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Issue Date: April 2006, Posted On: 11/26/2007


Pick the type with the performance you want

You shouldn’t judge a book by its cover, but you can tell a great deal about what to expect from an ATV engine by its type and specifications. This guide will put you in the know about your four-stroke ATV or the next one you plan to buy.

You may have noticed that the highest performance ATVs are all liquid-cooled. This is not a coincidence, or a conspiracy to keep radiator and coolant factories humming.
Liquid cooling is the best means of drawing away excess heat and stabilizing engine temperatures as engine loads, speeds and riding conditions change. Typical systems are simple and similar. Most use ethylene glycol coolant, a liquid that suppresses corrosion and has a higher boiling point than water. The liquid is circulated by a small engine driven pump through passages in the cylinder(s) and head(s) and then to a radiator. The radiator uses a network of coolant transport tubes surrounded by fins to allow air to draw heat from the coolant. The system is sealed and engineered to operate at 12-16 pounds of pressure. This raises the boiling point of the coolant further. Many systems also employ a thermostatically-triggered radiator fan to assist air flow in high-heat or low-speed conditions. Coolant recovery bottles are the final feature of most ATV systems. These are catch tanks that hold coolant expelled if the system boils over. A siphon tube in the tank lets the system suck coolant back in when it cools.

Big bore, short stroke, liquid cooled, double overhead cam (DOHC), four or five valves per cylinder.
To achieve the highest horsepower-per-cc of engine displacement, nothing beats the big bore, short stroke, DOHC, multivalve four-stroke.This is the dominant engine design in all the current 450 race quads. Very low reciprocating mass through the use of small valves and nearly skirtless pistons and low friction from the rockerless valvetrain allow revs to build quickly and freely. Multiple small valves create the valve area to let the engine breathe efficiently. Relatively large bores create room in the head for the multiple valves. With the large bore, the stroke can be short to keep piston speeds reasonable, even at engine speeds well above 10,000 rpm. Best of all, the cams directly open the valves in these engines, so there is no rocker weight to burden the valve springs at high rpm.

Single overhead cam (SOHC), four or five valve.
SOHC multivalve engines are nearly as efficient at producing power as DOHC types. The chief difference is, a single cam operates the valves using rocker arms. SOHC engines with roller bearing rockers can match the low friction levels of DOHC engines. Rocker weight is an almost insignificant penalty on these engines in typical ATV engines.

Single overhead cam, two valve.
This is the workhorse of the ATV world. Simple and effective, the SOHC two valver does a lot with very little. It’s capable of impressive power; just ride a Suzuki Ozark or Z250 and you’ll feel it. What you have here is a single cam operating a single intake and a single exhaust valve via rocker arms. Friction and reciprocating weight keep these engines from matching the power-producing and rev-building characteristics of their higher performance brethren.

Pushrod engines are lower-tech than sportier overhead cam types, but they are tough to kill, so they appeal to riders who like to keep things simple—and running. Highly tuned pushrod engines can make good power up to speeds of about 10,000 rpm or so, but typical ATV versions are all done producing power by 8000 rpm. Reciprocating weight is the problem. In these engines, the cam is buried deep in the engine’s cases. It operates the valves through long pushrods which work the valves via rockers. All that machinery between the cam and the valves doesn’t like to be rushed. Since these engines don’t do their best work at high rpm, manufacturers generally use more “square” bore/stroke relationships with pushrod engines to make them smooth and easy to ride.

WARNING: Much of the action de­pict­­ed in this magazine is potentially dan­gerous. Virtually all of the riders seen in our photos are experienced ex­­perts or professionals. Do not at­tempt to duplicate any stunts that are be­­yond your own capabilities. Always wear the appropriate safety gear.
Copyright 2012 Hi-Torque Publications, Inc. All rights reserved.
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