Dirk_Diggler

But I still don't have a clue what the VDI is and what it does.......is the VDI related to the 6 ports?? Please be gentle to the noobie.......

Thanks......

vaughnc

I know there are several 2nd gen sites that explain this. All I know is VDI & 6 ports are intake related and control low end torque & high end HP. Primaries & secondaries are fuel injection related and as rpms increase you need more fuel as compression increases.

No7Yet

vaughnc: Uh, no.

VDI stands for Variable Dynamic Intake.

You see, the diameter and length of intake runners (the "tubes" in the intake manifold) effects the speed and amount of air that flows into the engine. Long intake runners have lower overall flow numbers, but higher air velocities, while short runners flow more air, but at a lower velocity. What does this mean for your engine? Well, at low engine speeds, when the intake port opens, faster flowing air results in a higher intake port "pressure", meaning better VE (volumetric efficiency - a measure of what percentage of the intake's "potential" volume is actually used).

Mazda designed a system (independant of the 6PI, which alters port timing and "duration") that is the best of both worlds - VDI. At 5500 RPM, the long intake runners (runners tuned for torque) start to run out of breath, so a secondary intake path opens up. This new intake path is shorter and more direct, hence better engine "breathing". This prevents a power dropoff, and gives you m@d t0p 3nD, y0!

Something that Mazda also designed into their manifolds takes advantage of something called "port reversion". As the intake port opens, air flows faster and faster into it. Ideally, the intake port closes just barely after the size of the intake port begins to close. In a rotary, the next intake event happens immediately, but the size of the intake port, combined with exhaust event overlap, cause a small time where air is forced "backwards" in the intake runner - that's reversion. Mazda took advantage of this inherent reversion in their 13B design, but really made it shine in the 89+ manifolds. The modern VDI design uses this reversion to force more air into the alternate intake port (other rotor). The alternate intake port closing results in even more reversion, which is used to force air into the first rotor, in an increasing cycle. (Of course, it doesn't go on forever, but) The effect increases as engine speed does. According to Brian Smith, Mazda has recorded LIM air pressures of about 1 PSI when this reversion effect is in full swing. Very cool!

Brandon

EvilAvator:

TurboHead

Right on "No7yet"! Sucks though! Sucks because they DIDN'T take full advantage of that BEFORE the 89's! I don't believe there IS anyway to mod this into a pre-89, is there? Sure would be choice if there WAS a way!

No7Yet

Yes, you can put the 89+ VDI manifold on the 88- cars. You'll have to grind a small bump off of the engine in one spot, but it's not too terribly difficult. (I wouldn't do it as a first mod, though)

Problem is, while it'll gain you an easy 10-15 hp in the top end, the 88- cars simply run out of revs! The VDI doesn't take effect until 5500 RPM, and 1500 RPM is just too short a time. It still works great, though, and is a great mod for anyone who has the coin to throw down.

Brandon