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How to Build a Custom Off Road Exhaust

Author: Harry Wagner | 11/12/2020 < Back to Motor Life Home
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Rusty Wagner stuffed a 468ci big block engine into his FJ40 back in the early '80s. His goal was to produce the torque necessary to crawl over rocks long before super low-geared transfer cases were a viable option for torque multiplication. He had to build every support system for the larger engine, from the cooling system to the exhaust system. The exhaust, in particular, left room for improvement, and it was one of the first things we addressed when getting this family heirloom back on the road. You won’t be shocked to learn that there isn’t an off-the-shelf exhaust system for a Big Block in an FJ40 Land Cruiser, so Garrett Davidson of B-Radd’s Customs (BRC) built us a custom exhaust with components from Flowmaster.


While it makes more sense to run the exhaust inside the frame rails on a rockcrawler so the components are protected from damage, we wanted to retain the old school fender well headers on our FJ40 Land Cruiser. What we didn’t want was an obnoxiously loud exhaust, droning at highway speeds, or to burn our leg every time we got out of the vehicle. Flowmaster had all the components we needed and we didn’t have to take out a second mortgage to afford them.


Exhaust Routing


On a vehicle used for rock crawling, we recommend running the exhaust inside the frame and tucked up as high as possible in order to maximize ground clearance and minimize the chances that the exhaust will get smashed on the trail. That isn’t what we did with this Land Cruiser, which has custom headers that run outside of the frame rails. There is no crossover pipe, each side has frame rail headers that dump the exhaust out right in front of the rear tire. Block hugger headers would be the practical choice, but nostalgia took precedent here as the author’s father built the headers from scratch decades ago, and they are part of what makes the vehicle so distinct.


Flowmaster’s ball flanges (PN 15925) offer a better seal and easier alignment than the traditional three bolt flange connector. They also allow for some movement in the exhaust, which is nice when the torquey Big Block flexes the chassis when we are rockcrawling. The ball flange collectors are constructed from 16-gauge aluminized steel and are 3-inch on the collector side and 2.5-inch at the muffler end.


Exhaust Flanges


The last time the exhaust was modified, the exhaust tubing was just welded directly to the header collector. While this is simple, it requires the entire exhaust header to be removed to work on the exhaust. We added ball flange collectors to make the exhaust easier to remove in the future and allow us to access the body and frame near the exhaust system.


Flanges come in three basic flavors: Three-bolt flanges, ball flanges, and V-band clamps. Three-bolt flanges are the most common and least expensive. As the name implies, they use three bolts to connect the two flanges with a gasket in between. Ball flanges don’t need a gasket to seal, reducing the likelihood of leaking. They also allow for some misalignment and movement, which was useful for our application with a torquey engine and off-road use in our future.


V-band clamps are the more expensive option, but you get a lot of features for the price. They are low profile, which is useful in off-road applications where ground clearance is key, and any other application in tight confines where you might not be able to reach all the hardware on the front and back of flanges. A V-band flange is welded onto each exhaust tube to be mated together and a clamp is inserted around the mating grooves of both V-band flanges. When the clamp is tightened it draws the two flanges together creating a seal. They are very simple to install and remove.


Flowmaster's FlowFX mufflers use a straight-through design with a perforated core surrounded by fiberglass to quiet the big block under the hood of our Land Cruiser. They are economically priced, particularly given the 409S stainless steel construction. We wouldn’t call them “quiet”, but we were willing to give up some sound deadening for the compact packaging.


Muffler Choices


The original exhaust was incredibly loud when the big block was first swapped into the Land Cruiser. While it sounded great for the first few minutes, after hours on the trail it felt like the hearing loss would be permanent. For these reasons, the author’s father welded a second 3-inch glasspack-style muffler behind the first muffler. Aesthetics be damned, the new exhaust was in fact quieter. It also seemed to hit every rock on the trail.


With limited space under the Land Cruiser, we chose a Flowmaster FlowFX muffler, which uses a perforated core and is extremely space efficient.


The attribute that lead us to Flowmaster’s FlowFX mufflers over their famous chambered mufflers was the compact size, that fits nicely under the tub of our Land Cruiser next to the frame rails. The FlowFX mufflers we used (PN 71416) have 2.5-inch inlets and outlets with only a 4-inch diameter for the body.


The original exhaust was bulky, hung up on rocks, and, how do we put this… ugly. They also didn’t even have a flange at the collector, so they had to go in favor of an exhaust that was more compact and functional.

Putting together your own exhaust system allows for a certain level of customization. Our old Land Cruiser has rear fender flares on it to cover where the fenders were opened up to fit larger tires. Garrett Davidson positioned the Flowmaster exhaust tips (PN 15341) to protrude just enough to match the flare and rotated them to an angle to perfectly match the downward angle of the flares.

Garrett Davidson burned together the Flowmaster components we used on our Land Cruiser. Exhaust work can be tricky given the relatively thin material, which is easy to blow through if you are accustomed to welding together thick plate such as used on bumpers. If you don’t feel up to the task, we recommend going to a reputable local exhaust shop to have the Flowmaster parts installed.


The brushed stainless 3-inch Flowmaster exhaust tips (PN 15341) add the finishing touch to our new exhaust. We like the angle of the turn downs, as they split the difference between straight down, where the exhaust blows dust all over the place on the trail, and straight out, where they leave soot on whatever the Land Cruiser is parked next to when the choke is set.


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