This is an article I put together years ago but nobody picked it up. The Charts did not transfer to this text well but you get the idea. I did seven different runs with different jetting and timing on my 340.
Exhausted: Competition Tuning for Maximum Performance
By Chris Holley
Photography by the author
In an effort to maximize the performance of the internal combustion engine, the use of exhaust gas temperature (EGT) thermocouple probes and the use of wideband oxygen (O2) sensors have allowed professional and sportsman drag racers to tune their engines for maximum performance. While the use of these monitoring devices is nothing new to drag racing, the expense of these devices has dropped to a range that can be afforded by even a racer on a limited budget. With that in mind and the fact that it was time to replace the headers on the ’69 Dart, it seemed an opportune time invest in EGT and O2 monitoring to see if a bit more performance could be extracted from the old 340.
The use of EGT monitoring without the use of an O2 sensor can result in confusing results. Both a rich mixture and a lean mixture will result in high exhaust temperatures; rich mixtures will have burning air/fuel moving out of the head into the exhaust manifold or header while a lean mixture will result in a poorly running engine. In addition to fuel mixture, ignition timing has a great impact on the EGT. A proper air/fuel ratio in conjunction with retarded ignition timing will result in high EGT readings. Engine detonation will decrease the EGT due to the heat being absorbed in the combustion chamber allowing less heat to move out of the head into the exhaust. For most applications the EGT ranges between 1250 degrees F to about 1600 degrees F at WOT. Most applications will make maximum power with an air/fuel ratio between 12:1 to 13.5:1 but each combination will have its own specific air/fuel ratio. There is also no optimum EGT range for each application; any change of the camshaft, compression ratio, cylinder head(s), or exhaust components will have an effect on the optimum EGT range of the application.
For the EGT monitoring the Percy’s digital exhaust temperature indicator kit was selected with the two ¼” NPT thread-in probes with 6-foot leads (one probe comes with the kit the other was purchased separately). The probes thread into bungs welded to the header(s) or Percy offers clamp-on probes if welding is not an option. The kit includes the digital display unit for monitoring; the display has a hold button for recording measurements, the abilities to record the maximum temperature reached, and provisions to monitor two cylinders at once. The unit is housed in a tough rubber molded case that provides excellent protection. Additional bung and plug kits were purchased to provide EGT monitoring of every primary header tube (only two at any time).
For the O2 monitoring the FAST’s racing dual sensor wide band air/fuel meter was selected. The dual O2 sensors allow the measurement of the air/fuel ratio of each bank or the ability to average both banks together. The features include a built-in data logger with on-screen playback and provisions for outputs to external data loggers; the unit’s built-in calibration and quick start-up delivers air/fuel readings in under a minute.
After the arrival of the new headers, Hooker Headers (1 5/8” primaries into a 3” collector), work proceeded on welding the thermocouple and O2 bungs to the header pipes. The thermocouple bungs were welded to the primary pipes about 1” downstream from the mounting flange of the header. The driver’s side pipe provided the greatest challenge as the #3 pipe extends over pipes #5 and #7; the thermocouple bung for pipe #3 had to be placed strategically to clear the spark plug and wire while pipes #5 and #7 required the bung to be rotated to clear pipe #3. I made sure there was clearance around the inner fenders of the engine bay especially on the passenger’s side. The O2 sensor bungs were welded into the collector approximately 8” from the collector opening. The O2 sensor bungs had to be positioned so there was zero interference with the transmission pan.
The installation of the headers was the normal routine with the added requirements of running the thermocouple leads from the Percy unit to the header thermocouple bungs. A small hole and grommet was required in the firewall to allow the leads to pass from the driver’s compartment to the engine compartment. The unused length of the leads was zip-tied under the passenger compartment dash to prevent the leads from getting tangled on something in the engine bay. With the thermocouple probes attached to the primary tubes of cylinders one and two, (front tubes of each bank) it was possible to attach the leads to the Percy unit. The leads attach to the Percy unit with a simple push motion sliding the male connector leads into the female ports on the Percy unit. After the battery installation the unit was powered up and the unit displayed the ambient temperature of T1, bank one; depressing of button T2 the unit displayed the ambient temperature of bank two.
To power the FAST air/fuel meter a cigarette lighter power adapter was supplied but this would not work with on the Dart as the 12-volt supply to the cigarette lighter was no longer operating. Wiring a fused 12-volt source and the ground wires to the air/fuel meter had the unit operating in short order. The two O2 sensor wire looms were run through the floor, one to each bank. The O2 sensors were screwed in the collector bungs located about 8” from the collector flange.
Upon completion of the installation it was time to put the new headers and monitoring devices to the test; a trip to Numidia Dragway, located in Central PA, was required. Numidia was having a test and tune session that would allow at least six passes on their newly poured concrete and polished track. A lot of money has been spent on safety and track improvements at Numidia and I was more than happy to test at their track.
Once at the track the 340 was warmed up and run through technical inspection. There were zero problems with the car so it was back to the trailer to prep for the first run. The prep consisted of topping off the fuel cell with Sunoco 93 octane fuel, checking the tire pressure, cleaning the glass, and peeking under the hood for any loose components or leaks. With that everything was ready to go.
Run One - Off the Trailer
The left lane was selected to perform all the tests for the day. To establish a baseline, the timing set at 36 degrees and the jetting on the Holley 830 CFM carburetor was set to 84s square. The Dart was shifted at 6400 RPM, with engine temperature about 170 degrees F, and the manually shifted automatic transmission temperature about 160 degrees F. The ambient temperature for the first pass was 70 degrees F, with a barometer reading of 29.03”, and relative humidity of 39.0%; this resulted in an altitude density of 2241’ above sea level. Additional components to consider were the track temperature of 84 degrees F and a tail wind speed of approximately 6 MPH. The first run produced an elapsed time of 11.757 at 112.38 MPH. The FAST air/fuel meter’s display showed an 11.4:1 A/F ratio and an EGT temperature of 1407 degrees F.
Run Two
With the baseline established it was time for an adjustment. With the A/F ratio indicating too little oxygen or a “rich” condition it seemed that a jet change was necessary. The primary jets were jetted down to 82s while the secondaries were left jetted with 84s. The ambient temperature for pass two was 71.5 degrees F, with a baro reading of 29.01”, and a relative humidity of 40.8% which established an altitude density of 2380’. The track temperature had risen to 91 degrees F. After the run the O2 sensors monitor reported an A/F ratio of 12.0:1 and a decent EGT of 1398 degrees F but the results were less than acceptable with an elapsed time of 11.856 at 112.46 MPH.
Run Three
With the data provided from the first two runs it was decided that more timing would be needed to lower the EGT readings and lower the elapsed time. So with the jetting remaining the same as in run two, the timing was bumped up three degrees for a total of 39 degrees total advance. With very similar atmospheric conditions of the first two runs, ambient temperature 72.7 degrees F, a baro reading of 29.01”, and the relative humidity of 41.9%; the resulting density altitude was 2479’. The track temperature had risen to 93.1 degrees F. After the run the EGT reading was 1371 degrees F and the A/F ratio remained at 12.0:1 but the timing really improved the elapsed time. The ET dropped to an 11.731 at 113.34 MPH.
Run Four
The timing really got the Dart moving so an additional three degrees was added to increase the total timing to 42 degrees. The ambient temp remained 72.7 degrees F for this run but the baro reading had dropped to 29.00”, with a relative humidity of 40.5% which resulted in an density altitude of 2465’. The track temperature dropped slightly to 92.1 degrees F. The run produced an EGT reading was 1343 degrees F and the O2 sensor reading resulted in a 12.2:1 A/F ratio. The ET really responded to the timing as the Dart slipped down to an 11.632 at 114.30 MPH.
Run Five
With the A/F ratio of 12.2:1 it seemed that more jetting was required so the carburetor jetting was set back to the off the trailer setting of 84s square. The ambient temperature had risen to 73.9 degrees F, and the baro reading dropped to 28.97”, with the relative humidity reading of 38.9%. The altitude density had risen to 2577’. With the jetting set at 84s square the EGT reading at the end of the run was 1329 degrees F and the O2 sensor reading was 11.9:1 A/F ratio. The additional fuel really picked up the Dart, dropping the ET to 11.573 at 115.44 MPH.
Run Six
The old 340 seemed to like the additional fuel so a little more surely couldn’t hurt. It was time to remove the jets from the secondaries; the 84s were replaced with 86s. The atmospheric conditions had improved slightly with the ambient temperature dropping to 72.8 degrees F, a baro reading of 28.97” and the relative humidity remaining the same as run five at 38.9 %. The altitude density dropped to 2507’. At the completion of run six the EGT readings increased ten degrees to 1339 degrees F and the O2 sensor reading was 11.7:1 A/F ratio. The ET dropped again this time all the way down to an 11.505 at 116.12 MPH.
Run Seven
The last run of the day, a high 11.40 run had to be in the cards. Although the O2 sensor readings were indicating low oxygen, a “rich” condition, a jetting change was still selected to try to increase the performance of the Dart. The primary jets, 84s, were replaced with 85s. The ambient temperature had dropped a tenth to 72.7 degrees F, the baro had dropped to 28.95”, and the relative humidity was up to 40.4% resulting in the altitude density as 2531’. The last run of the day produced EGT readings that increased to 1350 degrees F, while the O2 sensor readings dropped to 11.5:1 A/F ratio. The result speaks for themselves, an 11.499 at 116.15 MPH corrected to sea level the run would have produced an 11.338 at 117.89 MPH; this was the best corrected ET run ever for this engine combination.
Run
Time Time After Last Run
ET 60FT
ET 1/8TH
MPH 1/8TH
ET ¼ Mile
MPH ¼ Mile ET ¼ Mile Sea Level MPH ¼ Mile Sea Level
EGT (F)
A/F Ratio
Total Timing
JettingPri/Sec
1 11:14 - 1.599 7.428 91.160 11.757 112.38 11.592 114.07 1407 11.4:1 36 84/84
2 11:50 0:36 1.619 7.504 90.240 11.856 112.46 11.690 114.15 1398 12.0:1 36 82/84
3 12:29 0:39 1.584 7.419 91.180 11.731 113.34 11.567 115.04 1371 12.0:1 39 82/84
4 13:05 0:36 1.575 7.352 91.920 11.632 114.30 11.469 116.01 1343 12.2:1 42 82/84
5 13:46 0:41 1.567 7.322 92.280 11.573 115.44 11.411 117.17 1329 11.9:1 42 84/84
6 14:17 0:31 1.559 7.282 92.940 11.505 116.12 11.343 117.86 1339 11.7:1 42 84/86
7 14:53 0:36 1.555 7.276 92.990 11.499 116.15 11.338 117.89 1350 11.5:1 42 85/86
Run Lane Air Temp Barometric Pressure Relative Humidity Track Temp Altitude Density
1 L 70 29.03” 39% 84.0 2241’
2 L 72 29.01” 41% 91.0 2380’
3 L 73 29.01” 42% 93.1 2479’
4 L 73 29.00” 41% 92.1 2465’
5 L 74 28.97” 39% 94.1 2577’
6 L 73 28.97” 39% 94.9 2507’
7 L 73 28.95” 40% 95.5 2531’
Conclusion
The Percy digital exhaust temperature indicator kit was easy to set up and very simple to use. It provided valuable information that was easily read on the monitor. The FAST air/fuel monitor was a very effective tool for tuning; the data logger and playback selection was a great feature that allowed me to review an entire completed run. The combination of the two units together provided the greatest amount of useful information. Was the investment of the EGT and O2 monitoring worth the money? I believe it was; reducing the elapsed time by .258 seconds in the quarter mile in one afternoon was well worth the cost and effort. Also, picking up almost 4 MPH indicated there was an increase in horsepower. There may have been slightly more power to be found but this was an excellent increase in performance for a single day’s work.
About the Dart
The Dart is a 1969 Dart Custom that left the factory in late ’68 with a 318 engine, a 904 Torqueflite transmission and an 8 ¾” rear end. When I purchased the Dart in 1989 the 318 was long gone and a ’68 GTS 340 was nestled between the frame rails. The original 904 and 8 ¾”, with 2.76 gears, were still present. The 340 has a factory 1053 steel crank spinning SCAT h-beam rods with TRW forged pistons. A .557 lift 296 duration Mopar Performance Purple camshaft is spun by the crankshaft via a double roller timing chain. The heads are Edelbrock aluminum units with 2.02 intake, 1.60 exhaust valves. The valves are opened by Crane Cams 1.5:1 rocker arms. The intake is an Edelbrock Torquer II topped with a Holley 830 CFM carburetor. The headers are Hooker as described in the article. The 904 is a reverse manual with an A&A valve body and a deep pan. The 8 ¾” is fitted with 4.10:1 Richmond gears that rotate 26” X 9” Hoosier slicks mounted on 15” X 7” Weld Draglites. The Dart sports plenty of fiberglass parts including the bumpers, hood, and fenders and tips the scales at 3033lbs with the driver in the car.
All the stock NHRA 340 motors I've help build and raced like 30 to 32 BTDC at sea level, I have heard of other 340 racers using 40 or 42 BTDC, but once they tried 34 to 36 BTDC there cars ran faster 
test, test and test some more 
