CHT temp bias, Very important Options

[bd1308]

Okay, I was told by James that the CHT is inaccurate when the temp isnt 70F, which was the calibration temperature used at the factory. Well its 40F outside, and i'm running 400F on the highway, do I have a problem? James said that the CHT reads high when cold (delta T higher) and reads lower when warm (delta T lower). Is this true?

[Bleyseng]

yeah.....Whats your AFR

[Spoke]

James said that the CHT reads high when cold (delta T higher) and reads lower when warm (delta T lower). Is this true?

Most temperature sensors which use a variable resistance use a negative temperature coefficient (NTC) resistor. The oil temp gauge is like this and I suspect the CHT gauge is the same. I tested my oil temp gauge with a potentiometer. With the pot set to high resistance or not connected at all (infinite resistance), the gauge had no deflection from the coldest reading. When the pot was turned towards very low resistance, the gauge deflected all the way to hot.

I don't know if 400F is a high number or not.

Spoke

[bd1308]

I suspect my AFR is somewhere close to stoich mixture, since CHTs tend to be highest at that point. I was running very rich before though, except at idle. Now I have corrected things and i'm a little closer to where I should be. Just a bit more tuning.

but 375+30=405F, which is where I was running on my way to work, except when I climbed hills, which raised it to 420F or so, which would make sense if the sensor was off at other temps.

Mind you it was fine when it was 70F out like two days ago.

[Jake Raby]

VDO gauges are not temp compensated. they are only 100% accurate when the ambient temp of the sender is 76F..

For every degree above 76F the gauge will show about 1 degree COOLER reading..

For every 1 degree below the gauge will show 1 degree HOTTER than the actual engine temp.

Keep tuning, you are too hot.. The max temp you should be seeing is 400F and only on big hills in 5th gear, only for a few seconds...
Above 400 and start saying prayers- your valve seats can fall out at any minute...

[bd1308]

Jake, its 40F outside. so I should be heading for 405F on the straights, right?

405F-30F (its currently 46F here) = 375F on the straights

405+25= 430F max temp under load going up hills in 5th

[Bartlett 914]

I think adding a ballast resistor (in series with the CHT) will richen up your a/f. Try 50 ohms. I think 1/2 watt will be good.

[bd1308]

i'm referring to the VDO Cylinder Head Temperature guage with the spark plug sender unit thing...

[Bartlett 914]

i'm referring to the VDO Cylinder Head Temperature guage with the spark plug sender unit thing...

Oops

(IMG:style_emoticons/default/laugh.gif)

[ejm]

i'm referring to the VDO Cylinder Head Temperature guage with the spark plug sender unit thing...

This gauge does not work like most gauges. It uses a thermocouple to produce voltage rather than a NTC sensor that varies in resistance. The instructions say to never shorten the wiring, the gauge is calibrated to the length of the wire, shorter wire=less resistance=higher reading. Try using an infrared thermometer to verify the gauge....shut it down hot, pull the plug wire and get a bunch of reading down around the plug...shiny areas tend to read wrong....see if you get a reading close to what the gauge says...my turbo climbs to only 350F on a long hill with 7lbs boost... if your gauge is anywhere near correct you are way too hot....

[Jake Raby]

In recent applications myself and one of my racers (member here, Kevin Groot) have found that the newer VDO gauges are very slow to respond and may also be as much as 75 degrees off in their temperature readings.
I am currently outlining a big test to verify all the instruments on the market (VDO, Westach, SPA, Dakota Digital, etc) in true "Myth busting" style.
The issue has been finding a "control" to base things from, but I am working on a partnership with Georgia Tech on this one as they have all the calibration equipment to ensure the control is accurate...

BTW- Infrared temp guns aren't of much value at all in my experience...

[dmenche914]

The stoock CHT sender is not a thermal couple, and hence not dependant on a referance temp, This is a semiconductor resistance device, with a negitive temp coeff. (hotter is lower resistance) all the reading is done at the CHT sender and it goes tot eh brain. after market CHT units are usually thermocouples, and they are dependent in reading on two things, the temp at teh sender,a dn the temp at the gauge, hot day, the cockpit gaguge is warm, cold day the cock pit gauge is cool, the thermal couple measures the difference betweent eh sender and the gauge, hence on a hot day, with a warm gauge, the temp will read lower than actual, adn visa versa on a cold day. the delta reading will still be acurrate, just is the reference temp at the gauge has changed.

Fancy gauges can overcome this by installing a resistance temp sender to measure the gauge temp and use that in the circutry to compensate for variuos gauge temps, thus giving an accurate reading regardless of ambient temp.

Thermocouples read the delta, semiconductor gages read the actual when calibrated (more free carriers at higher temp mean less resistance)

the thermocouple uses two disimular wires, that creat a small voltage (usually millivolts) dependant ont eh temp difference between one end of the wire pair, and the other, Thermocouples require no external power source to operate, resistance type require a power source to work