TDC

When the ev­i­dence is hid­ing in plain sight

Cycle World - - Contents - By KEVIN CAMERON

When Honda first went GP rac­ing in the early 1960s, its four-cylin­der 250 was built just as Honda’s pro­duc­tion twins were—with a hor­i­zon­tally split crank­case hav­ing main bear­ing sad­dles bored half in one case, half in the other. When the crank­shaft with its ball or roller main bear­ings was set into the up­per case and then the lower case was bolted into place, the crank was well-sup­ported by the en­tire struc­ture. Honda’s early fours also in­cluded both gear­box shafts in this struc­ture, the sad­dles for their bear­ings like­wise be­ing bored half in the up­per, half in the lower case.

For its rac­ing en­gines, Honda soon gave up this ap­par­ently ra­tio­nal and ro­bust con­struc­tion, switch­ing to one in which crank­shaft main bear­ings, in the form of “pil­low blocks” (a bear­ing in a footed hous­ing that can be bolted to a flat sur­face), were bolted to the un­der­side of the cylin­der block. The whole idea of strongly sup­port­ing the crank bear­ings be­tween up­per and lower case halves was given up, and the lower case served only to con­tain oil, with no struc­tural func­tion. Its ma­te­rial was changed from the alu­minum it had been in ear­lier de­signs to lower-strength mag­ne­sium. This is the way the RC165/166 in-line six-cylin­der en­gines of 1964 to ’67 were built.

This puz­zled me, but I some­how dis­missed the “why” ques­tion by not­ing that Honda, at the same time, gave up the use of heavy, large-di­am­e­ter ball main bear­ings (on pro­duc­tion twins of the 1960s, these are re­ally hefty) in fa­vor of much more com­pact small-di­am­e­ter nee­dle rollers. This saved both weight and fric­tion. Yet that did not ex­plain what made Honda engi­neers (prob­a­bly none other than Shoichiro Iri­ma­jiri him­self) will­ing to give up the struc­tural ro­bust­ness of clamp­ing crank­shaft bear­ings be­tween crank­case halves.

A red herring also en­tered my think­ing. When Gil­era built its first air-cooled 500cc four in 1947, it was given pil­low-block-style main roller bear­ings that bolted to the un­der­side of the cylin­der. This is sim­i­lar to how count­less mil­lions of auto en­gines have been and still are built. Half of each main bear­ing hous­ing is ma­chined in the un­der­side of the cylin­der block (look at any tra­di­tional Amer­i­can V-8), and the other half is ma­chined into a bolted-on main bear­ing cap. It’s not a pil­low block but is the next thing to it.

It’s no mys­tery why the early MV four-cylin­der rac­ers were de­signed the same way—count Agusta had hired away Gil­era’s de­signer Piero Re­mor. But when the MV Triples ap­peared, Re­mor had left the pic­ture. The first Triple was a 350, but in rapid in­creases of bore and stroke, it be­came the 500 that would be Gi­a­como Agostini’s fa­vorite ma­chine. The Triples had full pil­low-block

Those jew­ellike sixes peaked at 18,000 rpm...

con­struc­tion us­ing small-di­am­e­ter split-outer-race nee­dle/roller main bear­ings—very sim­i­lar in ef­fect to what was com­ing from Honda’s race depart­ment.

A sim­ple-minded the­ory would pro­pose that one com­pany copied the other, but it’s far more likely that both were re­spond­ing to com­mon prob­lems of high-rpm en­gines.

In re­cent years, Ja­panese new-model brochures have made much of sav­ing horse­power by putting vent holes through main-bear­ing sad­dle-sup­port webs so air be­ing pushed into the crank­case by a de­scend­ing pis­ton can more eas­ily and with re­duced pump­ing loss reach and fol­low the nearby ris­ing pis­ton. With­out the holes, this back-and-forth pump­ing process is very much like those screen-door closers that hiss as they close be­cause they keep the door from bang­ing by pump­ing air through a re­stric­tion. At high rpm, sev­eral horse­power can be saved by pro­vid­ing cylin­der-to-cylin­der crank­case vent holes.

Yamaha in Mo­togp has used one For­mula 1 ap­proach to solv­ing this pump­ing-loss prob­lem: It pro­vides a crank­case evac­u­a­tion pump that pulls the pres­sure of crank­case air and va­por low enough to nearly elim­i­nate pump­ing/windage loss. In the 1990s, when we cheered fa­vorites in AMA 600 Su­per­sport rac­ing, we had no idea that sim­i­lar crank­case evac pumps had been clev­erly hid­den in the gear­boxes of cer­tain brands. Very hush-hush.

The other ap­proach is to iso­late each cylin­der’s (or each V pair’s, in the case of V en­gines) crank­case, pro­vide each with its own scav­enge oil pump, and just let the pis­ton(s) com­press and ex­pand the air be­neath with­out loss be­cause it is not be­ing forced through ori­fices or con­fined spa­ces.

Honda engi­neers were well-aware of this kind of loss. A look at the lower crank­case of its RC-161 four shows fair-size holes to al­low oil drain-back, and easy cylin­der-to-cylin­der crank­case air pump­ing.

By adopt­ing pil­low-block con­struc­tion, Iri­ma­jiri elim­i­nated nearly all re­sis­tance to cylin­der-to-cylin­der crank­case air­flow. I was em­bar­rassed at how ob­vi­ous it was.

Those jewel-like sixes peaked at 18,000 rpm, so their po­ten­tial for losses from crank­case pump­ing/ windage was huge. The pil­low-block so­lu­tion, by elim­i­nat­ing the tra­di­tional ob­struc­tive bear­ing sad­dles and their sup­port­ing webs be­tween cylin­ders, must have saved a bunch of horse­power that would oth­er­wise have been con­sumed huff­ing and puff­ing that air back and forth through re­stric­tive holes, slots, or other half-mea­sures.

Then I re­mem­bered another thing. The V-5 four-stroke that Kenny Roberts’ GP team de­signed and built in the early to mid-2000s never came close to its horse­power goals, and the back chat was that the prob­lem was “some­thing to do with the crank­case.” Could it be that the miss­ing horse­power was “gone with the wind,” blow­ing rapidly from cylin­der to cylin­der at 16,000 rpm?

I be­lieve that Honda’s present RC213V V-4 Mo­togp en­gine em­ploys the loss-free F1 sep­a­rate­and-sealed crank­case sys­tem. In­stead of push­ing crank­case air back and forth through re­stric­tions, each V pair of pis­tons just com­presses and ex­pands the air un­der them—with­out loss. Each sealed crank­case has its own scav­enge oil pump. Honda coyly calls this sys­tem a “semi-wet sump” be­cause, al­though the two crankcases (one for each crankpin) are sep­a­rated from one another, there is a com­mon un­der­engine oil sump be­low.

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