Stock exhaust system capacity with upgraded DP's

Albrigtsen

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Aug 20, 2017
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Hi Guys

I have a question I have struggled to find the answer on by searching this forum.
How much HP (or exhaust flow) can the stock exhaust system handle with just a 3” catless DP upgrade before the backpressure would cause problems for the turbos or other equipment?

The reason for my question: I have been running TTE550 turbos at max target 21 PSI for 2 000km (1 242 miles) and the axial thrust bearing on one of the turbos is completely worn out. The car has mainly been used as a daily driver forth and back from work. The car has FBO but as mention stock exhaust after the DP’s. The fault analyses I have received is a concern of too high back pressure on exhaust side. When searching it seems like several other people run 550-600 crank hp on just DP upgrade, is this a correct supposition?

Thanks
 

dyezak

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Basically with good mufflers (i.e. BMW performance or 335is) you're not going to gain a lot from an exhaust upgrade over dual 2.5" unless you are making in excess of 600hp.
 
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Albrigtsen

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Aug 20, 2017
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Thanks for your reply. I’m not really looking for any power gain but are wondering if 550/600 hp on a stock system (only DP upgrade) makes so high exhaust pressure that it’s harmful for the turbos? And this will cause the axial thrust bearing to be worn out after 2 000 km (1 242 miles)?
 

doublespaces

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I run the stock exhaust on my single turbo car, lol. With the catless downpipe, it actually sounds pretty good. I've got an exhaust system planned though. You can do it, you're just leaving a few horsetorques on the table
 

langsbr

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I think Payam said he made 750 thru the stock exhaust. He said he only tested backpressure at the turbo and didn't see any difference with or without the catback.

I'm tempted to drop my stock mufflers and take a log to see if there is a noticeable gain though.
 

fmorelli

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I don't know how the E9x cars are, but my E89 had the most retarded exhaust I've ever seen. I don't have numbers to back it up, but the passenger side pipe was heavily pinched in its design. I have a custom fabricated 2 1/2" dual system now that is a work of art, courtesy of a huge German mandrel bending machine and some CAD magic at MBS. I hope the E90 crowd has a better stock exhaust!

Filippo
 

dyezak

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You know, this platform is a decade old. There's nothing wrong with trying new things...but there's little point in retesting things that have been proven true for nearly 10y.

Imagine it. The year is 2007. New car. New platform. Not a single aftermarket part exists for this untouched virgin. Turbocharged engine sitting under the hood....there's got to be some easy horsepower under there. What do you think the first three things were that everyone tried? First, intake. Then, exhaust. Then more boost.
 
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langsbr

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I would concede that on stock turbos, but what about the current hybrids? When you get to the 550 - 600 whp range I would think that we would potentially see some gains from different exhaust.

I need to do the math behind it but 300whp thru a 2.25 exhaust seems less than ideal.
 

dyezak

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I would concede that on stock turbos, but what about the current hybrids? When you get to the 550 - 600 whp range I would think that we would potentially see some gains from different exhaust.

I need to do the math behind it but 300whp thru a 2.25 exhaust seems less than ideal.

The e90/91/92/93 has dual 2.5" mandrel bent tubing. You want to know the performance capabilities of a dual 2.5 mandrel system you can listen to all the people who have ran it up to 600hp and said they didn't gain a ton by upgrading. That's an option. Or you can watch the video above where they perform all the testing you are thinking about in a controlled dyno room on a 600hp engine. Or you can do some math. Or spend some money and do some testing.

Just like life there's tons of ways to go about getting to the answer, but like most things the answer is going to be the same. Very little benefit in upgrading the factory exhaust if you are under 600hp no matter what turbos you have.

ETA: Very little performance benefit. If you want a better sound, or are doing it for a lighter weight exhaust that could be beneficial too.
 

doublespaces

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Yeap, the exhaust is not a big upgrade. Its low on my list of priorities, right up there with carbon hood and trunk.
 

langsbr

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Interesting that it's 2.5" mandrel bent. I had read elsewhere that it was 60mm (2.36") and I never paid enough attention to measure it myself. Its also interesting to note that the Engine Masters video that was linked showed a gain of what, 20HP across the board by going from dual 2.5 to dual 3" at the 600 crank hp mark (~520whp estimate), so I didn't think my speculation was that far off, especially when considering hybrids. Granted, turbos change the discussion vs NA, and also the further from the engine, the less exhaust size matters in general. With hybrids, the choke point is likely the turbine itself, then the downpipe.

As for the platform's "it's been done and we know what works and what doesn't", let's just say that inlets were grossly ignored until Mauricio did, ahem, the math, and then applied the math, ahem, by testing, and got the results he expected. Now inlets are table stakes for an FBO car to perform at its peak. Based on that alone, it's clear that there is too much group think going on in this platform. We should be glad Mauricio didn't just trust that "the answer was going to be the same" right? ;)

What CAN be done vs what is optimal are not nearly the same. 900HP CAN be pushed through a single 3" exhaust, but it's not ideal. Even the LS guys are turning their heads when 2" primary headers are making more power than 1.75" on small displacement motors. Sometimes conventional wisdom is not always very wise.

I'm off to put my stock inlets back on my MMPs since they don't serve any purpose and weren't even thought to be a restriction for the first 7 years on this platform. Be back after I fight that rear inlet. :p
 
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doublespaces

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Bigger exhaust has always been better... Just not a very good value in some cases.
 

fmorelli

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Bigger exhaust has always been better... Just not a very good value in some cases.
In turbo motors, pretty much. I'd say exhaust design is not rocket science since the turbo provides the gating back-pressure to the engine. Normally aspirated is a different animal. We mostly need to get the exhaust out with little fuss.

Filippo
 

dyezak

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Interesting that it's 2.5" mandrel bent. I had read elsewhere that it was 60mm (2.36") and I never paid enough attention to measure it myself. Its also interesting to note that the Engine Masters video that was linked showed a gain of what, 20HP across the board by going from dual 2.5 to dual 3" at the 600 crank hp mark (~520whp estimate), so I didn't think my speculation was that far off, especially when considering hybrids. Granted, turbos change the discussion vs NA, and also the further from the engine, the less exhaust size matters in general. With hybrids, the choke point is likely the turbine itself, then the downpipe.

As for the platform's "it's been done and we know what works and what doesn't", let's just say that inlets were grossly ignored until Mauricio did, ahem, the math, and then applied the math, ahem, by testing, and got the results he expected. Now inlets are table stakes for an FBO car to perform at its peak. Based on that alone, it's clear that there is too much group think going on in this platform. We should be glad Mauricio didn't just trust that "the answer was going to be the same" right? ;)

What CAN be done vs what is optimal are not nearly the same. 900HP CAN be pushed through a single 3" exhaust, but it's not ideal. Even the LS guys are turning their heads when 2" primary headers are making more power than 1.75" on small displacement motors. Sometimes conventional wisdom is not always very wise.

I'm off to put my stock inlets back on my MMPs since they don't serve any purpose and weren't even thought to be a restriction for the first 7 years on this platform. Be back after I fight that rear inlet. :p

I mean you can talk theory and reference forum posts you read about 60mm whatever all day long. You could pull your copy of any factory service manual or Bentley's manual and see what the real values are. Or you can measure. I'll measure for you, let me run out to the garage and grab my calipers and I'll brb:

full?d=1509456655.jpg


As for the 20hp gained, I'll do the math for you. That's a 3% improvement on an NA engine. For a dual 3" on an e9x car you'll spend in the neighborhood of $1500. If you are making 580hp on a turbo car you'll get FAR more than 20hp by just turning the boost up 1psi, or finding 1deg of timing, or just a bit better fuel. Seriously.

Or you can listen to someone who has gone through 4 exhausts over the last 9 years of owning 3 of these cars spending a combined $4000 (give or take). I've bought and sold a single 3.5", two dual 3" (both sucked for fitment), and a BMW Performance. Guess what, I run the BMW Performance now because when I dyno tuned these back in 2011 at COBB racing their Mustang dyno only saw a 6hp difference. And that difference is questionable because we saw a bigger change in dyno numbers from the temperature change during the day.


These dynos were back in 2011, so maybe physics changed since then (or muffler tech). So I bought another exhaust just last year. Fancy ceramic coated tips with a new cross flow design wide open muffler. And instead of trying it on one car why not try it on multiple?

full?d=1509457701.jpg


The point is the been there done that. I've dynoed them on multiple cars, logged them, swapped them around. It's fun and a good way to spend some money. But we honestly saw a bigger variation in power from the outside temperature changing from 85f to 75f.

If you are after the weight loss then hell yea! That stock exhaust is heavy as sin!

If you are after a different sound then I can't fault you.

But if you are under ~600hp you're not going to gain much HP at all from an exhaust. Especially for the cost. It's a fact that's been proven multiple times by multiple people.
 

doublespaces

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I mean you can talk theory and reference forum posts you read about 60mm whatever all day long. You could pull your copy of any factory service manual or Bentley's manual and see what the real values are. Or you can measure. I'll measure for you, let me run out to the garage and grab my calipers and I'll brb:

Yeah but you're measuring at midpipe. I just installed 2.25" tips on my stock exhaust, and it had to be welded because they were almost the exact same size. I guess I'm saying, i'm 99% certain the exhaust diameter is not constant throughout, not that I really care about any of this anyway. The confusion over exhaust diameter is because it varies.
 
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Interesting that it's 2.5" mandrel bent. I had read elsewhere that it was 60mm (2.36") and I never paid enough attention to measure it myself. Its also interesting to note that the Engine Masters video that was linked showed a gain of what, 20HP across the board by going from dual 2.5 to dual 3" at the 600 crank hp mark (~520whp estimate), so I didn't think my speculation was that far off, especially when considering hybrids. Granted, turbos change the discussion vs NA, and also the further from the engine, the less exhaust size matters in general. With hybrids, the choke point is likely the turbine itself, then the downpipe.

As for the platform's "it's been done and we know what works and what doesn't", let's just say that inlets were grossly ignored until Mauricio did, ahem, the math, and then applied the math, ahem, by testing, and got the results he expected. Now inlets are table stakes for an FBO car to perform at its peak. Based on that alone, it's clear that there is too much group think going on in this platform. We should be glad Mauricio didn't just trust that "the answer was going to be the same" right? ;)

What CAN be done vs what is optimal are not nearly the same. 900HP CAN be pushed through a single 3" exhaust, but it's not ideal. Even the LS guys are turning their heads when 2" primary headers are making more power than 1.75" on small displacement motors. Sometimes conventional wisdom is not always very wise.

I'm off to put my stock inlets back on my MMPs since they don't serve any purpose and weren't even thought to be a restriction for the first 7 years on this platform. Be back after I fight that rear inlet. :p

Just a small correction, Inlets were grossly ignored until Brian May/Trueform Technology brought inlets to the forefront & did the math.