Old 11-24-2021, 07:04 PM
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Sirrotica Sirrotica is online now
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One of the hardest to beat inline engines for the new OHV V8s of the 50s was the 308 cubic inch Hudson Hornet engine. It was a flathead inline 6 with a bore and stroke that in todays world isn't in favor for a high performance build.

Bore of the 308 is 3.8125" and the stroke is 4.500". I can attest to the unusual bore and stroke as I disassembled one of these engines as a teenager. The connecting rods were extremely long. They measured out at 8.125 inches. I realize that the engine the OP is building is unusual and not run of the mill for a Pontiac build, but I wouldn't discount the specs as impossible to be in a high performance build considering how fast the Hudson engines were.

The other thing the Hudson has in common with a Pontiac is the tall deck, 12.375 inches from crank center to the top of the block. One other dimension is a 2.250 rod throw.

The thing that gave them an edge was the long stroke that produced torque at a lower RPM. This build should do the same thing. I'm looking forward to the end result, hopefully a dyno run to see where the HP and torque end up at.

With a 4.25 stroke his piston speed will only be a small percentage more than a 455, with less reciprocating weight hanging on the rod because of the smaller bore and piston size. Plenty of 4.25 stroke large bore engines have been built, and no one bats an eye when they stroke a 400 block to 4.25.

This should be an interesting build, looking forward to it............

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Old 11-24-2021, 07:14 PM
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steve25 steve25 is offline
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The most highly loaded component in the short block are the rods.
If we go by the stock reciprocating weight of parts we get these numbers if the motor is run to 6000 rpm.

Just going from the stock 3.750” stroke up to 4.250” will increase the already over 6700 pounds of load trying to rip the rod apart up to over 7800 pounds of load.

That makes for a 15.5% increase!

If you simply lower the reciprocating weight by 50 grams you are then down to a hair over 7500 pounds of load which is 4.2% less then what the load is with stock parts and the longer 4.250” stroke.

Conversely if you only run the motor to 5500 rpm instead of 6000 the load would back down to the 6700 pounds it was with the stock 3.750” stroke.

Loads on the reciprocating assembly go up and down by the square of the rpm, as in the loads seen at 1000 rpm are 36 times higher at 6000 rpm!!

With stock # 11 castings and that 4.250” stroke the hp numbers that the motor can make will be well nosed over by 5500 rpm anyway, so no sense in buzzing it any higher!

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Last edited by steve25; 11-24-2021 at 07:25 PM.
Old 11-24-2021, 09:05 PM
Steve C. Steve C. is offline
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It has been stated the troubles lie not in the piston's speed but stopping and reversing its direction twice per crank rotation.

“Rather than focus on mean piston speed, look at the effect of inertia force on the piston"



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Last edited by Steve C.; 11-24-2021 at 09:11 PM.
Old 11-24-2021, 09:29 PM
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I have not check to see how my calculation are verses that web site.

If I hold the parts weigh constant and just change the RPM and stroke.

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Old 11-24-2021, 10:04 PM
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Mr Anonymous Mr Anonymous is offline
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It'll make a great torquey engine at lower rpm. Strangely enough, your combo is an almost a perfect copy of the 1968-1969 Oldsmobile 400 as used in A-body applications (4-4-2 and vista). 4.25 stroke crank as well. People assumed that since it was the same cubic inches as the earlier 4.00x3.97 combo from the 65-67 4-4-2, that it would rev like it too. Big mistake. Ergo, very few "late-400s" exist. Any 68-69 4-4-2 with its original 400 are a rarity. They didn't breathe all that well with a small bore (3.87), and wouldn't do the rpm to make up the difference.

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Old 11-24-2021, 10:51 PM
74Grandville 74Grandville is online now
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th350 will probably need some work to live a happy life. should put 5 clutchs in the direct drum and double feed it. Other mods may be warranted as well.

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Old 11-25-2021, 05:23 AM
Schurkey Schurkey is offline
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Originally Posted by Mr Anonymous View Post
your combo is an almost a perfect copy of the 1968-1969 Oldsmobile 400 as used in A-body applications (4-4-2 and vista). 4.25 stroke crank as well.
The "long-stroke" 400 engine is HATED by Olds people. It was an obvious cost-cutting measure; Olds only had to machine one stroke on their crank castings instead of having the old 400/425 stroke and the "new" 455 stroke. They didn't rev, but they did blow up.

I would have put the original engine in a plastic bag, tossed in a couple of desiccant pouches, vacuumed the air out of it, and sealed it up. Stash it under a work bench in the garage.

I'd build an engine with more bore, less stroke. More bore allows bigger valves 'n' ports; but does not require bigger valves and ports. Bigger bore reduces bore shrouding of the valves--heads flow better on a large bore than a small bore.

Torque is a function of displacement and gas velocity through the ports/manifolds, not stroke length. A Buick 455 makes as much or more torque than an Olds or Pontiac 455, but has a much bigger bore and a shorter stroke.

A similar mild-cam 400 short-block, with your original heads would likely be more-tractable and have a wider power-band than the long-stroke 350 short-block with the same heads.

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