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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old January 21st, 2021, 10:47 AM
Prowbar Prowbar is offline
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Default Impco article, running 401 V6's on LP

My dad found this article a while ago in one of his Impco service manuals.
It is about the problems they had while trying to convert 401 V6's from gasoline to LP.

Interesting article, but not much details of work done on the engine to make it ready for LP (hardened valve seats, components used etc.)

Thought it might be interesting for the forum folks.

Does anyone have experience converting these engines to LP?
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Old January 24th, 2021, 04:32 PM
Prowbar Prowbar is offline
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Default Re: Impco article, running 401 V6's on LP

Any comments on the weak valve springs and positive crankcase ventilation causing 4 of 6 cylinder to run lean? Is this a known problem/experience with these engines?

I'd appreciate any comments.

Thanks.
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Old January 24th, 2021, 10:45 PM
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Default Re: Impco article, running 401 V6's on LP

Thanks for posting this Probar!

It is interesting reading and has some tips too. I haven't built up a V6, but I have experienced the backfires on the exhaust in the past (but I never blew out a muffler). The Ford valve springs seems like an easy upgrade.

I know we have a few on here with the expertise to address some of those engine modification comments, and I'd like to hear more about it as well.
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Old January 26th, 2021, 05:36 AM
BobBray BobBray is offline
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Default Re: Impco article, running 401 V6's on LP

I grew up in Los Angeles, and for many years the L.A. Times ran V-6 GMC Steel Tilts on LP. I wonder if that's the newspaper that had the issues! Von's Grocery also ran V-6 H7500 series tractors on LP. I have a lot of experience with CNG conversions on modern engines and not much is needed beyond hard valves and seats, as CNG is a 'dry' fuel. Never had any issues with the V-6's PCV system, though on light trucks there are only 2 valves. Never had a valve spring issues either, I wonder if the newspaper in question was trying to use old worn out valve springs on their rebuilds. Interesting article, thanks for posting!

Last edited by BobBray; January 26th, 2021 at 05:47 AM.
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Old January 26th, 2021, 07:29 AM
Prowbar Prowbar is offline
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Default Re: Impco article, running 401 V6's on LP

Quote:
Originally Posted by BobBray View Post
I grew up in Los Angeles, and for many years the L.A. Times ran V-6 GMC Steel Tilts on LP. I wonder if that's the newspaper that had the issues! Von's Grocery also ran V-6 H7500 series tractors on LP. I have a lot of experience with CNG conversions on modern engines and not much is needed beyond hard valves and seats, as CNG is a 'dry' fuel. Never had any issues with the V-6's PCV system, though on light trucks there are only 2 valves. Never had a valve spring issues either, I wonder if the newspaper in question was trying to use old worn out valve springs on their rebuilds. Interesting article, thanks for posting!
Thanks for the reply. Could be very likely that this is about the L.A. times. Yeah it makes you wonder what went into the rebuild of these engines. I read in the 65-66 manual that the 351, 401 and 478 have a hardened exhaust (not intake) valve seat made from 'Eatonite'. The exhaust valves on these engines are also of the sodium cooled type. Makes you wonder why, as these engines had excellent cooling capacity with the high flow water pump.

The article also mentions turning up the idle speed, which I doubt is useful as the camshaft dips its lobes into an oil bath so it could not be a lubrication issue?

When I start using LP as a fuel for my 305E, I'll have the heads machined for Stellite valve seats with new valves.
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