Anyone know wrist pin weights? Wiseco,spi,stock

j2hizzo

j2hizzo

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Probably going with an spi top end kit and want to know if anyone knows the weights? I know wiseco is lightweight compared to stock. How does api measure up?
 

wouldn't lightening the piston and rod assembly throw out the counter weight on the crank?
 
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No, if you lighten the assembly you'll rev faster building power quicker but if you use a heavier assembly you'll have more torque. How much effect there will be in just a few grams difference I doubt the average guy would notice, draggers will notice though. In the dirt world adding 5-13oz to the flywheel is common and back in the day the mx guys would put WR 450 cranks into their yfz 450 wheelers to make them rev quicker.
 
Snowman - Much of what your saying is correct. But i dont think the engine makes more or less torque with the weight of the crank. Using a lighter weight Viper crank will rev up quicker than a heavier SRX crank. We used to do the reverse trick to our enduro MX bikes. We would send out the flywheel and have a weight ring welded to flywheel to make it heavier to prevent wheel spin slowing down the rev.
 
Mac said:
Snowman - Much of what your saying is correct. But i dont think the engine makes more or less torque with the weight of the crank. Using a lighter weight Viper crank will rev up quicker than a heavier SRX crank. We used to do the reverse trick to our enduro MX bikes. We would send out the flywheel and have a weight ring welded to flywheel to make it heavier to prevent wheel spin slowing down the rev.
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According to the dyno results for Steahly flywheel weights his and independent study's shown an increase in hp and torque with the flywheel weight. I don't have a dyno but testing with my yz250 and wifes rm125 they are a lot harder to stall out and can lug through terrain better.

Sorry for stealing the post a bit. 1 month earlier I would've weighed my spi stuff for you before I installed them.
 
I totally agree the heavier weight of the crank /flywheel rod piston assembly lugs the engine. That is stored energy.

Torque /hp relate to the efficiency of the combustion event. The efficiency of that event boosts or detracts from the engine's torque/HP. Port timing, optimizing intake exhaust flow, increasing compression ratio, providing optimum shaped combustion chamber, and optimum ignition timing, etc. All these things affect combustion efficiency and in turn torque. A heavy or light flywheel does not appreciably affect the combustion event and therefore can not affect the torque/HP the engine produces.
---mac---
 
Very clarifying! My use of increased torque was a bit loose should have been more directed towards the torque curve.
Again OP sorry for invading your thread! If you haven't found out let me know. I will try and get a hold of a guy that stocks them to see if he will weigh it forward me.
 
Another note a bit off topic. Wristpin weight doesn't effect the engine the same as flywheel weight. Pistons and Wristpins etc are linear mass not true rotational mass like the crank and flywheel
 
staggs65 said:
Another note a bit off topic. Wristpin weight doesn't effect the engine the same as flywheel weight. Pistons and Wristpins etc are linear mass not true rotational mass like the crank and flywheel
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i was thinking the same, rotational inertia is different than linear.

wouldn't lightening the piston and rod assembly affect the balance between it and the counter weight?
 
On multi cylinder engines I don't believe it will affect it like it does in a dirt bike. In my friends 290f big bore it picked a large vibration from the bigger slug.
 
I have some spx parts here I will weigh them and post back

Sent from my E6560C using Tapatalk
 
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