Our BMW 435i’s best TTN lap points us toward what’s next

Every on-track experience is also a testing opportunity, particularly with a car like our 2015 BMW 435i where we’re building a fun and dependable track car–but only after we solved some critical shortcomings.

After a podium finish at the 2024 Tire Rack SCCA Time Trials Nationals, we still see a lot of potential being left on the table, especially after comparing our best lap data to that of our class’s winner.

Although our BMW went home with a third-place trophy in the 15-car Tuner 3 class, our best lap was still a whopping 3-plus seconds behind the best lap of Matt Hugenschmidt and his E46-chassis M3. Comparing his videos to ours made it clear what our car does well and where it still needs some help.

The good news? Our 435i pulls like a freight train at the bottom and top of its acceleration curve. The bad news? It also handles a lot like a freight train.

Between the torquey, turbocharged 3.0-liter N55 inline-six and the quick-shifting, eight-speed automatic, the 435i came off corners as hard or harder than the peakier S54-powered E46.

Somewhat surprisingly, the 435i also topped out on most straights faster than the M3. The 435i’s tiny turbo makes for great torque, but not high-winding power–we shift the car well below redline for best performance, actually–but the ZF 8HP transmission’s ability to keep the N55 in its ideal powerband made for some pretty high top speeds.

The E46 has great midrange, though, and a tall third gear that makes the most of its high redline. Its performance between the corners and short acceleration bursts out of faster sections are both impressive.

When those straights turned to corners, though, things got dicey. Our cornering speeds were down noticeably to the E46 in almost all types of corners. Particularly long, steady state sections like NCM’s Sinkhole, we were down an mph or two and losing lots of time through those long sections.

Weight probably has a lot of effect here–the E46 has around a 300-pound advantage over our more modern BMW–but unfortunately, there’s not a lot we can do to solve that in the Tuner class. And, particularly for a car we want to retain good street manners, there’s just not a lot of weight to drop outside of stuff like exhaust, which will be a low-double-digit saving at best.

So while we really can’t materially adjust the load or the load transfer, we can change the effect that load transfer has on the contact patch. Photos of our car in corners showed those outside tires doing a lot of work with their outside edges. Flattening that dynamic contact patch and keeping it stable through the use of more sophisticated coil-over dampers and carefully chosen spring rates should help.

Smaller-diameter springs should also allow us to get a bit more static negative camber–the -3.1º we have now isn’t quite cutting it–and further let those contact patches do their job.

Some stiffer bushings in key locations should help stabilize those patches as well. BMWs are notoriously soft in the bushing department, but that compliance means a lot of dynamic instability for the contact patches.

While we don’t want to just wholesale swap out all the chassis and suspension bushings and create an NVH nightmare, we feel like we can start with a few high-impact spots and replace our worn-out rubber bushes with a less compliant, more precise alternative and see some real benefits, particularly in the area of braking stability.

The other big mystery on the table is differential performance. During our video, you may hear a few instances of high-pitched squealing from the brakes. We intentionally left the anti-squeal shims out of the rear brakes as a bit of an experiment.

As we suspected, a few squeals rang out in the middle of corners or when we were riding curbs and definitely NOT hitting the brakes. That’s the 435i’s E-diff E-diffing.

It’s individually pulsing the rear brakes to create an ersatz differential effect, but also likely hurting momentum. Unfortunately, this function can’t be turned off through a button combo or pedal dance, but it can be shut off through ECU tuning.

The other complication here is that adding a mechanical LSD is a fairly complicated process. The stock ring gear is welded to the diff carrier, not bolted in like in a normal car.

So the stock gear needs to be machined off the carrier. Then the diff is installed and the gear is re-welded back on. Obviously, proper balance and zero heat warping are critical to this process being successful. So, yeah, that’s gonna be fun.

Still, there’s no disappointment here. We ran an underprepared car against some solid competition and came away with a podium finish and, best of all, we can see a clear path forward. Now the question becomes how much of the gap we can close, and whether or not we can do it while retaining some daily-driver manners.