I've seen this referred to in a number of articles and am only guessing as to what it is, why it's important, and roughly how it works.
Any gearheads wanna clue me?  Thanks.

well usually its on a Digital igniton and there is a circuit that sees how many times a spark plug is firing in a set amount of time (giving a rough rpm) and if it sees that it is above the set limit (depends on bike/setting/mfgr)  it will stop the spark plugs from firing, thus limiting how fast the engine can turn.  

The really cheap ones on GoKarts and such just limit how far open the throttle can open.

edit* and why its important is because there is a design limit for the engine.  Such as the size of crankshaft or the type of bearings that are used.  or the oil pressure could be so high it might blowout.  plenty of things can go wrong if the engine is only designed to run up to 11,500rpm and you try and take it up to 14,000.  not good.

hope this helps.

By the way, the GS500 has no limiter.

Sure it does,  its called valve float!  happens at about 12K  

Quote from: JamesGSure it does,  its called valve float!  happens at about 12K  

james canyou explain that term a little for me. I've heard it tossed around here a few times before

thanks in advance

jake

i can give you a rough explanation, think of it this way, the engine is moving so fast the valves dont have timne to close, they open again before having the opportunity to close, and the process is repeated. it sounds kinda weird (cam lobes move faster than the valve springs can close the valves

I heard that Ducatis have an extra cam lobe per valve, which shuts the valve. (Not sure how they worked that!).  That way, the valve will always close without needing to rely on spring quality.

what he said.  Basically the valves go spastic and the engine stops internal combustioning.  Its real obvious by the rapid metalic clatter, so it acts sort of like a rev limiter, but there is very real threat of causing damage to the motor that way ( and I have done so several times.   ).

On the street you usually just "find it" after a missed shift, or while trying to haul booty.

Yeah, that Ducati Desmo is a hell of a thing! With only 4 times the mechanical complexity and cost, clearances that must be adjusted to 0.020 mm, plus/minus nothing at insanely short intervals, they can rev almost as high as all the bikes with valve springs.

Unless of course the rocker arms flake hardened bits through your whole engine, which they've only been doing for 30 years. Then the thing comes to a grinding standstill. Oh yeah, that isn't covered under warranty, it's a feature.

In WSB last year, the Ducatis had a 500 km re-build schedule, the Honda SP2 that won was re-built every 5,000 km. Hmmm.

QuoteIn WSB last year, the Ducatis had a 500 km re-build schedule, the Honda SP2 that won was re-built every 5,000 km. Hmmm.

 wow!

I saw an SP2 (RC51 here) today, damn that thing sounded nice!

John

QuoteThe really cheap ones on GoKarts and such just limit how far open the throttle can open.

this item is called a govener, that prevents overspeed by closeing the throttle. this is done by weights on crank, or by air pressure on a flap connected to the throttle linkage.

not sure how hard it would be to make a rev limiter, figure it shouldnt be too difficult, in theory

Redline is well past our peak power so you probably want to shift before then anyway.

I always associated valve float with hydraulic lifters but I guess it happens when your springs aren't stiff enough either.  Max revs is also determined by piston/connecting rod weights and crankshaft composition.  Piston velocity can get too high and create more tension/compression forces than the con rods or crank can handle.  I know on small block V8's you'll need a crank thats better than your basic cast crank if you want to spin higher than 6500.  Maybe you'll go for a forged 4340 steel crank instead.  A shorter stroke engine will rev higher also because the piston velocities are lower.  You can stroke a 350 to a 383 but with the longer stroke it will put more stress on the engine spinning it as high as the 350.  

There are a lot of factors contributing to max revs but the main one is valve float and your oiling system.  To get max acceleration you want to shift at your peak power.  Once you get past the peak acceleration is not as good.  I guess if you just want to go that high hear it, then thats cool

Quote from: JamesGwhat he said.  Basically the valves go spastic and the engine stops internal combustioning.  Its real obvious by the rapid metalic clatter, so it acts sort of like a rev limiter, but there is very real threat of causing damage to the motor that way ( and I have done so several times.   ).

On the street you usually just "find it" after a missed shift, or while trying to haul booty.

What are the after effects of missing a shift or shifting above 12K occasionally? What gets damaged and what are some signs these things have happened?

lotsa little dimples in the piston roof? the nasty sound of intake and exhuast valves turning into swizzle sticks.  Beefer springs will allow higher revs without float, but no useable power up there.cant put toomuch duration into the camprofile as the valves will tangle, highlift is also a relitive term, as pistons love valves.  now, to design a new head where velve angles are so steep, this would be nice be able to drop in TL 1000 valves. perhaps getting 250 TSCC head to work on there

The moving mass of the valve assembly is an important factor in 'valve float'.  More inertia means less ability to change direction quickly which is what the valve has to do as it opens and closes.  

Valve assemblies in racing engines are engineered to be as light as possible, i.e. with minimum inertia, and often made of exotic alloys and with hollow stems, to try and beat 'valve float'.  This occurs when the valve motion can't keep up with the demands of the cams.  These are not robust enough for mass production consumer-style motors.

Actually, poppet valves (either spring driven or actively closed like Ducati desmodromic valves) are, basically, crap.  What you need is sleeve valves that run concentric to the piston inside the cylinder.  Search the web for the Napier Sabre, an aero piston engine with the best specific power (hp/lb) ever acheived in a production engine.

I actually think I followed most of that...thanks, everyone....net time I'll ask an easy question about, oh, octane or something...

Since I now little about engines, well more than nothing, but less than a mechanic:

why couldn't you get valve springs from a GSXR 750, R6 and raise your redline to 14-15K

Wouldn't that be an easy way to get more performance?

...err make that KNOW little about engines

Nope! It isn't only the valve springs that are different. GSXR's and such have 4 tiny little valves per cylinder, rather than two big heavy ones the GS has. The whole valve train is very carefully engineered for minimum weight, sometimes with exotic materials. And they have shim under bucket lifter designs for lower mass, and to avoid having the valve shims spit out.

The 600 and 750 size 4 cylinder bikes have much smaller valves and pistons. The pistons are all the latest "skirtless" lightweight designs. Less reciprocating mass equals less stress on the crankshaft, camshafts, etc. They also have water cooling which eliminates a a lot of engine hot spots.

There are about 1,000 reasons the GS isn't going to like reving any higher. In fact, it is pretty impressive that it will survive 11,000 rpm!

Big fat shims over the buckets that like to squirt out like bullets are a big one too...