FI Plenum Design, MS'ing my 2270 Options

[Tom Perso]

Hey Guys,

I am designing a MS system for my 2270 (163.86b cam, 48x38 valves, future Triad header). I am looking at running stock intake runners and possibly making a new plenum to run my Ford TB (probably from a 4.0L Exploder/Ranger).

Are there any "design guidelines" that should be followed when making a new plenum, such as a certian volume required for engine displacement?

Looking at space requirements, I see running the TB on top of the plenum is the only way to go. That, of course - begs the question... Is the stock 914 2.0L plenum a good starting point to modify to allow for the Ford TB?

Here are some pics for reference... I had a Bus 2.0L intake system to borrow, so I am basing my measurements and ideas off of that.

http://www.qtm.net/~persot/MS/

Any ideas, suggestions, etc, are appreciated!

Thanks,
Tom

[Brett W]

QUOTE

What the goal is, is to use the small venturi ( velocity increases magnanomously, with no loss of flow) which rams the air into the large plenum, changing the velocity to pressure---hence, " Ram Effect "

Sort of. The ram effect does not necessarily come from that. The reason plenums taper into the TB flange is that you can just bolt a flange to the side of an open box, you will kill the airflow. Air does not like radical changes in direction, it will seperate from the boundary layer and create turbulence, killing your flow. Airflow likes to "see" less than a 15 degree change in boundary conditions, anymore and it tends to seperate.

QUOTE

This is the only way turbines will survive at high speeds--just extropolate this practice into automotive--the practice is the same--but different--with turbines, the air has to be slowed down, or the engine will stall and flame out--piston engines are spark dependant, and never get to this speed, and hence can use this effect to an advantage--much testing has shown that when you increase the velocity of the air into the plenum beyond supersonic--and then have a large plenum to extract the velocity and turn into pressure ....

Turbine engines have to slow air down because it chokes at levels supersonic and beyond. When airflow through an orifice chokes it will stall. Supersonic airflow is a very different animal than subsonic flow. I have seen what happens when airflow goes supersonic inside an induction system. An engine I was working with last year would flatline at 11200, exactly the point at where the restrictor caused the air in the intake to go supersonic. It is a phenomenon know as choke flow. This engine should have continued to make power into the 15500 range.

Velocity is good to a point in an intake tract. Too much is just as bad as too little. The airspeed in the intake ports should not see any more than about 300-400 fps, any more and you run into problems. Remember High velocity does not create high pressure. High Velocity creates low pressure. In the intake you high pressure not high velocity.

The "ram effect" that you mentioned is caused by "wave" tuning. Air is a really cool medium. It is a fluid but it also behaves as a gas. A fluid has mass and direction, but you can't compress it. A gas on the other hand can be compressed. Airflow can not be stopped and restarted instantaneously.

When the valves in a cylinder are opened pressure drop moves air into the engine, it will keep flowing even when the piston is starting to move back up the cylinder. This inertia will draw more air into the engine. When the valve closes the column of air moving through the intake tract can't just stop and wait for the valve to open again. It bounces off the back of the valve creating a "negative direction wave". As this wave reverts back through the intake port it has a certian velocity and mass. It will hit the plenum and either proceed to disipate somewhat or it will flow into another cylinder. The size of the plenum will determine how much a dampening effect is applied to the "wave" . When the wave bounces off the plenum wall it is headed back for the intake port. If the cam timing is matched correctly to the intake characteristics then this wave will have some extra velocity and can help push more mixture into the cylinder. The problem with "ram tuning" is that it only works in certian rpm ranges.

Remember all engines are a compromise. Factory stock engines will have compromises made in order to:
make the engine fit the engine bay
Meet emmisions and noise regulations
Make the car easy for grandma to drive to the grocery store
ETC.

Assuming that Porsche built it right from the factory is not the way to settle an argument. Porsche made the 3.0 litre S2 with 208 hp. Not really impressive if you ask me. Yes it is good but hell the 1.8 litre Intergra engine made 180hp from 1.8 litres. The first production car to make 100Hp per litre.