Can our BMW 435i run with the GR Corolla or Civic Type R?

One of our hopes for our BMW 435i project is not just to build a fun, track-capable car, but to build a car that can actually compete in the Sport division of the SCCA’s Time Trials program.

The Sport division has its philosophy rooted deeply in “dual-duty” enthusiast culture, and the prep rules are structured around building a car that can be driven not only to and from the track, but used for daily duty in those weeks between track adventures.

As such, the prep rules are pretty limited, and based around off-the-shelf parts that can be bolted on by the average enthusiast.

So, the bottom line here is the base car you start with in the Sport division is going to largely define your competitive fortune in the long run. Yes, you can work within the rules to solve a few inherent issues with the car, but you can’t completely re-engineer things to make the proverbial silk purse out of a sow’s ear.

With that in mind, after we were able to lay down a baseline lap on our test course at the Florida International Rally and Motorsport Park, we wanted to study the data a little further and see what it could tell us about how competitive a choice this 435i may have been for our intended venue.

We had a lap time–1:23.61–which was certainly not scorchingly fast, but looking at the total lap time gives a pretty incomplete picture of performance. Hopefully there were some elements of that lap that gave us hope for making up some pace.

So we brought up the data chart of our BMW in VBox’s Circuit Tools software, and compared it to the fast lap of one of its mates in the Sport 3 class: the Toyota GR Corolla.

 

[VBox HD Lite: Big data power in a tiny, efficient package]

Sport 3 comprises many modern hot-hatches like the Toyota GR Corolla, Hyundai Elantra N, Mk7 Golf R and FL5-chassis Honda Civic Type R, as well as mid-power sport sedans like our 300-horsepower, N55-powered BMW 4 Series.

On the track, the Corolla was basically 2 full seconds faster than the BMW, although we expected the BMW to lag behind a bit due to being fitted with 540tw all-season Michelins.

[Which is faster, Honda Civic Type R or Toyota GR Corolla?]

And that’s the first thing that stands out in the data traces as well: cornering speeds. The grippy summer tires on the GR Corolla simply held on in the corners with greater tenacity. This was particularly true in the fast right-hand kink of Turn 4, where the Corolla held a steady 3 mph advantage through the cornering phase (figure 1), despite the BMW being slightly faster on the approach.

That faster approach is one of the things we can stick in the “plus” category for the BMW, however. It does seem to have similar thrust to another key car in its class, as most of the acceleration curves precisely match the GR Corolla’s, and the BMW even uses its quick-shifting ZF8HP automatic to outpace the Corolla in a straight line on straights requiring multiple shifts.

Particularly refreshing is a particular sequence (figure 2) where the BMW gets a terrible exit from the slowest corner on track. The speed graph shows a classic operator error of getting on the power too early and having to back out before the exit.

Circuit Tools’ delta graph, which details where exactly time is being lost or gained between the comparative traces, shows that this poor exit instantly costs the BMW some time, but is strong acceleration through down the next straight rips back the disadvantage and begins to erase the deficit. By the next braking zone, the 500-plus-pound heavier BMW has accelerated to a 3 mph terminal speed advantage.

Of course, that speed advantage was instantly erased by its lack of cornering grip in the next sequence (figure 3), but we’re taking our wins where we can at this point. When you’re testing on 540tw tires, you look hard for bright spots.

Braking is not particularly one of those bright spots at the moment, although we suspect that this has more to do with lack of tire traction than anything. The brake deceleration traces are consistent–there’s a good pedal and the big M-package brakes grab solidly–but there’s just not an overabundance of traction to call on. The angle of the BMW’s brake traces (figure 4) is notably shallower than those of the Toyota at the moment.

So, what’s the verdict here? Well, we may not have the ringer car for the Sport 3 class, but once we get some proper 200tw rubber on there, we should seriously close the additional grip gap. Of course, everyone in the class gets access to the same 200tw tires, but at least at that point we’ll be comparing apples to apples.

If we can close that lateral grip gap with tires and some suspension upgrades, then we seem to already have a solid formula in the straight-line thrust department.

We’ll also be allowed to do some ECU tuning, and while there isn’t a ton of additional power to be had from the small-but-responsive stock turbo on the N55, we should be able to free up another 25-30 horsepower with a bit more boost.

The real verdict, of course, will come from event results. But our VBox data at least shows we’re not bringing a pickleball paddle to the tennis courts. Our racket may be slightly heavier and more unwieldy than the ones the cool kids have, but it also has heated leather seats and looks more appropriate parked in front of the club than that flared-out Honda.

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