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Vespa Engine Temperature Sensor Info

Here are some old notes I had lying around on my PC. Haven't bothered reading them to see if they are complete but it may be useful to someone...

Here are just some things that we have found out about EGT (Exhaust Gas Temperature) and CHT (Cylinder Head Temperature). Paul has a CHT/ EGT on a PX with Pinasco 213 kit and Derek as a CHT on a standard PX engine with SIP exhaust.

There seems to be a lot of negative comments about temp gauges and that they are useless. We have found that yes, you are unlikely to be staring at a gauge the second that your engine heat seizes, so they will not prevent you seeing a seizure event happening. However, a CHT will give you invaluable long before, if you know how to read it and therefore are invaluable. You will be able to instantly judge the impact of fuel or jetting changes and adjust your driving accordingly… magic… it can make you relax or worry, where before you were ignorant.

CHT - Cylinder Head Temperature

CHT – We have found the CHT the most useful. Before the heat seizure Paul found that he could sit all day with the engine running at 350 F. I suspect that when the heat seizure happened that engine was running hotter, as I have found that some fuels cause the engine to run hotter and slower (by as much as 50 F), and I had just change fuels before the incident.

Now knowing the “blow up” point of my scooter, I changed jetting and spark plug rating. I keep an ear on the engine noise when the scooter sits at around 350 F and relax when it is running below that e.g. 320 F. I have found that with good fuels the engine will run colder and faster. Forcing the engine to run colder (I have a trick for this) does not make the scooter go faster but I believe makes it less likely to heat seize. I have found the CHT invaluable for having some idea of the effects of fuel and change is atmosphere.

EGT – Exhaust Gas Temperature

EGT – the EGT I have found to be next to useless* The EGT does show you a more precise indication of what heat the engine is producing with each stroke, however this is the problem. Unless you are traveling at an exact rev range, a change in temperature would mean nothing.

The EGT (and also CHT) would be better if I could calculate the temp based on revs. It would also be better if some warning indicator to be included to let you know when things are going bad. This to me is the only way these instruments would actually prevent a seizure at the time that it happens (since you are unlikely to be looking at the gauges non-stop).

Another reason why the EGT is not so good is because in theory the temperature change is not linear, at some pints the temp can go up if the mixture is too rich (fuel burning in the exhaust) or go down if it is too lean (air reducing the heat produced)! So a heat seize might look like CHT getting hot and EGT getting cold. There might be a point on the heat curve where you can detect when things get too hot but this requires a bit of calculation and knowing how your engine should behave.

* Maybe it would be good for determining pipe temperature as this affects tuned length


Paul McIntosh,
Mar 24, 2015, 11:37 PM