Cycle News - Archive Issues - 1990's

~ ~Q!!BENCH ~ ...... .... 0.. ~ Exhaust systems: More thanjust blowing smoke there's a period wh ere the 'exhaust, intak e and transfer ports are all open. Therefore, som e of the fresh charge escapes into the exha us t po rt. This problem is solved by the baffle cone, wh ich is the sectio n of p ipe that constr icts the exha ust flow into the stinge r, wh ich then disperses spe nt gasses into the ai r th rough the silencer . When th e flo w of gas is constricted, it crea tes a negative pressu re wave that travels back to the cylinder and stu ffs the fresh cha rge back into the combustion cha mber. A fo ur-stroke ex haust works in mu ch the same way. Back p ressure is crea ted by a reverse-cone mega ph one, whi ch helps to stu ff the inco ming cha rge back into the cyli nder , creating a kind of turbocharger effect. The tri ck to designing an effective pipe is to tim e the back-pressure wave so that it reaches the cylinder at the right time . Cone lengths are deter- H ow abo ut doing a colu mn on the theory of exha ust systems, both twoand four- stroke? I do not full y understa nd the effects of back pressure, son ic waves, etc. Steve Hyduke Collierville, TN teve isn't alo ne in his lack of u nderstanding of the finer po ints of exha ust tunin g. It 's a relativel y co mp lica ted sc ie nce wi th vario us scientific a nd mathema tical formulas. Books have been written on the subject, but we'll exp la in the very basics in the space of thi s column. We cons ulted a few of the expe rts in the exhaust pipe field - Tom Turner of Two Stroke Racing (TSR), Paul Thede of Race Tech and Pro S I ' i i !' 8==:::::J::J:::>=== -HJJbd' 'f Pressure waves produced by megaphone and expansion chamber pipes hom Gordon Blair and M.B. Johnson's SAE Paper. Figures derived hom mathematical fonnulas are inserted into Tom Turner's blueprint before being sent to the exhaust pipe fabricator. - -• A/ 22. ~ OIF, L1 L HEADER (. °1 POR BAFFLE DWELL 7 I( ---- ( 0) 0) OjF Of LL7 L ( 0) I °1 OIF ~ 02 3 ~ L ~ STINGER BAF BAF DWELL 1~ 03 ......... 01 ST 05 07 06 ! I TUNED Cir cuit's Mitch Payton - to help exp la in the fun ction of an exha ust pipe. There's a lot more to it than just cha nneling smo ke from the engine! An exha us t pipe, be it an after mar ket o r stoc k unit,'doesn ' t really add power to an eng ine. Rather, th e pipe enhances th e po wer output an engi ne is capa ble of producing and has the mo st affect o n a n engine's powerband, which is th e sp read of power an engine prod uces over various rpm. Gen erally. the lo w end of a powerband is torquey, th e midrange generally produces an abrupt surge of power, and the top end acts as an aft erburner to keep producing power as the rpm soars. " If an (two-stro ke) engine worked over on e rpm range, you wouldn 't see the various steps of an exhaust pipe," says Thede, who wrote his senior thesis on expansion chamber analysis on his way to obtaining a degree in mechanical engineering. " An engine is used over wider ranges of rpm, therefore you don't get ultimate power." The primary function of the pipe is to draw the spent charge (burnt fuel) out of the engine. In order to pull this gas out, the pipe expands from the head pipe into the diffuser. or meg aphone. As exhaust gas is drawn out of the cylinder. a fresh charge of the fuel/air mixture is drawn in. The more gas that's pulled out, the more fresh charge then can be pulled in. But this creates a problem. Since the new charge drawn into the cylinder also helps push the spent charge out, LENGTH , mined using mathemati cal formulas, dyn am ometers and co mp uters. Both Turner and Payton use computers programmed to comp u te the basic pipe design from the mathematical formulas. An engi ne is disassem bled to measure pi ston stro ke, rod length, port o penings and po rt timin g. T his informati on is then entered into the computer wh ich formulat es the , dimension s for an ideal pi pe based o n the da ta. After tra nsferri ng the dimen sions to a blueprint, a pi pe bu ilder shapes the exhaust p ipe from sheet metal and welds the cones togeth er to fit the particular motorcycle on which the pipe is to be "hung." The design is then tested , both on a dyno and on the track . Usually. tra ck testing gives the best indication of how a pipe will perform. "Some (p ipe s) work well on the dyno but suck on the track," says Pa yton . After testing, the pipe can either be changed to produce a different powerband, or put directly into production. As mentioned above, exhaust theory is a complicated subject, For more detailed exhaust pipe design theory. Turner recommends reading Roy Bacon's Two Stroke Tuning, Gordon Jennings' Two-Stroke Tuner's Handbook or Cesare Bossaglia's Two-Stroke High Performance Tuning. EDITOR'S MEGAPHONE By Brian Catterson s the 1990 Uni ted States Grand Prix draws near , I ca n' t help but remember my first trip to La guna Seca Raceway. It was four years ago, J ul y, 1986, when the AMA still had a Formula I roa d racing class and the Ch ampion Spa rk Plug 200 at Laguna Seca still drew world-class riders. Havin g swel tered in a southern California warehouse most of th at su mmer, and with the circled date on my calendar drawing nearer, I was eagerly anticipating the cool ride up the Pacific Coast for three days of spectating at what would be the closest thing to a Grand Prix to grace North America that year. Come departure day. however, I had some misgivings. Maybe it was getting a speeding ticket just five miles from my house; maybe it was the fact that no less than three of my buddies canceled their plans to accompany me at the last possible moment - whatever the reason , a bad feeling settled in the pit of my stomach as I glanced over my left shoulder to merge with th e steady stream of fast-moving traffic and pointed my Cagiva Alazzurra northbound on the San Di ego Freeway. Alone. The Montere y Peninsula, where Laguna Seca is located, is an approximatel y seven-ho u r ride from L os Angel es, which depending on your ro ute can be heaven or hell. Hi ghway I along the breezy, scen ic Pacifi c Coast is the best way to go and Interstate 5 through the flat . d ry and dusty San Joaquin Valle y th e worst, with the 101 Freewa y falling in between . Fortunately, som e friends had helped me map out a pl easant blend of mountain and coastal roads with a bare minimum of freeways, and hooking up with fellow en th usiasts en route to our co m m o n destination , th e rid e reminded me of wh y we rode motorcycles in the first place - for th e camraderie, the fun . Th is was my first trip to Monterey, and altho ugh I pl anned on cam p ing. I hadn 't bothered to bring a tent. What the hey, I th ought, sleeping under th e summer stars would be a bla st ... Of co urse, no o ne had told me abo ut the peninsula 's peculiar weather, and arriving a t th e track at 7:30 p .m. Thursday night. I couldn't believe my eyes. That is, if it was possible to see anything in the first pl ace! The central California pea soup was so thick that it was all one cou ld do to slowly follow the curbs outlining the track's access road. Eventually managing to locate the infield campground, I hit the kill switch and sat there in silence with my bike 's headlight piercing a hol e , through the mist, wishing I'd brought a tent. "Hey, wanna beer?" came a voice from the fog. The words seemed to be coming from nowhere, but whoever was talking had what I needed most right then. Well . with the possible exception of a tent. . . ' It turned out that the voice belonged to Scott Johnson, a southern California club road racer and dedicated racing fan who'd ridden his Honda XL250 all the way from Los Angeles! (Scott later went on to start his own A company. Tomcat Fabrication; many Angelinos would recognize SCOtt'S present street bike, a Yamaha FZ600 painted to resemble a Navy F-14.) SCOtt was accompanied by another club racer whose name I've forgotten, but I do remember that his predicament was even worse than mine. He was waiting for his girlfriend to arrive with the motorhome (she never did), and together we spe nt the evening sipp in g beer and telling lies, all the while p o stponing th e i ne vi ta b le en co u n te r with our sub -standard sleep ing arrangem ents. Fin ally, my tired bod y cried "eno ug h" a n d I cra wled int o my sleepi ng bag, pulled it over my head and fantasized that I was home in bed. Awakenin g Friday morn in g to the so u n d of ra cing tw o-strokes, and di scovering that my newfound friend had spent the night sleep ing in the ca m p g ro u nd' s showe rs, it became apparent that th ere really was someone worse off than me. From that point on, the weekend only got better. This was before Laguna Seca had added the new infield section to meet the FIM's minimumlength standards, and meeting a group of fellow Italian motorcycle enthusiasts who'd ridden down from Oregon, we watched Saturday's Superbike race from the inside of the old. fast turn two , comparing notes on our respective steeds while waiting for the leader, Honda-mounted Wayne Rainey, to reappear each lap on his way to an , unchallenged win. "Imagine that instead of just three of those exquisite V-4 two-strokes, there were a dozen or more ..." Sunday was the day of The 200 (fo r kilometers, not miles). and the cam pground was buzzing at th e crack of dawn. The da y was not eworthy in that it marked Kevin Schwantz ' Formula 2 (250cc) debut, riding a Ma cLean Honda (he cras hed. ) And in the Battl e of the Twins (I wish it was still called that! ), former World Champion Marco Lucchinelli won on his Ducati. That Team Kool Roberts' Mik e Baldwin beat his th en -teammate Randy Marnola tto win Sunday's F-I race, and that Team Marlboro Agostini 's Eddie Lawson cras hed after being run off th e track by a backmarker almost didn 't matter. What mattered was that we were seeing some of the best racers in the world, and having a n absolute blast cam p ing, partying, bench racing and making new friends. That night, with the adrena line of the weekend 's racing still rushing through my bloodstream, I raced the setting sundown Highway I to San Luis Obispo, where in the inte rest of getting home at something resembling a decent time, I opted to tak e the 101 Freeway south. It was there. on that long, droning expanse of superslab, that it occurred to me that what the U.S. needed more than anything was a GP of its own. We already had the best riders - why not show them off in our own country? Imagine that instead of just three of those exquisite V-4 two-strokes. there were a dozen or more from a variety of manufacturers, ridden not just by a handful of Americans but by literally the best road racers from around the world, with legions of fans cheering for their favorites. So as the 1990 United States Grand Prix draws near, I can 't help but remember my first trip to Laguna Seca Raceway, and to be thankful that apparently I wasn't the only one who felt the way I did on the way home that day in 1986. or

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