RPM's on different Engines

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Old 04-14-04, 07:18 PM
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RPM's on different Engines

Cheese. I read somewhere that every 4-cycle engine max's out at 3600 RPM. IS this true? I am thinking that is impossible because of different engine sises. Woulden't you think that a 25HP engine would have a different max RPM than a 3HP engine???
 
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Old 04-14-04, 08:24 PM
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I have a four cycle engine in my pickup truck that can turn more than 3600 rpm. A lot of the B & S racing engines turn much faster as well. Max RPM's depend on engine design & construction. Smaller engines in general can turn faster than larger engines. There is less metal mass in smaller engines so the stress is much less on engine parts when the piston changes direction at the top & bottom of it's stroke
 
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Old 04-14-04, 10:17 PM
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RPM does not increase when engines get into larger horsepower necessarily. The HP of an engine does not dictate the rpm. A 25 hp engine could be made to run at 2000 rpm, or 7000 rpm. It depends on the engine and it's intended use and construction. Think about a large diesel. If you revved it to 5000 rpm, it would come apart, yet it may have 450 hp. Then look at a v8 chevy engine. It could go to 5000 rpm easily, but most don't create much more than 275hp. RPM and horsepower have no relation to each other, until torque is introduced to the equation.
 
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Old 04-14-04, 10:21 PM
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Thanks guys for helping me set that straight. I thought that magizne was full of it. I just could not see how that would be, but now I understand. Torque plus RPM's will give you a good approximite to what you HP is. But HP is not what we are talking about here. Just RPM's

Thanks guys...
 
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Old 04-14-04, 10:57 PM
gmcfan
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All the older B&S engines I have seen have a lable that states the maximum HP is achieved at 3600 RPM. Maybe that was what the magazine meant? I have seen B&S engines used on karts that turn 9000 RPM, with a lot of modding, of course.
 
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Old 09-15-04, 11:11 AM
mxz600
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How can you tell if it is above this rpm. I suspect my Ariens 5hp Snowblower is over reving. I have a diagnostic tach but I believe it was meant for 4,6, and 8 cylinder cars. Not small engines. Is it easy to over rev. I rebuilt the carb this summer but find that to get the high speed screw running smoothly it seems to rev high? Is this possible or am I making things up?
 
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Old 09-15-04, 03:14 PM
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mxz600
What you want to use for small engine purposes is a vibra tach. It can be gotten from a small motor repair shop or order it through Briggs or Tecumseh.
They are a must for working on small motors.

snoman
 
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Old 09-15-04, 06:13 PM
mxz600
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Ok, I may have to look into that but what would cause it to over rev? I'm not understanding this because shouldn't it only be able to rev so high based on the air and fuel that the carb lets in? Same carb so why would it rev higher and how would I stop it from reving to high? continue adjusting the high speed screw? thanks for telling me about the vibra tach. I'm off to find one.
 
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Old 09-16-04, 03:40 PM
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mxz600
Your RPM is controled by the governor and spring. The governor spring is specific to your engine, if the spring has been shortened it will cause your motor to overrev. There is also a setting of the governor to make sure that it works properly.
A governor works this way:
When the motor speeds up to the governed speed, the weights on the governor (going through the governor arm) pull back on the throttle linkage.(it overrides the spring at this time) Now when the rpm of the motor decreases because of this, then the tension on the governor spring overrides the governor and pulls the throttle linkage to allow the motor to get back up to proper speed again. All this happens very quickly and constantly while the engine is running. You can tell when the governor really kicks in when you are cutting grass and get into some heavy stuff where it will start to bog the engine down and then it comes back up to proper rpm again.
What I just described is a weight style governor , there are also air vane governors, they run off the wind produced by the flywheel but do exactly the same function.

snoman
 
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