Dual Fuel Setup and Calibration Question

[FrankGRUN]

I'm finally about to finish restoring my 1982 E350-based Dual Fuel Motorhome. I had to replace the stock motor with a remanufactured 460 long block. The replacement has been built with a Melling MTF-3 RV cam, and the cam timing has been set to 2 degrees advanced from the 8 degrees retard factory setting. I'm using an Edelbrock 1411 750 cfm carb with an Edelbrock Performer 2166 manifold. The smog hardware is disabled (EGR and air pump dysfunctional) and the exhaust system consists of Headman Elite Long Pipe ceramic coated headers feeding a 3" single exhaust through a very large high flow muffler. The right and left side exhaust pipes from the headers meet about halfway from the right side exhaust manifold to the inlet to the muffler. Duals were not possible because of interference with the motorhome holding tanks. The ignition system uses the Ford Duraspark II distributor, recurved for RV applications, and a Crane Hi-6s ignition multispark system. The plugs are Autolite Iridium XP-25. The transmission is a C6 automatic with RV torque converter and a Gear Vendors Overdrive. In overdrive I'm turning 2300 rpm at 55 mph. The motorhome is 24 ft. long, weighs 11,000 pounds and resembles a large barn door moving down the highway. I'm trying to build the motorhome for the best economy I can at moderate cost.

When I added the headers, the airflow in the stock engine increased to the point that the 1411 was running rich and I had to rejet to 1 stage leaner main jet and metering rod.

Now as I convert to dual fuel, I will be bolting an IMPCO 425 to a right angle adapter to the top of the 1411 carburetor. I will use an IMPCO Model E converter and a Dual Curve Timing adapter as well as the IMPCO Commander Controller for the 425. Another right angle adapter will be used to connect to the high flow in-line air cleaner.

My question is this: With the right angle connection to the IMPCO 425 together with the air flow restrictions intrinsic to the 425 mixer when operated with the gas valve retracted (in the gasoline mode), will these restrictions impede the air flow seen by the Edelbrock 1411 to such an extent that the 1411 has to be rejetted or recalibrated from the calibration achieved before mounting the IMPCO 425? I have been unable to find and information or specification as to air flow restriction for the un-operating 425 in a horizontal or vertical position.

Any help would be greatly appreciated.

Thanks

[C3H8]

That's a lot of info. Initially I would say you would have troubles even with the valve lifted at higher RPM. Your other issue is the elbow. It will likely cause more of a problem than the mixer or in combination with the mixer the two may restrict the airflow. It does not sound like you plan on running very high on the RPM band though. I would estimate that under normal circumstances neither the elbow or mixer will cause you problems at anything under 4000 RPM. You may have to rejet the carb again for running on gas.

[FrankGRUN]

Thanks for the advice and comment. I have put together an amateur flow bench and can't measure any significant pressure differences with my manometer. I will be using the Innovate LM-1 wide band system to compare air/fuel ratios this week, with and without the mixer in place. I now have the option of using an Impco 425 or a X-450a mixer but have to use an Impco AA2-46 air horn low height adapter because of space restrictions in the engine bay. As soon as I can decide which mixer to use, I'll go ahead with the calibration.

[Frank]

Welcome to the forum! Can you post some photos of your flow bench?

[C3H8]

A couple of things to keep in mind. Besides your flow bench, which is an excellent idea, you can also use engine vacuum to test for additional restriction. One of the tests we used to use in designing venturrii styled mixers was to operate a vehicle on gasoline, testing the engine vacuum at specific speeds with and without the mixer installed. We used to do this both on the road and using a chassis dyno. This worked well to show us if there was a big change we had to be concerned about once the mixer was installed. Restrictions can also cause issues with the carburetor venting.

This will also be important if your secondaries are vacuum operated. Any restriction caused by the mixer and adaptor could change its opening rate and corrections might be required to compensate for this. You haven't mentioned if your secondary operation is mechanical or vacuum and I am not familiar with the Edelbrock carbs.

I also have a question or comments regarding your use of the commander controller. These controllers are designed to run stoichiometric or 15.5:1 fuel/air ratios to satisfy the needs of a catalytic convertor. A standard IMPCO mixer will normally operate leaner then this during light to medium loads. These controllers are not made to maximize economy. They are made to reduce emissions when used with a cat. They do help economy when an air filter begins to get fairly dirty as it will correct the fuel mixtures as they richen due to air filter restriction. It will also help to control the mixtures during acceleration. The key question in using a fuel controller on this vintage of vehicle is how well is the vehicle maintained? If it is well maintained and driven reasonably it is quite possible to achieve better economy without the controller. In saying this the other factor is your vehicle, as you stated, is 11,000 pounds and shaped like a barn. The idea of light or medium loads may not even be viable so the controller may help a lot.

By the way, my personal preference for this side mounted application would be the 425.

[C3H8]

Further to the controller. I have seen some dealers use this device with the standard IMPCO valve using it to keep acceleration mixtures and high load mixtures a little leaner. Once the mixtures lean out during cruise mode the controller just reaches a 100% off status. On heavy vehicles this can be very beneficial climbing hills and in strong headwinds to help economy as long as it is not pulling the mixtures too lean for the conditions.

[FrankGRUN]

On the flow bench, I followed, with some variations, the homebuilt flow bench described in detail by Tony Drew at this link:

http://www.tonydrews.com/Flowbench/FlowBench.htm

In my modifications, I use two Drywer manometers with a full scale of 10 inches of water. Both purchased years ago at an industrial salvage store. I use a unused Central House Vacuum for a 200 cfm vacuum source drawing through a Corvair 4BBL central plenum on which I mounted the Edelbrock 1411 carburetor. For these measurements, I measured the internal pressure above the primary and secondary venturi. I tested the relative pressure difference for the stock 14" air cleaner, an Impco right angle adapter connected to a Spectre 6" diameter in line air filter/airbox assembly, then the Impco AA2-46 air horn adapter with the 425 mounted and the previous Impco right angle adapter connected to the in line air box. Basically, all I found were small changes of 0.1 to 0.5 inches of water. Variations small enough to be possibly caused by room temperature variations, and operating parameter differences (real speed of Central Vacuum, etc. In the past, I have used the manometers to form cold air intakes and manipulate pressure differentials in flowing air through the radiator and the engine compartment, evaluating the effectiveness of air dams, etc. These differences at this CFM level, close to my cruising speed requirement, just suggest that at these flow rates, there is little impact of the air horn adapters and the fully retracted mixers.

[FrankGRUN]

Thanks for your thoughtful and very helpful comments. I'm now preparing to first calibrate the carburetor circuits with the Innovate LM-1 system using the wide band air fuel sensor. I have a manifold vacuum digital sensor that I will add to the LM-1 aux log capability as well as a g-force accelerometer sensor. All this will be done with the 14" air cleaner and then checked against the AA2-46 air horn adapter connected to the Spectre in line air box. Then I'll add the mixer and study the impact on the calibration.

The Edelbrock 1411 has vacuum operated secondaries.

After following your comments about the Commander, I'm inclined to first implement the LPG system without it. I can always reconfigure the Impco system for it later. If I can figure out how the shut off (retract) the OHG X-450a system during dual fuel gasoline operation, I'm inclined to go with the 450 - just because of the implied higher technology. Of course everything here is seriously limited by the lack of space in the engine compartment!