With the VVIS plate’s butterflies (in Red) closed, the throttle body feeds the relatively shallow and divided plenum, which feed the runners that go into the heads. Due to the small volume, low end torque is improved.With the VVIS butterflies open, the intake manifold plenum is no longer separated from the lower plenum. Since the lower plenum is only connected to the throttle body via the six butterflies, the volume is indeed increased but there is no additional air that flows through the butterflies from the lower plenum.
While the VVIS plate improves low-end torque by reducing the plenum’s volume at low RPM, in forced induction applications there is a benefit at both low and high RPM by reducing the restriction and turbulence of the intake by gutting the system altogether.
We first modified the VVIS plate by cutting out the butterflies and welding up the holes for the actuator.Next we modified the upper plenum by cutting out the bridge and center wall that divided the runners on each bank. Now after the air passes through the throttle body, there is a single, open plenum which feeds all six runners.With the VVIS plate gutted and the center dividing wall of the manifold removed, air enters a single, open plenum with a smooth path to the intake runners. This has proven to be advantageous in forced induction applications, which is good because our turbo motor is currently being assembled.
I eliminated the VVIS plate on my NA 1 C30 and cut the splitter out of the post throttle body area. I also blended the intake runners flush to the floors aand increase vertical radii. Finally I accentuated cross flow alignment with more discrete cutting – which is due to metal thickness somewhat limited but nonetheless noticeable in appearance. Having run the car on dyno prior to this it produced RWHP just under 270 with open headers through glass dual outlet mufflers. It also made 203 RWft.#s from 5300 to 6500 rpm.
After the intake trick the car performed better with restrictor tips in the mufflers than without prior to the intake mod. The low and midrange are unaffected, but from 7000 rpm to 8000 rpm the revs come much quicker. My guess is with restrictor tips out there is likely 8 – 10 more RWHP.
The motor is now out for rebuild. I will be porting the heads mildly which means a primary focus on deep unshrouding of the valve pockets and floors with more emphasis on the exhausts. While I could add a camshaft set, there is room in the heads for 10% better flow with an average of less than .5mm metal removal and improved contour below the stem boss.
Going with a 3.2 liter overbore kit and the aforementioned mods, it is easy to imagine 300 rwhp , so that is what we are shooting for. I found the JDM short transmission gears have drastically reduced the 0 -60 time bu at least 1/2 second and put me at the torque peak a couple hundred feet before the trap in fourth gear based on the engine before teardown. I hope the next test will reveal a livelier performance.
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I eliminated the VVIS plate on my NA 1 C30 and cut the splitter out of the post throttle body area. I also blended the intake runners flush to the floors aand increase vertical radii. Finally I accentuated cross flow alignment with more discrete cutting – which is due to metal thickness somewhat limited but nonetheless noticeable in appearance. Having run the car on dyno prior to this it produced RWHP just under 270 with open headers through glass dual outlet mufflers. It also made 203 RWft.#s from 5300 to 6500 rpm.
After the intake trick the car performed better with restrictor tips in the mufflers than without prior to the intake mod. The low and midrange are unaffected, but from 7000 rpm to 8000 rpm the revs come much quicker. My guess is with restrictor tips out there is likely 8 – 10 more RWHP.
The motor is now out for rebuild. I will be porting the heads mildly which means a primary focus on deep unshrouding of the valve pockets and floors with more emphasis on the exhausts. While I could add a camshaft set, there is room in the heads for 10% better flow with an average of less than .5mm metal removal and improved contour below the stem boss.
Going with a 3.2 liter overbore kit and the aforementioned mods, it is easy to imagine 300 rwhp , so that is what we are shooting for. I found the JDM short transmission gears have drastically reduced the 0 -60 time bu at least 1/2 second and put me at the torque peak a couple hundred feet before the trap in fourth gear based on the engine before teardown. I hope the next test will reveal a livelier performance.