Bang for buck Part 3

Exhaust and fly­wheel.

Scootering - - Contents -

Hav­ing al­ready run the stock 200 en­gine through a se­ries of tests and mi­nor ad­just­ments in Part 1, and then ad­dress­ing the nec­es­sary base-level up­grades in Part 2, we now look at a batch of tests which were car­ried out to take the en­gine to the next level. When do­ing this it is im­por­tant to get a fair com­par­i­son of re­sults on the same day with sim­i­lar con­di­tions, which are both me­chan­i­cally and en­vi­ron­men­tally in­flu­enced. The dyno has a ‘cor­rec­tion fac­tor’ which is ap­plied by the soft­ware, which takes read­ings of the room tem­per­a­ture, hu­mid­ity and air pres­sure via a probe. This is be­cause such fac­tors af­fect en­gine per­for­mance, so if con­di­tions one day are ex­tremely good for power (like high air pres­sure) then a higher power read­ing will be dis­played if a cor­rec­tion fac­tor is not used.

How­ever, a test done a week later with low air pres­sure may show 1bhp less. With no other changes the cor­rec­tion-fac­tor in the soft­ware goes some way to bal­anc­ing these changes out, to pro­vide a fairer com­par­i­son be­tween tests. The un­cor­rected ref­er­ence fig­ure is sim­ply shown as 1.00 which means 100% of the ac­tual power fig­ure achieved will be dis­played. So if the con­di­tions in the dyno room force a change of read­ing via the cor­rec­tion fac­tor soft­ware (which is fairly mi­nor) it will of­ten re­sult in the cor­rec­tion fac­tor be­ing a lit­tle lower at say 0.98 or a lit­tle higher at 1.05 for ex­am­ple. These fig­ures are of­ten dis­played on dyno-graph print­outs if you hap­pen to spot them .

When try­ing to eval­u­ate small changes it’s also im­por­tant to check a num­ber of small items, in or­der to achieve con­sis­tent fig­ures and re­sults which are com­pa­ra­ble. Tyre pres­sure was set at 30psi for all tests, and it is also im­por­tant to do a few ‘dummy runs’ first to al­low the en­gine tem­per­a­ture to sta­bilise as the very first runs that are done on colder crankcases

on air-cooled mo­tors are of­ten a lit­tle bet­ter. The strap­ping down of a scooter or the seated/non seated test­ing (or even dif­fer­ent tester/op­er­a­tor weights) can make a change on some dyno sys­tems, par­tic­u­larly those that have a poor knurl­ing on the drum which causes lots of in­ac­cu­ra­cies. But hav­ing done back-to­back tests on this sys­tem it dis­plays no change al­though a con­stant seated dyno run was used through­out any­way.

The first month’s tun­ing ar­ti­cle left off with a test of the Ancillotti Club­man pipe which had been length­ened on its slip joint, so to start the day with a base­line run I de­cided to test length­en­ing the header pipe even fur­ther to its max­i­mum length on the slip joints (read­ing 5 in red: Ancillotti max length) this moved peak power down the rev range to 6400rpm and showed a power in­crease at 13.62bhp with 12.7lb-ft torque. The torque fig­ure see­ing the big­gest jump as torque is al­ways higher when bhp is made lower down the rpm range.

Next test is the in­ter­est­ing one – fit­ting an ex­pan­sion pipe to see what it would achieve. The pipe fit­ted was the TSR Evo in its stan­dard sup­plied form as I felt its long tuned length would work well, see­ing as the pre­vi­ous Club­man had re­sponded so well to be­ing length­ened. Fit­ting up the exhaust stub showed was an okay match to the SIL cylin­der which was a bonus.

First im­pressions felt promis­ing, in fact very promis­ing… un­til the first prob­lem struck, namely a very small touch of clutch slip from the stan­dard SIL clutch assem­bly. This in­for­ma­tion be­comes quite note­wor­thy as we now have a power/torque fig­ure to help with the de­ci­sion at what point a STD SIL clutch will need up­grad­ing or re­plac­ing. Rather than pause now to change clutch plates, just when things were get­ting in­ter­est­ing, I de­cided to carry on with the test and gen­tly built some heat into the plates by slowly build­ing up the power and dump­ing the clutch re­peat­edly to scrub the oil film off.

That’s some re­sult as we are now in the realms of re­sults seen from some af­ter­mar­ket kits and around just 25% down on the most pop­u­lar 225 kits, yet giv­ing away 25cc to them!

I didn’t want to change any­thing yet as it’s the last of the batch of tests which I’d call ‘bolt on or bolt off’ and knew the up­grades (which were cov­ered in Part 2) would get done any­way. Now the dyno is set to record three runs in each batch, to al­low the ef­fects of heat on the case and pipe to build and sta­bilise, this gen­er­ally sees power peak on the sec­ond run, and then sta­bilises on the third. The clutch still slipped a lit­tle on the first run but man­aged a sec­ond and third run with no ap­par­ent slip. The re­sults were very pleas­ing as seen in graph test read­ings of 15.33bhp and 15.41tq, but in­ter­est­ingly it was at a very low 5400rpm hence why we have an al­most match­ing power and torque fig­ure. To re­cap on the mo­tor’s spec, it’s a stock SIL 200cc mo­tor us­ing a stock carb which has been re-jet­ted to suit, run­ning through a breathe-sweet type air fil­ter. It has a com­pres­sion in­crease and squish clear­ance re­duc­tion as a re­sult of re­mov­ing the stock head gas­ket, and an ex­pan­sion pipe fit­ted. That’s some re­sult as we are now in the realms of re­sults seen from some af­ter­mar­ket kits and around just 25% down on the most pop­u­lar 225 kits, yet giv­ing away 25cc to them!

Next test was the re­moval of the orig­i­nal heavy/stan­dard fly­wheel which weighed 2.65kg and has tall cool­ing fins, as can be seen in the pic­tures. I re­placed it with the pop­u­lar mid-weight ver­sion at 1.85kg which also has short cool­ing fins, to see how it per­formed. I had got the clutch slip tem­po­rar­ily fixed with a bit of scrub­bing in and backed the cable right off too, to make sure the runs were ac­cu­rate, and they were again fine on the later sec­ond and third runs. The re­sult was quite in­ter­est­ing, as per­for­mance was very sim­i­lar up to peak power (5400rpm) but from there to 7500rpm the lighter short-fin fly­wheel was around 1bhp up.

This sug­gests that with in­creas­ing rpm, either the air drag/re­sis­tance of the longer fans blades is in­creas­ing, or the ex­tra weight is sap­ping the mo­tor’s power. But in either case, at this spec, it’s of lit­tle ben­e­fit at all. The small im­prove­ment in the over-rev area will never be used on the road, as power has fallen to well be­low what it is us­able, and rapidly drops away past 6000rpm. Al­though on a higher rpm set-up it’s easy to see how and where a use­ful power im­prove­ment can be made.

Ad­di­tional tests were done at this stage to both jet­ting and ig­ni­tion tim­ing to check if either op­ti­mised the new pipe fur­ther, but there were no fur­ther gains found at this stage. It would have been nice to try a dif­fer­ent carb again with the ex­pan­sion, but clutch slip was just be­ing con­tained and any more im­prove­ment I feared would not be shown. Pre­vi­ous tests at this rpm showed the orig­i­nal carb to be fine though, so the bolt-on sec­tion comes to an end, with the over­all re­sults start­ing at 9.69bhp as sup­plied, to the fi­nal 15.33bhp and the orig­i­nal torque fig­ure 8.8tq up to the fi­nal 15.41tq.

Time to drop the mo­tor and do a touch of port­ing work. Stay tuned. Words & pho­to­graphs: Dar­rell Tay­lor

Start with the ba­sics and cre­ate some­thing spe­cial.

De­vel­op­ment con­tin­ues...

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