Street_Machine_302

What is the absolute fastest RPM that you can spin on a 350's cast crank.

I am running a set of forged I beams with forged flat top pistons.

Thanks J

Z28SORR

Depends on HP level. I think most would say to keep it under 6800rpm.

mdacton

pistons and rods will go before the crank in most cases.....

What kind of rods you have?

Street_Machine_302

I have a set of speed pro forged aluminum pistons (flat top with -8cc valve relief total)

The rods were manufactured by "STAR" and have 7/16 ARP studs. They are made form 4340 forged cromolly steel. They are I-beams.

The engine will have 12.5 : 1 compression.

I like high rpm, 7200 - 7500 realistically is where I plan to shift, but if I can go higher, I may.

mdacton

your pushing the linit as it is.....

never heard of those rods....

byrons1502

i ran my cast crank hard but it is a time bomb good luck

redneckgames

one of the big limiting factors as far as bottom end goes is if the pistons are press fit bushings or floating. floating will take rpms alot better.

rskrause

How many times or how much time at a given rpm are what counts more than anything else. It would probably spin 10,000rpm, once. Regardless, to make it last at high rpm a good harmonic balancer is key. There is really no single answer to your question. What usually causes a part like a crank to fail is stress over time, not a one time load. The stress is a product of rpm and hp. More of either = more stress.

If you are talking about a drag car that sees 7,000rpm briefly at the shift point I think the longevity will be acceptable provided the rotating assembly is balanced properly, has a good harmonic balancer, and it has an automatic or at least you avoid high rpm clutch dumps if it's a manual. If you want to run it at 7,000rpm for hours on end, that would be entirely different. Most people overspend on the bottom end and skimp on the top end. How many broken cranks, including stockers, have any of you ever seen? I'd put it together right and spin it up. Just don't expect 100,000 miles out of it.

Rich

tomcowle

You could spin it as high as you want to as long as it has a great balance job and you use the lightest possible rotating assembly, cylinder heads that can flow and excellent valvetrain.
A harmonic balancer is NOT needed when you use an automatic transmission, the convertor acts the part of the harmonic balancer. many Comp eliminator cars run the empty alum shell for the timing marks alone.

rskrause

Tom: how often do they rebuild those motors?

Rich

tomcowle

Was that the question?

They get rebuilt as often as engines that run a harmonic balancer operating under the same conditions.

I've also yet to see a harmonic balancer on a serious roots blown application either, the belt and drive set-up acts as the balancer.

ATI, Romac, Innovators West and I'm sure a few others offer the empty shells

rskrause

Was that a question?

Rich

tomcowle

I was referring to the original posters question to the amount of rpms, then you asked me on rebuilds. He didn't inquire about frequency of rebuilds, I answered his question regarding rpms.

If any of that made sence let me know. lol!

I'm going to be driving thru buffalo this monday on the way to Canada, how is that Buffalo Naval Park?

rskrause

Tom: it's nothing special. Buffalo is a funny place - it's a nice place to live but not much reason to visit. Opposite of many other places in the US, IMHO. As far as "must see" touristy things around here, Niagara Falls is about it.

As the original question and the question about the question: I don't think your point about CE cars applies unless he is building a high-level race car. Judging by the parts he is using, I am assuming that isn't the case. Like I was trying to point out, accumulated stress over many miles (revs) is what usually kills parts in typical usage. High-level race cars get rebuilt routinely, before anything fails (hopefully).

Rich