The truth about those bolt-on mods

The aftermarket bolt-on industry has been around for some time and it seems like everyone has items such as cat-back exhausts, or at least a cold-air intake kit on their car. Some of these things can and do make a positive difference, but you’re probably not aware that there is trade-offs to these things or that they may actually decrease power.

If you think about it, the companies that make these bolt-on parts are up against an entire car company who has invested countless millions into your car trying to optimize every component so that it functions best. So you should know it’s going to be very hard to make noticeable gains without doing anything drastic.

So everyone has some sort of coil-air intake system on their car. This may actually free up a few horsepower, but probably not enough that you would actually notice any difference. What it does do is causes you to have a positive psychological experience! Let’s face it; if you add 3-5 hp at the engine, which is probably the best you could get with one of these systems, you’re not going to notice it. The reason why big gains are not going to be made by changing the air filter or going to a cold-air set up is because the engineers that designed the engine of your car know its breathing requirements; and they designed the intake and filtration system to meet those requirements. There’s simply not much room for improvement.

Now, there’s a trade off to these systems as well. So if the filter is allowing more air to go through, you can bet that more dust is being allowed in as well! I guess the concept is pretty simple, but it’s confirmed by our friends at the oil labs. If you check out a forum like the one at, you will see that aftermarket high flow filters have a reputation of letting more silicon (aka Dirt) into the engine. I’m not saying that all of these filters are bad, but how are you going to know unless you send out an oil sample? Is this worth that extra 3 hp?

Everyone’s other favorite bolt-on is exhaust systems. If your only goal is to have a nicer sounding exhaust note, then go ahead. But if you’re looking for some big gains; you’re probably not going to find them. With a stock engine, at best you’re only going to find a few extra hp. At worst, you may actually lose power.

Going farther up the exhaust system maybe even worse. A good example of this is the Honda s2000. Honda was very proud of it header design for the F20C engine and openly acknowledged that the exhaust they chose for production was the result of many iterations and high dollar testing. The OEM Honda header is really a fantastically engineered piece, but that didn’t stop the aftermarket from offering ‘upgrades’. I don’t think any aftermarket header for that engine makes any more power than stock. Though, there may be a 4 into 1 design out there that increase peak hp but with big losses in the lower RPM range with a negative net effect. I don’t think many s2000 owners are looking for less power off the line and a shorter, more peaky power band.

I guess it depends on your goals, but if you’re looking for big power gains; these bolt-on mods are going to have a hard time finding some extra horsepower. The reason is because today’s manufactures engineer their cars just that well.


This posting is about modern cars and engines. Older cars designed before engineers had access to the great fluid dynamic/flow modeling software we have today can really benefit from some of these mods. These older cars are how the industry started, and people still think what worked backed then still works today. Times have changed!

Turbo cars do not apply to this posting, but do suffer their own problems.

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6 Comments to "The truth about those bolt-on mods"

  1. Jonathan's Gravatar Jonathan
    April 25, 2011 - 7:24 pm | Permalink

    It really depends on what you call a “bolt-on” application. Some cars are very sensitive to adjustments in how air flows in the intake, engine and exhaust systems on cars and even after changing a part, to really see an advantage must have a tune done on a dyno, Just because a bolt on allows someone to force more air into the engine, doesn’t mean that the car is using this air efficiently, or expelling it efficiently. This is especially true when people start eliminating sensors that are designed to be used in the system. Some of what worked back then does work now, for instance porting and polishing can still provide noticeable gains, when done correctly and a tune is accompanied with the p&p.

    I do agree with you though that turbo’s fit into an entirely different category as they require a certain amount of back pressure and just throwing on a more free flowing exhaust may or may not allow for greater horsepower gains.

  2. Axel's Gravatar Axel
    December 7, 2012 - 9:43 am | Permalink

    I could bother telling you about the design principles of exhaust systems and manifolds, tuning by how long the primary pipes from the engine are and the effects of making them inequal, backpressures and the different effects from a 4 into 1 as opposed to a 4 into 2 into 1. But I’m just going to say that your denotes that you own something, when the word YOU’RE looking for is a contraction of two words, you are. Cheers.

  3. james's Gravatar james
    January 28, 2013 - 12:44 pm | Permalink

    Axel, please please tell us where you studied engineering.

  4. Murray's Gravatar Murray
    April 9, 2019 - 6:21 am | Permalink

    if engineers are making cars reliable and fuel effiecent, doesn’t that mean they are restricting the potential performance of the engine? if I want a little more hp and not worry about mpg, can I slap on a cold air intake, exhaust and idk, maybe a down pipe or something? I’m not an mechanic, engineer or English teacher. I just want to know if getting a few mods is worth the money?

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