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Author Topic: Measuring Air/Fuel Ratio for Tuning  (Read 2730 times)

Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« on: April 23, 2017, 12:47:48 am »
Come on Jerry, many of us with your pipe and setup would like numbers for proper EGT. Please pick a certain probe, a certain gauge and a certain distance. Putting more Precision in HPR.

In the old days we used EGT as a tuning crutch on the enduro go karts with limited success.  The RPM changed slow enough that the durable 3/16'" diameter EGT probes would have time to saturate and keep up with the engine RPM.  In order for a guy to know what EGT was critical to his engine package he usually had to sacrifice a few pistons to determine what the critical temperature was for his engine. 

EGT changes with RPM.  It is not uncommon for the EGT to change 100 to 200 degrees from the start of a dyno run to the end of the dyno run.  Durable EGT probes often take many seconds to heat up or cool down to the temperature of the exhaust flowing over them.  Fast responding probes are fragile and often break due to the vibration and high temperature.  Most of the portable EGT gauge/meters I have used, sample 2 time or so per second.  The data acquisition system in the dyno room has the ability to sample 100K per second.   

One jet size will usually change the EGT on most engine 25 to 50 degrees F.  Moving the timing 1 degree can change the EGT around 50 degrees F.  One compression ratio point can change the EGT 50 to 100 degrees F.  Engines with transmissions  will accelerate 2000 to 3000 RPM per second in the lower gears.

As the compression ratio goes up EGT goes down.   Lower compression  makes the EGT go up.  Advancing the timing lowers EGT.

Considering the above information one can easily see that it is difficult to get accurate EGT readings in the field to correlate with what we see in the dyno room.  Most guys will burn a piston trying to get the EGTs we see on the dyno. 

Use the jet that makes the engine ACCELERATE the best and NOT the jet that gives the highest top speed.  If you optimize the gearing and jetting, the highest top speed will occur when the engine is at it's power peak at about 7600 to 7800 when you have one of our engine packages with the HPR 19 pipe and silencer. 

Running down a road with stock gearing, in high gear, will have the engine RPM way past the power peak where the jetting is usually a little rich.

If you have about stock gearing like most guys run, jetting  a little lean will give the engine more over rev (power after the power peak) while hurting power in the mid range and at the peak.  Higher EGT will shift the power curve to a slightly higher RPM thus giving a slightly higher top speed with the possibility of burning a piston. 


 

 

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