Here we go...
As usual, dyno testing for me brought about excitement and nerves! Finally, I get to see what this engine concoction is capable of, but, was it all worth it?! What if it is a total flop?!
Thankfully, the first pulls (4 runs per test) showed an ok power curve right away - 21hp peak with a nice curve - meaning jetting was pretty close from my guess work. I thought ok, well, it’s more than the stock 18hp (same engine - albeit stock, same dyno, 6-7 years ago) but it’s less than that engine made with the Jadus header, intake kit and advance brackets - that made 22hp. After the test the dyno operator noticed that the rear tyre smelt a bit and after inspection was really warm and sticky - huge friction losses. This was because the air pressure had gotten down to 18psi! For consistency's sake, all tests are done with 35psi rear tyre pressure.
Once we had filled the tyre with air, we ran another test, with the fourth test being a drivetrain loss test - the operator cuts throttle and pulls in the clutch instantaneously, waits for the dyno drum to slow some, then releases the clutch. The program then can measure engine power at the crank. The results were much better! This time, 24hp peak, again with a nice curve, and around 28-29hp at the crank.
The next 8 tests (32 pulls total) consisted of:
-With and without foam air filter
-With long intake set up (similar to Jadus intake kit, just shorter for higher revs for this engine)
-With short bellmouth/velocity stack
-With and without the PCV system hooked up
-With a 115 main jet and a 120 main jet
-With the jet needle in positions 3 and 2.5 (dropped to next position down but shimmed with a washer)
-With and without the Jadus ignition advance kit (stock timing vs 5 degrees advance)
-With screwdriver held over exhaust opening (covering 10-15% of it)
-A few tweaks of the air/fuel screw
The shorter bellmouth/velocity stack lost power, as did running no filter. The explanation being that yes, more air may flow at high rpm, but there is less of a vacuum signal at the carbs venturi to lift the right amount of fuel from the emulsion tube. Plus, the shorter bellmouth might not allow the airflow to be fully laminar when it hits the venturi. Plus the length (if calculated correctly) of the longer intake should give the 3rd harmonic effect.
The PCV system didn’t do anything, so that is going in the bin, but hey, worth testing the idea. Once we disconnected it again, I put my finger over the crankcase ventilation opening on the engine throughout an entire dyno pull just to feel what happens there and the vacuum I felt was marginal, so very little to be gained there. These kinds of systems must have more effect on larger displacement and or multi-cylinder engines.
Changing back to stock ignition timing lost power everywhere as well - about 1hp up top but across the whole rpm register and more down below 6000rpm. The operator thought that the cam I chose might want less timing but that theory didn’t hold. Worth testing though to put that one to bed!
The bigger main jet (120) smoothed the curve slightly above 7000rpm but worsened it slightly below, so that is why we adjusted the needle position slightly and that made the curve better everywhere.
The screwdriver over the exhaust opening is a classic test. Holding it there blocks about 10-15% of the opening and this allows you to see if it makes more power like that - meaning you have got too little ‘back pressure’ in your system for what the engine wants. Fortunately in this case, we lost power everywhere, meaning the system is correctly dimensioned (could probably be even better)?
Then we did a pull through all of the gears. What we saw was surprising and awesome. The bike was making over 26hp in third, dropping off to around 24hp in 5th. Meaning the gearing I calculated is way too tall. So the remaining few tests we did all in 3rd gear and managed to get a peak power figure of 26.8hp (DIN) at around 9000rpm and peak torque of 25.3Nm at 6500rpm, with over 22Nm of torque from 4500 to 8500, a pretty nice spread of power! Also, in the same final runs we did the drivetrain loss test again and it was consistent with the first test - 29.5hp at the crank.
With the gearing I chose, the bike could spin the dyno drum up to 190-200kph (118-124mph) but in real life, with wind resistance, it won’t have the power to get there. With the dyno software, you can lay over a graph of wind resistance and see how much power you will need for what speed with what type of bike and the prediction was that I would need around 24hp. So with over 26hp it should be achievable, but I need to do something about the gearing. The fact that it can reach 130kph in third is just wrong. I have checked a few calculators again and I will be ordering a 14, 15 and 16 tooth front sprocket to test. That should put peak power rpm closer to 160kph instead of 190kph.
Are these good results? One thought was, ‘all that work for almost 27hp’?! Then again, looking at rules of thumb, 100hp/l is still considered good for NA engines, so 29.5hp (at the crank) for an air cooled, 263cc 2-valver is not that bad! I also wonder which modifications gave the most effect… Or would it have always been the combination of all of the mods together? 26.8hp at the wheel is around 46% more power than the stock 18.2hp (again, same dyno) and around 20% more power than the same engine with the Jadus bolt on mods at 22.7hp. All of these numbers are pretty close to my predictions as well, so with that said, I am pretty happy. This together with the weight reductions makes the bike pretty fun to ride and it handles very well. Now just to see if it will do the ton...