Light Weight Crankshafts

Anyone have any thoughts about using a light weight crankshaft on the street? I'm thinking of the Scat Q-Lite at 40 pounds or the Superlite at 44 pounds.

compstar comet

Thanks Dwayne I will look into them. ;)

im running eagles lightweight crank and i love it. it revs so much quicker, this is in my daily driver

Thanks for the reply. I did think there would be any problem running a lightweight crank on the street as long as it was balanced properly. That is good to hear.

I read a post about them freeing up lots of hp??? True?
Something about a light weight rotating assembly will produce more power??

I don't think it produces more power but does allow the motor to rev up faster. Less rotating mass for the toque to overcome.

You end up with more HP at the flywheel, because less HP is consumed in acceleratiing the mass of the rotating assembly. How much HP you gain on a dyno will depend on the engine rev rate, in RPM/sec.

I am actually looking at a 3.750 crank from Ohio Crankshaft, weighs 44 lbs. and about $550 I believe.. Thats less weight for a little under the price of the 52 lb eagle or scat.. I still have the Eagle H-Beams that weigh 630g, so dunno if it will do me any good? Any opinions?

I am actually looking at a 3.750 crank from Ohio Crankshaft, weighs 44 lbs. and about $550 I believe.. Thats less weight for a little under the price of the 52 lb eagle or scat.. I still have the Eagle H-Beams that weigh 630g, so dunno if it will do me any good? Any opinions?


Run far and fast. Those ohio cranks are all over the friggen place on tolorences last time I saw somebody try to use one it woulda cost 3x what he paid to actually get it in shape for use.

You wont notice a lighter crank unless you're road racing and matching revs is really important. In that case a light crank and flywheel is where its at you can get the engine where you need it a bit faster than the heavy crank.

The only other thing is, the energy your crank soaks up instead of sending it to the drive wheels is put back into the car at a different time. The inertia is given back to the car when you shift and the engine speed slows down... thats the surge of speed or the little bump you get during a shift at wot or anytime really.

If the price difference of the light stuff is insignificant then by all means go for it. If its going to cost more my opinion on that is, every dollar you put into the heads is worth three dollars anywhere else.

I would not lose sleep over my crank being 10lbs heavier than it coulda been. Also keep your eye on the target bobwieghts of the cranks you're thinkig of buying.

ahh I got ya, well if I can get a Eagle 4340 Crank for $200 plus shipping with 1800 miles on it, would it be wise and cost effective to buy it?

Assuming theres nothing wrong with it, by all means. I would at the least have it polished and checked for cracks and runout though, figure that into your bill.

Stay away from Ohio Crankshaft.

Rich

Rich, I hate to disagree with you but they are great to work with. I've had them "correct" several cranks supplied by thier competitors (Eagle especially, scat, and etc) and for very reasonable costs.

I've bought cranks from them and if they had any run-out or straightness issues they gladly corrected the problems, no questions asked.

Rich, I hate to disagree with you but they are great to work with. I've had them "correct" several cranks supplied by thier competitors (Eagle especially, scat, and etc) and for very reasonable costs.

I've bought cranks from them and if they had any run-out or straightness issues they gladly corrected the problems, no questions asked.

Their service may be good, but there are plenty of horror stories about their import cranks.

Rich

I've bought cranks from them and if they had any run-out or straightness issues they gladly corrected the problems, no questions asked.

If they want me to work quality control for them they need to send me a weekly check. A few of the problems with thier work that I have seen is things that should not even get out the door ever. Sending stuff back and giving them a second shot at it with more than one crank is not acceptable imo.

Heck I've had issues with LA Enterprises billet cranks, it is your job to double check somebody elses work if your building an engine. I've disassembled countless oil pumps with machining shavings in them, MSD distributors with loose parts floating around inside.

A good friend of mine bought a race engine from somebody in this board, it had a 9/16 socket under the valve cover, rust on two of the connecting rods and sand packed under the oilpan baffle. When questioned about it they claimed these were his issues. On top of that the clearancing on the block appeared to be performed with a stump grinder and it leaked water.

You shouldn't have to but you should check everything.

Anybody who builds engines knows all that. When I check and its off and they have to try again, and they send me a replacement and its off as bad or worse than the last one and this repeats several times with different makes and models. I reserve the right to not do business with them anymore and give them a poor review. The list of companies I buy parts from is pretty short. The list of companies that have imo high quality parts is even shorter.

Just for comparison, the stock crank is like 62lbs on an admittedly cheap bathroom scale, so something like a Comet is 20lbs lighter. What lenghts have some of you gone to to remove 20lbs of dead weight much less rotating? How much did you spend on fancy light drg wheels and front skinnies. Granted wheels and tires are on a larger radius so the weight has more impact but you get the idea.

I thin Injuneer is right that it will only showup as acceleration HP gain, don't think it would register in a steady rpm test, but then again acceleration is what all this is about.

I got a good deal on a SCAT Ultra Light crankshaft. On my bathroom scale it comes in at 43.6 lbs. So if my stocker weighs close to the 62 pounds that Dwayne found then there is a 18.4 pound savings. That is a lot of rotating mass saved. ;)

A lighter rotating assembly is always better if the strength is there. But for some uses it is not going to make a big difference. Most drag cars operate in a fairly narrow rpm band. Example: my bracket car operates between 5,500 and 7,100rpm so there would not be much point in going to any great lengths making the crank light. Light pistons will lower stress on the rods, that's where taking weight off the RA is going to the most good in that sort of application. A street car or a road race car or even a manny tranny drag car is going to benefit a lot more than a drag car with an automatic. Not knocking the light crank idea, just trying to give some perspective. I have an expensive crank with lightening in the Camaro, the race car has a fairly heavy crank. Sounds backwards, but it's not.

Rich

Just look up angular momentum, and then apply conservation of energy to it. That will be the end of the story and end all speculation.

Lets ignore bearing drag or any frictional losses they will be roughly the same. You spin up a disc that wieghs 62lbs, and you spin up a disc that wieghs 48lbs. Both of them are giong the same speed, its harder to slow down or speed up the 62lb disc because it can store more momentum. It takes the same amount of energy to slow it down 2000rpm as it does to speed it up 2000rpm.

That is what happens in an engine when you go down the racetrack. Yes the crank/disc will store more hp while its accelerating instead of sending it to the wheels, it will also give it all back to you when you shift right throught he wheels. The only place you're going to get ripped off on the energy is in high gear and you dont get to absorb that energy back (no shift).

Sure sending more to the back wheels is a good thing as you run through the gears. Imo its not worth losing sleep over because the gains will be very marginal.

The moment of inertia is more important in regaurds to how a crank will act rather than its overall weight.

Anybody who builds engines knows all that. When I check and its off and they have to try again, and they send me a replacement and its off as bad or worse than the last one and this repeats several times with different makes and models. I reserve the right to not do business with them anymore and give them a poor review. The list of companies I buy parts from is pretty short. The list of companies that have imo high quality parts is even shorter.

Amen!

It pisses me off, and it pisses off the customers when they're told that time is money, and them trying to save a few bucks on the cheapo crank is costing them because I have to measure multiple cranks before they find a decent one. We just went through this with an "RPM International" crank for a 383 mopar - 2 "hand selected" cranks with over .002" bend and journals up to .001" over spec and .002" variance across the width, so bad actually that the connecting rod would not slide up against the radius. Then on top of that the 2nd crank had counterweights that were not cam cut enough and they actually hit the block before the journals laid on the bearings.

After 2 bad cranks, he finally let us supply a good one.

Yeah thats kind of another thing why I don't go through that. Average Joe hot rodder doesnt check taper runout straightness or verify fillet radius and wont have it checked for cracks after all its NEW aint it? Hell only a few shops I know of check surface finish. Add to that the average person doesnt know what a stress riser even is how we apply it to mechanical design or the effect it can have on a part or what to look for in that regaurd.

I do my best to make sure companies supplying crap parts don't get any money from me or people I help out. Because everyday people will end up getting screwed when they buy a half ass part and dont have the smarts to verify everything.

On the track we see a pretty consistant .01-.015 improvement in ET with lightweight cranks in BBC dragsters but they don't live nearly as long. Not a huge gain. I prefer to use std weight cranks unless it is a heads up kind of deal and then it would be torn down on a regular basis for inspection.

On the track we see a pretty consistant .01-.015 improvement in ET with lightweight cranks in BBC dragsters but they don't live nearly as long. Not a huge gain. I prefer to use std weight cranks unless it is a heads up kind of deal and then it would be torn down on a regular basis for inspection.

Amen.

Rich

its more of a butt dyno thing you dont get that much hp from it but you accelerate faster due to quicker revs

Not according to post #25. :)

A whole .015! :eek: Where do I sign up :lol:.

Thats probably a 700hp+ engine to boot.

Here is an interesting thread for those interested in the subject. Some of the top engine builders in the country are commenting in the thread.

http://www.speedtalk.com/forum/viewtopic.php?t=5063&highlight=lightweight+crankshaft

As Rich was saying this may not help a race car much since they can keep the engine in a narrow band, but a street car drivetrain probably wont. Like I said before, even if you ignore the rotating weight part, what have you guys done to save 20lbs of dead weight, off the nose of the car no less?