Because it has a terrible flow pattern. It gives up flow for no other reason than saying its a closed chamber.
I can't remember how many hours I put into W5 chambers but it was a bunch. It took a weird valve job to make them happy. So does the CPPA chamber. The exhaust valve is too big on the W5 (and the W2 for that matter) and I suspect it's too big on the CPPA heads as well.
If I had one I would flow it, but I wouldn't want to spend a bunch of time with it. I'm too old to be fixing crap any more.
