Re: increasing horse power?
Standing in the middle here. Operating a surgery on a motor to increase factory standard power output is one side and leaving it as it is the other. Going through most threads on power differences of same displacement motors (for example the 40-55 HP 2-cylinder line, which is my case), what is being asked, is if and how the owner of the lower HP - restricted motor can open it up. Going for a racing motor is another matter and possibly would result to a non lasting motor which no one here wants.
On the cost side, say you are the manufacturer and have to produce a 2-cylinder line with 40,45,48,50 and 55HP outputs. What do you do? different displacement to each output? you're out of business !!! So you build up your motor to it's full capabilities and then shut it down for market needs. For us, the owners, it is secure to think that the less costly the manufacturer suppressed the motors the less costly we can open them up !!!
Searching part numbers we see that the 48 and 50 are exactly the same motors, the 45 and 55 differ on the carbs only, all four above wear the same reed leaf stopper, the 40,45 and 55 wear the same reed leaf (that basically concludes that not much differences on reed vales altogether between all motors, except the 40HP stopper which may be more restricting than the others), 45 and 55 wear the same crankcase and exhaust housing as do the 40,48 and 50 and finally 40 and 45 wear slim throat carbs with relevant orifices plugs and 48,50 and 55 wear the wider ones.
So for us 40HP owners who want that costless 10HP, wide throat carbs is one for start (got mine for free of a '85 scrapped motor). Reed valves nothing much other than getting rid of these stoppers altogether (retain the holder !!!). Did those two it on mine, got loads more torque and 300rpm top end. Had been worried of spitting out liquid fuel through the exhaust but no spark plug color is as healthy as before (light brown).
On the exhaust side we have the 40,48 and 50 wearing the same housing and then the 45 and 55 wearing a two part housing. Manufacturer doesn't like many moulds! and just as well, his low cost restrictions may be our low cost too! There is question here. Is the exhaust housings where the 5 more HP come from on the 45 and 55? ...(1).
Last are the crankcases. We have a part number for the 40, one for the 48 and 50 and one for the 45 and 55 HP. Shaft, bearings and all inners are the same so we're left with the manifold, port sizes and position ...(2). We have seen before that manufacturer does not like many moulds, and i would guess that the case is one mould one manifold. After all it is extremely tight in there.
On port sizes is where there may be a costless difference as they are cut individually. A not cross examined info i have, is that the exhaust port on the 50 with respect to the 40, is wider upwards in the cylinder sleeve by 0.048". That would give the 50 model extra top end power with a compromise on torque low and midrange which is anyway somewhat accounted for by the wider throat carbs.
It is possible that between two identical motors as far as parts are concerned, the 48 and 50, nominal power output difference is of timing adjustments only ...(3).
There are three interesting (to me and possibly others) questions here. It is not that i will be grinding the exhaust port on my motor, most of it is having sashed it out.
