Running Downpipe Without Tuning (Should You?)

I am guessing by now you already know that an aftermarket downpipe can give you even more horsepower. A downpipe is not that expensive – about $100 – $150 for a decent one. I can save up for that, you’d say. But do you also need to tune? You need to save up even more then!

For a quick answer, you can equip aftermarket downpipe without tuning – no problem. But it’s recommended to tune because you can get more horsepower out of it and you can make sure the check engine light (CEL)  doesn’t turn on. An aftermarket downpipe can provide 25 horsepower without tuning and up to 50 horsepower with tuning. 

That’s double – but is it worth the additional expense to tune? We will cover that in this article: pros, cons, price and also my recommendation.

Before we discuss to tune or not to tune, we must first understand the basics. How does an aftermarket downpipe provide horsepower? If you are already familiar, just skip the recap sections. These are for the beginner enthusiasts. I got all of you covered 🙂

For now, you should know that downpipe is only used if your car has a turbo. Turbo is an air compressor that sucks additional air and pumps it directly into the combustion chamber – for a more powerful combustion and increased horsepower.

Turbo is very efficient – it’s self powered by a turbine that gets spooled by incoming exhaust gases. Increased exhaust flow will generate more power for the turbo. Causing the turbo to work even harder and provide even more horsepower. You can read more about turbo in my other article Which is better: Supercharger or Turbocharger?

Downpipe is a pipe that connects turbo with the exhaust system – allowing exhaust gases to flow through the turbo and then exit into the exhaust system, where they eventually exit the car. 

Stock downpipes are usually restrictive. They are too narrow and are equipped with restrictive components like catalytic converters (CAT) – creating back pressure that prevents exhaust gases from flowing easily. Remember, you want a lot of exhaust gases to flow hard so your turbo has more power. Back pressure is an enemy here.

Aftermarket downpipe fixes this issue by having a wider piping and equipping CATs that are less restrictive – allowing exhaust gases to flow easily. Some downpipes even go as far as removing the CAT – this is called catless downpipe. Read more about downpipe in this article Up-pipe Vs. Down-pipe (And Why Modify Them) 

Dyno tuning means changing the values in your car’s computer (ECU) so you can customize how your car behaves  in different situations. For example, you can tune your car to produce more power at the lower RPM range for a more aggressive feel. 

You can do a lot of stuff through tuning. But what I want to focus here is why you should tune your car every time you install an aftermarket part. You see, your car’s ECU has fixed behaviors on what they would do in each scenario.

For example, if your car’s air flow sensor detects incoming air of 100 g/s then 1 ml of fuel is injected into the combustion chamber. These values work well for all the stock parts. But once you equip aftermarket parts, then these values should also change. 

With aftermarket downpipe, there should be more air in the combustion chamber because the turbo works harder at sucking additional air. Without tuning, your car wouldn’t know this and would inject the same amount of fuel – resulting in no or minimum horsepower gain.

With tuning, we can instruct the car to inject more fuel because we know that there will be more air because of the aftermarket downpipe. This is an oversimplified explanation but it should be good enough to make you understand 🙂 (I hope so, at least…)

Also, the numbers 100 g/s and 1 ml are not accurate. They are used only for the sake of explanation. Don’t take it too seriously!  If you want to know more about tuning, head over to this article How Much HP Can You Gain From A Dyno Tune?

Downpipe does not require tuning to work. If you really don’t want a tune or you can’t afford it – you can install downpipe without a tune. The main reasons why I recommend you tune after installing a downpipe are horsepower and check engine light. 

I expect an aftermarket downpipe to provide 25 horsepower if not tuned and 50 horsepower if tuned. This number can be slightly more if you go with the catless downpipe. But I don’t recommend going catless at all. 

Catless downpipe has horrible emission and gives out a bad smell when your car idles. You can definitely smell it when you get out of the car to grab something from the trunk. In more severe cases, you can even smell it from inside the car. 

Also, tuning your car after a downpipe install has more benefits than just the horsepower. Let’s list down all the pros and cons so you can decide whether to tune or not. These are just quick lists. If you want to know more about dyno tuning in general, check out my other article How Much HP Can You Gain From Dyno Tune?

1. More Horsepower

We touched on this already. With tuning, we can instruct your car’s computer (ECU) to inject more fuel for each air flow reading. This should increase horsepower because the combustion chamber now has more air because of the downpipe. More air + More injected fuel = more horsepower.

2. Reduce Wear And Tear

Your car’s computer (ECU) controls when should the exhaust valve open / close. The ECU does this by measuring the back pressure in the exhaust. If it reaches the right amount, the exhaust valve opens – allowing exhaust gases to flow out from the combustion chamber and allow fresh air to enter.

The valve opening / closing works well for stock back pressure. When you install an aftermarket downpipe, the back pressure will decrease. Without dyno tuning, your car’s ECU will mess up the timing a little. The ECU will open / close the valve slightly late because the back pressure reading is lower with downpipe. 

This means exhaust gases will not be released at the right time, causing it to occupy the combustion chamber a little longer. This can reduce performance or even cause wear and tear damages if left un-tuned.

3. No Check Engine Light (CEL)

When equipping aftermarket downpipe, it’s common for Check Engine Light (CEL) to turn on. This happens because aftermarket downpipe equips a less restrictive catalytic converter (CAT) – good for exhaust flow but bad for emission.

Your car’s ECU assumes that you are still using the stock CAT and thinks that the CAT is not functioning properly. Thus, the check engine light. You could tune your car to keep the CEL off even when equipping a less restrictive CAT.

4. More Aggressive Car

When tuning your car after installing a downpipe, it does not mean that you could only tune your car to work with the down pipe. You could go a step further and tune the other parts of your car. 

For example, editing the Engine Torque Map to give you more power in the lower RPM range – so your car feels more responsive and aggressive. If you want to know about what else you could tune for peak performance, then read my other article. How Much HP Can You Gain From A Dyno Tune? 

1. Additional Cost

This is probably the biggest con in tuning – it’s not cheap. A good tuner could charge you a hefty price of $300 or more. ‘But that’s even more than the downpipe itself!’. Yeah, I know. But that’s the price for a tune.

You could probably negotiate with the tuner and get a cheaper price if you are only tuning a few parts. I don’t really recommend this but it’s an option. I cover what I really recommend later. So keep reading 🙂 

2. Bad Tuner = Bad Results

Tuning largely depends on the tuner. All the pros I mentioned above apply only if your tuner is reliable and they know what they are doing. Otherwise, you wouldn’t see any changes. Worse, your car could run worse! 

If you don’t know any tuner, I suggest you research the tuners in your area and read the online reviews and hit the forums.

If you are serious about performance gain, you should always tune your car after installing any performance mods. If you only care about sound and minor performance gain, then don’t waste your money.

My suggestion is that you bring your car to a tune after you have installed a couple performance mods. If you only have downpipe now, then save the tuning for later. Install other mods like catback exhaust, cold air intake and exhaust header first. 

Once you have installed all of them, then bring your car for a tune for the maximum performance gain 🙂