Nass cowls - the effect of ram air
 

I thought I would dust-off my data from my road race days and try to apply it to my latest obsession, outboard Skaters and the effect of Nass cowls on the 300xs motor. I have attached some hypothetical calculations based upon proven equations, realistic engine metrics and tested constants. The goal was to quantify the probable effects of ram air on horsepower of a Mercury 300xs. Enjoy and give me your comments! Thanks, David

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QMan you are making my head hurt.

when I graduated high school I was going to be an engineer..now I remember why I changed my mind :faint2:

how did you estimate the manifold loss? and are these just hypothetical estimates or measured while an engine was on a dyno?
signed a junior undergrad en

Why deal with all that logic. You just need one of the 12 volt "electric superchargers" off of eBay and you will pick up at least a 10% horsepower increase.

As I stated in the thread, the hypothetical component is the manifold efficiency. I used a VERY conservative value for both scenarios . When it gets warmer and can test, I hope to get instrumentation to data log it. All other data/constants are real undisputed dyno results or just physics. I was hoping that someone would connect DBR's increases in HP to the increase of pressure from the ram air cowls, with a tweek in tune to add fuel and RPM, spark timing, etc. My conjecture with how DBR improves on a "tight" Mercury tune on the 300xs"s, though obviously very lean (easy to window one) is to increase air pressure demand tune.

10 thread cross reference points for you young man...

This is good stuff Taboma. How will data logging confirm the variable of ram air efficiency, and wether or not it does stabilize intake manifold pressures back to the atmospheric basis? I'm sure a ram air system could be built to achieve these results at 100 mph, but will the Nass cowls be enough to overcome some of the vacuum created at WOT? What about 110 mph, or 120 mph? Is there a constant for the equation?

Over my head for sure, but I'll be interested in the test results.

Hey guys to throw in one more variable we are assuming the cowling is in clean air coming of the boat. What is the aire flow over the boat is pushed up a bit. May not be as much?
This is good stuff though David.
I cant wait for us to test to the computers in a few weeks with Jim.

You're obviously way more technically knowledgeable than I am on this subject David, but I can almost guarantee you're not going to see any true "ram-air" benefits in performance. First of all, the cowls are not sealed, which would be necessary to provide true "ram-air" induction to the throttle bore. They have vents/drains in other areas of the cowl that allow air to escape, negating true "ram-air" effect. And if you rigged a sealed tube from the air intake to the throttle bore for true "ram-air" intake, you'd be risking ingestion of water if you ever took spray into the intakes (which does happen). As an example, we run the injector hats backwards on the K-boats we race out here in California so as to not injest water spray from other boats. When we go to try and set a kilo record, we don't turn the hat around to face forwards, as they go just as fast with the hats facing backwards, even though there's 140+ mph ram-air right into the butterflies when facing forwards! And, I can tell you from 15 years of experience with 28s that these rigs take more than small gains of 3-5hp per engine to even be noticeable on the speedo head, or in the seat of your pants. I do really like your enthusiasm and detailed knowledge of performance equipment though! Let us all know how she runs..........