Popping noises when flowing air at low valve lift

[SirYun]

Hi.

when I suck out grit by sticking a vacuum in the intake port and sucking it out I noticed that when you put in a valve and slightly open it you get a very very noisy and turbulent flow which crackles and pops and generates quite a bit of shockwave action as the entire hose of the vacuum resonates along (you can feel the pops in the hose).

I know that for exhaust ports this is generally considered bad and a more smooth sounding exhaust ports usualy give better results (according to Darin Morgan (Reher Morrison), and he spends most of his working days flowtesting and dynoing pro stock and other large V8's) .

But this is the intake and the valve seat is finished, bar some final tiny tweaks

Throat diameter 90% of valve diameter with the minimum diameter about 3mm down from the lower edge of the 45deg. seat. 70 downcut is blended to the rest of the port with a 45 degr. seat leading to a tiny 20 topcut. Seat is matched to the valve and the valve has a tiny tiny backcut (not much more than edge removal due to the flat shape).

Any experience of/or idea what is going on? Is the flow supersonic at low lift?

I do have to get my own flowbench one of these days..

[Guy Croft]

Well, the valve flow is not going to be representative as you're sucking out of the inlet port with the valve in, the air is going the other way on the engine, right? Interesting but functionally not much use to you.

Is the airflow supersonic? That I don't know...

What is the noise? Most likely vortex shedding. Somewhere at the very high velocity near the valve throat, high pressure air is rotating (curling up)to low a pressure region, forming a vortex pattern and these vortices are trailing off down the pipe. It disappears as you lift the valve and the velocity drops, am I right?

Will it happen when you flow the air the true direction? Most likely not.
I sometimes see vortices in 16v heads, and then not often, doing valve-in tests, they form between the short side radius and the splitter. There's a picture in 'Integrale 16 Evo 2 head prep' in GC Q&A; there I am blowing the air thru the inlet port; the ports generate vortices rotating (see cotton thread RH barrel) extremely fast and powerfully out of the throat in the clockwise direction as viewed as the airflow builds up, and the airstream certainly breaks away totally from the SSR. No idea what happens exactly there with the valve in, hopefully not total separation and a major vortex, but I have no way of really observing that.

GC