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Exhaust Tube Sizing: I did your Arithmetic for you!!

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Tony D    143

I got one like that. But is that ending in two 50mm pipes, or two 60mm pipes?

 

They have both options on that header. The Z432 had twin 60's all the way back. The L24's had twin 50's, something about them not running to 10K rpms with 15:1 CR's at the time (like the S20's)

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josh817    15

I got one like that. But is that ending in two 50mm pipes, or two 60mm pipes?

 

They have both options on that header. The Z432 had twin 60's all the way back. The L24's had twin 50's, something about them not running to 10K rpms with 15:1 CR's at the time (like the S20's)

Oh-oh-oh-oh can I see?!

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Leon    35

Here's a good article by Steve Dinan, which echoes what I've explained. Click on "Dinan's E90-92 M3 Exhaust."

 

From his Exhaust Theory section:

After the exhaust manifold or header, optimum performance comes from making the balance of the exhaust system as short and large as possible. This approach will result in greater engine efficiency for maximum power, as well as minimizing the weight of the system. Probably the best example of an optimized, nocompromise exhaust system would be that of an F1 racecar. If you have ever had the opportunity to hear a F1 exhaust note, I think you will agree that it is best described as deafening. Clearly an exhaust system that even approached such a volume level in a performance street-car would draw far too much of the wrong sort of attention. Therefore, a modern street-car exhaust represents a number of performance compromises in order to achieve an acceptable exhaust volume, as well as meeting emissions standards.

 

In order to accommodate the various components and baffling necessary for a street-car, the exhaust system becomes longer and the flow of gasses more circuitous as noise and emissions standards are addressed. Each bend in the exhaust tubing, catalytic converter, resonator and so forth introduces restrictions to the exhaust flow, particularly at higher rpm where flow is most critical. Exhaust flow can actually reach hundreds of miles per hour when the engine is producing maximum power, which results in power robbing friction along the exhaust tubing walls, particularly when the gasses must change direction. This friction results in increased backpressure that can be quantified with a pressure gauge. This backpressure restricts the amount of gasses that can be passed through the engine, resulting in a reduction of peak power.

 

I’m fairly certain that many of you have been exposed to a “bench racing legend†that would have you believe that increased backpressure will improve low rpm power and that low backpressure will increase high rpm output. Nothing could be further from the truth. An exhaust system is sized for maximum flow at wide-open-throttle and peak rpm. All exhaust systems are “oversized†for lower engine speeds (rpm), as backpressure is so insignificant that it can’t even be measured. Less backpressure always results in more power at higher rpm, with no negative effect on lower engine speed performance. The amount of power that can be extracted from an engine at a given rpm as a result of exhaust design is really limited by the exhaust manifold or header. After the header, less backpressure is always better. The real challenge when tuning a street-car exhaust is to increase flow without making the system loud or eliminating catalyst that will prevent you from registering your car because of your local emissions standards. It is also important to understand that vehicle manufacturers must meet more stringent maximum noise requirements than aftermarket manufacturers.

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Tony D    143

Actually I believe the diameter of the piping is inverse to dB. Larger usually means quieter. This was the way it was on the Corvairs which basically had mufflers right on the header collector and then out the back. The larger diameter exhausts which you thought would be louder had a much lower and mellow tone. Smaller diameters on the Z seem to have a 'snarl'---much more 'pop' rather than a low-throated rumble. It may be where the auditory effects are masked. People find an F15 at idle wildly loud (and it is...) but mostly because the noise is RIGHT in the irritating zone of human hearing Hz - Wise. Perhaps the larger diameter pipes will 'meter' as loud, and it's just the human ear that 'seems to think' it's lower noise.

 

And really it's that perception that will get you the ticket! If it 'sounds loud' it is, and the ticket gets written. :(

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zredbaron    6

Its obvious an exhaust system can choke an engine if it's too small, but is there a too big?

I've always taken exhaust systems the same way as header primary diameters, smaller to keep up velocities for low end torque and bigger for the higher RPM horsepower.

 

I believe you are referring to post-collector piping, correct? This has of course been answered since your post and I won't repeat others' posts.

 

If by exhaust "system" you include header primaries and collector, yes. Keep in mind he's talking about header primaries' diameter selection and NOT post-collector piping, but this is directly from Stahl headers:

 

Hi Mark,

Jere here using Judy's computer. In the early years when 280Z was top dog in SCCA CP we would go to 1-3/4" around 295 to 300 HP. Of course they were interested only in power above 5000. I have no modern dyno data on 6 cyl Datsun engines so I have no idea what the shape of the power curve is from 1500 to 5000 and that can be a major issue in the driveability of an auto cross vehicle. However, my experience with engines proves that intake port shape has more to do with driveability than any other single component including exhaust and camshafts. Certainly going to EFI will be an extremely wise choice and allow you to properly tune with the handicaps of carburetors. Just remember, that too large in the exhaust is far worse than too small.

 

It's my understanding that too large (for primaries) is devastating because it decreases velocity and therefore scavenging effect, which is of course crucial for normally aspirated engines, especially ones with lots of overlap. I elected to not go with 1-3/4" but instead 1-5/8" primaries for a wider torque band, despite that my race motor was in this HP range with street headers. JonhC and Stahl both agreed that for an autocross motor this would suit my best interests.

 

 

[...]

 

 

Actually I believe the diameter of the piping is inverse to dB. Larger usually means quieter. This was the way it was on the Corvairs which basically had mufflers right on the header collector and then out the back. The larger diameter exhausts which you thought would be louder had a much lower and mellow tone. Smaller diameters on the Z seem to have a 'snarl'---much more 'pop' rather than a low-throated rumble. It may be where the auditory effects are masked. People find an F15 at idle wildly loud (and it is...) but mostly because the noise is RIGHT in the irritating zone of human hearing Hz - Wise. Perhaps the larger diameter pipes will 'meter' as loud, and it's just the human ear that 'seems to think' it's lower noise.

 

Great point, Tony. I hadn't considered the irritating frequency range, and I would agree that when tone deepens (with larger pipes) it's less irritating and therefore seems less loud.

 

 

[...]

 

 

Also, the earlier discussions regarding post-collector piping have made me curious. I just might dyno the car with race headers+merge collector (no piping or muffler) and then once again after installing the full exhaust (3" mandrel). I don't think I'd be interested in cutting pipes to find the ideal length, but I am curious how much of a loss might be incurred due to post-collector head loss (friction). If I do this, I'll be sure and share the dyno results and pics of the exhaust setup(s).

 

If nothing else, I'll have an excuse to hear the race headers + merge collector roar without a muffler... :twisted:

Edited by zredbaron

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josh817    15

I'm slated to go on the dyno August 20th and will test a few exhaust configurations on my L24 with triple Webers and stock exhaust manifold. I'll report the results when I have them!

Don't forget pictures and video so we don't have to attack you. :malebitch

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Leon    35

Dyno Testing a 260Z

 

This is an analysis based on my dyno testing from yesterday (8-20-11).

 

Test Parameters

 

The reason for the testing was that I wanted to log a baseline for my 260Z before going further with more engine modifications and to make a comparison between different configurations.

 

The baseline settings are as follows:

  • early 260Z with 1972 L24/E88 that was installed by the previous owner
  • triple 40DCOE-18 Weber carbs with K&N-style filters, not jetted yet, just bolted on and sync'd
  • Cannon long-runner manifold
  • stock exhaust manifold to single 2.5" crush-bent pipe
  • stock cam and internals
  • refreshed cooling system with aluminum radiator, new water pump and hoses
  • heater loop is blocked off at both ends
  • Mobil1 synthetic 10W-30 in engine, Redline 75W-90 in transmission (GL-4) and differential (GL-5)
  • MSD coil, NGK plug wires

 

100_9996-1.jpg

 

The two pipes to be tested:

 

101_0033-1.jpg\

 

 

Test 1: Long 2.5" pipe with straight-through muffler in the rear

 

P1020004.jpg

 

 

 

Initial Thoughts and Numbers:

 

I know actual dyno numbers don't mean a whole lot but I came in thinking that I'll be happy to break 100hp! The Z feels very torquey and quick but we all know that the almighty butt-dyno isn't very representative of reality, so for all I knew I could've been making 85hp...

 

 

I was also pretty convinced that I would not be over 120 hp as the carbs are not jetted and I have not even touched the spark timing. However, according to the dyno that day, I am making 121 hp @ 5200 rpm and 136 lb-ft @ 4000 rpm! At the very least, this indicated that my engine is fairly healthy and that my tune is not completely off. A happy side-note but we're here for comparison purposes not peak numbers. On to part two!

 

 

Test 2: Stock downpipe, cut after the merge

 

101_0035.jpg

 

 

 

Results

 

SOLID lines are long-pipe runs

DOTTED lines are short-pipe runs

 

Dyno-Leon8-20-11.jpg

*Note: there are local peaks at about 5850 rpm, and they are there for both runs so it very well could be due to the tune.

 

The long-pipe made slightly more torque on the top end, but the short-pipe made more low-end torque.

 

Analysis

 

On the surface, the results look counter-intuitive. After all, shouldn't a short pipe be better for top end and a long pipe better for low end? How about the "mechanic" that's been "doing this for years" that says "well son, when there's less backpressure you lose all yer low end torque. You need some backpressure." Well in this case, running it with no muffler and a cut after the merge increased low end torque.

 

So please, get any thoughts of backpressure out of your head when speaking of exhaust tuning. In fact, forgot the word completely, it is not a very useful term in the exhaust theory glossary, besides wanting as little of it as possible.

 

The results come out as expected after you look at the differences in the two pipes. The critical difference between the two being the length of the secondaries before the merge into a single pipe. Take a look at the second picture from the beginning of the post, showing both collectors. Take note of the differences. The long pipe exhaust merges the secondary pipes quite a bit earlier than the short pipe, meaning that the short pipe secondaries are actually longer.

 

Thinking back to wave tuning, whenever there is a discontinuity in a pipe (e.g. a change in pipe diameter or merge), the compression wave coming from the open exhaust valve gets reflected back as an expansion wave. A tuned length of pipe allows the expansion wave to travel back to the exhaust valve just before it closes, further dropping pressure in the cylinder. This draws out more burned exhaust gasses as well as draws in more air-fuel mixture during the valve overlap period (scavenging). Keep in mind that a tuned length is only good for one engine speed. As pipe length before the discontinuity (merge) changes, so does the time it takes for the wave to travel out the exhaust valve, down the pipe, reflect and come back to the exhaust valve. A shorter pipe before the merge decreases the length of time it takes for the wave to reach the exhaust valve, therefore it is more efficient at higher engine speeds. The opposite is true for a longer length of pipe before the merge (longer wave travel time = lower rpm efficiency).

 

This test also shows that a single 2.5" pipe is ample for my L24 spinning to 6000 rpm, because it essentially mirrors the short pipe and actually outperforms it at high rpm. The large diameter pipe allows for the collector to do its work without hindering performance post-collector.

 

Conclusion and Discussion

 

The results nicely agree with the exhaust theory that we've covered in this thread.

 

If I took the long pipe with the muffler and cut it after the collector, it would perform better, but the difference would likely not be discernible. On the other hand, if the pipe coming off the collector were smaller than 2.5" then pumping losses would begin to climb and performance will decrease across the board, especially at high rpm since frictional losses in a pipe increase with the square of velocity.

 

In the near future, I am having twice pipes made and I'm curious as to the effect of keeping the secondaries separate. It may cause a small hit in performance, judging by how much of an effect moving the secondary merge point had, but carb and spark timing tuning will be a much more drastic change. I broke off a stud at the exhaust manifold-to-exhaust connection point, and although there was no detectable leak, I may just put on my Nissan Motorsports header and build the pipes off of that. :D

 

101_0049.jpg

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killaimdie    0
Actually I believe the diameter of the piping is inverse to dB. Larger usually means quieter. This was the way it was on the Corvairs which basically had mufflers right on the header collector and then out the back. The larger diameter exhausts which you thought would be louder had a much lower and mellow tone. Smaller diameters on the Z seem to have a 'snarl'---much more 'pop' rather than a low-throated rumble. It may be where the auditory effects are masked. People find an F15 at idle wildly loud (and it is...) but mostly because the noise is RIGHT in the irritating zone of human hearing Hz - Wise. Perhaps the larger diameter pipes will 'meter' as loud, and it's just the human ear that 'seems to think' it's lower noise.

 

Science supporting different perceptions of audio tones. The article talks about Psychoacoustics and human perceptions of different audio frequencies.

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Justinp551    0

This thread is great reading. I have the MA 6-1 with a 2.5" system on my 280na. After reading this thread I am considering having a 3" made up and dyno both pipes to see what differences we can find. If I am understanding this correctly though the main loss is friction of the walls. Is my example of a whole bunch of people running through a narrow hall vs running though a room sound about right? My only concern is the 2.5" flange from the headers to the exhaust pipe. 3" collector through a 2.5" flange back to a 3" system all the way back. Is this small kink a huge problem? I understand it is a restriction but it's only 1/2 inch or so. Would this kink be a small issue, good for exhaust wave reflection, or a big issue?

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Leon    35

This thread is great reading. I have the MA 6-1 with a 2.5" system on my 280na. After reading this thread I am considering having a 3" made up and dyno both pipes to see what differences we can find. If I am understanding this correctly though the main loss is friction of the walls. Is my example of a whole bunch of people running through a narrow hall vs running though a room sound about right?

That's a way of looking at it, yes.

 

My only concern is the 2.5" flange from the headers to the exhaust pipe. 3" collector through a 2.5" flange back to a 3" system all the way back. Is this small kink a huge problem? I understand it is a restriction but it's only 1/2 inch or so. Would this kink be a small issue, good for exhaust wave reflection, or a big issue?

I would prefer a 3" flange, and an even better option is a 2.5" flange to 3" pipe with a megaphone in between. In your case, I would prefer to integrate a megaphone post-collector, that tapers from 2.5" back to 3" diameter. Taper angle and location will alter its effect, as discussed within this thread.

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Leon    35

Interesting solution. I like it. When I get the car together and running smoothly I will build a 2.5" to 3" exhaust and post dyne results back to back. Thanks for the idea Leon. I have come across the megaphone idea on a few different threads. Will research it a bit further.

No problem. Some dyno results would be great!

 

And best place to find a 13" 2.5" to 3" megaphone?

I've used Burns Stainless in the past, check their site.

 

You can have a custom one made, cut up an existing one, or make it yourself. For option 2, you can buy one that has too small of an inlet and/or too big of an outlet, do some quick math, and cut them to size.

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Leon    35

Thanks for the input guys. John C's pic of the megaphone stated 13". And suggestions on a more desired length?

 

That was just an example picture. 13" is not a hard and fast number and I actually don't know the proper length for an open megaphone on a L6 engine. Never built one.

Furthermore, the proper length for one guy is not necessarily the proper one for the next. In order to properly size your megaphone, you must take into account the causalities posted in this thread (longer megaphone = wider bandwidth/smaller magnitude, and vice versa) as well as perform resonance calculations, taking into account gas temps, pipe diameter, valve timing, etc. This, along with your goals (location and width of powerband) will determine megaphone size and location.

 

Of course, you can always say the hell with it, and slap it in there. Just don't expect to get exactly what you want.

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Justinp551    0

Oh boy in over me head now. Haha. Just looking to make the exhaust as efficient as practical without using a race team budget. Fresh N42 combo with CAI and small increase in cam. Cleaned up and refreshed n42 head w/ MSA 6-1 headers. Stock FI. I understand the theory of the megaphone and am all for fabbing it. 2.5 to 3" megaphone- With that small of a step... What general length should I be considering. Leon I understand the different requirements on desired performance. Just looking for that "general" recommendation if one exists. î—

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Leon    35

I'll reiterate the generalizations, from which you can then approximate "where" and "how big". Longer means you get a boost over a longer range of rpm, but at the same time it decreases the amount of extra torque. A bigger difference in inlet/outlet diameters will increase the effect, but since you have already set your inlet and outlet diameters, then you can't do anything with that. Placing the megaphone upstream shifts its powerband influence to higher rpm, and placing it further downstream shifts its influence to lower rpm.

 

Now, if you want take the suggestions given in David's (1 tuff z) link, they imply that a 9 degree transition is a good compromise. Whether that's true or not is up to interpretation, but it's a start. Given that you're not going to be doing testing of different exhaust combinations, I think it's safe to say that you'll be in the 7-9 degree range.

 

If you do the math, the length comes out to about 1.6 inches for a 9 degree taper, if I did that correctly.

Edited by Leon

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