Front-Blown "Potvin"-style fuel injection plumbing.

[Ace-Garageguy]

Landman PM'd me asking for some assistance interpreting the maze of hoses that are sometimes seen on these somewhat unusual drag-racing blower / mechanical fuel-injection setups.

Thinking this might be of interest to other modelers here, I've decided to put the answer up in tutorial form for everyone to see and bookmark for future reference.

There are two distinct plumbing styles for these setups, one of which is simple and only runs lines to the injector bodies mounted to the side of the blower itself, and a more complex arrangement that routes lines to individual injectors (one for each cylinder) drilled into the ports on the intake manifold.

The simple arrangement was the more common in the time-space continuum I inhabited when real competition cars still ran these things, but we'll cover both types (some rather famous cars did in fact appear with both types of plumbing at different times).

The shot below is of the famous old Mooneyes Dragster, and it's shown here running the same relatively simple setup that the equally famous Orange Crate appeared with (as was modeled by Revell). Both of these cars also appeared at different points in time with the more complex setup, which may have been because of mixture-distribution problems arising from the simpler style.

This is kinda the "standard" setup, and if you understand what's happening here and generally how it works, you'll have no trouble interpreting photos of cars that have other plumbing arrangements.

Notice that the injection pump is bolted to and driven from the front of the blower housing. Also notice that the injector body has 4 ports cast as a unit, and that there are no injector lines on top of the engine.

There is usually a large supply hose running from the tank to one side of the pump, and a slightly smaller return line running from the pump back to the tank. Tuning is accomplished by changing the size of the orifice in the restrictor "pill" on the return side. A pill with a larger hole allows more fuel to bypass the engine and return to the tank, making the mixture leaner. A smaller hole in the pill forces more fuel to flow TO the engine, richening the mixture. These things were designed to really only work at WOT (wide-open-throttle), so tuning wasn't very precise under other engine conditions. From the high-pressure side of the pump, a line runs to the throttle valve (sometimes called the "barrel valve") which is connected via linkage to the throttle pedal and butterflies. As the throttle is opened, this valve also opens, allowing more fuel to pass through it. From the throttle valve, fuel flows to the injectors themselves. The barrel valve can be seen just aft of the injector body in this shot (still looking at the photo above) with 4 lines running to the injectors...and that's about it.

Here's a shot of the Orange Crate, running the same basic setup, but with only two injector ports bolted to the blower this time.

The more complex variation on the same basic theme is shown below. The pump is again mounted to and driven from the front of the blower, and the supply and return lines and tank are fairly clearly visible. The barrel-valve is also located on the other side we can't see, linked to the throttle as before. However, here an additional line goes up to a "spider" mounted on top of the manifold. This can be nothing more than a fuel-distribution block (or if it's known as a "metering block" other tuning adjustments may be possible, and the throttle linkage may also control opening and closing, synchronized with the butterflies) and eight individual pressure lines run to eight individual injectors mounted directly in the intake ports. This arrangement lessens the likelihood of uneven fuel distribution due to poor mixing or pooling of the fuel in the intake tract, and can also make a blower explosion a little less likely (the facts that the blower was lying on its side, and that the long curved intake pipes had to travel UP to get to the intake manifold, could tend to cause uneven fuel / air distribution to the cylinders; atomized fuel could easily fall out of suspension in the air stream, due to the twists and turns and sharp corners in the intake tract). You'll almost always see a backfire "popoff" valve on the rectangular box the long intake pipes come out of, too.

It's really pretty simple once you understand how it's supposed to work. All the Hilborn-style mechanical injection systems work basically the same way, though there can be many specific detail, plumbing and linkage differences from car to car, and as noted earlier, even on the same car at different times.  

                                                                                         Any questions?    

 

 

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Edited January 26, 2017 by Ace-Garageguy

[jwrass]

Ace,

Well that was easy...... Clear, Concise and very informative.... No???? here!

Great Post!

"RASS"

[DonW]

When I was racing my BMW 2002 turbo we used an extra fuel injector into the plenum chamber, to enrich the mixture (it originally had a single injector into the throttle body). We tried a set-up with an extra injector per individual inlet port but that drowned it out at anything under 7000 rpm, so wasn't of much use! If only we'd had a bigger development budget...

[DonW]

And thanks Bill for the lovely shot of the crate motor. The greatest hot rod never properly raced, I love it anyway.