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with Bill Johnston
Upgrading an Exhaust System
Talk to any of the aftermarket accessory folks out there, and they will tell you that the biggest leap in power (for the cost) for a Samurai is to replace the restrictive factory exhaust manifold with a less restrictive aftermarket collector. Otherwise known as a header. The trick is to find one that is made well, lasts even with offroad abuse and is street legal in all fifty states. The best example that we could find was from Doug Thorley.
A stock exhaust system can get abused after many years of slamming it on the rocks. The muffler on the left finally gave up it’s life when the exhaust hanger that was welded to it got ripped loose. You can also see how the exhaust pipe on the right has been reshaped countless times. The original restrictive exhaust was written off on this one. Time to replace it too. 
Although the new header came with a new exhaust gasket, the stock gasket was still in great shape from a previous engine replacement, so it was retained. Another good reason to use a stock gasket is the fact that it is larger, thicker and has built in heat shields for the plug wire boots. The flange on the Thorley is very thick, so we know it isn’t going to be warping anytime soon. Because of this thick flange, you need to make sure the rear-most bolt (actually the only bolt – the rest are nuts) is long enough to do the job. You need to make sure the bolt threads in at least 4 complete turns to make use of the full strength of the bolt. 
Once the header is secured at the engine, the oxygen sensor needs to be installed. Be careful as there is very limited room between the header and the fender well. Also be sure to use anti-seize compound on the sensor threads. This will make things easier when it has to be changed at a later time. 
The stock exhaust system has a spring loaded retaining clamp that allows movement of the system when under stress. The flexible pipe on the left was used to put that same capability into the new system. When selecting a muffler, I wanted a low profile / low restriction unit but without being abusively loud. The side by side tubes inside this unit will fit the bill. 
The experts at Ernies Tire and Muffler concluded that a two inch exhaust system would work well with the small engine and low restriction muffler. There is enough back pressure to assure low end torque without being too restrictive and limiting top end. 
Ever try to change something under the vehicle and find that the exhaust system is in the way? I had them put in an extra flange at the back of the transfer case so I could drop the center section if I needed the elbow room. The tip of the exhaust pipe was also routed out through the stock location. The shape of the Samurai is such that if the fumes go straight out the back, they curl back over the tailgate and into the passenger cabin. 
Once the exhaust was installed at a local shop, we took it out on the road for a test run. The power increase was measurable. With the stock exhaust, the 0-60mph time was 42 seconds. With the new header and 2″ exhaust it only took 28 seconds. Still not a corvette, but a pretty impressive gain for Zook!
08/11/10 15:19
