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For anyone learning how to tune; the basic concepts are important. But it’s just as important to know your target values. Below are some ideal values for a completed tune.
AFR (Air to Fuel Ratio)
The AFR is the blend ratio of air and fuel. It is measured by the O2 (wideband) sensor. Numbers that are higher than 14.7 are lean. Numbers that are less than 14.7 are rich. The number 14.7 is a “stoich” value for gasoline, and is the ideal ratio for normal conditions with a gas tune. When you tune, try to use the target AFR values below.
To keep your tune extra safe, you can subtract 0.5 from all of these numbers.The engine will run rich, and be more forgiving to tuning mistakes.
Boost
The amount of boost you can apply to your engine is a direct relation to your fuel’s octane rating. Most SyTy’s have a similar compression ratio, so these Boost levels should be accurate. When you tune, try to set your timing table first, and slowly increase the boost level.
Ignition Timing
The ignition timing advance in the engine controls the fuel burn. Higher RPMs require more timing advance. However, more boost requires less timing advance, depending on fuel. During normal conditions, your engine operates like a non-turbo’d V6. Therefore, the proper ignition timing for a 4.3 applies. Once the engine goes into boost, you must reduce the timing advance as boost increases. Depending on your fuel octane, you can reduce timing at 1:1 (1 degree for 1psi of boost), or you can reduce at 2:1 (2 degrees for 1psi of boost). Either way you ideally want to keep the minimum timing at 15 degrees in boost. Try not to reduce the timing below that. You will notice that increasing the timing at part-throttle gives the truck better throttle response, makes the engine feel peppy, and gives better gas mileage. Increasing the timing in boost will add more horsepower. However, you need to be careful, as increasing the timing too much can hurt the engine. Keep an eye out for knock when increasing timing.
When using racing fuel, Alky or E85 you can raise the bottom timing advance while in boost. You will see the most benefit with timing in the mid 20s.
These values will give you a ballpark idea of where you want to be with your tune. Every engine is different, and one tune does not fit all trucks. A safe tune will make your engine last longer. Running your engine at peak power all the time is a sure way to engine failure. You can have a safe tune by lowering the target AFRs (0.5), reducing the boost pressure (3psi), and reducing the ignition timing (2deg). Remember that when you use pump gas your tune is limited by the octane rating. The engine will knock way before you get to peak horsepower. If you use racing fuel, your tune is limited by engine part failing. So you can get every last horsepower out of your engine by using racing fuel, but you will have parts break first.
AFR (Air to Fuel Ratio)
The AFR is the blend ratio of air and fuel. It is measured by the O2 (wideband) sensor. Numbers that are higher than 14.7 are lean. Numbers that are less than 14.7 are rich. The number 14.7 is a “stoich” value for gasoline, and is the ideal ratio for normal conditions with a gas tune. When you tune, try to use the target AFR values below.
- 14.7 - 14.0 during Idle & light-throttle (less than 25%) Cruising
- 13.5 – 13.0 during part-throttle (50% or more) Cruising & Spoolup
- 12.0 – 11.5 while in Boost
- 11.5 – 11.0 when using Alky Injection in Boost
To keep your tune extra safe, you can subtract 0.5 from all of these numbers.The engine will run rich, and be more forgiving to tuning mistakes.
Boost
The amount of boost you can apply to your engine is a direct relation to your fuel’s octane rating. Most SyTy’s have a similar compression ratio, so these Boost levels should be accurate. When you tune, try to set your timing table first, and slowly increase the boost level.
- 91-93 octane pump gas, 15psi – 18psi of boost
- Alky Injection and pump gas, 18psi – 25psi of boost
- 110 octane racing gas, 18psi – 25psi of boost
- 116 octane racing gas, 20psi – 35psi of boost
- E85 pump fuel, 18psi – 35psi of boost
Ignition Timing
The ignition timing advance in the engine controls the fuel burn. Higher RPMs require more timing advance. However, more boost requires less timing advance, depending on fuel. During normal conditions, your engine operates like a non-turbo’d V6. Therefore, the proper ignition timing for a 4.3 applies. Once the engine goes into boost, you must reduce the timing advance as boost increases. Depending on your fuel octane, you can reduce timing at 1:1 (1 degree for 1psi of boost), or you can reduce at 2:1 (2 degrees for 1psi of boost). Either way you ideally want to keep the minimum timing at 15 degrees in boost. Try not to reduce the timing below that. You will notice that increasing the timing at part-throttle gives the truck better throttle response, makes the engine feel peppy, and gives better gas mileage. Increasing the timing in boost will add more horsepower. However, you need to be careful, as increasing the timing too much can hurt the engine. Keep an eye out for knock when increasing timing.
- Idle timing should be in the mid 20s
- Part throttle timing should be around 30deg (+/- 5deg)
- Spool-up before boost should be in the mid-high 20s
- 0psi to 3psi hold the same timing as Spool-up
- 3psi+ start reducing timing as boost increase.Bottom out at 15deg (pump gas).
When using racing fuel, Alky or E85 you can raise the bottom timing advance while in boost. You will see the most benefit with timing in the mid 20s.
These values will give you a ballpark idea of where you want to be with your tune. Every engine is different, and one tune does not fit all trucks. A safe tune will make your engine last longer. Running your engine at peak power all the time is a sure way to engine failure. You can have a safe tune by lowering the target AFRs (0.5), reducing the boost pressure (3psi), and reducing the ignition timing (2deg). Remember that when you use pump gas your tune is limited by the octane rating. The engine will knock way before you get to peak horsepower. If you use racing fuel, your tune is limited by engine part failing. So you can get every last horsepower out of your engine by using racing fuel, but you will have parts break first.
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