A very pertinent question. There are a lot of cases where either packaging precludes the optimal solution, or simply 'good enough' does the job. This is particularly true of turbos, where you can, to a point, over come flow problems with more boost.



But your base assertion is correct. First you should design for optimal flow (preferably on a flow bench) and then tune runner lengths and plenum volumes to get the resonant mode you want working for you. its not easy or quick to get it right, but the gains can be considerable.



A good example with rotaries is the curve in the inlet runners, even with an IDA. The packaging in the car precludes a straight or near straight shot at the ports (although I was lucky in that respect due to the car i have). When you turn a corner you introduce turbulence and shear in the airstream. This is bad. however the additional inertial supercharging overcomes the flow loss, and there are things that can be done with tapers to try and get the air back together.



Fascinating subject, but you can spend a lifetime studying it.



Bill



P.S. Adam. Good to see another MS going on a rotary.

 

errrm. No. Its a Megasquirt

 

d jet is map based batch fired efi, same as mega squirt

 

So that would make haltech, wolf, SDS etc all D-jet ripoffs.



The Megasquirt is a great gift to the DIY community. Calling it a ripoff is both incorrect and somewhat offensive.

 

I'm not trying to **** you guys off, i was just simply pointing out the fact that the vdi uses a completly differnt set of pulses to tune. Also it's not using reversion, its using an acousitc pulse from the high pressue of the exhaust, which doesnt have enough voume to revert the intake (do stock motors have intake reversion problems? no). Also i think you guys dissmiss it in perfomance applications to quickly. a turbo is always going to have back pressure, and a BP is going to have even more overlap, both of which would ensure a strong pulse. I am not trying to act smarter then you guys, i know i am not, however i think you dissmiss vdi tunning to quickly as a real option.

 

Actually, the VDI manifold doesnt work on bridgeports because there isnt a period when the port is completely closed like a SP. I can vouch for that from personal experience also, because my aux bridge ran like total **** with the VDI manifold.



We all understand that is uses a different form of tuning, and if you would actually read and understand, you would have realized that from our posts. The VDI manifold has a more limited range of uses than a typical plenum setup, or ITB's. For one, it requires backpressure, and on top of that, it requires a non-bridged side port. You can even read in that SAE paper that is uses backpressure to form the high pressure wave. What happens is as the port closes, the air slams into the rotor face, compressing it. As the intake port opens, the backpressure from the exhaust port during this overlap period forces that high pressure area of air back up the runner. There is the positive pressure wave. Think about how restrictive the stock exhaust is, and now you see why the VDI manifold was chosen over a typical manifold. Backpressure hurts a typical manifold MUCH more than the VDI manifold, since the VDI actually utilizes the backpressure to increase the VE. You put on a free-flowing exhaust, and the gains from the VDI manifold start to fade. Also, because of the high amount of overlap, bridgeports are especially sensative to backpressure. They need a free flowing exhaust to make power.

 

I see your point on the bridge port, and since i have no real world experience in this matter, i will take your word for it. I still believe you guys are underestimating the stock VDI setup, especially for turbo applications, however i think we should agree to disagree.

 

why doesn't the pulse resonance carry over to the exhaust side as in pistons?

For years I wondered why there wouldn't be a benefit to the N/A rotary especially since now exhaust length and muffler are mandatory.

Could this apply for the N/A?(balance pipes or crossovers)

 

Be more specific on your question. Are you referring to true duals? Any collected exhaust does what youre talking about. Thats the purpose of the collector. When a positive pulse passes into a collector from one of the pipes, a negative pulse is sent back up the other pipes in the collector. The distance of the collector from the exhaust port determines the rpm the exhaust is tuned for. The tuned rpm is the point where that netagive pulse is trapped in the chamber by the closing of the port. Its called the scavenging effect.



Drago, there are SO many variables associated with intake tuning, and the VDI manifold is much more complex from an acoustic standpoint than an ITB setup, or a typical plenum type intake manifold. Very few people have the resources and knowledge to make it work well. But that also brings up another point. There are many things that seem like a good idea at first, but there are very obscure reasons why it wouldnt which arent readily apparent. Often, if something hasnt been done, its not because noone has thought of it. Its because it hasnt worked, or hasnt worked well enough. Im not saying not try new things. Actually, trying new things is very important. BUT, you should always look back, and try to find reasons it hasnt been done. Try to prove to yourself that it cant be done, and if you can't do that, press on.

 

One year I decided to run American Sedan in SCCA. Having a small block I was reading on tunning etc. Its been 8 years but the guy (Smokey) was credited with early NASCAR/Drag engine R&D. Any way I recall it saying the V8 balance pipe in the exhaust should be approximatley 18 inches from manifold no closer than 15,(but this was not a collector)

for the scavenging effect. So would the exhaust balance pipe application do the same or do you just place the two into one collector at the optimal distance per porting and then continue single to the muffler/exit?

Also (on the intake side of things) in SCCA they wont allow any venturies connecting indidual runners or a common space in between the carb and plenum.(If I recall) Wonder if this applies because of the afore mention pulse damping

threads you and Bill S. were speaking of but why would it matter?

Thanks