For Don and Redline

Don, Red & anyone who would like to take a shot at this

I saw that post about a 300XS with a 250XS Cowl made me think about cowl mods for the XS

There is a company that makes light weight carbon fiber cowls for the XS line they weigh only about 9 lbs which is like 1/5 the weight of a regular cowl and they have an increased air flow capacity. Any thoughts on performance gains with this mod and would the increased air flow help HP production, would the ECU's adjust for the additional air flow etc.

Now the hood scoop mod for the EFI 250 XB has been a real winner but I know EFI is not DFI technology.




Modified by esdbass at 8:33 PM 8/11/2008

On the EFI's, increases in airflow often resulted in HP gains. Those motors were usually running "fat" (rich) under most conditions, and although they could not adjust for the added airflow (without manual adjustment or mods), they did see improvement from more (or cleaner) airflow.

DFI (Optimax) engines are NOT running excessively "fat" (rich). They are emissions rated engines... and essentially are run as "lean" as they can be, while still "rich" enough to prevent failures. Since there is no means for the PCM to adjust for additional airflow, adding additional air will ABSOLUTELY result in the engine running leaner under some operating conditions. The fuel mapping works on a "speed-density" system... there is no calibration to allow for changes in airflow, or to compensate fuel-flow based on additional airflow.

LEAN= FAST... but NOT FOR LONG. "BING-BANG-BOOM" will be the inevitable result... the only question would be "when". http://www.bassboatcentral.com/smileys/wink.gif

My biggest concern would be when cowling mods resulted in "positive" pressure inside the cowl (barometric pressure in excess of one atmosphere). This would essentially be the same as "boost" from a supercharger (although not as strong, obviously). When we are talking about an engine traveling at 80+MPH, with a scoop-fed cowling, this would be a real possibility. The end result would, inevitably be "a lean running condition".

There's my .02 http://www.bassboatcentral.com/smileys/glasses.gif

Don: The existing lean condition of the XS's was why I doubted the increased air flow would work. I too think that it would lean out the motor even more and the thing would go boom.

I was asking you and Red to see what your thoughts would be as to if the motor would adjust or not and if it did would it just dump more fuel into the motor and negate any power increase?

In almost every motor till these new opti's which I haven't a clue if you give them more air they run better. I wonder how the guys that have these cowls are making out and what their computers are reading after running one of these increased air flow cowls??? The weight reduction is real nice though http://www.bassboatcentral.com/smileys/smile.gif

Nope- it won't adjust... as a matter of fact, it will not have any idea that the airflow changed, and that's exactly where the problems would start. http://www.bassboatcentral.com/smileys/wink.gif

Just an added footnote: If the "scooped cowl" were used, and resulted in "higher than one atmosphere air pressure"... it would be possible that "MAP" or "TPS" faults could be set, resulting in "Engine Guardian" and reduced power. This would be due to a "mismatch" of actual TPS position vs MAP pressure vs Barometric Pressure at key-up. The tables used for fuel delivery are a "fixed table"... essentially there is a "expected range" for each of these values programmed into the calibration. If any one of the sensor's ranges falls outside of the "expected table"... let's just say: "You've got Faults".... and Guardian, and reduced power. That would be, at the least, counterproductive. http://www.bassboatcentral.com/smileys/crying.gif