As someone who's been part of the W-Body community for 15 years now (that's a lot time, I feel old), I must admit that I find the LS4 community to be a bit of an enigma. There's a lot of segregation, little cross sharing of info from the V6 platforms, and LS4 stigmas are hard to break. I preface with this because I want to see LS4 owners make good choices that allow the platform to reach its potential. Part of this I hope is to look at a small outfit like Overkill Motorsports who has special interest in these vehicles, rather than the big brand companies, for your performance needs.

GM did a lot of engineering work to fit the LS4 5.3 V8 into a W-Body, however, in typical GM fashion, there are shorter comings in the finishing details, V6 parts in particular made to work on the V8, and that's where power gains can be had.

The air filter box is the same as the 3500/3900 V6 Impala/Monte, and so is the MAF sensor. While there are commercial aftermarket intakes available, they all reuse this V6 MAF, which is only 3" ID at its exit. We faced this issue on the B-Body Impalas as well, for some reason we had the V6 MAF sensor, while the F-Bodys Corvettes and pickup trucks had a larger MAF. The bottom line here is that a MAF upgrade is worthwhile whenever doing an intake system.

We offer two MAF upgrade solutions. One is the popular LS7 card MAF in a 4" housing, the other is our own sourced ASI MAF upgrade. We like our MAF solution because its very essentially a larger MAF version of the stock MAF, it even looks the same on the exterior if you didn't have the two side by side for context. Its 4" inlet, 3.75" outlet, it plugs into the stock wiring harness without modification, only tuning is required which we offer confidently for mail order tunes.

In our initial testing on an LS4 vehicle that had only simple bolt-on modifications, we noted that there was a 1 KPA reduction in MAP readings at WOT. What this means is there was a reduction in intake vacuum. When there's intake restriction, a vacuum is created trying to draw air intake the intake manifold; flow more air and that vacuum is reduced. So we knew there was an increase in airflow, and we wanted to dyno test.

Using Nathan Shutt's 08 GXP which had headers, 3" exhaust, Overkill tuning and an aftermarket intake using the stock MAF, we ran on the dyno as is, then switched to our Overkill ASI intake with MAF upgrade solution, tuned only for the MAF calibration otherwise keeping our Overkill tune comparable, and we picked up 5-7 wheel horsepower across the entire powerband, not just at peak.

So long as you can stand not having a shiny aluminum box or a brand name cache, you'll make more power with an Overkill ASI with MAF upgrade intake and you'll spend less money, that's our promise.

K&N has made some very good intakes over the years, I'm not here to rag on them. But, as I love to point out (as someone who chooses to specialize in platforms): sometimes large Do-All companys do things poorly.

The main issue with the K&N system is the tubing. A customer sent me the following photo:

The inner diameter of the tubing is 2.75". That's smaller than the small MAF sensor. That's smaller than the throttle body. That's small, period. They likely did this because the 3.25" outer diameter is about the right match to the stock MAF and TB, but because their plastic material is 1/4" thick, it narrows the inner diameter significantly. While there are gains to be had with the K&N over the stock air filter housing, there's too much left to gain to make the K&N a good choice. The good news we do make a retrofit kit that will replace the K&N plastic tube and the small stock MAF but reuse the K&N filter and shield, giving you the look you originally wanted but a noticeable improvement in performance

Moving on from the small MAF, the LS4 throttle body is a 76mm unit, borrowed from from the Trailblazer 6-cylinder motors and Colorado/Canyon 5-cylinder motors. Again, parts bin diving from GM, yet they didn't dive the V8 parts bin did they. Its smaller than the 87mm truck or 90mm Camaro/Corvette units. It is however similar in size to the first LS1 engines. The bore has protrusions around the throttle blade, meant to help the electric throttle blade with idle quality, where small movements provide limit airflow change thanks to these lips which contour to the opening rotation of the blade. These can be ported out with little to no negative idle effect, and turn the throttle body into a true 76mm bore.

The LS4 intake manifold has a pinch in the throat just after the throttle body. This is to accomodate the oil pressure sensor, not an issue in all other LS engines where the intake manifold faces the other way. LS1, LS6 and LS2 manifold swaps are becoming more common but they're not easy, you have to delete DoD including the valley tray, you have to solve the oil pressure sensor, you have to adapt a throttle body if you use an LS1 or LS6 unit, its not so simply to just willy nilly toss something else on and pick up power. Non the less, its not impossible.

Using the truck 87mm or car 90mm throttle body, 2008-09 LS4 throttle body wiring is plug and play, while 05-07 will need an 8 pin to 6 pin wiring adapter. Eliminating DoD is the easiest way to go, which will require removing the heads to then swap the lifters to standard lifters, and install an LS3 non-DoD valley plate to eliminate the DoD electrical connector. The oil pressure sensor can be relocated to down by the oil pan where oil cooler fittings are currently blocked off; either install the sensor down there and extend the wiring harness, or use a braided line and fitting to reach that port up to the oil sensor by the intake manifold. A cut to the alternator bracket is required. More details are on LS4 web forums with a google search. You'll need to be handy and realistically expect to spend upwards of $1000 on proper parts and incidentals.

The LS4, and also the Impala 3.9 V6, have the largest stock W-Body exhaust systems. They're 2.5" single pipe, back to a dual 1 7/8" split to the dual mufflers behind the gas tank.

Still, a 2.5" system isn't exactly big. GM pickup trucks use 3" systems from factory on identical motors. While 1/2" may not sound like a big deal, it represents a roughly 45 % increase in pipe area size, that's a big deal and a big difference.

The common counter argument is this: A 2.5" exhaust supports XXX Horsepower. I hate this argument. I guess if you took a 600 horsepower engine that was running a 3" exhaust, then stuck a 2.5" exhaust on it and it made 565 horsepower, you could say that the 2.5" exhaust supports 565 horsepower, but why the hell would you give up horsepower over a simple exhaust system swap? The real question is how much is gained swapping to a 3" system. My answer to that at the moment is I don't have a solid figure to give you, the proper way to test would be to take a car with a 3" downpipe or header pipe, 3" cat, that has a 2.5" cat-back currently, dyno it, swap to 3" from the cat back, with identical rear mufflers, and re dyno. I haven't done that yet, noone to my knowledge has, but I've driven enough LS4s as part of my job to know there's a difference. With headers, I'm confident that the 3" gains would be around 10 wheel horsepower on a dyno.

I think most people get hung up on the 2.5" number, forgetting that theres only one. If you do the math of the area of a circle (cause exhaust piping is, you know, circular), here are some numbers to consider: So, looking at the above, a few things to consider:
#1 - The factory dual 1-7/8" split doesn't choke the single 2.5" pipe that feeds it
#2 - A single 3" exhaust may sound enormous but doesn't have the same air volume capacity as a dual 2.25" pipe setup
#3 - Increasing from 2.5" to 3" single exhaust piping may not sound like much, but its actually an increase of 45% in surface area.

The Magnaflow in particular is very popular, but the system is 2.5" like the stock cat-back. You'll gain power with the free flowing mufflers. This system shouldnt be purchased as a performance power upgrade, but rather as an OE replacement with a nice sound.

CounterArgument: The Magnaflow improves performance because it replaces the dual 1 7/8 piping with true 2.5" all the way back. I've heard this right from a Magnaflow sales rep. This drives me nuts. Fact is, no it doesn't, I've already explained the math above.

To clarify, this is not to knock Magnaflow, there's no reason for them to many anything but a 2.5" system when the factory cat-back connection is 2.5". Magnaflow is another Do-All company. Unless you're going to specialize in starting 3" from the manifolds back you won't gain anything starting 3" from the cat-back and I'm sure Magnaflow's interest in doing this for the LS4 platform is zero. There are other companies out there that specialize in this platform that offer this...

ZZP is now the only company doing a 3" downpipe for the LS4, but its a good starting point for any exhaust work. For $190-240, you remove a very significant restriction in the LS4 exhaust system.

The stock LS4 downpipe is super restrictive. The factory flex pipe narrows to around 2" inner diameter right at the downpipe inlet, and then you see the rear O2 sensor pinch.

Even though the ZZP unit narrows down to 2.5" after the cat, that does allow for use with the stock exhaust system there back, and you can easily add 3" from the cat-back. ZZP also offers a 3" complete exhaust system for the Grand Prix GXP and Monte Carlo SS. The 04+ Grand Prix system on their website fits the GXP perfect, while their Monte Carlo system you'll see a 3" option on the product page. Impala SS owners you're SOL, except that you can select the 04+ Grand Prix system without mufflers, and then you'll just need yourself a center in and out muffler with custom tips to complete.

Having run my fair share of W-Body exhaust systems, here's the setup that I like: The headers are a no brainer and both are the same, Doug Thorley is the originator, OBX is the offshore copy cat, however you'll find OBX more available on ebay.

The Magnaflow 5" cat converter allows more airflow than the regular 4" body spun units since the 5" body catalyst has a larger surface area with more holes for exhaust to flow through compared to a narrow body.

The nice thing about ZZP is you can buy their cat-back without resonator or mufflers to select your own, and you'll also need to ask them for the tail section of a 3" downpipe, just saying that you have a 2.5" exit and want something prefabbed to cut and replace with 3".

If you don't want drone or rasp, you want a large and long muffler in the resonator area; infact if you want it loud, just do this Magnaflow muffler with no rear mufflers, it'll sound way better than doing no resonator and just rear mufflers, but in either case this Magnaflow unit is the key to a good sounding W-Body exhaust.

The Flowmaster 60 series gives that Flowmaster sound that I like, but the 60 series is nice and quiet; most people complain that Flows are too loud yet they chose their loudest mufflers like the Super 44s, well try a quieter version, you'll be happier. The Impalas benefit from an offset outlet to help the tips line up. Tips are cosmetic, your preference.

That's my favorite W-Body exhaust that I've run, hopefully it gives you some ideas for your own!

More writeup to come...