Exhaust Performance Question

One or two questions regarding exhaust performance, on my NA.

I know certain words in that sentence might not necessarily go together but anyway....

I've recently fitted an OBX manifold to my NA, and also have a 2.5" 2nd decat pipe to fit in the coming weeks.

I'll then have two exhausts in my possession to choose from, one is a Powerflow one with a 3" outlet and a second resonator type box:

The other one is a Motoria single box type with a 4" outlet:

So I've been reading up on these two threads from years ago, as well as a few elsewhere:

http://www.mkivsupra.net/vbb/showthread.php?96937-Quiet-or-otherwise-aftermarket-exhausts/page3

http://www.mkivsupra.net/vbb/showthread.php?22670-back-pressure-(normally-aspirated)

What I'm basically wondering is what exhaust will be best for performance, or does it make absolutely no difference whatsoever once the emissions pass through the 2nd decat pipe?

Obviously the 4" one will be asbo loud so I need to think about that but at the moment I'm curious just in terms of performance.

If it makes one single HP difference one way or the other Id be really interested in learning why and how, or if one will make a difference in certain rev ranges etc.

The best will be the one that most nearly approaches a resonant state at the point in the power band you want most power or torque. It could possibly be predicted by simulation, but practically it needs dyno time with one fitted after another.

I never heard if the you do need or you don't need backpressure to improve bhp discussion ever reached a conclusion. People still argue over that on various forums. Probably needs someone to go dig out data but people sometimes don't like data and just go with what looks best so the 4 inch will win every time.

I'd go with the four inch one, I have a double decat 3' straight pipe with a 4' exit. Stupidly loud but with a bung in its the same as my old 3' single decat with a exit silencer so I wouldn't worry too much about noise

N/A exhaust design is far more than just back pressure, which is a bit of a misnomer, they are usually designed to perform as an extraction device through pulse reversion, so length and diameter are all taken into account, however they are also a compromise because of noise and length, i would shy away from anything that increases the std diameter by more than 0.5" any larger and you will probably notice a change in where peak torque is produced and power will also suffer...been there, done that on other cars.

BTW thats not a 4'' exhaust system, just the muffler is, the rest of the pipe is 3' which will be your constraint'. Diameter of the muffler makes no real difference.

I used to have the same one, tried it a few different exhaust setups. Dosen't make a huge difference really. Only noticable difference was some low end grunt loss when I went full decat. In the end I went back to a catted system.

N/A exhaust design is far more than just back pressure, which is a bit of a misnomer, they are usually designed to perform as an extraction device through pulse reversion, so length and diameter are all taken into account

This is to do with the exhaust manifold design, the distance from the back of the exhaust valve to the collector is a key dimension along with the diameter of the pipes to this point.

After the collector, the dimensions of the exhaust has very little influence on the performance. I've done a little bit of work on this in both a chassis dyno and engine simulation software with similar +/- 1% changes from minor dimensional changes (say 2.5"-3" in this case).

The shape of the exhaust and the quality of the bends seemed to have more of an influencing factor on the chassis dyno, tested 4 or 5 'cat backs' IIRC, the straightest one made the most gain, albeit still only a couple of % increase peak.

So in your case, the Powerflow is full of sharp bends, all of which are crush bent and not mandrel bent; so they will be even smaller in dia, compromising performance.

The Motoria one which is pretty much straight, that would be the one I would fit.

Thanks for the feedback chaps.

Interesting discussion.

Will go with the Motoria and get some ear plugs.

When I had my NA MKIV it was fitted with a Blitz Nur Spec, sounded great but was loud.

I changed back to a stock exhaust and low down torque and throttle response felt much better.

Will go with the Motoria and get some ear plugs.

Speaking as someone who tracked a practically unsilenced and stripped NA from 2009 to 2013, and has had the pleasure of constant (and I do mean 24/7/52) tinnitus since, I'd strongly recommend you do.

This is to do with the exhaust manifold design, the distance from the back of the exhaust valve to the collector is a key dimension along with the diameter of the pipes to this point.

 

 

After the collector, the dimensions of the exhaust has very little influence on the performance. I've done a little bit of work on this in both a chassis dyno and engine simulation software with similar +/- 1% changes from minor dimensional changes (say 2.5"-3" in this case).

 

The shape of the exhaust and the quality of the bends seemed to have more of an influencing factor on the chassis dyno, tested 4 or 5 'cat backs' IIRC, the straightest one made the most gain, albeit still only a couple of % increase peak.

 

So in your case, the Powerflow is full of sharp bends, all of which are crush bent and not mandrel bent; so they will be even smaller in dia, compromising performance.

 

The Motoria one which is pretty much straight, that would be the one I would fit.

Just going back to this then, and thinking about noise.

Would a bung reduce performance in any noticeable way?

This is to do with the exhaust manifold design, the distance from the back of the exhaust valve to the collector is a key dimension along with the diameter of the pipes to this point.

 

 

After the collector, the dimensions of the exhaust has very little influence on the performance. I've done a little bit of work on this in both a chassis dyno and engine simulation software with similar +/- 1% changes from minor dimensional changes (say 2.5"-3" in this case).

 

The shape of the exhaust and the quality of the bends seemed to have more of an influencing factor on the chassis dyno, tested 4 or 5 'cat backs' IIRC, the straightest one made the most gain, albeit still only a couple of % increase peak.

 

So in your case, the Powerflow is full of sharp bends, all of which are crush bent and not mandrel bent; so they will be even smaller in dia, compromising performance.

 

The Motoria one which is pretty much straight, that would be the one I would fit.

I'm afraid i beg to differ, i have seen first hand how much power and torque an oversized exhaust can rob using the same manifold, so if it only the manifold that has any influence i must be dreaming then.

I'm afraid i beg to differ, i have seen first hand how much power and torque an oversized exhaust can rob using the same manifold, so if it only the manifold that has any influence i must be dreaming then.

No problem - happy to discuss.

How oversize was the exhaust you tried and what were the changes you saw?

Just going back to this then, and thinking about noise.

 

Would a bung reduce performance in any noticeable way?

Yes they make a noticeable difference, to reduce noise an additional silencer on the mid pipe would be better performance wise.

No problem - happy to discuss.

 

How oversize was the exhaust you tried and what were the changes you saw?

It was on a 2lt VAG engine and older ford, increased exhaust size from 1.75" to 2.5" and lost around 10bhp and 12ftlb especially in the peak torque areas, and that was a so called sport/performance exhaust with good bends and less restriction, but i don't really have any further specifics as it was a long time ago, seen similar effects on bikes, although its slightly different due the the lack of a dedicated manifold.

What power were they making? Sub 150bhp?

Yes they make a noticeable difference, to reduce noise an additional silencer on the mid pipe would be better performance wise.

In which case am I back to looking at installing the Powerflow one?

There has been a video on exhaust size on motortrends engine masters in the last week re engines with sub 400bhp and exhaust size. Well worth a watch!

There has been a video on exhaust size on motortrends engine masters in the last week re engines with sub 400bhp and exhaust size. Well worth a watch!

 

/QUOTE]

 

Thanks mate I'll see if I can find it.

Thanks mate I'll see if I can find it.

Cheers, interesting video that.

Obviously at high end the bigger bore seems better, although that's on a 600hp engine.

Interesting also how at low revs (3200 ish) the smaller diameter is actually better, which is where I'd imagine most of my driving will be.

Food for thought.

Not that one this one-

[video=youtube;bXQ-cXeri1Y]https://www.youtube.com/watch?v=bXQ-cXeri1Y

this is the follow up, more applicable to your power level. go against alot of what you read on forums. and im sure every engine is different but food for thought

Lots of rubbish spoken and written about N/A 4 stroke exhausts, here's some decent info from Jack Kane. I have the original but it's copyright, so I can't scan it.

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

https://motordyneengineering.com/exhaust-backpressure-and-scavenging/

That's all very technical Chris and I know there are many who do struggle with the technical aspects. Do you have an overview, summary, key points snapshot kind of idiots guide to understanding exhaust dynamics relative to variables of size, bends, packing? Otherwise everyone will just go big bore every time because its noisier and looks mean.

That's all very technical Chris and I know there are many who do struggle with the technical aspects. Do you have an overview, summary, key points snapshot kind of idiots guide to understanding exhaust dynamics relative to variables of size, bends, packing? Otherwise everyone will just go big bore every time because its noisier and looks mean.

The whole first doc relates to manifold design, bar the 6,000bhp funny car section saying they don't run a collector (where the individual manifold pipes meet, if you didn't know) because it would restrict the exhaust gas flow (this isn't applicable to any N/A engine below something like 1,000bhp, i.e. v8 f1 cars still had collectors at 700+bhp)

The second link is again talking a lot about manifolds to begin with, with some data about trying different diameters post the collector, but is also discussing 350z exhausts which are usually twin pipes and I didn't see that it specifies that or not, so I wouldn't take the diameters as a direct comparison.

The video Ripper Fear put up about mufflers is good, it compares two 2.5" pipe directly to two 3" pipe on a 600bhp V8 engine (300bhp per bank/pipe) with literally no change in bhp - that is the closest piece of info relating to your original question between the two exhausts.

The muffler comparison from the video shouldn't apply to your two exhausts, my old Powerflow was a perforated tube design muffler so I would imagine yours is too, the Motoria is also going to be this sort of muffler too. (The cheap muffler from the video, if it wasn't clear, has a thick 'brick' usually of fibrous material in the box blocking the exhaust gases, seriously restricting the exhaust gas flow and reducing performance, this is not the same as a 'packed' muffler where there is a perforated tube between the in and out holes of the box, and again usually the same fibrous material around the perf tube)