EGT Reading for Max Tuning

[Tom Rix]

I have run 2 EGT probes on my race cars for several years and have found them to be an excellent tuning/jetting tool. I usually install them on the hottest cylinder on each bank as shown on Dyno. Most typical gas fuel engines like between 1250-1450 degrees. I run my Super Stocker close to 1450 degrees and the Top Stocker at about 1350. Keep in mind that no 2 engines are alike and many things effect the exhaust gas temperature including: engine design, compression ratio, fuel mixture, ignition timing, spark plug heat range, fuel, operating RPM, location and type of thermocouple, etc.

Assuming you are using EGT's and have found the "right" temperature for max performance then write down the conditions BECAUSE there is not just 1 "right" temperature as it CHANGES with weather conditions!! As an example, let's say your engine performed the best with an EGT of 1385 with an inlet air temp of 72 degrees. Then the next time you race the air inlet temp is 87 degrees. If you "shoot" for the same 1385 EGT you will be more than likely RICH! The probable ideal temp for that day for max power would be 1425+. As air inlet temps go up versus your baseline EGT's must also increase to reach max power! Conversely, as air inlet temps go down versus baseline the EGT's need to drop and jetting needs to increase to reach max power levels.

Remember not only to keep track of outside temp but what temp the air is going into your carb! Unless you have a hood scoop your "inlet" air temp can change from run to run as your radiator/water temp changes or just your overall underhood temperature changes! Hmmm, would running as consistent air inlet temps as possible be of benefit??? Food for thought.

Good Luck at Morocco!
Tom Rix

[Nitro71455]

Thanks for the tips!!! I just installed a EGT in perperation of my Procharger :).

I didn't install the prob in my header tube (I know I know) but my headers are coated and I didn't want to mess up the finnish. I put the probe right after the collecter. What temp do you think I should be running with the prob there?

I know it's not going to be quite as accurate, but it's at least going to give me some tunning to go by once the charger is on.

Rich

[flynbuick]

Tom

I have considerable experience with this concept for aircraft piston engines both with carbs and with fuel injection. On aircraft engines we have the advantage of manually leaning the engine's fuel air ratio from the dash while watching an exhaust gas temp gauge. On less sophisticated aircraft this could be a one probe set up meaning only one typical worst case (Hottest) cylinder is being monitored. The better systems have probes on all cylinders and we lean looking at them all which brings me to the point. It seems like the same cylinder should theoretically reach peak EGT each time you lean the engine but this is not always the case. Best power on an EGT to power curve is in fact not even at peak EGT but generally around 50 degrees rich of peak. Now you combine this with the fact that carbed engines do a lousy job of equally distributing the fuel air ratio among the cylinders. Even for fuel injected engines, because the intake runs are different distances, you must have the injector nozzles individually sized to equalize the mixtures and even then for various power settings give the various conditions a different cylinder may peak EGT first. The important point to be drawn from this experience is that in order to protect the cylinder(s) which naturally tend(s) to be the first to peak under the conditions from an overlean condition you are relegated to tune for best power, close to 50 degrees rich of peak, for that one cylinder. This compomise tune is going to be too rich for the remaining seven to make best power. I presume this is in turn is going to be reflected on the spark plugs deposits as a too rich condition overall. The ability of fuel injected systems to somewhat overcome this inherent problem of carbed engines is how they may more power for a given set up.

Jim Lore

[Tom Rix]

Hi Jim,

Good post and I agree with your points. I probably should have said "max" power on safe side. Many of the fastest "stock" cars out there run individual cylinder timing (MSD 7 or bend reluctors in distributor), staggered jetting and different cam duration on each lobe trying to get identical power out of each cylinder! Data loggers and EGT's from all 8 cylinders are common. However if you only have 1 or 2 EGT's the key is that the EG temp will change as air temp conditions change. Too many racers "shoot" for the same EGT every time and do not always reach their max power. Thanks for the further explanation.

Tom Rix

[GSXMEN]

Tom and Jim,

Excellent tips!!:TU: I had always been told that EGT's were a great way to tune your engine properly - this really helps!!:bglasses: