Need help finding topic on exhaust size. pros/cons on hp & torque 3.6

[martensson]

Hi all.
Tried to find this topic during the entire evening yesterday. I tried searching, checked all my saved topics...nothing...

It contains dyno charts & compares mufflers & piping. One person compares 2.0", 2.25" & 2.5" systems with comments on pros/cons on horse power & torque.
The conclusion, from testing & dyno figures, was that the 2.25" had overall better mid range torque compared to 2.5".

If anybody can help me find that topic it would be appreciated.

[Dpmulvan]

[Dpmulvan]

Try google search for pelican site, I tried to copy link but wouldn't work. Apple wants to sell me a new IPad.

[3rd_gear_Ted]

General result rule I've heard:
Two pipes into one has a better torque profile
Single straight pipes with tuned length creates best top end HP

[Bill Verburg]

I just love it when my work is posted w/o attribution

Here's the amended version


Acoustic tuning effects are largely absent when a muffler is used so it's mostly flow that you need to be concerned w/. As you can see from the chart there is overlap that is rppm dependant for example @6k rpm a 2.7 has the same flow demand as a 3.8 @ 4.2k rpm and since the system is usually not capable of changing flow capacity you select to emphasize on particular regime of the rpm spectrum.

For a 3.6 that is used below 4k or so you might want to use 35mm ID pipes(SSI) as they keep the exhaust gas speed up and flow fine in that regime, but if you want the full performance potential of the motor at 7k you want a bigger pipe. The factory used 42mm( 1.65" ID) or 45mm1.78" ID) on their 3.8 Cup engines, I use 44.5mm(1.75" ID) on my 3.8.

The firing order and pipe connectivity can also affect the design. Our flat 6s are even firing engines , each collector see a pulse of gas 240 c/s° apart for this reason a smaller collector can be used and some of the tricks used to optimize acoustic tuning effects in other engines don't apply.

Here's the data set for a Porsche 6


Here's the data set for new style Chevrolet V8, These are odd fire engines as are all 90° crank V8s, these engines always have 2 consecutive firing cylinders on each side


Here's an old style Chevrolet slightly different but the same issue of consecutively firing cylinders on the same side, this issue results in only 90 c/s° of pulse separation, this gives them the lumpy idle everyone loves but also requires some tricks in the exhaust, best would be a cross over where a left cyl is connected to a right collector and vice versa, this can be done but is expensive in $ and space used, alternatively you can do what Ford does on it's Coyote V8 and Fewrri does in it's V8s, ie use a flat plane crank, which then provides an even fire engine w/ 180 c/s° pulse separation. What is usually done on these 90° V8s is to pair 2 cylinders on each side, for new Chevrolets that would be 2 - 4 and 6 - 8 on the left and 1 - 5 and 3 - 7 on the right and also to provide another cross over further downstream.



The easiest way to see the benefits of cylinder pairs is in a 4, This is a Honda even fire 4 w/ a 2 plane 90° crank, When 1 - 4 and 2 - 3 are paired it provides 180 c/s° pulse separation

[3rd_gear_Ted]

Originally Posted by Bill Verburg

I just love it when my work is posted w/o attribution

Here's the amended version


Acoustic tuning effects are largely absent when a muffler is used so it's mostly flow that you need to be concerned w/. As you can see from the chart there is overlap that is rppm dependant for example @6k rpm a 2.7 has the same flow demand as a 3.8 @ 4.2k rpm and since the system is usually not capable of changing flow capacity you select to emphasize on particular regime of the rpm spectrum.

For a 3.6 that is used below 4k or so you might want to use 35mm ID pipes(SSI) as they keep the exhaust gas speed up and flow fine in that regime, but if you want the full performance potential of the motor at 7k you want a bigger pipe. The factory used 42mm( 1.65" ID) or 45mm1.78" ID) on their 3.8 Cup engines, I use 44.5mm(1.75" ID) on my 3.8.

The firing order and pipe connectivity can also affect the design. Our flat 6s are even firing engines , each collector see a pulse of gas 240 c/s° apart for this reason a smaller collector can be used and some of the tricks used to optimize acoustic tuning effects in other engines don't apply.

Here's the data set for a Porsche 6


Here's the data set for new style Chevrolet V8, These are odd fire engines as are all 90° crank V8s, these engines always have 2 consecutive firing cylinders on each side


Here's an old style Chevrolet slightly different but the same issue of consecutively firing cylinders on the same side, this issue results in only 90 c/s° of pulse separation, this gives them the lumpy idle everyone loves but also requires some tricks in the exhaust, best would be a cross over where a left cyl is connected to a right collector and vice versa, this can be done but is expensive in $ and space used, alternatively you can do what Ford does on it's Coyote V8 and Fewrri does in it's V8s, ie use a flat plane crank, which then provides an even fire engine w/ 180 c/s° pulse separation. What is usually done on these 90° V8s is to pair 2 cylinders on each side, for new Chevrolets that would be 2 - 4 and 6 - 8 on the left and 1 - 5 and 3 - 7 on the right and also to provide another cross over further downstream.



The easiest way to see the benefits of cylinder pairs is in a 4, This is a Honda even fire 4 w/ a 2 plane 90° crank, When 1 - 4 and 2 - 3 are paired it provides 180 c/s° pulse separation

Bill, side question I always wondered about till I saw this data.

6500 rpm @ 162.5 pulses is about 40HZ for the engine harmonic frequency; correct??

[GG Allin]

See post #68 here:

https://rennlist.com/forums/911-forum/1134151-new-motor-almost-done-porsche-porn-inside-5.html

I have those same headers and a 3.6, thinking about using the same muffler.

[Bill Verburg]

Originally Posted by 3rd_gear_Ted

Bill, side question I always wondered about till I saw this data.

6500 rpm @ 162.5 pulses is about 40HZ for the engine harmonic frequency; correct??

That's to each collector, total is 325 pulses/s, so the frequency of that is 325Hz

[eastbay]

Originally Posted by Bill Verburg

I just love it when my work is posted w/o attribution

Hey Bill, if you care, you should sign or watermark your stuff, goes for anyone else also. Then we know where it comes from if we have questions or sometimes corrections.

I have tons of charts and files plucked from the this and other sites and virtually none of it has any attributions

Of course, that is the nature and promise of the Internet, collaboration without attribution.

[martensson]

Originally Posted by Bill Verburg

Acoustic tuning effects are largely absent when a muffler is used so it's mostly flow that you need to be concerned w/. As you can see from the chart there is overlap that is rppm dependant for example @6k rpm a 2.7 has the same flow demand as a 3.8 @ 4.2k rpm and since the system is usually not capable of changing flow capacity you select to emphasize on particular regime of the rpm spectrum.

For a 3.6 that is used below 4k or so you might want to use 35mm ID pipes(SSI) as they keep the exhaust gas speed up and flow fine in that regime, but if you want the full performance potential of the motor at 7k you want a bigger pipe. The factory used 42mm( 1.65" ID) or 45mm1.78" ID) on their 3.8 Cup engines, I use 44.5mm(1.75" ID) on my 3.8.

I love your input on these matters.

I think you made a comment in that specific topic as well. The thing that makes me confused is, that you basically say the best size for a daily driver 3.6 would be 35-42mm ID pipes. Pea-shooters
In the topic i´m referring to our pelican fellow compared 2", 2.25" & 2.5". The conclusion was to go with the 2.25" for best mid range torque, IF i remember correctly... 1.65" & 2.25" is a big difference....& so far i have not seen anybody compare numbers between the two

My choice is to go full custom with stainless steel piping & mufflers. Perforated pipe mufflers with, as much as possible, full flow all the way.
But, there is always a BUT, i would like to get the most out of the engine when it comes to power/torque. I´m starting off with a set of stripped 993 headers. These have an exit port of approximately 2.5". This means i would have to merge the piping down to 2"-1.65"...

[winders]

1 5/8” or 1 3/4” is pipe diameter before the merge collector. The 2”, 2.25”, and 2.5” sizes you are talking about are after the collector.

[martensson]

Originally Posted by winders

1 5/8” or 1 3/4” is pipe diameter before the merge collector. The 2”, 2.25”, and 2.5” sizes you are talking about are after the collector.

Yes, that is correct Sorry, assuming nobody is running 2.5" primaries I didn't bother to clarify this.

[Bill Verburg]

Originally Posted by martensson

I love your input on these matters.

I think you made a comment in that specific topic as well. The thing that makes me confused is, that you basically say the best size for a daily driver 3.6 would be 35-42mm ID pipes. Pea-shooters
In the topic i´m referring to our pelican fellow compared 2", 2.25" & 2.5". The conclusion was to go with the 2.25" for best mid range torque, IF i remember correctly... 1.65" & 2.25" is a big difference....& so far i have not seen anybody compare numbers between the two

My choice is to go full custom with stainless steel piping & mufflers. Perforated pipe mufflers with, as much as possible, full flow all the way.
But, there is always a BUT, i would like to get the most out of the engine when it comes to power/torque. I´m starting off with a set of stripped 993 headers. These have an exit port of approximately 2.5". This means i would have to merge the piping down to 2"-1.65"...

It's best to avoid steps, If the collector exit is 2.5" then stay 2.5 all the way out. But keep in mind that any thing that happens after the muffler has little effect upstream as long as it's not restrictive.

[RarlyL8]

Power and torque come from properly designed headers. The muffler's job is to quiet the tone without harming the power/torque. Contrary to popular belief OEM 993 Bischoff exchangers are not a performance part. The only thing good about them is the internal diameter size of the primaries.
To retain velocity/torque the primary ID needs to be very close to the exhaust port size. Primaries must be equal length based on pulse timing. Collectors must be a smooth merge that properly organizes the pulses to create scavenging at the collector outlet. Secondary system must be sized just large enough to retain velocity. Muffler must do the least harm impeding the flow while quieting the tone. The entire system will have one target RPM. Using Bill's info and your chosen RPM target you can calculate the ideal sizing. For a bone stock 3.6L I would use 1 5/8" primaries and 2.25" secondaries.

[faapgar]

I have been building step headers since 82.More to come.I love this forum.Fred

[Bill Verburg]

The thing is that when you have an open exhaust all the design details like equal length, proper merges, appropriate length, proper collectors, careful transitions etc are very important to making torque & hp and too where in the rpm range it is produced, but the minute you install a muffler most of that gets pushed to the background, sure its better to have good design and construction but the muffler just negates most of the acoustic tuning benefits provided by design and construction detail. W/ a muffled system flow capacity is the #1 design criteria followed by sound. And yes a proper header does provides the good flow it's just not going to be as helpful as when running unmuffled.

By acoustic tuning I am referring the resonances in the pipes, w/ a properly designed open system there are 2 wave phenomena that can help the engine make power both can be seen here, you want a low pressure rarefaction and a reflected sonic wave to be present at the exhaust port during valve overlap, these are always going to be out of phase w/ each other and the desired valve event and will only be in phase for a small rpm window. When out of phase they can do as much harm as good when in phase.



The other thing that headers rely on besides good design and construction is at least some TDC powerstroke valve overlap, unfortunately the 911s built before vari cam have little to no such valve timing specs due to the emissions constraints placed on the cars.

[Bill Verburg]

I came across this from back in the day when I raced bikes, it shows the details the factories incorporate in their designs to tune the pipes for one quality or another

[martensson]

Originally Posted by RarlyL8

Power and torque come from properly designed headers. The muffler's job is to quiet the tone without harming the power/torque. Contrary to popular belief OEM 993 Bischoff exchangers are not a performance part. The only thing good about them is the internal diameter size of the primaries.
To retain velocity/torque the primary ID needs to be very close to the exhaust port size. Primaries must be equal length based on pulse timing. Collectors must be a smooth merge that properly organizes the pulses to create scavenging at the collector outlet. Secondary system must be sized just large enough to retain velocity. Muffler must do the least harm impeding the flow while quieting the tone. The entire system will have one target RPM. Using Bill's info and your chosen RPM target you can calculate the ideal sizing. For a bone stock 3.6L I would use 1 5/8" primaries and 2.25" secondaries.

Great input from both you & Bill

This is what i got. If i remember correctly these are 1.5-ish" ID.
I really don't know how good or bad these are?
What would a nearly perfect set of headers look like? What would the gain be between these 993 & a perfect set?
On the other hand i believe these 993 are still better than many other..so...i don't know. Is it worth the effort?
I know that a perfectly designed exhaust also includes the inlet, but that's science beyond this car...

According to your, Bill, theoretical input on design the 993 is far from the best. But for us mortal, is there anything out there considered to be of a better design?
I still have the option to custom fab my headers. The system has to be silenced since we have sound restrictions & since the car is to be used as a daily driver, firstly. Compromises are never good, i know.

As for the engine everything is "stock", besides less restrictive airways all the way from throttle body to exhaust ports.
Besides that, balanced bottom end & lightweight flywheel.

[Dpmulvan]

Originally Posted by Bill Verburg

I came across this from back in the day when I raced bikes, it shows the details the factories incorporate in their designs to tune the pipes for one quality or another

I wonder who was the author of this sketch, maybe a footnote would be appropriate.

[RarlyL8]

Yes adding a muffler changes the dynamic and masks the details of exhaust tuning, however it is better to start off with a tuned system and have the opportunity to utilize it than to not have a tuned system at all.

A variation of the headers below will be offered for 3.4L-3.6L engines. Primaries will be sized appropriately for the exhaust ports, steps will be an option and collectors will terminate with v-bands to accept various secondary and muffler systems.