Home Up Contents Search Links

Model 425
Up Products Discussion Automotive Show-Offs

 

 

While I was driving both of my propane vehicles, it seemed to me that the fuel economy for both cars was lower than I was expecting. I was expecting propane gas mileage to be somewhere in the neighborhood of 80% of gasoline gas mileage. The driving I was doing at the time was highway commuting with steady speeds of 60 mph.

It seemed to me that my New Yorker with the 440 engine would be cruising with a richer than necessary mixture just because its airflow would be at the upper end of the range for the 425 mixer. Later on, I was experimenting with supplying the 77 Pontiac with cooler fuel by throttling back on the water to the evaporator. I think that the 425 mixer was better sized for the 350 CID engine but the fuel mixture was too rich due to the cooler fuel.

My understanding of Impco gas valves is that the valve profile provides a richer fuel mixture as air flow increases. The only place I found gas valves mentioned is Jay Storer's book "Economy or Performance Propane Fuel Conversions for Automotive Engines" but he didn't discuss it in any detail. I emailed my Impco dealer and he confirmed that Impco makes a leaner and richer valve (3 valves in total) for the 425 mixer. He also said that he knew of some experimentation with different valve profiles but didn't have any further information to give me. I don't recall water flow to the evaporator discussed in any detail. The Impco service manual I have is not helpful nor is Impco.

Thanks for the explanation. I understand that engines are essentially an air pumps operating at different displacements. I was hoping to learn about fuel mixture enrichment at higher air flows or with larger engine displacements, if that is actually the case. It almost sounds as if you are saying the standard gas valve is designed to provide a stoichometric fuel mixture for all air flows.

The 425 mixer may have been designed with a 350 CID engine in mind under normal conditions. However, my understanding until now was that the the fuel mixture becomes enriched at higher air flows or loads. That is why I was suspecting that my 440 CID engine with the same mixer (and gas valve) had higher than expected fuel consumption because it was getting a richer mixture than necessary due to the profile of the gas valve.

Regarding fuel temperature, I was searching the postings on www.eng-tips.com and noticed a posting concerning this topic (http://www.eng-tips.com/viewthread.cfm?qid=53767). You recommended placing a control valve on the outlet of the converter to control exiting water temperature to 75°F. I tried looking for automotive-grade temperature control valves without success so far. Which valve did you find to work well in this case?

When you say that cooling the fuel has been shown to be minimally beneficial, this has to do with the minimal effect cooled fuel has on increased power from its cooling the air. As I suspected, cooler fuel does require an adjustment to the fuel mixture. However, the only fuel mixture adjustments on 425 mixers are on the idle and power circuits. The gas valve profile controls the cruise fuel mixture and is not adjustable.

If fuel temperature does have an effect on fuel mixture, does the evaporator have a design temperature? It seems to me that the correct installation of an Impco converter is for it to receive water from the heater core. The maximum water temperature in this case would be 160°F to 195°F during the summer.

I have heard about formation of oils in the converter with high temperatures. However, I have also read somewhere that this oil originates from the residues in the pipes at the refinery, which then contaminate the propane. If the oil is formed from high temperatures, is there a critical temperature below which oil will not form?

I have done some research into Impco gas valves and received some information from Impco’s technical department. I have paraphrased their answers for this posting. They did not provide specific profile information, as this is proprietary intellectual information.

There are four gas valves available for the Impco 425 mixer. The AV1-16 is the standard valve that comes with the 425 mixer. The AV1-1637 valve supplies a rich mixture and is used with the EC1 system, which is now obsolete. The AV1-1644 valve supplies a lean mixture and is used with 370 CID engines and above. The AV1-1651 gas valve is required for the feed back carburetor.

The gas valves do not necessarily keep a stoichiometric fuel/air ratio across the entire flow range. That would be only in a perfect world. The airflow remains constant with each air/gas valve assembly based on the lift of the assembly at different load points. The difference in the assemblies is the gas valve portion that is cone shaped. Each gas valve has it's own unique contour and length thus resulting in different gas flow at the same air flow or lift point of the diaphragm assembly. In other words, the flow curves are fairly linear and the rate at which the fuel mixture becomes richer with increasing airflow is different for each gas valve.

The 425 mixer has adjustability at both idle and full flow. The AV1-16 is fairly linear in the flow curve. The AV1-1637 is also linear but is richer throughout its flow curve than the AV1-16. As far as use with the EC1 system, Impco’s technical department does NOT recommend nor do they believe that IMPCO continues to offer this option. The EC1 was quite complicated to adjust and was used to provide a lean cruise mixture. This was achieved mechanically via the use of vacuum source on the atmospheric side of the regulator. The AV1-1644 is also linear but is leaner throughout its flow curve than the AV1-16. The AV1-1651 is richer throughout it's flow curve relying upon a processor to evaluate the air/fuel ratio via an oxygen sensor signal and thus adjusting the output from the regulator by varying the atmospheric pressure through a fuel control valve. Although Impco did not say it, I believe that the AV1-1651 valve must maintain a constant ratio of fuel to air so that the feedback pressure regulator in the converter could regulate the fuel mixture by adjusting the fuel pressure (and thus density) supplied to the mixer.

Although it does not state this in the Impco catalogue, the 425 mixer will bolt directly to most all Holley 4 barrel throttle bodies without the need of an adapter. You can order a mixer with the standard or feedback air/gas valves already installed. As the EC1 valve is obsolete and as you would not want to install any other valve in the feedback mixer, the only option available for changing gas valves is to install the AV1-1644 valve in the standard mixer.

Even if you were to find an AV1-1637 valve, there is no reason to install it because engines respond to richer gaseous fuels opposite to the way they respond to liquid fuels. Richer mixtures burn slower and result in burned exhaust valves. They also cause the engine to be more susceptible to detonation or pinging.

My understanding of carburetion flow ratings is that there is a characteristic pressure drop for every flow across the mixer. At a flow of 460 CFM, the 425 mixer will have a pressure drop of 1.5” Hg. At higher flows, the pressure drop will be higher and at lower flows, the pressure drop will be lower. However, the important thing is that the gas valve is all the way open at 460 CFM.

Impco responded that the 1.5" Hg is manifold vacuum that is very close to wide-open throttle on any engine. 460 CFMs is the maximum air that the 425 can flow at that manifold depression. Since the air/gas valve is variable, if the engine does not require 460 CFM of air, the air/gas valve will not open all the way. If the application requires more then 460 CFM's of air, then the air/gas valve will be all the way open and the engine will starve for air above 460 CFM and the 425 would not be the carburetor of choice.

You need to remember that these are Air / Gas valve assemblies. The amount of air and fuel are both controlled by the movement of the diaphragm. The capacity of any carburetor is defined by its ability to flow air. The maximum amount of air that the 425 can flow on a normally aspirated engine is 460 CFM. Generally you can adjust the high flow fuel adjustment to be very rich even when the air/gas valve is at the full open position unless the regulator being used has also reached it's full flow capacity. Even if you have a 600 CID engine capable of 10,000 RPM and if you had the 425 carburetor on it you would only be able to flow a maximum of 460 CFM's of air, thus starving the engine for air

I disagree somewhat on this point, as a 600 CID engine capable of 10,000 RPM will flow more than 460 CFM with a 425 mixer. My disagreement is that that engine will continue to draw more air with increasing RPM but the pressure drop across the mixer will become larger. The engine will become starved for air because the density of air filling the cylinders will become less due to the increasing pressure drop. That is, the large pressure drop across the mixer will dramatically reduce the volumetric efficiency of the engine. The more important problem will be that the fuel mixture will be uncontrolled at excessive airflows because the gas valve reaches the limit of its travel at 460 CFM.

 

 

Send mail to fraso AT mergetel.com with questions or comments about this web site.
Last modified: November 11, 2004
Hosted by www.Geocities.ws

1