Honda Crf450r Tech

HONDA CRF450R TECH

Formula One technology for the dirt

KEVIN CAMERON

FEDEX BROUGHT ME A PADDED ENVELOPE WITH A LUMP in it. Inside was a piston from a pre-production Honda CRF450R. Secret prototype stuff!

This forging is little more than a disc to hold the rings, with little mini-skirts, only 35 percent as tall as the piston is wide. As with a Formula One race-car piston, set it on a table and the wristpin bosses touch first-they’re longer than the skirts!

It’s light-only 272 grams with its two piston rings, but without its stubby 19mm wristpin. The 7-8mm-thick crown is perfectly flat, save for four shallow valve cutouts for 36mm intakes and 31mm exhausts. Forged pistons have the toughness to withstand the fatigue of high revs, and the roller connecting rod is surface-hardened by carburizing for the same reason. Flat piston crowns offer minimum heat-gathering area to the combustion flame. The highdomed pistons of yesteryear looked radical but ran hot and interfered with combustion speed.

A single compression ring is usual in race applications because although two gas rings seal better over the long haul, race engines are freshened up often enough to benefit from the single ring’s lower friction. The production engine uses a “barrel-faced” compression ring of conventional type. The prototype piston shows, by having a different compression-ring type, that alternatives were still being tested recently. Each type of compression ring has advantages in particular circumstances. This piston’s design has roots in F-l, where extreme engine speeds up to 18,000 rpm require the lightest possible components. Honda’s own CRF literature refers to this F-l kinship.

But while both the CRF and F-l applications need extreme light piston weight, they have very different duty cycles. The F-l engine will probably be started and operated only once after break-in, so its piston rings must offer low friction and excellent seal immediately-even if life is short. The CRF, on the other hand, must run for many hours.

Each ring type offers a different “sweet spot,’’ in which its seal is good and its friction is low. The prototype ring I saw was probably closer to an F-l ring-perhaps offering immediate sealing and low friction-compromised by short life.

The production, barrel-faced ring no doubt offers a longer lasting “sweet spot’’ after break-in.

Another area of interest is the three-piece oil control ring. To return the oil scraped by such rings, there are normally drain-back holes, drilled behind and below this ring’s groove. In this piston, such holes exist only in the skirt regions-not all the way around the piston. We’re used to sportbike engines with plain crank and rod bearings that throw much more oil onto the cylinder walls, which must be aggressively scraped off by high-friction oil rings. But this 450 has a ball-and-roller bottom end that needs only a few cc’s of oil per minute-much less than a plain-bearing engine needs. The few drain-back holes indicate there’s little oil to scrape, and that it’s mostly where the con-rod would throw it, in the plane of the skirts.

When little oil reaches cylinder walls, oil rings of lower pressure and therefore lower friction can be used, as they were in late-model RC45 roadrace engines. That is surely part of the CRF’s low overall friction. The lightness of a “disc-piston’’ cuts vibration and bearing friction. A low-pressure oil ring works in an oil-poor environment. The 450 engine peaks at 9000 rpm for a moderate 3700 feet-perminute piston speed, but can safely overrev to 11,200 rpm, a racy 4600 fpm!

The engine’s claimed 55 horsepower tells us it breathes well, so aftermarket parts will soon be pushing it to make more power at higher revs. I think I’ve been here before.