This is my timing map, and I'm just blown away at how little advance it wants for my propane powered slant six.

This is the graphical display from MegaJolt Jr configurator.

It seems like VERY little advance, but it runs quite good, and as the weather warms up, I keep backing the timing off due to some pinging while running hard at part throttle ( 3200 rpm up a steep hill).

Any thoughts?

BTW, the very low load above 3000 area never is used, so I haven't re-written it.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Drop the heat by using a 160 degree stat and something like Be-Cool water wetter, use cold ram air intake, colder plugs and richen the mixture slightly. You could also use alcohol/water injection which will help drop combustion temps a little like EGR does. I know you have a heat deflector.......so that is good.

I do all the above and still run 52-55 degrees total. Even on the hottest days I can keep it from pinging. The engine prefers no more than 10 degrees initial, 20 mechanical and 22 vacuum from the vacuum advance.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I already am at 160 thermostat, cold air intake, and the coldest resistor plugs I could find. I was considering removing the thermostat for summer use.

Can't richen it... it's a vapor fuel, which means that LEANING it burns cooler, not richer. If you richen it, it burns hotter, not cooler.

I just keep backing off the timing, and it runs better and better.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

First, which intake manifold are you using? As the Propane-Air vapor mixture is perfectly homogeneous, you don't need Carburetor heat (actually a Hyper-Pak intake would be perfect for Propane AutoGas and the Clifford Manifold is not a bad choice either!).

Second, if you are using a stock, Offenhauser or Weiand intake manifold, did you block off the heat riser and disconnect the EGR, use headers or divorce the exhaust manifold from the intake? You need either Dutra-Duals or Headers as well, just to let the slant six breathe.

Third, how much compression (static) do you have? You need at least 9.5:1 and a moderate duration camshaft to build cylinder pressure (dynamic compression ratio).

Finally, are you getting any oil into the combustion chamber (worn valve guides of rings)? Oil has an octane rating of about 40 and kills the 115 octane rating of propane autogas.

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David Gibson

Everything except the exhaust is noted in my sig. I have a mild cam advanced several degrees. I have a super six intake and a stock exhaust, but there's a steel plate and an insulator between in the intake and exhaust manifold. I don't have the money to do otherwise. The headers I bought turned out to not fit.

The head has been rebuilt, there's no oil going through the guides. It has stainless oversize valves, bronze guides, hardened intake and exhaust seats.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

One thing that might be easy (even though you've got the 3/8" steel "block-off Plate") would be to remove the heat riser valve. I did this on my wife's 1979 AMC Concord DL Coupe, back in 1986, tapping the holes and installing grade 5 bolts coated with Lock-Tite Blue. This will allow most of the exhaust to immediately exit toward the outlet. The lack of exhaust heat to the intake manifold is "a major cause" of poor running on a gasoline engined vehicle, as the fuel will no longer vaporize in the intake, but helps on a propane vehicle.

What size exhaust pipe are you running? Are you still running a stock catalyst? Are you running a "turbo-style muffler?" Freeing up the back pressure, gets rid of the excessive heat, which should help. Finally, a cold air intake (like the aftermarket Spectre brand - MoparAction had an article about how to adapt one of these to a Jeep Cherokee with a 4.0 liter inline AMC six) to draw air from outside of the engine compartment will get those air intake temps down and help even more. [/b]

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David Gibson

I think the hot intake air is certainly a likely culprit.with conventional fuel heat is absorbed through vaporising the liquid fuel,propane obviously doesn't have that effect so it's really essential to reduce the temp of that manifold. Can you modify the headers to fit? If not perhaps do what I mentioned in a previous post,cut away the section of the manifold that carries the flapper valve and weld on a blanking plate. if your not keen about welding maybe mill or even grind down the exhaust hot box gasket face as much as possible and then bolt a plate onto the exhaust manifold directly,then grind away the hot box mating face on the intake in order to get an air gap between the two? I'm not sure about leaning mixtures to run cooler,rich for hotter,not saying its not correct,just that from what I've seen propane is still a fuel and behaves the same as any other.....if you find " peak exhaust temp" for a given power and lean it from that point exhaust temps will drop,economy will improve and power will decrease,richen slightly from peak and its best power,and temps will again drop ,continue enrichening the mixture and your wasting fuel.....BUT if your not at peak EGT , for example on the lean of peak zone then a leaner mixture will certainly produce a cooler temp,and much less power,in contrast to running richer and the temp will increase as you get back towards the peak EGT condition which is in the ball park you want to play in.....all your doing is getting the most thermal efficiency out of burning your fuel so the engine can do its job as good as possible..... I Hope that made some sort of sense! By the way, that timing isn't really far off what I would have considered good for propane ,21' all in by 2600 and 42 under light loads seems feasible,I always thought propane used a fairly fast curve with less total than conventional fuels? I know edis is set at 10 static and the megajolt takes it from there,I assume the map displays total advance,not what the megajolt is adding to the existing 10'? I'm sure you've checked,but are all your timing marks correct and does a timing light reflect what the map says? Whatever you do don't let the engine ping too much.....propane is a great fuel,I am a big fan of it,but if not treated with caution it can torch pistons real fast!

The timing is what the map reads, not 10+ the map.

I've checked, and that's how it works.

The MJ LJ has the ability to tweak the timing for having a slightly displaced wheel or sensor. I have set it. There a jumper to pull to force default, which is 10, and you put the jumper back in, and if the map is asking for 10... You have 10.

I, too, believe that propane wants a little more initial, a quick jump, LOTS of light load (comparatively) and with real high compression, not a lot of total at WOT. At least that's how it seems to want to run. I'm coming to think I have too much initial, as well. It doesn't ping, though.

I'm part of a propane fuel forum, and it's confirmed that lean = cool and rich = hot. When I started, I followed what people suggested should happen in terms of advance between propane and gas. I originally had a map very similar to this, but with LOTS more light load. It barely ran, but eventually I found a fuel system fault, it was running extremely lean. But not before I had added about 12 degrees more advance. Then after fixing it, I've been undoing that.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

AFAIK, propane has a faster burn rate than gasoline, so you don't need or want as much advance in order to have peak combustion pressure at 12-16* ATDC. In other words, I believe that a advance map that works with gasoline would cause detonation due to over-advance on propane.

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63 Valiant Wagon
225 - 4 bbl