Engine tuning changes the balance between airflow, fuel delivery, ignition timing, emissions, and fuel economy. The figures below combine manufacturer setup charts with EPA testing from different periods, vehicle types, and test cycles, so each measurement period and scope matters.
Contents
- Nitrous jetting by pressure and power
- Fuel-pressure effects
- Timing and spark-plug setup
- Baseline checks and mixture optimization
- Tune-up effects on heavy-duty vehicles
- Passenger-car fuel economy
- Measured ECU tune outcomes
Nitrous jetting by pressure and power
The Nitrous Express Single Nozzle Jetting chart lists matched nitrous and gasoline jets for several claimed horsepower settings. Its pressure rows are manufacturer-chart specifications, not a universal calibration for every engine. The chart is undated and was accessed in 2026.
At 5 psi fuel pressure, the 35-hp setting uses a No. 31 nitrous jet with a No. 18 gasoline jet. The 50-hp setting uses No. 35 nitrous and No. 20 gasoline, while 75 hp uses No. 41 nitrous and No. 24 gasoline. At 100 hp, the listed pair is No. 52 nitrous and No. 30 gasoline; at 150 hp, it is No. 62 nitrous and No. 37 gasoline.
The chart’s 10 psi row keeps the nitrous-jet sequence unchanged for these five settings: No. 31, 35, 41, 52, and 62 for 35, 50, 75, 100, and 150 hp. The gasoline jets are No. 18, 20, 23, 28, and 37 respectively.
| Claimed setting | Nitrous jet | Gasoline jet at 5 psi | Gasoline jet at 10 psi |
|---|---|---|---|
| 35 hp | 31 | 18 | 18 |
| 50 hp | 35 | 20 | 20 |
| 75 hp | 41 | 24 | 23 |
| 100 hp | 52 | 30 | 28 |
| 150 hp | 62 | 37 | 37 |
This comparison shows where the chart changes between the two pressure rows: 75 hp moves from gasoline jet 24 to 23, and 100 hp moves from 30 to 28. The 35-hp, 50-hp, and 150-hp gasoline-jet entries remain the same in those rows. These are chart entries, not independently verified legacy measurements.
Fuel-pressure effects
The same Nitrous Express chart lists additional fuel-pressure rows. At 15 psi, the 35-hp setting uses nitrous jet 31 and gasoline jet 17; 50 hp uses 35 and 19; 75 hp uses 41 and 23; 100 hp uses 52 and 28; and 150 hp uses 62 and 36.
At 20 psi, the nitrous jets remain 31, 35, 41, 52, and 62 for the same five claimed outputs. The gasoline jets are 17, 19, 23, 28, and 34. Across the 15 psi and 20 psi rows, the 35-, 50-, 75-, and 100-hp gasoline entries are identical, while the 150-hp entry changes from 36 to 34.
| Fuel pressure | 35 hp | 50 hp | 75 hp | 100 hp | 150 hp |
|---|---|---|---|---|---|
| 15 psi | 31/17 | 35/19 | 41/23 | 52/28 | 62/36 |
| 20 psi | 31/17 | 35/19 | 41/23 | 52/28 | 62/34 |
Each cell is nitrous jet/gasoline jet. The chart therefore records relatively stable gasoline-jet entries in the 15–20 psi range, with the listed difference appearing at the 150-hp setting. Actual setup decisions still depend on the specific system and engine; the chart itself does not establish a universal result for all applications.
Timing and spark-plug setup
Holley’s Nitrous Oxide General & Technical Information technical sheet, dated 2004, gives a motorcycle fogger baseline in horsepower per cylinder. A nitrous jet 14 paired with fuel jets 16 and 14 is rated at 6 hp per cylinder. Jet 16 with fuel jets 18 and 16 is rated at 9 hp per cylinder; jet 18 with fuel jets 22 and 18 is rated at 12 hp per cylinder.
The same chart rates nitrous jet 20 with fuel jets 24 and 20 at 15 hp per cylinder. Nitrous jet 24 with fuel jets 28 and 24 is rated at 20 hp per cylinder, while jet 28 with fuel jets 32 and 28 is rated at 30 hp per cylinder. These are baseline chart pairings from the 2004 sheet, not a modern fleet average.
Holley’s stated timing baseline calls for 1.5–2 degrees of ignition retard per 50 hp of nitrous. The Nitrous Express chart specifies copper-core spark plugs two heat ranges colder than stock, with a gap no larger than 0.035 inch. Plug type, heat range, and gap are therefore quantified parts of the published setup guidance, while the correct application remains system-specific.
Baseline checks and mixture optimization
The NOS Powershot owner’s manual describes a baseline calibrated for 900 psi bottle pressure and 5–6 psi flowing fuel pressure. Its pre-operation checkout sets engine speed to 2,000 rpm before the microswitch is activated briefly. Those values describe the Powershot procedure and should not be generalized to every nitrous kit.
The manual recommends changing the fuel jet by two sizes when spark plugs indicate an excessively rich or lean mixture. Its rich-mixture example changes a No. 61 fuel jet to No. 59, a two-size reduction. The same manual’s 100-hp timing example treats the increase as requiring 4 degrees of retard, based on 2 degrees per 50 hp, and then adds an initial 2-degree safety retard.
The manual also says the initial timing setting should be 2–3 degrees more retarded than the expected optimum. In the same document, nitrous oxide is stated to be 36% oxygen by weight. These numbers are procedural and technical references from an undated owner’s manual accessed in 2026, not results from a controlled comparison across engines.
Tune-up effects on heavy-duty vehicles
The EPA’s Study of Control Strategies for In-Use Heavy Duty Vehicles Final Report analyzed 181 before-and-after tune-up test pairs. The testing occurred in the 1970s, and the report was published in 1981. Manufacturer-specification tune-ups reduced average hydrocarbon emissions by about 36% and average carbon-monoxide emissions by about 33% in that sample.
Fuel economy increased almost 4% after tune-up in the 181-pair heavy-duty sample. The report valued that improvement at $8.80 per 1,000 miles using a fuel price of $1.30 per gallon. The EPA study evaluated the statistical significance of nitrogen-oxide changes at the 95% confidence level; the supplied result does not state a corresponding percentage change for NOx.
These figures describe older in-service heavy-duty vehicles and manufacturer-specification tune-ups. They should be read as historical measured outcomes for that sample and period, not as a forecast for a current nitrous-equipped passenger car.
Passenger-car fuel economy
The EPA’s Passenger Car Fuel Economy: EPA and Road - Report to the Congress reported an average 6% mpg improvement for tuned versus as-received pre-1974 cars. The report was issued in 1980, and the vehicle population was explicitly pre-1974.
For 300 1975–1976 models, tune-up changed fuel economy by +1.9% in city testing and +0.5% on the highway. The city and highway results are separate measurements, so they should not be combined into an unsupported overall percentage. The different result sizes also show why tuning statistics need a defined vehicle group and test cycle.
Measured ECU tune outcomes
The EPA Derive Emissions Testing Summary Report, published in 2016 from 2015 vehicle testing, measured a 2013 Ford F-150 with a 3.5-liter EcoBoost and an SCT 93-octane tune. The results vary materially by test cycle.
In EPA FTP testing, carbon monoxide rose from 0.536 to 0.578 g/mi, an 8% increase. NOx rose from 0.017 to 0.023 g/mi, a 36% increase. NMHC changed from 0.024 to 0.023 g/mi, a 7% decrease, while fuel economy changed from 15.57 to 15.65 mpg, a 1% increase.
The same tune in EPA HWFE testing raised CO from 0.063 to 0.108 g/mi, a 71% increase, and NOx from 0.004 to 0.005 g/mi, a 25% increase. The report’s supplied HWFE results do not include a separate fuel-economy change here.
In EPA US06 testing, CO rose from 1.02 to 8.75 g/mi, a 762% increase, while NOx fell from 0.107 to 0.053 g/mi, a 50% decrease. In EPA SC03 testing, CO fell from 0.856 to 0.545 g/mi, a 36% decrease. Fuel economy in SC03 changed from 15.26 to 15.27 mpg, rounded to a 0% change.
The ECU results do not support one universal direction for every pollutant or cycle. The same SCT tune shows increases in some FTP and HWFE pollutants, a very large US06 CO increase, a US06 NOx decrease, and an SC03 CO decrease. The measurements are specific to the 2013 Ford F-150, the 93-octane tune, the stated EPA cycles, and the 2015 test period reported in 2016.