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So, dyno-man, how do I get 100 HP?
There are lots of ways to get 100 HP. Oh, you mean from an RG500?
OK, still lots of ways.
In the end, you have to burn enough fuel (efficiently), in a short enough time span, to push those little pistons up and down and spin that crank hard enough and fast enough so a dyno tells you that you have 100HP. Seriously. Right?
This is basic, I know, but you have to understand what that 100HP means. HP is torque, times RPM, all divided by 5252. So, it’s a combination of torque and RPM. If you could make 100 ft-lbs at 5252 rpm, you’d have 100 HP. Or, you can make 25 ft-lbs at 21,008 and still have 100HP. Properly geared, those two bikes will top out at exactly the same speed. Huh!
How do you make torque? Push down hard on those pistons! As fuel burns, you develop pressure in the cylinder above the piston. That pressure pushes the piston down- it’s just that simple. Since the pressure in the cylinder during the burn phase varies a huge amount from start to finish, the math guys like to work with an average for the cylinder pressure- makes things simpler- what they call Brake Mean Effective Pressure, or BMEP.
It figures, there is a formula to relate BMEP to your torque output. And here it is:
2-Stroke BMEP = torque x 1.23/L
where L is displacement in Liters.
So you see, your torque is directly related to BMEP.
here’s another:
BHP = PLAN/33,000
P is brake mean effective pressure, in PSI
L is piston stroke, in feet
A is the area of one piston, in square inches
N is the number of power strokes per minute
Piston Speed
or 2-Stroke BMEP = (HP x 6500)/(L x RPM)
Thanks for the math lesson, Mister Science.
No really, what it boils down to, is how hard you push on the pistons, and at what RPM you do it.
We could get finicky and worry about reducing friction and inertial losses but let’s stick with making power, not how to reduce losses. Also, BMEP figures go along more with gross HP numbers, you lose a lot of power in the drivetrain, maybe 10-15% or even more, so let’s for the sake of the exercise, let's figure you’re actually making 10% more at the crankshaft.
BMEP tells you, "how much PUSH am I getting on those pistons?"
A two-stroke that breathes poorly, can’t draw a decent breath, has lousy pipes /poor scavenging and has its fresh charge contaminated with a bunch of old, half-burned residue from the previous cycle, is NOT going to make a lot of cylinder pressure. So remember, it’s how hard you push on the pistons, and at what RPM you do it.
Isn’t this where we started?
Maybe, but now we all know what’s involved, and we get to talk like fancy pants tech guys while we’re at it. We have two tasks ahead of us:
A) increase cylinder pressure (BMEP)
B) increase RPMs at which we develop this BMEP
Of course, the easiest way to have 100 HP is simply claim that you have 100HP. It works for a lot of people, and they sleep well enough at night, so this is a proven approach! But, that doesn’t work so well when accurate dynos are involved, so let’s actually make the 100HP and get it over with.
An average, stock Gamma makes about 75 net (wheel) HP at 9500 rpm and about 42-43 ft-lbs of torque. That = ~83 gross (crankshaft) HP. 83HP @ 9500 rpm gives us 113 psi for our BMEP. Which is very poor, by the way.
A new 250MX bike will make 46 HP @ 8000 rpm. So, 51 Gross HP @ 8000 = 165 psi. The BMEP plot will be exactly the same shape as the torque plot, so a 250 MX bike is VERY GOOD at turning gasoline into force against those pistons, and it does that over a good rev range as well.
I have a dyno chart from a stock 1994 TZ250 making 78 HP at 11600 rpm. Figure 86 gross HP, = 192 psi! And the latest 125 2-strokes are somewhere around 215 PSI, which is just staggering. The kicker, though, is they can do that at 13,000 RPM, resulting in ~53 hp from a 125. A GP engine is extremely good at turning fuel into cylinder pressure, but generally only over a narrow rev range. It has been tuned for MAXIMUM BMEP and to do that it has to sacrifice low end, etc. The 113 psi RG500 is exposed for what it is, a very low-output, inefficient, strangled motor.
That stinks!!!!
Yeah, but the good news is, we have a LOT of room for improvement.
Suppose we just change things around so it revs higher? Why not put on some screaming pipes? Well, the RG doesn’t breathe all that well at 9500 rpm. Will it breathe any better at 10500 or 11500 rpm? Maybe yes, because a better exhaust pipe can help draw air into the motor as well as pump better on the exhaust side. But, trust me, your BMEP is not going to go up just by making the engine spin higher, in fact, it usually falls if you try to make the same old ports, carbs, etc do their job at even higher RPM. Try sprinting while breathing through a clogged dust mask. The faster you go, the worse it’s gonna be.
But, lets say we can put on some pipes and keep our BMEP of 113psi (47 ft-lbs gross) as we raise the RPM.
To make 110 gross HP (100 net HP) with 47 ft-lbs, we will need to develop 47 ft-lb at , oh... 12,230 RPM. How does that sound?
By the way, I have seen dyno charts from RGs making 100HP at 11,800 and such. It’s pretty scary. That’s just being mean to your crankshafts. And nobody likes mean people.
OK, just revving higher isn’t the answer. But if a 250MX can make 160 psi, why can’t we?
Well, those 250s do have the latest cylinders, etc. But a goal of 150 psi for an RG is entirely realistic.
To make our 110 gross HP, with 150psi BMEP, (that’s ~ 61 ft-lbs gross, or 55 ft-lb net) we will only have to spin 9470 RPM.
I like the sound of that MUCH better.
So, there you go- how do you make a 100HP Gamma?
Just produce 55 net ft-lbs at 9500 RPM!
8-)
You Dork!
That was no help at all!
Hey, my 516cc bike made 100 net HP at 8500 RPM with the RG cylinders. And went on to 127 net HP/ 66 ft-lbs at 10200 RPM (that’s about 180 psi) . Trust me, this stuff works. I’ll show you- read the engine tech pages - that would be under “teaching a man to fish”!
100 HP from your RG500