My question is this - I spin 8,000rpms going down the freeway at about 65 indicated, 60ish true on my KLX250s. The red line is 10-5, so I am thinking that a sustained 8,000 rpms might be ok, but is it really???

Consider this:

At any rpm the piston is going from top dead center, down to the bottom of the stroke and back up, allowing the crank to complete one turn. In that one revolution, the piston is completing two strokes. One down, and one up. So lets take the 8,000 rpms and multiply it by two strokes per revolution resulting in 16,000 up/down strokes per minute.

At this point we know that @ 8,000 rpm our little piston is completing one up or down stroke 16,000 times every minute. Now lets convert into hours by multiplying by 60. 16,000 strokes X 60 minutes = 960,000 strokes per hour. Wow!! That's freeking crazy to me, but it makes sense.

Now, if we consider that our little KLX engines have about a 3 inch stroke, (this is just an estimation based upon what I noticed when doing a head gasket swap one afternoon. It may be a little more or less) Lets multiply our 960,000 strokes per hour by 3 inches to get the number of inches travelled by the piston per hour. 960,000 strokes ph X 3 inches = 2,880,000 inches travelled per hour. So for every hour that the engine is run at 8000 rpm, the piston is travelling 2,880,000 linear inches. Damn Scotty, that's a lot of inches!! But we still need more power...

Next, if we want to know how many miles per hour our poor little piston is traveling up and down in the cylinder, we'll do the simple math: the piston is traveling at 2,880,000 inches per hour / 12 (inches in a foot) = 240,000 feet per hour / 5,280 (feet in a mile) = a little more than 45 mph.

So at 8000 rpm, our piston that has a travel of 3 inches on each stroke, is traveling at 45 miles per hour between going from the top of the stroke to the bottom of the stroke and back. And it's doing that 16,000 times a minute.

This tells us that at 8000 rpm resulting in 60 mph, the piston has a total travel of 3/4 of a mile for every mile that we are moving forward on the bike.
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With all this in mind, is sustained 8000rpm travel down the freeway going to damage the engine over time?

 

I remember hearing a rule of thumb that the rpm's for sustained cruising shouldn't be more than 2/3 of redline, and only to go over for passing or something. That would be just shy of 7K on the KLX I think.

I can get to most local riding areas on county roads but to get up to the Crystal Basin area I have to do a 15 minute burn on highway 50 pretty much flat out, surrounded by SUV's and trucks towing boats and motorhomes up to Tahoe at breakneck speeds, probably the least enjoyable road segment I do with any regularity.

 

What gets me most about running the bike @8000 is the sound of that damn cam chain. I have recently put a click in the adjuster, but you can hear it click away up around 8000. Otherwise, the bike ran clean and strong like it wanted to scream down the interstate all day long.
The wind does get to me though.

 

Somebody had a little extra time today
2,880,000 linear inches an hour, that's an average of 45 miles an hour including a complete stop and start every 3 inches.
Dan

 

Uhhhhhhhh I was told there would be no math when I joined this forum.

 

I've gone an hour at 7-8,000 with my big bore (niaskil? plated) on some hot days this summer. It felt like I was killing it but other than the fenders getting a bit hot it was fine. Worst part was that I was deaf from the wind blowing in my ears.

 

SF is geared taller, it is about 62mph @ 6500 Rpm. It pulls fine. If I get much slower and I use 5th. I don't think we can hurt it as long as it has clean oil and stays cool. Go watch a moto X Race. I rode sunday about 100 miles all in the "torque curve" . I had a blast! Hit the rev limiter a few times. I was following a KTM 950 adventure. Its not a rocket but if I drive it like a 2 stroke its much better. We rarely got over 55 mph in the Adirondacks. 3rd or 4th gear.

I add a manual click to the automatic adjuster just by removing the spring and pushing the adjuster from behind. Below 5,500 I can not hear the timing chain at all. If that changes I will look closer.

David
Thinking 351....

 

I sort of have used 7k as a "hold it there for a long time" number. Only because many bikes gear highway cruise close to the torque peak. Best roll on is usually between the torque peak and the hp peak. My bikes dyno runs show the torque peak at ~ 7k. Not that there's any other logic to it. I subscribe to the "if it can't take it, it deserves to die" school of thought. So far it's took it. There's been many rides where I held it wide open for many miles in a hurry to get home.

 

I used to have a Honda NX250 that was geared down alot, I'd run that thing 9-10 grand for an hour or two at a time on the road and I was willing to kill it but it never died or really even complained all that much. I hope this bike does it also, with-in reason as it's not geared down as much.

 

In my opinion that is the joy of the bike. My other bikes are much bigger. This one I can beat the snot out of and not worry. Its made to take it and vibration is almost none. Water cooling keeps things fitting the same no matter how hard you run it.

I do not think it hurts it a bit.

Forget piston speed, I just beat on it.

The rev limiter is a wonderful thing. On my BMW 750 its the intake valve. (bent it once)

David