Decision Time: X-450a or Impco 425

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FrankGRUN
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Joined: Tue Jul 22, 2014 8:31 pm

Decision Time: X-450a or Impco 425

Post by FrankGRUN »

Just today I obtained an MM X-450a C8-1 mixer and X-1 vaporizer. The mixer is clean and I'm assured that both components were taken from a working dual-fuel truck. I also have an Impco 425 mixer with rebuilding kit and a Model E regulator with rebuilding kit.

My intention is to mount one of these on my 1982 motorhome Ford 460 engine and operate it in dual fuel mode. I have assembled all the necessary parts including a Dual Curve Timing Module, Impco Commander controller for closed loop operation and the FSA-1000 tuning module. The engine is freshly rebuilt with a high torque (2200-2800 rpm) cam, cam timing at 2 degrees advanced, stock 7.8:1 compression, Edelbrock Performer 2166 dual plane intake manifold and Edelbrock 1411 carburetor. My key problem is a lack of engine bay space and specifically height over the carb air horn. This forces me to use an Impco AA2-46 low height air horn adapter to mount the mixer, and the mixer must be mounted on its side.

So back to my need for help: Which mixer should I use? I need the best operating efficiency I can get in cruise together with good operability at idle and lower speeds from the LPG system. Cruise is typically 2300 to 3000 rpm. In reading this forum and Franz's Diagnostic Guide, operating the 425 or the x450 on its side is less than ideal, but is there an advantage to one or the other? I'm lead to believe that the x450a is the more advanced design. Does this mean more fuel efficiency? I also understand that the x450 flow rating is closer to a match for this engine. I will use the Model E converter and electric lock-offs as I understand that it is more reliable. I am assuming that closed lop control will give better economy, and that both mixers are compatible with the Commander controller.

Finally, if I use the X-450, how does one lock up the air valve to be able to operate in the gasoline mode with minimal air flow obstruction?

Any help will be highly appreciated!

FrankGRUN
Posts: 10
Joined: Tue Jul 22, 2014 8:31 pm

Re: Decision Time: X-450a or Impco 425

Post by FrankGRUN »

After giving it much thought, I have decided to go with the X-450a and the X-1 converter, mounted with a MM low clearance adapter. So again I have to request additional info about the X-450a. The key issue is that this will be a dual fuel application. There is very limited information on the internet about the X-450a and how to use it in a dual fuel mode. Specifically, i need to know if I have to implement a vacuum lift to retract the air valve to minimize the restriction in the air inlet path when operating in the gasoline mode. If I need to do this, as required if I had chosen to use the IMPCO 425, I need some directions as to how to go about implementing a vacuum lift on this OHG unit.

Any assistance would be greatly appreciated!

franz
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Re: Decision Time: X-450a or Impco 425

Post by franz »

The X-450 can be modified for vacuum lift for dual fuel use. You will need to remove the top cover and drill the very top for a vacuum fitting, then to a solenoid connected to a switch when on gasoline. Be sure the vacuum fitting is in the very top and not the side. Clearance may be an issue. Be advised that this is not always successful since the vacuum piston will drop at idle and low load, bleeding the vacuum in the top cover and allowing the gas valve to drop, increasing the inlet air restriction and probably upsetting the airflow through the carburetor.

The X-450 was introduced at the very end of the carburetor days and works best on fuel injected vehicles where an inlet restriction is not an issue. I was always a fan of the X-450 but there are times when it is not the best candidate.

For what its worth, the Impco 425 was more successful since the gas valve doesnt have the power piston to contend with. In this case, I would favor the 425 over the 450. The height of the 425 bolted directly to a Holley baseplate wouldnt be much different than an X-450 with an adapter.

Hopefully this will help some.

Franz

FrankGRUN
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Joined: Tue Jul 22, 2014 8:31 pm

Re: Decision Time: X-450a or Impco 425

Post by FrankGRUN »

Thanks very much for your reply. It is unfortunate that there is so little information available about the X-450. I have been studying the unit and have been very impressed with the design as best I can understand it from disassembled parts. Prior to your reply, I did a U.S. Patent Office search trying to find a cross-sectional drawing and more functional details. I searched on OHG as well all of the principals that you had listed in your post on the history of OHG. unfortunately, Imcould only find material on the IMPCO designs and on the apparent precourser to the X-1 converter.

Following your comments, I plan to modify the X-450a and use the X-1 converter, but hold the IMPCO CA425 and it's air horn adapter in reserve, should the lift fail. In view of your warning that the vacuum lift could be overcome by the vacuum piston at idle or low load, I plannto make a large diameter port in the center top (0.5 inch bore) and fabricate a small vacuum plenum there with a 3/8inch ID hose through the air horn adapter to manifold through the standard vacuum lift switch. I'm hoping the higher effective flow through the lift circuit would dominate the path to the vacuum piston. Unfortunately, it is not clear to me how the two conditions of idle (high manifold vacuum) and low load (perhaps 6 to 7 inches less) would largely differ from cruise (probably 3 to 4 inches less than that at idle).

A final point of confusion here. When I purchased the X-450a, X-1 and airhornnadapter it was assembled as one piece with an IMPCO air filter mounted to the mixer. It was dirty and the interior of the airhorn adapter and the interior of the X-450a was covered with a layer of black oily deposits, indicating use. The airhorn clearly bolts to the standard 5.25 inch 4bbl card air cleaner flange. The son of the original (now deceased) owner told me that his father had run his GMC truck in a dual fuel mode for some time, but was unhappy and removed it setting it aside for years. Examining the unit as I bought it, there are no vacuum lift modifications that I can see. Just two ports, one to either side of the idle mixer screw - one with a impressed brass tube (to the right) and a threaded 0.125 inch pipe pier with plastic right angle fitting. There is also a threaded 1/4-20 port just above the manifold mounting flange, below the plane of the other three ports (counting the idle mixture screw) that is filled with a socket cap screw. If, as I must assume, this setup had been operated in a dual fuel mode, how could it have run? Is the intake velocity and flowing air mass moving for any given throttle opening adequate for proper carb gasoline metering? Are there any measurements of air flow restriction that suggest that cfm delivered at any throttle level is adequate for proper running without the addition of the vacuum lift? Finally, is it possible that any input restriction could be compensated for by carb rejetting?

Sorry for the endless questions, but the X-450 is technically fascinating. When I first saw your comment about the oblong wear possibility for the metering rids when the x450 is mounted sideways, I immediately thought of having the metering rod bores coated with Si3N4 or Al2O3. In my pre-retired life we developed a system to coat surfaces with a few 10s of monolayers of these very hard materials to effectively eliminate mechanical wear on frictional surfaces.

FrankGRUN
Posts: 10
Joined: Tue Jul 22, 2014 8:31 pm

Re: Decision Time: X-450a or Impco 425

Post by FrankGRUN »

While awaiting responses on this forum, I have contacted Athol Mullen, of Mullem Automotive Engineering in Australia, about the dual fuel use of the OHG/MM X-450a. In my internet searches, I found several excellent archived posts by Athol on the tuning of the X-450 and I requested his comments and opinion. I quote his reply :
Mullen Automotive Engineering 4:11am Sep 10
Any variable venturi mixer such as an Impco or OHG will work without being locked open, but will present a greater restriction to flow. While Impco run 1.5" vacuum constant depression, OHG only run 0.5", so the restriction is enough less that the rare times that they are used in dual fuel applications, they are used with no force open mechanism. There really is no way to make a vacuum lift work, as the slide and power valve air bleed port are designed to deliberately leak vacuum.

To assist in lifting the slide further open and to prevent the petrol mixture being upset by a vacuum leak, you can fit a solenoid in the power valve line so that when on petrol, there is no vacuum on that port on the mixer. The spring on the power valve will therefore "close" the air bleed port, which will increase the opening position of the slide, reducing restriction further.

The alternative is to have some form of bypass air valve that seals shut when running on LPG and opens when on petrol.

Just in case you're not aware of it, the X-450 will not run properly unless the power valve is Teed into the vacuum line between the PCV valve and manifold vacuum. The PCV acts as a regulator to prevent the valve from fluttering as it will if connected directly to manifold vacuum. The bottom of the power valve also delivers LPG down the vacuum line when the valve closes, so if it flutters the mixture varies wildly until the engine stalls.
His comments are, of course, consistent with Franz's comment about the power valve leakage.

Following his comments about the air restriction represented by the X450, I looked into measured air restrictions with air cleaners and air boxes. EPA studies of air cleaners (new and clogged) and their effect on fuel economy of both carbureted and fuel injected engines show that the average impact on air flow by adding a new air cleaner is 1.75 inches of water. The impact of a clogged air cleaner (as defined by the EPA as needing replacement) is 10.5 inches of water. In tests of 5.0L and larger engines, carbonated vehicles showed an average loss of 2.5% in fuel economy when operated with the clogged filters.

This and Athol's comments suggest that a direct placement of the X-450a along with solenoid control of the power valve system should be quite effective for operation of the Ford 460 below 4500 rpm. Given that I will be calibrating the Edelbrock 1411 carburetor with the data logging Innovate LM-1 wide band air fuel ration meter, which also logs rpm and manifold vacuum, I plan to repeat the calibration runs with the X-450a mounted with the final air cleaner arrangements and determine its impact on the system without vacuum lift implementation. I hope to get to this within 2 weeks, if all goes well.

I'm continuing this posting in case others are interested in the same pathway.

storm
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Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: Decision Time: X-450a or Impco 425

Post by storm »

Athol Mullen is brilliant, he has a facebook page for his business that he posts bits and pieces of information on so people can keep up to date with what's happening in the automotive scene (specifically engineering) in NSW Australia.

I'd be very interested to see your logs and the difference between the MAP or Vacuum before and after fitting the X450.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

franz
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Re: Decision Time: X-450a or Impco 425

Post by franz »

I've been very busy working on my consulting business that I missed the last couple of posts. I do not know of Mr. Mullen and look forward to reading his input as well. My experience with ANY of the air-valve mixers when placed above a standard carburetor is that it alters the metering, enough that it may cause excessive rich running. On the last setup I tried, an Impco 425 above a Rochester Quadrajet of the late 1980's, we could let the engine idle without the mixer, then just place the mixer with adapter directly on the carburetor. It would cause the engine to die, every time. The placement of the mixer above the carburetor acts like a choke, it artificially decreases the inlet pressure (increases the vacuum).

Hope this helps a bit.

Franz

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