Hi Kris, I stand corrected on the HP issue. As for the load concern you expressed, We have already taken that into consideration. The functioning of the PRSU (easy short way of writihg prop re-drive) can be likened to a two bicyclers. One is a huge bruiser who is pumping slowly up a steep hill pounding it with his tree trunk legs which have to endure tremendous strain. If a skinny little shrimp were forced to do this, (besides the fact he couldnt) It would breack his legs pull muscles etc... But give that same skinney little guy a bicycle with the proper gears and he could easly keep pace with the big bruiser and would not be experiencing / feeling the strain like his big friend would. His little legs would be spinning like mad but the "Strain" would not be "felt" the same.
Clear as mud right
I have many friends using auto engine conversions with PSRUs for many hours of flight time with no problems. Some of them are running 100hp. + VW engines with 2.47/1 ratios and turning BIG props just fine with excellent reliability. If it weren't for the PSRU your concern about load would be very valid. As it is an engine used in an aircraft application would not have as long a life but it will still serve nicely.
Regards;
Troy W.
OK Troy, I'm a bit confused. A PSRU, or "Propeller Speed Reduction Unit" is really just a fancy name for a single ratio gear, chain or belt reduction unit. You won't increase the HP by reducing the RPM. What you WILL do though is increase the efective propeller torque at any given engine RPM by using a PSRU. If you look at the following formula, you can see that while the engine is producing a constant HP at a specific crank RPM, the PSRU will affect the TORQUE and not the HP that is applied to the prop.
TORQUE = HP x 5252 ÷ RPM
So, if your engine is producing say 150HP at 3300 RPM, you would be producing 238.72 lb-ft of torque.
150 X 5252 ÷ 3300 = 238.727272.....
If you reduce that RPM by say 50% in a 2:1 reduction PSRU The formula would look like this:
150 X 5252 ÷ 1650 = 477.454545.....
Yor torque at the prop would then be 477.45 lb-ft, but the HP is still 150.
I'd be concerned about one other bit of info that I found concerning the load model of an automotive engine in an aircraft application:
The automotive load model deserves mention here, because some products use critical parts taken from automotive systems. Be aware that, depending on the aerodynamics of your vehicle, it only takes between 30 and 60 HP to move your car along the road at 60 MPH. Doubt that? My station wagon gets 21 MPG at a steady 60 MPH. That's a fuel burn of 17.1 pounds per hour. Assuming a BSFC of 0.42, the engine is producing 40.8 HP at that fuel burn. Certainly, more power is required for acceleration and hill-climbing, but if you think about it, most of the operational time in your vehicle is spent in some form of cruise.
Most automotive propulsion systems are designed around a load model in which the maximum rated load occurs during only 5% to 10% of the design life, and 75% or more of the design life is at less than 25% of maximum output. Compare that model to an aircraft load model in which a large portion of the operation (90% or more) is at an engine power output of more than 75% of rated engine power. Contemplation of that fundamental difference should give an insight into why automotive parts are generally unsuitable for use at their rated level in aircraft applications
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Not trying to discourage you, but that 250 may not last long at those kinds of loads.
Kris[/quote]
Does anyone know what harmonic balancer I can use on a 250? Will a Ford 302 one fit? I can't seam to get my local parts store to find one :roll: 
I'll be using a drive from Tracey Crook. 
