Thanks for your input, guys.
Tomcat:
If I knew what I know now when I was buying the Vortech box, I would have definitely bought your Supercooler setup. It certainly looks a lot nicer. So does the dyno sheet, from what I've seen
I agree that Vortech box plumbing has too many tees and 90 deg. turns, but I still can't see it being that much of a restriction to cause the problem I'm having.
Cooltoys: Thanks. Thats the kind of info I was looking for. If Mike Urick is right, then I don't see how the .110 seat assm. could be the problem. I don't know if I'm at 840+ HP. I don't think so, but I would like to be.

Tomcat: I know you and Dean Nickerson are the Gurus of Centrifugal setups - so, do you think it might simply be just too lean? Could it be that I might be rich at mid- range but way lean at full boost/high RPM?

I guess if I don't sort it out this weekend, I'm just gonna have to give up and give Dean a call.
Thanks again guys.

cooltoys61:

We have seen 1000 HP with 0.6 lb/hr, so that's in line with your calculation.

Hardcore:

Thanks for the detailed test results. I reread the whole thread and this is what the tech section is all about.

I am not a carb expert but if you have two power valves and the channel restriction size is 0.080" that's a lot of fuel. What are your power valve numbers and what are your manifold vacuum and fuel pressure when cruising on the primaries only? What I'm trying to get at is the differential pressure acting on the power valve.

I'm not sure about the P/V orifices because I didn't measure them, but I can see that they are quite a bit bigger than what I have seen in other 850's. I'll measure them this weekend. I have a bunch of very small drill-bits (from my former Nitrous-jetting days) so I might open up a bit so I don't have to go so big on the mains.
I also haven't really studied the boost/vacuum gauge at cruise (I know I should have) but from what I have seen the motor normally pulls around 7 - 3 " of vac. at cruise. This is why I believe it runs real rich at 1/4 throttle or so - because the Power Valves(6.5") are tipping in. I have a couple each of 4.5" and 10.5" P/V's on order that I'm going to play with.
Also, I have like 8-10lbs of fuel pressure at cruise and 15+ under full boost.
I have seen that I can cruise at about 2/3 to 3/4 throttle at 85mph with the vacuum/boost gauge @ 0" boost, so I figure with even just 5lbs. of boost 105+ should be no problem.
(BTW: The boost gauge is referenced off the Vortech box base plate BELOW the carb, and the Regulator is referenced out of the box.)

Too bad I have to frickin work for a living or I could be testing now. (Well I guess if I didn't have to work, I would have had a Skater 46' w/ Turbines!! and my own pit crew!)

At least if I don't sort it out I should have some data for Nickerson to work with

Oh, I forgot to mention that this problem is more noticeable when the motor is at a warmer temp, like 140+ deg water temp. I was almost tempted to think it might be vapor-lock, but I'm sure the carb doesn't get that hot - I think

I'm the air flow guy, Nickerson's the fuel flow guy, but I can share some of my thoughts on the blowthrough carb issue, maybe it will help you. By the way, I don't think I would have the nerve to tackle the carb, and my hat's off to you because you obviously have a lot of experience and seem to be taking things one at a time.

OK, this may be long-winded so I apologize in advance if you already know it. I just need to start at the beginning to keep it straight in my own head.

Let's say an engine makes 500 HP naturally aspirated and the carb is jetted for that. Then we add a blower and intercooler and WOT air density is increased by 50%. Now there's 750 HP worth of air and the carb needs to add 50% more fuel. However, actual CFM through the venturis has not changed, since the engine is a positive displacement pump. So air velocity is the same and the signal to the fuel metering passages is the same.

This is the problem. We have made a huge density change, but the carb doesn't know it. This isn't quite true since venturis do respond somewhat to the density increase, meaning the signal to the fuel metering passages does increase a bit with density, but let's ignore that to be on the rich side.

This problem is the reason you have to richen the jets when you move a carb to sea level. But that's easy; there the higher atmospheric pressure is constant. A blowthrough carb sees a range of pressures. So we have to start thinking about the different operating points.

The easiest is WOT. Let's say that the naturally aspirated carb had 78 primaries, a 6.5 primary power valve and a power valve channel restriction (PVCR) of 0.040" which would add 25% enrichment on the primary side, 94 secondaries and no secondary power valve. We can calculate the total metering area of this combination, and the additional metering area needed to make up for the 50% increase in density.

I did this calculation for the example above, and if we added all the additional metering area to the secondary side, we would need 130 jets. Procharger used to tell people to drill their secondary jets out to 140.

So now we are covered for WOT, and we have the original primary jets so off plane and docking should be fine. What about cruising on the primaries?

If there was 9 psi fuel pressure at cruise on a 6 psi base, we would have 3 psi boost in the box. That's about a 20% density increase, so if we were cruising on the 78 primaries, we'd be lean.

What is the power valve doing? If we had 5" of vacuum at the same time as the 3 psi in the carb box then the total pressure difference across the power valve is:

(3 psi X 2) + 5 " Hg = 11 " Hg

The 6.5 power valve is closed. It will only open when the combination of carb box boost and intake manifold vacuum fall below 6.5" Hg.

If we get into the secondaries from this part throttle position we will initially be way too rich since we put all of our extra metering into the secondary jets. As RPM and boost climb we eventually get to the point where the mixture is right.

Richening the primary jets will help the lean cruising condition, but we would need 85 jets to add 20% more metering area. The limit here is how much this affects low speed operations like docking. It might be better to add some of this 20% by enlarging the primary PVCRs and playing with power valves to get it synchronized with the load and boost curve.

Leaning out the secondaries would help the overly rich condition just above part throttle cruising, but whatever we take out of the jets has to go into the primary PVCRs to maintain a safe WOT mixture. If we use a secondary power valve, we could add it too the secondary PVCRs.

That's my theory and what it is too!

An additional proble that I always noticed is with the carbs sideways, fuel from the secondaries did not like to cross over to 2,4,6,8 when it was stepped up for WOT. The primaries would feed 1,3,5,7 and didn't seem to cross over during cruising. The end result was always having one side of the motor richer or leaner and not a happy medium. I ended up cutting the divider out of the manifold to the floor and still had this issue to a degree. I gave up and bought fuel injection.

Just a bit more about the hypothetical described above. If we decided to add all the extra metering area to the primary PVCRs we would have to drill them out to about 0.100". If we split the extra metering area between the primary and secondary PVCRs, then we would have to drill them out to about 0.075".

I have been thinking about your testing procedure and I wondered if it would make sense to first determine what primary jet will support your cruising speed on primaries only, no power valve enrichment. This should end up with a smaller primary jet than 90s. Then do your WOT testing to select the secondary jets. This will satisfy the two extremes and probably result in too rich in the midrange. Then you can start playing with power valves and the PVCRs.

PVCRs come anywhere from 0.030" to 0.120". If yours look like 0.080" then your current metering area with the 90s is 75% more than my naturally aspirated carb example. Without knowing charge air temperature and pressure I can't calculate air density, but I doubt if it is 75% higher so you sound rich to me.

We too have experienced the lugging at WOT throttle and Nickerson tells us to decrease secondary jet size. I must admit it is hard to take his advice when we see the white plugs.

One thing that complicates matters is the fact that you have a 100+MPH jet of air blowing across the top of the carb venturis. This probably does some weird things to venturi operation, making your tuning job tougher. Procharger tells people to run a small brass flame arrestor inside their box to address this turbulence, but we have gained 75 HP on the dyno by taking them out.

Good luck with your testing!

Tom

Whew! Dammnn Tomcat, it's a good thing you're not the "carb guy"! I'm going to have to read all of that over again for it to sink in! Thanks for taking the time to help a poor fool like me out!!

I'm starting to think that it's just that the motor wants a ton of fuel at full boost/W.O.T. and when I fatten the mains up it runs too fat in the middle. So, I think I'm going to install the new inlets (just to be safe) and I'm going to measure and increase (drill out) the Power Valve orifices by about 10-15%. What the hell - if I screw up I'll just have to buy two new metering blocks is all.
I'm going to go a bit bigger on the secondary main jets as well.
I didn't realize that Procharger used to tell you to drill out to #140 jets- Sh!t! that's big! Alcohol-jet territory, even. (the biggest jets I have are #99s)
Hmmnn, I wonder if anyone ever tried using/ tuning an alcohol calibrated carb for a blow-through setup... Big jets, big fuel calibration all round, seems like the perfect candidate for an "artificial atmosphere" setup!

As for the power valves, I take it then that I need to increase the opening point # (say to 10.5") to ensure that they come in earlier? BTW: if I have my boost reference for the gauge below the carb where the power valve is also referenced from, isn't what I'm seeing effectively the same vacuum reading as the P/V?
Thanks again for all your help.

Tomcat, I've just seen your most recent post about what Nickerson recommends with the lugging- Do you think it could be possible that I'm too rich? I wonder if this is why the motor lugs more when it is hotter than when it is relatively cold? Colder engine, colder air charge, denser singlet air, leaner mixture, right?
If this is so, I have to say that I'm kinda worried about leaning the jetting down because the insulator is white. The threads are black (sooty) though and the Procharger manual says that you must ignore everything else about the plug and concentrate on the threads. They say that once the first two threads are nice and black/sooty then the jetting is right on. It may be then, that regular plug reading technique doesn't apply to blown applications since we are definitely generating higher combustion temperatures.
Man, what a dilemma! I guess I'll have to check the PVCRs first and that should give me an idea of whether I'm rich or not.

After fighting the above jetting problem along with what you are fighting for 2 years I also went with fuel injection.

Hardcore. Your power valve needs to be smaller 4.5 or so to come in earler. Bigger to come in latter. Remeber the power valve is seeing the difference between top & bottom of carb. When I was crusing @ 2500 rpms I had 3+ lbs of boost in box & had 6-5inches of vacuum in maifold.

Tom- You are the man.

Here's another theory:
Carbs are designed to be "sucked" through and so the venturis are designed to cause a pressure differential (a drop in pressure, or Vacuum) so as to pull fuel from through the jets and out of boosters. The vacuum comes from the air being displaced through the engine and this causes a pressure drop in the manifold plenum and so forth.
So, when we "pressurize" the whole intake tract with a positive displacement of air through the carb, we adversely affect the pressure differential at the venturi/booster. The booster therefore produces a much weaker signal and (everything else remaining equal) less fuel is pulled through the same jet.
However, air is still flowing through the carb throat so there is still enough of a pressure drop at the booster to pull fuel through it and the engine still runs, albeit seemingly lean for the size jetting it has.
This is why I think a smaller venturi and perhaps an annular booster is needed to generate enough fuel flow through the carb to keep up with the air. This might be why Nickerson (and some others) changes to a 750 main body on some applications (or so I've heard), so as to speed up the air flow at the venturis and thereby provide better signal to the boosters.
Has anybody tried changing to annular boosters and/or a smaller main body and if so what were the results?