Please Stop: Common Mods that Ruin Your Diesel's Looks and Performance

• November 20, 2021
• Story By
  Mike McGlothlin

We’ve been analyzing and covering the diesel performance industry for going on two decades now, and in that time we’ve watched various trends come and go. But we have to admit, we’ve seen some real doozy’s infiltrate the ranks of the compression-ignition crowd over the years. From dumb mods like smoke switches to “hey, look at me” mods such as giant exhaust tips, many diesel upgrades (or should we call them downgrades?) are made in poor taste. Other mods, like stacked fueling boxes and aggressively stretched tire fitments, are performed with added performance or improved curb appeal in mind, but come with safety risks.

Whether they’re a hideous looking addition, bad for performance, detrimental to longevity or just flat-out dumb, the following foul-ups are about as bad as it gets in the diesel truck game.

Smoke Switches

This one is at the top of the list for a reason: It’s stupid. If ever there was a cause for the diesel industry earning a black eye, installing a smoke switch would be it. In late-model diesels, manipulating the solenoid for the variable geometry turbocharger (VGT), coupled with fueling mods, can yield excessive smoke out the tailpipe. It accomplishes this by disabling vane (or nozzle) movement on the exhaust side of the turbo, which eliminates the VGT’s ability to build boost quickly when the engine is being lugged at low rpm.

Bull Haulers And Homemade Exhaust Stack Systems

Exhaust stacks are on the decline in the diesel aftermarket, but we won’t be happy until all bull hauler and especially homemade systems are gone from sight. Not only are they hideous looking in most circumstances, but they call for you to cut irreparable holes in your truck’s bed. Of course, you can get around cutting holes in the box floor by keeping the piping external, but let’s face it, that’s equally horrendous, if not even worse.

Stacking Boxes

Luckily this doesn’t happen with nearly the frequency it used to, but nevertheless someone out there at this very moment is probably about to stack an old timing box over another ancient product that does the same. Trust us, just because you score two stackable devices for next to nothing doesn’t mean your engine wants anything to do with them. Lethal combos where both boxes advance timing can scatter an engine, not to mention cause some pretty awful injector rattle and scary rail pressure. With as far as custom tuning software has come for diesels over the past 10-15 years, it’s best to stick with platforms such as EFI Live and keep your engine safe!

Giant Exhaust Tips

Calling attention to a diesel’s exhaust system has always been a “thing” in the diesel performance segment (possibly due to the coolness associated with it in the Class 8 world), but some examples are simply over the top. Unfortunately, while comedic in some circles, the implication with most oversized exhaust tips is the truck’s ability to belch copious amounts of unburnt fuel. When this fad fades completely out of existence, we’ll gladly bring the shovel and bury it ourselves. Just look at the size of the tip in relation to the truck’s rear differential…

The “Local Guy’s” Transmission Build

We’re not saying your local slushbox wizard is incapable of building a diesel-specific automatic transmission as well as the major players in the diesel game, but many cannot. While the corner tranny shop may know how to build one heck of an automatic trans for a gas vehicle, they might not take care to accommodate the abundance of torque a diesel-rated one will be exposed to. In both transmission and engine failure, the torque produced by a diesel causes the most damage, not the horsepower. After you have to pull out a locally-built transmission more than once, you’ll quickly realize your mistake.

Buying Too Much Turbo

Arguably no other mod can change the way a diesel drives more than a turbo. Unfortunately, the bigger is better theory that runs rampant in this particular niche can make a truck a chore to drive. Neglecting or failing to seek advice on your turbo upgrade can lead to a laggy, lazy driving experience or even a turbo that sits on the surge line of its map while cruising at low rpm. Trust us, there are scores of diesel owners who bought into the hype, purchased a bigger turbo, got tired of the lag and eventually went back to stock. Our advice: do your research and seek professional guidance for the right turbo for your specific needs. Failure to get this right can make you despise driving your truck.

A Heavy-Duty Dual Disc Clutch In A Daily Driver

They can hold some serious torque, but some of the multi-disc clutches in the diesel aftermarket are extremely noisy. With the floater plate (or intermediate flywheel) that’s located between the clutch discs being the cause but also an inevitable component in a dual-disc system, many enthusiasts accept the rattle as part of the power-adding process. Still, many others come to loathe the noise, which is especially pronounced at idle when the clutch is disengaged. Throw in the stiff clutch pedal that comes with an aggressive, dual-disc clutch that’s designed to handle 1,500-plus lb-ft of torque, and some diesel owners eventually find themselves parking the truck that was once their pride and joy. Full disclosure: We’re not saying you shouldn’t upgrade your clutch, but stick to a single disc if you can. If a dual disc is the only solution, you’ve been warned.

Stretched Tires

Any automotive enthusiast wants the vehicle they’ve poured their passion into to stand out. One of the most common ways of doing that at the present time is through huge, high-dollar wheels with tires that’ve been stretched to fit them. Diesel owners do it to make their wheel “pop” or protrude and attract attention.

Unfortunately, aggressive stretch fitments are dangerous because the strength of the sidewall is removed. Ultimately, this can lead to a damaged sidewall or the tire de-beading from the wheel. Aggressive tire-stretching is a bad idea. Even the trucks you find in the show ‘n shine area at any given diesel event are typically still used to tow with or get daily driven.

Please be safe out there.