I’m beat. The car’s ready to start but the heat and humidity were enough to call it a day a bit early. Well, that, and neighbors talking forever in their driveway and I didn’t feel like an audience – and a wife wanting to go to the store.
Anyway, the pictures pretty much tell the story. On a whim I decided to have the wastegate exhausts stay separate from the turbo. Performance-wise it’s a good idea, though noise-wise we’ll have to see. It isn’t that different since before, the wastegates dumped into the exhaust pipe about 4″ from the end, so that didn’t make any sense running it that way without a muffler. Also on a whim I decided the exhausts would look nice exiting on centerline directly below the license plate. Though they don’t look like it, they are long enough that they’ll extend through the screen once it’s added.
The wastegate exhaust tubing is rather floppy due to the flex joints, which is a good thing. Still to fabricate is a small “shelf” that the exhausts will be rest on, held in place with springs.
Forgot to mention that when the wastegates were added, small “scoops” were created, extending part way into the main tubes to redirect a portion of the gas flow toward the wastegates. While the scoops may impede flow slightly during cruise, that’s also when boost and flow are minimal so it shouldn’t be an issue. Under full throttle though, the wastegates need as much flow as possible, so the scoops benefit that condition. Time will tell if this is a brilliant idea or whether the scoops will be abraded by the heat and gas flow, with little bits of stainless getting blown into the turbine…
The new C-clip pliers are far more robust than the now-broken tool. They did sort of over-engineer them though, canting the tips inward in apparent anticipation of large C-clips exerting enough force to bend the tips straight. The problem is that because the tips are canted, they don’t fit into the C-clip with it laying flat without modifying the holes slightly – okay, done – but for some reason the C-clip still wasn’t snapping into its groove. It “seemed” to be seated but I suspected it might just be holding the compressor section in by friction alone. That could catastrophic if the turbo heats up and the car hits a bump, the compressor section could shift allow compressor-to-housing contact. The worst thing is that would allow ground-up metal bits to get sucked into the engine, destroying it. After some messing about I finally noticed that the C-clip has a slight taper on one side – and of course I’d be trying to install it upside down. Flipped it over and presto, in it went with a satisfying click. Glad I didn’t leave it as “probably fine.”
Here’s “boost creep”: an uncontrolled increase in manifold pressure, seen here as the light blue “MAP” trace at the center of the figure. This is with a 4-lb spring which in a perfect world limits boost to 4 psi, but the manufacturer notes that actual boost will vary engine to engine. In mine it’s about 7 psi (150kpa), which is fine if it stays there. It holds pretty well until about 5000 rpm, but then the exhaust gas volume becomes too much for it to vent off and the turbine speeds up and increases boost. I let off the gas when it hit 13 psi at 6100 rpm because it was trending upward pretty fast (and because I was already going 84 mph). The right way is to do all this on a dyno – maybe after boost is under control.
Parts were (again) ordered to change to a dual 38mm wastegate setup. I figure that worst case, the two don’t do the job and the header has to be redesigned, and both will still be needed in that case so they aren’t a waste. Somewhat related, I found this funny quote which is a pretty good description of what it’s like to live with a turbo engine, where:
… it’s like having a cokehead pornstar girlfriend. There’s going to be unparalleled excitement and thrills, but a lot of unexplainable downtime and a likely violent ending that leaves you broke and insane…
Back-filled 2011 ; all early posts are under the “Archive” link above.
Disconnected the upper wastegate hose to ensure there’s nothing keeping it from working properly but boost creep persists. I had a talk with a buddy who really knows turbo:
“You increased compression 8:1 to 9:1 (~12% more flow), right?” Yes
“You added cams that flow more air.” Yes
“You switched from a 0.82 AR turbine housing to a 1.06.” Yes
“You’re running E85, which produces a lot more exhaust gas than gasoline.” Yes
“You built an exhaust header with poor flow to the wastegate.” Um, yes
He went on to explain that the 1.06 AR turbine housing is far less restrictive to exhaust flow. That’s a very good thing but means that given the choice, exhaust gas would much rather go through a low-restriction turbine housing instead of making a hard turn into a smallish wastegate. He recommended one or more of the following:
Change the geometry of the header to provide a more direct shot at the wastegate: This is the right way but there’s no room
Add a second 38mm wastegate: There’s room, just
Replace the single 38mm wastegate with a 60mm unit: The “nuclear option.” Builders use these to regulate 1200-hp engines, but much more often, as a Band-Aid to limit boost creep caused by poor wastegate routing. Not an option because chassis tubing blocks the area.
Of the above, adding a second 38mm unit works the best short of starting over. For what it’s worth, two 38mm wastegates have the equivalent valve surface area of a 54mm unit, so it’s not vastly different than going with the 60mm unit.
Well that didn’t last long. Woke up this morning realizing I didn’t need some of the parts ordered last night for the exhaust modification, while a couple other parts had been forgotten. Canceled those parts, then found the wastegates are backordered with an unknown delivery date (at least from this vendor) so everything was canceled. It’s not entirely bad as it gives time to do a few more tests, like disconnecting the upper hose from the wastegate (which can keep the wastegate valve shut if pressurized, increasing boost). It “should be” zero but if it isn’t that would do it, but if it’s zero, that leaves wastegate routing and/or the engine changes (compression, cams, and larger turbine) as causing the wastegate to be unable to regulate a setpoint.
It’s too warm and humid to be doing much in the garage so the time was spent back-filling the 2012 blog.