[enlimalima]

Hi...

I've looked into some of the threads and not sure if everyone have discussed this in much depth regarding the exhaust design and diameter size of the pipes for maximum performance. I've read threads on which sounds best but not enough discussion on performance...

My 335i is on PS2 + HPFP RWD, ER downpipe with custom exhaust all the way back...

Here's the design of my current exhaust (3" pipe all the way back), i was too reliant on my exhaust tuner and he made it JDM style like this...it looks really cool tho...but, I do feel there is a bit too many bends that would create backpressure which is not too good for turbocharged cars...

My car makes 400whp+ and Ive been receiving advice that my exhaust should be 3.5" or 4" to ensure maximum velocity of the exhaust gas...

Any take on this?

[aps]

no pics showing up

[enlimalima]

Originally Posted by aps

no pics showing up

Sorry, this is fixed now...

[enlimalima]

Akrapovic came with an interesting idea of splitting the mid pipe into a 2 x 2" pipe - which is interesting to me and it is a race-tested brand...

My exhaust tuner says he can make it like this, so I'm really intrigued but too afraid if the end result (after spending $$$) would not have any effect or even reduce the dyno numbers...

[thejeremyman9]

Originally Posted by enlimalima

Hi...

I've looked into some of the threads and not sure if everyone have discussed this in much depth regarding the exhaust design and diameter size of the pipes for maximum performance. I've read threads on which sounds best but not enough discussion on performance...

My 335i is on PS2 + HPFP RWD, ER downpipe with custom exhaust all the way back...

Here's the design of my current exhaust (3" pipe all the way back), i was too reliant on my exhaust tuner and he made it JDM style like this...it looks really cool tho...but, I do feel there is a bit too many bends that would create backpressure which is not too good for turbocharged cars...

My car makes 400whp+ and Ive been receiving advice that my exhaust should be 3.5" or 4" to ensure maximum velocity of the exhaust gas...

Any take on this?

It is highly unlikely that your exhaust is causing any significant restriction at all at that power level. People make 500+whp with PS2 and stock exhaust (not counting DP of course). The bends in your exhaust look either mandel bent or with pie cuts, meaning they do not reduce diameter due to the radius of the bend - ideal for flow.

If you REALLY want to know how much restriction your exhaust is causing, get a dyno session, do 3 pulls with the exhaust on, pull off the axle back, or if your exhaust is one piece, pull off the entire exhaust and run open DP, and do another 3 pulls and see what the difference is. I have seen this test done on other platforms with stock exhausts and the difference is usually only 2-3%. A well designed 3'' exhaust is plenty up to 500hp and likely beyond.

But again, i wouldn't lose sleep over it. Also, whoever said you want to increase diameter of the exhaust to gain velocity has it backwards - increasing the diameter might allow more flow if backpressure is truly an issue, but its going to reduce velocity - a smaller pipe has more air velocity at a given flow than a larger one.

[mike@x-ph.com]

All the dynos i have see show 5-8 HP more with high flow exhaust.

Does this setup offer more power?

[5w20]

Originally Posted by enlimalima

Hi...

I've looked into some of the threads and not sure if everyone have discussed this in much depth regarding the exhaust design and diameter size of the pipes for maximum performance. I've read threads on which sounds best but not enough discussion on performance...

My 335i is on PS2 + HPFP RWD, ER downpipe with custom exhaust all the way back...

Here's the design of my current exhaust (3" pipe all the way back), i was too reliant on my exhaust tuner and he made it JDM style like this...it looks really cool tho...but, I do feel there is a bit too many bends that would create backpressure which is not too good for turbocharged cars...

My car makes 400whp+ and Ive been receiving advice that my exhaust should be 3.5" or 4" to ensure maximum velocity of the exhaust gas...

Any take on this?

The exhaust will only flow as much as its biggest restriction is capable of flowing. I wouldn't change the rear section after the pipe splits into two, but I would change the merge where it splits into a 4" pipe all the way up to the downpipe and custom make the downpipe's exit to match the 4" pipe. I've been contemplating on doing this myself.

[thejeremyman9]

Originally Posted by 5w20

The exhaust will only flow as much as its biggest restriction is capable of flowing. I wouldn't change the rear section after the pipe splits into two, but I would change the merge where it splits into a 4" pipe all the way up to the downpipe and custom make the downpipe's exit to match the 4" pipe. I've been contemplating on doing this myself.

This is going to be a lot of fab work, and the cost benefit doesnt really make sense if you are seeking HP gains. Money is better spent elsewhere. Also, you might run into clearance issues with a 4'' circular pipe when the stock is very much oval. If you do it, lets see it

CG precision sells a DP with a 3.5'' exit and a 3.5'' exhaust to pair it with, IIRC. However like i said this is probably not going to net you gobs of HP, your talking maybe 10, not something like 50.

[enlimalima]

Originally Posted by thejeremyman9

This is going to be a lot of fab work, and the cost benefit doesnt really make sense if you are seeking HP gains. Money is better spent elsewhere. Also, you might run into clearance issues with a 4'' circular pipe when the stock is very much oval. If you do it, lets see it

I think this is where Akrapovic 335i exhaust made sense, it split the midpipe into 2x2" - to keep the ground clearance high enough that the pipes wouldnt rub on the ashpalt, though curious that it merged into a 3.5"...

yeah, it is a big investment to bet on...

[enlimalima]

Originally Posted by thejeremyman9

It is highly unlikely that your exhaust is causing any significant restriction at all at that power level. People make 500+whp with PS2 and stock exhaust (not counting DP of course). The bends in your exhaust look either mandel bent or with pie cuts, meaning they do not reduce diameter due to the radius of the bend - ideal for flow.

If you REALLY want to know how much restriction your exhaust is causing, get a dyno session, do 3 pulls with the exhaust on, pull off the axle back, or if your exhaust is one piece, pull off the entire exhaust and run open DP, and do another 3 pulls and see what the difference is. I have seen this test done on other platforms with stock exhausts and the difference is usually only 2-3%. A well designed 3'' exhaust is plenty up to 500hp and likely beyond.

But again, i wouldn't lose sleep over it. Also, whoever said you want to increase diameter of the exhaust to gain velocity has it backwards - increasing the diameter might allow more flow if backpressure is truly an issue, but its going to reduce velocity - a smaller pipe has more air velocity at a given flow than a larger one.

Maybe i am too hung on this comparison video, it might not be an apple to apple comparison but looks to me the video makes a lot of sense...(or I'm just understanding the video wrong)

Also I found this on youtube, again this may not be the right comparison to BMW engines...(youtube link:

)

[thejeremyman9]

Originally Posted by enlimalima

I think this is where Akrapovic 335i exhaust made sense, it split the midpipe into 2x2" - to keep the ground clearance high enough that the pipes wouldnt rub on the ashpalt, though curious that it merged into a 3.5"...

yeah, it is a big investment to bet on...

I am fairly certain that the split is for acoustics and not for performance. You do realize that 2x2'' pipes is less cross sectional area than a single 3.5'' pipe right? The area perpendicular to the flow goes up with the square of the radius, so 2x2'' pipes is only 6.28'' of area and a single 3.5'' is about 9.72''... even a single 3'' is bigger than 2x2''. So that split doesnt make sense if you are trying to just trying to minimize back pressure and maximize flow.

In response to the videos, i have seen engine masters (probably all episodes), but you can't extrapolate a engine like that to ours for numerous reasons... its not even forced induction, its not running a load or boost based tune, etc etc. While of course you want to minimize back pressure on any forced induction engine, what we are saying and what testing has shown is that that even the stock exhaust is not a major constraint at and above your power level. Also as mentioned above, the "choke point" is going to determine where the power loss occurs, which might not even be the stock exhaust.

Bottom line, if you want to try it go for it, but i think you will be disappointed in the result.

[AmuroRay]

I did a LOT of research on exhaust gas theory for my last car (N/A) and back pressure is never good for any car. The stock exhaust may be more restrictive as power levels rise - if you want to see how much it’s holding you back, unbolt it from the down pipe back - and then log.

[thejeremyman9]

Originally Posted by AmuroRay

I did a LOT of research on exhaust gas theory for my last car (N/A) and back pressure is never good for any car. The stock exhaust may be more restrictive as power levels rise - if you want to see how much it’s holding you back, unbolt it from the down pipe back - and then log.

This is exactly what i said to do lol. Run back to back with the exhaust then with open DP. Someone on the 3000gt forums did that and even with the stock catback on that car, which looks awfully restrictive, he only made like 15 more whp running open DP compared to running the full catback at 500whp power level. The F3x exhaust is pretty well designed, so i think you are leaving maybe 10-15 hp on the table at similar power levels. This is also assuming you have a catless DP, because i bet if you have a cat, the exhaust isnt really making that much of a difference.

[AmuroRay]

Originally Posted by thejeremyman9

This is exactly what i said to do lol. Run back to back with the exhaust then with open DP. Someone on the 3000gt forums did that and even with the stock catback on that car, which looks awfully restrictive, he only made like 15 more whp running open DP compared to running the full catback at 500whp power level. The F3x exhaust is pretty well designed, so i think you are leaving maybe 10-15 hp on the table at similar power levels. This is also assuming you have a catless DP, because i bet if you have a cat, the exhaust isnt really making that much of a difference.

The down pipe is another area I feel improvements can be made, but I’m in agreement.

[AmuroRay]

Question, was the exhaust dyno done on a stock or modded car? How much boost?

[thejeremyman9]

Originally Posted by AmuroRay

Question, was the exhaust dyno done on a stock or modded car? How much boost?

Was that a question for me?

[AmuroRay]

Originally Posted by thejeremyman9

Was that a question for me?

Yes

[thejeremyman9]

Originally Posted by AmuroRay

Yes

Went back and found the thread:

https://www.3si.org/threads/how-to-b...-stock.708873/

Just read the first post and watch the first two videos. Back to back dyno runs with and without the stock catback in place. It went from 690 to 706 HP.... Of course he had upgraded downpipes and whatnot, just like we do, but the catback was stock. For reference i attached a picture of what the stock catback looks like - its just under 3'' ID, very similar to the F3x stock exhaust, but has tiny tip outlets and its not even a true dual in the sense that its one pipe into the muffler then runs through to the other side.

(Its the one on the right, the left is a N/A exhaust)

[AmuroRay]

Originally Posted by thejeremyman9

Went back and found the thread:

https://www.3si.org/threads/how-to-b...-stock.708873/

Just read the first post and watch the first two videos. Back to back dyno runs with and without the stock catback in place. It went from 690 to 706 HP.... Of course he had upgraded downpipes and whatnot, just like we do, but the catback was stock. For reference i attached a picture of what the stock catback looks like - its just under 3'' ID, very similar to the F3x stock exhaust, but has tiny tip outlets and its not even a true dual in the sense that its one pipe into the muffler then runs through to the other side.

(Its the one on the right, the left is a N/A exhaust)

Yeah, I read through and watched it, but replacing one crappy exhaust with another doesn’t mean there aren’t more gains to be had.

http://www.superstreetonline.com/how...aust-test-tech

Do all of our DP terminate at 3”?

[thejeremyman9]

Originally Posted by AmuroRay

Yeah, I read through and watched it, but replacing one crappy exhaust with another doesn’t mean there aren’t more gains to be had.

http://www.superstreetonline.com/how...aust-test-tech

Do all of our DP terminate at 3”?

What do you mean replacing one crappy exhaust with another? Those back to back dyno runs were with (1) stock catback from the DP back and (2) open DP, so literally no exhaust at all after the DP. In theory #2 would be the maximum gains by replacing the exhaust with one that does not create any additional back pressure. The only way #2 is not a upper limit is if you think that adding an exhaust is going to somehow facilitate extra HP over just running a dump at the DP. I would note that the DP he is running is a 3'' exit. You can't really fit anything bigger on these cars. Maybe with a bigger DP that necked down to the stock catback, the catback would make more of a difference.

Ill take a look at that article a little later, it looks like they are claiming some bigger HP gains by sizing up the exhaust (notably at 600+whp level as well). I would suspect they were running a bigger DP than exhaust, which might be why the exhaust was proving to be such a restriction.

For our platform, the only DP i am aware of that is >3'' at the catback is the CG precision, and they specifically offered a DP with a 3.5'' outlet to pair with their 3.5'' catback. I am not sure if the kits like speedtech offer a custom DP for a larger diameter exhaust.

I think the only way you would see any significant gains from going bigger on the exhaust for our platform is if you had a catless DP that was 4'' at the turbo (EWG), and then continued to be 4'' or maybe down to 3.5'' at the catback, then maintained that same size all the way back. To my knowedge this doesn't exist in off the shelf parts (I think CG precision is only catted, and i have not seen a single person running their 3.5'' exit option paired with a 3.5'' exhaust). Using a DP that is 3'' at the catback then running a 3.5'' or 4'' exhaust is probably going to be negligible gains over the same DP with the OEM exhaust.