First, a couple pictures of the defective engine mount; it was pretty disgusting with the rusty-looking oil stains everywhere.
Regarding the mystery modification, can you spot it in the engine bay? The car was tuned at Church Automotive to tune the new GTX3576R turbocharger! The gain was about 10% due almost entirely to running higher boost than when the previous turbo was tuned. The graph overlays the new turbo results over the original GT3071R. What’s striking is how the larger turbo spins up at virtually the same engine rpm as the old one, and in driving the car, I can’t feel any additional lag. The new turbo has three things going for it in that respect: a new compressor blade design, twin scroll, and ball bearing. With the larger turbine wheel, exhaust backpressure is lower which allows more oxygen into the cylinders. Also, the larger turbo allows the engine to pull hard all the way to redline with no sense of power dropping off. I’ve had a number of cars that at mid-rpm, where right when you start thinking, “man, this thing’s really going to take off”, it sort of dies off at the high end – not so with this turbo.
One side effect of a twin scroll setup is that boost pressure is often different than the advertised wastegate spring rate, and with the larger turbo the difference was even more pronounced; with 13 lbs springs, boost never rose above 11.5 psi. Tial (the spring manufacturer) notes that boost will only match the spring rate if the intake and exhaust back pressure are the same – clearly mine is not.
Afterwards, the tuner said that 11-12 psi should be good for trackday events, though I’ll add some race gas to ensure no issues on-course. He also noted that “if this was my car, I’d be running 15-17 psi on the street.” The reason this can be done is because the car’s so light that it’s simply not possible to stay on the gas long enough to build up engine heat – the car will be going at “ludicrous speed” by then.
The only bummer on the trip is that I recorded the tuning session with my old camcorder – completely forgetting that my new PC doesn’t have a Fire Wire port (remember those)? There’s simply no way to get the video out of the camcorder – the pc recognizes it as a USB drive but can’t see the video files (only still photos). To make up for this, here’s the video that probably had the greatest effect on me for deciding to go with a larger turbo, the Ferrari F40 LM – be sure to turn it up good and loud. I’ve read that Porsche and Ferrari use large turbos to lift the high end of the power band instead of the low or mid-band and I’ve not seen another video that demonstrates this better – enjoy!