Exhaust valve timing..?

[John]

Has anyone investigated (verified) the opening times for the exhaust valve in relation to engine RPM?s?

If you are familiar with the principle of how the exhaust valves operate, you know that the opening times are controlled by a spring loaded mechanism.

It would be possible to change the opening times by fiddling with the spring pre-load and the K value of the spring itself.

I am not saying that this is something I particularly want to do, I was just wondering if someone has done anything in this area and would like to share their findings.

I think it would be possible to set up the whole axle with the balls and spring in a lathe and crank up the rews until it opens. I guess there are lot?s of opportunities for false reading given that the axle is not in its native environment - lack of friction and resistance and all.

Just wondering?


On the same subject;

Would there be any benefits in changing the exhaust valve timings? Maybe this could be a cheap (and easy) tuning tip?

//John

[YUNGGUNNAZ]

john,good question to explore.my opinion is there should be some gains in changing the exhaust timings if other factors such as transfer hgts.,exhaust widths,intake openings etc.as well as pipe and intended use of bike. did not have time this year to experiment with governor spring or ramps to see what i could come up with.sure there are gains here.did tinker with rm250  spring and tension on return spring and it picked up revs quicker.i'm pondering blocking off the exhaust port drumm openings and center valve in order to control timing more for my use,which is drag race.this thread should be interesting.

[IridiumRed]

This would be an interesting test to do - running the gov in a lathe and watching the behavior at various rpm

I wonder how much the governor behavior (such as the RPMs when it begins to open, fully opens, etc)

I know the powervalve makes a HUGE difference in the way my bike runs.  I should know, I had problems with mine for a while (dirty powervalve system, overclosing valves, etc), so I played around with it, disengaging it completely (leaving it closed), leaving it wide open, etc

This might be a fairly substantial reason why one KX500 runs differently from another

[John]

Just writing something to keep the topic going. I can't be bothered to do anything myself .  

//John

[YUNGGUNNAZ]

john,i don't have a lathe here to turn it that fast.but i did take a look again at the rams and balls and decided to modify ramps some to see how it actually makes the bike react.i assume it will allow the balls to climb ramps faster and open valve sooner maybe getting motor on the pipe a little quicker.still plan to spinn it to visualize effect.

[John]

If you alread have it apart, could you count the coggs on the different gears involved to be able to calculate the total ratio agains the crank revolutions. I assume it's not 1:1

Be careful so that you don't modify too much. One of these balls went through the engine cover on my 125, it did a nice big hole. That was due to wear / exhaustion rather than modification. Expensive anyway!

Cheers!

//John

[YUNGGUNNAZ]

ok.will do that tomorrow.good point.won't grind on it too much.will make sure at full extension that the balls cannot come out.will also mike them for size as with lockup clutches, gram weight makes a difference with spinning mass.

[YUNGGUNNAZ]

here is somespec's on 99 k5 governor : balls-7.9mm(.311)WASHERS-1.02MM(.040),same thickness as trans washers.spring hgt. uncompressed-25mm(.985)coil dia.2.04mm(.080).plastic gear was 18 teeth and 13 on the crank.looking for my angle finder so i can maybe get the degree of angle of the ramp.anyone know whether there are optional ramps available to the public.heard some factory sponsored bikes try different ones.

[John]

Nice work!

This would mean that for every crank revolution the small axle will rotate 13/18 of a revolution. So if 6000 crank RPM's is the theoretical opening time for the govenor, it would calculate out as 6000 x (13 / 18) = 4333 RPM's on the small axle.

Does anyone know what the specified opening times (RPM's) are supposed to be for the exhaust valves?
Does anyone know what the ideal opening times (RPM's) should be for the exhaust valves?
Could anyone make a general statement of how the exhaust valve opening times affects the general behaviour of the engine?
I assume fiddling with the timing would require a different jetting - needle anyway.

//John

[doordie]

Harder spring in KX250.
(Nearly the same maximum rpm for kx250 and kx500).
I have a kx250 spring for testing later in winter.


//doordie

[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