I didn't see this covered in the stickies, but I was wondering, is there any general consensus in terms of the diameter of different components of the exhaust system? For example, is it better to have 2.5 HFCs -> 2.5 inch exhaust?

I was reading that if you go from 2.5 inch cats to a smaller exhaust (2.25 exhaust for example) you'll be creating too much backpressure and inefficiency? I know that the OEM system is 2.25 inch cats -> 2.25 inch y-pipe -> 2.5 inch exhaust piping.

Basically, I was wondering how the flow of exhaust gases corresponds to low end vs top end power.

 

I found this quote from Tony @ Motordyne from a random thread, thought it was interesting:




Back pressure (at least on our engine) is NEVER a good thing for NA applications.

It is a widely spread myth that some back pressure is good, but it is 100% FALSE.

There clearly are situations where smaller diameter pipes can outperform larger diameter pipes but this is not because of back pressure. This is because of scavenging. And when tuned properly, scavenging actually reduces back pressure as seen by the engine.

But be careful in the assumption that smaller automatically equals better performance. It is highly dependent on where the smaller diameter pipes are being used.

Small pipes near the engine can be good for scavenging and power, but as the pipes move farther and farther away from the engine the effect of scavenging rapidly diminishes. If scavenging cannot be taken advantage of, then small/restrictive pipes must be completely avoided.

Back pressure can only reduce power. Do not confuse this with scavenging.
Scavenging actually reduces back pressure on a tuned cyclical basis.

With each pulse released during the exhaust stroke of the engine, the pulse travels like a shotgun blast down the exhaust pipes. The high intensity blast creates a shockwave with a large positive pressure at the wave front. This wave front is traveling so fast that even when the piston reaches TDC and all the gas is expelled by the piston, the fast moving slug of exhaust gas doesn't stop and it rarefies the gas and creates a vacuum behind the shockwave.

It is the vacuum behind the shockwave that sucks out any remaining exhaust gas from the cylinder. This vacuum also pulls more fuel/air mixture through the intake valves during the intake/exhaust valve overlap period. And this is how more power is made. This is the scavenging effect. It vacuums exhaust gas out of your engine!:thumbup:

Adding back pressure can only kill off this vacuum that you want.

NOW HERE IS HOW THE MYTH STARTED.
IT WAS A MISINTERPRETATION OF TEST RESULTS.

Sombody a long time ago probably did the same series of dyno tests I did on varying pipe diameters. Like I did, they probably found that smaller diameter pipes can yeild higher HP and TQ. They probably mistook this for back pressure and put it out in the public. Smaller diameter pipes can provide higher performance when used properly. But small diameter pipes are only desirable when they are very close to the engine.

For example:
I did a series of dyno tests on various diameter test pipes ranging from 2.5", 2.25" and 2.0".

Before conducting the tests, my initial guess was that the larger diameter pipes would produce the highest HP with lowest TQ. And the smaller diameter pipes would produce the lowest HP and the highest TQ.

Well... I was 1/2 right...

As expected, the dyno testing showed the 2.5" diameter test pipes made the lowest TQ. And as expected, the dyno testing showed the 2.0" diameter test pipes made the highest TQ.

But here's the kicker. The 2.0" test pipes made 2 more HP than the 2.5" test pipes! ...It left me thinking "COOL.:thumbup: Smaller diameter test pipes make more TQ and more HP. That's a wining combination!"

So sombody a long time ago probably misinterpreted the smaller diameter as adding performance by being more restrictive. But this is not the case. It is because of increased scavenging. Smaller diameter pipes near the engine increase the velocity of the shockwave and thereby increasing the effect of scavenging. It was a misinterpretation of the results.

So I continued down this line of testing at the Y-pipe primaries. Using the 2.0" test pipes, I then tested various Y-pipe primary diameters. 2.0", 2.25" and 2.5".

The expectation was to see similar results... but not quite this time. At least not at the Y-pipe.

The 2.0" Y-pipe primaries did indeed provide the highest TQ, but it brought a good portion of the HP down. 2.25" primaries were better but could still be improved upon. The 2.5" Y-pipe primaries provided the best peak power and the best average power.

So dyno testing proved the best test pipe diameter is 2.0" diameter and the best Y-pipe primary diameter is 2.5".

I then continued further down this line of testing on the mid-pipe and made some more interesting observations. Testing mid-pipe diameters at 2.5", 3.0" and then a fully open Y-pipe.

What I did find was that there was no scavenging effect possible after the Y-pipe. There was nothing to gain from the smaller diameter what so ever. In fact, the only thing that had any effect was simple back pressure.

Using a open Y-pipe as the baseline I found that connecting a 3" single exhaust had no effect on TQ and with only a small 1.5 HP decrease.
The 2.5" midpipe slightly reduced TQ and was ~2.5HP down from than the 3" midpipe.

This series of tests established:
1) There was no scavenging possible after the Y-pipe.
2) A smaller diameter midpipe can only decrease HP&TQ
3) There will be rapidly diminishing returns beyond a 3" midpipe
4) With power to weight ratios taken into consideration a 3" midpipe can be considered optimum. 3" also allows more headroom for medium boost FI applications.

Going from 3" to a 3.5" midpipe may at best provide a 0.5HP increase. So from a weight point of view, going larger than 3.0" would be counter productive for NA applications.

I then conducted another series of tests at the end of the Y-pipe.
1) Attaching a 3" diameter butterfly valve with variable position restriction plate.
2) Attaching a 6" diameter parabolic diffuser to reduces pressure drop below that of a 3" open pipe.

The purpose of the butterfly valve restriction plate was to directly test the effect of raw back pressure on performance. And the results were very clear.
BACK PRESSURE RAPIDLY REDUCES PERFORMANCE.

I dyno tested the valve at various levels of flow restriction. From wide open to almost fully closed as back pressure was increased, performance rapidly decreased.


This set of dyno plots is proof positive that back pressure is the enemy of power and torque.

Let the myth of back pressure be permanently dispelled from the vocabulary of this forum!

After that series of tests I started another set of tests that decreased exhaust pressure beyond that of a simple open ended 3" pipe.
A 6" diameter parabolic diffuser was clamped onto the end of the Y-pipe. This was used to decrease flow resistance below that of a open pipe.

Dyno tests of the diffuser showed an instant 4-6HP increase over that of a open Y-pipe!:thumbup:

This picture below is a picture of a 5" linear diffuser. It doesn't perform quite as good as the 6" parabolic diffuser but the 6" diffuser is completely impractical for fitment and production reasons.
6" is too big for fitment under the Z and the parabolic shape also gives it a curvature that makes the production process MUCH more difficult.

This is the diffuser used on the MD ShockWave single exhaust system. It can also be attached directly to the Y-pipe for drag race applications.



So while there are rapidly diminishing returns with going to larger and larger diameter tubing after the Y-pipe, significant gains can still be made by use of diffusers.

The back to back dyno testing shown below was a simple open Y-pipe as the baseline and then with the diffuser attached.

 

i don't have an answer to the question but i just replaced my ('06x) rotted oem y-pipe with an aftermarket one. i was surprised to find the OD of the oem y-pipe was about 2.1" so I don't know what the ID is. I measured somewhere after the flex section not right at the flange.

 

Don't the sedans have a different sized system since it's only single exit?

 

This is a pretty difficult question to answer, but it seems like you're doing the proper research on the topic and not just going to throw parts together and hoping it works.

the snippet from tony at motordyne really does answer the question, however I'm not sure anyone without an engineering or science background can really understand the content and digest it.

I'll try to sum it up a little without giving a lecture on fluid mechanics. The reason you want a smaller diameter test pipe is to increase scavenging as said in the article, but the reason it happens is because it increases flow velocity in the pipe. Having too large of a pipe without sufficient flow makes it turbulent and causes "backpressure" (friction) to happen within that portion of exhaust. This also happens when you step down in exhaust pipe sizing, so your logic on the right track.

With that being said, you want to start off with a smaller diameter near the motor to improve exhaust scavenging, and move onto a larger diameter as you exit the Y-pipe to improve flow capacity and minimize pipe losses.

I don't have the ability to switch parts around and do dyno testing to further back up any claims, but for an NA application I feel that 2.5" test pipes/HFC -> Y-pipe with 2.5" inlet and 3" outlet -> 3" single exit exhaust is ideal in terms of performance while keeping the weight down.

 

Aww no lecture on fluid dynamics?

I just don't like the asymmetry, otherwise I would totally consider Tomei/Agency Power

 

The best exhaust system will have the same inside diameter from header to tips! Smaller diameter HFCs will show down the flow and cause rasp across the rpm range. My authentic HKS true dual exhaust is 2.5 in. in diameter and so is the Magnaflow x-pipe and Fast Intention HFCs! The x-pipe equalizes the pulses from each bank eliminating the HKS faulty crossover pipe! Having SLICK tuned I didn't lose WHP or torque at any RPM until the top.
Gary