Author Topic: Stock engine RPMs  (Read 1700 times)

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Offline RoMow

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Stock engine RPMs
« on: April 25, 2021, 10:56:14 am »
Heymowers, 

   In the Driveline section, there is a post entitled "Getting speed out of Drive Shaft Mowers?" and one of the questions was about tweaking the governor to get more than 3600 RPM from the engine.  One of the Replies cautioned about going beyond 3800 RPM.  Allow me to share an incident from a factory service school:

   In February of 1999, I attended the Kohler Engine Level 2 Factory School in Kohler, Wisconsin, and on Thursday afternoon of the 5-day school, the class was given two Command 25hp V-twin engines to "do with whatever you'd like."  Since in our shops, we only see an engine needing repairs after it's been damaged, we decided we'd run one wide open and the other without oil.  For the wide-open engine, the instructor wired the governor open, set the engine stand outside (just in case), and started it up.  The tachometer read 5700 RPM -- for 45 minutes!  It never missed a beat, and would have run faster but the valve springs began floating, so 5700 was its maximum RPM-- well above the "recommended" governed 3600-3800 RPM.  Even with the factory muffler, the sound was impressive!

   Engine RPM limits are dependent on keeping with the design engineering of a maximum metal stress in a stock engine of 2,500 feet-per-minute (FPM) of travel, with allowable momentary speeds of 3,500 FPM.  An engine with a 3" stroke will travel 6" per revolution.  This engine turning 5000 RPM is at the 2,500 FPM industry-acceptable limit.  And with the precision manufacturing and materials of today's engines, these stresses are even more controlled.  An engine with a  2.75" (2-3/4") stroke can run over 5400 RPM to reach the 2500 FPM limit.  The stroke on the CH25 Kohler engines we maxed out is 2.64" which, at the 2500 FPM stress limit, is 5,681 RPM. 

   This applies not just to Kohler engines, but to all engines -- Tecumseh, Briggs & Stratton, Kawasaki, Honda, etc.

   Hope this helps. 

Respectfully, 
Rodney Rom 
Rom's Reworks 
Butler, MO
   
"No matter how many material possessions or awards you may acquire in your lifetime, the only thing you will ever truly own is your reputation." (Age 73)

Offline cycloneracer

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Re: Stock engine RPMs
« Reply #1 on: April 25, 2021, 01:08:10 pm »
If racers want to keep their insurance they won?t spin engines faster than the recommended 3650 with a stock flywheel. 


This has been a long standing stance by heymow as well.

We don?t discuss or recommend dangerous practices.  This includes modifying a flywheel or running a stock flywheel faster than 3650.

Paul Krueger

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Offline RoMow

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Re: Stock engine RPMs
« Reply #2 on: April 25, 2021, 08:49:54 pm »
Paul, 

   So, any engine run over 3650 RPM requires a billet flywheel?  What if the manufacturer's allowable maximum RPM is 3800-4000?  Some are --depends on the engine's Type or Spec Number. 

   I didn't post this to create issues -- my apologies if it did -- I was just noting that the engines can do this without self-destructing internally. 

Rodney
"No matter how many material possessions or awards you may acquire in your lifetime, the only thing you will ever truly own is your reputation." (Age 73)

Offline Knoot

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Re: Stock engine RPMs
« Reply #3 on: April 25, 2021, 09:02:29 pm »
I think you saw my post, kudos for being able to give the guy better information than me, I don't know a bunch about those hydrostatic setups.

Thing is, you might be able to spin a honda engine that fast, but if you spin a B&S OHV at 5000 rpm, well lets say you're going to become very familiar with the internals very quickly. I've seen people blow up stock B&S engines in otherwise great shape even at 4000rpm. See

I wouldn't tell people to rev their briggs over 5000 rpm  :worried: at the very least they're going to have to buy a new engine, at the worst someone could get serious injuries when the rod decides to take a trip through the block and into their face.



Offline cycloneracer

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Re: Stock engine RPMs
« Reply #4 on: April 25, 2021, 09:07:33 pm »
According to the insurance companies 3650 is Max for stock flywheels.  Heymow agrees. 

I don?t make the rules

If I did they would look much different!!! And I?d probably still be racing.....
Paul Krueger

****2017 USLMRA FXT points Champion****

#150 Worlds First "sidewinder" FX (now dismantled)

#150 FXT  Sponsored by Zach Kerber Machine. http://zkbrmachine.com/

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Offline RoMow

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Re: Stock engine RPMs
« Reply #5 on: May 05, 2021, 01:12:45 pm »
   Part of Kohler's engine test lab includes spin-testing flywheels.  Each flywheel is properly installed to a clean, dry, motor-driven-and-monitored shaft, then hand-torqued just like the manual instructs.  The flywheel is mounted into what resembles a large safe with very thick walls and a big door.  RPM is then run up to the point of burst.  They need to survive at least 2X the normal rated speed of 3,600 RPM.   

   After bursting, everything that still stays on the shaft is out of balance and the lab floor will shake as the speed comes back down. When the door is opened, there will be a pile of broken parts with magnet bits stuck all over the inside of the chamber.  All flywheels pass the 2X3600 mark -- 7,200 RPM -- and many of them hit 10,000 RPM. 

   Anytime a new flywheel is developed, whether for a new engine or as a new flywheel design for a current engine, it gets qualified/tested. And anytime a modification is made to an existing flywheel, it gets qualified/tested. 

   Many Kohler engines are set to run 3750 no-load speed.

   Keep in mind that lubricating the tapers or using an impact wrench to torque a flywheel are always bad ideas.  Broken flywheels result from lubricating the tapered shaft or over-torquing the flywheel bolt/nut. But properly hand-torquing a stock Kohler flywheel on a clean, dry taper will safely allow in excess of 5,000-6,000 RPM.   

   As noted above, this is Kohler's flywheel-test information.  I'm still trying to get information on Tecumseh, Briggs, Kawasaki and Honda flywheels. 

   Now, having said that, and having watched Knoot's video, I need to say that the Kohler we ran to 5,700 RPM did not have any counterbalance mechanism inside, just a well-balanced crank, rods, pistons and cam.  The Tecumseh V-twins being raced don't have counterbalancing mechanisms, either, but I don't know what RPMs they're producing.  What I saw on Knoot's Briggs video looked like the oscillating counterbalance weight self-destructing, and the oil that came out of the crankcase looked quite dirty and black, which would tell me that the internals had a lot of wear from the dirty oil.  This may have helped contribute to the self-destruct.  What was the RPM at the point of disintegration? 

   And Paul keeps mentioning "insurance companies and a 3,650 RPM maximum."  Which insurance companies?  Would a stock Kohler running it's governed 3,750 RPM be uninsurable?  Maybe the insurance companies are using outdated information from when the flywheels included the cast-in cooling fins.  The new unfinned flywheels with separate plastic fans can safely spin much faster, as Kohler's tests -- and my own experience -- prove. 

Respectfully, 
Rodney 

"No matter how many material possessions or awards you may acquire in your lifetime, the only thing you will ever truly own is your reputation." (Age 73)


Offline cycloneracer

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Re: Stock engine RPMs
« Reply #6 on: May 05, 2021, 03:15:42 pm »
Insurance companies that insure racing and racers. 

I don?t know how they came up with that number, nor do I care.   That is the standard.  It isn?t going to change. 

Paul Krueger

****2017 USLMRA FXT points Champion****

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#150 FXT  Sponsored by Zach Kerber Machine. http://zkbrmachine.com/

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Offline Tom Cole

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Re: Stock engine RPMs
« Reply #7 on: August 21, 2021, 09:36:47 am »
I can't speak for individual organizations or insurance companies, but if you alter the engine in a way that removes the governor's stock max RPM limiting function, the safest plan for both yourself and other racers around you is to switch to a flywheel that is designed for racing.
I have been on the track when a competitor's stock flywheel came apart right in front of me.  His chain broke and the engine immediately accelerated to destruction.  This was a Yamaha "F200" engine where the base engine was either made by Yamaha for Kohler, or for Kohler by Yamaha.  Our "racing" max RPM target was about 6k rpm but that was exceeded quickly when the chain broke.  That is when billet became the rule in that class.  Almost every single time a rule has been made to mandate billet flywheels when a stock flywheel used to be allowed, it was because of an on track/dyno/rev-stand failure that far too often resulted in injury. 
Heymow does not advocate the use of a stock flywheel in an instance where the governor on the engine has been removed.  If the governor is removed, an SFI certified billet flywheel should be used.  I'm not going to entertain an argument or "what if" scenarios about this.  I don't want to hear "So and so organization allows..." because I don't make or influence someone else's rules.  Seriously, don't test me.  I don't have time for it.
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