Pro Motor Cowling Design Question

[Al Peffley]

Most Pro racers have an airdam panel inside their fibergass or carbon fiber composite motor cowling to reduce or control the airflow into forward-facing carbs. I noticed that many Europeans in O-250 have a fairly large front air scoop opening on their cabover-designed Pro hydros. My guess is some of the Euro 250 teams have an air filter panel or an adjustable door panel inside the scoop to control intake air flow to the carbs.

How does one select the right airdam design for a new motor cowling? How does one calculate the optimum distance from the front of the carbs for the baffle location? Too much air & you stick it, too little air and you load the motor up in certain climate & running conditions...

Any fresh thoughts on this carburation air feed subject? One local person here suggested a space of about 11 inches between the front of the carbs and an internal airdam panel to break excessive air flow coming in from the front opening of a cowling. Is there a good book on fairing intake designs (car or boat) that I could read and get more educated on this subject?

Al
(not an Aerodynamics Engineering kind of guy..)

[Dave M]

Simple

Call the motor manufacturer (ie: Rossi, VRP, Etc) and ask them. They will let you know what the optimum distance is, and then you can place your deal according to what is best reccomended. If memory serves me right, for the 3cyl Rossi 350CC it is recomended to be between 8" - 12". Also, I don't think the intedned purpose of the part is to restrict air flow....... it does something different than that.

[B1PRORACER]

engine cowl....

I learned the hard way when I went from Stock racing to PRO racing that you had to do something about a spray shield. Early on, I would go into a turn and get washed out and stop...! I hated that! I wanted to race anywhere on the race course and not worry about getting washed out. That is what a Stock racer takes for granted!
Here is what I found: a 250cc engine for example pumps approximately 15 cubic inches of air per revolution and it does it 13,000 times per minute. That is 195,000 cubic inches of air per minute that your engine sucks in. The engine is a vac*** cleaner! It pulls in air from everywhere, even from around corners! Problem is- if there is any water in that air that it is drawing in, you will stop.
You have to do something to remove the air from the water- best if the engine doesn't draw air from outside the cowl. An additional problem to all of this is the fact that there is always a lower pressure area behind any blunt object that is dragged fast through the air. This lower pressure will act to draw air from around the outside area of the cowl.
Just being able to cross over close behind another boat in a turn with out being washed out is a performance increase. What the europeans are doing probably isn't a speed increase, but does help performance.

[Al Peffley]

Euro F250 Cowl Designs

Does anyone out there have information or specific drawings on how the European cowl designs are configured for the O-250 class?

Al

[GRM]

Hello,the cowl is very important,the follow photo is from a new Betta boat 250 and the cowl is the best design wich we have in Italy,tomorrow more photo with more light...

[Al Peffley]

Thank You Very Much!

Great photo! EXACTLY what I needed to help me fit my new cowling and Rossi motor on my existing 250 hull. I look forward to seeing any additional photos you have to offer to me. How many centimeters or millimeters deep (approximately) should the rear cowling bulge's nose be in respect to the front of the motor's two carburetors? Does GRM offer exploded view parts diagram(s) of the GRM Rossi 250cc powerhead that I may purchase from you here in the US through Rex Hall?

Best Regards,
Al

[sam]

The parts list is online: http://www.grmracing.com/G250.asp

[Al Peffley]

Drawing

I have that list already, but thanks for the website hotlink tip...

Al

[Al Peffley]

Polland Euro Boat Picture with Crew Working on Engine

I noticed a large grey duct tube and black plenum air box in front of the carbs on a F-250 hydro at the recent UIM races in Polland (was it the Demmler Team boat?) Do other European O-series hydroplanes have this same type air induction control system installed inside the rear cowl/fairing?

Al

[Mike Schmidt]

No...

No, most of the Euro cowls are just wide open on the inside. The 125 of Rossi's that won the Worlds for the last two years is just wid open. No air dams at all.

I tried this in the past with my pistom port Yamato and did not finish a lot of races. I guess the modern reed valve Italian motors can digest a lot more water.

Michael D-1

[reed28n]

Gas

The parts list is online: http://www.grmracing.com/G250.asp

What happens if you run ths motor on gasoline alone?

Would it run?

Would the pipe be different?

[sam]

What happens if you run ths motor on gasoline alone? Would it run? Would the pipe be different?

I think you mean gasoline and oil; no it will not run on gas unless the powerhead is modified. The pipe would be the same.

The compression ratio is too high to run on gas, it might start esp. if you can change the timing to TDC or very close, but would likely overheat and stop in a minute or two.

A temporary mod to run on gas would be to add some kind of shim to the head gasket to lower the compression ... the permanent best power mod would be to alter the cylinder head to lower the compression and keep more of the original power.

If you are thinking of this kind of motor as a future Stock racer ... I'd say it is possible in these days of CNC machining. If there was a set of computerized plans to feed the CNC machines and they could be done by machines owned by boat racers or their friends during off peak time (over night, third shift etc). The price could be affordable to some, but it would never be "cheap". A lot of CNC equipment is so expensive that it is run 3 shifts a day to pay off the loans ... but some cleaver guys retrofit existing used machines to run CNC.

[Lights]

What happens if you run ths motor on gasoline alone?

Would it run?

Would the pipe be different?

They will run great on gas with a simple jet and timing change. We fire them up on gas and oil after a day of racing all the time to flush out the methanol and caster.

Mark

[sam]

Firing to clear the alcohol and running on a boat under load are different