ATV PROJECT: RATH RACING YAMAHA YFZ/YZ450 HYBRID

Adding modern moto power to a 450 quad By the staff of Dirt Wheels Photos by Wayne Davis

Yamaha makes the most advanced race quad available, and recently its YZ450F motocross motorcycle has been dominating. Rath Racing combined the YZ engine with the YFZ chassis.

For those who are devotees of the straight-axle sport quad, times are rough but with spots of brightness. In the youth quad market, the sport quad blueprint holds strong, but that doesn’t help an adult interested in throwing roost and getting air. Of the big-name manufacturers, Yamaha is the only brand that continues to build high-performance 450cc and 700cc sport quads. While change doesn’t come yearly to Yamaha’s sport quads, the YFZ450, however, did receive regular updates, and the 700 earned less frequent but steady changes. That fact impresses us about Yamaha. The company literally has no competition in the class, yet pride in the brand ensures that development continues.

Some people demand being on the forward edge of performance, though, and there is no arguing that in 450cc motocross engine development, motocross motorcycles get the attention and the technology. Rath Racing has years of experience constructing hybrid race quads, while Yamaha has the most advanced sport quad for sale, and it has the best availability of production and aftermarket parts for its YFZ450. Most of the hybrids Rath builds start with the YFZ chassis. In this case, Rath transplanted a 2018 Yamaha YZ450 motocross engine into a 2014 YFZ chassis.

For those who don’t follow motocross motorcycles, beginning in 2010 the Yamaha YZ450F engine had the top end reversed. The intake was on the front of the head and the exhaust exited from the rear.

Of course, the YFZ quad engine was based on the pre-2010 Yamaha YZ and WR450 engine, with a conventional engine layout with the intake in the rear and the exhaust in the front. Despite that radically different engine configuration, Rath was able to construct a hybrid that looked like Yamaha built it.

Using the YZ450F engine offers more power, less weight, better gear ratios, less vibration and better weight distribution. It also demands a lot of custom fabrication to fit this cleanly.

HISTORY REPEATS

Hybrid quads with full production or complete aftermarket frames and motorcycle engines have a long history in the sport of quad racing. During other times of limited OEM involvement, the aftermarket has stepped up in a big way.

There are a few reasons that hasn’t happened in current national pro racing. There is a production rule in effect for ATVMX National-class racing for the national number-one plate. No surprise, Yamaha and Chad Wienen have all but dominated the series for at least the last decade.

It is interesting that ATVMX national amateur racing has no production rule. The Youth classes see many hybrids racing and winning. The most impressive of these are seen in a thriving 250cc class that is made up almost entirely of hybrid machinery, because hybrids are extremely fast and capable.

Aside from the visual excitement of the late-model motocross engine, what are the advantages to swapping in a 2018 YZ450F engine? First, Rath claims the YZ engine has a linear powerband. Many of his latest hybrid conversions have involved implanting a compact and powerful KTM or Husqvarna 450 motocross engine into the Yamaha YFZ chassis. The Yamaha has less vibration than the KTM or Husky engine. It has less vibration than a Honda TRX450R as well.

A modern YZ engine is both faster and lighter than the WR450-based YFZ engine. Quads are technically off-road trail machines, and federal emissions and sound standards for OHVs made it smart to start with a WR-type engine. Back when the YFZ was created, only the WR-based engines had the electric start vital to a MX-to-quad conversion. KTM has had e-start for its four-stroke motocross models for a long time, and it got to the point that no motocross brand could afford to ignore electric start. Now, nearly all modern 450cc motocross machines come with electric start.

If you watch a modern Supercross race you’ll see that electric start has transformed racing. Tipping over or stalling the engine no longer ruins your race as it did when most of the machinery was kickstart.

Motocross is motocross whether you are talking about a motorcycle or a quad. Statistically, very few 450 quads are seriously used exclusively as motocross machines or even closed-course competition machines. Motocross machines are most effective with close-ratio gearboxes. As a result, the YZ engine with its motocross transmission has gear ratios that feel good and natural on the track.

It would be easy to believe that this is a production Yamaha. Keeping the engine and frame in the same family makes the swap easier, despite reversing the intake and exhaust. It looks natural.

WHAT IT TAKES

The conversion is easier than ever with stock e-start on the YZ. The engine starts easily with the button, and it makes for a nice, simple package. Despite the reversed cylinder head, the engine fits and the necessary plumbing works. That doesn’t mean it jumps right in there.

The YZ450F induction system takes up all the space that the standard fuel tank requires, and the rear exhaust needs the space where the stock intake and airbox normally reside.

Removing the stock intake and airbox makes room for the exhaust routing, and gutting the fuel tank area for the EFI intake and air filtration is an elegant fix. If you are keeping track, that leaves no room for fuel. Rath fabricated a 1.7-gallon aluminum fuel container that mounts right in front of the engine—only possible since the exhaust no longer needs that real estate. The bottom of the tank is built to accept the YZ fuel pump. We worry about engine heat warming the fuel but having the tank right above the hot engine isn’t great for fuel temperatures, either.

Even beginning with a Yamaha chassis and engine, it takes work to make the conversion clean. Just the factory OEM parts required to make the swap possible run for $485! Of course, you are looking at a completely custom RP Race Performance intake and exhaust, which is never as cost-effective as parts that are mass-produced. Rath doesn’t list a price for the actual conversion. The condition of the chassis and engine have a great effect on the conversion process.

Rath is hoping to enter a relationship with Yamaha that will let him buy YZ250F and YZ450F engines rather than start with two complete machines. Current conversions require buying a YFZ450 and a YZ450R. Most folks recoup $2500 to $3500 selling off the YZ450F rolling chassis. The difference depends on how much work you want to do. If you sell the chassis as parts, you get more than selling it in one piece. You can sell the YFZ engine as well.

In addition to $500 in production parts, fitting the modern YZ450F engine into the YFZ chassis requires custom mounts and a fabricated aluminum fuel tank in front of the engine.
This is a Rath TT front end. It has aluminum upper A-arms, chromoly lower arms, a sway bar, PEP shocks, billet front hubs, DWT rims and Hoosier tires. The stem is titanium.

WHAT FOR

Rath feels like the YZ conversion is worth the work and expense for MX, dunes or other types of racing. As with so many Rath builds, this one was aimed at TT racing, but the style of racing doesn’t really matter. Quad chassis setup for high-end racing is always highly specialized, and stock doesn’t cut it.

All the suspension components are routinely replaced, and that includes the front A-arms and rear swingarm. Rath TT upper A-arms are aluminum with chromoly lowers. They are bolstered with Pro Series tie-rods and a titanium steering stem with a billet flag and oversize (Fat bar) handlebar clamps. PEP controls everything with PB1 ZPS front shocks. PEP front brake lines and Rath triple-bearing billet front hubs join with light DWT rims and Hoosier tires to complete the front end. Since this is a TT build, it uses a standard Rath sway bar system. It wouldn’t be used off-road or for motocross, but is used for circle track, TT, ice racing or Supermoto.

Rear suspension starts with a TT -1.875-inch swingarm and TT link. A shorter swingarm increases traction for acceleration and turn exits. That shorter swingarm requires a new PEP brake line and a PEP ZPS shock. Baldwin Motorsports handles the bearing carrier and anti-fade nut. Rebel Gears makes the 36-tooth rear sprocket and DWT makes the rear wheels. Hoosier is the name for TT tires, and the rear runs wide ones.

You don’t stop your TT race testing in the winter. In Minnesota you simply shift over to ice to find a place to ride. This TT-focused YFZ/YZ450F hybrid corners flat and throws ice.

THE PACKAGE

There are a variety of other parts and accessories that must come together to complete a race machine at this level. Rath builds a fine race machine, and this one is no different. Most ATV builds include significant engine mods, but since this one employs the YZ engine, it lists only a Hinson clutch and VP MS109 race fuel for engine mods.

Rath claims that this hybrid is faster on his personal track than any other quad that he owns. Building a hybrid doesn’t make sense for every enthusiast, but it has definite advantages. Those advantages are worth it to a percentage of those seeking ultimate performance. Depending on the year and source of the engine you get the donor motocross engine from, you can get a hydraulic clutch, standard slipper clutch, electric start and the latest in EFI engine performance. For popular motocross engines, there are loads of performance products available as well.

Some of the new national ATVMX pros we talk to speak very fondly of the hybrid 250cc machines they raced as amateurs. Machines they claim are better in many ways than the production-based 450s they race in national competition.

While a hybrid isn’t for everyone, it is something to consider. As more people handle conversions like Rath is making, there will be greater access to the parts to make them happen. We can’t wait to try one.

Full-coverage nerfs and grippy pegs are super important for TT racing. The custom fuel tank must accept the YZ450F fuel pump. The short TT swingarm puts the tires close to the nerfs.
A Rath TT rear setup has a swingarm almost 2 inches shorter than stock for better traction. It employs a LoneStar axle, Baldwin bearing carrier and a PEP shock.

PARTS AND SUPPLIERS:

75 DESIGNS: (763) 439-1289

Graphics: $250 and up

BALDWIN MOTORSPORTS: (440) 224-2734, www.facebook.com/baldwinmotorsportsbaldwin

Bearing carrier: $256.95

Anti-fade hub: $249.95

DWT RACING: (800) Race-rim, www.dwtracing.com

Black A5 wheels: Front $78 ea., rear $94 ea.

FLY RACING: (208) 319-3079, www.flyracing.com

Aero Flex CR high-bend handlebar: $52.95

WPS O-ring chain: $95

FOURWERX CARBON: (262) 501-9696, www.fwcarbon.com

Billet oversized brake pedal: $75

Seat cover: $170

HINSON RACING: (909) 946-2942, www.hinsonracing.com

Full conventional clutch kit: $1250

RATH RACING: (320) 234-7223, www.rathracing.com

TT nerfs, monster pegs, Gen 2 nets: $555.95

SS bumper: $154.95

Rear grab bar: $98.95

Ti steering stem w/ billet flag, 1 1/8” handlebar clamps: $725

TT A-arms, chromoly lowers, aluminum uppers: $995

Pro Series tie-rods: $230

PEP front brake line: $135

PEP PB1 ZPS front shocks: $1995

Triple bearing billet front hubs: $445.95

Standard front sway bar: $405.95

Aluminum 1.5-gallon gas tank made to accept YFZ fuel pump: $670

TT -1.875-inch swingarm: $925

TT link: $325

PEP rear brake line: $65

PEP rear ZPS shock: $1185

Engine swap kit parts: $485

Engine swap labor: Varies

REBEL GEARS: (931) 788-1617, www.rebelgears.com

13-tooth counter: $48

36-tooth rear: $65

RP RACE PERFORMANCE: www.rprace.com

Exhaust: $590 and up

Intake: $315

WORKS CONNECTION: (530) 642-9488, www.worksconnection.com

Clutch perch assembly: $204.95

ATV TestRath RacingYamahayfzYZ450F