Pros and cons of a lightweight crankshaft?

[soonersublime]

I have read a couple of opinions on this subject and would like to hear some more. I've seen a few engine builders who prefer heavier 4340 cranks over the lightened ones in certain applications with the normal excuse being that the heavier crank will absorb more stress with some builders claiming the same thing for the lightened ones.

[fast_crew]

You won't see a heavy crank in a sprint car motor, and your not gonna get any more stress than that. They don't run a balancer, but the crank does drive the water pump. (fluid dampener in a sense.) Which isn't recommended when running an actual balancer. Get a good balancer and don't worry about it.

[soonersublime]

You won't see a heavy crank in a sprint car motor, and your not gonna get any more stress than that. They don't run a balancer, but the crank does drive the water pump. (fluid dampener in a sense.) Which isn't recommended when running an actual balancer. Get a good balancer and don't worry about it.

Sprint cars also rebuild their engines every 4-5 nights out.

A late model engine would be a better example IMO.

[stockcar5]

i know some sprint car drivers and builders sometimes use a heavier crank to keep rpm up on a bigger high hp track. slamming that big rr into a cushion at 100+ mph and not having enough mass to keep that rpm up is a problem for some guys.

[fast_crew]

Engines are rebuilt about every 8-18 races depending on the team, usually due to valve train issues, and poor air filtration. Cranks aren't replaced upon a normal rebuild. A longer driveshaft should absorb more shock (stress) then the shorter drive shaft that is basically bolted directly to the crankshaft of a sprinter. A RR tire/wheel weighs as much as both rear tires off of a latemodel. A winged sprint is usually hammer down and hooked up, bouncing off the cushion in the turns lugging the motor down, even though the throttle is, or near w/o. A latemodel is on and off the throttle which does cause a alot of stress (twisting crank opposite directions) but they also spin the tires more often then a winged sprint, and the chassis/ suspension itself absorbs some of the shock (stress) when lifting the chassis. And how many latemodels don't run a balancer?

Some companies know the crank it self can be designed to cause less stress with careful placement of the counter weights. As mentioned cranks with smaller journals and larger strokes are weaker by nature due to less overlap between the main/rod journals. But this doesn't really apply to standard journal light weight under cut counter weights or gun drilled main cranks (doesn't remove metal from the overlap area.

[powerslide]

i'm with living truth on this one. a 2350 pd car hitting bumps and the wheels coming off the ground then slamming back down seems like it would be more stressful than a 1300pd car doing the same.
and you only factored in the tire/wheel what about the rotors and hubs? i dont see how a sprinter would have anywhere near the rotational mass a late model would.

[fast_crew]

A lightweight crank has lightening holes drilled into the rod journals, and the mains gun drilled, along with undercut throws, and counter weights. And when all said and done is fairly close to the same designed bob weight numbers as a heavy crank, it's where the weight is removed not how much. Our 43lb crower, w/66?g rods, 46?g pistons+pins only has 2 IIRC on each end,slugs of mallory. Now granted I have seen some hack jobs with 4 or more so pieces in each end. And a 42lb compstar comet w/530g rods, 43? IIRC pistons, still needed around 190-200 grams off each end.
Those nascar guys with 850hp better get rid of there 283 or smaller mains and there 1.850 rod journal 35lb cranks before they run them over, with there 3400 lb car.

[soonersublime]

Your right in many ways fast crew. NASCAR runs billet cranks though. Whole different animal. Rod and piston wise I don't know. I wouldn't recommend the typical 4340 lightweight crank w typical h beams for 500+ rods.

[stockcar5]

nascar uses $10,000 MCT billet cranks and have 25 million dollar budgets. they replace them cause they can..not because they need to. now roush is putting used cup parts into motors built for dlm's and sprint cars.

[hucktyson]

Nascar uses a lot of sonny bryant and windberg cranks just like cornett, clements, propower etc. Big name engine builders are using cranks in the mid to low 40's with 4" strokes to build there 430s. By your estimation of a 51lb crank only being good up to 550 hp what the hell do you run with 850 plus hp ??? a 200lb crank ????

[fast_crew]

yeah but there made from that special unobtanium huck... Guys ask questions they already have the all the answers to.

[sj valley dave]

We have a 46 lb Callies Magnum Pro in our 819 HP 422 with a 3.875 stroke with multiple seasons on it....

[hpontap]

Grain structure, how "clean" the material is, amount of overlap between the rod and main journals,where material is removed etc. all play a part in the equation. Thats why the higher the quality, the more you pay for it.
The advantage of the billet crank is you can go lighter and still retain the strength of a forged crank.
Unless you are building some kind of monster, a good quality forged crank in the mid 40's will usually take what you can use on any dirt car.
Now if you go drag racing and use power adders, you want a heavier crank for inertia factor in the launches and as much material as you can get in the crank to withstand the added stresses.

[Hendrens Racing Engines]

The cranks we use are Crower 3.875 stroke/400 main 2.00 rod 42/44 lbs ready to run.All of them are in engines that make a minimum of 850 H.P.with around 30% of them making over 865H.P.
Our failure rate in the last 5 Years has been zero.Bill

[soonersublime]

The cranks we use are Crower 3.875 stroke/400 main 2.00 rod 42/44 lbs ready to run.All of them are in engines that make a minimum of 850 H.P.with around 30% of them making over 865H.P.
Our failure rate in the last 5 Years has been zero.Bill

Is that a billet crank? What's your opinion on 4340 cranks if not?

[Hendrens Racing Engines]

Is that a billet crank? What's your opinion on 4340 cranks if not?

They are 4340,plenty strong with the combinations we build.Bill