Scooterotica

We discuss EGT probes and how far down the pipe they're mounted, but I found out recently that it's quite important to have an EGT probe tip in the correct position for the guage to work correctly.

There are two types, a bullet with a rounded tip and a stinger with a wire protruding. I thought the tip should be placed as near to the centre of the exhaust pipe as possible, but was told the bullet type should extend into the pipe by around 4mm and the stinger by 2mm so that the guage can read correctly. This could explain some of the wildly high temperatures some have been reading, particularly with the types of analogue guages used on aircraft where fitting instructions aren't included.

With the screw in type you need the probe AND a weld on boss with gasket too, like this boss, click here . Some sites don't advertise the boss and I have welded the fitting with the probe directly to the pipe. This would be incorrect although I think the probe depth can be adjusted to suit.

It would seem difficult to obtain optimum probe depth with the clamp on type, perhaps this is more suited to the Rotax type pipes they're designed for

So, what guages are people using and what probes, and how deep is your probe? Is it a welded boss or clamp on type? What temperatures are you getting?

I have a Falcon gauge with a screw in type probe(welded on threaded stub) which is 100mm from the piston skirt and the probes tip sits central in the Devtour header. When riding it shows between 5 -650 C but occasionally creeps up to near 700C especially if cruising with throttle 1/4 open so I either open it up more or shut down and the gauge drops. Interestingly a mate bought a Stage 6 digital one and after fitting it was cheesed off to find it would not work so removed probe ,heated it up with a butane torch and away it went.Mb's told him the probe is too short to work in a scooter pipe as they are for autos.So that shows a short probe is not the way to go as I would have thought the hot gasses are more likely to be at max temp/velocity in the centre of the header.But then again I am not a scientist

got a koso with clamp on probe to center of devtour 100mm from piston skirt and getting 450 to 680, still fiddling with carb etc.

a little alarmed with temps at first but still a good guideline for jetting etc.

I can't see how it would make any difference whether on an auto or a Lambretta as it's a thermocouple and will read temperature regardless

My information is on good authority and he is quite knowledgeable on using EGT guages on a Lambretta and would have experimented extensively with them, we barely know what we're doing with them and assume they're fitted as they would be on a Rotax engine or similar to other two strokes and liquid cooled motors, whereas Lambrettas seem to be an anomoly in most aspects, certainly with shape of downpipe which may or may not affect it.

I've been using EGT guages for a couple of years but I hope to understand it a lot more when it comes to getting mine up and seeing how it should be used for myself.

i am thinking about getting one of thes ones

keep,er lit wrote:i am thinking about getting one of thes ones

Be careful there's nothing on there to say they are waterproof- assuming it's going on a scooter that is

keep,er lit wrote:i am thinking about getting one of thes ones

Ive got one of those. the gauge is quite long about 80-90mm the probes seem to be the weak point as ive had two so far.
If you get on dont get the smoked one unless you only ride at night as they are difficult to see in the day i had to take mine apart, take the smoked lens out and replaced with a piece of old visor
I thing they are a good buy, as an entry level gauge.
Ive now got a koso one (on a different scoot) as im now convinced of there usefulness, but understanding what they are telling us is not as simple as most think. i think we all have a lot to learn with them.

Russ

The EGT must surely read its hottest when complete combustion takes place, that is when all the fuel and air combine in the correct ratio and burn completely and at the right time so as not to detonate, this would, or should be when the engine is running efficiently at whatever throttle opening, and all things being well with the engine then maximum power for that throttle opening is being obtained.
The temperatures obtained at this point could well exceed what some consider safe, and this could be when the engine is delivering maximum power (For that throttle position). If the engine cant now dissapate the heat developed adequately at this point and the temperature builds up, pre ignition or detonation could first show as a loss of temperature on the gauge just prior to a catastrophic melt down.

This drop in temperature could be caused by an incomplete burning of the air/fuel mix (just prior to melt down) through detonation or pre ignition ?

A correctly positioned thermo couple would most likely react quicker to any change in temperature, giving an earlier warning of imminent failure.

This is most likely why some tuners rather than trying to get the most power and close to 650-700 degree C EGT's might suggest keeping a road scooter closer to 550 degrees C.

The bikes Ive seen using them have the thermo couple at the front of the down pipe directly in-line with the exhaust ports leading edge, The scooter down pipe twists away from the exhaust port and positioning the thermo couple in line with the ports leading edge is not quite so easy.

I think Soulsurfer has a very valid point about the EGT positioning which needs more looking into.

You make a valid point Mike.

J1MS wrote:The EGT must surely read its hottest when complete combustion takes place, that is when all the fuel and air combine in the correct ratio and burn completely and at the right time so as not to detonate, this would, or should be when the engine is running efficiently at whatever throttle opening, and all things being well with the engine then maximum power for that throttle opening is being obtained.
The temperatures obtained at this point could well exceed what some consider safe, and this could be when the engine is delivering maximum power (For that throttle position). If the engine cant now dissapate the heat developed adequately at this point and the temperature builds up, pre ignition or detonation could first show as a loss of temperature on the gauge just prior to a catastrophic melt down.

This drop in temperature could be caused by an incomplete burning of the air/fuel mix (just prior to melt down) through detonation or pre ignition ?

I thought a rise in temp would indicate imminent danger ?
Also load seems to play a part in temp rises as well.

J1MS wrote:
A correctly positioned thermo couple would most likely react quicker to any change in temperature, giving an earlier warning of imminent failure.

This is most likely why some tuners rather than trying to get the most power and close to 650-700 degree C EGT's might suggest keeping a road scooter closer to 550 degrees C.

The bikes Ive seen using them have the thermo couple at the front of the down pipe directly in-line with the exhaust ports leading edge, The scooter down pipe twists away from the exhaust port and positioning the thermo couple in line with the ports leading edge is not quite so easy.

I think Soulsurfer has a very valid point about the EGT positioning which needs more looking into.

You make a valid point Mike.

I think the position of the probe is very important, and shows the an EGT is just another tool to be used along with plug chops, dynos etc. With different egts on different engines and in different positions, it stands to reason that temps will vary, and you need to know your own engine and optimum temps.

reading... might be informative to you all...??

http://blog.oopsclunkthud.com/tuning/EGT-CHT-gauges.pdf