PVCR Calculation for Six Pack

[EngineerDoug]

Hello,

Running a six pack setup, I am a little uncertain on how to compute/estimate my PVCR size. I am familiar with the formula that adds the area of the primary jet to the area of the PVCR to arrive at the suggested area of the secondary jet.

The fly in the ointment here is that I am not dealing with a square bore carburetor setup. The CFM ratings of the primary and secondaries are not the same. At first blush you might think this requires a scale factor to account for the end carburetor vs center carburetor CFM ratings (500/350), but I'd like to hear about any flaws you might see in this reasoning.

At present I am running #66 primary jets, 0.052" PVCR, and stock metering plates (0.089") on the secondary.

Thanks!

 

[lewtot184]

Not sure I understand the question. PVCR is a center carb only thing, unless your trying to figure this by combining all three carbs. From my years of experience with 6paks I don't think that's a hoop that needs to be jumped through. There are some jetting tips from the old Mopar bulletins. For me I just kept it simple, .093 in the corners and 65's in the center. From experience and reading some tests that seems to be pretty good for a mufflers car. The pvcr on factory carbs was staggered but I think any mods throws that out the window. I really think after a certain point is reached the real issue is the manifold and not the carbs. I think one needs to keep in mind the Era of design and what it's true purpose was; it's not a race setup.

 

[EngineerDoug]

I agree this is something that may not warrant my attention.

For a square bore Holley carburetor, a general starting point for jetting (secondary versus primary) assumes the airflow rate through the secondary and primary barrels to be the same at WOT. Roughly speaking, then, you would want the same fuel flow rate through the secondary and primary systems. That means the same area for secondary jets and the primary jets + PVCR. That is the rule of thumb as I understand it.

I was viewing the outboard carburetors as the secondaries and trying to apply the same logic. More than anything else, I would like to understand why this might not be the case.

Chalk it up to curiosity I suppose.

 

[lewtot184]

Wouldn't comparing the end carbs to the center be an apples to oranges comparison? Their functions are different. Different venturi and throttle bores between them would dictate different jet area I'd think? Sometimes I think the only purpose for the center carb is cruise and opening the end carbs. I know that's an over simplification but reality is it doesn't make the power. Anyhow, keep playing with ideas. Maybe we'll both learn something.

 

[lewtot184]

Another thought; when I was getting serious about this stuff some years back I found that fuel delivery made more of a performance gain with these carbs than jetting did.

 

[EngineerDoug]

On a related note - I installed a bypass style fuel pressure regulator and it made a big difference in driveability. I use a mechanical pump with plenty of output volume and the regulator bypasses the extra back to the tank. Keeps the fuel a lot cooler and performance is more consistent over temperature.

I also found that moving the IFR to the low position made a big difference in idle quality. Reduced this to 0.028" and increased off-idle AFR. Here again I found the idle to be more consistent as driving conditions change.

 

[lewtot184]

I found this from June 1971 race bulletin jet specs.
Front carb; .093 throttle side, .089 diaphragm side.
Center carb; #65 jets, .052 pvcr
End carb; .093 throttle side, .099 diaphragm side.
I'm sure this is for a stock manifold, open exhaust, cheater cam, good heads class racer. Definitely some distribution issues.

 

[70sixpkrt]

This is from the Mopar Engines Speed Secrets book.

 

[6PKRTSE]

I don't go by anyone else's recommendations or whatever manual or charts say to do. I tune and go by what my car likes.
All my buddies always comment how flawless my sixpack cars run. In fact my one buddy always laughs and comments is this thing fuel injected? Because it always fires right up and idles perfect.
In fact, I should probably look at some things this winter. I don't think I have even removed the air filter off of it in years.

 

[EngineerDoug]

Thanks!

 

[Aron Gleason]

I know when you go up in pvcr size, the afrs at wot should be 12.5-13.0. As far as figuring pvcr size to jetting I can't answer that.

 

[Bird 426]

I am with post #9. I have gone through all of the do this and do that's. One of the most significant changes that I did was a muffler change, and no one ever mentioned that. I installed quiet flow mufflers because of my sensitive hearing issues. Hard to explain how the performance was cut, totally different animal. Guess that I need electric cutouts!