Maintenance & Technical > KX500 Original

Exhaust valve timing..?

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YUNGGUNNAZ:
let's say the harder spring will start the opening rate a little slower.would we then assume the motor will have a stronger bottom power curve yet still pull harder on top or slugguish thru the mid range transition.i guess the type of riding comes into play as well as the porting done to everything.i'm going to research this thing as far as i can.

John:
Hum,

I was assuming that a harder spring would change not only the timing (RPM) when the exhaust valves would open but also the time it takes for the valves to become fully open. Then I did the mistake of thinking again?

Once the engine reaches the critical RPM the balls are starting to move out from the centre of the axle. The further away they are from the centre of the axle the greater centrifugal force they will be subject to. This would mean that the opening of the valve would be almost binary (closed or opened). I don?t think it would ever be in a state of half open.

I don?t know if that?s good or bad or even interesting but it will simplify things?

A stronger spring moves the opening time further up the RMP?s and the opposite for a weaker spring. The opening times (time spent half open) would theoretically not change with different springs.

So the question remains; what kind of impact would this modification have on the behaviour of the engine characteristics?

Doordie, Mun, anyone?

//John

KXcam22:
I can't resist diving into this.  In my experience, raising the exhaust roof increases the "pipe-iness" of the engine.  In the KX, the powervalve also opens the sub ports, increasing the effective width.  Both increase hp.  I would expect that the stiffer spring would delaying the action of the power valve, and spread the opening over a wider rpm band.  This would make the powerband more mellow but then make the top end rush a little more drastic as the engine RPM will be higher when the valve actually opens fully.  I disagree on the binary opening.  When riding you would feel a drastic step-change in the power characteristics when the valve snapped open.  I think the increasing compression of the spring would balance the increasing centrifical force (not entirely unless it is a rising rate spring) and slow the action so that the full opening would take place over a few thousand RPM. This may be pure speculation but it's fun to analyse. Cam.

mun:
More speculation.

My story is easier with picture. There are old dynos of MuN old -86 KX with Slechten- and my own MuN-pipe. Power on my pipe came between 5500-6000 too fast. I even crash on tarmack track for this reason.

I ask multiple Finland Champ (iceracing, RR, dirt track, enduro,..) Ilkka Heinaaho: If i added washer to increase spring preload. He says: "Are you sure to add preload help you? My experience with KTM is different." How much i try to change washer? "1 mm is lot" he say.

I fix my problem with raising tranfers, so I don't tested with washer and have no experience with it.

John:
Cheers Mun,

When would you say that the exhaust valves are opening? Some drastic things happening around 5.5kRPM's (with your pipe that is...).

//John

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