As a general rule, outboards are simple to maintain and tune — but for those who like to run their engines on the ragged edge, a normally easy-going outboard can
become a high-strung beast. If tuned improperly, it can quickly morph into a high-dollar pile of scrap aluminum. Experienced engine tuners and performance enthusiasts know that to avoid meltdown, an outboard’s air-to-fuel mixture must be carefully adjusted for maximum performance, while not going “over the edge.”

Getting that mixture just right requires a good background in two-stroke tuning, an assortment of carburetor jets for carbureted engines or fuel-injection-tuning tools for EFI outboards, and the know-how to get the engine tuned without burning down a piston.

UNDERSTANDING EGT FUNCTIONS

Fortunately, two-stroke tuning has become a lot easier with the advent of exhaust gas temperature (EGT) gauges. Sometimes called pyrometers, these handy instruments have been in use for years in aviation, as well as snowmobile, motorcycle and automobile racing, and are indispensable to tuners who are trying to eke out that last bit of power — without sending the engine into meltdown mode.

EGT gauges rely on metal temperature-sensing probes inserted into an engine’s exhaust pipe (close to the exhaust ports) to measure the heat from exhaust gasses. In turn, an experienced tuner can tell from the dash-mounted gauge how rich or lean the fuel mixture is, and can adjust it accordingly.

Two-strokes generally run best when the exhaust temperature (measured approximately 3 to 6 inches from the exhaust port) falls between 1150 and 1300 degrees Fahrenheit at wide-open throttle (WOT). Partial throttle readings will typically measure quite a bit less. A well-tuned outboard usually shows temperatures in the 700- to 800-degree range at idle to off-idle speeds, climbing to about 850 or 900 degrees in the midrange, then finally to about 1200 degrees at three-quarters to full throttle. Usually, more than 1300 degrees at full bore is too lean (not enough fuel), and the engine may preignite or detonate. Conversely, less than 1200 degrees is typically too rich — with a corresponding drop in performance as WOT exhaust gas temperatures drop and the engine tries to burn the excess fuel.

CHOOSE YOUR GAUGE STYLE

EGT gauges vary from simple, needle-style analog or digital instruments (that match other dash-mounted gauges), to elaborate systems suited for serious tuners and racers. Companies such as Gaffrig, Hewitt and Nordskog Performance Products sell basic units, while specialty racing outfitters such as Racepak/CSI and Exhaust Gas Technologies offer pro-style systems with “high temperature” memory recall. Some gauges also record tachometer readings, and can play back an entire full-throttle run so you can see exactly where EGT readings vary across a measured timespan.

Gauge kits are easy to install. In fact, it should take the average hotrod outboarder only an afternoon of careful labor to complete the job. Most in-dash EGT gauges use two wires to send the temperature signal from the engine-mounted probes to the gauge, and two more wires to power the internal light (for low-light operation). Digatron’s battery-powered gauges attach to dash tops via bracket mounts, and their probe wire hook-ups make them easy to install and remove for storage or to prevent theft.

Most Mercury V-6 2.0L, 2.4L and 2.5L outboards accept probe installations quite readily. Gaffrig’s Mercury probe kits use fast-readout, thin-wire probes that mount in the two, bottom center bolts on the exhaust cover plate. Their drilled barrel bolts replace the stock bolts and allow the EGT probes to easily slip into place. Compression-fitting nuts secure them and the probes can be bent slightly to fit into tight locations. This is important, as the rear portion of the lower cowling is where they’ll be routed from, and that’s a snug fit on most engines.

WATCH WHERE YOU’RE DRILLING

Other outboards, such as the larger Evinrude, Johnson and Yamaha V-6s (as well as Mercury’s 3.0L V-6), also will accept EGT probe installations. However, these jobs are more involved. Basically, holes to accept the probe mounts must be drilled and tapped in the exhaust adapter below the lower cowl pan. It’s important when drilling these holes that only the exhaust adapter wall is drilled — not the water jacket that surrounds it. If you’re unsure of the water jacket’s location, remove the outboard powerhead and you’ll be able to peer down into the adapter to check for the correct positioning of the probes. Drilling and tapping will be easier with the powerhead removed, as you’ll have more direct access. Each probe should be centered in the exhaust cavity.

While it’s OK to install an EGT probe only in one side of the exhaust, be aware that you’ll only be measuring the temperature of one bank of cylinders. To accurately monitor both banks, you’ll need two probes and a twin-readout gauge. Many of these engines have lower-cowl “skirts” that may need to be trimmed or drilled to create clearance for the probe wires.

COMPLETING THE INSTALL

The wires are routed from the lower cowl pan into the boat, then along the gunwale and to the helm, where they connect to the gauge. Most digital units are able to read temperature the moment they’re powered up; analog units require the engine to be running before they’ll show a reading.

In-dash EGT gauges are available in standard round gauge sizes, and installation is no different than with any other round gauge. A holesaw is used to bore the mounting hole and then the supplied brackets are used to hold the gauge head in place.

With a battery-powered unit such as the Digatron DT-32, external brackets are used to secure the gauge assembly to the top or front of the dash. When planning your install, be sure it’s easy to remove the unit to replace the AA batteries. Typically, these gauges chew through batteries, and you’ll have to replace them a few times each season.

INTERPRETING THE READINGS

Having an EGT gauge doesn’t mean you’ll instantly be able to assess an engine’s state of tune. First, you’ll need to establish a base of comparison on which to gauge your outboard’s EGT readings. This means taking a high-speed run, then cutting off the engine “clean” with the ignition switch, and then “reading” the spark plugs (or piston tops) for proper color.

The reason for killing the engine at high speed is so that low-speed running doesn’t cause miscoloring of the plugs. At speeds other than full bore, the engine is running somewhat richer, and plug readings will reflect that. Be careful when cutting the ignition at these speeds — particularly if your hull is flighty or reacts violently to sudden throttle reduction at speed. In this case, trim in slightly and “settle” the boat for just a second (while still at full throttle) before turning off the ignition. When the outboard stops, remove the spark plugs (carefully noting which one came from which cylinder). The insulator nose of each plug should be a dark, coffee brown.

Darker shades mean the engine is running too rich, while lighter shades indicate it’s too lean and needs more fuel. A white tip means you’re right at the edge of burn down, and should enrich the fuel mixture immediately to avoid costly damage. Conversely, an extreme dark brown or black tip means the outboard is running very rich — and leaning it out several steps can improve its performance.

WORTH EVERY PENNY

It’s important to take several spark plug readings, and then correlate them to the EGT gauge display; this way, by glancing at the gauge you’ll know if the engine is running at the proper temperature for your particular application. The reason for correlating the two readings is that gauge temperatures will vary according to where the EGT probe is located in relation to the exhaust ports. Probes placed 3 inches from the ports will give different readings than probes placed 6 inches from the ports.

One trick racers use with analog gauges is to place a strip of tape at the proper mark (1200 degrees or thereabouts), so that the gauge needle noses up against the tape when the engine is running right. If the needle disappears under the tape, you know you’re in the “danger zone” without having to closely study the gauge (something that’s not easy to do at high speeds).

Outboard racers and tuners know that an EGT gauge is a valuable diagnostic and fine-tuning tool. Considering that it lets you push an outboard to the limit — without going over the edge — the cost is relatively low. Twin-probe, analog-gauge kits sell for less than $500, while the racer’s choice digital units sell for upward of $600 to $1500, depending on features. If you’re serious about speed, keeping tabs on exhaust temperature will help you dial in top-end performance. And by installing an EGT system, you join the ranks of boaters who are true keepers of the flame.

SIDE BAR

EGT Gauge Suppliers

Digatron Instruments
Dept. BWB, 8102 North Freya St., Spokane, WA 99217 509/467-3128 digatron.cc

Exhaust Gas Technologies
Dept. BWB, 1381 N. Kraemer Blvd., Anaheim, CA 92806
800/348-4678 exhaustgas.com

Gaffrig by Livorsi Marine
Dept. BWB, 10 Gaffrig Drive, Grayslake, IL 60030
847/548-5900 livorsi.com

Hewitt Industries
Dept. BWB, 5492 Bolsa Ave., Huntington Beach, CA 92649; 714/891-9300 hewittindustries.com

Hydro-Dynamics
Dept. BWB, 1 Hydro Lane, P.O. Box 418, Benton, LA 71006
318/965-0201(no website)

Nordskog Performance Products
Dept. BWB, 1120 Yarnell Place, Oxnard, CA 93033
805/483-2411 nordskogperformance.net

Racepak/CSI
Dept. BWB, 30402 Esperanza, Rancho Santa Margarita, CA 92688; 949/709-5555 racepak.com