Rapido 250 TUNE

[Darrell Taylor]

Ive had last few days developing/testing a rapido 250 tune that i did a few month ago
motors now got 100+miles on it after doing the whitby run so freeing up a little
the customer had an existing rapido 250 with a 35 carb and reed valve fitted with a boost port added so made a baseline for comparison

heres the starting point 21.59 bhp 17.75 torque
rapido 250 +35mmcarb reedvalve and boost port/piston windows jl3 pipe

[Darrell Taylor]

the customer wanted a powerful touring motor with the use of his old reedvalve and 35 carb transferring to his new motor
brief given i set about porting the cases and cylinder to standard gasket sizes only including both the inlet and exhaust ports matched to gaskets only but added some of my case porting
head was machined to suit the tune
cylinder was ported to a conventional style single ex port 4 transfer with only a single boost port added as be aware the next oversize piston has pegs in a different location to the standard piston so wouldnt be any use doing 2 or 3 boost ports come rebore time to find barrell is scrap
the first dyno runs were done on jl3 and showed the following results

[Darrell Taylor]

this was next test with franspeed race fitted

[Darrell Taylor]

wasnt sure about ignition so tried bit of adv and retard along with tests static with augusto disconneted to no real gain,out with strobe and found ign was advancing up when running static but ok when augusto fitted,eventually tried a few parts and found a replacement flywheel helped out but still have suspicions a full new ignition system would help out

[Darrell Taylor]

decided the kit would benefit from some extra duration on transfers and exhaust so stripped top end down and did another hours porting
heres result 29.35 bhp 23.29 torque

[Darrell Taylor]

Heres a few graphs overlaid which shows progress from std to finished tune
theres more to come as can be seen no loss of low down power/torque has resulted from the extra porting work but thats as far as im going as now ticks all the boxes.

[Darrell Taylor]

this runs on approx 4.2 gearing and pulls all gears with ease a few more miles needed to see what it will do in top with sat nav
this motor has prompted me to do some power graph comparisons which work as a better comparison of engines
thought id check power at fixed points at 4k 5k 6k 7k 8k on a number of motors and was quite amazed with the results i found ,despite nearly 30bhp not sounding as good as some of the 35-37 motors out there its low rpm power figures are top notch

15bhp between 4k and 5k 15 torque
25bhp at 6k 22 torque

[corrado]

25 bhp at 6,000 rpm is impressive with that gearing it must be a great motorway cruiser.

i normally start runs as a motor comes onto pipe for a franspeed race approx 5200rpm but im often dropping to 3000rpm to please graph critics

I wouldn't class myself as a critic but as most of my riding is probably sat around 5,000 rpm on A roads, so the 5 - 6,000 rpm range is of far more interest to me as far as real world riding is concerned.

[Dimitrios_231]

This is very impressive Darrell,
well done.

What transfer timing have you chosen?
Is it higher or below 130 degrees?

Cheers.

[Darrell Taylor]

thanks Dimitrios
transfers are above 130 but also has huge transfer areas more than any other cylinder ive done before as the big bore/ long stroke is allowing greater width and depth,i do like the rapido and its bang for buck ratio is very appealing, would have loved for it to have been just piston port too instead of chucking that restriction in the way
ive compiled a dyno results comparison chart that tells a better story of how a motor performs compared to others and it makes for very interesting reading ,quite often the bigger picture is clouded by a high or low bhp peak figure and think it warrants a seperate topic for discussion
perhaps eden can compile a chart where figures can be added to and updated