Photograph by Chris Tropea
As we alluded to in the wrap-up of the kickoff of the FIRM’s Track Attack series, our first session in our BMW 435i was fine, even if the track wasn’t. We were there competing but also trying out some new Vitour P1 tires that have been performing well in our recent tire tests.
[Has the Vitour Tempesta P1 earned its place among 200tw benchmarks?]
Photograph by J.G. Pasterjak
During our first session, even though the track was still damp and sandy from the previous day’s off-road festival at the FIRM, and the tires were fresh off the FedEx truck with the mold release barely scrubbed off, our minimum speeds in several key corners were within 1-2% of the speeds we saw during our fastest ever laps on Bridgestone RE-71RS tires. So far, so good.
After that initial scrub in, though, things took a turn. When we went out for a subsequent session, the power felt soft, and a quick glance at the iPad displaying the MHD gauge data showed 11psi of peak boost, down from the typical 14-15psi we’d see when running our track tune. That 11psi was consistent with the base tune, however, so our first assumption was that the base tune had been reloaded.
So we pulled in, reloaded the track tune and headed back out… to 11psi once again.
At this point, we decided to record the data stream to see if we could find any telltale signs in the boost curve, so we loaded the track tune once again, hit record, and headed back out. This time we barely held 3psi of sustained boost.
A review of the data log shows the boost climbing unsteadily when full throttle is applied, then slowly falling off. Subsequent boost hits are sloppy and don’t hold, either. Our initial impression is that this pattern is consistent with a mechanical boost leak, so that’s going to be our first exploration.
At various points over the past year, we have touched pretty much every connection in the pressurized intake circuit. Both charge pipes and the intercooler have been upgraded at various point.
[Improving our BMW 435i’s durability by upgrading the charge pipes]
It’s all been in the name of reliability, so our prime suspicions lie not with hard parts but human error. Our bet is we’ll find a slipped band clamp, or a Henn clamp that wasn’t fully seated, or some other fairly obvious joint that was fat-fingered into place, or not fully seated, or not fully clamped.
Our first order of business is a visual inspection, particularly of those joints that we never really loved when installing them (intercooler inlet, we’re looking at you first), followed by a pressurized smoke test if the visual doesn’t show anything obvious.
Stay tuned for the results of our troubleshooting, and our plan to eliminate the possibility of a repeat performance.
Hopefully the engine sensed the issue and boost was being limited due to turbo speed or something, though I would think you'd get a warning lamp for that. Otherwise a massive boost leak + some laps at WOT makes for some significant turbo overspeed.
What is the waste-gate duty cycle saying? If WGDC is 100%, but you're not hitting the boost target, this could signal a boost leak.
I don't want to jinx you but it's about time for the valvetronic system to act up.
Vacuum or electric waste gate control, n55 can have both. I'm a little surprised you did not get a failsafe when boost targets were not hit.
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