Cruise Egt Temps?

[BellevilleMXZ]

Today was the first time I got to try the sled, 00 MXZ700, and I see on the EGT that I put on this year, it runs about 1250-1300 on a cruise, 50-60 mph 6500 or so on the tach. This is round slides not flats. Only mods so far are Dynoport pipe, and an extra 1/4 turn out on the air screw, as it always seemed a little fat down low last year. WOT is only 1100 or so, so will jet down there. I know this si warm, but warm enough to worry about? I seem to remember these were always hot on a cruise.

[LB700]

My '00-700 is reading about the same as yours (mine is stock, except for Carbon Tech reeds (which means nothing)). I'm running one size lower main jets (270 if memory serves).

Rotax engines always seem to run REALLY hot in the midrange, and I'm not sure if it's actually hot, or some anomaly creating a hot reading on the EGT's. The plugs look fine after a long 1/2 throttle run, so I think it might be a false-high EGT reading in the mid-range (I could be wrong!).

I've got my redline set at 1350 and sometimes when running at half throttle I get nervous, and either pin-it, or slow down to bring the temp back in line. Temps drop QUICKLY either way.

So far I've had no meltdowns (knock on wood!). I'm going to play around more with my needle settings once we get some snow to test on.

[BellevilleMXZ]

Thanks! It seems to be alot crisper and cleaner with the idle air screw open 1/4 turn more, and I didn't think that would effect it that much. I'll keep an eye, and see when its highest. What you running at WOT?

[zeng62]

I stuck my 96 F-SS 670 at 1/2 throttle with a 1400 exh. temp.
I was to close to the edge got a load of bad gas, plugs were
light tan (too lean), wot was usually 1200 max. I always use 92
octane, stock motor except 501 RV. If you are worried try a size bigger
needle jet if they are changeable or raise your needles 1 clip pos.

[BellevilleMXZ]

Well, i'm not so much worried, as I didn't change much, was more curious what other people were seeing.

[zx 800]

There was a small article in Snowgoer Canada's annual buyers guide on egt tuning. It talks about the misconseption of tuning to egt temps.

[zx 800]

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THis was in snowgoer by Kevin Cameron

Hot Lessons
When multi-cylinder two-stroke motorcycle engines came into wide use during the 1970s, tuners had to deal with problems arising from pipe temperature. In particular, the Yamaha TZ750D — an inline-four not unlike the Yamaha Vmax-4 sled — routed three of its pipes under the engine, while the fourth one circled around behind the cylinders. Those who worked with this engine noticed that the “behind” pipe had been made 35 mm longer than the other three. This was not a mistake. The three “under” pipes were exposed to direct air blast and so ran cooler than the single behind pipe, sheltered as it was from direct airflow. The extra 35 mm of length was necessary to make the behind pipe peak at the same rpm as the under pipes.
The reason for this is that the speed of sound in a gas strongly depends on its temperature. Sound is a wave of pressure, transmitted by molecular collisions, so the speed of the wave obviously depends on the average speed of the molecules. Temperature is a measure of this average molecular speed. At room temperature, the speed of sound is close to 340 metres per second, but in hot combustion gas it can exceed 600 m/sec. Because pipes get hot, it’s obvious that heat flows from the exhaust gas to the pipe metal, cooling the gas in the process. Sound travels slower in the cooler gas, making the exhaust pipe act as if it were longer than it is.
This is why exhaust pipes for grass dragsters are typically made shorter than those for the same engine used in a trail sled. On the trail sled, operating for long periods, the pipes have time to get hot and stay hot. On the dragster, the pipes are cold at the start of the run, so to make them develop power at the desired rpm during the all-important start, they must be made shorter.
Engine tuners have noticed horsepower losses between the dyno room and the outdoors, resulting from differences in pipe temperature from the hot dyno room to the cold outdoor air. In an attempt to correct this, some tuners wrap their pipes end-to-end with insulating tape, hoping this will keep the pipes at constant temperature on dyno or trail. Unfortunately, with some pipes the increased temperature in the header section causes detonation. The suspected cause is hotter gas being pushed back into the cylinders by the returning pipe wave.
Another pipe-temperature effect led to some midrange seizures of early throttle-body EFI engines in the early 1990s. These engines used a system that employed map fueling, which means that the engine control computer would note engine rpm and throttle opening, then look up the “correct” fuel quantity for those values on a stored map, and then inject that amount. The control map was made on a dyno by operating an example of that engine type at many rpm and throttle-opening points, and finding and recording the optimum fueling for each point.
The problem with this was that the fuel map assumes the same pipe temperatures as had occurred during the original dyno mapping procedure. On the trail, in the great outdoors, the pipes could cool to a much lower temperature, and thus would perform their maximum pumping at a lower-than-normal rpm. The result would be extra air being pumped into the cylinders, but no extra fuel. The computer, assuming hot pipes, would send in the normal charge of fuel for middle-rpm running — and it wouldn’t be enough. The engine would detonate, overheat, and seize.
Knowing of this problem, I was very interested in 1993 to look at Honda’s (at that time) fuel-injected NSR500 racing motorcycle and discover that it had a temperature probe in the fat part of each pipe, rather than in the usual location in the header section. Clearly Honda had had this same problem, and had added a pipe centre-section temperature sensor to enable the engine computer to fuel correctly with hot or cold pipes.
Lots of people assume that if they install exhaust-gas temperature (EGT) probes in their header pipes, they will be adding valuable protection against seizing. If the gauges go up too high, you’ll know to back off. There are two problems with this simple assumption. The first is that when an engine begins to detonate, its exhaust-gas temperature falls rather than rises. Here is why. When an engine begins to detonate — even lightly — the shock waves produced by abnormal combustion have the effect of scouring away the insulating layer of stagnant gas that lies next to engine internal surfaces — the boundary layer. Deprived of this insulation, engine parts now take up heat faster from combustion gas, causing the rise in coolant temperature associated with detonation. Since more heat is being lost from combustion gas to engine parts, there is less heat remaining to go out the exhaust port. Hence, exhaust-gas temperature falls during detonation.
The second wrong assumption is that there is a single, correct maximum exhaust-gas temperature for all engines. In fact, exhaust-gas temperature varies with degree of cylinder-filling (which varies from one engine design to another, and with changes in altitude) and with compression ratio and exhaust-port height (the higher the ratio, the lower the EGT; the higher the port, the higher the EGT). These are heat engines, so maximum power naturally comes with maximum combustion temperature — provided there is adequate piston and cylinder cooling. Tuning to a certain arbitrary EGT forces you to jet your engine seriously over-rich in order to force the EGT down, throwing away a lot of power by drowning it in fuel.
A kart-racing friend of mine who was new to motorcycle engines tuned his TZ250 Yamaha twin to the same low EGT as his old 100 cc kart engine. Hopelessly rich as a result, the twin produced only about 70 percent of its normal horsepower.
The right way to use EGT is to tune your engine for maximum power using the reliable normal tuning indications from spark plug and piston-crown appearance. Then install the EGT gauge and see what EGT is associated with this already established safe maximum performance. Don’t be surprised or frightened if you see high numbers. Spark plugs and piston crowns don’t lie

[zeng62]

xotar-very, very, very well said, reading plugs and piston tops to
establish a desired baseline, remember the leaner you go the higher
the octane rating to resist detination. SWRules

[LB700]

Kevin Cameron is a god!

Sure wish I could get him to work on my sled LOL.

[BellevilleMXZ]

Well after a fair bit of riding today, hottest it gets on a criuse is 1275, and 1200 on a long long lake run. Long enought to trounch the guy with the ZR800 hehheh plugs look good.