Supercharged 3800s and 5.3 LS4 V8s run the 4T65E-HD, HD standing for heavy duty. Changes are actually quite minimal in the HD, mainly consisting of a larger torque converter and a larger differential, neither of which really add much strength which makes me wonder why GM bothered at all. In any case, let that preface the following tech info on the 4T65E.

Many of the gear and chain sets advertised are misleadingly labeled according to an HD transmission, so I wish to clarify that first. The differential is on the passenger’s side of the transmission right at the axle shaft, while the drive gears and chain(s) is inside the transmission closer to the driver’s side. The drive gears and chain transfers the energy from the input shaft rearwards, from in alignment with the engine to align with the rest of the transmission innards. The heavy duty 4T65E (found in all supercharged 3800s and 5.3 LS4s) all have a 3.29 heavy duty differential ratio but a 2.93 final drive ratio (except paddle shift competition group package GTPs which have 3.29). This is because they use a 37 drive, 33 driven gearset, making a 1.121 ratio. When you divide 3.29 by 1.121, you get the 2.93. The issue is these 1.121 gears are mislabeled 2.93 gears, only because they’re only “2.93 gears” when combined with a 3.29 differential. The other gearsets available are 35/35 or 1:1 ratio, most common and mislabeled 3.29s, and 33/37 or 0.89 ratio, mislabeled 3.69. Non-HD transmissions come with several different differential ratios, none heavy duty size, and since most of them use 35/35 or 1:1 gearsets already, you cannot purchase “3.29” gearsets to alter the final drive since they’re what you already have. To change the gear ratio, you’ll need to swap the diff, and since they’re easy to swap you may find you want to look at a swap to improve performance or reversely fuel mileage. As you can see from above, vehicles that have a 2.86 differential such as the 06+ Impala 3500s could benefit in acceleration from a 3.29 differential swap, for example, from an 00-05 Monte 3800, 97-03 Grand Prix or 06+ 3900 Impala. Overall I find the 3.29 gearing the best feeling and best performing, and its worth the swap in either way you need to perform it.

Non-HD 3.29 differentials are easy to find on ebay, remanufactured and being sold with the diff, case, sun gear etc, all you’d need for the swap, at typically $100 or less. Look for adds that mention a 34 tooth count on the sun gear, those are 3.29 differentials; 38 tooth count is 3.05 and 42 tooth count is 2.86, for reference. The differential is easy to swap for a trained tech, the trans doesn't need removing and all work can be done from the passenger's wheel well. The PCM/TCM will need reprogramming, of course.

If you have an HD unit and want to swap in 35/35 gears to make a 3.29 final drive, this is luckily just as straight forward to do, all you need are the 35/35 gears found in nearly any non-HD transmission. The drive chain length is the same size for all gearsets and can be reused if in good shape. New part numbers are 24216063 Drive and 24212633 Driven but you’ll find them on ebay, used but good shape for around $50 for a set. The install is more complex than the differential swap, the side cover needs to come off and valve body removed, but the trans doesn't need to be dropped and all work can be done in the driver's wheel well. While in there, a shift kit in the 3-4 accumulator, a new PCS solenoid and some other valve body updates from Sonnax can always be installed.

The HD transmission uses a larger 258mm torque converter versus the non-HD 245mm converter. Rather than try and rely on the quality of an aftermarket converter, my experience is that a factory converter is far away your best bet for reliability, and when borrowed from other GM applications we can find different stall speeds to improve performance. If you have a 3800 Supercharged application, it’s my recommendation to switch to the 245mm converter size. The reduced weight on the input shaft improves the life expectancy of the shaft. My specific recommendation for most supercharged rebuilds is the FDQB converter. It comes from factory in a W-Body only in the 3400 Impala/Monte Carlo to my knowledge (FDQB also comes in Aura/G6 equipped with 2.4 Ecotec). This converter acts around a 2800 stall behind a supercharged 3800 and feels tight and controlled. I’ve also used this converter in turbo builds and its shown itself reliable behind 350+ wheel horsepower.

To use the FDQB converter on a supercharged 3800, you’ll need to purchase a non-supercharged 3800 flexplate, roughly $50 brand new from GM online, or you may be lucky to have a dual-pattern flexplate if you are using an L26 bottom end for your supercharger build. The FDQB is a direct replacement for all non-supercharged 3800s and will also work very well in place of the factory FLQB converter.

For non-supercharged 3800, I have also used one step higher. FZFB is used in 05-10 Cobalt 2.2 and 2.4s and is the highest rated stall. It was noticeabley loose and provided great launching power. The vehicle was also equipped with 3.69 final drive which works well with higher stalls, since engine speed is built so much faster versus road speed, and a tight converter can feel very jerky. The FZFB’s use in forced induction hasn’t been tested yet by me.

I am currently researching 3500s and 3900s. Impala and Monte 2006+ with either V6 use the same converter, GM p/n 12491317. I do not have sufficient information on whether this is a 258mm or 245mm converter, the info gathered so far suggests it’s both, which is obviously false.

If you require a higher stalling 258mm converter, the 3800 and LS4 both use the JZFM in most of their applications and JXFM is the highest rated stalling 258mm converter. It was used in the 3.5 DOHC Intrigue (remember that thing?). In my more limited experience using the JXFM converter, it feels more loose than the FDQB 245mm but stalls at around the same engine speeds. LS4s I find the factory converter loose feeling and you may wish to run a tighter converter to improve fuel economy, throttle response and potentially pick up some horsepower at higher rpms due to less slippage; the JSFM and JTFM are the lowest rated stalling 258mm converters and was used on some Buick 3800 Supercharged vehicles.

Here are the part numbers that I have for each converter:

I covered the differentials and the incorrectly labeled aftermarket gearsets available briefly above. The drive and driven gears transfer the engine’s force input transversely in the case from inline with the crankshaft to inline with the rest of the transmission. This can be a weaklink as you start to build more power.

The stock setup is dual 3/8” chains. The reason they went to dual chains instead of a single is they offset the teeth on each gear for each chain in an effort to quiet down the tooth engagement. The reason they can fail is if the gear flexes sideways, it can be extra uneven force on one chain over the other, snapping that chain and then the other. This really isn’t a concern though until you start to produce in excess of 350 horsepower.

An inexpensive upgrade, when doing a standard rebuild, is to use the chains that were developed for the LS4 platform, and were made standard replacements for all 4T65Es as well. GM says they’re stronger, we’ll take their word. They can be identified by a blue link in the chain. They’re inexpensive, using p/n 24229085, quantity two required. These have been used reliabley in 300 wheel horsepower cammed M90 and mild turbo builds, and since chains stretch over time I’d consider these a required purchase for any rebuild.

7/8” or 15/16" single chainsets are being sold by several vendors as strength upgrades and that they are. They should comfortabley hold up to 400 wheel horsepower, so any M90 supercharged application, and most turbo builds. It’s been documented how you’re looking for the chainsets that have the rabbit ear or waterdrop shaped chain links, not the round links, as they’re stronger by allowing less chain flex. If you purchase your single chainsets from ZZP or Triple Edge, you’re basically guaranteed to be getting the right chain, but other vendors not sure. I could consider doing the single chainset above 300 wheel horsepower.

Lots to be written about brakes for our W-Bodies, because thankfully there's lots of upgrade options, and lots of platforms that could use an upgrade!

First off, front brakes, what our vehicles came with: So now, lets talk upgrades.

For all single piston front calipers, I recommend you use 2000-2005 Impala police spec pads, GM OES (original equipment stock, what the car came from the factory with) p/n 18029828 or AC Delco 171603; these run about $60. While of course these fit 00-05 Impala, they also fit the same year Monte Carlo, they also fit the 10.9" 97-03 Grand Prix and 97-04 Regal setup, they indeed also fit the 11.7" 04-08 Grand Prix and 05-09 Buick Allure V6.

For dual piston iron caliper setups (which excludes GXP), I again recommend police spec pads, this time from the 2006-2013 Impala. GM p/n 19207421 is what I have, or Delco 17D1159MHPV.

For the 12.7 Grand Prix GXP, I have no specific pad recommendation at this time. The police pads may very well fit the GXP calipers but I have not held them back to back to confirm.

For rotors, Impalas and Monte Carlos, use the police spec Impala rotors. All others, an offshore rotor made by a good brand name can suffice. Centric is often a rotor I take a liking to, even their lower end rotors, while Raybestos Advanced Tech top of the line is a rotor I've used very happily in several vehicles.

This stage basically upgrades all single piston caliper setups to the 11.9 dual piston iron caliper setup used in the 2006-2013 Impala and 2006-2007 Monte Carlo.

Dual piston upgrades from other applications have been sourced before but the Impala calipers are true bolt on replacements, no custom brake hose lengths or special bolts required. Best of all, they're everywhere, in every wrecking yard across the country. You'll find them on ebay, you'll find them remanufactured at your local parts store, and you'll find lots of pad and rotor choices.

For pads and rotors, I recommend the police spec pads, always. For rotors, I again recommend police rotors, but if on a budget, again stick to a name brand offshore made white box rotor. But don't cheap out on the pads.

This stage upgrades almost everyone to the Cadillac DTS brakes, as I prefer to refer to them as. They're also used on the 08-09 Buick Super, and the 12-13 Impala police package. In particular, while us Grand Prix owners have always been buying Impala brake pads to upgrade to, now I'm looking at your 06-09 Impala SS and Monte SS owners, this is your upgrade to catch up to the other LS4 owners and put some good brakes behind that power. These are harder to find in the wreckers but not impossible to be sure. If you wish to buy new, they're certainly not unreasonable; using p/ns 25983762 and 25983763 you'll find them at for around $100 each side, and you'll need two caliper brackets p/n 25983766.

The Impala police spec pads 19207421 will fit and there are no police spec 12.7" rotors but again use good judgement, GM OES rotors are always a good bet or good quality Raybestos or Centric.

You will need to run a 17" rim to fit the Stage 3 upgrade.

Stage 4

Now if you want even even more, credit the good people on the web forums for learning that multiple Brembo caliper setups work, the simplest being the 2010+ Camaro 4 piston Brembos, and the cheapest being the 04-07 CTS-V requiring only minor mods to accomodate the 12mm caliper bracket bolts (14mm stock). 18" wheels are required and enough offset is required as well to clear the caliper body or wheel spacers get involved and it gets messy. Still, the cost is certainly not outrageous, and these brakes do stop well for sure. See this thread for more info

Rear brakes

The only real upgrade is to Grand Prix GXP. The calipers are hard to find, best to order new. These will bolt on to older Grand Prix, Regal, all Impala/Monte carlo but 04-08 Grand Prix will require a swap to spindles that use the parking brake on the inside of the rotor drum on the vehicles previously mentioned. I would consider this update only if doing the Brembos or 12.7" front rotor setup, but wow these plus Brembos infront would stop well.


Dual piston calipers don't require a master cylinder change, however if you want more bite you can run the 2006+ Impala/MC master cylinder, which has a larger bore to move more fluid. You may wish to match it however with the 06+ Impala brake pedal, the one vehicle (small sample size) that had the master cylinder with a stock 97-03 GP brake pedal had some initial dead travel before the brakes started to bite, and then they bit hard.

Also, it's rumoured that the Grand Prix GXP uses a shorter lever brake pedal assembley, same as above with the Impala, but I have not tested in my own vehicles.

In my own vehicles, I've run both the 11.9" dual piston Impala setup, the 12.7" DTS setup, amungst other outdated brake upgrade setups. I wouldn't feel I ever needed more than the 11.9" dual piston Impala for normal driving, and the 12.7" was awesome on the highway. Both are cheap and easy to set up. If you're going for show or autocross, the Brembo setup is worth looking into.

Upper Strut Retainers

Commonly referred to as "anti-pogo retainers" on the forums, these are not just for lowering springs, THIS IS A MOD THAT EVERYONE SHOULD DO. How it works is the front upper strut mounts are a floating design, you can see the gap between the rubber mount and the metal retainer if you pop your hood. It allows the mount to have play before the strut shaft reacts to movements from the spring. The top retainer can be replaced with GM p/n 10203412, a similar size retainer from the subframe with a shallow cup, you'll need two. Installed, that gap between the rubber on the mount will be completely taken up, and the strut will now react to any spring movement. With the car on the ground, use an air or electric impact gun to loosen the upper shaft nut, re&re the retainers and reinstall the nut. This is a $25 and takes a few minutes with the right tools.

Underhood Braces The GM strut tower brakes are a great mod on the cheap for all 3800 W-Bodies not already equipped. GM installed front strut tower bars on Buick Regals from 99-02 and Impala/Monte Carlos from 00-05 so pop your hood, ensure there's no bar running between the strut towers already. GM p/n 12498648 buys you a kit of two, 12456148 gets you just a single bar, you pay the same price roughly for each p/n so obviously just buy the kit of two. Brackets can be purchased from "don rome" to install these for around $85, however the traditional way is simply to mark the front towers with the bar at each bolt hole, then drill and install the mounting studs, all of which is much simpler than it sounds, then just bolt the bar on. In the rear, enlarge one hole at each end of the bar, and you'll find they fit over one of the upper strut studs on each side, so loosen that nut, put the bar inplace and tighten the nut back down and you're done. This is a $30 mod, will take roughly an hour to install with drilling. This is a forum thread on installing the front bar using the drill method:

LS4 owners: The front strut tower bar mentioned above doesn't fit due to the alternator, but don't dispair. GM actually had a different front strut tower bar for the 3400 equipped W-Bodies with the same problem. GM part number 12458546 is no longer available from the dealer but google it, ebay it, companies that sell GM NOS (New Old Stock) will have it, they'll probably ask in the $80-90 range. If you're handy of course, go look in the wrecking yards.

GM updated the cross diagonal bars in around 2004. If you have the older U-Shape looking bars, you can update to the more rounded bars. I haven't looked up these numbers for you but you'll find them, but don't forget the wreckers as well, any 04+ Grand Prix or 06+ Impala/MC are sure to be updated.

Sway Bars

GM updated the sway bars on the Police/Taxi 2000+ Impalas, and they're also sold rebranded under GM Performance Parts as handling upgrades for other W-Bodies, and yes they are. The police/gmpp front sway bar is 34mm and the rear sway bar 19mm.

Front bar, 2004-2008 Grand Prix owners, you're an exception to the following, so see below. Now depending on what suspension package you have, you'll either have a 30mm, 32mm or 34mm front bar already, you can always mic your sway bar to find out for sure. Not huge differences either way, but the good news is the GMPP/police bar is cheap, the police front sway bar p/n 25861196 goes for around $55 online, while the gmpp kit which includes frame bushings and end links 12498643 goes for around $70. Now 2004-08 GPs, you have a 20mm front sway bar which attaches to the strut body, unlike all others which attach to the lower control arm. The upgrade for you is to switch to the bar-to-lca design and put a 34mm front bar on; to do that you'll need to switch your LCAs, see below for more info.

Rear bar, the police/GMPP 19mm is an upgrade for most with 16mm stock bars, though if you have a GXP, CompG or Chevy SS you may have the police rear bar already, so double check. Either way, what I actually recommend is going aftermarket in the rear. Addco makes two different rear sway bars, a solid 22mm rear bar p/n 639 and a 25mm hollow bar 2179, both running around $200 but well worth the extra over the GMPP rear bar. I recommend the 25mm rear bar to match the GMPP 34mm front bar, which is also hollow.

Hollow vs solid sway bars: Hollow sway bars (also called tubular) save a noticeable amount of weight and are just as structurally stiff as solid bars. Solid bars are easier to manufacture, so more common. The factory bars are hollow. My recommendation is a GMPP 34mm hollow front bar with the Addco 25mm hollow rear bar. If you're a solid kind of person, then go for the Addco rear 22mm rear bar, and look at Dorman 927100 which is a 33mm solid front sway bar kit that runs about $80.

End links can be updated in the rear with Moog K6662 units, which feature a much thicker shaft than the factory units. In 04-08 Grand Prixs, which have the same type of end link up front as used in the rear where it attaches to the strut, Moog also sells a thicker end link there too but consider doing a full sway bar upgrade if you really want to feel a difference. Also, Mevotech sells a cheaper version, under MK6662 part number. These rear end links will run less than $50 for the pair and are easy to install with the rear wheels off, worth doing on even a stock sway bar setup.

Lower Control Arms

The lower control arm assemblies are fairly modest pieces. The front bar with vertical bolt attachment is a common failure point leading to a clunk in the steering and excessive torque steer under acceleration. Also, 2004+ Grand Prix owners do not have a sway bar attaching bracket, in order to upgrade their sway bar. If you need to replace the control arms for any reason, GM does make police specification control arms, and not only that there's been a recall on 08-12 Impala Police LCAs, with new lower price control arms available. shows the recall notice, and if you pull p/n 22947665 and 22947666 out of the document you'll see they're available for around $60-65 each side online, for a complete LCA with OEM quality police spec bushings and ball joint. One of each control arm, plus a GMPP 34mm front sway bar kit on a 2004-2008 (including GXPs, you're not exception) makes a big difference in handling.

Lowering Springs

Only one manufacturer I recommend is Superior Sport Chassis, or their retail branch Canuk Motorsports. And from their lineup, ONLY BUY THE 97-03 Grand Prix springs is my recommendation, SSC p/n 24-5211 or Canuk p/n 2452.41100. There's an important reason for this. Years ago, when my friend Paul "Shimmy" approached SSC about developing a softer better riding lowering spring for our W-Bodies, he also worked with them to fix the rear sag issue where the rear looks noticeable lower than the front. This fix of a taller rear spring was only applied to the 97-03 GPs, since that's what we owned and thats what we specifically worked with them on. So whether you have a Regal, Impala, any engine size etc, speaking from my personal experience using these springs on many other vehicles other than just 97-03 GPs, this is what you want to buy. It will create an even wheel gap front to rear, with a slight rake to the body, and its the perfect stance. But again, you want SSC p/n 24-5211 or Canuk p/n 2452.41100 only, in my opinion, no matter what W-Body you own. These can be bought online from Canuk Motorsports off their website


Once you start buying lowering springs, the quality of the strut becomes very important. KYB AGX struts offer the best ride quality that I've experienced, and I've been in a lot of crappy riding lowered W-Bodies and a few good ones too. The AGX struts dampen the hard hitting potholes and control the pogo/bounce, they can really make or break the suspension. If you have an 04+ Grand Prix, these lack the sway bar mounting bracket, but I'd recommend going as far as doing the police LCAs and police/gmpp sway bar upgrade in order to run the AGX struts up front. What about the GXP FE4 Bilsteins? They're nice, but buy the AGX, in my opinion. KYB AGX front struts are p/n 734058 and rear strut p/n 734059; you'll need two of each and from they'll run around $500 for the complete set of 4.

Now if you're going to the trouble of AGX struts and some nice lowering springs, you'll want to replace the upper mounts, that leaves only a few parts that you'd be disassembling your old struts to obtain, why not just go all new. When I constructed and sold complete units, I used Monroe parts to complete them. Good quality, particularly the upper front mounts where the strut shaft can be poorly located on other mount brands. Here's what you'll need to complete the units along with the struts and springs:

Front Struts: Rear Struts: Trailing Arms

The factory U-shape trailing arms are junk, plain and simple. The rest of the rear suspension arms are tubular and just fine. Now GMPP used to repackage the police trailing arms, which were once again upgraded for "heavy duty use", and sell them as a handling upgrade; they no longer do. The GMPP/police arms were tubular. GM does continue to sell improved trailing arms that are now boxed rectangular bodies, as used on the Grand Prix GXP and newer police vehicles, under p/n 25990164, they run $30 per side. Dorman however continues to sell a GMPP style tubular arm, p/n 905501 and they run about half that cost, google/ebay/rockauto. A tip for you, the front connection point is a bracket that bolts to the body, and its typically rusty and disgusting if your vehicle is anything but fairly new, so use p/n 10262701 quantity 2 to buy 2 new brackets for roughly $10 each.