391 Piston upgrade

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Jun 26, 2013 | 10:32 PM

imabass

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391 Piston upgrade

mid 70 Blue Bird 40' bus/RV conversion of my brothers. It has 391 with super low compression and can barely pull itself up a hill. We pulled the engine about a year ago and I am finally getting started ordering parts. I got all the valve covers, oil pan, intake manifold, all brackets, crossmembers, transmission covers, (everything but the heads and block) powdercoated gloss black. I have almost everything but cam, lifters, gaskets, and rings. I just got my pistons in and thought I would share. These are 360 pistons.

Jul 3, 2013 | 01:33 AM

Rusty_Old_F250

Fleet Mechanic

Joined: Aug 2009

Posts: 1,771

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From: Western Washington

Looks good!

Just a word of warning, be careful with the compression ratios. If it's up too high pinging can be a real issue. It would just a shame to put it all together and then have trouble...

Sam

Jul 3, 2013 | 07:16 AM

80broncoman

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Joined: Nov 2003

Posts: 898

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From: southeren Oh

Quote:

Originally Posted by Rusty_Old_F250

X2 I agree on that.
Ford set the compression lower on the Bigger trucks for a reason.

Jul 3, 2013 | 07:55 AM

HOTWRENCH

Elder User

Joined: May 2004

Posts: 565

Likes: 3

From: saxton pa.

Pistons

I doubt that those lite duty pistons will last very long , FT's had the heavy duty low compression pistons for a reason -- durability. I'v seen people try to substitute a car or lite truck engine for the heavyduty version , it does'nt last usually it takes the top ring land out or melt a piston from detonation that you can't hear. Something else to remember if you use those lite duty pistons instead of the HD pistons you will need to rebalence the rotating assembly. If you build a 391HD to factory specs it will last and do the job , most of the time when you crossbreed FT-FE parts it is a disaster especially if its loaded all the time. FE pistons and heads won't hold up under continous heavy loading in a truck application.

Just My Opinion --- Hotwrench

Jul 3, 2013 | 12:45 PM

3414

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Joined: Jun 2008

Posts: 693

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From: N E Ohio

That 40' bus/RV conversion will put a really heavy load on that engine, putting alot of heat into the pistons. Looking at your pictures the new pistons are a much lighter construction. The combination of high load, high heat, call for a heavy duty piston. And then you add higher compressionto the mix. Hotwrench has it right, avoid trouble down the road and use the proper piston design.

Jul 4, 2013 | 01:36 AM

dmanlyr

Fleet Mechanic

Joined: May 2005

Posts: 1,575

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From: Puyallup, WA

As many have said. Gas medium and heavy duty engines are built the way they are to have a reasonable service life. And this is just not Ford, EVERY medium and heavy duty gas engines followed this build up.

You said a 391 piston upgrade? Well not really, if you do what you are suggesting, it would be a piston downgrade.

Normal hot rod tricks do nothing except to give you a short life engine. These beasts have around a 7.2 to 7.4:1 compression ratio and are designed to slog along all day hammer down without detonation. As mentioned, EVERY manufacture of medium and heavy duty gas engines followed this same low compression ratio for there engines without any exceptions.

Unfortunately for these engines and older trucks, they are unappreciated as people nowadays want to blast up hills as fast as on the level, and these engines were never designed for that and cannot be made to do that reliably. There was a time when trucks were just slow, almost always in the right lane and these trucks live up to that time!

Just food for thought.

David

Jul 5, 2013 | 05:38 AM

big job

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Joined: Dec 2005

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A 40' bluebirds a hell of a lot lighter that a 391 dump truck pulling trailer with 25,000
dozer of couse you have to shift it even with the newest. Those new pistons will work
fine with airplane gas at 10 bucks a gal, Other than that you lug it, then it gets hammered
to death. I have my first car a Merc /430 10:1 & I got to run it retarded to run on this
junk gas, Its designed to run on 100 octane thats a fact