With the finished transmission on the way, it’s looking like it’ll go into the car at the end of the month during a marathon garage session. While I’m doing that I’ll also be figuring out how to attach the rear wing assembly. On the Midlana forum I received a good tip about making the mount hinge so that the assembly can be rotated down out of the way for engine access. Thanks, Bill!
Back when the oil and smoke incident occurred during the dyno tuning session, the theory at the time was that maybe the turbo oil return line got sucked flat by the dry sump scavenge pumps. My buddy Dave asked what crankcase pressure was; how did I know that it wasn’t just the opposite, that under heavy boost, maybe blow-by (past the rings) was pressurizing the crankcase to a positive pressure and actually pushed oil back up the return line? I said that can’t happen because it’s a dry sump; the crankcase is always at a vacuum. He countered with, “do you know that for sure; have you measured it, or are you just guessing?” Ugh, he was right, it’s bad science to just decide something without knowing for sure, and even worse to make decisions based upon it.
A test setup was assembled consisting of a 200kpa (+/-15) psi gauge, hose, and valve cover adaptor. I prefer kpa because it’s clear what vacuum is – 0, and ambient is 101. With English units, “zero” can be confusing because it depends upon context; it can either be ambient pressure or a perfect vacuum. Ironically, the gauge manufacturer scaled the meter wrong; there’s no such thing as negative kpa, zero is zero, a perfect vacuum.
Warmed up the car and crankcase pressure settled out at 55 kpa (-13 in. Hg). The picture below was a couple minutes after starting it and before oil and coolant came up to temperature. Took it out for a test drive and during cruise, crankcase pressure fell (meaning vacuum increased) to about 40 kpa (-18 in. Hg). Found a deserted stretches of road, cranked up the boost to 15 psi and did a few 4th gear pulls*. Vacuum dropped to 55 kpa (-13 in. Hg), the same as at idle. This was a relief because that’s what’s supposed to happen; the dry sump pump maintaining a negative pressure even when producing maximum power. So for now at least, the collapsed turbo oil return hose still seems the most likely cause of the engine spitting out all the smoke and oil. Being immersed in hot oil for extended periods of time under vacuum very likely softened the rubber hose enough to allow atmospheric pressure to squash it flat. Once that happened, the oil couldn’t leave the turbocharger and filled up the center section and pushed past the seals into the inlet and exhaust sides. The anomaly hasn’t happened since, but than again it hadn’t happened before, and the rubber hose was since replaced with a Teflon part.
After the test and after shutting off the ignition, it took about 30 seconds for crankcase pressure to slowly rise back to ambient pressure – sort of a poor man’s leak-down test and a reassuring sign that there aren’t any major crankcase leaks.
*Ever since the retune, the car is a serious handful at full boost. Even a very slight bump in the road causes some wheel spin at even triple digit speeds. It’s one reason why there’s a knob on the dash for boost and it’s normally kept turned down to keep both me and the car out of trouble. That said, flooring it in fourth on the freeway at full boost is – frankly – effing awesome, as it’s as if everyone else put their brakes on 🙂
All the transmission parts are on the way to the builder, who’s also sourcing the core. This avoids the concern about shipping him a used tranny of unknown history, bought from a stranger from out of state, and having the builder possibly say, “this core is trash.” After checking what used transmissions go for and what the shop’s charging, their price is very fair so we both benefit.
Regarding the LSD, I found some good information in an unexpected place, Porsche 911 forums. 911s have an higher rear weight bias than Midlana, and being Porsches, many owners track their cars, so the topic of limited slip differentials often comes up (“what’s ‘best’ “) . There are many brands that fit 911s and the Giken holds its own. It was informative to read how the Giken helps stabilize the tail-heavy 911 under braking, during partial mid-turn throttle, and during full-throttle corner exit, all things I’m interested in. Of course, the Internet being what it is, you have to stand back to get an overall view of impressions instead of fixating on only desired posts.
Another good tidbit was that you shouldn’t install just any LSD into a mid- or rear-engine car because the chassis dynamics are different from front engine cars, be they FWD or RWD. Because of the differences, a proper clutch-type LSD for a mid-engine car is a “reverse 1.5-way”, and you can tell from the name that it works opposite what a front-engine configuration needs. Very glad I found that out before having it installed! As an aside, I also found that OS Giken is owned by Toyota, which seems like a good thing.
Speaking of 911s, I found this post which I can relate to:
“One of the things I had to force myself to accept when I first was learning to race [rear-engine Porsches] was that “more throttle equals more rear grip”. It was very counter-intuitive.
Luckily I was driving my instructor’s car and he kept pushing me to “Get on the gas!” at corner apex when I “knew” I was at the limit of adhesion.
So I remember thinking “Well I’ll show him, I’ll do what he asks and then he’ll see that the car is going to spin”. I mashed the throttle and the car hooked up and flew out of the corner. “Wait, what?!”
I’m hoping that the transmission shows up before the end of the month because with the wife out of town, there’s an uninterrupted week to do the laborious gearbox swap. It’s going to be a ton of work but it’ll be worth it. Another reason to do it while she’s out of town is so I don’t have to hear about her car sitting outside for the duration.
In my spare time I’ve been (re)reading my aerodynamics books. Until now I’ve skipped the wings chapters because all I cared about at the time was cooling system design and whole-vehicle airflow.
So yeah, I’m a little excited, looking forward to a better track experience, but I also realize that the above changes aren’t a magic bullet and don’t correct bad driving habits.