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Propane only or dual?


treetrimmer
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I am going to build a 4.6 for a 95 D1. I was thinking dual fuel but from what I've read it looks easier to just build for propane only. Also, would it be easier to do away with the fuel injection if I do this? Would this affect the engine performance? Any thoughts on this and any advice on the rebuild for running propane exclusively? This is going to be a mild build up, not a monster.

Thanks

Tim

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I am going to build a 4.6 for a 95 D1. I was thinking dual fuel but from what I've read it looks easier to just build for propane only. Also, would it be easier to do away with the fuel injection if I do this? Would this affect the engine performance? Any thoughts on this and any advice on the rebuild for running propane exclusively? This is going to be a mild build up, not a monster.

Thanks

Tim

Depends what you're building it for - if it's just a toy and you can guarantee always being able to get LPG for it, then propane only might be the way to go. If it's a daily runner I'd be worried about running out - in some areas LPG is hard to come by, especially if you need to fill up at weekends or late at night, and if you run out you can't just hitch to the nearest petrol station and buy a can of LPG to get you going again.

The existing fuel injection is only relevant to petrol fueling, so if you went for propane only you could do away with it completely. You have two options for LPG fuelling - venturi type or multi-point injection. Venturi is cheaper and easier, but less efficient and suffers from poor idle stability and a tendency to blow back (frontfire?) if you start on gas, which can damage your air intake, particularly if you have the old flapper type air meter. I have a venturi kit, but if I was doing a new conversion I'd shell out the extra for injection.

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I think I can do propane only for daily driving, I have a couple of options for fueling near or at home. Extended trips might take some advanced research and planning. First I am more interested in whether or not the engine can be set up better and easier to do only propane. I've read a little about the RPI multi-port injection. Would that be installed in my current intake manifold in lieu of my current EFI? It just seems simpler to build the engine with the right compression and proper ignition for LP only. Also, could I eliminate some of the engine management systems with LP only? i.e. would I keep the hotwire system? etc. I read some mention of that on another website.

Thanks,

Tim

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I think I can do propane only for daily driving, I have a couple of options for fueling near or at home. Extended trips might take some advanced research and planning. First I am more interested in whether or not the engine can be set up better and easier to do only propane. I've read a little about the RPI multi-port injection. Would that be installed in my current intake manifold in lieu of my current EFI? It just seems simpler to build the engine with the right compression and proper ignition for LP only. Also, could I eliminate some of the engine management systems with LP only? i.e. would I keep the hotwire system? etc. I read some mention of that on another website.

I can do propane only for my daily drive, but I've been 'caught short' a few time away from home. Usually when I discover the only LPG station in the area has restricted hours or has run out. Not a major problem if you can fall back on petrol to get to the next one, but otherwise you're going to be making a rather embarrasing appointment with the man from the RAC... Combined with the the RV8s considerable appetite for fuel, you're going to hit this problem sooner or later.

I'm not that up on LPG injection - there are others on this forum who can give you far more reliable answers on that subject. However; The Lucas Hotwire EFI system only manages fuel injection. Depending what LPG solution you went for it may be completely redundant if you go propane only, but be careful - at least some of the LPG injection kits calculate their fuelling by monitoring the injector outputs of the original petrol injection system.

Obviously, running only one fuel means less need to compromise and/or less complication. Depending what ignition solution you go for ignition timing may not need to be compromised - with a dizzy you advance it as far as you can get away with on petrol, which isn't as much advance as you'd ideally run with propane. Some aftermarket electronic ignitions allow you to offset the ignition advance for one of the fuels from the default, or something like Megasquirt'n'EDIS will allow you to have completely custom ignition maps for each fuel. Decreasing compromise in return for increasing complexity.

Compression ratio is also to some extent dependant on your ignition if you're running dual fuel - I had a discussion about this with Steve Lundlack from Lund Engines a while back when I was ordering some bits off him - he suggested skimming the heads (I was doing head gaskets at the time) to get 10.1:1 compression ratio if running a single ignition timing, or 10.3:1 if you have the ability to run massively retarded ignition when on petrol. Both are aimed at getting the best out of propane at the expensive of petrol, the latter will reduce petrol to a 'limp home' mode, as the performance and economy will be attrocious, which sounds like it would be fine for your purposes.

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  • 3 weeks later...
Couldn't the CR be higher for propane only, like maybe close to 12:1? Also, shouldn't the ignition put out more spark for propane only?

Not sure - I only discussed dual fuel with Steve, not an engine that would no longer be able to run on petrol. You need a good spark for LPG, but I'm not sure how much (if anything) you'd gain by going beyond what a good petrol ignition system gives you. I think it's much more about the ignition timing (LPG burns slower so you want to ignite it earlier).

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Couldn't the CR be higher for propane only, like maybe close to 12:1? Also, shouldn't the ignition put out more spark for propane only?

As has been mentioned, I would not go LPG only because you will find places that have very few LPG outlets (Wales springs to mind!), places run out, or pumps that are out of order.

Because LPG is still relatively unknown or cared about, when pumps are broken, garages don't seem so bothered about fixing them or even apologising about the lack of fuel! I bet if they ran out of petrol they would be bothered...

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when pumps are broken, garages don't seem so bothered about fixing them or even apologising about the lack of fuel!

Not just in Wales sadly :angry:

I would stick with dual, especially if you are new to LPG, apart from that the petrol will always be there as a safety net even if you don't use it.

See if you can get hold of a copy of Practical Performance Car Nov issue no.19, there is quite a good article on the subject if you are new to it, good mag by the way.

Just had the sandbag's new Hyundai 2.0 converted with the latest SGI system, haven't been home to test it yet but she is very impressed with it and says it starts directly onto LPG even from cold, taking that with a pinch of salt till I have checked it myself. She is used to her old Astra 1600 that has done 1k miles on an older closed loop non injection system so she has a lot of experience actually dailly driving with LPG. I think the new system is OMVL but again I will have to check.

One of the guys in the Rangie register in Durham converted his carbed V8 by removing the carb diaphragms and blanking them, he used a very old and basic mixer system, his only comment was that you have to go easy with the loud pedal for the first few miles until you get some heat in the system or leave it to idle for 5 mins before driving. To be fair I found much the same with all three previously converted vehicles that I have owned, all mixer systems, and the only way I got around it was to fit a Kenlowe pre heater system, which has benefits all round and I would recommend it.

Edited by Niall_CSK
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Perfectly possible to go LPG only, but I agree with others that unless you are very confident of getting LPG whenever and wherever you need to, dual fuel would be adviseable.

Having said that, I have only run out of LPG once in the last 4 years(on holiday on the Isle of Wight) because I couldn't find anywhere to refill. I only use petrol to check it still works occasionally.

With regard to compression ratio, LPG(autogas) has a variable octane rating which changes according to the mix of gases which make up LPG. In the UK the (autogas) is generally 100% propane, though the mix varies according to season and may vary slightly between areas too. On the continent the mix may be very different, varying from 50/50 propane/butane or even 35/65 propane/butane.(which is why many users report better performance/economy on holiday in France).

There isn't a set standard used for the octane rating of Autogas in the uk(as there is for petrol), so it is unwise to work on the basis that the autogas has an octane rating of more than 100. As a result, in my opinion, 11:1 compression is the max you should consider for an lpg ONLY engine. In my opinion, the max compression for a dual fuel would be around 10:1 if you use Shell optimax or similar, and even then with some engines pinking may be a problem.

The problem of pinking is compounded by the fact that petrol and lpg have different ignition advance curve timing requirements. Generally speaking, most engines running on lpg run better with more initial and early advance, BUT a similar advance at the top end of the rev range when on LPG.

With regard to starting in cold weather on LPG only. A simple, well set up and suitable mixer system can start and run perfactly well even in the coldest weather(after priming). So if you don't want to use petrol ever again, You don't have to!

Hope this helps,

Regards,

Diff

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