Piston speed=broken parts

Kennedy_Power

New Member
Apr 8, 2008
642
20
0
Rochester,Michigan
This question came from another member, piston speed; please explain.
There are several units we can do the math on. Let’s use feet/minute. The rule of thumb is 3000 to 3500 feet/minute. You can go beyond that point; but the pistons will have to be changed several times. NHRA Pro Stock engines now spin over 11,000rpm. Back in 1995 they turned 8,800RPM. Big changes have happened and technology as evolved. However for us we do not have pockets full of money. A single cylinder two cycle engine like the blaster has a stock stroke of 57mm. If we spin the engine at 8500RPM that would be 3179feet/minute; safe and we have a durable engine. If we install a +3mm crank or 60mm
The piston speed would be 3364feet/minute. Most of the blaster crowd does not drag race; they just want good power for the trails. I am not by any means saying that power pro pipe is no good. They do spin fast; just be aware to check your piston for cracks. I run my own engines close to 4500feet/minute and they do need to be replaced often. When you drag race there is a price to pay in parts replacement. In the trails you are off and on the throttle, so the piston speed is averaged over the time and is under the safe limit.
I hope this helps
Feel free to PM anytime.
 
i heard on another site wit motorcycles and wat not that its good to increase your bore when you increase your stroke is this true?
 
probably be a good idea, but ultimately depends on where you want to make power and how fast you want to turn your motor. increasing stroke creates a cylinder that is more "under square" than it was before which means that the stroke is longer than the cylinder is wide. a motor that is under square will make better power (read: torque) down low, but won't be able to turn as fast. a motor that is over square (bigger cylinder than stroke) is better for high-reving and top end power. for example f1 cars are pretty oversquare and turn much faster than 10,000 rpm. contrast this with the motor in my brothers jeep (4.2l strait 6 bore x stroke >3.75 x 3.89-inch) which makes peak torque at 1800 rpm's and redlines at like 4,500 rpm's....
 
usually when you stroke your bike, it causes the engine not to rev higher, as there is more weight to turn.
another thing that is bad for the engine is constant quick rpm change.
 
Hopefully it's pretty well understood that any serious engine modification like stroking or dramatically increasing the bore (not just boring out to the next size) while increasing the amount of power will decrease the engine's service life and reliability. How much depends on how much you mod it, but for most the decreased reliability is worth the extra power!!

That being said, even most casual riders with relatively stock bikes (ie not stroked or big bored) do a top end at least yearly. If you race, maybe weekly!! It's all fun and games though, right?
 
weekly? are you f*ckin sh*ttin me? i know a top end can last a lot longer than a year. i am going on over 2 years on my top end. my brothers shee has only one top end rebuild in the 7 years he has owned it. and dont say we dont ride often, we ride about 3 times a week through the year.
 
weekly? are you f*ckin sh*ttin me? i know a top end can last a lot longer than a year. i am going on over 2 years on my top end. my brothers shee has only one top end rebuild in the 7 years he has owned it. and dont say we dont ride often, we ride about 3 times a week through the year.

Racers man. Weekly is kinda extreme, but I know guys who race and they do at least just the rings pretty damn frequently. Yes, the beat the ever living turd out of the bikes....that's why you never buy a bike that's been raced!! Professional racers, they're not going into a race with an old top end...they want it fresh every time! Same with nascar, they tear those engines down every race.


If you're still rocking a stock top end from 88, I would rebuild!! It may have good compression and run fine, but if you rebuild it, you'll love the increased power!!