Hi all.

This is going to sound a bit Noobish but anyway.....

Right can any one explain in laymens terms why the Primary is Smaller then the secondary (ie doen't have exactly the same timing)

I thought for years the Throttles opened the primarys first which used the smaller bore runners and smaller port to keep the air speed up?

But when i think about it if you WOT at 2k on the dyno for example all the Throttles are open off boost so it makes no diffrence? and i did think the secondary throttles where shut so they shut off the secondarys but that would apear to be wrong.

So i am trying to get my head round why the Primary is so much smaller then the secondary while they are effectivly open at the same time? Does that mean i can port match the primary to the secondary and gain a bit of extra flow without getting into and timing issues? or more importantly loosing any low end torque or HP?

 

No one?

 

I'll take a stab at it, but I was hoping someone more knowledgeable would chime in. The way I understand it, and you touched on it, is that they're smaller for low end torque and smoother operation at lower rpm. People do open them up a bit, but if you go real large (think full PP for extreme example) you'll have rough idle (or have to maintain a higher idle) and un-smooth driving at low rpm.

 

^Based from what I know, it's like what Baldy has said.

The RX-7 are build as a street car, so under low rpm it is not necessary to have bigger primary. However, as you mod the engine, add more power, and move away from the factory spec, it is recommended to upgrade the primary (even the secondary sometimes) bigger to allow the abuse.

This applies to pretty much anything, the more power you want, the bigger/higher flow and more reliable design you need.

 

Flow velocity is the key to HP. Imagine that the rotor is a blade in a meat slicer. Small diameter runners produce the very highest velocity
for a given depression (low manifold pressure). So the slugs of air or sausage if that picture helps, will be longer than if velocity is low.
See the Bernoulli site. Also when the RPMs are high the large secondary ports can also have high velocity as the engine ingests more air, so the secondaries open up for high power output, and good drivablity at low speed/RPM.

http://mitchellscience.com/bernoulli_principle_animation
http://www.turborx7.com/porttiming.htm
http://www.mototuneusa.com/think_fast.htm

Lynn E. Hanover

 

I understand there is a second Wind approach to this but wouldn't you need to stop the air going through the secondary’s until the air speed increased?

I can understand the Part Throttle / Primary Throttle is used for off boost cruising around town the Secondary’s are effectively Closed so the stock engine can Dial out all the opening timing and get better MPG and torque low down. But if i have this correct if you WOT it at 2000rpm in third gear and the secondary Throttle open up the air speed will not be quick enough to make the revs pick up? to make it pick up you would have to drop a gear so the rev's are higher and there for the air speed higher to fill the extra volume before the engine would start to pick up? Does that sound about right?

So if that’s right on a dyno when the operator hits WOT at 2k for a full Pull both Primary and Secondary’s will be open effectively slowing down the pick up because ideally the Secondary’s should be closed until the air speed is fast enough to take advantage of the extra volume.

So if you where to use for example a RW 90mm single butterfly Throttle it would Kill stone dead the off boost around town low end Torque because you would loose the advantage of the Small volume Primary system at part throttle? or have i still not got it?

 

The explanation covers 4 port and 6 port NA engines. It is used by the primary ports in the 4 barrel carb, where the primary throttle is controlled
by the driver and the secondary ports are opend by vacuum pots. The idea of small primary runners is valid for turbo engines however if the driver controls the secondaries, it becomes much less effective. If you dyno from 2,000 RPM at full throttle the timing must be retarded.

Lynn E. Hanover