as well ???

Seems strange that in the Mighty Demon line of carbs (650-850) all have an 8 jet split, and all have 0.059" PVRC's which seems logical........

Untill..............

You calc the areas of the jets used in them

The 650 has 71/79 split for a difference of 0.001967 sq/in per jet
The 850 has 85/93 split for a difference of 0.000805 " " "

Yet both have 0.059" PVRC's ????

The entry conditions for the larger jets must change their flow properties alot.

Ease of machining I suppose ????

[lluciano77]

I never understood the .059" PVCR as delivered from BG. Most of the time the end up too small from what I have seen. Some of their other restrictions are goofy too. The MAB are too big. The e-holes were too big at .033" and there were too many per metering block. I modded a lot to make my BG work. Now it shows up with a nice curve on my WBO2.

The jet versus PVCR relationship shouldn't be based on differences between the primary and secondary sides. It should only be based on what is needed to enrich the carb where it is needed. Don't forget, a lot of the Holley squarbore carbs are more like a spreadbore. The primary venturi and secondary aren't always the same. Not to mention airflow can change based on how you drive. Lugging around most of the time in traffic and never going to WOT could gain from a slightly leaner jet. How much is the secondary contributing and when.

Do you idle and cruise most of the time on the primaries? The primaries basically meter on their own through low speed to high speed cruise. The secondaries meter the additional mixture required when both sides are open. So, the calculation to compare isn't exactly a 1+1=2, or a 2+2=4 to be more accurate. Some people get the mixture right on the primary side with the secondaries disconnected and the tune the secondary jetting in later with the overall. In this case the primary is trying to cover WOT on its own and secondary airflow combined isn't taken into account. Then when its added it changes the total flow. But I can't argue with this method because it is simple, gets you close and quite frankly, I can't think of a better way to keep the primary and secondary relationships and differences sorted out.

[amcenthusiast]

The math to answer this question is found in Dave Emmanuel's book "Holley Carburetors".

-the old school Holley power valve design is actually a cheaper? way to make a 'mid jet' metering circuit -using vacuum signal instead of a dedicated air bleed

-has no precise orifice/air bleed to admit air; just a crude hole to operate by vacuum (negative barometric pressure instead)

-the PVCR (power valve channel restriction) in the metering block is a fuel jet, but the circuit has no air bleed orifice (just half of a 'real' metering circuit)

This design has caused untold air/fuel metering problems for millions of hot rodders and poorer quality air fuel ratio (bad gas mileage) for consumers ever since.

50 years later and still only the freaks/carb gurus can make that carb work spectacular.

Hence, a 'good carburetor' will feature no power valve at all, but feature a continuously variable metering rod system instead -better than any mid jet or 3 jet carb design.

Hint: Rochester Quadrajet, Carter Thermoquad, and now Speed Demon goggle valve carb.

[ClassAct]

The math to answer this question is found in Dave Emmanuel's book "Holley Carburetors".

-the old school Holley power valve design is actually a cheaper? way to make a 'mid jet' metering circuit -using vacuum signal instead of a dedicated air bleed

-has no precise orifice/air bleed to admit air; just a crude hole to operate by vacuum (negative barometric pressure instead)

-the PVCR (power valve channel restriction) in the metering block is a fuel jet, but the circuit has no air bleed orifice (just half of a 'real' metering circuit)

This design has caused untold air/fuel metering problems for millions of hot rodders and poorer quality air fuel ratio (bad gas mileage) for consumers ever since.

50 years later and still only the freaks/carb gurus can make that carb work spectacular.

Hence, a 'good carburetor' will feature no power valve at all, but feature a continuously variable metering rod system instead -better than any mid jet or 3 jet carb design.

Hint: Rochester Quadrajet, Carter Thermoquad, and now Speed Demon goggle valve carb.

There is so much wrong here that it’s not worth addressing. Just all wrong.