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propane performance tuning

propane performance tuning

have recently converted my 86 chev 3/4 ton 4x4 350 engine over to propane and would like some information on tuning the ignition and mixer. i had build the engine with specs that i thought would be good for propane. block is bored 30over decked and all the other goodies camshaft is a comp cams extreme 4x4 designed for low-mid performance intake is edelbrock performer heads are world products 67cc chamber high compression heads with 202 160 valves. approx 10.5 -1 compression ration. im also running comp cams 1.6ratio roller rockers for increased lift headers and good dual exhaust. on gas this engine seemed very strong but now on propane it seems to be lacking. any suggestions or recommendations on tuning. thanks

RE: propane performance tuning

Hi there,

I'm not an expert by any means but have been experimenting with Propane on my own vehicle for a couple of years.

I suspect the performance loss you're noticing is down to a number of factors. Since you mention a mixer I'm assuming you have a venturi unit installed in the intake to add propane gas to the air. This will lead to a breathing restriction which will have a greater effect the higher the revs, also the volumetric efficiency will be lower due to the addition of gaseous rather than liquid fuel. On top of this the calorific value of propane is lower than gasoline which will also reduce power.

Since you already have a mixer installed I guess you're stuck with that. I'm assuming you're already running closed loop fuelling (ie: controlled by an oxygen sensor in the exhaust) so the mixture ought to be resonable at least in steady state conditions. Your best bet for extracting more power is probably through ignition advance. On my own car I have a unit which alters the ignition advance when running on propane and it currently is set to add 12 degrees of advance over the gasoline settings. I don't necessarily believe the settings but it definitely advances the ignition on gas and has a dramatic effect on the pick up and general performace. Curiously it doesn't seem to have any measurable effect on the fuel consumption (at least on my rough and ready tests). Propane apparently does not suffer from pre-ignition so can stand more ignition advance safely. Optimisation would be best done on a rolling road.

On my own car (a 3.9litre V8 Land Rover Discovery) I originally had a venturi (mixer) based system fitted which performed quite well but had a noticeablty lower performance than running on gasoline with the unmodified engine. Even on gasoline the performance was reduced by the effect of the venturi restriction.

About eight months ago I replaced the front end of the system with a continuous gas phase injection system which removes the need for the venturi. This has returned the performance on gasoline back to where it was originally and after a considerable amount of tuning (fiddling!) the fuel consumption on gas is now only slightly less than when running on gasoline and if anything the car feels quicker.

If you want optimum power then liquid injection would seem to be the way to go because this overcomes the volumetric efficiency loss at a stroke and gives a good charge cooling effect. Having said this I don't know what systems there are commercially available at the moment. I know Ford in Europe fitted a liquid phase injection system to the 1.8 litre Focus but this suffered a lot of problems due to the injectors freezing and/or blocks of ice forming in the manifold.

Hope this helps



RE: propane performance tuning

thanks for the reply i guess i should have been more specific. im running a straight lpg system with a impco 425 mixer. what type of cost is associated with converting over to a continuous gas phase injection system ?

RE: propane performance tuning

I'm not sure what is available in the USA. The system I've got is a BRC Just Heavy which uses a different vaporiser connected to a flow control valve linked by pipes into injection points in the manifold. The control valve contiuously regulates the gas flow into the manifold as directed by the ECU.

Better systems use individual injectors which allow pulsed injection which can be done sequentially for better power and emissions.

As regards liquid phase injection I don't know of any commercially available at the moment, but I'm quite keen to give it a go myself.


RE: propane performance tuning

Although injection is popular in Europe and Australia, it has yet to take a grip here.  The old standby carburettor is pretty much the standard.  It is hard to vaporize enough fuel at pressure to feed a 5.7 L engine (and larger) without specialized equipment.  Liquid injection only works with very fuel specialized engines, and certainly not for the timid, or aftermarket.  Fuel injector pressures can reach 300 psig on hot days.  Stock injectors WILL NOT WORK HERE! ! !


RE: propane performance tuning

what do you recommend for being the best system for both performance and economy?

RE: propane performance tuning

Boy, thats a loaded question!

The basic venturi system is simple, robust, and performs well.  The mechanical carburetor is generally the mainstay here in the US, but is not as popular in other parts of the world.  I personally prefer the venturi for high speed work and the mechanical carburetor for low speed performance, but there are exceptions to both.  Vapor injection is without a doubt the most ideal all around system, but they are generally vehicle specific as for programming.

You need to identify what will be the target usage for your application, then play from there.


RE: propane performance tuning

most of my driving consists of offroading where low end performance outways highend 9 times out of ten. as well i do a fair bit of pulling my travel trailer out camping. thanks

RE: propane performance tuning

At the Advanced Transportation Technologies Center we did a lot of work with various aftermarket system back when conversions were popular. I like the GFI (Gaseous Fuel Injection?) which is a Canadian company but also distributed in the states. The key component of this system is the Compuvalve which is the computer and injection control module all in one. The injection volume is controlled by 4 or 5 solenoid valves which are either on or off. There are then two pulse-width modulated injectors to fine tune the delivery volume. This is a close-loop fuel metering system using a speed density strategy. One the calibrated fuel leaves the compuvalve you can deliver it anyway you like, or more accureately anyway that works. We have converted throttle body systems by simply fabricating a spray bar and inserting it the adapter on top of the throttle body. This works OK if throttle response is not an issue. We also did some where we used a port injection manifold, pulled out the injectors and fabricated discharge nozzles with a small hole at the port end (usually about .060"-090"). You can flow the nozzles and size them appropriately. This overcomes the transport delay time inherent in these systems. GFI has actually made some of these pieces available. I believe they were also doing a Chev. TPI conversion manifold for police vehicles. This would be a great set-up for you since TPI are readily available used and have some advantages in air flow capacity and tuning. I believe they were fabricating small lines and routing through the manifold internally.

I'm not doing a commercial but the GFI is kind of neat. It is software driven and the company has appropriate calibrations for many popular conversions or the calibration can be customized from a laptop. The laptop is also used to monitor and diagnose the system. They have a monitor that displays sensor and accuator data in digital, analog, and graphic forms, in color. On these tystem the software also has an injector flush utility. We found that this is very critical on LPG systems. The fuel contains compressor oil among other things and the oil creates a shock absorber effect in the solenoid valves which creates performance problems. We use alcohol and nitrogen for flushing. You will be suprised at the amount of junk that comes out. By the way we had similar problems on the IMPCO system that you are using. You indicated that you are using a 425 mixer and I assume you have a series "E" regulator. We found that the inlet at the regulator would slowly fill-up with solids and in extreme cases nearly shut off fuel supply. These have to be cleaned by disassembling the regulator.

It is important to get your fuel from a modern station with newer equipment. A lot of the LPG systems did not originally use a coalesing filter, but it is a good idea.

RE: propane performance tuning


I too have use the GFI, but hesitate recommending it for the lay person (no offense intended comptech).  One thing, this forum is not intended to do plug-ola for products which is why I did not mention specific systems.  The GFI FCAL tables and FMON may not be accessible to the public (I am pretty sure it is not) so specific programming is not really an option.  Some custom EFI or SEFI programs may be consumer tunable and may actually be a better solution.  One person recently converted a Dodge van from OEM CNG to LPG and is in the process of field testing the fuel injection system.  I am sure he will report his results.

The occlusion on the fuel inlet of the 425 will also happen with the orifices on the compuvalve, which is a real problem now.  The valve has a heater circuit which tends to pull the paraffins out of the fuel and plug the fuel passages.  This is the biggest problem with these injection systems on LPG.  Some parts of the world have far worse conditions than here in the US.


RE: propane performance tuning

You are absolutely right. I thought the GFI system held a lot of promise at first but ultimately it suffered some problems but is still a system you can work with. We have experimented with several other systems as well. You are right that the average user will not have access to FMON or FCAL but there should be some local support. We have not worked with conversions for the last couple of years and instead have concentrated on OEM light, medium, and heavy duty applications. Our latest projects have been with LNG.

RE: propane performance tuning

The BRC Just Heavy system I'm currently using is a simple continuous injection system that uses a flow control valve with a non linear characteristic coupled with a vaporiser that increases the gas pressure based on MAP. This makes a system that's much more resistant to clogging that the multiple port types at the expense of some response time. The gas is fed into the manifold by small pipes which can give rise to some transient fuelling issues.

Basic set up is not too complicated because all the default settigns can be used quite successfully and the fuelling map (flow valve openeing for engine speed verses MAP) is generated by driving at about seven set points while the software uses the lambda sensor to set the fuelling. This does require the use of a laptop, but the software is pretty good and for most applications the default set up will work acceptably.

The big advantage I've found is that the lack of a venturi in the system means the engine breathing is much improved (the petrol side is by fuel injection so no carburetor). Higher speed power is much greater on this system but curiously the fuel economy is virtually the same - I guess the venturi is pretty good at mixing. The vaporisers can cope with engines up to 180kW, for higher power levels than this two are used in parallel. The flow control calves cope up to about 200 kW - again two can be used in parallel.

I'm not sure if the BRC kit is available in the states and certainly in the UK the software is restricted generally to registered installers. I got mine by attending a training course run by the importers.

RE: propane performance tuning

It is interesting to see what remarks are made about the use of LPG in performance applications.  I would like to throw in some data that can be chewed over in relation to some points that have been raised.  LPG does have a lower BTU count than liquid fuel.  However this does not mean that you will produce less power but simply use more fuel to produce the same power.  LPG is used in vapour form.  Vapour takes up space, lost air flow, and so this is where a large part of the power is lost compared to a petrol equivalent.  If you want to make the same power on LPG across the same rpm range of the engine, make the intake bigger, this includes manifold, head flow, cam profile and of course induction system.  Of course not all of us have the money or desire to build an engine specific for straight LPG but if it was the case, bigger is always better for comparable power with a gaseous fuel.  The systems that are used in the US on the large capacity engines are laking in two key areas, flow and metering.  The reliability of the Impco system is excellent however the flow of a single 425 mixer is around 450cfm.  The fuel metering of a 425 is in some cases appalling at part throttle speeds, generally where you want your engine response.  Sure you can tune the units but you still need a depression (vacuum) above the piston diaphragm for it to move.  A vacuum is generated by a restriction and, well, a restriction limits air flow and hence power.  Ignition timing I find is always best set for a short advance curve.  In other words you would have an advance curve of around 10-12degs mechanical with an approx. initial advance of 18deg going through to 28-30degs.  This will give best low end performance as well as best top end performance.  Obviously this is a guide and will certainly depend on all things from engine through to LPG sytem.  I like the GRA products from Australia.  You can see them on  The best part of these systems are that they are SIMPLE.  The most desirable feature you could want in a fuel system.  Forget the injector systems that will gum up, forget the dificencies of the 425 mixer, these fully metered throttle bodies are probably the best keep secret in the LPG industry world wide.  They are fully tuneable, giving you the reliability of a simple converter with the flexability of a fixed fuel metering device.  In Australia, the company selling them will give a money back guarantee that the power and economy will improve over any LPG system fitted to that vehicle.  They can be used in any naturally aspirated or forced induction application, again totally flexible.  By the way, these systems in twin throttle body form are good for upto 600hp (If the engine is capable).  Simple and effective.

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