Reciprocating Weight

[SCOOBY DOO]

I noticed that RJC's race crank pulley is 5 lbs lighter than the stock crank pulley. Would removing 5 lbs of reciprocating weight from the crank assembly allow the engine to rev up quicker and thusly allow the turbo to spool quicker. Has anyone ever changed over to the RJC race crank pulley and noticed a difference?

 

[turbo39151]

I cannot vouch for the RJC piece as I don't have it but I performed the same mod on my WRX and the spool was definately faster. Better throttle response immediately.

I read somewhere (not sure how accurate this is) that reciprocating weight was to be calculated as a ratio of 8lbs for every 1 lb as opposed to stationary weight. Thus, 5 lbs of reciprocating weight would actually be more like 40 lbs (5 x 8) while stationary weight like the 50lbs in your truck (spare and jack) would still be 50lbs.

So, the idea is to remove as much recip. weight as possible (wheels are a biggie) GN rims are like 20lbs or so. A rough estimate (not including tires): 20x8= 160 x 4=640. Weld prostars at 15lbs each (15 x8=120x4 =480) 640-480=160lbs of weight savings? (that's like taking a person out of the car!)

Please note that I am not an expert and I have not been able to confirm if this information is acurate. However, based upon my experience, using light weight pullies and wheels has resulted in a better feeling and faster feeling car (according to my Butt Dyno)

Can any experts elaborate?

 

[chevyII]

1:8 is correct and can be applied to and rotating pieces in or on the driveline.

 

[turbo39151]

Thanks

 

[DR BOOST]

so does this mean that steel cranks inherently cause a loss of power?

 

[Mike Licht]

just because they are steel does not mean they are heavier. Lighter components will spin up faster however
Mike

 

[DR BOOST]

is a steel crank heavier than a stock crank?
i believe a bhj balancer is heavier than a stock hub and a jw flywheel heavier than a stock.

 

[pacecarta]

is a steel crank heavier than a stock crank?
i believe a bhj balancer is heavier than a stock hub and a jw flywheel heavier than a stock.

in sure they are heavier than stock ,and while the stock can be rebalanced for use on the forged cranks remember if the car is running 10.99 or faster it requires sfi certified balancer and flywheel on an NHRA track
so whether forged or stock crank BHJ/JW should be there .

 

[chevyII]

why do you think the throws on the crank are lightened and alum. rods? Or a small 3 dics clutch instead of 1 large.

 

[Alky V6]

Going with a lighter pulley would be a reduction in rotating mass, not reciprocating mass.

 

[turboclam]

Maybe?

Wouldn't you be able to launch harder with the heavier pulleys? As far as a track car goes. Would the extra weight be beneficial on the line while boosting up?

 

[yullose]

Going with a lighter pulley would be a reduction in rotating mass, not reciprocating mass.

THANK YOU.

I see this confusion all the time and it drives me nutz.

Reciprocating weight is weight that changes direction. (rods/pistons/pins/rings)

Rotating weight is anything spinning. (crank/balancer/pulleys/flexplate/torque convertor)

Lightening any of these items will improve throttle response.

Example... Two convertors with the exact same 3200 stall speed.
One is a 12", one is a 10".
The 10" will spool faster and have a crisper throttle respone due to being several pounds lighter.
Also... the further the weight is from the crankshaft centerline, the more difficult it is to accelerate.
Example... Two convertors weigh the same 15lbs. One is an 8" unit, the other is a 12" unit. The 8" unit will rev faster due to the weight being closer to the crank centerline.

 

[Alky V6]

Wouldn't you be able to launch harder with the heavier pulleys? As far as a track car goes. Would the extra weight be beneficial on the line while boosting up?

Back in my old 4 speed manual days, the rule of thumb was 10 lbs. of flywheel weight for every 1,000 lbs. of overall car weight. This provided enough inertia force to get the car launched out of the hole. Very effective for helping a small engine with a heavy car. Especially if you had the ghonads to redline the engine before side stepping the clutch. WFO! The draw back was that the flywheel weight also slowed the rate of rpm increase after the launch. With an automatic transmission that rule of thumb does not apply. Light weight is the ticket.

 

[turbo39151]

my apologies for the confusion. I was thinking rotating and writing reciprocating...:redface:

 

[yullose]

my apologies for the confusion. I was thinking rotating and writing reciprocating...:redface:

That's OK... RJC's site has it wrong too.