Consequence of raising the rev limiter?

Thanks for the replies

Firstly, I am trying to work around my problem by changing the gearing and if my calculations are right, I think I'll be safe. This was just a last ditch quick fix on a race weekend.

As a side point though and out of interest, using the Race ECU raises the 675's limit by around 1000rpm. The only mechanical kit parts that MUST be fitted with this are new inlet/exhaust cams, cam sprocket and valve springs - all top end stuff. Does that not suggest that the standard bottom end, rods etc. are built to a spec to work with the increase limit as well?

That said, I won't be back at Taupo for a while, so it is a bit of a moot point.

This is a twin though, i would have thought a twin is a lot more sensitive to increase rev limits.

I chewed my well maintained engine on my SV - popped at 120mph on the motorway while pinned. It was a K3 also, which i've just found out has the slightly higher limit.

I wouldnt bother if i were you.

Sorry.

My bad, you talking about the 675....

[embee]

Could be OK, depends what it was designed for.

The thing with valve train is that it will work up to the design speed before loss of contact occurs, the limit is clearly defined. Valves break usually when it's been over-speed and there's been piston contact causing a slight bend in the underhead stem. This then flexes when it seats and eventually the repeated flexing causes fatigue and the head pops off.

The problem with bottom end is that the limiting factor is usually fatigue limit of rod-caps, bolts, or sometimes the rod itself. The critical areas are usually in the thinned section near the bolts. It can reach a point where clamping is lost at exhaust TDC and shells can rotate in the rod (many modern rods don't have bearing tangs). If the stresses are within the design fatigue limit the life is more or less infinite (except with light alloys, not common these days for rods). If run beyond the fatigue limit the life becomes finite, the greater the stress cycling the shorter the life. You can run beyond the design stress limit, but plan on replacing the rods fairly regularly. Even crack testing doesn't help much, once a crack starts the life is very short so unless you're rebuilding after every few races you probably won't catch it. If you know the material and stress you can predict the fatigue life, otherwise you're guessing.

Modern pistons rarely suffer fatigue fractures, being light alloy and running at high temps they have to be designed with a high safety factor anyway.

It will be fine with 500rpm. For 1000rpm then you would need to change to the appropriate retainers/springs etc for the valves.

Most modern sports bikes have a rev limit thats well under what the bike will take. If you go upto 1000rpm then you really do need to be changing valves/rods etc at a much reduced interval. Speak to someone like JT's who specialise in tuning the 675 and they will let you know from experience what is required.

I run +500rpm on my supersport GSXR600 but run the Yoshi retainers and cotters to help reduce the stress off the valve and valve rock.

Originally Posted by embee

Could be OK, depends what it was designed for.

The thing with valve train is that it will work up to the design speed before loss of contact occurs, the limit is clearly defined. Valves break usually when it's been over-speed and there's been piston contact causing a slight bend in the underhead stem. This then flexes when it seats and eventually the repeated flexing causes fatigue and the head pops off.

The problem with bottom end is that the limiting factor is usually fatigue limit of rod-caps, bolts, or sometimes the rod itself. The critical areas are usually in the thinned section near the bolts. It can reach a point where clamping is lost at exhaust TDC and shells can rotate in the rod (many modern rods don't have bearing tangs). If the stresses are within the design fatigue limit the life is more or less infinite (except with light alloys, not common these days for rods). If run beyond the fatigue limit the life becomes finite, the greater the stress cycling the shorter the life. You can run beyond the design stress limit, but plan on replacing the rods fairly regularly. Even crack testing doesn't help much, once a crack starts the life is very short so unless you're rebuilding after every few races you probably won't catch it. If you know the material and stress you can predict the fatigue life, otherwise you're guessing.

Modern pistons rarely suffer fatigue fractures, being light alloy and running at high temps they have to be designed with a high safety factor anyway.

Embee - I love your posts - very interesting and informative, thank you

Originally Posted by SV650Racer

It will be fine with 500rpm. For 1000rpm then you would need to change to the appropriate retainers/springs etc for the valves.

Most modern sports bikes have a rev limit thats well under what the bike will take. If you go upto 1000rpm then you really do need to be changing valves/rods etc at a much reduced interval. Speak to someone like JT's who specialise in tuning the 675 and they will let you know from experience what is required.

I run +500rpm on my supersport GSXR600 but run the Yoshi retainers and cotters to help reduce the stress off the valve and valve rock.

Thanks Sarah. I'll keep it as is for now, but nice to have that in my back pocket in the future. It certainly would've helped this weekend.

My bike is with the tuners at the moment, so I could pick his brain on the 675. I'm really keen to keep the engine stock for Stock Prod. racing next year. I can swap out my full system and change the mapping easy enough, but stripping the engine to revert back to stock...