Exhaust upgrades 101

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Staff member
Sep 12, 2023
North Ridgeville, OH
Vehicle Details
1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Upgrading the exhaust on your MN12 isn't difficult, but you need to have realistic expectations on what will change when you start upgrading pipes or swapping mufflers.

The factory exhaust and mufflers are not terribly restrictive, especially with stock (or near-stock) powertrains. The 1989-1991 exhaust design routes the pipes around the gas tank rather than under it, which increases bend severity and quantity - if you have an older MN12 and have the stomach for it, swap to the newer 1992-1997 gas tank design when you upgrade your pipes to reduce the number of severe bends.

If your goal is simply to make the car louder, replacing the resonator with a straight-pipe and/or cutting off or replacing the factory mufflers is an easy and relatively inexpensive place to start. Others have gone so far as to remove or gut their catalytic converters. Search your favorite video website for videos with sound clips to help you gauge the sound before you start cutting!

If you're looking to make more power, there are several things to be aware of.

First, backpressure is bad. I've seen some claim that "some" back pressure is required for the engine to run properly. This is a myth! Any amount of pressure in the exhaust system is an obstacle that the engine must overcome by sacrificing a part of its power output to overcome. Pressure in the exhaust also prevents an increased percentage of exhaust gases from evacuating from the cylinders after the combustion and exhaust stroke; residual exhaust gases in the cylinders means less fresh air and fuel is drawn in, all meaning LESS POWER is making it to the wheels!

Second, larger pipes and more flow is not always beneficial. Pipe diameter must be matched to engine power targets. If the pipe diameter is too small, exhaust gases cannot flow through them fast enough to prevent creating detrimental backpressure, and the engine must sacrifice power to squeeze its own exhaust out through the pipes. Conversely, pipes that are too large reduce exhaust velocity and limit the effect of scavenging - a critical asset when installing aftermarket cams or using headers.

Third, crossover pipes (X-pipes or H-pipes) in dual exhaust systems serve an important purpose. The exhaust gasses do not flow through the exhaust in a continuous manner. There is a pressure pulse generated each time an exhaust valve opens, and these pulses travel through the whole exhaust system. Since the firing order on a V8 does not alternate back and forth between each bank, there are times when there are 2 back to back pulses on one bank, and none on the opposite bank. If each bank has its own pipe with no crossover, then you will have increased pressure in one pipe while the other does nothing, and vice versa. By having a crossover between the pipes, the pressures can equalize, and the exhaust can take the path of least resistance, which helps with both power and sound! As for which one to choose, an X-pipe will have a slight advantage at high RPM and so is preferred when peak power production is favored. An H-pipe helps increase the scavenging effects at a lower RPM than an X-pipe, and tends to enhance low and mid-RPM torque production slightly better than the X-pipe, however it will fall short of the X-pipe on peak power numbers. Really the difference in power between the two styles is negligible, at best only a few hp difference, however comparing an exhaust with any crossover compared to separate pipes can be a 15-20hp improvement! So run a crossover in your dual exhaust!

Fourth, scavenging is your friend. The pulsing of the gases moving through the exhaust creates areas of low pressure in the pipes. Scavenging describes the effect where a low pressure area in the exhaust pipe is present at the exhaust port on the cylinder head as the exhaust gases are exiting. This helps draw the exhaust gases out of the cylinder, and also helps draw the fresh charge of air and fuel into the cylinders for the next combustion stroke.

Finally, the factory exhaust manifolds on the MN12 are quite restrictive. Years ago, enthusiasts discovered that the factory exhaust manifolds could be enhanced with careful porting to improve raw flow, however the benefits to scavenging using any factory style manifold or short primary header is negligible. For any meaningful improvement to exhaust performance, mid-length or full-length headers are required.

Kooks makes stainless 3/4 length headers for the 4.6 that will fit the MN12 without modification. There are no manufacturers of long tube headers for the MN12 at this time, however long tube headers designed for the 4.6 in the Mustang are plentiful. They can be used, but the steering shaft must be modified to allow them to fit - or the driver's side primaries need to be rerouted to clear the steering shaft. 3/4 length headers generate sufficient scavenging to offer major benefits to peak power production. To also get maximum possible low/midrange torque improvements that benefit heavy cars like our MN12s, full length headers are the only option.

In terms of numbers, upgrading the factory exhaust to 2.5" duals with an X-pipe while keeping stock manifolds may net an increase of 3-5 HP on stock or near-stock (factory heads or cams) combos. Integrating long tube headers with the upgrade can improve expected yields by another 3-5 HP. However, on a vehicle with a fatter cam and/or cylinder head upgrades, the difference could be 15-20 HP or more.
I would add: 2.5" Trubends exhaust kit sounds the best; use an x-pipe, and cheap $25 mufflers, and it sounds loud without driving you nuts.

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