Can an exhaust be too big for your car?

Can a performance exhaust be too big for your car? This is a question I have heard many times. The answer to that question is yes and no. It all depends on if the engine is naturally aspirated and if it is forced induction. So why is it different for the two setups you may ask. Well, I will try and explain why exhaust size is crucial.

As long as I can remember people always spoke about back pressure on the exhaust. “You need to have the right amount of backpressure on you exhaust”. I hated those words as no one could really tell me why you need backpressure.

What I have learned running engines on engine dyno’s and playing with different setups is the following. You don’t need backpressure, what you need is a tuned exhaust. It is all about the pulses running up and down the exhaust. What a proper exhaust need to do is to move the exhaust gas away from the cylinder head in such a manner that scavenges on the other ports. This means as one cylinder exhausts its gas there must be suction on the other exhaust runners so that when the next cylinders valves open it sucks out the exhaust gas. While this is happening the intake valves open and you are able to fill the cylinder with the fresh gas mixture.

What you don’t want is to have exhaust gas stuck in the cylinder, which is really bad for performance.

Exhaust for naturally aspirated engines.

To build a proper exhaust system for a fully race built engine can be difficult. The “bigger the better” does not work. To get a naturally aspirated engine to perform at its best it needs a lot of time on the dyno. It doesn’t help to run a header pipe of which the ID is bigger than the exhaust port. You need to have the same or slightly smaller. What you want to achieve is to keep the gas speed as high as possible. You will have to have a set of headers of which you can change the primary lengths before the collector.

As intake manifold lengths are different from engine to engine you need to find the primary length that will be in sync with the intake manifold length. Yes, this means you will have to run after run on a dyno until you get the sweet spot. This can be a time-consuming part of your development.

Once you have your primaries sorted out you can play with the exhaust size after the collector. Once again you can’t go to big, too big and you will lose gas speed. If you lose gas speed through your collector you will lose some of your scavenging effect on the other runners. You want your exhaust small enough to keep the gas speed up without choking the gas coming out.

To know if you have a good design header there is an easy test. Blowdown one of the header from the head flange side with compressed air. Put a piece of paper on the other ports. If the scavenging of the exhaust works well the paper will be sucked down hard onto the ports. Now try it the same way one each runner. They should all have the same effect.

Exhaust for Turbo cars

Exhaust for turbo cars is completely different from Naturally aspirated cars. Most turbo cars don’t even have a proper exhaust manifold. Some of the are just a straight pipe connecting all the ports and the turbo is mounted on that. it works but you don’t get the maximum out of the turbo.

Proper turbo manifolds work the best and having custom-built manifold even better. It all depends on how much you want to spend on your build.

An important part is what happens after the turbo. Your exhaust can never be too big after the turbo. You don’t want anything that can cause back pressure after a turbo. This is one of the big reasons that people remove the cats on their cars. It holds the turbo back. The turbo has to work so much harder to generate the boost. I am not saying go and put a 4″ exhaust on your turbo car. In most cases, a 3″ exhaust will do the job.

The more restrictive the exhaust is after the turbo the longer the turbo will take to spool up. The max performance of the turbo will also be limited as it will battle to get the exhaust gas out. There is one downside to running big exhaust on turbo cars and that is the drone. Most of the newer cars are well isolated and you will not have an issue with the drone.

You could also go for 2 smaller pipes but the sectional area of the 2 smaller pipes need to be the same as a 3″ pipe. This will help a bit with the drone. The downside of this is you may add a bit more weight to the car.

Exhaust for a supercharged engine

The rules for a supercharged exhaust is a lot the same as naturally aspirated cars. You need to have good scavenging on your headers to get the gas out, your pipe from the collector may be a bit bigger to that you can get the big volume of gas out. Supercharged engines move a lot of air, meaning they have a lot of exhaust gas coming out of the cylinders.

What you don’t want with a supercharged engine is to have exhaust gas recirculation. Having exhaust gas stay behind in a cylinder on a performance engine is never a good thing.

What happens if exhaust gas stay behind in cylinders?

There are a few reasons why is not good for exhaust gas to stay behind in the cylinders. One of the reasons is if there is old burnt gas trapped in the cylinder then there is less space for the fresh gas mixture. This means less power because it is less fuel burnt.

Another reason is if there is exhaust gas left behind they cylinder temp starts out hotter and then it can have a snowball effect on your cylinder temps. The temperature spike can cause sharp edges on the cylinder head or even the spark plug to glow. This will cause premature detonation. If this happens while an engine is being tuned the tuner will take ignition timing out of the map and this will result in less power.

All engines need proper exhaust scavenging. The more exhaust gas you can get out the more fresh gas you will get in the more power you will make.