Tom and Jerry,

 

Grayhawk is piping up even when Tom is right.

 

 Generally,

KIAS Hg."

150   1.1

175   1.5

200   2.0

225   2.5

250   3.1

 

These numbers are not theoretical since this is how the pitot/static system converts ram pressure differential into indicated airspeed.

 

I get very confused about all these engine configurations. I have an IO 320 B with a 9:1 CR.  The sump was modified to accept the throttle body on the front where the 1&2 exhaust crossover to 2 into 1 under the sump along with the 3&4 just behind the sump and where both exit on the pilot side.  It is very tightly packed under the cylinders making cooling more difficult.  My ram air intake is smooth walled (plumber's sleeve connection from cowl to throttle) of decreasing, then constant diameter all the way to the throttle - no filter, no airbox.  I see very close to the calculated rise although it is difficult to measure as the absolute pressure sensor used in the intake system is not like the TSO'd measurement of atmospheric ambient pressure.

 

The only true measurement is to fly side by side thru the same air mass.

 

Grayhawk    

 

Jerry,

Chris was a little too strong in saying that cowling mods are a must.  I did it without required mods.  The starting point was an IO-360B1F.  The pan was replaced with one from a B1B (aft facing fuel injection servo).  Performance Aero Engines mods that sump to put the entrance on the front.  It is true that you need a custom exhaust.  Mine is a four-into-two-into-one that exits on the pilot side.  The crossover is under the much shallower pan.  The only reason I had a cowl mod is that I wanted to have a Legacy style tunnel exit and I wanted the total exit area to be smaller.  The cowling sides were unaffected.  The larger question is how much benefit is to be gained by forward facing induction.  Before you change anything, observe what your engine off manifold pressure is and then immediately go fly.  Try to find a nearby place with low enough elevation that you can safely fly, flat out, at your field elevation.  Record what your highest manifold pressure is at that time.

Pipe up right here Grayhawk if I am wrong, but I think the theoretical maximum ram air pressure gain obtainable is about 2" of mercury.  So if you record more than an one inch of MAP above what you had before engine start, you already have a pretty good induction system.  Getting more is really tough.  When you move the servo forward there is very little room to have an expanding air box to convert that high velocity air into increased MAP.  The updraft actually does pretty well, even though it looks like it shouldn't.  With a great deal of effort, I have built an expanding airbox on the front of my engine and I do get between 1.5 and 2 inches above ambient, but it was a huge amount of work.  It is definitely producing some speed increase.  Hard to tell how much, because the airplane is in the middle of many drag reduction mods and half un-painted right now.  Too late for this year's racing season, but there is always next year.  BTW, in the current, very dirty configuration, I get better than 210 Kts flat out at 2700 on 180 HP at 4500 DALT.