Choosing the right exhaust size

Hi, everybody

I tried to use the search, but it didn't give me the results I needed, so I decided to create a new topic.

I was puzzled by the question of choosing the right exhaust size for a particular project.

Is there some kind of working, proven table or maybe a formula for calculation?

What I found on the internet doesn't look like real numbers.

Basically, values such as 300-350HP for one 76mm pipe and 450-500 for 89mm pipe are offered.

For example, I have several projects such as:

- LS7 with a volume of 427ci(7.0l) with a Whipple 4.0l supercharger which has a power of 935Hp with a 108mm throttle and about 760-800HP with a 90mm throttle. It has a straight short exhaust 2x76mm (outer diameter, inner 74) with H-pipe.

- 1.5JZ-GTE 3.0L 600HP with one 76mm pipe (74 inner diameter).

- My friends from HGK Motorsport use an exhaust system on their N/A LS7 7.1 in which 2x76mm pipes converge into one 89mm (again 87mm inner diameter) and got 750HP without nitrous oxide and +120HP with nitrous oxide.

Next season I plan to install backpressure sensors to make sure that the performance is sufficient, but I want to do it as correctly as possible so as not to redo it later...

No specific suggestions, there are plent of guides on-line and I'd suggest checking several of them and using the closest pipe size to the average - rounding down for lower rpm response and quiesness, up for peak power with, maybe, some additional noise.

That said, remember that the efficiency of the engine is potentially a big factor - for the same output power the NA will have least exhaust gases, the turbo-charged may be producing 10-20% more, and the supercharged 20-40% more.

Then there is the affect of the mufflers, if used, and whether there's a cross-over/H-pipe, or similar, to share some of the flow, or whether they're individual?

TBH, I would expect the rest of the components to have at least as much affect on the gas flow as 'close enough' pipe diameter.

When you're in the 700+ HP range, you are going to need to depend on experts, or do you own simulation / testing to find the best solution.

You mention primary tube sizes, but not length, steps or merges. There can be a lot to extracting maximum performance. as this article mentions:

https://www.chevyhardcore.com/tech-stories/exhaust/header-design-choosing-the-best-headers-for-your-application/

I suggest looking into engine simulation software that allows exhaust system design as well.

I could add my cents but honestly his explanation is worth more.

https://youtu.be/DvMBbojZpII

Thanks for your answers, guys

If anyone else has information, I will be glad of any help