Max rpm 3" vs 3.25" Mains
 

How would the max safe rpm for a cutdown crank kit (Eagle 455 in a 400 block, 3" main) compare to a full size 3.25" stock type 455?

More meat in the bulkheads, less bearing surface speeds.

I believe there is a chart on main size to RPM.I think H.O had it.Tom

ironhead,

"Safe" is a relative term. Compared to what? We rev our pump gas "street" engines with the Eagle (3") crank to about 6,600 regularly. Rick Holladay revs his 4" stroke 3.25" main 440 to 6,800. Scott Roberts revved his 406 to 6,600 with the hydraulic cam, and to 7,200 with the solid. The Drunken Injun (3" main) revs to 8,800. With the exception of "DI" (only 3 passes on it so not enough "hard" data), ALL of these engines are racing and driving for more than a year or two without refreshing.

RPM is more a function of head flow and valve train than it is "bottom end". You do, of course, need high quality parts in there. The "weak link" in a stock engine is the connecting rod. They gotta go! Rod bearing "speed" is another area that can use some attention. IMO, the only real drawback to the larger main journal, aside from the weaker block (NOT insignificant), is the added heat it puts "in" the oil. I would think that's only a big issue when high RPM is used for extended periods.

It is commonly believed a 455 crank is stronger than a 400. True enough. But again, measured against WHAT? The main journal in a BBC is 2 3/4". So is a Hemi. Those cranks are plenty tough, so the added 1/4" of diameter in a "large journal" Pontiac is not necessary. The crank in a 400, even a "stroker", is far stronger than the block.

Jim

come to think of it it was oil pressure required for main size to RPMs.Tom

If we "normalize" the 3.25" & 3" Main crankshafts as both stronger than the Factory Blocks then:

We cannot ignore the plentiful Factory 3.25" Block Main fractures.

Soooo, seems to me the Factory 3" main blocks are holding up. Or....are they?
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I don't draw a decision on the Main diameter for Aftermarket cranks in aftermarket Blocks.

That's why I have half a notion to go ahead and built a 4.50 stroke 3" main bottom end using either a real nice 66 389 block that I have or a real beefy early 67 400 block. I wish Jim Zeek had made more passes with his car this year just to see how the block would have faired. I would try to keep the rpms under 6500. Makes my think... Are there many 9 second range 4.50 stroke 3" main motors out there?

Has anybody had any success cutting down the rod journal on these cranks? What would be the benefits if any?

Eagle's "stroker" cranks ALL have the 2.2" BBC rod journal (not the 4.21" stroke "replacement" 455). Most of Ohio's offerings do, as well.

We've seen them as small as 2.1" (SBC) incorporating 7" rods, making OBSCENE RPMs (8,000-plus) and living. Our T/A 475 engine uses BBC journals and revs to just under 9,000.

Can't say about the 4.5" stroke in a 400 block, but there are MANY 9 second 461s (4.25" stroke) with 3" mains out there doing it. There really is no logical reason for 3.25" mains in anything but a specific restoration.

Jim

Crankshaft/Pontiac Gregg
 

Factory 400 Nodular Cranks in my solid lifter motors 7400 rpm , That's the plain Jane motors.Crower rods USA Made , cost more $$
428 4 bolt's with Arma Steel cranks 7400 rpm Crower rods
455's With billet 4 bolt caps and arp studs, Nodular crank, roller cam, 7200 rpm.
Bill Miller Alum rods and Crower Super

Now that cast or forged China stuff, I don't know, but all the people who saved $$$
by not buying American made are coming home to roost.

We just took apart the Flying Piglet race car motor( Fast Eddy) the parting line of the rod bearings in the Eagle rods are showing copper. That means the rod forging is going out of round, now the arp( USA made) rod bolts were the only thing that kept it togther, 4.25 stroker motor. WE NEVER SAW THIS WITH AMERICAN MADE RODS !!! Think people, you are cutting your own throats !!

Pontiac Gregg

My 462 saw 7200 down the track most every time out. Eagle 3.25 4.21 stroke Howard Aluminum rods. 70 455 block 3 center billit caps. Took it apart after a couple seasons and magnafluxed it all was good. Not exactly stock but not too exotic.
I must have got a good one...
John

My 510" IA2, 3.25" main, 2.100" rod, 4.375" stroke crank, aluminum 6.80" rods goes through the traps at 7700.

We just took it apart, the bottom end looks good.

The reason we went with this crank is because it is a forged Moldex from many years ago. We had it in our old factory block when we were running mid 9s at 3200 lbs.
We then purchased a 3.25" main IA2 because we had the crank for it.

This winter we will be stepping up to an Ohio forged crank. 4.5" stroke, 3.25" main and 2.200" rods. We are going with new rods and pistons as well, going from a 4.31 piston to probably a 4.32. Should be about 527". Should still go through the traps at at least 7500 RPM.

with aluminum rods the sky is the limit..as long as you can keep the bearing cooled and lubed..they take the shock load off the journals..impart a softer "push"..i believe imbalance and bad tuning is what break things..i have run cast 400 rods past 7000 rpm many times..455 over 6000..

those mains may be big, but they are durable..light rods and pistons will make sure of no issues..if you break it, ya tuned it wrong..imo

All this focus on Crank & rods is neat.
Perhaps a discriminating focus on Aftermarket Block vs Factory block needs/leaves $3000 in limbo.
Finally, them 7000+RPMs need the valvetrain (at what cost again?)

So, back to 3" vs 3.25" : no bearing whatsoever inlight of Block & Valvetrain needed.
So, back to Rod journal Factory vs 2.2 BBC: well Bigger is better I think. yet perhaps no bearing whatsoever in light of Block & Valvetrain needed.

I could ponder the Long stroke NEEDING the larger Rod journal for crank stiffness.
More Journal is better to improve fatigue life when delivering HP throught that last 7-8torsional bar huh. Seems the best case is for the 1-2 journal to be smallest, while the 7-8 journal be "Factory" size .. Anybody do thaat?

3inch to 3.25 inch with same rod journal the 3.25 has more journal overlap(if same stroke) thus stronger. Important more so with cast vs forged IMO.

A forged crank will "move" a lot more than a cast crank. "Stiffness" in crankshafts is not necessarily "good". A cast crank is dimensionally more stable than a forging. That's where the trade-offs begin. For racing, a more flexible crank is more "forgiving" and can stand higher revs. Internal harmonics play a huge role once RPMs go over 5,000. The "steel" crank (forged, NOT cast) is the crank of choice for this. Eagle lists their crank as "cast steel". That refers to the material, not the "process". When someone says "steel crank" in the vernacular, they MEAN "forged".

What Bruce says is correct, the larger main journal makes for a stronger crank. But again, stronger than what? The 3" main crank is NOT "weak", even WITH a 4.5" stroke. The block is of much more concern than the crankshaft in a Pontiac. Consider that big block Chevy has had a 4.5" stroke for many years in the aftermarket. They use 2.75" mains and 2.2" rods without issue.

Larger rod journals would also increase the sheer strength of the crankshaft. Again, no need.

Lastly, Eagle rods go out-of-round on the big end when the bolts are over-torqued. Been there, done that. We've had MANY BBCs over 1,000 HP with Eagle rods that show ZERO out-of-round after 500 1/4 mile passes. We have yet to experience a rod-related failure in a Pontiac equipped with Eagle rods. Agreed, Crower Sportsman rods and Crowerods (the famous billet) ARE very good rods. The latter perhaps the best steel rod available anywhere at any price. The Sportsman, however, is not competitively priced with other rods at the same level.

One of my purposes here is not to point at others and say "Wrong". It's to help bring Pontiac racers into the modern world by dispellng myths and superstitions. Since our shop builds as many non-Pontiacs as we do Pontiacs, we have an "edge" in this arena, as we see more "leading edge" stuff. What we learn from SBC, SBF and BBC (among others) applies directly to the old Injun, and improves it. My point here is to make sure it's clear to all, compare apples to apples, not to donuts. The Pontiac 3" main crank has another "edge" over the BBC and Dodge B and RB cranks, it's shorter. By about 3" from front main to rear. So let's not get carried away worrying about the strength of the crankshaft. The block will self-destruct LONG before the crank is at risk, provided clearances and oiling are correct.

FWIW

Jim

The Drunken Injun is on safari! (we're hunting elephants...)

"The block will self-destruct LONG before the crank is at risk, provided clearances and oiling are correct."

Agree. As long as the balancer doesn't do it in first. LOL

Does the 3" main crank have an advantage over the 3.25" in the oiling department at high rpm's? I don't remember for sure but it had something to do with the oil hole location or passages or something was different limiting rpm on the 3.25" main crank. I dunno, just something I remember being told in the past.

I run a factory 455 crank cut down to 3", in a 400 block. 455CI trapping at 6400. Has gone 9's. I will be pulling it apart this winter (only because it will be freshened and sold, had no issues, and good oil pressure) and will report on findings.

I believe all 3.25'' cranks were cross drilled, all 3'' were not.