Power, Rpm, Torque - Horsepower increase for my small engine

In summary: The current setup, which has a centrifugal clutch and two sprockets, is not powerful enough for my needs. The main factors affecting engine power are brake mean effective pressure, mean piston speed, total bore area, and volumetric efficiency. Doubling any of these factors will double the power, but since stroke and bore cannot be changed, the best options are increasing BMEP through fuel or oxygen, increasing mean piston speed at the cost of reliability, and improving volumetric efficiency through turbo
  • #1
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Hello all, the short story is, I am basically trying to get the same ( or a little more) “power” out of a larger lawn mower type gas engine ( vertical shaft) as one of the largest (120 ish cc) chainsaws ( ref the Stihl MS 880 or newer 881 at approx 10-12k rpm and 9 Hp)
The parameters are : a relatively small form and weight as close to that of the chainsaw as feasible. (but I realize at least double is likely). With off the shelf parts for the most part. I have already built an aluminum “power plate” to hold such a motor and trans which simply has 2 bearings and 5/8” shafts to transfer the power with a centrifugal clutch on the motor side with an approx 2 inch sprocket and an approx 8” sprocket on the driven side. It does not have enough “ power” to drive my machine. TIA to all who participate.
 
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  • #2
The engine power is directly related to:
  • Brake Mean Effective Pressure (BMEP)
  • Mean piston speed (= RPM X Stroke)
  • Total Bore area (= piston bore area X number of pistons)
  • Volumetric efficiency
Double any of those (without altering the others) and you will double your power. Or increase each of them by 19% and you will also double the power.

Stroke and bore seem to be what you don't want to change so this leaves:
  • BMEP: Doubling it is very unlikely. Your best bet is changing the fuel and/or adding oxygen (i.e. injecting nitrous).
  • Mean piston speed (through the RPM): Doubling it is very unlikely because it is usually optimized for power vs reliability. But if you can accept losing reliability, you might get some room here.
  • Volumetric efficiency: Probably your best bet overall. It is usually done with force induction (turbo or supercharger). But if your engine intake and exhaust systems are not already tuned, redesigning them would make a big difference without any reliability consequences.
Increasing each of these three things by 26% will also double your power.

Except for intake & exhaust systems tuning, any modification will most likely require stronger components (piston, connecting rod, crankshaft, etc.) for great power increases, or at least machining with tight tolerances (blueprinting).
 
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  • #3
Wow. Great response. Thank you.
 
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  • #4
ThinkR said:
Hello all, the short story is, I am basically trying to get the same ( or a little more) “power” out of a larger lawn mower type gas engine ( vertical shaft) as one of the largest (120 ish cc) chainsaws ( ref the Stihl MS 880 or newer 881 at approx 10-12k rpm and 9 Hp)
The parameters are : a relatively small form and weight as close to that of the chainsaw as feasible. (but I realize at least double is likely). With off the shelf parts for the most part. I have already built an aluminum “power plate” to hold such a motor and trans which simply has 2 bearings and 5/8” shafts to transfer the power with a centrifugal clutch on the motor side with an approx 2 inch sprocket and an approx 8” sprocket on the driven side. It does not have enough “ power” to drive my machine. TIA to all who participate.
jack action said:
The engine power is directly related to:
  • Brake Mean Effective Pressure (BMEP)
  • Mean piston speed (= RPM X Stroke)
  • Total Bore area (= piston bore area X number of pistons)
  • Volumetric efficiency
Double any of those (without altering the others) and you will double your power. Or increase each of them by 19% and you will also double the power.

Stroke and bore seem to be what you don't want to change so this leaves:
  • BMEP: Doubling it is very unlikely. Your best bet is changing the fuel and/or adding oxygen (i.e. injecting nitrous).
  • Mean piston speed (through the RPM): Doubling it is very unlikely because it is usually optimized for power vs reliability. But if you can accept losing reliability, you might get some room here.
  • Volumetric efficiency: Probably your best bet overall. It is usually done with force induction (turbo or supercharger). But if your engine intake and exhaust systems are not already tuned, redesigning them would make a big difference without any reliability consequences.
Increasing each of these three things by 26% will also double your power.

Except for intake & exhaust systems tuning, any modification will most likely require stronger components (piston, connecting rod, crankshaft, etc.) for great power increases, or at least machining with tight tolerances (blueprinting).
I guess what I am asking is, for those who really know the math, will the 16hp John Deere be able to make a little more power ( however calculated) than the Stihl Ms 880 chainsaw without major modifications or is there another powerplant out there that will do the job. What I am trying to avoid is the cost ($2000) and noise of the big chainsaw.
 
  • #5
ThinkR said:
I guess what I am asking is, for those who really know the math, will the 16hp John Deere be able to make a little more power ( however calculated) than the Stihl Ms 880 chainsaw without major modifications or is there another powerplant out there that will do the job. What I am trying to avoid is the cost ($2000) and noise of the big chainsaw.
If you have a 16hp John Deere, it’s already about double the stock HP of that chainsaw?
 
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