Exhaust backpressure fail...

[julex]

This is allroad, BillyBoat performance exhaust (dual 2.25") without any cats too just the resonator and mufflers so fairly quiet overall. 3in DPs. Pressure sensors located just before neckdown to 2.25" inside gutted main cat with smooth transitions everywhere.

Pull done with Tial 770 at 32psi of boost...



Yes, that about 10psi of backpressure in exhaust at 7k rpms... That's just horrible and explains my less than stellar top-end and torque fall off once getting into 6k rpms area, it pulls like hell of wheels 5-5.5k and then mellows down like it was k04 turbo..


I will be welding in boost activaed (pneumatic) cut outs sometime in near future, like these:

[landfill]

Those cutouts looks cool but why not just run a bigger exhaust?

[LINDW4LL]

Those cutouts looks cool but why not just run a bigger exhaust?

Does he really need a bigger exhaust?

Edit: Logic fail.

[erroob0977]

Does he really need a bigger exhaust? He has a combined 5" catback.

In terms of cross-sectional area it falls somewhere between a 3" and 3.25" catback. I'm definitely interested in seeing how significant of an effect the cutouts have!

[rtl5009]

Does he really need a bigger exhaust? He has a combined 5" catback.

Area of a circle is a nonlinear function.

[LINDW4LL]

Area of a circles is a nonlinear function.

Ahh, good point- duh.

[NOTORIOUS VR]

Those cutouts looks cool but why not just run a bigger exhaust?

Agreed. Much, much better option IMO.

[julex]

Agreed. Much, much better option IMO.

Question of budget I guess. I like current system the way it is, it looks good, it is in excellent condition (all SS everywhere) and works very well for DDing. Only top end seems to be affected which can be remedied by cutouts. Slapping cutouts would be far cheaper...

Anyway, sometime this week I will disconnect my exhaust at DP V-Bands (3in there) and pull it back to create large gap between DP and exhaust pipes, do a FAT pull or two to see what the difference is. Results will drive my decision about what to do next.

[NOTORIOUS VR]

Question of budget I guess. I like current system the way it is, it looks good, it is in excellent condition (all SS everywhere) and works very well for DDing. Only top end seems to be affected which can be remedied by cutouts. Slapping cutouts would be far cheaper...

Anyway, sometime this week I will disconnect my exhaust at DP V-Bands (3in there) and pull it back to create large gap between DP and exhaust pipes, do a FAT pull or two to see what the difference is. Results will drive my decision about what to do next.

Even if the problem is remedied for now with cut out's you have to deal with an absurd amount of noise pollution with the cutout's open. So it's kind of backwards from what you really want.

Re-doing the exhaust to properly fit your setup (high boost 770 turbos) is IMO your best option as you've going the wrong way from the beginning trying to keep a restrictive exhaust from the get go.

A larger diameter exhaust system can be quiet and still flow a ton without ever having to have a blaring cut out open that sounds like complete crap and draw attention to yourself. Plus you have to actively open and close it.

My vote is for a 3.5" exhaust and be done with it and sell your existing setup instead of hacking it up.

He wants civilized until wot.

I'm not sure I follow? Are you trying to say it's not possible to do with a larger exhaust? Because it is, quite easily at that.

[julex]

Even if the problem is remedied for now with cut out's you have to deal with an absurd amount of noise pollution with the cutout's open. So it's kind of backwards from what you really want.

Re-doing the exhaust to properly fit your setup (high boost 770 turbos) is IMO your best option as you've going the wrong way from the beginning trying to keep a restrictive exhaust from the get go.

A larger diameter exhaust system can be quiet and still flow a ton without ever having to have a blaring cut out open that sounds like complete crap and draw attention to yourself. Plus you have to actively open and close it.

My vote is for a 3.5" exhaust and be done with it and sell your existing setup instead of hacking it up.

I linked the cutouts I want to install above. They will be boost operated and open at around 15psi. So DD condition they will remain closed and automatically open once I cross 15psi which is where I still have no backpressure as that's about 1/2 in RPM to redline and at 2/3 the intake flow potential so about 0.33x the maximum exhaust flow my car tries to push now (I did some more measurement on that).

I will visit my ghetto muffler place to see what they say. If they give me killer price on dual 3", that's what I will do. If not, cutouts!

[rtl5009]

Agreed. Much, much better option IMO.

He wants civilized until wot.

[oppositelock]

Interesting data, but 2.5 PSI rise from where it's running strong to where it's not doesn't seem like enough to be causing the performance drop-off you're feeling. I'll be interested to see the follow-up data.

[julex]

Interesting data, but 2.5 PSI rise from where it's running strong to where it's not doesn't seem like enough to be causing the performance drop-off you're feeling. I'll be interested to see the follow-up data.

Several things are working against developing proper power as engine goes beyond max torque RPMS: lowering VE of engine (less and less time to get air in ends up in les and less charge in cylinder), increasing IATs of air (less and less oxygen as air heats up) and the rising backpressure resulting in less complete exhaust cycle as it goes up and up so less and less space for new air.

This log is just an illustration, I was only interested in seeing what kind of backpressure I would see at 6-7k rpms. I did this on local road, in 2nd gear, brake boosting the crap out of car so only really captured around 6-7k window reliably,

Seeing 10psi just dropped my jaw...

[oppositelock]

Several things are working against developing proper power as engine goes beyond max torque RPMS: lowering VE of engine (less and less time to get air in ends up in les and less charge in cylinder), increasing IATs of air (less and less oxygen as air heats up) and the rising backpressure resulting in less complete exhaust cycle as it goes up and up so less and less space for new air.

This log is just an illustration, I was only interested in seeing what kind of backpressure I would see at 6-7k rpms. I did this on local road, in 2nd gear, brake boosting the crap out of car so only really captured around 6-7k window reliably,

Seeing 10psi just dropped my jaw...

Definitely agree that increasing backpressure + decreasing VE == no bueno. It will be very interesting to see not only delta P but delta T! (FATS) Make sure to wear earplugs on long WOT pulls.

BTW, what sensor are you running? I searched for 'exhaust pressure sensor' and didn't find much, just some Ford diesel truck stuff on eBay.

[julex]

I just used 18mm o2 style to quick connect air hose adapter (my compression tester kit had one) and run the line to my zeitronix sensor which normally reads from manifold vacuum line.

Nothing melted as it was a brief test :)

[landfill]

Did I read wrong I interpreted he had too much
constriction up top. I guess I goofed.

[julex]

I don't want to redo the exhaust, I like the one I have,very civilized. Adding cutouts will accomplish what I want, no restriction at WOT and while quiet and flowing ok in normal driving.

Just letting you guys know that unless you have some straight 4" pipes there will always be backpressure in the exhaust stealing your HP since cylinders have to overcome this additional pressure (backpresssure raises pre turbine pressure) to expel spend gases - which means they don't expel as many out as they could with open exhaust.

Good article on this:

http://www.autospeed.com/cms/article...x-Blows&A=1770

[dr.sah]

This is interesting, i have same problem with my car, at 6500-7200 rpm wont go smooth as is should.

[bigern45]

I should be good with 4" single

[Blue AWD]

When I switched from a 3" Borla to 2.5" dual and instantly was over boosting 4psi on my gt28s. Was surprised at the restriction of the 3".

[FlyboyS4]

Nice info. Kind of surprised at the amount of back pressure, though I don't know what the BB exhaust piping layout looks like.

When I switched from a 3" Borla to 2.5" dual and instantly was over boosting 4psi on my gt28s. Was surprised at the restriction of the 3".

The operating pressure is an important factor in this, what were you operating at? A 3 psi increase, from 21 to 24, made a difference when I tried swapping exhausts.

[Blue AWD]

Nice info. Kind of surprised at the amount of back pressure, though I don't know what the BB exhaust piping layout looks like.



The operating pressure is an important factor in this, what were you operating at? A 3 psi increase, from 21 to 24, made a difference when I tried swapping exhausts.

I run 30 and when I switched was hitting 34.

[rtl5009]

Bold move.

[Meow]

http://www.exhaustvideos.com/faq/how...pipe-diameter/

"A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:"

[FlyboyS4]

http://www.exhaustvideos.com/faq/how...pipe-diameter/

"A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:"

There's no mention of the pressure drop for those flows, I'm guessing that it is assuming a NA engine which is not a good reference for a forced induction engine.

[GrapeBandit]

maybe im interpreting this wrong, but a 3" single exhaust can only flow enough to make 339hp
doesnt make sense to me, like every BT b5 a4 uses 3" and puts down well over 339

[julex]

maybe im interpreting this wrong, but a 3" single exhaust can only flow enough to make 339hp
doesnt make sense to me, like every BT b5 a4 uses 3" and puts down well over 339

It is not like 3" will suddenly stop flowing at 340hp you know.

My example mirrors yours almost to the dot. I still make ~550WHP on dual 2.125" (estimated ID with 2.25 OD) which is exact equivalent of single 3" pipe (7.09 vs 7.06 in^2). But, I can probably make 600WHP by just upping the exhaust to dual 3" as per table above which will cause the backpressure to go from 10psi+ to 2-3psi, if even that.

What the table tells us that with piping properly sized, you will have negligible amount of backpressure and thus very minor power loss due to hampered VE.

[FlyboyS4]

maybe im interpreting this wrong, but a 3" single exhaust can only flow enough to make 339hp
doesnt make sense to me, like every BT b5 a4 uses 3" and puts down well over 339

The table is unlikely to have been intended for a forced induction engine. Leaving the amount of pressure drop out is a critical omission for a turbocharged setup.

Sent from my PC36100 using Tapatalk 2

[Meow]

The table is unlikely to have been intended for a forced induction engine. Leaving the amount of pressure drop out is a critical omission for a turbocharged setup.

Sent from my PC36100 using Tapatalk 2

airflow is airflow, leaving out backpressure means the table was likely intended for turbocharged vehicles as having enough backpressure is tossed out the window when talking about turbocharged car performance and only a discussion of the required airflow is necessary. Only NA cars worry about needing back pressure for maintaining midrange torque. Stop posting in this thread, you have nothing to add and your comments are irrelevant and ignorant.

[FlyboyS4]

airflow is airflow, leaving out backpressure means the table was likely intended for turbocharged vehicles as having enough backpressure is tossed out the window when talking about turbocharged car performance and only a discussion of the required airflow is necessary. Only NA cars worry about needing back pressure for maintaining midrange torque. Stop posting in this thread, you have nothing to add and your comments are irrelevant and ignorant.

I said pressure drop, and when determining air flow rate it's necessary to specify at what pressure drop the flow rate is being measured.

What you are thinking of is a completely different subject and not what I am referring to.

[Monty23]

A 3.5" exhaust with dual 14151 mufflers is VERY tame sounding FYI. It sounds almost stock from the inside of the car IMO.

[Monty23]

I have never personally used cuts, but I hear they don't like backfires and they usually leak when closed. Just something to keep in mind.

[NOTORIOUS VR]

I have never personally used cuts, but I hear they don't like backfires and they usually leak when closed. Just something to keep in mind.

yep... they never seal well, especially once the carbon gets up in there... To me exhaust leaks are quite annoying, and another reason why I would never consider a cut out. The first reason being they make the car sound like a pile of junk.

[Meow]

Pressure sensors located just before neckdown to 2.25" inside gutted main cat with smooth transitions everywhere.

Your pressure sensor location can have a large impact on the readings you are getting. You placed it in a location with transitions, nozzles and diffusers all over the place in there depending on how you located it in the main cat. Though I am thoroughly confused now reading your post. You say you have 3 inch downpipes but still have a gutted main cats? Can you clarify the location. Im not doubting your exhaust is too small and that you have back pressure issues, but I am curious of where you located the sensor and what your full exhaust system really is.

Placing it too close to a transition like a taper down from 3 inch to 2.25 inches can impact the readings.

[julex]

Anybody has logs from a GT car with a nice open exhaust? I want to compare my logs (where I calculated my engine's relative VE which seems like be tracking RL_W exactly so for all intended purposes RL_W is "virtual" VE...) so that I can see what's my % drop off in RPM range.

Thanks.

[julex]

Part of backpressure mystery solved (besides small dia pipe). View when looking inside of muffler...:



Yup, series of perforated triangular baffles so the exhaust gas has to go through a series of these and there is also a bend at the end on that obstacle course (or beginning rather). Looks more and more that doing dumps might be in my future (I just can't afford 1k for mandrel SS exhaust right now).

[FlyboyS4]

Nice that you were able to get some better reason for the pressure reading. How's the pressure sensor oriented, could you be picking up dynamic pressure from the airflow?

[julex]

Nice that you were able to get some better reason for the pressure reading. How's the pressure sensor oriented, could you be picking up dynamic pressure from the airflow?

Probably. I am not concerned about the sensor placement, I just wanted to see a big ball park. I will do some WOT pulls tomorrow wit exhaust disconnected and plot relative VE to see how things improve. I will continue this thread for others' awareness.

[Meow]

Unbolt your exhaust and do the same log

[bobkatkat]

Unbolt your exhaust and do the same log

My 3" Billyboat on my ACR SRT-4 has the same style baffles in the resonator. The system is unbearably loud. I didn't think they were that restricting. This thread makes me wonder just how bad my 200 cell spun cats are on the S4? Looks like they must go.