Update 21 OCT 2013: The car pulled 285 whp and 225 ft-lbs wheel torque on the dyno.
3.07 --> 3.64
Pros – 18.6% more torque to the ground; wider range of gear choices
Cons – 18.6% increased highway rpm with lower gas mileage
2500 rpm at 60 mph
2900 rpm at 70 mph
3300 rpm at 80 mph
The car is a whole lot more fun around town with the 3.64 final drive. Without a doubt it’s the best bang for the buck “power” mod (we all know it doesn’t really add horsepower). A stock car will feel like it’s gained 40 horsepower, and forced induction cars will feel like they have gained around 60 horsepower… plus all that extra torque throughout the rev range.
A 3.64 pairs extremely well with a low boost centrifugal supercharger kit. For NA cars, I would recommend a raised redline, either through the Shark Injector or other tuning software. After driving the 3.64 around for a few hundred miles, I think I could even go as high as a 3.91 on a NA car. For a car with a centrifugal supercharger, a 3.64 is about right if highway cruising isn’t the norm.
Highway rpm is similar to a 2.5i manual. Regular short-distance runs on the highway are a non-issue. If the car spends most of its life on the highway, this mod is on the aggressive side. If you have a 3.07 and want to know how it feels, try driving around in 5th on the highway.
I wouldn’t say that I shift more often. I think that only applies if you’re one of those people that feels the need to row through every gear before settling. I skip gears almost every time I shift. With the 3.64, it’s much easier to do so. The car is happier going from say 3rd to 5th than it was with the 3.07.
I now have a wider range of gear choices. With the 3.07, there were hills I had to navigate in 2nd gear. Now I can be in 2nd or 3rd. Similarly, when I would have previously been forced to choose between downshifting into 2nd at 50 mph (wringing the car out more than I would have liked), I can drop to 3rd and have plenty of torque to get the job done.
I like the diff more than I thought I would. I bought it off of a 17k mile 2.5i auto, and I couldn’t be happier.
Ceramic Coated Headers
Pros – finally makes the M54 sound mean, flames, spitting, popping, horsepower increase, cooler operating temps, better fuel economy, easier on the engine
Cons – eliminates the only set of cats (emissions), fitment, resonance, rasp, spitting, popping, drone, gas fumes
It’s harder for me to recommend this mod than the 3.64 final drive, because there’s more baggage that comes with it. Where a 3.64 has no other side effects than higher rpm on the highway, headers are a mixed bag. Personally, I’m thrilled with them. You may not be, depending on what you want with your car.
I decided to do headers as soon as I saw a picture of the stock exhaust manifold. Long story short, the stock exhaust is terrible. I couldn’t believe how much it sucked. Knowing how much better the Supersprint headers were, and seeing the ones I bought sit around the house for almost two years, kept the engineer in me awake at night. When we got the stockers off, the guy that did most of the work independently commented, “It doesn’t look like BMW spent much time on these.”
I would guess that these headers uncork at least as much power on a supercharged non-M as a stock Z4M, because a Z4M’s stock headers are better designed, and supercharged cars gain significant benefits from free-flowing exhausts. I had previously installed a Supersprint Race exhaust, including a Y-pipe and muffler. With the addition of Supersprint headers, the car went from tame to loud. It spits and pops more than I would prefer, but I’ll be installing cats soon to appease the environmentalist in me, which will also moderate the noise a bit.
Comments on the exhaust note are overwhelmingly positive. Every person that has heard the car has said two things – “God that’s LOUD" and "It sounds amazing.” Down low it’s bassy, similar to the Diablos on my M, and up high it roars. There is some rasp, but don’t confuse “rasp” with a tinny, buzzy, hollow noise. It’s like Satan is blowing his vocal cords out at redline. I can see it being a bit much for some. Oh yeah, and it shoots flames during high rpm let off… evidence of a rich supercharger tune and catless exhaust.
There were some fitment issues in two places. The first is where the headers mate up to the rest of the exhaust (S-pipe on a Z4M). Even if I gave Supersprint the benefit of the doubt, saying the exhaust shop that installed the Y-pipe and muffler somehow influenced the remaining resonated “S-pipe”… it doesn’t excuse the fact that you can’t put both secondary O2 sensors in the remaining bungs simultaneously. Fortunately I had already planned to relocate the O2 sensors, so I went ahead and did it. I put my wideband in the second slot on bank 4-6, but beware: if you plan to put your secondary O2 sensors in the headers (rather than allowing them to dangle around somewhere in the engine bay), you’ll have to weld in a new bung. With my headers being ceramic coated, this wouldn’t have been easy.
There are other downsides too. While everyone that’s heard the car has been highly complimentary, an audiophile would notice increased resonance. Louder is not always better. The resonance reminds me of the same reverberations I heard when I removed the foam during the sound generator mod. Some of it could be due to the combination of the headers and other Supersprint exhaust pieces. At redline it screams, but below 4000 rpm it’s a bit boomy. I expect the planned aftermarket cats to mitigate this. Right now it’s just a quirk, and it doesn’t annoy me enough to spend hundreds of dollars to fix it.
The last negative is the smell of gas that sometimes enters the cabin, which I’m sure other motorists have to endure. This is something that I know will get on my nerves once the new mod feeling wears off. Again, cats are in the near future. If you do this mod and don’t plan to get cats, expect every nearby motorist that knows anything about cars to immediately label you an asshole.
With the combination of the headers and 3.64 final drive, I have no doubts that this car would smoke a stock M in a straight line. I can shift it right into its powerband at any speed, where the supercharger happily provides its contribution to the engine’s increased power to the ground. Overall I’d give the 3.64 final drive an “A” and headers a “B”.