Rebuild engine on LPG

[Plymouth1966]

Hello everybody,
Thank you for accepting me on the forum.

I’m rebuilding my chrysler 400ci to put it in my 1966 Plymouth Belvedere. I increased the compression a bit to ~10-1 because I want to run it on LPG (I’m from the Netherlands and it WAY cheaper hahaha)

The rotating assembly is done and I’m trying to get the engine started for the first time this month.

Now comes the ‘problem’ my engine builder isn’t a big fan of running engines on LPG and especially not breaking them in on LPG.

He says i should drive about 1000 miles on petrol before even starting to think about switching to LPG the reason for this he gave me is as following:

“When running on petrol the pistons will get a thin carbon coating on top of them, this has to cover the complete piston because this will protect it when you run it on LPG later.”

I cant find any info on this online so I whould like your input on how true this is and if it whould be smart/a problem to break in the engine on LPG and start driving with LPG right away.

Thanks in advance,
Marco

[Tom68]

LPG only cars don't get broken in on petrol.

The old reason for not breaking in on LP was said to be glazed bores, loading the engine during break should stop that with any fuel.

[Plymouth1966]

Hi Tom,

Thank you for the reply, good to hear it will not be a problem to break in the car on LPG. I’m planning to run LPG only so that would be no problem.

Yep I’m planning to indeed load the engine when breaking in.

Thanks,
Marco

[C3H8]

Engine rebuilders typically make this statement because they have experience with chrome rings that were used in some engines and probably still available today. Dodge used a top chrome compression ring in the slant six engines that was miserable to seat. These engines burned oil for the first 1500 km's on petrol and progressively burned less as the mileage increased. IMPCO used to refer to these rings and even suggested various methods, which I will not repeat, to seat them if the engine was operated from day one on propane. Done wrong they could completely ruin an engine. That being said chrome rings would seldom seat properly on propane. Basically the rings were difficult to get them to wear in or I guess break in and operating on propane it took a very long time. A local taxi company using the slat six actually gave up trying to break in his engines on gasoline as they continued to burn some oil even after 40K to 50K km's. He just went ahead and converted them anyways.

Dodge was one of the last manufacturers to quit using chrome rings. The typical alloy rings used today will break in fine on propane but they will take a little longer. The only caveat to this is "what type of rings did your rebuilder use"? If he actually used rings with a top chrome ring break-in could be an issue. IMPCO used to recommend that when rebuilding an engine stay away from chrome rings. I've never hear of carbon build up being a desirable stand on any fuel. I don't get his rationale on that. Even todays carefully controlled fuel injected engines don't have much carbon build up on them. Take into consideration to that used forklifts converted to propane usually command a higher sell price just because the lower end of the engine is usually in such great. This means a minimal amut of carbon build up on the pistons and minimal ring ridge or wear on the cylinder walls.