I made this post because i was bored. I thought i might share why a dual (split) exhaust system is better for performance than a single exhaust system, "in terms of HP/TQ." (not weight) Some of you may already know this, but i can asure you alot of people do not.

The theory is that when the exhaust splits into two seperate pipes, the combined area of the two pipes is greater then that of a single pipe of the same diameter. For example if you had a 2.5 inch exhaust system that split into two 2.5 inch pipes, it would have alot more room for the air to flow then if it was a single 2.5 inch pipe all the way back.

So if you wanted to run one single exhaust pipe all the way back you would have to make it alot larger in order for it to equal the same area, compaired to having two pipes.

Also, alot of people think having two 2.5 inch pipes is like having one single 5 inch pipe. This is not true.

For those of you who like to get down to the nitty gritty, here is an example of the formula used to solve for what size pipe you would need in order to equal two smaller sized pipes.

The example below is to find out what size pipe you would need for a single exhaust in order to have the same area as a split 2.5 inch exhaust system.

Ok first part (figuring out how big two 2.5 inch pipes are):

area = (3.14)(r)(r)

area = 4.9

therefor the area of the two 2.5 inch pipes is 9.8

to find the radius of the pipe with the same area (as the two 2.5in pipes)

9.8 = (3.14)(r squared)

take 9.8 divide it by 3.14

3.12 = (r squared)

take the square root of 3.12

1.77

thats your radius

you are solving for R

multiply that by two for your diameter

answer = 3.54

so if you wanted to have one single exhuast pipe, that does the same as two 2.5inch pipes...you would need roughly one 3.54 inch exhuast pipe.

If your exhaust split into two 3 inch pipes you would need one single 4.24 inch pipe to flow the same amount.

So in theory, people with a Magnaflow exhaust system (or any other system that splits into 2.5inches) Have a 3.54 single exhaust pipe after the "T" or split. (just talking about pipe size, not mufflers or anything else)

This is why a "split" exhaust out performs most single exhaust systems. It just isnt practical trying to fit such a huge single exhaust pipe under the car when you can make two smaller ones that do just as much.

If you wanted to use the formula for other sizes of pipes, you can just plug in the different numbers.

Have fun

 

This ignores the fact that the first ~7 feet of the exhaust is a single pipe. So a single 3" will still have a total flow superior to the, for example, Magnaflow.

Cool math though

 

I havent seen many 3 inch single exhaust systems for the mini, only 2.5

But yes you would loose half an inch in the first part and gain half an inch in the rear

Edit: unless you went with a 3 inch exhaust and had it split into two 2.5inches then you would be the same/more then a single 3inch

 

You're right. The future Webb exhaust system is 3" it's all I know of.
Fits quite well.

 

3" cat back is mighty large. It all starts with the head. What's the exhaust flow? Then headers? How big are the primaries? What kind of collector? Does it step down? Cat flow rate? Then comes cat back considerations? I've tested a bunch; but didn't choose 3".

 

I was just trying to explain some theories incase people didnt know, not indivual exhaust systems, (although i did reference one as an example.)

I plan to custom build my exhaust in the near future, wish me luck! (i will need it )

 

i was under the impression that exhaust design (for N/A and Supercharger engines) is more about getting the exhaust pulsewaves correct etc... than fitting as big a tube as physically possible

 

Hello Mini Mizer,

What are you thoughts on the following system:
2.5" main section, splitting into 2 pipes of 1 3/4
these are the dimensions of my larini exhaust.
let me know
thanks for your thread

Victor

 

I wasn't talking about ideal flow for the MINI. Just discussing pros and cons.

As in anything performance oriented, heck life in general, you need to consider the whole system but that doesn't preclude discussion of subcomponents and their merit or lack thereof.

Now let's see that exhaust

 

Good luck

 

Alot more goes into the design of an exhaust system, then just pipe diameter. It involves resonators/bends/back-pressure/muffler flow and of course sound level. Which is a big factor when creating a street setup. Not everyone likes roaring exhaust systems. (especially not the cops )

But as a "whole" the engine is a giant air pump, the more air you can get into it and out of it, and the faster you do this, the more HP created. As long as you have the fuel to match the amount of air.

When it comes to exhaust its all about restriction. The more free flowing the engine can be the better, so larger pipes do infact help HP.

Im not sure about pulsewaves or anything like that, infact i havent heard about that until just now. (i dont know everything ) But i can tell you that no matter the application, the more free flowing the exhuast the better for "max performace" and there is alot of different ways to make an exhaust system flow. Restriction is the enemy when it comes to exhaust, period.

You can see "extreme" examples of this, on any true race car. IE: F-1 cars, NASCAR, Top Fuel dragsters. Most of these cars have little to no exhaust at all and are performing at peak levels.

For example, Top fuel dragsters have super chargers along with about 2 feet of curved tubing comming off the head. Im not sure what this does for pulsewaves but they all seem to run the same setup.

The thing is with street systems is that people are trying to make the most HP/TQ with the least amount of noise, and doing this is truely an art. Because normally the more free flowing an exhaust is the louder it tends to get.

But restriction will always be the enemy!

 

Actually that is most important in the header itself. Once the exhaust leaves the header and enters the jumble that is the rest of the exhuast pulse timing is pretty much a done deal. Just keep out of the way.

 

All I know is, I love my ALTA Sportone Single Pipe Exhaust. I love the look of it. I love the sound of it. I love how my car runs with it. And I love its gorgeous big tips.

 

I'm very hesitant to post pictures due to the enormous development cost incurred. We learned a lot from the work we did on Porsche Grand Am cars that we support. At the track those with "secrets" like to keep it that way.

 

The exhaust is only going to be as good as its most restrictive part. In the MINIs case that would be the header cat assembly. So unless you are willing to replace these parts, the size of a dual or single sided system really won't matter above a certain point (3" for a single sided system ala WMS)

 

Top fuel pipes do this....
Get heat away from the vehicle, Produce downforce( you would be amazed how much), and help keep cold air from striking the exhaust valves.

Not strictly true. Tubing diameter and length for instance can be used to optimize or tune torque curve.

 

I was quasi joking. I figured if you were going to you would have.

The downside is very few will buy anything sight unseen. Myself included. <---Insert shrug emoticon here

 

This, is true although restriction normally creates back pressure and back pressure normally makes low end torque. But i was orginally talking about max performance as in the red-line of the engine. Restriction will hurt red-line performance without a doubt. Me personally i would rather have top end performance over low end torque. Just my preference.

 

i'm not an engineer or anything, but i am an enthusiast and a numbers junkie. but i do remember that for years on my dad's Harleys when researching exhausts on motors not much smaller than the MINI's, the 2-into-1 exhausts always made more power than the more traditional 2-into-2 systems. in motorcycles it's not the size of the tubing so much as it is the flow design of the muffler's internals. i just feel that 3" is just too big for a small 1.6L motor like ours. but like i said i'm no engineer so i am probably wrong but i appreciate good technical discussions.

 

From my experience on NAM, pictures do seem necessary to most. But to the customers I am more accustomed to dealing with all that matters are results. If you walked through a Grand Am paddock you would see blankets covering aero devices and engine bays covered. Ever been to a high level motorcycle race? You'd see nothing. But many covet what they think makes the other guy fast, sight unseen. You simply trust the builder and judge him by his results.

The cat back I build is nothing special; it is 2 ½” all the way, has a 10” ss interlocking flex piece (if my header is used), a Magneflow resonator, a “Y,” two Magnaflow mufflers, and exits the rear at both sides.

The header is another story. Unfortunately, the cost will be prohibitive to most. I took a considerable amount of development work (3+ years) that went into some Grand Am, HSR, and PCA cars I mess with a transferred it to the Mini. But for those who want more than an OBX but less than mine I have an MTH header that works.

***A heads up: for those who have installed a header there is a considerable amount of play in the connection between the head and header. You must make sure the ports are lined up and the gasket is not in the way. And this is not as easy as you might think. To do it properly you need a scope camera.

 

Yeah, bored.

 

the "pulse waves" that have been mentioned are paramount! they create the vacuum at the exhaust valve that actually sucks the gas out of the cylinder. Sure you can make the flow performance of the exhaust better, but what good is it when you can optimize the scavenging effect so much more? What usually makes the power? Headers or catbacks?

For flow, think of it like this. You want to blow up a balloon with a pipe from your mouth to the balloon. What pipe do you use? A thin straw? A toilet paper roll? Both will be bad. Sure the flow of air is fast through the straw, but it doesn't flow much air at all, so it doesn't flow well. The toilet paper roll has too much capacity to make a fast flow of air. It would work well if you attached a reverse-cycle vacuum cleaner to it, but for humans? nope. You have to find the optimal pipe diameter and length for the MCS (and for each particular car).

By the way, having split pipes has also a lot to do with pressure, not merely the area. Take an electrical analogy. Two resistors in parallel of a resistance of 4ohm will be equivalent as running one 2 ohm resistor. That's two resistors, both twice the size of the other resistor......take flow as current, and voltage as pressure, and you have a similar process for gas flow

 

Ok you lost me with the electrical thing (im horrid at electrical)

My OP was never about headers/cats/mufflers/resonators. It was more about area and volume. I was orginally talking about a split system and a single system of the same diameter.

As quoted from my orginal post.

"The theory is that when the exhaust splits into two seperate pipes, the combined area of the two pipes is greater then that of a single pipe of the same diameter."

I also wrote i was only talking about pipe size.

"(just talking about pipe size, not mufflers or anything else)"

I was only trying to discuss the rear area of the exhaust system where it splits, as opposed to continuing in a single pipe. Not anything else.

As for the balloon example, i dont get it Why try to blow up the balloon?

I think the example would be made better if you said, "try to breath through a small straw all day long, or breath through a paper towel roll Which one is easier to breath through?" The paper towel roll, because its larger and provides less restriction/back pressure. Same thing goes for an engine. Large area of piping allows less backpressure which is ideal for making top end HP.

This is direct quote from the Magnaflow website in reguards to their "race" mufflers.

The Race Series is designed for the ultimate in high performance and an aggressive racecar sound. It features a true straight-through design for maximum high RPM pulling power and a 2.5" or 3" perforated core to virtually eliminate backpressure and maximize exhaust flow. The Race Series is recommended for the serious racer, turbo, nitrous oxide fuel systems, or wild cam applications.

They also agree that eliminating backpressure is good to maximize "high RPM pulling power"

Like i have said before, restriction is the enemy, and i stand by it.

 

Well in that case, as the pulsing/scavenging has little to do with the catback (as you said), i wont' pursue headers etc

All i was saying is that i understand that the area would be obviously greater for two pipes, but since we are discussing flow, which also depends of the backpressure, you treat the flow through two pipes as if it's current flow through two resistors (or two pieces of wire). Having two pipes of the same diameter as that of the single pipe would mean that all three pipes have equal resistance to flow. Having two pipes in parallel however, means half the total resistance than the single pipe. The resistance of the pipe is directly related to the pipe diameter.....

Blowing a balloon is just an analogy. Let's say we're trying to flow as much air as possible out of our mouths (which would blow a balloon as quickly as possible). A paper towel roll is easy to breath through (almost zero backpressure), but if you want to create the fastest possible flow at the other end of the roll, you need a certain amount of backpressure. Otherwise all exhaust companies would make 5" catbacks for power. Car's with pullies and headers etc, flow more gas, so a catback with a larger pipe diameter would flow exhaust gases better, but if you don't have the lung capacity to blow through a paper towel roll with any sort of force out the other end (if you're a toddler), you'd get higher CFM with a smaller diameter pipe. Drag engines are giants compared to our "toddler", so a macdonalds straw would be a better option. Do you understand the concept i'm trying to explain? It's hard across the net :(

Restriction is the enemy ONLY if you have the power to flow enough gas through it (which most race cars do)

 

with respect to the backpressure/balloon analogy: cfm isn't going to change with opening size until you max out what the source can provide or throttle down the opening to limit flow. what will change is gas velocity.