Hey, this is my first post here. I hope someone can help.

I've been training on and off for 11 years so I know what I'm doing. The only issue is that I'm only just getting to know HIIT training. Now I know it's a great method to lose fat and spare muscle.

I'm busy bulking right now. I'm 5'11" and weigh around 217lbs with 15-20% bodyfat. Of course I'd like to control it and later cut it down.

I'm reading alot of conflicting information on HIIT during bulking and it's muscle sparing effects.

Now here's where I'm having an issue.
Would anyone here recommend doing a 20 minute session twice a week on non-weight days?
Should I keep my pre/post nutrition the same as for weight days?
Will HIIT spare muscle or will I end up losing a significant amount?

As long as you are eating enough, you probably won't lose any muscle. Just recalculate your calorie needs with 2 20 minute HIIT sessions a week in addtion to your weight training.

I can't seem to find any real figures on how many calories are actually burnt with HIIT.
Especially based on the fact that it effectively burns for up to 24 hours after.

As far as I understand, the exact # of calories burned will depend on your heart-rate for any given session. You might be able to find some estimates. Or, you could invest in a heart-monitor that will calculate the amount of calories burned.

As far as adjusting your diet, redo your BMI and figure for a high level of activity. Then adjust as you go. If you find yourself losing muscle, eat more.