This often over looked piece of the exhaust system plays an important part in exhaust flow and emissions. Here are some ideas to help you upgrade this rare part. If you have more information about what is available, please E-mail us.
Before we upgrade, let's have a look at the original piece. This is the pipe that is under the car between the Downpipe (Primary Cat) and the Catback Exhaust System. It is a long pipe ranging from 2.25" to 2.5" in diameter. It has a flexible section and a small secondary catalytic converter, (now referred to as CAT). The last half of the pipe is actually not so bad, and even mandrel bent. It's the part that runs over the suspension crossmember that is terrible. It is crushed down to a silly diameter. In addition the smallish CAT that is in this pipe is probably clogged and useless on your 100k mile car. Here are some pictures of what one looks like:
As with the primary CAT, you can benefit greatly from "gutting" the CAT. This process involves removing the midpipe and using something long enough to get through the piping to the CAT to punch out the material. A better, faster option is to have a willing shop cut the CAT out and replace with a straight pipe. To remove the midpipe you must first remove the lower crossmember. There are 6 17mm bolts holding this up, as well as a single 14mm bolt in the middle. Once removed, there are 3 14mm bolts that must be removed from the downpipe at the front, and 2 14mm bolts from the rear that attaches it to the exhaust system. Prepare for the worst as these may be rusted and could easily strip or break off. Have new Toyota studs/nuts/bolts standing by, or better stainless replacements. Once each end is removed, a 12mm bolt is holding a clamp in the front that helps support the midpipe that must be removed. Finally, the hangar that goes through the rubber support must be wiggled off, and your pipe will fall to the ground. After having the cat punched out somehow, replace it in reverse order. Make sure that if you punch out the material that you do not breathe the dust, it is toxic. It would be best if you poured some oil into the pipe and let it settle into the cat to make it soft, and make less dust.
JDM - If by some chance you find a JDM midpipe, it has no secondary cat, and will fit right in place.
Mongoose/Scorpion - These UK based exhaust companies include a 2.5" midpipe replacement with their exhaust system. See the Exhaust System Page.
Aussie - Aussie Exhaust now offers a 3" mandrel middle pipe replacement that bolts right on. The company is in Australia, but the US exchange rate is in our favor and the prices come out very nice even with freight. It is offered with or without a CAT, and does contain the proper flexible system. So far the impressions and fitments have been great. It is for ST185, but will fit ST165 with slight modification to the stock clamp support. ST165 notes will follow. Here is a picture, as well as an installed picture of the CAT version. It comes with a nice HPC coating in a couple of colors and styles. Visit our site sponsor www.935motorsports.com to order.
C-One - If you have a Japan connection, C-One offers a polished stainless steel mandrel 2.5" pipe for ST205 and ST185. Based on what we have seen it should also fit ST165.
Custom - A midpipe is not terribly difficult to make on the car using mandrel pipe pieces. Going over the middle crossmember looks like a challenge, and most shops are not willing to even try it. Even so, altering or replacing the CAT is not legal in most areas. Any decent race shop should be able to build one for you on the car with a CAT, but the labor price may not be cost effective.
Q: Are ST165 / ST185 midpipes interchangable?
A: Yes, but depending on which way you go, some modification is necessary. From examples seen, it's easier to put an ST185 midpipe on an ST165, then the other way around. It also depends on the design of that midpipe. In most cases you shouldn't have a problem.
Q: Does the midpipe have to go over the crossmember?
A: No, it could go under, but it is not recommended. Even when the pipe goes over the crossmember, it still touches the ground under some conditions. A pipe that would run under, would quickly be broken or flattened by being too close to the ground. If you are looking for more clearance to make a midpipe, a single thick washer can be used on the crossmember bolts on top of the crossmember to lower it just a smidge. On ST165, there is a cast iron piece that goes on top of the crossmember where the exhaust lays on it. This thick and heavy piece can be unbolted, providing more midpipe clearance. ST185 does not have this piece.
Q: Does the midpipe have to have a flexible section?
A: No, but it is highly recommended. There is alot of engine movement on the GT-four. The midpipe flexible section helps keep the engine movement from going into the rest of the exhaust system. This will help prevent exhaust leaks. Several midpipes have been made and installed without a flexible section, however these were all 2.5" (65mm), which might say that 3" without a flexible section could be a problem.
Q: My flexible section is leaking, what do I do?
A: This would be a good excuse to upgrade to a performance midpipe. You can replace the flexible sections, but most exhaust shops will have trouble installing a new one. It would then be more likely to leak again due to improper installation. Expect to pay over $100 for the new flexible piece and labor. If you really cannot afford it, just have them weld in a new straight pipe until you can find a suitable replacement.
Q: What size and type of midpipe should I make?
A: Make sure you at least make the midpipe 65mm and mandrel bent style. Stainless is not required, but the pipe should be coated with rust protectant and high heat spray after welding to prevent rust. Use a high quality flexible section if possible. A 3" midpipe is not necessary, but will provide for maximum HP potential. Your midpipe diameter should be equal to or larger than your downpipe for the best effect. If you are not looking to go over 300 HP, and the car is a daily driver, a 2.5" mandrel downpipe and midpipe will offer more low end torque and still flow enough to support your car. Mandrel parts and flexible sections can be ordered from JCWhitney.com or similar surplus warehouses.
Q: My CAT on the new custom pipe is much farther than the original, will this work?
A: Yes, but the further downstream it is, the more temperature it will take to work. If you have no primary cat and are only depending on this CAT, just make sure that your car is nice and hot for emissions. In fact, some have their CAT way in the rear of the exhaust system and have still passed. Take your car for a good long drive before the emissions test. Make sure that your car is not turned off and cooled before the emissions test. The primary cat is not required to pass the emissions test. However, without a primary cat in place you may fail a visual test if the tester happens to know a thing or 2.
Q: I am making a custom midpipe, but how do I find the 3 point flange to bolt to the stock downpipe