I have a 86 that has a mild street port. I just ported the intermediate housing and removed the 5/6 ports. I used a RB template, the front and end housing ports are similar size as the template. Oh to the point. I have a S5 manifold and I was concidering putting it on. I noticed the VDI actually doesnt do anything more than combine the front and rear runners. It doesnt change the length of them. So I was wondering it I should even bother. I dont really think it will gain any lowend torque. Will it really affect my top end.Heres my mods.
86
rebuild street port, ne emmisions, TB mod, intake, RB header/presilencer, catback.

 

Many people say that there is not as much power to be had from the S5 manifold on a streetported motor as there is from the S4 manifold. I don't personally know though. Just by looking at the two next to each other I would think that the S4 manifold would flow better but the VDI affect seems to play a VERY large role in its success. Your peak torque number won't really be affected but your average torque throughout the entire rpm range should be improved judging by the longer runners. Someone else on this forum has dyno comparisons between the two with the only difference between runs being the manifold change. It shows about a 15 hp peak gain at the wheels increasing over the stock S4 manifold after about 5500 rpm. This is about where VDI kicks in. I think this was on a stock port motor though. If you have both and a little time I say try each one and see which works better. Then post the results

 

I have an s5, is there anything I can do to the VDI to make it work better? ie, can you make it open up earlier or later, or is it pretty useless to do so?

 

Remember that the 13B engine has a dynamic surge, (closing of the intake on one side, "super charges intake charge into the other rotor) so when the VDI actuates; that is part of the dynamic surge.

VDI does not effect low end torque, it helps high range (effectivly straight lining the torque curve through most of the whole RPM range). While disabling the VDI often increases HP, disabled it also tends to roll off/lower torque Ft/lbs at upper RPMs (which in the long run limits top speed and shift points). Of course that is one of the reasons that the VDI was added to the FC in the first place.

 

I thought the VDi was closed at low rpms and open at high?
woudn't this imply that it's meant for low-rpm driveability?
Or am i thinking wrong?

 

use the VDI manifold, but port it , i dont think the RB ports are big enough to cause problems with flow that would require the s4 manifold, thats more for like small bp's and large streets, the s5 will give you 15+ more top end because of the pressure wave tunning magical stuff it does. Vdi Is closed for low rpm, It is for low rpm drivability,.. well its how they sneaked in a racing tuned manifold that comes on at 5500 rpm, and still made it have enough torque that people didnt complain,.. much