Nemesis is partners with BOR and do a TON of hemi installs in Denver. They are also alot more active on here and Paul/Rob/Aaron are all very easy to talk to and can answer all of your questions. You may want to shoot them a PM (SN: Nemesis Industries) or give them a call because I believe their kit doesnt have some of the problems you mentioned.
As for mpg, the BIGGEST factor that I have experienced first hand is tire size. Everyone can claim that the LS is significantly better than a hemi in that department but until you slap a set of 37s on it, you havent seen how close the two really are.
I dont have a hemi but I personally lost 2-4 mpg going from the stock BFGs to a set of 37" MTRs on Walker Evans beadlocks.
I know Nemesis is on this board & surfaces now & again, and I've sent them PM's and even emails before. Their responses are not only slow (like over a week) but generally don't contain the answers to the questions I posed - only a "we'll get back to you". (More often than not, and especially in the last few months). While they seem like nice guys, for my purposes they are ineffective.
When I called BOR directly ~9 months ago to begin my swap research, the guys on the phone weren't very knowledgeable when it got to more than price & donor year/make/model. Nor did they offer to get the answers to the questions (which were primarily around overheating).
AEV at least put me on the phone w/ Dave Harrington when "phone guy" couldn't answer the question. But the Truck Hemi won me over, and Todd answered all questions and then some.
Originally Posted by RiverCityOffroad
My response is in direct comparison of the 5.7L Hemi (VVT model) and the 5.3L GM motor with AFM. The Hemi was in our '10 WK and the 5.3L is in our '11 GMC Crew Cab.
The HEMI runs way hotter in terms of performance. It is very unrefined in terms of power delivery and definitely gives you that seat of the pants, head tossed into the headrest, smile on your face reaction when you push the throttle. But that's where the joy of ownership ended for me.
The MDS is absolutely horrible on that engine. It cycles in and out of 4CYL mode almost constantly and the drone, even with stock exhaust, was enough to drive me insane. It also averaged 11 -13 MPG with Jennie driving it to work. We have alot of traffic, but that's ridiculous for such a small vehicle with that much torque and horsepower pushing it down the road.
The transmission also made alot of racket and helped dilute my faith in the whole package. I really could not see that motor and transmission going 100K miles with the noises it was making at 15-20K.
On the other hand the 5.3 in our new truck is silky smooth, quiet on the road, AFM (active fuel management) is seamless and only kicks in on the highway. It has plenty of power, but it's a much more linear power band in comparison to the HEMI.
We just returned from Houston and on 23 gallons of gas we averaged 18.26 miles per gallon. I drove from South Austin to Kingwood at an average speed of 75 - 80 MPH and then we drove all over Houston visiting family and rarely used the highway. I filled up on my way out on Sunday night with about an 1/8th of a tank remaining. It held about 23 gallons.
My vote goes to the GM engines. I've never had any issues with them and they are plenty strong for pulling the JK around. Loaded down or not. My last GMFS was on 35" Mud Grapplers with heavy 20 x 10 KMC wheels and 4.56 gears. I towed our wake board boat from Houston to Austin frequently without any issues. I never paid attention to the mileage at that time in my life.
If I were you I'd lean heavily on the 5.3. I know how you are and I think you'll nitpick that HEMI like I did. A little more work for the silky smooth power delivery that the GM setup will give you is well worth it in my eyes.
Here's my impressions after my 45 minute drive in the 2012 5.3 Tahoe I rented up here in Colorado (with the 6-speed auto): yucch. Kind of boring. It may be the gears, or the altitude (?), but it seemed to be hunting gears up & down, always in the wrong one & wouldn't hold it there as we climbed the mountain. Now, it was smooth, I'll give you that, but it seemed lethargic - like I had to really get into it to wake it up. It wasn't "fun" in the least bit. It did do everything I asked of it, and I'll check it out all this week, but its benefits weren't power. I will need to check the gear ratio somehow using the vin, because if it's 3.07 or so I'd feel a little better about putting it in a 4.88 JK w/ 35's.
Here's my scorecard so far:
+Mileage (although needs more verification w/ 35's & 37's)
+Runs cooler (no need for AEV Hood)
+Cheaper (if you pick a 5.3)
+6L80 6-speed auto seems to be a bonus
+Don't have to mod the steering column hole in the firewall
-Major wiring harness hacking
-Requires a change in direction from what I've started on
-Won't work with my Rubicon transfer case. This is huge, but if I choose a 5.3, I can pay for an Atlas & be just about straight across with the Hemi.
5.7VVT Truck Hemi:
+There's one sitting in my garage
+Lots of reports of truly happy customers (the fun factor)
+Well established swap procedures
+Plenty of comparison data from lots of swaps (35's, etc)
+Plug & Play wiring
-Mileage is often an issue
-Overheating (many reports of this, but not universal)
(Overheating and mileage are my top 2 concerns)
So it's not a slam dunk for me just yet. Here's what I want to find out:
1) More driving of this Tahoe
2) Confirm Tahoe gearing
3) Get real mileage reports from both 5.3's & 6.2's with 35's & 37's
4) Solve my transfer case issue. (I just can't see downgrading my Rubi transfer case to a regular one)