This GReddy Type-S BOV uses two springs to control how much the valve opens under part-throttle and full boost operation. The screw on top of the cap adjusts the pre-load of the springs pushing down on the black rubber diaphragm. When there is enough boost to overcome the spring and air pressure on the top side of the BOV, the piston on the bottom will push up and allow excess boost to dump out of the housing. The Type-S valve also comes with an additional nipple on the boost plumbing side of the diaphragm.
The second force is from a diaphragm at the other end of the valve. That vacuum hose provides air pressure from the intake manifold to this diaphragm. If the diaphragm is, say, two square inches, the same 18psi in the intake manifold will be pushing the valve closed with 32 pounds of force. 32 beats 18, so the valve stays shut. When the throttle is closed, though, there's still 18psi in the pipe, but the intake manifold is now pulling 10psi of vacuum. Suddenly, there are 18 pounds pushing up from the valve and 20 pounds pulling up from the diaphragm. The side door slams open and the party rushes outside.
This is all so elegantly simple, so why do we need a third force? The problem arises at part throttle, where the car might be cruising along with a slight vacuum in the intake manifold, but an eager turbo pumping boost into the system. In this situation, a dual-force blow-off valve would blow open. A blow-off valve that's open at cruise adds an annoyingly inconsistent delay when opening the throttle, so every blow-off valve also has a spring holding the valve shut.
This is where the adjustment is on adjustable blow-off valves; they're simply adjusting the pre-load on this spring.
There's no hard rule on how to adjust these valves. On a street car, it needs to open quickly when needed, so it should be on the softest setting that will still let it stay closed at cruise. On a recirculated system, it's even OK to let it leak under some cruise conditions to get this quick response. This setup usually gives the best boost response after the shift, but I've seen cars built by people who know a thing or two-like Prodrive-with valves set so tight (and deliberately undersized) that some boost remains in the intercooler plumbing during the shift.
Personally, I set them loose.
Q. Diesel Rotary
I have a Mazda RX-8 with all the basic bolt-ons and I absolutely love it. I fell in love with the rotary engine the day I understood cars. I also love diesels and I was wondering if there was any way of making a diesel rotary? I think that that would get the gas mileage up in a rotary and would sound really cool. I think it's possible, but I'm sure you will make me feel like an ass when you tell me why it can't work.
Will Aronson
Coolsville, ID
A. If you're an ass, so were the blokes at Rolls-Royce who tried to do just that in the '70s. There are two fundamental problems with making diesel rotaries. First, diesels require very high compression ratios and rotaries can't mechanically reach that compression without enlarging the rotor to ridiculous dimensions. Rolls-Royce solved this problem by using two rotors to do the compression. A larger rotor was used to pre-compress the intake charge and feed it into a second rotor, which then finished the compression, went bang, made some power, then fed the still-expanding exhaust back into the big rotor where it could expand some more and make even more power.
By stacking two of these two-stage, double-rotor contraptions into one engine, R-R managed to make one 929-pound monster that made 350bhp. Shockingly, this was about half the weight of a comparable piston diesel for the same application (tanks) and would have been perfect for the job at hand, which was to make tanks light enough to drop from airplanes. Before the design was refined enough for production, though, the British military realized that instead of blowing things up by dropping tanks nearby, it was much easier to just blow them up directly from the plane, eliminating the middle man and making the 900-pound diesel rotary nothing but an odd historical footnote.