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Topic Review (Newest First)
06-02-2011 11:16 AM
john1edwards
thanks

Thanks for the replys, I thought about a gearbox (reduction drive) Right now I am running a 250 inline it does surpriseingly well. I have a 292 on the garage floor and ready to put in boat. will post video one day. I do have an adjustable pitch prop. currently set were it turns max 2600 rpm.
05-28-2011 07:31 AM
curtis73
Quote:
Originally Posted by techinspector1
A gearbox is the first thing that popped into my mind. Heck, set it up with a 3-speed manual trans (low buck) and run it in second gear. You could also run it in first gear for low-speed, high-torque. Or run it in reverse to back up. "Hey dude, check it out, my airboat backs up". .
I had a similar idea, but it involved a powerglide
05-27-2011 06:28 PM
ap72 Make the tq curve run parallel to the prop efficentcy curve, whil making it as high as you can under 3000prm.
05-27-2011 03:48 PM
crowerjunky call dave at waterthunder performance his site is www.waterthunderperformance.com
05-27-2011 03:43 PM
techinspector1 A gearbox is the first thing that popped into my mind. Heck, set it up with a 3-speed manual trans (low buck) and run it in second gear. You could also run it in first gear for low-speed, high-torque. Or run it in reverse to back up. "Hey dude, check it out, my airboat backs up". Build the motor at 9.5:1 with a tight squish and Crane 204541 cam, 216/228, 112, 0.530"/0.560" power range 1800-5400.
http://www.summitracing.com/parts/CRN-204541/
For coming right off the crank to the prop, the stock cam will probably produce the best torque numbers at 3000.
05-27-2011 03:02 PM
curtis73 F-bird... that's a fine idea, but airboats like to explode props above about 3000. He's limited to about 3000 prop RPM.

If you're intent on sticking with a direct drive, your engine needs to peak its HP at 3000. That means a tiny cam. If you increase your HP at 3000 rpm, then you will (like F-bird says) increase prop pitch to keep it at 3000.

For the most part, direct drive applications that want more power require more cubes. The best way to increase torque (and consequentially HP) when the RPM limit is fixed is to increase displacement. That's why most direct drive airboats use big blocks. Most small block airboats with big power need to use a reducer gearbox. That way you can spin the engine to 5000 and make nice power while maintaining safe prop speed.
05-27-2011 12:20 PM
F-BIRD'88 If you change the torque curve of the motor by changing the camshaft, etc you may have to change the prop pitch.

If you get carried away with the cam selection you may loose 10hp @3000.

Big cams trade low end power for more top end power.
3000 rpm is mid low rpm.

Larger cams need more compression ratio. and tend to enhance power at higher rpms. Don;t get carried away.

If you really want more engine power, you need to improve the induction and exhaust system (porting, larger valves, better intake, exhaust etc) to go along with a new cam.
(the cam is part of the induction, exhaust system, it all has to work together)

if the engine then makes a good bit more power @3000 rpm and the prop must run at 3000rpm (limit) then the prop will need to be changed (pitch, diameter).

You'll need a accurate tach and a radar gun to evaluate your boat's performance change after moding the motor.

Many of the cam companies have high perf cams for this motor even if you don;t see it in the catalog. Call a few of them for tech help.
They have probabily dealt with this before. You are surely not the first to want to cam up your air boat.
05-27-2011 09:47 AM
john1edwards
292 cam on airboat direct drive

putting an inline 292 on airboat. prop will turn 3000 max. if I spent money got a cam made. how much could I increase horsepower at 3000. Would it be 10 horse power or more. maybe horsepowr is not the hole ball of wax. maybe its torque. any thoughts.

I know you can get lots of horse power by increasing rpms, but don't have that luxury.

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