I want to use zwift to do my FTP test. (Im to zwift and I and using a Wahoo Kickr Snap indoor trainer). When i went to ride it i had my gears as high as they would go and the trainer wasnt increasing resistance. Am I supposed to change it to ERG mode? Im also curious if i can do an FTP test using a Garmin Edge 520 plus.
I used a 1030 for my FTP test, my 520+ also has the same capability.
For your FTP test I would recommend not using ERG mode. ERG mode will hold you to one specific watt. If we knew exactly what power you could hold for 20 minutes there would be no reason to do a test. You don’t want to get held back because the trainer is controlling the power. But with that said you still want to stay in a range early on so you don’t blow up 10 minutes in. Go in based on what you believe it is. Hold that range and see how it goes.
You can use Zwift, but I wouldn’t have it control your resistance in game for the FTP test. You don’t want to feel the effects of hills and drafting. However there maybe a FTP test workout in Zwift already that you could use. This should keep the resistance smooth.
You can set the resistance on your trainer using the Wahoo app on your phone. Sounds like the resistance was set far too low since you were in the biggest gear. So you would need to increase the resistance on the trainer. That will need to be adjusted for the workouts as well I’m assuming since you were already in your biggest gear.
Why not use the Ramp Test in Zwift? Yes, it uses ERG mode but that’s the point. It changes the wattage requirements by I believe 20 watts every 60 seconds. It then computes your FTP by using a percentage from the last 60 second period completed. I find it seems to work well for its intended purpose.
The 20 minute Field Test. Test outdoors - more realistic. Ramp tests are for the lab (which you don’t race in)
Give our podcast on field tests a listen too + read the training tip we include with your plan (so technically #FtFP 
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Thanks ill try it. Just trying to get an accurate honest baseline.