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Hello all,

What's the maximum safe RPM for my engine? It's a 250, tuned to Mach 1 specification: 10:1 piston, big valves, shim valve adjustment. I'm not certain of the cam, but evidence suggests it's the 'red' profile. As painted on the Veglia tach, redline is 8500 - but is that correct in this case?

Thanks,

Dave

" What's the maximum safe RPM for my engine? "

____ Well it seems that no-one cares to suppose what top/max.RPM is "safe" for 'YOUR' particular engine.
__ I guess it depends on what you consider to be 'safe'. _ Cuz ya can rev-out the 250-engine to 9-grand in 1st.gear and then possibly miss-shift (for 2nd.gear) and have the engine then spin-up out of control to who knows what RPM that's way-past any imaginable red-line,, yet still, the engine recovers (back to a normal-RPM) and continues to motor-on (without having 'blown-up'). _ So, as it seems, there's no top-RPM that's unsafe.
However, 'red-line' is an informed guess of what the (relatively new) engine-parts can handle without undue/excessive wear/stress,, and as those parts become 'aged', the red-line ought to be adjusted downward somewhat, leaving you-yourself to be the judge of what red-line ought to be on YOUR-own motor.


" It's a 250, tuned to Mach 1 specification: 10:1 piston, big valves, "

____ I believe that the (stock) 250-piston which was designed to be matched with the large-valves, provides a slightly less than "10:1" CR., (possibly just 9.8 or 9.5).
Have you done any compression-check ?


" I'm not certain of the cam, but evidence suggests it's the 'red' profile. "

____ What exactly is your supposed "evidence" ?
If indeed a Red-cam, then it's really not correct to have claimed that your 250 has been "tuned to Mach 1 specification",, and 'Mark 3' (rather than 'Mach I') may be a more appropriate claim.
Can you provide any picture of your camshaft, so that I may then possibly confirm it's model ?
__ The combination of a Red-cam with the large-valves is a particular combo that the factory itself never provided (on std.production models), and I-myself (back in 1974) only once ever installed a Red-cam into a large-valve cyl.head (which belonged to a customer).
I don't really recall if the cyl.head was next installed on a 250 or a 350 motor (as the customer had both engine-sizes), but the owner of the w-c.Duke-project was fairly pleased with the outcome of his engine's improved performance.
__ So I assume that YOUR 250-motor is a n-c.model ? _ Whatever, how do you like it with the Red-cam ?
And what kind of exhaust-system/muffler do you run with it ?


" As painted on the Veglia tach, redline is 8500 - but is that correct in this case? "

____ Not being positively sure of exactly what "in this case" may actually be, nobody could thus-then be certainly-sure. _ But it's still likely as good as any red-line to go by.


Duke-Cheers,
-Bob

I find this a very interesting question for those of us that have singles that we like to ride hard. I have a 68 250 desmo that according to specs is good for north of 9000 rpm. I think DCT Bob has summed it all up:

__ I guess it depends on what you consider to be 'safe'. _ Cuz ya can rev-out the 250-engine to 9-grand in 1st.gear and then possibly miss-shift (for 2nd.gear) and have the engine then spin-up out of control to who knows what RPM that's way-past any imaginable red-line,, yet still, the engine recovers (back to a normal-RPM) and continues to motor-on (without having 'blown-up'). _ So, as it seems, there's no top-RPM that's unsafe.
However, 'red-line' is an informed guess of what the (relatively new) engine-parts can handle without undue/excessive wear/stress,, and as those parts become 'aged', the red-line ought to be adjusted downward somewhat, leaving you-yourself to be the judge of what red-line ought to be on YOUR-own motor.

And I feel the same. I don't feel good about winding my 250 M3D up to the top because it is basically "old metal" and I don't want to trash it. I stongly suspect it would handle it fine, but I don't want to risk it. I sometimes shudder when I miss a gear and then engine revs well past what it should - but it hasn't been a problem so far. I don't think it's unique about protecting old bike revs. Ago and Paul Smart don't want to do that with their old race bikes.

My feeling is that you don't need to redline them to enjoy them unless you're racing.

Funnily enough, for some reason, I don't feel the same with my roundcase twins. It makes no sense, I know...

The Sport getting some excerise north of 7000 rpm a few weeks ago...



Kev