2021 GMC Sierra AT4 6.2L Supercharger build

[SierraStorm21]

I searched for about a year, including posting here looking for references, and finally found someone who had a solid reputation to do it and pulled the trigger on purchasing the Whipple in June of 2023. However he was 3 months backed up, so it didn't go in till middle of September 2023 and I got it back a few weeks later when I returned from vacation.

 

At the urging of the shop owner, I opted for the Trans tuning as well and I'm certainly glad I did.  There was a few hiccups along the way, mostly logistics between HP Tuners and Whipple, but in the end it was all sorted out. I also splurged and got the red version.

 

It certainly pulls and I had a lot of fun driving back home and since. The tune he did on the Transmission made a world of difference too. Shifts are crisp and in the earlier gears, harder and more concise. When just driving around town, it shifts earlier in the RPM range and I like it alot. The SC itself is pretty quiet. I can hear it because I'm used to it, but friends I've taken for a ride can't really tell unless I point it out. I wish it had more of the SC whine, but I can live without it. Like AEMedic mentioned in his SC build, I think the Whipple tune leaves some to be desired. It has some surging and I mostly notice it under light loads up any incline at cruising speeds. It's like it doesn't know what to do under certain circumstances. It's a minor annoyance for now which I plan to have tuned out later this year.

 

Since around 1000 miles, I've had the AFE CAI, Borla S-type cat back exhaust and the Edge Pulsar LT module installed. I knew the truck felt fast, but I really didn't know how fast. I bought a dragy several weeks before I drove it up to have the work done and got a baseline of what the truck could do in 0-60, 1/8 and 1/4 in a spot I've found that is 3/4 mile long and is around 112 feet in elevation. Did about 12 runs over several days. They were all done earlier in the morning and with 3/4 to full tank of fuel. I also should note that I have the 6.5' bed on mine.

 

When I got it back, I did the same thing over a few days and was pleasantly surprised at the results. Attached are a few screenshots.

 

While I love it, I feel like the Whipple tune is pretty conservative and while I get that and the reasons why, I think I want a little more. In discussing this with the shop that did the work, he suggested the next step would be meth injection kit with a slightly smaller pulley and a custom tune. He said he might be able to get another 30-50 rwhp by custom tuning alone, but that using meth injection would allow for a reduction of the intake temps and stresses throughout, and also allow easier tuning. He said they do alot of those kits on a multitude of rigs.

 

I'm on the fence about whether to move forward with it and would love to hear from those that have more knowledge and wisdom about the subject than I certainly do. I've done a little research and it makes sense. I just don't know if I want to deal with having to fill up the reservoir and more importantly, wonder if doing so would cause other failures in the drivetrain. I should add that this is my 'sports car' if you will and not a daily driver

 

[Gangly]

Thanks for the nice writeup and great pictures.  12's is a VERY respectable number for a 1/2 ton truck in the quarter-mile, impressive!

[kodiakdenali]

Thanks for sharing!

Awesome setup and solid numbers.

I know it will be really difficult to judge because it got the blower at the same time, but what was your impression of the transmission tuning? it shifts earlier in the rpm range? is the same true for when your truck is in sport mode? Usually holding out the gears a bit longer is desirable, but it all depends on so many factors. I’m not a fan of the factory shift program on this transmission. it certainly gets the job done, but I’ve always felt like it could be improved. Although if you tell the truck you really want it to go it can. Especially in sport mode I can get some very snappy shifts when I’m on the throttle but as soon as you let off its back to sloppy gear changes. The manual says in that sport mode is essentially an arming type system. as in even when you're in sport mode it only gets sporty when it detects that you’re driving more aggressively and not all the time. There’s definitely a lot of variation in how it shifts, but I can’t get it to shift the way I want it to consistently. Tuning is probably the answer.

[SierraStorm21]

The shifts are much more defined and felt in the lower gears...1-4. It shifts sooner and crisper under light to medium loads. WOT is the same and the shift points there are probably the same as stock when doing that. You just feel it shifting unlike the stock tune where I could hardly tell.

 

The 1-2 and 2-3 are very definitive and even 3-4 you can feel. After that they are less felt but you can still tell. At first I was going to have him lighten the 1-2 because it was pretty harsh. Especially on the drive home and hitting Portland rush hour traffic. I'd get going 15-20mph and then have to let off the gas again just as it shifted and it'd jerk my head a bit. However after some time it's softened up a bit and I've grown to like it. 

 

I'm always in Sport mode anymore. I think I drove it home in normal, but the shifting wasn't any different between the two from what I recall. 

[johnnyquick]

May I ask which shop you used?

EPC racing in Spokane, Wa seems to be knowledgeable about the installs AND  more importantly to me the dyno tuning.

[SierraStorm21]

2 hours ago, johnnyquick said:

May I ask which shop you used?

EPC racing in Spokane, Wa seems to be knowledgeable about the installs AND  more importantly to me the dyno tuning.

That's who I used. They are great. I found them by just searching online and then @kodiakdenali referred me to them as well. Unfortunately they are a 12 hour round trip for me.

Just so your aware, doing the Whipple gets you the Whipple static tune. There is no customizing it beyond that. Whipple encrypts it so he's unable to see what they've done.

 

If you want to do the Whipple and a custom tune, that's something different where he has to start from scratch essentially. If this were my daily driver, the Whipple tune is pretty good and fairly refined. It leaves some HP on the table as I've mentioned previously, but if you want a good amount of power and reliability, the stock Whipple tune is fine. His trans tuning brings it all together though imo. If this were my daily, I might have it adjusted slightly to be less harsh on the 1-2 and 2-3 shifts, but for me, it's great.

[Aubie_01]

I remember seeing that the aluminum driveshafts on the K2 trucks could come apart above a certain speed (and saw the pictures).  Is that a concern with these trucks?

[kodiakdenali]

EPC had one from a 2019 under his desk. Wish i took a picture of it. Broke right in half. Said it happened around 140. I had him leave my speed limiter at 125. 

[johnnyquick]

So, did you go with the Alky?

[SierraStorm21]

5 minutes ago, johnnyquick said:

So, did you go with the Alky?

I did, but I'm not sure I'd do it again for a few reasons.

I have to keep the Windshield Wiper fluid reservoir 2/3 or more full in order to not have a cavitation problem with the system when driving. So I fill it up every time I go to drive it.

 

That's apparently due to the angle and design of the factory reservoir where the feed comes out and EPC offered to make a custom reservoir, but I decided against it. I even had a leak a few weeks after it was done and had to drive it back up there to have it resolved. The problem was that the fittings Alky sends aren't a perfect fit for a truck setup. So they came up with their own custom fittings solution and it's been rock solid since.

 

Secondly it's not something that is going to be maintenance free. It's kind of complex setup with a pump in the fender well and modules under the dash and while I understand the concept, it's not something I want to mess with. Additionally, since I'm not local to the shop that did it, if anything were to go wrong, I'd have to deal with it on my own or find a way to get it 5.5 hours to the shop or find someone else closer, which I have not yet found.

For the positives, it's like a mini NOS boost when it engages, kind of fun. Additionally, it's cooling down the intake temps and lastly, it's keeping everything clean internally. 

I'd probably opt for a different route, like a DoD delete with new pushrods, lifters, springs and cam. But that's going to be much more costly.

I might end up doing that in a few years anyway and that point, decide whether to keep the Alky or remove it.

[johnnyquick]

Doing a Texas Speed dod delete kit and custom cam designed for use with the supercharger, and putting on Long tube headers and a heavy duty driveshaft as per LMR in Texas.

I am hoping to end up with 650Hp at the crank with all this and dyno tune with TCU tune as well.

Did you get any dyno numbers on yours?

Funny thing is wifeys Escalade V smokes everything I have came across yet, this fueled my upgrade plans to the tilt...lol Unless I REPO the V from her that is

BTW, just coming off warranty in 1500 Miles thus I am free to mod to the max now. 

Edited May 29, 2025 by johnnyquick

[SierraStorm21]

1 minute ago, johnnyquick said:

Doing a Texas Speed dod delete kit and custom cam designed for use with the supercharger, and putting on Long tube headers and a heavy duty driveshaft as per LMR in Texas.

I am hoping to end up with 650Hp at the crank with all this and dyno tune with TCU tune as well.

Did you get any dyno numbers on yours?

 

Yeah that's the way to go.

 

I did, stock was 347.3 WHP / 388 /WTQ and after both the Whipple, Alky Control and custom tune, I was at 554 WHP / 504 WTQ. Using an online calculator, it estimates I'm at 670 HP / 610 TQ at the crank.

Who are you going with to do it?

[johnnyquick]

17 hours ago, SierraStorm21 said:

Yeah that's the way to go.

 

I did, stock was 347.3 WHP / 388 /WTQ and after both the Whipple, Alky Control and custom tune, I was at 554 WHP / 504 WTQ. Using an online calculator, it estimates I'm at 670 HP / 610 TQ at the crank.

Who are you going with to do it?

Same guy as you in Spokane is the plan!

[kodiakdenali]

Just stopped by over there to say hi and have him do a quick checkup on my setup yesterday. Couldn’t possibly recommend him more. Supercharged at around 2500 miles I think, a bit over 40k now. It's has been flawless.