Cast crank rpm limits?
 

I have enough parts to build a back up short block for the race car. I have a forged crank and billet 4 bolt main caps in current engine. What would be the upper limits of a cast crank with studded 2 bolt mains?
I shift a 6400 and go through the traps at 6800 Making about 700hp.

Mike,
While I don't have an answer for you. One would need to know the stroke also.

Stan

455? If so you are right at what I consider the limit. Aluminum rods would help the situation.

Balance quality is the deal; Counterweight momentum (weight) needs to be matched to the slug&rod momentum (weight, to minimize counterweight "wafting" which exercises the rodpin armpit for pit cracks.

PMD Crank strength is quite good when flexure is constrained to NA pulsed torsional TQ.

BTW Some strong AL (steel too) rods are now lighter than the BME AL rods i had, by 50 grams.

I made 610hp on the motor, and sprayed 400(jet sizes) of nos through it...I did that with a stock nodular crank, super duty rods, and a heavy TRW dome piston...Car saw many many street miles, and passes down the track over the 5 years that I ran it...The same crank will go back in that engine if I ever find something to stick the engine in...

I agree that quality balance work is a must...Mine regularly saw 7500rpm, and was one of the smoothest engines that i've ever messed with...

I'm sorry, yes 455 with ross pistons and eagle rods.

More then having a concern about crank strength up at 6800 rpm and 700 Hp I would 1st off be far more concerned with the strength of those 2 bolt plain grey cast iron main caps.

Yes, there plain cast iron just like the block, not Armasteel or modular iron like the crank!

Then 2nd if the block was drilled and tapped for 4 bolt main caps I would worry about the loss of strength by having those 6 unused bolt holes open with no grade 8 Allen head bolt very well loctited in to help limit flexing which then provids a good place for a crack to start.

In terms of increasing the crank strength you can grind and polish the hell out of it, it will take about 2.5 hours to do so.

Set it in the block in two old bearings and remove all the sharp edges not only the casting seams..

While your at it you can somewhat knife edge the leading edge of the counter weights if the crank will need a re-balance job.

Note also that there are a lot of sharp machining edges left in the main web area of the block from the ruff cutting work to get the cylinder bores started.

In order, …..Ping test, check oil galleries. Start spending money,…. mag, check see if straight, grind it, bal it, run it, hehe

....wish the fella that ran a blown cask crank into the 6 sec ET would post the prep work he did.

There is only so much that can be done but wouldn't mind hearing his process. We used to cut down the counter weights, SBC crank pin, large radius on the mains, larger on the rods, chrome, tufftride or heat treat the journals, shot peen, radius/knife edge the counter weights and use about fifteen slugs of Mallory to correct the balance. In today's dollars probably about two thirds the cost of a billet crank.

Back in the old days it was Doug Rowe at Crankshaft Specialties that did a lot of Pontiac cranks with the modifications Paul mentions above. Jim Butler used to send him a lot of work !

( They had some nice product when it was right. Some people had rough dealings with them )

.

The block I have is 2 bolt main and not drilled for 4 bolt, I remember years ago when I was using studded 2 bolt mains I cracked the block where one of the studs screwed in. Forgot which main but it was in the middle. Alot of people said is was from using a 2-step and trans brake with solid motor mounts. I since put a mid plate and motor plate in.

If you used the 2 bolt early style motor mount, it only straddles #3 main, so all the force is concentrated on one bulkhead. The later style 3 bolt mount was moved further towards the front of the block, and straddles #2 , and #3 bulkheads. It was redesigned in 1970 when the longer stroke 455 made it's debut. Evidently Pontiac engineers had the higher torque output of the 455 in mind when they redesigned the motor mount locations for the 455, which later trickled down to all displacements using the improved location.

Of course you have now circumvented the side mounts by fitting the car with motor plates, probably one of the best choices for eliminating side stress on the bulkheads completely, especially if you're using PMD blocks, as opposed to aftermarket blocks.

Splayed 455 caps with front one too boot, 275$. Not bad.

https://forums.maxperformanceinc.com...d.php?t=863212

A two step beats the livin Hell out of a block and crank if the crank is not a bilet with all 8 counter weights .

That was a 400 crank and I am pretty sure made a post on what he did. I believe he had plans on using one of those forged Ohio 400 cranks that can be hard to get sometimes in the future.
I think he a a member but he did mot make many posts.
From Fresno ? Or maybe Bakersfield, somewhere in that area.

When I build my 421 SD almost 25 years ago,used a 4 bolt 428 block,Armasteal crank,knife edged,nitrited.Carillo rods.Raced it for a few years shifting at 6200.Still in the car running great.Tom

There was a guy in our Pontiac bracket racing series who had an 8-second roadster. He was running a 455 with ported 614 RAIV heads and a factory 455 crank with Pontiac journal stock length rods (unknown maker or material) who shifted at 7500. I had a habit of asking general questions about the engines of the faster cars.

This was 25 or 30 years ago.

Years ago I bought this crank off E Bay that the seller said it was a forged 455 crank. So I gambled had it sent to Rowe at CS and had them do the works to it. Weld the thrust and cut it to 3" mains. Double knife edge it and gun drill the thing.
So I get it and it has the 455 square cheeks and since I have never seen a forged Pontiac crank with those I had my doubts.
The thing is VERY shiny, especially on one of the counterweights they profiled.
I call Rowe up and ask him about all the steel-casting deal. You would think the guy who welded up the thrust and machined the thing would immediately know if its iron or a forging. But never got a straight answer. Maybe its cast steel so it would weld OK with normal rod-wire.
The guy I bought it from originally bought it from Crankshaft Specialist, it even had their logo on the crank in the E Bay pictures.
I still have it and consider it cast. Maybe I will use it with some blocks/rods I have laying around for use when I am a geezer still bracket racing.

We ran the Grocery Getter wagon for years with a stock block 455 and an N cast crank. 3 different blocks. After years and years of running around 1000-1100 HP. Zero broken or cracked N cranks. Ended up with 3 broken blocks. All cracked through the #2 main oil feed hole up to the cam tunnel. But our combination was a little different than yours. We had 5 billet main caps. Also aluminum connecting rods. But we ran it to 7500 rpms hundreds of times with a manual transmission and did 15-20 second 1/8 mile burnouts @ 7000 RPM's. Man, that was fun. IMO, with 4-bolt main caps, the crankshaft will be the last part to fail. Rods first, then the block, last the crankshaft. I have never run big HP with a 2-bolt main cap Pontiac. So can't help you on that part.

I had an issue with cap walk at 2 and 3 at a lower level than what your trying to build in a .060 over 469 at 6500. I ended up putting the the available longer dowel pins for the main caps to help tighten everything back up.

We ran a 455 with stock crank, stock 2 bolt main caps and bolts, Eagle rods and Ross pistons. All steel 67 GTO w/ full interior and 9" slicks. Best was 10.51 @ 127. Shifted at 6300 and went through the traps about 6700. And although we didn't race every weekend, we did it for years.

It’s very easy to tell if a 100% polished out crank is cast or steel.

Drill a shallow 3/16” hole in the the flywheel end of the crank as close as you can get it to the crank centerline.
If the crank is cast you’ll drill out chips, if it’s steel you’ll get long shards.

My bet is since you say it has a center square cheek that it’s a factor nodular iron crank.

The crank is not 100% polished out. The profiled counterweight is just cut on a lathe. Very shinny.
I did do the drill thing years ago. Scat forged came out as spirals. OEM cast came out very small chips, powder like.
This crank had a bit larger pieces than the PMD cast crank.
I am thinking it started out life as a Eagle cast steel crank since they did not weld up the thrust with Ni rod-wire.

I would like to modify my answer as I misunderstood the question. Your crank is good for a lot more power than you're are asking about. The two bolt main caps are the week link at your power level. Anything you can due to reduce reciprocating weight will help along with main studs and over size main cap locating pins.

Easier to say larger dowel pins :D

The transmission alignment dowel pins? :D

Factory 2-bolt Main Caps will be fine with NA and well-Balanced. sure there are better tings.

You're probably right, all the way until the number two main cap separates and becomes two one bolt caps.... and assuming you don't mind some main cap fretting

So Mike S ( the OP) your good to go it seems!

Half inch stud will guarantee you that up at the 700 hp level with your motor being NA with its two bolt caps, that a good balance job is the ticket to keeping it all together!

Well no, but saving money on a 1/4 mile grenade is a balancing act. Post #21 reads well to me.

Whatever the ECM does is the right answer, if you got the money to pay out for that solution. I don't.

See, i went with Studded and redoweled Steel MainCaps awhile ago: the Block showed a split up #2 web. Must have been the heavy TRWs, SD Rods and typical sized balancer. And the steel mounts.

I bought a 2-bolt drilled 455 block, cast caps stock bolts that had several hundred high rpm rounds in the low 9s. Inspected well. Sold it last year to a fella that will race it. Had/has aluminum rods and super light sbc 400 slugs and thinwall 927 pins.

Midplate and front plate seem to be the economical feature that keeps the main webs intact.

Lots of good stuff here, I use longer bigger dowls I make my own
two bolt studded mains and alluminium rods and a good bal job.
My allum rod 464 SM headed motor seen 7000 quite a bit with no prob's

GT.