Exhausts with good flow velocity

[nikitopo]

Going too big in an exhaust diameter is not always optimal. It will decrease the back pressure which is good, but at same time it will decrease the flow velocity which is bad. A good flow velocity aids the complete expulsion of burnt gases and increases the amount of the air-to-fuel mixture being drawn into the combustion chamber. When exhaust gases exit the engine with a higher flow velocity, they create low pressure behind them. This low pressure continues to suck out exhaust gases at an extremely high rate, creating a valuable cycle.

An example of such exhausts is the family of the STI performance mufflers:



Are you aware of any other exhausts that decrease the back pressure and at same time have a good flow velocity (e.g., by keeping the stock exhaust diameter size)?

[Ashikabi]

Anything aftermarket that isn't 3 inch should do you just fine. 3 inch of almost certainly too big but I don't have Dyno results to back that up

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[humfrz]

Originally Posted by nikitopo

Going too big in an exhaust diameter is not always optimal. ..........

NOW ...... you tell me!

[PulsarBeeerz]

You are only paying for noise with an axle back. There aren't any performance benefits so just choose whatever floats your fancy. If it is a cat-back on an otherwise stock car a few JDM manufactures make 60mm cat-backs. But once again there is no performance benefit. Just money spent and noise.

[EndlessAzure]

[nikitopo]

Originally Posted by Ashikabi

Anything aftermarket that isn't 3 inch should do you just fine. 3 inch of almost certainly too big but I don't have Dyno results to back that up

Here is a dyno graph showing how power is affected if you go with a 3" full exhaust. Peak power in stock exhaust is around 6800-7000 rpm, while peak power in bigger exhaust falls into the 6300-6500 rpm cause of the worse flow velocity even though it is used a less restricted air filter. Overall gains are irrelevant here, because Stages 1.1-1.3 have a more aggressive tune and Stages 1.2-1.3 a sport cat and catless header.

[nikitopo]

Originally Posted by PulsarBeeerz

You are only paying for noise with an axle back. There aren't any performance benefits so just choose whatever floats your fancy. If it is a cat-back on an otherwise stock car a few JDM manufactures make 60mm cat-backs. But once again there is no performance benefit. Just money spent and noise.

This is a false perception from the first years, because the engine was new and there wasn't much R&D. For example, the STI performance muffler was made available in '16. Before they were selling the STI sports muffler without any improvement in back pressure and as you mentioned with a change just in sound.

[PulsarBeeerz]

Originally Posted by nikitopo

This is a false perception from the first years, because the engine was new and there wasn't much R&D. For example, the STI performance muffler was made available in '16. Before they were selling the STI sports muffler without any improvement in back pressure and as you mentioned with a change just in sound.

No, thinking that changing the last 3ft of exhaust will make an appreciable performance difference is just silly at this point. Just admit you want a different noise out of the car. There is nothing wrong with that. You will never notice the 1-3bhp you think you are gaining.

[SydBRZ]

NA wise, your header will be the main area where flow velocity will play a role. The remaining exhaust system will only play a small role, assuming you don't choose a massive pipe diameter.

The flow velocity of the exhaust system behind the header, all the way back to the rear muffler, will be more critical in forced induction applications.

[Ashikabi]

Originally Posted by nikitopo

Here is a dyno graph showing how power is affected if you go with a 3" full exhaust. Peak power in stock exhaust is around 6800-7000 rpm, while peak power in bigger exhaust falls into the 6300-6500 rpm cause of the worse flow velocity even though it is used a less restricted air filter. Overall gains are irrelevant here, because Stages 1.1-1.3 have a more aggressive tune and Stages 1.2-1.3 a sport cat and catless header.

I assume manifold is referring to the header. That's a very interesting Dyno chart since I've always been under the impression that the cat back was not very valuable for power and most is gained through the header. This chart seems to indicate otherwise

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[nikitopo]

Originally Posted by Ashikabi

I assume manifold is referring to the header. That's a very interesting Dyno chart since I've always been under the impression that the cat back was not very valuable for power and most is gained through the header. This chart seems to indicate otherwise

The OEM catback is quite restrictive with high back pressure. Most gains of a bigger catback are coming by advancing ignition timing.

[ZDan]

In short, I don't believe those plots. NObody's *really* gaining real-world +20 hp with a catback and "more aggressive tune" vs. stock on any year FT86...

Regarding "good flow velocity", it's a bit of a myth... From what I've seen on 150rwhp 2.4 liter ITS 240Z dynos, bigger exhaust always helped but going from 2.5" to 3" was a small, almost negligible gain, point of diminishing returns.

You could run a 1" catback and get massive gains in "flow velocity" and if you measured at a static pressure port in that 1" section it would show low pressure due to Bernoulli. However you will have lost power due to increased pressure upstream...

[spcmafia]

The HKS Hi Power Spec L claims to improve low end torque and horsepower. I have to say that compared to my last catback, Q300, the car did feel lighter after installation, I was able to develop lower speeds slightly faster and going from 65MPH to 80MPH feels easier. I did have JDL UEL headers while having both exhausts. It just "felt" better with the HKS one.

[alphasaur]

Originally Posted by spcmafia

The HKS Hi Power Spec L claims to improve low end torque and horsepower. I have to say that compared to my last catback, Q300, the car did feel lighter after installation, I was able to develop lower speeds slightly faster and going from 65MPH to 80MPH feels easier. I did have JDL UEL headers while having both exhausts. It just "felt" better with the HKS one.

I think the spec L just sounds good down low so it feels faster. Spec L gang