My beloved Aprilia RS50 acquired a new Doppler Vortex 50cc kit recently. I bought it because I liked the look of the wide bridged exhaust port and the wide and (I believe) nicely-angled transfer ports. It's advertised as a road kit so it (as I have discovered) has relatively mild port timing.
There's a video showing the inside of the kit here:
https://www.youtube.com/watch?v=mUeeycpkpc8 .
The standard Aprilia top end with some exhaust port mods made a bit over 11hp on the ESE dyno. So we put her back on the dyno and, lo and behold, with the Doppler kit she was making just under 10hp with a dip in the power band the size of the Grand Canyon.
But ... we took advantage of the large number of exhaust pipes to be found in the ESE workshop to see if a different pipe might conjure up another pony or two.
So the Conti pipe (which had been largely responsible for an earlier increase from 5hp to 8hp) came off and another pipe went on. Still a large dip in the power band, but substantially more power. Back up to just over 11hp. This pipe was a bit large and only fitted a couple of mm into the barrel. So we shaved a bit off the OD so it would fit in better.
It was probably about 10-12mm shorter when properly fitted, and was a disaster. Back down to just over 10hp. What?? Okay ... so we tried a different pipe. The result was MUCH better. Nearly 12hp with a much nicer looking power curve.
And then for the hell of it, we tried the original Conti pipe with a shorter adaptor between the pipe and the barrel (probably by about 12-15mm). Based on our previous experience of a longer pipe being better, I bet that this shorter adaptor would make things worse. WRONG!!! There was still a chunky dip in the power band, but it was back up to nearly 12hp.
The moral of the story ... the proof is in the dyno! Below centre is a photo of the 5 dyno charts. You can see the range of different results obtained from 3 different pipes, with two of them tried twice each with shorter or longer adaptors. The maximum power varies by around 20 percent, but 4 out of 5 have a similar dip. One doesn't. This is very promising.
The photo on the left illustrates the huge difference that just having a 10mm or so longer adaptor between the pipe and the barrel can make. The photo second from right shows the two adaptors.
So on went the degree wheel and some considered measurements were taken with the result being that we're going to raise the barrel a bit with a spacer, machine the head to fit it into the top of the cylinder. Tweak the squish band and stick her back on the dyno. More exhaust port duration is apparently what is required here.
Two-strokes and dynos ... a match made in, well, if not Heaven, then at least Wonderland. Tim
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