[gtome]

I was just browsing Butlers site and I see they have a kit with a 4.5" stroke forged crank. I dont read everything here, but is there some reason these arent used much? Seems on the site they are actually cheaper than the 4.25" stroke kit. Hope this isnt a super dumb question, but im pretty new at this. "No replacement for displacement" isnt true in this case?

[74Grandville]

i'm not an expert, but i think side load and piston speed might be deterrents in stock blocks.

I'd like to hear more on this topic as I was thinking of going this route as well. what is the recommended safe rpm with a stock 3" main block.

[RocktimusPryme]

Maybe the 4.5 would require clearancing in the block where the 4.25 doesnt

[lust4speed]

Piston speed by itself shouldn't be a limiting factor. A 4.25" stroke will have the same piston speed at 6,400 RPM as that of a 4.5" stroke at 6,000. So really not a deal breaker. Side loading shouldn't be much of a problem either if the block is sonic checked and wall thickness is decent. I built out a 389 2-bolt block with the 4.5" stroke for a friend and chipped it at 6,000 RPM just to give the block a fighting chance. Built over a year ago and he's still shredding tires. The real danger is to his drivetrain.

Also installed a 4.5" stroke forged crank into a friend's 428 4-bolt main block with billet steel caps that's in his drag car. He runs 9.6 in the quarter, and races it like he stole it.

[Formulajones]

Really no reason not to use the bigger cranks. My father just had Bischoff build a stroker with a 4.75" crank, shoved in an MR1 block. Bischoff said it would spin 7,000 easily and he would spec a cam to make power up there if my father wanted to do that.

My opinion, when building a stroker pontiac I wouldn't even look too hard at the 4.250 cranks considering the 455's are already 4.210 stock. You aren't gaining much in size for the money spent, it's still a relatively small engine by todays standards when it comes to stroker engines. I'd go straight for a 4.5 incher.

[Gary H]

I've built two different engines using a 4.5" crank and stock block over the last 3 years for customers. Both have ran great and haven't had any issues at all. One used a 400 block and the other was a 455 block. Both had Edelbrock round port heads and hyd roller cams. The torque for the street is unreal!

[Formulajones]

4.5" in a stock 455 block was the original plan, Tony said not a problem. Until the block was found to be unservicable, cracks all around the mains.
Rather than use his stash of engines that seem to be getting harder to find, he decided why not go aftermarket and step up the crank size. It's a domino affect. In the end he was much happier with the results.

[Singleton]

A question for those that replied above.
What are you using for rod length, and compression height on the Pistons? Any issue with the pin in the ring lands?

I have a 4.5 crank that I am going to put in a 428 block, 2 bolt main with studs. Don't plan on spinning past 5800, so hopefully it can handle it.

[Gary H]

Originally Posted by Singleton

A question for those that replied above.
What are you using for rod length, and compression height on the Pistons? Any issue with the pin in the ring lands?

I have a 4.5 crank that I am going to put in a 428 block, 2 bolt main with studs. Don't plan on spinning past 5800, so hopefully it can handle it.

I've used 6.7" rods. Yes, the pin get's into the ring land area but you use the supplied spacers and they work fine. No issues at all.

[71 T/A]

Butler says you must then go with steel billet caps, which requires align boring of the block.

[Bob-o]

I just got done building mine. ( a couple of months ago,Still haven't run it yet) Its a 71 block with 2 bolt mains studded. Special attention was paid to balancing and I don't plan on revving past 5500. Lots of opinions but Butler and SD both seemed to believe Id be fine as long as I kept the revs down. In the end I got a great deal on a forged crank, a better deal on a set of 6.8" rods and a ridiculous deal on my probe pistons. I had to give it a shot!

[ponjohn]

I thought the main reason was because the piston skirts do not have enough support at the bottom of the bore?

In addition to the side loading stressing the block.

[gtome]

So from what im reading here it seems its key to keep the revs down on a 4.5" stroker. Would it be better to go less stroke and more rpm's (from a performance standpoint) or more stroke and less rpms?

[Bob-o]

Originally Posted by gtome

So from what im reading here it seems its key to keep the revs down on a 4.5" stroker. Would it be better to go less stroke and more rpm's (from a performance standpoint) or more stroke and less rpms?

That seems to be the "Golden Question". Overwhelming response when I was asking was to just go with the 4.250 stroke. Others said splayed 4 bolt mains were a must. Some were speaking from experience others might have been 12yrs old and repeating something they read somewhere. That's the good and bad about the internet, great way to research but who knows who is answering the questions! I feel confident from my actual communication with 2 very reputable resources that I will be fine as long as I stay within my self imposed boundrys. (btwn 500-525 hp and 5500 rpms) I hope to bigger in the future and intend to use an aftermarket block when I do. As long as I don't damage parts in the mean time, I'll have my crank, rods, and heads when the time comes

[Formulajones]

I wouldn't say low RPM is key. I don't know or use Butler or SD, so I'm not sure how they would prep the block with plans to use a 4.5" crank.

I know in the original 455 block/4.5" crank plan, Tony wanted to hard block it, steel splayed mains, they were doing a solid roller, and they were going to spin it up and make some power. Unfortunately the block was toast from a broken oil pump shaft.

The MR1 block they went with is a stout piece, doesn't need hard block and the main saddle area doesn't need extra attention. So they went with a 4.75 crank, and Tony wasn't afraid to spin that one up on the dyno, saying 7,000 would be no sweat. Being a street engine, they wanted something easy on valvetrain parts, no spring changes every season, etc...so Tony spec'd a mild solid roller that made peak power at 5700. They must have made 40 or more pulls trying different combos over the course of 3 or 4 weekends, and every time it was pulled to 6,000. This thing also raps up like a Nascar engine and RPM's quickly, not what you would expect from something with such a long arm in it.

Frankly, I'd leave it up to your engine builder. If he says he isn't comfortable spinning past 5500, then so be it. With smaller heads and milder camshafts you wouldn't need to spin it up that high anyway, and just taylor the rest of the build to the rpm limit he suggests.

[gtome]

Well thats kind of where my problem is. I would like my engine builder to be me this time. Ive never really done it and would love to dive in and lean some more. That is if I even do it at all. I dont see much point "for me" to step up the rotaing assembly to a forged crank or fancy pistons if im not going to be spinning it any faster than what I already do. But then again, piece of mind would be nice too if I did put some more robust parts in it. Or if I could spin it another 1000+ rpms, then I could see it being worth while.

I guess I saw that rotating assembly and thought that if I was going to do it.....bigger is always better right?!!

[Formulajones]

Understandable. I like the idea of big crank, small head and cam and make tons of grunt. Always fun on the street. You could always put good parts in the bottom end for a solid foundation, and keep the head/cam choice mild, make locomotive power and enjoy that for a while. Then you would have a solid foundation for later if you got board and wanted to step it up a bit.

[gtome]

Thats true, but its also where im already at now too! Kinda bored. I probably only have 4-5k miles on this combo. But I havent been able to drive it much lately. That should change once I have it all restored. But I think you may be right. It would be a good step toward learning engines and give me a nice foundaion to throw better heads on etc....

Heck maybe with a forged crank and some I or H beam rods I could turn it a little faster for a little bit more fun. Or maybe I dont even need to spin it faster....so much to learn.

[lust4speed]

Originally Posted by gtome

So from what im reading here it seems its key to keep the revs down on a 4.5" stroker. Would it be better to go less stroke and more rpm's (from a performance standpoint) or more stroke and less rpms?

More stroke, more RPM's, and more cam is the way to go. Otherwise all you are doing with a bigger stroke is moving the same air at a lower RPM and will be producing the same horsepower maybe 500 RPM lower. Most of our combinations are flow limited depending on our heads, and the bigger engine is simply going to max out our heads at a lower RPM. So while much of this thread seemed to imply that the engine was mechanically limited on RPM, it might be better to say that the larger stroke will be flow limited if using the same intake system.

[HWYSTR455]

It was explained to me at some point that due to Pontiac engine parameters, the results of using a 4.25 stroke and a 6.80 rod are very favorable. It provides a great balance of power and integrity, and supports many different combos. You CAN go bigger and be successful, but it takes a more careful approach and selection of parts to see useful returns. Cost to output starts sloping off.

Not sure how true that is, I'm no rocket scientist, but it has worked for me.

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