MPG would be very bad. You are think pump gas CR here with turbo. Think differently. You could run 10 to one or better and a super charger (I do not think I would use a turbo here because of the way the gas needs to mix and you would not want it in a hot turbo) and you would not have any knock problems and have great power too. Propane is around 110 octane so adjust CR accordingly

 

as high as you can and still have a usable motor...again...the higher the compression the more stress, heat and driveability issues. I doubt that you want to go over 9:1 DCR and ya best be concerned about DCR...any solid info to refute what I am saying would be welcome with open arms as i would love to build a 14:1 motor that would last more than 10k miles.

You are right in that the compression allows the propane to make more energy and make up some of what is lost...propane is a great fuel. But the other engine building rules still apply.

 

Stress from what? The higher CR with not stress motor negatively and propane motors actaully run a bit cooler because of the cooling effect of fuel in intake and higher CR also reduces the amout of heat energy from combustion due to increased expansion of expanding gasses and converting it to work. The more work you extra from fuel through improve expansion capture mean less heat is lost to cooling system and exhaust. It is really silly to build a engine with 9 to 1 CR to run on 110 octane propane when so much more power and economy can be achived with 12 to 1 or higher CR ratios when using this fuel.

 

Just to throw in my 2 cents my work truck is gas with propane you cant tell the difference in power and it is definetly more effiecent.But we dont have emissions here anymore its been years but they started it when I started driving my 82 ford truck hadd true duals with cherrybombs and edelbrock intake and 4 barrell and headers I got lucky and found a loophole the 302 we put in came out of an old maverick so I got the form and because of the year of the motor I was exempt I just showed my papers every year and drove right through.appreciate ya I will post my propane system .

 

This forum has been a very interesting read- it's great to see the input from guys who have had experience with propane . Some of the various suggestions made ( higher c.r., turbos, etc.) really make a guy think.

I think propane is viable if it's done right. I would like to do it right so I've been doing a bit of research as time allows. I recently bought a low mileage 97 F-250 crewcab with a 7.5L and auto. I'm inclined to go with a Vortech blower, stock c.r. and a few extras such as headers, electric fans on the rad and a decent propane conversion to start with. I'm 90% sure that I'm going the straight propane route. This truck will mainly be a highway runner and will pull a trailer on occasion.

I'd be interested to hear your thoughts.

 

straight propane is probably best- keep us up on the turbo etc.

 

Has anyone had any luck finding a conversion kit for an older engine? I have a 390FE (in a 68 F100) and would very much like to convert it, but was told a few years ago by the local propane supplier that kits were no longer available.

 

try propaneguy.com. He either has a kit or can get one for you.

 

If not at least he has all the parts. But he can probly put a good kit together.

 

very cool, thanks!

 

I was just reading some of the earlier posts in this thread and thought I would put my 2¢ in.

• The octane rating (actually anti-knock index) of pure propane and HD-5 propane is 104. (DOE Fuels Table). Commercial propane AKI can be much lower.
• Street engines will survive much longer at 11:1 compression than 12:1 or higher. (Compression Guide)
• While it's true that propane is a dry gas, its effect on exhaust valve seats is similar to that of unleaded gasoline. Because it is a dry gas, it does not wash off lubricating oil from the cylinder walls like gasoline, which is a major reason why propane engines last longer.
• Propane engines can run leaner than gasoline and leaner mixtures burn at a lower temperature. Propane engines respond opposite to gasoline engines with a rich mixture as it causes them to detonate and burn exhaust valves.

Frank
Raso Enterprises

 

Fraso- I really like your input- some of the stuff I've been trying to get accross myself regarding compression. Still, it's the dynamic compression that is crucial- ie a 12:1 static bled down to 8:1 that burn 91 octane is not as damaging as an 11:1 with an rv type cam grind that only bleeds dynamic down to 9.1 which requires race fuel.

Also...your saying that lean propane burns cooler than rich and that isn't true with gas...right? odd

 

While I don't have very much experience with building engines, I think you're correct about effect of dynamic compression ratio. Interestingly, Popular Hot Rodding has a tech article in their June 2006 issue entitled “Compression Comprehension” that includes a discussion about dynamic compression. You should be able find it online next month in their tech article section.

Most propane engines are built for maximum fuel economy and low RPM torque. Normally, an RV cam is used to help accomplish this and, as you noted, the dynamic compression would be closer to the static compression than that of a high RPM cam. Probably a better guide for estimating the octane requirements of a high RPM camshaft grind would be cranking pressure.

Unfortunately, I haven’t come across such a guide except for a compression pressure section in the “Compression Comprehension” article suggesting that 93-octane gasoline would be required for 200 psi and that compression pressure should be about 5 psi less for every octane number less than 93 to avoid detonation. Any such guide would have to be very general as the design of an engine’s combustion chamber has a major effect on the octane it requires for a given CR, static or dynamic.

Yes, I am saying that lean propane burns cooler than rich and that isn't true with gasoline. Rich fuel mixtures tend to cause gasoline-fuelled engines to run cooler because of the evaporative cooling effect of the atomized liquid gasoline. Because propane is already vaporized before it is metered into the engine, propane vapor cannot provide a similar cooling effect.

In addition, a rich propane fuel mixture will burn hotter than a stoichiometric (ideal - neither rich nor lean) fuel mixture. Also, both lean and rich fuel mixtures burn with a slower flame speed than a stoichiometric mixture. The combination of hotter combustion and slower flame speed can result in a fuel mixture that is still burning when the exhaust valve opens. Severe cases of over-rich fuel mixtures can burn exhaust valves but slightly over-rich mixtures can still overheat exhaust valves to potentially cause detonation.

If you are going to run high static compression with a cam of any kind, be very careful to keep your fuel mixture in control. Impco recommends having an exhaust CO reading of 1.0% to 3.0% for maximum full throttle power and this is best set up on a dyno. Part throttle cruising mixtures should be lean for better fuel economy. However, I haven’t yet come across a target lean fuel mixture recommendation for lowest fuel consumption.

With gas valve mixers, it is difficult to set both a lean part-throttle and a rich full-throttle fuel mixture because the fuel mixture at any given air flow is determined by the shape of the gas valve. Generally, Impco only offers standard, lean, and feedback gas valves for their mixers. The standard gas valve is designed to supply the proper fuel mixture for the mid-range of engine size for which the mixer was designed. An engine on the larger side of the size range will run too rich with the standard gas valve and the lean gas valve provides these engines with a leaner part-throttle fuel mixture for better fuel economy. The feedback gas valve is designed for use with a feedback (exhaust O2 sensor) control system to maintain a stoichiometric fuel mixture at all times.

Don’t forget, because the spark plug has a harder time ionizing a propane fuel mixture (especially a lean fuel mixture), to keep your ignition system in top condition to avoid lean misfires or backfires.

 

Wouldn't the technocarb system be able to deal with this since it is computer controlled. The only draw back is it is for 87 and older 4x4s. It is realatively pricey at ~1300 but has the bling look rather than the impco industrial look.

 

The CR talk in this thread has got me to thinking. I have gottent the impression that iron heads are preferred over aluminum due to their heat characteristics. However, anything over 10:1 CR is not good with iron heads. This seems to be a catch 22 when building a propane specific engine.

Also, how does head design theory work in regards to propane vs. gasoline? I've been looking for months and I can't find crap.

I'm in the process of building a propane 351W stroker and can't figure out if I should go with my E7TE heads, my E6SE heads, or my World Windsor Sr. heads.

PropaneGuy told me that my Stealth intake was no good for propane and use a stoke 4v intake. He also suggested I get a custom made came specifically for propane and application.

The head choice seems to be a big mystery. I wish the 700 hp Wilys had more head information regarding intake runner and exhaust runner volume and
dimmensions.