PVCR Area to Main Jet Area Ratio ??

[63qcar]

Since it`s raining ( again ) I have some time on my hands . I have a carb ( Holley HP950 ) That was originally jetted square ( 82 mj ) with no Primary PV . From what I remember , to go to a PV , you drop the MJ 6 to 8 steps down ..... which leads to my question . If you calculate the primary mj area at the 82 size and then drop that jet to say 75 , you have reduced the mj area by a certain % ( also flow through the jet ) . Does the existing PVCR ( that was plugged ) go up by that same amount and is there some magic ratio between the two ? Say the primary mj is an 82 , which has an area of .00636 sq. in. and the PVRC is .060" dia. for an area of .0028 sq. in. and you go to a 76 mj which has an area of .00503 sq. in. , those two areas don`t add up to the 82 mj area .
Realizing that an A/F gauge is what is really needed to set the ratios , I was just curious if there was a way to get in the ballpark by doing some preliminary calculations .... or am I just overthinking this ?
Keep in mind that I came up with this while having some apple pie moonshine last night !

BTW , I realize that Holley rates their mj by flow and not actual hole diameters ..... and somewhere I have those numbers as given by Holley for some jets I have . Just looking for some thoughts/ideas here .

[turbo2256b]

wELL i go with having a power valve main jetting 7 to 10 sizes smaller.
With a power valve the the main jet ends up a cruse jet. So going with that figure your max cruse speed. I often go with between 80 to 90 MPH a bit over what I WOULD REALLY CRUSE AT. Test for this i use a block off or a 2.5 or so power valve to keep it from opening. Once jetting is close to optimal for cruse I up the jet size until it runs best at full throttle. The differance in main jet area between cruse and full throttle is the what the PVR should be.

[pamotorman]

doing the areas will get you close. you still need to adjust for altitude and humidity.

[andyf]

In theory you should just balance out the area of the jets and it should work. Whatever you decrease from the main jet area you add to the PVCR area.

One thing that I do not know is if the PVCR acts the same way as the main jet. It should since they both add fuel to the main well but the PVCR is kind of obstructed so perhaps there is some reduction of flow thru it.

I found out recently during some testing that the intermediate circuit in a 3 circuit carb flows a lot more fuel than the area would suggest. So the pressure drop on the intermediate circuit must be a lot higher than it is on the main circuit. But you are working with main circuit jets so the pressure drop should be the same across them so therefore you should be able to just work with equal areas and have it be fairly close.

[MadBill]

I've wondered about the PVCR vs. main jet area too. A few considerations:
o Holley listings show anywhere from 4 to 12 # difference from PV to non-PV barrels.
o The PVCRs are not as deep in the fuel so have less delta P.
o Their fuel passages appear be a little longer than than a jet's, thus more restriction.
o Main jet area is not the sole determinant of flow; a given drill size in a number of cases covers three jet numbers.


My thoughts are that the tuning sequence should be something like: idle and off idle, then cruise via main jet, next primaries-only WOT via PVCR changes, then primaries and secondaries together, re-jetting only the secondaries.

[cjperformance]

If doing a 'primary ONLY wot run' to check mains and PV i would only run up to the aprox RPM that you would see the secondaries for your given application begin to open. There is little point to tuning just the primaries at wot and at rpm past the secondary open rpm. At this point the primaries will see differing intake vac, air flow characteristics etc with secondaries connected or disconnected.
Once you have the primaries working well up to the secondary point then you need to be accounting for the secondaries and working on their transition from closed to WOT.

[MadBill]

I didn't specify, but I was speaking re mechanical secondaries; hadn't considered the situation with vacuum ones. For the former though, yes the peak primary CFM will be a lot more without the secondary's contribution but I like to run it up fairly high just to broaden the data base but then jet for what's needed for the actual range of primary flow. A really well set up carb can have a very flat AFR if desired, so test pulls to say 600 CFM through the primaries even though they will only see 400 in normal operation can be useful.

[cjperformance]

Agreed Bill, definitely take data from above peak primary flow,, id be hesitant to change anything in this area IF it was the only area not right until I have checked with secondary operation.

[Walter R. Malik]

Every engine combination has a different requirement ... every carburetor size also has a different requirement; especially at part throttle versus wide open throttle ... the sub-sets are almost endless.

[jmarkaudio]

You can't use area, it takes more area from the PVCR to replace area from the main jet due to passage restrictions in the PV, PVCR passage, and as Bill pointed the level of the PVCR. Some of the billet blocs lower the PVCR's so that changes it a little. Best thing to do is run the car at the track with jets squared and jet for best performance, find what it wants for AFR with a wide band. Then jet the primary down with the PV in, find where it runs as lean as it will tolerate, the get the AFR's back by sizing the PVCR's as needed for WOT. You can do it on a chassis dyno as well.

[cjperformance]

You can't use area, it takes more area from the PVCR to replace area from the main jet due to passage restrictions in the PV, PVCR passage, and as Bill pointed the level of the PVCR. Some of the billet blocs lower the PVCR's so that changes it a little. Best thing to do is run the car at the track with jets squared and jet for best performance, find what it wants for AFR with a wide band. Then jet the primary down with the PV in, find where it runs as lean as it will tolerate, the get the AFR's back by sizing the PVCR's as needed for WOT. You can do it on a chassis dyno as well.

Yes and the less restrictive the jets the less responsive a given pvcr area will be for a given combo so totally agree area is not a good reference point.
Yes, can be done on a chassis dyno but its not often that something comes off of a chassis dyno and is optimised for driveability. Most just dont get this right. So yes , wideband o2 is imo by far the best.

[BradH]

You can't use area, it takes more area from the PVCR to replace area from the main jet due to passage restrictions in the PV, PVCR passage, and as Bill pointed the level of the PVCR. Some of the billet blocs lower the PVCR's so that changes it a little. Best thing to do is run the car at the track with jets squared and jet for best performance, find what it wants for AFR with a wide band. Then jet the primary down with the PV in, find where it runs as lean as it will tolerate, the get the AFR's back by sizing the PVCR's as needed for WOT. You can do it on a chassis dyno as well.

Yes and the less restrictive the jets the less responsive a given pvcr area will be for a given combo so totally agree area is not a good reference point.
Yes, can be done on a chassis dyno but its not often that something comes off of a chassis dyno and is optimised for driveability. Most just dont get this right. So yes , wideband o2 is imo by far the best.

This topic is interesting to me because of the number of PVCR area vs jet size calcs I've done for a variety of scenarios. Some of my #s for estimated jet # spreads factoring in the PVCR areas are quite different than published specs for carbs similar to mine.

[andyf]

You can't use area, it takes more area from the PVCR to replace area from the main jet due to passage restrictions in the PV, PVCR passage, and as Bill pointed the level of the PVCR. Some of the billet blocs lower the PVCR's so that changes it a little. Best thing to do is run the car at the track with jets squared and jet for best performance, find what it wants for AFR with a wide band. Then jet the primary down with the PV in, find where it runs as lean as it will tolerate, the get the AFR's back by sizing the PVCR's as needed for WOT. You can do it on a chassis dyno as well.

I think it would be more accurate to say "you can't use area by itself". I think we have to use the PVCR area but then add a derating factor to it. Perhaps the PVCR only flows 80% as much fuel as the main jet or something like that. I do not know what the ratio is but I know the PVCR does flow fuel so it isn't 0%. But it also does not appear to be 100%. A little bit of testing on the dyno should confirm the number. I'll start the bidding at 80% but I would not be surprised at anything between 70% to 90% effective.

I was shocked to see the data that told me that the intermediate circuit flows 3x the fuel of the main circuit. I would not have thought it was anything close to that. Still not sure I believe it so I might do some more testing.

As a side note - how come none of the many books on Holley carbs cover this type of information? Wouldn't this information be useful to people? Always wondered why so many people have written so many useless books on Holley carbs.

[statsystems]

You can't use area, it takes more area from the PVCR to replace area from the main jet due to passage restrictions in the PV, PVCR passage, and as Bill pointed the level of the PVCR. Some of the billet blocs lower the PVCR's so that changes it a little. Best thing to do is run the car at the track with jets squared and jet for best performance, find what it wants for AFR with a wide band. Then jet the primary down with the PV in, find where it runs as lean as it will tolerate, the get the AFR's back by sizing the PVCR's as needed for WOT. You can do it on a chassis dyno as well.

I think it would be more accurate to say "you can't use area by itself". I think we have to use the PVCR area but then add a derating factor to it. Perhaps the PVCR only flows 80% as much fuel as the main jet or something like that. I do not know what the ratio is but I know the PVCR does flow fuel so it isn't 0%. But it also does not appear to be 100%. A little bit of testing on the dyno should confirm the number. I'll start the bidding at 80% but I would not be surprised at anything between 70% to 90% effective.

I was shocked to see the data that told me that the intermediate circuit flows 3x the fuel of the main circuit. I would not have thought it was anything close to that. Still not sure I believe it so I might do some more testing.





Andy,

Wouldn't the factor be influenced by the position and angle of the PVCR holes??? Jmark already posted that some metering blocks have altered PVCR positions. You'd have to test for a factor for every metering block with a different PVCR.

That would be time consuming, unless there was a formula to do math to figure the difference in position.

As a side note - how come none of the many books on Holley carbs cover this type of information? Wouldn't this information be useful to people? Always wondered why so many people have written so many useless books on Holley carbs.

[BradH]

I don't think there's one "number", unfortunately. There are a few variables that come to mind, some already mentioned above, that might mean there is a correction factor for the PVCR area applicable to one particular style of metering block. For example:

Is the channel drilled straight into the block or a replaceable jet?

Was the entrance to the passage chamfered or left sharp edged?

Does a Holley-type e-jet of a given size flow the same as longer hex-head screw such as used by BLP?

I'd expect that somebody has gone through this sort of testing regimen for their particular program, but I'd question whether their findings would be any more than another "rule of thumb" for people using different carb components.