1. home
  2. how to
  3. engine
  4. How To Gain Back Acceleration After Adding Larger Tires

How To Gain Back Acceleration After Adding Larger Tires

Get Your Groove Back

Harry WagnerPhotographer, Writer

If you have been following along, you’ll recall we added a set of 35-inch Toyo Open Country M/Ts to our orange LJ. That increased our ground clearance and traction at the expense of our acceleration and braking. Our next upgrade addressed the braking issue with upgraded Heavy Duty Orange brake pads and Ultimax USR slotted rotors from EBC. Then we regeared the axles to gain back some of the performance we lost. Now we are turning our attention to further improving acceleration through some simple engine mods.

Operating under the theory that an engine is a giant air pump, we used common parts from some of the most reputable manufacturers in the industry to gain back the passing power we lost when we increased our tire diameter and rotating weight. Starting on the intake side, we chose Volant’s PowerCore filter and closed intake box for this application. While power gains were important to us, we didn’t want to sacrifice filtration, particularly since these Jeep spends a lot of time on the trail. “The large element is the secret,” explained Volant Technical Service Representative Daniel Erickson. “If you cut out the PowerCore and rolled it out flat, there would be 23 square feet of surface area. We filter to 0.0001 micron.”

At the other end of the engine, we added a tubular exhaust manifold (aka header) and stainless steel exhaust from Rugged Ridge. The header was the most complicated installation we performed, and when we say “we,” we really mean Nate’s Precision. Installing the exhaust manifold on a 4.0L is an involved process the requires removal of the intake manifold as well, so rather than bust our knuckles and swearing like a sailor, we stuck to shooting photos while Eric Schilling had the exhaust manifolds swapped in a few hours. The new manifold uses equal length tubes with smooth bends to minimize restrictions and increase flow.

The Rugged Ridge tubular exhaust manifold bolts to the factory downpipes and catalytic converter. From there we added a stainless steel exhaust system from Rugged Ridge. This exhaust uses high-quality components like mandrel bends, stainless tubing, and fully welded hangers. Another benefit we really appreciate is that while the larger diameter tubing and free flowing muffler increase horsepower, they are still quiet. This is not the case with every aftermarket exhaust system or even necessarily the norm. A loud exhaust sounds cool from stoplight to stoplight, but after a long day on the trail, it can drive you crazy.

To complement the increased airflow in and out of the engine, we used our DiabloSport Trinity programmer again. We had previously used it to measure 0-60 times, and it can also calculate 1/4-mile times, as well as clear diagnostic trouble codes and recalibrate the speedometer for larger tires and gear changes. This time we took advantage of the DiabloSport’s performance engine tunes, which alter timing and are thus tailored to specific fuel requirements. We uploaded the 91 Octane Performance Tune since that is the highest-octane fuel readily available to us.

Prior to adding the performance parts, our 0-60 acceleration had decreased from 18.6 seconds to a dismal 29.6 seconds when we upgraded to 35-inch Toyo Open Country M/Ts. The Volant intake, Rugged Ridge exhaust, and Diablosport Trinity tuner, working with the new Yukon 4.88 gears, brought the average 0-60 mph time down to 20.0 seconds! Fuel economy and throttle response have also improved. Most of the gains can be attributed to the Trinity, which programs the transmission to hold each gear longer, shifting at 4500 rpm to keep the engine in its powerband. The acceleration gains, along with corresponding mileage gains, demonstrate the effectiveness of the aftermarket components that were chosen. While the Jeep is still not quite as quick as stock, the improvements are significant, especially when combined with the added capabilities we gained on the trail with the addition of the 35-inch Toyo Open Country M/Ts. Not only is there no going back now, there is no reason to.

When we started making mods, our 4.0L engine was bone stock, with 66,000 miles on it. The only addition under the hood was an Optima Yellow Top deep cycle battery.

The factory air box does a good job of filtering air and keeping the Jeep quiet, at the expense of air flow. That horn on the front of the air box and the intake tube that looks like a giant flexy straw are designed to reduce noise in the cab.

The Volant intake tube is smooth on the inside for improved airflow. It is also slightly larger in diameter but has provisions for the factory crankcase breather and temperature sensor. The new intake tube is constructed from 1/8-inch thick polyethylene that is strong and light, and it is not as susceptible to heat soak as a metal intake tube.

Some aftermarket intakes sacrifice filtering ability in order to increase airflow. This is not the case with the Volant intake, which uses a huge Donaldson PowerCore air filter. Volant is so confident in their PowerCore filter that it comes with a 4-year/100,000-mile warranty.

The two-piece factory exhaust manifold on our ’06 LJ is a much better design than was on the pre- 2000 4.0L engines, which were extremely prone to cracking. There remained room for improvement though, which prompted us to order a tubular header from Rugged Ridge.

The Rugged Ridge tubular exhaust manifold uses thick flanges to resist warping, which can lead to exhaust leaks. We ordered our stainless steel header bare, but they are also available in black and ceramic coating.

Of all the parts that we added, the Rugged Ridge exhaust header had the most complicated installation. While it can be installed at home with hand tools, the entire intake manifold has to come off and you have to get under the vehicle to mate the header to the factory downpipes.

The professionals at Nate’s Precision installed our Rugged Ridge header and exhaust. We installed the programmer and intake at home but felt that paying for the exhaust installation was money well spent given the complexity of the installation.

The Rugged Ridge header bolts up to the factory downpipes, which contain the catalytic converters we need to retain in order to comply with our local emissions requirements. This is the only part of the stock exhaust system that we retained.

The Rugged Ridge exhaust uses stainless steel tubing that will not rust, which is perfect if you live in an environment where they salt the roads in the winter. Rugged Ridge also uses mandrel bends, which retain their interior dimensions to optimize flow characteristics. Note how the cheaper tubing we replaced is crushed through the bends, creating a restriction.

The new exhaust from Rugged Ridge uses 2 1/4-inch diameter tubing that is slightly larger than the factory 2-inch tubing without being so large that exhaust gas velocity suffers. This velocity is necessary to scavenge the combustion chamber during the exhaust stroke and make room for a full charge of new clean air.

The new Rugged Ridge muffler is both smaller and lighter than the muffler we replaced. The added space is particularly useful if you are running a larger fuel tank like we are or want to run a triangulated four-link rear suspension.

Some aftermarket exhausts can be obnoxiously loud, particularly with an inline-six engine like our 4.0L. Fortunately, this is not the case with the Rugged Ridge exhaust system, which increased our power without additional decibels inside the Jeep.

Fully welded exhaust hangers mean that you never have to worry about the Rugged Ridge exhaust rattling loose while off-road. We did have some fitment issues during our installation, but after discussing these with Rugged Ridge, they revised their exhaust so this will not be a problem in the future.

DiabloSport’s Trinity programmer was a great tool to quantify our gains and losses in acceleration when we installed new components. If you have a JK, you can use the Trinity to adjust when the tire pressure monitor system warning light comes on, as well as to engage your lockers with the transfer case in any position.

We used the DiabloSport Trinity programmer to adjust our speedometer and odometer for the 35-inch Toyo Open Country M/T tires. You can also read and clear diagnostic trouble codes with the Trinity as well.

We used the DiabloSport Trinity programmer to adjust our speedometer and odometer for the 35-inch Toyo Open Country M/T tires. You can also read and clear diagnostic trouble codes with the Trinity as well.

Installing the intake, exhaust, and programmer together removed all of the bottlenecks in the engine, and it all worked great as a system. You can’t see the difference with the hood down, but we can definitely feel the results when we stab the gas pedal.