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AP vs OS, OTS/base map vs Protune, Street tune or Dyno Tune. It can be confusing, people define things different ways and use confusing terminology. There are also many misconceptions. I'm going to break it down for you.

First, we will start with some definitions.

AP - Accesport (v2 or v3 for the purpose of these discussions). This is a handheld tool that allows you to load aftermarket mapping to your ECU. The v2 and v3 are functionally IDENTICAL as far the car is concerned. For the user, the v3 gets you a nicer screen and interface. The AP comes with a set of basemaps specific to your car for very specific mod sets.

OS - Opensource. Direct ECU tuning. Generally you wont be loading maps yourself w OS, but some people purchase maps from XPT, TP, etc and load them on the car w a Tactrix cable. This is much less user friendly then the polished AP system, and requires you to use hardware and software from various places.

OTS/Basemap - These are literally base maps such as the ones provided with the AP. They are generic maps designed for the lowest common denominator car. They generally will run a car correctly, providing you have followed the parts list pretty much to the 't'. You leave some power, economy, response on the table using these type of maps. Sometimes quite a bit. You are also limited in your parts selection.

Street Tune - Tuning done without the use of a dyno. Properly done, there is no problem with this. A dyno is a good tool for extracting maximum power, and doing back to back comparisons of parts, but proper tuning can be accomplished on the street. Especially on more basic setups. It involves having a protuner basemap your car, log it, and make revisions. How much you leave on the table is mostly dependent on how good your tuner is, and how extreme/complex your setup is.

Dyno Tune - Tuning with the use of a dyno.  Personally I use a Mustang AWD-500 that allows me to do true road simulation so I can tune for all driving conditions while on the dyno. Many other dyno's do not allow for this, and therefore the car should be tested on the street as well for proper driving characteristics, load levels, etc.

Now we should all be on the same page with some basic info and definitions, lets talk about how you should decide what to do with all these options.

Should you get protuned or use a basemap? This is the easiest question to answer (and not because Im a protuner lol). Its always best to be protuned, there is no debate here.

Should you use an AP or OS? Most good subaru tuners will be able to do either. Below or some reasons you would want an AP

Your car is not supported on OS.
You want to run your car with Speed Density rather than MAF based.
You want to have maps for multiple fuel types.
You live far away from a protuner and need to be able to load basemaps, log, and do diagnostics easily yourself.
You have a high power 16bit based car (02-05 WRX ECU, Id say like 450whp and up) and want the extended functionality the AP provides for these models. I personally dont like using workarounds and hacks.
You like gadgets
People often talk about being able to easily flash the car back to stock, but the reality is that any tuner can flash a stock map on your car and really shouldnt charge you for it. Ive even flashed cars back to stock for people that werent my customers. It takes two minutes to do for any tuner and really isnt a big deal. Same goes for being able to read DTC' can get a $10 code reader to do that.
When can you not use an AP? If you are running a JDM ECU, or your car is N/A. Cobb never saw that market to support either of these platforms, which is strange because they make up a reasonable percentage of the cars I tune.

I prefer using the AP for the overall user experience, its just a useful tool and a relatively small cost in relation to your car, or your build.

Street tune or Dyno Tune? This is going to come down to tuner and personal preference. Some tuners dont ever use dynos, and some tuners only use dynos. I'll lay down my philosophy and advice based on my 7 years subie tuning experience and you can take it for what its worth.

I street tune quite a few Subarus. I can do this primarily because of my experience with the platform. Its the same reason I can generate a basemap for a car given almost any list of random parts, and it will start and run the car 90 percent correctly straight away. For very basic setups, its a reasonable cost saving option. I do prefer to use the dyno however.

The advantage to dyno tuning is the ability to do a comparison. I've extensively dyno'd stock subies, and ones of almost every mod level and configuration. This allows me to give good build advice, and to have a rough idea of what power each setup will make. I also know that on the dyno Im getting the most power possible out of a specific configuration, and its easier to spot mechanical problems w a setup. I require dyno tuning for certain level builds, as well as anything SD based.

How the car is tuned specifically is a discussion best had with your tuner directly. How to pick your tuner, well, that's a whole 'nother discussion.....maybe another note ­čśë

Wicked M@