tomcat

Blowthrough carb supercharger systems are very popular. There are a lot of Procharger systems out there. Complaints I have seen on this board include, black transoms, poor bottom end and poor fuel economy. When you ask Procharger for carb settings they say to call the tech line after you purchase a kit. There seems to be a secondary market of carb tuners prepping carbs for these systems. What's the story?

Cobra Marty says the solution is EFI. He's right, but what about all us carb guys who can't or won't switch? Some of you are happily running carbed Procharger systems. What was the secret? Who set up your carb? Do you see the need for any improvements?

I'm no expert on this but I do know that venturis do not respond as well to pressure as they do to flow. I forget the math but I think doubling the airflow through a venturi will double the amount of fuel delivered, however, doubling the density of the air will only result in about 40% more fuel. This means you have to go way up on jet size to get enough fuel under full boost WOT.

The compromise comes when you return to part throttle cruising where you spend 95% of your running time. Imagine for the moment that you only have a two barrel carb. The jets that were just right for WOT under full boost are now way too big for part throttle, low boost. Black soot and the other symptoms listed above are the results. A similar effect occurs just off idle where the primary jets begin to flow and are too rich.

Since you actually have four barrels you can reduce the size of the primary jets somewhat, and go really nuts on the secondary jet size, maybe even leave the secondary jets out altogether. Now you have some other problems. You can't go too far transferring the fuel carrying duties under boost to the secondaries or you run the risk of leaning out when cruising on the primaries only with boost in the carb box. And when the secondaries are opened suddenly and you aren't at full boost, you are way too rich and the transition is bad.

I would think that some tuning with the power valves could help the primary side and let you reduce the jets a bit more without leaning out the engine, but you still need big enough jets to deal with the full boost WOT condition. With the huge jets on the secondary side, I'm not sure that the secondary power valve would mean anything.

I assume that a skillful carb tuner can make the best of a bad situation, but I don't see any fundamental way of overcoming this compromise unless you could somehow change jet sizes "on the fly" according to the air density.

In the "old days" when taking your car from high altitude to sea level, you would be wise to increase jet size to adjust for the increased air density, otherwise your engine could run dangerously lean. But a blowthrough carb system makes a much bigger transition every time you open and close the throttle. And you obviously can't change the jets while doing so.

One possible solution is to have a tapered rod inside the large jets (like the Q-jet), with the height of the taper controlled by a diaphragm measuring boost in the carb box, referenced to atmosphere. Rochester actually produced a Q-jet that did something like this. It had a third primary jet with a rod that was attached to an aneroid, a little vacuum cannister that fit inside the fuel bowl. As atmospheric pressure increased, a lever on the cannister lifted the rod out of the jet, thereby richening the mixture. This system was sized to make the relatively small adjustments due to changes in altitude, but there is no reason why the idea wouldn't work on a larger scale. It has the advantage of full modulation of fuel flow according to pressure.

Another possibility that might be easier to implement on Holley carbs would be a modified two stage power valve. Bear with me, I have to set this up properly or I can't follow it myself.

Power valves, when open, allow additional fuel to enter the main well behind the main jets, through two holes called power valve channel restrictions (PVCR). The size of the PVCR holes is what determines the amount of fuel entering the main well, the valve just determines at what manifold vacuum this will happen. I have read that these holes are sized to provide from 25 - 40% added fuel compared to the amount of fuel that enters through the main jets. This extra fuel is needed when the engine is under load, as indicated by low manifold vacuum. Two stage power valves were originally designed for motor homes where a slightly enriched mixture was needed at 10.5 " Hg and full enrichment at a lower vacuum like 6.5 " Hg.

In a blowthrough system at part throttle we have some boost in the carb box, so denser air is entering the venturi. But we have put large jets in to deal with full boost, so the mixture is too rich. On top of that, the throttle blades are only partly open and there is still vacuum in the intake manifold. So the engine is not under a lot of load, and it doesn't need the super rich mixture we are giving it due to our huge "full boost" jets.

So what if you increased the size of the PVCR holes so that they could provide twice as much fuel, and reduced the size of the main jets accordingly? Now at low speed you would have a normal carb with regular jets. Off-idle performance would be normal, no rich jets. At full boost WOT the extra metering area would be provided by the enlarged PVCR holes. And at part throttle the two stage power valve would provide medium enrichment for the medium boost in the box.

Dave F has said that at cruising speed he has 5 lbs in the box and 3-4 " Hg in the intake. So he would need PVCR holes big enough to handle his 8-9? lbs of WOT boost and a two stage power valve that would allow enough fuel for 5 lbs of boost when intake vacuum dropped below 4.5 " Hg. He can't just use a 4.5 " single stage power valve, because he doesn't need the valve fully open until the intake manifold sees boost. His valve would be a 4.5/0.5 or something like that.

Now I know what you're thinking, it says right in the Holley book that two stage valves were restrictive and shouldn't be used in a high performance carb. So some redesign is necessary. And for this system to be tunable, you would need to have PVCR jets as well as main jets. So a new metering block casting is necessary. It is not a fully modulating system either just two stages, but much better than the compromise we have now.

Am I nuts or can this be done? Just a thought...

[ 11-30-2001: Message edited by: tomcat ]

[ 01-11-2002: Message edited by: tomcat ]

HPJunkie

Wowee!
Just 1 edit thats great! No really what a thought. What your saying completely logical. Just a new thought: I've got a nickerson carb, about 900 something, 850 baseplate and a so and so body, but the primary jets are 80's. Primary power valve is 4.5. Secondary jets are .144 (I drilled them out according to Dean N. I'm going to install EGT on the 7&2 cyl.(the ones I melted down) I think those are the lean ones, and record the temps. I've heard they should be 1300- 1350 ish, 1400 tops/borderline.
Last year was the first year with that combo. It started and ran well although a little "flat" or unresponsive ( or lean)between 3-4K then secondaries would open and what a rocket! At 4k I'd hammer it with a 25 mirage and the passengers heads would whip back just like a car. A bunch of us need to share ideas and get it right. Whew I wish I could type better. Thanks greg

kook

Solution: Get a Whipple?

pullmytrigger

Tom- Have you ever thought of selling your present motors and all your stuff you got kicking around in your garage and getting some 502mpi's and supercharging them?....carb problems solved. Doug

tomcat

HPjunkie - Thanks for the response. Where does you boost/vacuum gauge measure from, box or intake, and what are the readings from 3000 - 4000 RPM? Did the pistons melt at part throttle or WOT?

kook - Whipple and EFI may be the ultimate but I don't want to spend that much to solve a fuel metering problem.

Using HPjunkie's current jets I came up with the following calculations of fuel metering area, assuming that the PVCR holes could be enlarged to carry 100% more fuel and a two stage power valve would allow 0%, 50% or 100%.

Before

Jets 80 x 2 = .01005
add 25% PVCR = .01256 .01256
Jets 144 x 2 = .03257 .03257
no PVCR

Total metering area .04513 in2

After

Jets 72 x 2 = .00814
add 50% PVCR = .01221
add 100%PVCR = .01628 .01628
Jets 96 x 2 = .01448
add 100%PVCR = .02895 .02895

Total metering area .04523 in2

This accomplishes several things. Off-idle is leaner, part throttle is about the same (.01221 vs. .01256) with the first stage of the two stage valve, and if the throttle is opened a bit more, the second stage will allow the primary side to carry even more fuel than before. This should allow beter tuning of part throttle.

Now the secondary doesn't have to be so huge, and if the secondary throttles are opened at low RPM, the secondary side will provide enough fuel for low boost since that's all you will have in the box. As engine and compressor RPM come up and boost occurs in the intake manifold, the power valve allows full enrichment. It probably isn't necessary for the secondary power valve to be two stage, and the split in metering area between jets and PVCR holes is just a guess. I just figure that starting leaner and delaying the full rich mixture has to help the transition.

Total metering area at WOT is the same (.04523 vs. .04513) so full boost operation is not affected.

The Holley book says that PVCR holes can be enlarged and jet sizes reduced accordingly if that's what your engine needs. They also say that the holes can't be bigger than .120 inch or the power valve itself becomes the metering restriction. This sounds to me like the primary PVCR could carry as much fuel as a 120 jet minus whatever restriction the power valve presents to the flow. This could be in the ball park to double the flow with 72 jets as in my example above. On the secondary side, you may not be able to double the flow with 96 jets, I don't know.

Bruce and Dave F, jump in here, you both know more than I do. Can it be done?

[ 12-01-2001: Message edited by: tomcat ]

tomcat

I just thought of something else. Power valves measure the difference between the pressure in the fuel bowl and the pressure in the manifold. A 4.5" Hg valve opens when intake vacuum goes below 4.5" Hg and the fuel bowl is at atmospheric pressure. If the carb box is under pressure, so is the fuel bowl. One psi is equal to about 2" Hg, so if you have 5 psi in the box and and 4 " Hg in the intake, there is about 14 " Hg pressure differential across the diaphragm in the power valve holding it SHUT. It will not open until you have 5 psi in the box and 3 psi in the intake manifold. That means you are relying on the primary jets alone for cruising and must jet them rich enough since the power valve can't open until there is enough boost in the intake manifold.

This is good in one way since the second stage of the two stage power valve can be a normal 4.5" Hg spring. But the first stage would have to be set with a much stiffer spring. Holley's two stage power valve for motorhomes started opening the first stage at 12.5 " Hg. This would start to open at part throttle with 4 psi in the box and 4.5" Hg in the intake. The problem is that many high performance engines don't even idle with this much vacuum, so the power valve would be dumping 50% more fuel than necessary just off idle. I don't think that would be very much fun around the dock.

So let's say 8.5 " Hg for the first stage. Off idle should be OK and the first stage will open at 2 psi in the box and 4.5" Hg in the intake, or some similar combination. I'm using Dave F's numbers for boost above/vacuum below the carb and I'm wondering if his numbers are more extreme than a regular 5 - 7 psi kit. The higher the maximum boost of the kit, the faster the compressor is spinning at part throttle, so this above/below carb pressure differential becomes more extreme and the limitation of the power valve vs idle vacuum becomes more of a problem.

I wish I had access to a dyno. Who's interested in having someone like Bruce try this out?

Twin 29 - Money my friend, money...besides, I have better heads and cams in my 502s, to make as much power with the MPIs I would need head work, different cams, bigger injectors, a reprogrammed ECM etc.,etc. Better the devil you know...

[ 12-01-2001: Message edited by: tomcat ]

JimV

iTrader: (1)

Tomcat, this is "KAAMA" (I'm at JimV's shop). There's a marine engine building shop in S.W. Michigan that has built several Pro-Charger engines. The shop owner's name is Bruce Wolff...he's EXCELLENT when it comes to carbs & tuning various applications of engines. He can do the same stuff on carbs such as boost referencing as Nickerson for blower apps, etc, etc and is a little easier on your wallet. He seems to have a pretty good reputation when it comes to that stuff. You can try calling him at 616-422-1090 and tell him KAAMA (Mark) sent you.

tomcat

I talked to Bruce Wolff. Didn't get into detail about my idea since he honestly doesn't think a carb designed for drawthrough can ever work properly in a blowthrough application. He suggested a Roots blower with carbs was the best combination if you wanted to use carbs. Even better, just go big displacement with carbs.

I did some more reading in the Holley book, trying to find out if their power enrichment system can be altered to carry a lot more fuel. The answer is yes.

Most Holley carbs will have PVCR holes from .030 to .060 in diameter. But Dominator carbs have .0935 holes, and Holley makes a special single stage power valve #25BP-595A, that flows more fuel to feed these holes.

Carbs modified for alcohol also need about twice the fuel flow in all circuits. The PVCR holes need to be drilled to .125 to .130 and the same power valve as above is needed. Along with 95 jets on the primary side, this gets you in the ball park for alcohol. On the secondary side, you can remove the power valve, and take the jets out completely.

Because of these statements, I am satisfied that the PVCR enrichment system can be made to handle enough fuel for a reasonable amount of boost. It only remains to modify the high flow single stage valve listed above so that it has two stages.

To make the job of modifying a carb easier, it is also desirable that the power valve itself is the metering restriction, and is adjustable. That way you don't need PVCR jets. At the first stage of flow, the power valve will definitely be the restriction, but this too should be adjustable.

I am picturing a double spring instead of the single spring in this power valve now. The stiffer inner spring would force the valve open to an adjustable first position based on the length of that spring in its extended position. Then as manifold vacuum or rather the differential between carb box and intake decreased, the lighter outer spring would force the valve open to an adjustable final position.

This system would allow the carb tuner to simply drill big holes in the PVCR and then change power valves for different mixtures and opening points.

Holley's existing two stage power valve was meant to precisely meter a relatively small amount of enrichment at 10.5 - 12.5 " Hg. The first stage orifice is very small, and would never add the 50% extra fuel that could be needed. In fact, due to the design of the valve, the total flow of both stages probably wouldn't be enough.

By starting with the highest flow single stage power valve and restricting it for the first stage, this flow problem is avoided.

This is still just a two stage enrichment, not fully modulating according to pressure. But it would be better than what we have now. I would be happy with a system that had normal idle and off idle metering, an adjustable part throttle/low boost setting and a safe WOT setting. I could cruise at 75 mph and maybe break 100 at WOT. If for some reason, I wanted to cruise at 90, it would be too rich.

Is there a carb tuner (with a dyno) out there who wants to work with me on this?

Smitty

iTrader: (1)

This topic is the reason I bought a Whipple. I had a carbed magnum engine. I called Dustin for carb suggestion, he told me what to get and approx what jets to use. I did so and it runs better than it did stock!! To bad my boat is so heavy or it would really fly!!!

Dave

tomcat

No argument with the carbed whipple. It just doesn't fit under the hatch.