Why do oversized PV covers do what they do?

[motogroove]

Whenever I am faced with questions of subtle adjustments or modifications like this, I like to remove the subtle part and think in extremes.

If the PV chamber was tiny, let's say the size of a thimble, it would not break up these bad resonance waves or add significantly to immitating a longer pipe. So, the engineers need to go bigger than a thimble.

If the PV chamber was really big, let's say a gallon, it would immitate a pipe that was way too long (even for such low revs), and probably be way beyond the size needed to disturb the bad wave impulses. And of course it would be bulky, heavy, and more exposed to damage.

There's an optimal size range anyway.

Also note that our factory cover has cooling fins and the aftermarket units don't have room for that. Maybe that's the trade-off right there - a little extra cooling. Think about that little region of the head, pumping hot exhaust gasses more than any other region of the head. It's hot there.

I'm not good enough to work my 300 hard enough to be taxing the thermodynamic limits of the machine. But that doesn't mean that cooling isn't important for a top rider (who probably gets along fine with, maybe even enjoys a less linear power delivery).

[bchatt]

There's a good explanation of how PVs work on here

http://www.dirtrider.net/justkdx/kips.html

It doesn't go into volume details....

[(F5)]

I think people think of it as a Helmholtz resonator which it is not.

[husley]

Originally Posted by HuskyDude

Ya...what they said.

I have one on my 2010 300. I tried it with and without and I think it does have a positive effect at really low rpms. The bike doesn't stall as much and pulls real hard from a almost dead engine. Great for tight S/T, logs and stuff.

Exactly what I wanted to hear...........which cover did you purchase?

thanks!

J

[bchatt]

Originally Posted by (F5)

I think people think of it as a Helmholtz resonator which it is not.

Umm....please explain

[HuskyDude]

Originally Posted by husley

Exactly what I wanted to hear...........which cover did you purchase? thanks! J

[Bailey28]

I bought a PV cover for my 300 and wound up taking it off. At first it seemed like it added some low end which it did. But after testing for a few more rides, I felt that it muted and watered down the overall throttle response. In other words, the engine was not as crisp as it had been with the stock cover in place.

I have since sold the cover and returned to the stock one.

[HuskyDude]

Originally Posted by Bailey28

In other words, the engine was not as crisp as it had been with the stock cover in place. I have since sold the cover and returned to the stock one.

Hey Bailey, did you do any jetting changes with the big volume cover?

[(F5)]

Originally Posted by bchatt

Umm....please explain

Well they aren't so much about the effect such as a resonant airbox has, but more about disrupting the returning wave when it is out of phase (ie revs too low for pipe to be working in synch with the engine). Talking to tuners about it they have said that both in dyno & on SW sim packages, that you can change the pipe when the chamber is open & have no real effect on the power. That sort of indicates that they are not working to 'make the pipe appear longer' or whatnot, but merely disrupting the negative effects of the pipe when revs are too low.

That isn't to say that that chamber isn't too small std to create enough of a disruption & absorb the returning waves, but I am dubious about any bolt on item that is easy to machine & market yet strangely could be easily incorporated into next years model but the factory ignores it. Hmmm. . . Maybe they don't have the internet or after market manufacturers in Spain. . .

[Bailey28]

For jetting changes, I tried the new cover with the stock setup, N1EF-1, 40 175. Not much improvement. I went leaner everywhere, then richer everywhere to get a feel for what the changes would run like, mostly changing the pilot and experimenting with different needles.

I could never get rid of the softening of response with the cover. The bike seemed to make a bit, and I mean a tiny bit more power at the very bottom end. Anything above that it just seemed less crisp. The power was there, but the sharpness was lacking some from the stock cover.

The cover was well made and looked great but it wasn't working for me in my situation. Without the cover I finally settled on a 42, NOZH-1, 160 main. I know the 160 looks lean but with the rich final taper of the NOZx series and the 95+ degree weather it rocks.