crank weight?
#886551
12/23/10 02:48 PM
12/23/10 02:48 PM
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Joined: Jan 2003
Posts: 526
orange,tx
onebadstepside
OP
mopar
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OP
mopar
Joined: Jan 2003
Posts: 526
orange,tx
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I have been kicking around the idea of building a sb stroker.I was wondering about the scat cranks.They have a lightweight forged series which is 8lbs lighter than the regular forged crank.What are the advantages and dis-advantages with the lighter crank?Im I going to have trouble balancing?..do you need the reciprocating weight to get moving?...thanks in advance
Small blocks rule!!!!!
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Re: crank weight?
[Re: onebadstepside]
#886553
12/23/10 07:58 PM
12/23/10 07:58 PM
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Joined: Oct 2007
Posts: 4,205
New York
polyspheric
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master
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Posts: 4,205
New York
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The stored inertia in reciprocating weight is thrown away as heat (wasted, used up, does nothing) twice in every engine rotation: 233 times per second at 7,000 RPM. You don't want any at all (but of course it's not possible)
Boffin Emeritus
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Re: crank weight?
[Re: polyspheric]
#886554
12/23/10 08:19 PM
12/23/10 08:19 PM
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Joined: Mar 2007
Posts: 12,008
Finally a HUSKER again
Moparnut426
I Live Here
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I Live Here
Joined: Mar 2007
Posts: 12,008
Finally a HUSKER again
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The last dirt track Late model engine I built had a VERY light crank in it, it had a Kind custom crankshaft that only weighed 38lbs IIRC It started out as a 41 lb crank. In dirt track they use RPM as a break too. They spinn it up to 8500, get to the curve, let off the throttle set the car, and smash it again, the engine is constantlu at 8500, 2500, 8500, 1800, 8500, 3400, and so on. So the lighter the rotating weight in the engine the faster things will happen. They dont really need torque, or stored weight. They are always in the rpm, and they dont want it at say 5000, its either up or down. Thats also why a VERY strong rod is needed. I have seen some VERY nice stuff from Callies, King, Scat. Scat sells a nice crank thats 43lbs outa the box for a small block shebby. Thats all we ran was a 355 shebby. We had titanium valves, light cranks, light pistons, light rods, and a light lifters, and rockers. I dont know if you really want a super light set up in a street engine as you would sacrifice torque. I dont know If I kept any of my Specs as I usually gave them with the engine as they were sold, but Even with a super light crank, we hardly ever saw a failed crank. A good buddy of mine runs a Briant custom crank in his "open" late model engine, and it alone was over 5K for the crank. He wouldnt tell me how light it was. The only reason I know it was at least 5K is when he had it ordered, his machinist said it was not true, and when he was talking loudly on the phone the blippidy blippidy damn thing costed over 5k and its blippidy not true!!! Fun times  Kasey
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Re: crank weight?
[Re: polyspheric]
#886555
12/23/10 08:20 PM
12/23/10 08:20 PM
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Joined: Jul 2009
Posts: 413
Norway (old world)
Oyvind Mopar
mopar
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mopar
Joined: Jul 2009
Posts: 413
Norway (old world)
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Quote:
The stored inertia in reciprocating weight is thrown away as heat (wasted, used up, does nothing) twice in every engine rotation: 233 times per second at 7,000 RPM. You don't want any at all (but of course it's not possible)
Well, it would only be the friction losses that are waisted. The energy stored when accelerating the parts is restored when decelerating, minus the friction losses. Much of the friction losses in an engine is normally between the piston rings and the cylinder walls, but of course everything counts. Though, it is better to have heavy good parts than light fragile parts, so do not go extreme...
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Re: crank weight?
[Re: onebadstepside]
#886556
12/23/10 08:52 PM
12/23/10 08:52 PM
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Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
Performance Only
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top fuel
Joined: Jul 2010
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Delray beach, Florida
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use the kiss method. if you need every last horsepower out of your engine and it'll see rpm above 7500, go light. if that's not you and it's a street/strip or bracket car, the standard weight crank will do a fine job and live a long and happy life.
machine shop owner and engine builder
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Re: crank weight?
[Re: barracuda man]
#886558
12/23/10 10:06 PM
12/23/10 10:06 PM
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Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
Performance Only
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top fuel
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Delray beach, Florida
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Quote:
Quote:
use the kiss method. if you need every last horsepower out of your engine and it'll see rpm above 7500, go light. if that's not you and it's a street/strip or bracket car, the standard weight crank will do a fine job and live a long and happy life.
If the scat pro comp light weight 4.00 crank at 52 pounds will turn up quicker rpm.Do you think in a 416cu stroker its the way to go.I turn the 344cu 6500 rpm.I'd like to be the fox for a change instead of the rabbit.   [image]
a light rotating assembly is nice and depending on other factors can even be easier on some parts.
the bottom line for me is that there are a lot of other things i concern myself with first in order to make the power a customer requests. if your building a 8500 rpm w9 headed combo to go out and set the world on fire, go light. if your building a bracket motor that needs to run 9's or 10's, i can think of a lot better places to spend the extra money.
i don't want to say there isn't an advantage to having the lighter parts, but if your going with a lightened crank are you willing to spend the money for lighter rods and pistons and ring pack to go with it? if not, you might not see much if any benefit for a typical 650-700 HP bracket combo.
machine shop owner and engine builder
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Re: crank weight?
[Re: Performance Only]
#886559
12/23/10 11:14 PM
12/23/10 11:14 PM
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Joined: Jan 2003
Posts: 526
orange,tx
onebadstepside
OP
mopar
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OP
mopar
Joined: Jan 2003
Posts: 526
orange,tx
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So what I am trying to decide...is that the extra $230 for the light forged scat crank instead of the regular std forged crank would it be a waste of money say for something that will turn 7000rpm with h-beam rods and forged pistons?
Small blocks rule!!!!!
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Re: crank weight?
[Re: onebadstepside]
#886560
12/24/10 12:05 AM
12/24/10 12:05 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
So what I am trying to decide...is that the extra $230 for the light forged scat crank instead of the regular std forged crank would it be a waste of money say for something that will turn 7000rpm with h-beam rods and forged pistons?
I turn down the weights on most of my cranks but I
have a lathe and its my time.... I have spent more
than $230 on lightening and balancing on a crank...
if your a bracket racer you really dont need to BUT
if you trying to go all out $230 isnt much.... myself
I would but thats me... but as I say that , the engine
I'm building now wont have a turned down crank
(street/strip engine)
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Re: crank weight?
[Re: Oyvind Mopar]
#886561
12/24/10 12:36 AM
12/24/10 12:36 AM
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Joined: Oct 2007
Posts: 4,205
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,205
New York
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Reciprocating weight comes to a dead stop @ TDC & BDC. Zero. Not moving. It doesn't carry over anything.
It requires power to stop it, and power to start it. All of these are completely wasted when the direction changes.
The only stored inertial energy is rotational.
Boffin Emeritus
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Re: crank weight?
[Re: polyspheric]
#886563
12/24/10 12:56 AM
12/24/10 12:56 AM
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Joined: Jan 2003
Posts: 5,894
Florida
Locomotion
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master
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Florida
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You will be MUCH better off by spending the extra money on lighter pistons & rods. Again, as long as the rods are strong enough for the application. The lighter those get, the easier it will be on a quality cast crank, which is naturally lighter than steel.
SRP pistons are some of the lighter off-the-shelf pistons, Eagle makes a nice cast crank and you can consider SIR rods or go a little beefier with Scat I-beam or Eagle H-beam.
Last edited by Locomotion; 12/24/10 01:00 AM.
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Re: crank weight?
[Re: Locomotion]
#886564
01/11/11 10:29 PM
01/11/11 10:29 PM
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Joined: Feb 2009
Posts: 1,532
off the grid
340B5
pro stock
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pro stock
Joined: Feb 2009
Posts: 1,532
off the grid
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Good topic: I'm about to pull the trigger on a set of lighter pistons and lighter(but stronger) rods. Short of figuring high high a horse can lift 33,000 lbs in a minute, how much HP could one free up by lightening rods/pistons by 2320 grams (5.1 lbs)  for a set of eight? Keeping in mind that weight has to come off of the crank counterweights also.
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Re: crank weight?
[Re: onebadstepside]
#886566
01/12/11 12:13 AM
01/12/11 12:13 AM
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Joined: Jan 2003
Posts: 6,591
Canton, Ohio
Sport440
master
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master
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Posts: 6,591
Canton, Ohio
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Quote:
So what I am trying to decide...is that the extra $230 for the light forged scat crank instead of the regular std forged crank would it be a waste of money say for something that will turn 7000rpm with h-beam rods and forged pistons?
Put the Extra $230 towards a lighter reciprocating piston/pin and rod assembly. Thats more important then the overall weight of just the crank. To balance the lighter assembly, more weight will have to come off the cranks counter weights anyways making itself lighter in the process too. mike
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Re: crank weight?
[Re: emarine01]
#886567
01/12/11 12:03 PM
01/12/11 12:03 PM
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Joined: Oct 2007
Posts: 4,205
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,205
New York
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starting and stopping a mass 233 times per sec , taking off 5.1 lbs
Only the part that stops gets deducted - not the rod big ends.
Boffin Emeritus
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Re: crank weight?
[Re: polyspheric]
#886568
01/12/11 04:40 PM
01/12/11 04:40 PM
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Joined: Jun 2007
Posts: 2,646
Plymouth Meeting, PA
bigtimeauto
Trophy Winner
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Trophy Winner
Joined: Jun 2007
Posts: 2,646
Plymouth Meeting, PA
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On a stroker I would be more interested in a crank with center counter weights instead of overall weight, just sayin.
Now if it was a comp engine it would be rifled, undercut and star'd but it would still have the center counters.
BB, TT5,Procharged 3300lb Street Car 4.79/154
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Re: crank weight?
[Re: bigtimeauto]
#886569
01/12/11 04:59 PM
01/12/11 04:59 PM
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Joined: Aug 2003
Posts: 43,097
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,097
Bend,OR USA
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I believe the blown alcohol NHRA cars don't run center counterweights, 10,000+ RPM I have one of the lightweight Crower blown alcohol cranks (66lbs  ) in my 527 C.I bracket motor I have seen a 36 lb. SB Ford NASCAR type roundy rounder cranksahft,it was a piece of art 
Last edited by Cab_Burge; 01/12/11 05:25 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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, taking off 5.1 lbs has to help a lot, The block , crank, caps and bearings will like it. 
