Hi folks!
Short version:
What setup would you suggest for a slant six on LPG that is driven 50 miles a day, mainly highway? Cam, compression, etc.
Long Version:
I can get my hands on a nice 66Dart 2dr 3sp manual tranny, with an LPG system installed. I´m thinking seriously about making this car my daily driver, so mileage is the only concern. I have a little cash at hand, and tons of /6 parts in the garage, like 6 Engines, 5 trannies etc. BTW I live in germany and work at the mopar shop of germany (www.moparshop.de) so I can do all the fabrication myself (with a little help of my friends ). LPG is readily available here as well as high octane fuel.
I will run an A833 OD with 3.23 rear in this car. Now I´m thinking about the engine. I understand that LPG allows(needs) more compression, so I´ll shave a tenth off of the head. Or more? Shortblock will be fresh. I don´t want to go into bigger valves because of cost. Should I use a stock cam or the mopar performance 268° I have on the shelf? I only have stock 1bbl manifolds and carbs, but a set of dual headers.
I want to push compression and timing to the max to take best advantage of the LPG´s high octane rating. For starting and warm-up (how long do I have to run the engine on fuel until I can switch to lpg?) I have high octane fuel available.
Thanks for your input!
Chris

Your maximum compression ratio depends upon the amounts of fuels (propane, butane, etc) that comprise the LPG you have available. In North America, LPG motor fuel (HD-5) has an octane number of 104 [(R+M)/2] because it contains mainly propane. Increasing amounts of fuels such as butane will lower the octane number.

Propane has a critical compression ratio of 12:1, which means that higher CR will cause it to self-ignite. It's not that LPG needs higher compression, it's that LPG allows you to use higher compression because of it's higher octane number. If your LPG is similar to HD-5, you can safely run compression ratios of 10.5-11:1. If you have a longer duration cam, you can run higher compression ratios because the dynamic CR will effectively be lower. However, having a reasonable CR will ensure maximum reliability. On a bi-fuel vehicle, the maximum compression ratio should really be dictated by the gasoline you want to use.

If you want to maximize your fuel economy, you need to keep engine RPM as low as possible to minimize friction and pumping losses. I would stick to a stock or short duration, high lift cam if you're intent on changing it. I find that my fuel economy is better with a lean gas valve rather than the standard valve. I've found that keeping the converter hotter will also help fuel economy on a open-loop system.

If you an Impco-style LPG conversion, you will not need to start on gasoline before switching to LPG unless your converter is seriously undersized. I have started my own (mono-fuel) vehicle at -20°C (see video) quite easily and yours should start as easily too. I would run your car periodically on gasoline to prevent the gasoline from becoming stale.

More initial, but less total timing is the go.
10 deg initial and 25 deg all in by 2500 Rpm would be a good place to start.
I agree on the Impco cold start as well.
I run a 265 Hemi 6 with a 425 Impco on LPG only (not that it gets cold here, about 20 deg F or -6 C minimum)
It will start after about 5 seconds of cranking and will idle within about 10 seconds of firing.

_________________
Richard (aka Fingers)

Thanks for your input! That is lots of usefull information. I´m surprised, though, about the total timing. May a slant six ask for more advanced timing due to less eficient combustion chamber design?
Chris

The Woodward catalog's Technical Information has a good explanation of LPG engine timing. You might also be interested to read Franz Hofmann's Diagnostic Guide as well.

The reasons for the difference in timing between gasoline and LPG are LPG's higher octane, faster flame speed, and typically leaner fuel mixture. LPG's higher octane number allows you to use more initial advance but its higher flame speed does not require as much total timing. Because LPG fuel mixtures are typically leaner than gasoline mixtures, lean fuel mixtures require more vacuum advance.

Typically, an LPG engine should have its total centrifugal timing (no vacuum advance) set to around 28° BTDC. You can experiment with what is the ideal for maximum power on the drag strip or a chassis dynamometer. Once you've found the best centrifugal timing, you can adjust the vacuum advance to give the best fuel economy. This would typically be around 40° to 46°. See my propane ignition page for more information.

When setting timing, you can't just advance it until it pings and then back it off a few degrees. LPG's high octane prevents any pinging with gasoline engine CRs, which means the timing needs to be way overadvanced for pinging to occur.

LPG likes a healthy spark, time to get rid of the points ignition if you haven't already done so.

This is exactly the kind of information I need
Thanks guys!
Chris

bought the dart today

Can you post photos of the LPG system on your car?