Dear All,

I am planning to build my engine with a suggested cam RV15M (wicht lies next to my bed at the moment ). The question is, I also run on LPG with my current setup, will I be able to use LPG for street/daily rides? The octane in LPG is 104, more than gasoline, it should be fine, does anyone has an experience?

Thanks,

The important consideration when building an engine (propane or gasoline) is to match the carburetor and camshaft to your performance requirements. Your engine fueled with LPG should work fine with this camshaft. For best performance and durability, keep your CR below 11:1.

Generally, most people convert to propane to reduce their fuel costs. In conjunction with lower cost fuel, a low RPM torque camshaft for improved fuel economy would also be a good choice.

Are you planning to continuing to use a dual fuel system with the new cam?

Well, thanks for the info.

I plan to use dual fuel. My Static CR will be around 9.5:1 and my carb is Carter BBD dual barrel (from a 318 cid). I guess it will be enough for my engine needs. I also think LPG should do no harm.

You're right and properly set up engines actually last a lot longer on LPG. I've been using it for many years and still think it's great (see Propane Impala).

Propane and gasoline require different advance curves. Since the Dual Curve PN 8795 and PN 40622 are no longer available, you will have to pick the fuel you use most often and set up the advance curve for it. The other alternative is to use a programmable ignition controller like Megajolt Lite Jr. (like Autosport Labs MJLJ).

Although propane is a dry fuel like unleaded gasoline, you should still make sure that your valves and valve seats are in good condition. Running rich propane mixers tends to burn out exhaust valves so make sure that your fuel mixture is correctly set up. If you have the heads off, hardened valve seats are a good idea. Propane is also more demanding on ignition systems so you will need to make sure that you use top quality parts and keep them in good condition.

A lean running LPG engine will burn the exhaust valves and seats. So get a system that is big enough for your engine and get it properly tuned by a shop.

You can get almost anything running on LPG but you will lose power, depending on the system used. It's a good idea to convert to electronic ignition if you stll have points. LPG backfires can be very violent. A stock electronic ignition should be sufficient for street use.

LPG works opposite to gasoline with respect to fuel mixtures & exhaust valves. Because LPG is already vaporized in the carburetor, it cannot provide any cooling effect from the latent heat of vaporization like atomized gasoline can. Extremely lean LPG fuel mixtures and/or ignition cross-fire can result in intake backfires. Tuning on a chassis dynamometer with a exhaust gas analyzer would be ideal.

Ensuring that the spark advance is correct will also help to protect exhaust valves.

I totally understand your point, tuning on dyno with exhaust analyzer makes sense, also I agree to everyones thoughts about burning the valves... I have electronic ignition but I just can not understand how to recurve the distributor or understand if I did it correctly. How can I measure or check if I recurved it in a better way ?

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As I read many posts from different forums, I understand that its a trial and error method to get the best possible curve for ignition. I guess the aim is to advance the ignition as soon as possible without pinging? Is this formula correct?

Thanks again and again and again...

With 9.5:1 CR, you will likely not notice any pinging on LPG unless it is way over-advanced because of its 104 octane rating. LPG burns at a different speed than gasoline so I would not just arbitrarily advance the timing of your distributor. A general rule of thumb is 14-14-14. That is, 14° initial + 14° centrifugal + 14° vacuum, with the centrifugal advance all in by around 2500 RPM. Every engine is different so it's not a hard and fast rule. See the following pages for more information:
Recurving Ignition Advance
choosing an ignition curve
Carburetion Best Practices

The goal in setting the mechanical advance is to produce maximum power at full throttle. Since you are not limited by pinging, you have a lot more flexibility to select both initial and centrifugal advance. You can run a more initial advance, but you will likely not gain too much power with more than about 32° of mechanical advance.

The goal in setting the vacuum advance is to product maximum fuel economy. Once you've set the mechanical advance, start off with a moderate amount of vacuum advance (say 10°) and advance it until your fuel economy starts to decrease. Once you find this point, reduce the vacuum advance to the lowest timing that gave you the best fuel economy.

You might also want to try SlantSixDan's recommendation to use NGK ZFR5N spark plugs. The more central location of the gap of this plug will likely help to reduce advance requirements slightly.

While you're doing this, please remember that fuel mixtures affect advance requirements. You might want to consider using a feedback controller for low emissions and good overall fuel economy.

As for running 160°F thermostats, although this might theoretically improve power slightly on HD vehicles, I have found that using hotter water in the converter (180°F) provides better fuel economy.