Why do oversized PV covers do what they do?

[motogroove]

People are changing their power delivery using the oversized covers. Better low end I believe is the goal.

The action of the rotary valves that (I think) port the exhaust over into the PV cover(s) is kind of hidden, and I'm trying to understand the how and why.

When is the PV chamber in use? High revs or low?

My crude understanding is that the PV chamber volume is used sort of like stretching the exhaust like a trombone. But I could be way off and I still don't quite get the timing, etc.

Anybody want to take a crack at explaining it?

Thanks a bunch!

 

[noobi]

They work to boost bottom end response by increasing the resonating volume of the exhaust, I think?. They are only in effect when the PV is closed, when the PV opens at higher revs that area is blocked so no longer effects. It gives the bonus of adding length to the pipe for bottom end advantages with out the disadvantages of a longer pipe, which is obviously harder to make fit on the bike.

Someone else will have to chime in on the mechanics of it, I havent covered that part of my degree yet

 

[GMP]

Thats basically it. The left side drum valve in the PV is like a two position valve. At lower RPM, the exhaust sub port is open to the chamber formed by the cyl and PV cover. At higher RPM, the drum rotates and closes off this chamber as it opens directly to the main port/header.

The PV cover has more effect on the 200s and 250s than the 300s, and also more effect if you are running a mid to top end orientated pipe. I like how it works on a newer 250. With this, a head mod, and good jetting, it will pull from a crawl at idle to a screaming top end. An older 300 with an old Gnarly pipe will not benefit or need a high volume PV cover.

 

[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.

 

[bchatt]

OK, thanks for those explanations, I understand what you are saying but I have a question. I have been wondering why the actual volume of that left chamber is so important, and why the increase in volume gained by the addition of the larger PV cover has the effect that it does. I have made a 10mm spacer, and found it made no difference. The aftermarket covers all seem to be about 20mm. What's the significance of this volume and why don't the engine designers get it "right"? How did they come up with the volume that they did?

 

[wasp4banger]

OK, thanks for those explanations, I understand what you are saying but I have a question. I have been wondering why the actual volume of that left chamber is so important, and why the increase in volume gained by the addition of the larger PV cover has the effect that it does. I have made a 10mm spacer, and found it made no difference. The aftermarket covers all seem to be about 20mm. What's the significance of this volume and why don't the engine designers get it "right"? How did they come up with the volume that they did?

When you say how don't they get it right, any design is at best a compromise. We all ride with different styles over different terrain with differing levels of ability. The factory set up is a good starting point, most who know how to ride, set their bikes up to suit their own needs.
I wanted more bottom end tractor like power, so added a large volume PV cover, and reduced the base gasket thickness and jetted to suit. The bike is now awesome off bottom and no slouch at wide open throttle. My mate with a KTM 300 wishes his bike pulled like mine off bottom.

 

[bchatt]

My question was of a more technical nature - I am interested in the theory behind the chamber and its (specific) volume.
As for " all designs are a compromise" - I don't see a compromise with having more power down low, at no expense to the rest of the rev range, after all that's what exhuast valves are all about! Simply put, if for example the LTR cover is so much better, why didn't the engine designer come up with it? I'm not bagging, I'm just wondering about the design process.

 

[gasgasman]

why didn't the engine designer come up with it?

It's all about the mighty dollar.
Adding all the goodies will drive the bike prices too high.

 

[(F5)]

I'll risk pooing on internet knowledge

The chamber is open at low RPM as that is where the pipe is out of phase with the engine speed. On a conventional engine the returning waves are disruptive off freq as they are returning too early on a wide open port. The chamber open disrupts these returning waves. Typically a chamber size somewhere near the cylinder volume will work well enough but isn't critical.

Of course it would cost GG $0.2c to make bigger covers & I doubt they would make the same mistake year after year.

Most reports are anecdotal & involve other changes.

 

[gg3]

I have the same questions as Bchatt having read some power valve cover threads.At what point does the volume have 0 gains,or negative effect?
The cost at manufacture is not even worth discussing,so why not put a bigger cover on?Or is their a compromise somewhere?Just curious.
I have a larger cover I had on a 2002 but have never tried it on my 2010.

 

[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]

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]

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

Umm....please explain

 

[HuskyDude]

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]

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)]

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.