Will try another spin down calibration and if that doesn’t work I’ll return it. Might just spring for a direct drive.
120 on a 23 isn’t terrible (but you could go lower…particularly given the heat trainers impart to tires, which will bump up the pressure a bit. I remember tires blowing off of tri bikes in transition when people would set skinny tires to max psi in the cool of the morning, and they’d heat up as the day progressed 
)
I would however, entirely separate from your issue, look into getting a trainer tire. Or a Gatorskin. They are thicker tires, and made to withstand the heat of running on a wheel-on trainer. If yours is a Grand Sport Extra it should be a little tougher than a Grand Sport Race. But a trainer-specific tire would be a good idea. (Or a gatorskin, which is pretty much built the same.)
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At this point though I think tire pressure isn’t your issue with this trainer. It does sound like it’s not calibrating correctly.
One thing I’ll add about tightening the trainer, is that on wheel-ons I’ve had, the ‘tighten till you can’t make it slip’ has always meant ‘tighten till you can’t make it slip a lot’. When wheel-ons are tightened properly, if you hold the seat down and grab the wheel and jerk up, there will be a little slip. If you’ve got it tightened to the point that you can’t make it slip at all, that may be too tight.
Right. I wonder if I just have it too tight bc it really cranked it down.
Typically a given trainer will say for example to turn the knob until the roller just touches the wheel, and then X number of turns. (Although some don’t even say that in the manual and you have to find out by searching online.) But how many turns exactly will depend on things like tire pressure, and the construction of the trainer.
What you want is for the tire to not be slipping around when you’re riding. Even when tightened correctly, there may be some slippage if you get out of the saddle and hammer on the pedals–but in general, you want it tight enough so regular and consistent hard efforts don’t cause it to slip. Usually, that comes at a point where like I said, you could still make it slip a little by hand. It’s probably worth, before you send it back, playing around with the tightness a bit. Won’t hurt anything.
You give advice and link to an article that has absolutely nothing to do with trainer tyres.
And I was being polite not bringing familiar politics into the discussion.
Okay. Not sure what you mean by ‘familiar politics’. Are you sure you aren’t confusing me with someone else? Because I have no idea who you are. 
And I have no idea what sort of ‘politics’ are involved in talking about tire pressure. Honestly very confused here.
Regarding tires–trainer tires are thicker, more durable tires. There are other differences regarding tread patterns, but they still follow the same principles. Wider tires–larger volumes of air–don’t need the same pressures. Most places you care to look will suggest running trainer tires at the same pressures you’d ride a similar-sized tired on the road. If you’ve got some other sources, I’d appreciate seeing them.
Here’s Wahoo’s Kickr Snap page, for example: https://support.wahoofitness.com/hc/en-us/articles/115000296724-KICKR-SNAP-Smart-Trainer-Information-and-Setup
That’s where they say: “Inflate the bicycle’s rear tire. For 23c road bikes, we recommend 100 - 110 PSI. For other sizes and styles, use the manufacturer’s recommended tire pressure.” Given that 100-110 is in fact in the range for pretty much all 23 road tires, they are saying ‘run your tires on our trainer at the pressures you normally would.’
And let me also point out–he’s not running a trainer tire.
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I used a CycleOps Fluid 2 wheel-on (dumb) trainer for many years with normal road tyres, usually 23c. My tyre pressure was around 100 psi and I used to set the drum pressure with about 2.5 turns of the knob to give a good contact patch and avoid slippage. If the contact pressure of the tyre on the drum surface is too high it will feel almost impossible to turn the gear over.
The difference between tyres on the trainer/turbo and IRL is that, with the former, the pressure isn’t being optimised to support the weight of the rider. You are simply trying to establish good contact patch between the tyre and the rotating drum.
I would recommend reading your trainer manual to see what they recommend (number of turns for tightening the drum) and then adjust your tyre pressure to get you in that range.
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I used a Snap until 5 months ago. Try: no more than 110 psi. Turn the knob 2 full turns after it just barely engages with the tire. Perform a spin down using the wahoo app. If the spin down doesn’t seem to be working—eg, taking too long to stop, seems too easy or too hard to get to test speed— try a factory spin down. Google “Zwift insider Kickr factory spin down” for instructions. I’d also suggest a trainer tire—my Snap seemed to eat through regular tires—but some folks seem to have no problems.
There were a few times over 3 years, particularly at the beginning, when something seemed off, and I had to shut things down and reboot once or twice or three times. But for the most part, the snap worked extremely well. I changed to direct drive when Zwift was dumping its Cores at prices too low to resist.
So I think I figured it out. I read that a wahoo spin down should take 10-15 sec and mine were taking 30-45 seconds. So i did repeated spin downs tightening the roller against my wheel until I got 18 secs. Then I jumped into zwift and 3 two min intervals at 125 watts and it felt incredibly easy. But I really had to crank the roller down on my wheel.
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Nice!
Just out of curiosity, is it tightened down more than you had it before, or less? And can you get the wheel to slip a little by holding the saddle down and sharply pulling the wheel up? Just curious to see if that was part of the issue or not.
Really happy you got it working
I initially tightened it until I couldn’t get it to slip but this is several turns further. Really tight.
There is zero slip now even with really pulling on the wheel or sprinting
Interesting. I’m definitely in the camp of ‘do what works’ But now that it’s properly calibrated, I might take a look at the manual and see how it goes tightening it just to whatever tension they recommend. I’m pretty confident, including looking at Snap-specific experiences from other people here, that ‘can’t budge it at all’ is too tight. It’s possible that could cause your tire to wear faster, for example, or maybe even trainer parts to wear faster. If it worked to get it calibrated, great. But at the risk of tempting fate so it doesn’t work again 
I’d test it at the manual’s tension and see.
But I’m happy it’s working, good deal.
I sent wahoo an email and will see what they say
But the actual manual says tighten until you can’t make the tire slip at all while holding the roller still
Okay, well–I’d say go with that then
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Another route to getting there is to tighten against a partially deflated tire, then inflate the tire.
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i used to have a snap, and i put a mark on the knob so that i could tell how far i turned it. i used to do just under 2.5 turns – and that seems like A LOT of turning, especially if you don’t have a mark to show how far you’ve gone.
2-2.5 turns is normal – you need that spin down time to be around 12-15 seconds. spin down time is the key here – forget about slipping or PSI until you can get that!
once you have the spin down right, you’ll need to calibrate the trainer. ideally, you have warmed up the tire for 10 mins before you do the calibration. in a perfect world, you should re-calibrate the trainer after 10 mins warmup EVERY SINGLE RIDE. most people recommend using the wahoo app for the spin down. zwift used to have a calibration in-game, and that was more convenient, but i’m not sure if that is still the case.
finally, once you have all that working, you should THEN focus on PSI. did you do the spindown and the calibration at 120PSI? if so, then you should always re-inflate the tire to 120PSI before you start riding!
if you get this working, and you enjoy zwift, i recommend moving to a direct-drive trainer that has auto-calibration. it’s way more expensive, so not a great starting point, but if you are into zwift at all, not having to deal with all the crap above makes life much nicer!
Nah.
Auto-calibration is nice to have, but a direct drive trainer without auto-calibration (most of them) isn’t that expensive these days, and will give a much more consistent experience than a wheel-on trainer.