Safe rpm?

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Sep 26, 2018
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156
So I'm looking to revise my setup and changed the header length for my tuned pipe. The engine is balanced and I've revved it to 8500rpm without much problem. Its a GT-5 so piston speeds are lower but how far can I push the rpm limit on the stock piston and rings before the rings flutter and such? Its a windowed piston so its a bit lighter but whats the reliable limit?
 


mark20

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Sep 26, 2018
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So I'm looking to revise my setup and changed the header length for my tuned pipe. The engine is balanced and I've revved it to 8500rpm without much problem. Its a GT-5 so piston speeds are lower but how far can I push the rpm limit on the stock piston and rings before the rings flutter and such? Its a windowed piston so its a bit lighter but whats the reliable limit?
well you can rev it to the moon but you wont get much power at those rpms. 10k is about the limit. ive reved mine to 11k before (pulled in the clutch and let it rev!)
 
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well you can rev it to the moon but you wont get much power at those rpms. 10k is about the limit. ive reved mine to 11k before (pulled in the clutch and let it rev!)
With the pipe, it should scream to 12k rpm if I cut it back down to stock length (yz85 pipe). Porting will be modified according to the pipe spec. Rn, the pipe cuts at 7000 rpm. I know I can make more power up high due to shorter stroke but how high can I go before I blow rings or start getting issues? I'm reed inducted so no problems there. 21mm carb should handle 10k+ rpm
 

Frogslayer

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Jan 20, 2019
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Unfortunately, I don't think your question can be answered until after you've blown it up a few times. Must be balanced really well to turn those kinds of Rs. How fast does go? Just curious.
 
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Unfortunately, I don't think your question can be answered until after you've blown it up a few times. Must be balanced really well to turn those kinds of Rs. How fast does go? Just curious.
I had the bottom end built by Fred and I built the top end. Basic cleanup, squish tuning, and windowed piston. I ran it on a 700cc at 7500rpm approx and would hit 40mph. After that, the pipe had a hard limiter and I've gotten it to 50mph on a slight decline (about 8500rpm iirc). The frame snapped and I got a new bike (steel this time). I blew my old engine (stock with Reed, 21mm carb, and pipe) and blew it at 600 miles at 8000 rpm after a downhill. Piston fell apart and blew the cheap bearing to bits.
 

Street Ryderz

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Oct 14, 2016
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3,007
With the pipe, it should scream to 12k rpm if I cut it back down to stock length (yz85 pipe). Porting will be modified according to the pipe spec. Rn, the pipe cuts at 7000 rpm. I know I can make more power up high due to shorter stroke but how high can I go before I blow rings or start getting issues? I'm reed inducted so no problems there. 21mm carb should handle 10k+ rpm
If the pipe is currently dropping off at 7k it's way too short!Now a yz 85 is a square engine with a power valve and port design that we really can't copy,it also makes 29 hp @ 12k and a big part of that is due to electronic variable timming witch we also don't have,So shortening the pipe will not garuntee that it hit's 12k rpm and after 11k is where our engines start to rip themselfs apart.I try to make all my power below 10.6k to keep it working the longest possible.
 
Joined
Sep 26, 2018
Messages
156
If the pipe is currently dropping off at 7k it's way too short!Now a yz 85 is a square engine with a power valve and port design that we really can't copy,it also makes 29 hp @ 12k and a big part of that is due to electronic variable timming witch we also don't have,So shortening the pipe will not garuntee that it hit's 12k rpm and after 11k is where our engines start to rip themselfs apart.I try to make all my power below 10.6k to keep it working the longest possible.
Definately, 7k is pretty underwhelming. I made the header too long. Changing that once I get my hands on an angle grinder.

I'm not expecting the pipe to give me the same power output as the yz. My logic may be flawed but if i kept the tuned length the same between bikes, it should stay within the same effective range. But then there is exhaust port duration... :unsure:

Should I shorten it down all the way to stock (no added length)?

Also, what typically tears apart above 11k rpm? Wrist pin bearing? Crank bearing? Just curious.
 

Street Ryderz

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Joined
Oct 14, 2016
Messages
3,007
Definately, 7k is pretty underwhelming. I made the header too long. Changing that once I get my hands on an angle grinder.

I'm not expecting the pipe to give me the same power output as the yz. My logic may be flawed but if i kept the tuned length the same between bikes, it should stay within the same effective range. But then there is exhaust port duration... :unsure:

Should I shorten it down all the way to stock (no added length)?

Also, what typically tears apart above 11k rpm? Wrist pin bearing? Crank bearing? Just curious.
The stock length of the yz pipe is going to be closer to where you'd want to be for sure but check the length from piston face to 3/4 into the baffle cone.It should be around 30" though it may well be less so I would try it at 30" and then see where it hits and falls off.At 11k or more timming is a big issue and start's causing the rods little end to walk and put's pressure on the wrist pin clips and can eject them or break the ends off,also that's where the rings start to flutter and cause excesive wear in the blow down area taking chrome away,the big end also takes a beating from the timming being off and that just promotes even more rod walk at the top until it just let's go.
 
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