EGT and CHT reading and understanding

[KTy]

I am struggling to correctly jet a Polini 177, with anticipated rotary valve, normal Polini manifold, PHBH28 carb and Leo pipe.

Ignition is standard and set to 18° (IT).

Carb setup is:

Idle 51

Main 125

Atomiser AV262

Needle is x2 and slide is 20 (default).

Clip is now 2nd from bottom (4Ting, too rich at mid-rpm)

Also the Polini kit was new, I already pre-seized several time this engine, and I still want to understand what's wrong with the setup.

The combination of these component makes a picky engine, with lots of power at high-rpm. The point is, with a short gearing (original PX Lusso) it's funny to drive !

My feeling now is that I had seizure because the carb was properly setup for high and low RPM but was too lean at mid-rpm.

So I have an EGT sensor and a CHT one and I am looking at the numbers...

The picture below shows where is the EGT sensor, it is about 10-11cm from the piston side, as usually recommended.

What I am reading is still really strange to me; under heavy load (going 4th gear with 2 peoples on a serious slope) the EGT hardly reach 350°C while the CHT was actually scarely increasing up to 135°C... I remember having seizure when the CHT was showing around 140°C. And I actually believe the CHT is optimistic because it is not exactly under the sparkplug, it is just close to it... At idle the CHT shows around 80°C and the EGT is suprizingly low at 150°C.

Does anyone has experience jetting a setup with an EGT ? What are some target temperature numbers ??

Could it be that the EGT sensor (Koso/Stage6) is deffective and thus optimistic ?

Could it be that I have pre-seizure simply because of a lack of fuel ? (I have doubts about this carb anyway)

Thnaks !!

[Crusher]

Thats way to close to the exhaust!

[KTy]

But...

1. it's about 10cm from the skirt of the piston (usual recommandation)

2. if it is too close, I should read a much higher temperature, no ?!

[Brosi]

EGT Temp is abnormally low. Usually it´s about, let´s say, 400°C-600°C at an distance of 14cm to the piston. At least after my experience.

Bearbeitet 24. Juni 2008 von Brosi

[Crusher]

But...

1. it's about 10cm from the skirt of the piston (usual recommandation)

2. if it is too close, I should read a much higher temperature, no ?!

Haven't read all in your first post, sorry. But looks really close to the exhaust.

Temperature goes down if its too close to the exhaust, cause cool gas-air mixture which is drawn into the pipe cools down the sensor. This would also explain your unusal low temperature.

Bearbeitet 24. Juni 2008 von Crusher

[KTy]

Ok, then understood.

Please correct me but this distance, that I know is really important, is calculated from piston skirt /exhaust port, right ?

On a largeframe cylinder, the "exhaust manifold" is 7cm, this is why I put mine about ~3cm away...

[Buddi]

I´ve found something very interesting about the EGT and CHT meassuring:

http://www.iwt.com.au/EGT_CHT.htm

Bearbeitet 24. Juni 2008 von Buddi

[KTy]

Yeah, I know this page... It is interesting, but do not actually contain so much information...

[KTy]

I do not get it...

There should be a way to roughly calculate at what distance should the EGT probe be mounted ?!

Probably a simple equation combining engine displacement and RPM peak power, no ?!