AFR, Lambda, EGT and Engine heat

[Steve Cole]

It seems I'm getting a lot more calls and emails about what these all mean to the DIY people so here are a few guidelines. These are not carved in stone and you can push the limits but when you do, expect that things will not last as long. This is for a HD Big Twin engine.

ALL OF THESE THINGS MUST BE CHECKED.

So what this means is if you push the limit of one item you have to check the others to make sure they do not exceed there limit.

EGT Limit              1450 deg F
AFR Lean Limit       16.5:1
Lambda Lean Limit  1.12
Engine Heat Limit    325 F

EGT to be measure within the first 6" of the exhaust pipe leaving the cylinder head

AFR and Lambda must have accurate mounting into the exhaust stream for proper measurement and use calibrated test equipment capable of making the readings. Sorry, Broad Band sensors should not be used.

Engine heat as measured using the stock HD sensor and mounting position in the front cylinder head

So does this mean if you were racing your bike and at the and of the 1/4 mile you saw 1470 EGT you've ruined your engine........ NO. These are just practical limits to be used as guidelines for normal use on your bike. Would I like to run these limits all day long, NO.

The best power and fuel economy will be found running just to the rich side of Stoichometeric for the fuel you are using and the closer to Stoichometeric you run the better the fuel economy you will get. Using a Lambda measurement you would be between .925 - 1.00

Continuous operation should be kept below ALL of these limits and as for EGT's I am comfortable running 1250 deg F all day long provided all the others stay below there limits. Rider comfort is something that has to be determined by the conditions the bike is used in, so as an example what works for someone who's outside environment never exceeds 80 deg F is completely different than another that has to deal with 110 deg F. There is no magic setting that will get you the best of everything in all conditions, so you need to error on the side of safety.

So if you want your bike to be able to pull a trailer fully loaded riding two up in 110 deg F weather in the mountains your going to have to make sure that under those conditions you stay below ALL limits. Now when your riding 1 up with no trailer you will be much cooler and down on both fuel economy and power some amount. So pick your poison wisely for what you plan to do with your bike.

[Jeffd]

Steve, have you found the stock head temp sensors to be accurate?  It seems that mine seems lower then what I would expect and usually is under 250* whendoing the data scans. And great information thanks jeff

[Steve Cole]

It is pretty accurate as Engine Temperature but not as Head Temperature. While the sensor is in the head it is NOT a Head Temperature Sensor.

[FLTRI]

Well don't stop there! 
Please explain what the diferences are and how it might affect tuning stratgies.
Bob

[rbabos]

Head temp sensor tends to run a lower reading then at the sparkplug. Influenced by cooling radiating in from the fins. Head will be different temps within it's mass. Still, it's a good constant for tuning purposes. Good to see you back Bob.
Ron

[wurk_truk]