Neighbour pegged his mower at WOT

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Originally Posted By: meep

Newer OHV--- I suspect would rev higher--- though I don't mean that would be any safer.


Possibly the opposite. They might be able to rev themselves to failure, a flathead pretty much won't. The crank/rod/pin/piston can handle more RPM than the flahead can typically achieve.

I still prefer flathead mower engines, honestly. Just no need for the extra complexity, apart from emissions laws.
 
^^^exactly^^^. less valvetrain mass to float, etc. in a OHC, OHV a bit more efficient than flathead--- I think they'd rev and let something loose!
 
Originally Posted By: steve20
I believe top governed speed is 3400.
B&S flatheads max hp is usually measured at 3600
IMO they run about 4500 max--and not for very long (25-30 hours)


That's what I was thinking. Comparing the engine frequency to the governed speed (assuming 3600RPM), this guy was revving that thing at over double that! I mean, it's only a one cylinder, but sounded like a 4 cylinder at 4500rpm
crazy2.gif


At least on a flat head the piston cant smack the valves eh?
 
Originally Posted By: itguy08
I'm surprised nobody has hooked up a tiny tach to one and let her rip!

Hmmm...... May have to order one.


I did once on the old Tecumseh 12HP we had in our Massey Ferguson (it was a side-shaft motor). It sounded like it was spinning a lot faster than it actually was. I don't recall being able to get it over 3,800. Though it sounded like it was doing 6K.
 
Hmmmm I have one of those tachs for my riding mower. It would be kind of a pain to pull it off and put it back on again but I could do it. I'll think about it and see what my flathead briggs runs at true WOT
 
Originally Posted By: meep
^^^exactly^^^. less valvetrain mass to float, etc. in a OHC, OHV a bit more efficient than flathead--- I think they'd rev and let something loose!



Flatheads actually have even less valve train mass (think about it- no rocker, no pushrod, just the lifter acting straight on the valve).

But they don't BREATHE fast. The flow in and out of the cylinder is contorted and includes a complete U-turn from the valves into the cylinder proper. Its the air flow that suffocates a flathead at high RPM, the valves will snap away happily at WAY higher revs than the engine could possible turn
 
Originally Posted By: 440Magnum
Originally Posted By: meep
^^^exactly^^^. less valvetrain mass to float, etc. in a OHC, OHV a bit more efficient than flathead--- I think they'd rev and let something loose!



Flatheads actually have even less valve train mass (think about it- no rocker, no pushrod, just the lifter acting straight on the valve).

But they don't BREATHE fast. The flow in and out of the cylinder is contorted and includes a complete U-turn from the valves into the cylinder proper. Its the air flow that suffocates a flathead at high RPM, the valves will snap away happily at WAY higher revs than the engine could possible turn


But the springs have all the strength of the average clothes pin...

If you want to see one rev itself to pieces, run it without a blade, revs real quick... Of course you have to know how to start one without the mass of the blade helping to spin it over...
 
I'd be way more worried about the flywheel grenading. I witnesseed a Limited Modified Briggs grenade a flywheel and it was not a pretty sight. It was turning about 8,800 when it left loose.
 
From my experience, most small engines can go considerably above operating RPM (generally 3650), for a while with no harm. An exception to this would be the Tecumsehs with plunger style lubrication on a thick oil. Those pumps cannot handle the RPM and pump pressure which then blows out the side of the plunger bore causing the engine to starve for oil. At this point, the rod breaks in half and the crank part of the rod tears through the block.
 
Don't even need to overrev a Tecumseh to get it to break the crank. I had one that did so.

Looked like the rod began to weld itself to the crank. Eventually the crank broke.

Just junked the whole lawn mower at that point.

Now, with the HF Chonda's, I would have re-powered it.
 
Originally Posted By: TFB1
Originally Posted By: 440Magnum
Originally Posted By: meep
^^^exactly^^^. less valvetrain mass to float, etc. in a OHC, OHV a bit more efficient than flathead--- I think they'd rev and let something loose!



Flatheads actually have even less valve train mass (think about it- no rocker, no pushrod, just the lifter acting straight on the valve).

But they don't BREATHE fast. The flow in and out of the cylinder is contorted and includes a complete U-turn from the valves into the cylinder proper. Its the air flow that suffocates a flathead at high RPM, the valves will snap away happily at WAY higher revs than the engine could possible turn


But the springs have all the strength of the average clothes pin...

If you want to see one rev itself to pieces, run it without a blade, revs real quick... Of course you have to know how to start one without the mass of the blade helping to spin it over...


@Magnum and TFB1 -- good points on both counts!
 
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