HKS Downpipe (with Coke can for scale)
HKS Downpipe Parts
One of the most significant power gains on a turbocharged car can be realized by optimizing the exhaust flow. This is due to the fact that the turbocharger, while obtaining energy from the velocity of the exhaust stream, also creates a large restriction to the exhaust flow exiting the engine. If the exhaust system is not capable of releasing the exhaust being generated by the combustion process quickly enough (and at all rpm ranges), the engine will experience a lot of thermal loading due to the gases backing up. This will also thermally load the turbocharger as well and create undue heat stresses on the bearings.
Therefore, the goal to any performance enthusiast is to maximize that exhaust flow. This should not be the case, however, on a normally aspirated car where a certain percentage of back pressure is necessary for optimal torque characteristics. On the 7M-GTE Supra, there are 4 primary points of exhaust flow resistance to optimize. The largest (and largest resulting gain from an upgrade) comes from the muffler, the second is the primary catalytic converter, the third is the precatalyst, and the fourth is the pipe itself. All of the HKS (and other hi-po Japanese exhaust systems) eliminate the muffler restriction by going to a straight through perforated pipe with fiberglass packing as well as upgrading to a mandrel bent (no kinks at the bends) pipe from the cat back. The 65mm HKS exhaust was the first mod done to this car. I figured that since flow testing showed only a 3hp increase for the 75mm, I'd stick w/ the 65mm and have less chance of scraping the hell out of my new setup. And as far as numbers, either of the cat-back HKS setups will net you an approximate 30 hp gain at the rear wheels.
The next step was to remove the restriction seen at the pre-cat. This was done using the HKS Downpipe. This is a 3" mandrel bent pipe which eliminates the stock pre-cat, increases flow, and includes an even better than stock flex joint. You can see the dramatic differences in the pictures below:
Stock Downpipe (on top)
Closeup of HKS Downpipe
As you can also see, the pipe was ceramic coated using the same coating used on the exhaust manifold, plus the MCX liner which pushes its heat resistance to nearly 2500oF to decrease heat transmitted to the engine bay and interior as well as rust proofing everything. And, of course, while I was at it, I coated a bunch of miscellaneous parts and hardware which came with it for superior corrosion and heat protection (as well as a couple of others nearby). Below you can see (from left to right): the charcoal canister mount, the lower and upper turbo tiedowns, the turbo elbow (redone with the newer high temp liner just released), the side turbo tiedown, the downpipe hardware (in the bag), the turbo elbow-turbo metal gasket, and the upper heat shield (which was Cermachromed), and of course the downpipe below everything:
The Miscellaneous Coated Parts
Closeup of the Cermachromed Upper Heat Shield
The other heat shields were Cermachromed as well, which necessitated unbolting and sliding up the turbo a little bit to get to the front one, but the results were quite stunning. A little pitting is evident on the shield's base metal, but that's typical. It's now pretty damn awesome looking. The Cermachroming process also seals the metal and will allow gasoline and oil and stuff to simply be wiped off instead of staining or baking on. The stuff will take up to 1300oF normally (which your shields should never see unless you're running super-lean, but just in case, I added an MCX liner which adds an additional 400oF of protection. The same went for the turbo elbow as well. Both should be able to survive re-entry temperatures actually :-) Here's a picture of the installed setup:
Gotta love that shine...
And finally, here are some nice pics of the installation procedure:
My mechanic Pat doing his usual awesome work
Looking over Pat's shoulder as I help
The end result of this installation was a noticeable decrease in boost lag and a slight increase in overall power (should be able 10 rwhp on the dyno). The boost lag decrease was nice since the larger HKS Sport Turbo definately increases lag over stock yet hauls more ass in the upper ranges of the tach. So I'd have to say the lag is now just slightly more than the stock CT-26 instead of a bunch more. If I had to quantify it, I'd say a 50% decrease in lag. Whatever the numbers, though, it feels good.
The exhaust note became slightly deeper on my HKS 65mm setup with only an extremely small increase in loudness. Overall, the aural quality is still quite sweet. As far as ground clearance, the downpipe took away about another half inch i'd estimate. It's doesn't look as low as some other HKS downpipes I've seen. It's pretty level down there too. One thing which probably helped a little was that Pat (my mechanic) and I got out the oxy-acetylene torch and heated up the exhaust bracket hanger which comes off the transmission and made a few "modifications." On every car I've ever seen which has attempted to install this pipe, the HKS bracket would never fit the stock bracket as it should. There was always about an inch of space between them. So, we heated up the bracket and with various tools, bent it and conformed it to a much closer location than before. I'd say it's now got about an eighth to a quarter inch of space, which is closer to the way the stock setup was. This, after all, is what the flex joint is really for. It's not for the engine torquing over, since the transmission (which the pipe attaches to) is rigidly mounted to the engine and will torque over together and simultaneously. So anyways, with that space, it was easy to move the pipe against the bracket and clamp it down. We also double nutted the inside of the bracket so as not to deform it too greatly when clamping it down and possibly ruin the integrity of the bracket. We're going to monitor it and see how it "grows on us." One possibility is a custom bracket which we might make in the future as a one-off mod. We'll see....