According to choked flow, the limit would be when the exhaust is traveling at Mach 1. I haven't yet seen a car with exhaust coming out at the speed of sound. Additionally, this assumes upstream conditions (temperature and pressure) would remain constant which would not be the case for an IC engine.
It's important to realize that the calculations are based on engine flow required for a certain HP level (regardless of induction type). This flow is then required to move through the exhaust but at some *assumed* pressure drop. This assumed pressure drop equates to a steady-state flow, which is very unlike the pulsing of an engine.
The point here is that the chart is a rough calculation to determine "ideal" exhaust diameters for an engine at peak horsepower (peak flow rate). Putting the same pipe on two different engines simply will not produce the same power output (which I think we can all agree). The power levels associated with each diameter are the "ideal" levels of power that can be generated assuming the exhaust to be "loss-less". As previously mentioned, power generated above these levels is assumed to be lost but at a rate less than 100%.
Well, below are the assumptions they have made, to conclude that, a 2.5 inch dual exhaust can only support an engine that makes a max of 466 hp. How much more hp can we make on an engine hooked up to a 2.5 inch dual exhaust. Which of the below assumptions do you feel we should change? (also, I thought I read somewhere you are super-sonic at the exhaust valve). Also, this whole issue might be looked at a different way, ie size of intake, or cfm of head flow, to come up with a max hp, but that's another analysis. so far, for me, the 466 limit seems reasonable, but I'm open to any other possibilities.
""Breaking Down The Problem
While we’re not going to go through and list out all the formulas and calculations you need to figure this exactly, we will break down the problem, explain how you would go about figuring things out scientifically, and then leave you with some good quick-and-dirty exhaust system math as well as some interesting links.
The science goes like this…
1) Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases
Conservation of mass, right?
2) To calculate the volume of air the engine takes in, we multiply the displacement of the engine by the engine RPM and then divide by two (it takes two full revolutions for the engine to exhaust it’s entire air volume). We then convert that to volume to mass.
3) To make the calculations easy, you want to assume that combustion is perfect, i.e. there aren’t any byproducts, any unburned fuel, etc. It’s easier to assume perfect combustion and then “back in” to the actual numbers using an estimate after the fact.
4) Since you’re assuming perfect combustion, it’s easy to figure out how much fuel mass is added to the exhaust.
5) Once you know the mass of the exhaust gas, you just figure out how much volume that mass would occupy. Of course, you have to adjust for expansion due to the high exhaust gas temperature.
That’s it! Of course, when you sit down to figure it, you’ll find that getting a good scientific estimate takes a lot of work (which is why we don’t bother with it here).
Quick and Dirty Exhaust System Math
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.
Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).
Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:""
