Help me, I'm too stupid to figure this out, Decel Valve Options

[pbanders]

Weird - I posted a response to Clay's post last night, but in didn't show up - I probably didn't click the "Add this" button after previewing...

Anyway, I'm pretty sure I figured out HOW it works last night, and it doesn't involve pressure waves or throttling orifices (well, perhaps throttling is important, see below). I'll know for sure when I open one up.

The key is that it takes a pressure differential to open the valve. But if both sides of the diaphragm are evacuated, then how do you get a differential. Simple, both sides of the diaphragm are NOT evacuated. I thought the guts of this thing looked like the guts of the fuel pressure regulator, where the "output" port has a relatively small seat area on the diaphragm. In the decel valve, the end that supplies air must have a seat that trumpets out inside of the valve, so that it covers nearly all of the diaphragm area. That way, most of the non-control side of the diaphragm is at atmospheric pressure.

When you pull a vaccum on the control side of a sufficient level to overcome the spring tension, the diaphragm moves and the seat opens up, allowing air to pass through the valve and limit the vacuum level in the intake manifold. Where I think the throttling is important is that the vacuum in the control chamber side bleeds down slowly, allowing the valve to stay open longer.

Clearly, some fraction of the non-control side of the diaphragm must be under vacuum, which explains why when I pulled vacuum on both the control port and the manifold port, my valve didn't open. I've probably got mine adjusted way too high and that's why it acts that way - or perhaps, something is wrong with my valve.

[clarkcou]

I have a 75 BMW with L-Jetronic. I seem to remember the decel valve allows extra air in during high vacuum conditions (i.e. deceleration) leaning the mixture and slowing the apparent throttle resonse on deceleration.

I do know friends behind me report a brief flame when I decelerate with this valve disconnected (unburned fuel without the extra air). I keep it connected except during testing.

Randy

[pbanders]

OK, a bit more on this. I was poking around my parts box, and I found that I'd bought a new decel valve some time ago - same Bosch part design, but for a different application (it's a 0 280 160 133). Since it was nice and clean, I could look into the port on the side and see the operation of the valve.

Looks like my idea that the atmospheric seat trumpets out is bogus - the atmospheric valve seat is no bigger than the diameter of the port. I can also see a small helper spring between the port and the diaphragm.

The valve operates exactly as I would expect it to - when you pull a vacuum just on the control port, it opens at an onset level, set by the adjuster - in this case, about 15 psig. However, if you pull a vacuum on both the manifold port and the control port, the onset increases. If the vacuum on the manfold port is the same as the control port, for vacuum levels I was able to achieve with my hand pump (~ 25 inHg, about 3 inHg more than I've seen in overrun manifold pressure on my newly-rebuild 2.0L), the valve NEVER opens.

Still wondering how this thing works......

[bd1308]

You still need my donor decel valve?

[pbanders]

You still need my donor decel valve?

Yes, I want one to cut open. The one I have is new and a non-Porsche one, I'll put it up on ebay eventually.