Choosing the right fuel injectors is a critical aspect for performance as well as ease of tuning of your Harley Davidson Twin Cam or Milwaukee-8 engine. See the information below to determine which fuel injectors will work best for your application.
Fuel Injector sizing
Its important to understand adding larger fuel injectors will not create more horsepower, however they are necessary to support additional horsepower above & beyond what the stock injector is able to deliver. The OEM injectors used in some applications are large enough to support mild to moderate increases in horsepower without the need to replace them, however Harley Davidson has used an assortment of injector sizes which in some cases narrowly support mild upgrades such as an aftermarket camshaft; higher power levels may require larger injectors.
Here are the OEM injector sizes for H-D Delphi injected models:
- 4.38 gm/s – 2017-later Milwaukee-8 (all models)
- 4.35 gm/s – 2008-2017 Twin Cam Throttle by wire
- 3.91 gm/s – 2006-2017 Twin Cam w/cable throttle
- 4.35 gm/s – 2001-2005 Twin Cam
We put together the following chart to illustrate what we consider to be the optimum injector size based on rear wheel horsepower. This info is based on experience of 2 decades of dyno tuning as well as the wide range of data we work with day in day out. This is to be used as a general guideline and may vary based on the application.
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Increasing the injector size is necessary when your engine has exceeded the horsepower capacity of the OEM fuel injectors, above that point larger injectors are required to support the additional horsepower. Proper injector sizing is crucial; too small injectors and you risk running the injector out of duty cycle, losing fuel control of the injector, however using an injector too large can be an even bigger problem as it can cause low RPM drivability and tuning issues as you lose resolution on the injector when tuning at an excessively low duty cycle.
Duty Cycle
Injector duty cycle is the amount of time the fuel injector is energized during an engine cycle. To determine duty cycle for any given point you need to know the injector pulsewidth and specific RPM, you then then use the following equation:
(Pulsewidth * RPM) / 1200 = % Duty Cycle
An easy option if you have a Dynojet Power Vision is to configure Injector Duty Cycle Front & Rear and monitor or log it, this is also in the Thundermax Smartlink software as a monitor option.
The general rule traditionally for internal combustion engines is to keep max duty cycle within 80%, however we’ve actually established that you can run the duty cycle in HD applications up to about 90% ceiling in most applications very reliably & accurately. Using injectors that are not larger than necessary gives you the benefit of better injector control at lower RPM’s which is extremely beneficial on these engines for tunability especially with cams with a lot of overlap, and you still have more than enough ceiling in the upper RPM’s. These engines have a relatively low brake specific fuel consumption (BSFC), they have solid fuel pressure & delivery, and realistically these engines are not subjected to the same loads & conditions one extreme to the other as say a passenger car.
Choosing your fuel injectors
Here are some generalizations as to when you will be required to upgrade injectors.
- Milwaukee-8 engines: the 4.38 injectors on these engines work well in stock & stage 1 applications, in many cases they can still be utilized with an aftermarket camshaft up to about the mid-upper teens for HP. If you have a top performing cam & exhaust you may likely need to upgrade injectors, we generally go to the 5.5gm injectors. If you are installing a big bore kit larger injectors are mandatory, see the chart above based on your respective rear wheel horsepower, most applications will use the 5.5gm or 6.3gm injectors.
- Twin cam Throttle by Wire: the OEM 4.35 injectors in this application can be used up to the upper teens for horsepower, above 120Hp and larger injectors are mandatory. We use stock injectors in the great majority of the mild 107″ and 110″ big bore applications, however if we increase the size of the throttle body or in higher output builds we generally use the 5.3gm injector. For higher displacement engines see the chart above.
- Twin Cam Cable throttle 2006-2017: these models use the small 3.91gm fuel injectors, these injectors are good up to about mid 90’s horsepower. The OEM injectors will work in most 96″ and 103″ combinations with Stage 1, bolt in cams, or mild big bore kits with stock heads. Applications 100Hp or more require larger injectors, we generally use 4.9gm or 5.3gm injectors. See chart above for sizing for your combination.
- Twin cam Cable throttle 2001-2005: in most cases the stock injectors are large enough as they will support just over 115 Hp which cover bolt in cams and most big bore kits up to 100CI. If you have a larger displacement engine see the chart above.
For questions on fuel injectors or what may work best for your application we can be contacted at 920-423-3309 or info@fuelmotousa.com
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