[marcozen]

Originally Posted by Manboy

I like it. there is no whistle sound which is good. I believe the claim is improved throttle response and slightly more hp and tq. I feel zero change in throttle response from the spacer but there might be more pick up at low and mid rev range.

If could recommend one cheap mod to get it would actually be an air scoop for our cars. I don't know if I made any power but there was a huge difference in throttle response after installing the air scoop. I know it sounds silly but Its not in my head. there is a difference.

The air scoop is better.... but the throttle body spacer ... thats a waste ? There are some youtube vids on that ....

[Manboy]

Originally Posted by marcozen

The air scoop is better.... but the throttle body spacer ... thats a waste ? There are some youtube vids on that ....

I cant say. All i know is that there was no noticable change after installation, however with the air scoop i actually felt better throttle response. I was sure i wasnt going to notice, but i did.

[Manboy]

EDIT: Throttlebody spacer has now been removed from the car, no point in keeping it installed. One more thing that can brake, I felt.

Originally Posted by Manboy

I cant say. All i know is that there was no noticable change after installation, however with the air scoop i actually felt better throttle response. I was sure i wasnt going to notice, but i did.

[FinnF30]

Reviving this thread just for opinions and posterity lol.

So I recently suffered timing chain failure, which took the engine along with it. I’ve since decided to go fully forged, CP Carrillo pistons and rods, heavy duty wrist pins and calico coated bearings, simply because I’m a masochist and I like being the underdog lol. The car is currently at the engine builders shop, getting ready for the rebuild. I’ve purchased an MHI big turbo, VRSF catless down pipe, FTP charge and boost pipes, FTP intake inlet pipe, injen intake and VRSF intercooler. Planning on going with a Cary Jordan custom BM3 tune as well to bring it all together. Assuming I have enough fuel, what do you guys think I’ll see rwhp wise with this setup? Will I have enough fuel to max out the MHI turbo? I haven’t upgraded the fuel system at all, but the LPFP is brand new since the car had a recent LPFP failure and was replaced.

Additionally, for those that have gone this route, is there anything else you guys would recommend to do now that the engine is being rebuilt? Goal is around 350-375rwhp, I don’t really want to dive into upgrading the fuel system, just want something quick and fun but RELIABLE as well as this will still be my daily driver!

[sqwinny]

Originally Posted by FinnF30

Reviving this thread just for opinions and posterity lol.

So I recently suffered timing chain failure, which took the engine along with it. I've since decided to go fully forged, CP Carrillo pistons and rods, heavy duty wrist pins and calico coated bearings, simply because I'm a masochist and I like being the underdog lol. The car is currently at the engine builders shop, getting ready for the rebuild. I've purchased an MHI big turbo, VRSF catless down pipe, FTP charge and boost pipes, FTP intake inlet pipe, injen intake and VRSF intercooler. Planning on going with a Cary Jordan custom BM3 tune as well to bring it all together. Assuming I have enough fuel, what do you guys think I'll see rwhp wise with this setup? Will I have enough fuel to max out the MHI turbo? I haven't upgraded the fuel system at all, but the LPFP is brand new since the car had a recent LPFP failure and was replaced.

Additionally, for those that have gone this route, is there anything else you guys would recommend to do now that the engine is being rebuilt? Goal is around 350-375rwhp, I don't really want to dive into upgrading the fuel system, just want something quick and fun but RELIABLE as well as this will still be my daily driver!

You'll be fine fuel wise, if you are going to go this far, you should have a b58tu installed with this kit. That way your HPFP isn't limiting you at all. Also will allow you to run full e85. Running a "pure stage 2" near to you as well and I'm curious to see how it plays out. Is Miami engines in boyton building the block ? I'm just lazy to being mine up there and have them do the work. Graphs show I should be around 330-340whp but when the tune is done, we will see whats possible on 93 and e85.

https://navardituned.com/collections/bmwn20parts

also, if you are going balls to the wall, you'll want to have these installed for a bit more lift. A set og 6 for the n55 fit the n20, but you'll only use 4 obviously:

https://www.bimmermilvs.com

[FinnF30]

Originally Posted by sqwinny

You'll be fine fuel wise, if you are going to go this far, you should have a b58tu installed with this kit. That way your HPFP isn't limiting you at all. Also will allow you to run full e85. Running a "pure stage 2" near to you as well and I'm curious to see how it plays out. Is Miami engines in boyton building the block ? I'm just lazy to being mine up there and have them do the work. Graphs show I should be around 330-340whp but when the tune is done, we will see whats possible on 93 and e85.

https://navardituned.com/collections/bmwn20parts

also, if you are going balls to the wall, you'll want to have these installed for a bit more lift. A set og 6 for the n55 fit the n20, but you'll only use 4 obviously:

https://www.bimmermilvs.com

Thanks for the reply! Yeah I was looking at the b58tu upgrade, I just didn’t know whether or not I’d even need it. How much can I get out of the stock HPFP before running into problems? I don’t plan on running e85, likely 93 on Meth if anything. But yup Miami Engines will be doing the work, they’ve been really helpful and informative so far. What all is done to your car? I’d be happy with that kind of power from this little 4 banger!

[sqwinny]

Originally Posted by FinnF30

Thanks for the reply! Yeah I was looking at the b58tu upgrade, I just didn't know whether or not I'd even need it. How much can I get out of the stock HPFP before running into problems? I don't plan on running e85, likely 93 on Meth if anything. But yup Miami Engines will be doing the work, they've been really helpful and informative so far. What all is done to your car? I'd be happy with that kind of power from this little 4 banger!

Everything that was bolt on off the shelf minus turbo + modified intake valve. Dynoed the car at track out performance down near the zoo.

https://www.bimmerpost.com/forums/sh...php?p=27467237

Car sits the same, but with a TTE400 turbo (specs are the same as pure stage 2) + s63tu injectors. Car can't handle these larger (flow 40% more than stock) injectors with the stock hpfp thus a b58tu HPFP is being installed next week Tuesday. Should be fun to see what's possible on 93 octane. Will post up a dyno for comparison before dabbling with ethanol.

Fueling wise I'm not too sure but at 22-24 pounds of boost with the bigger turbo it held, however I'm sure you'll want to max it out at 30 if possible due to forged internals and the rating from Mitsubishi. This should be good information for you:

https://www.notion.so/N2x-B4x-HPFP-I...f2cd8b2c0b1038

Navardi has been awesome with everything, he has been doing very good with tuning my car as well. I'd give him a shout and get the car tuned by him from my experience.

[FinnF30]

Originally Posted by sqwinny

Everything that was bolt on off the shelf minus turbo + modified intake valve. Dynoed the car at track out performance down near the zoo.

https://www.bimmerpost.com/forums/sh...php?p=27467237

Car sits the same, but with a TTE400 turbo (specs are the same as pure stage 2) + s63tu injectors. Car can't handle these larger (flow 40% more than stock) injectors with the stock hpfp thus a b58tu HPFP is being installed next week Tuesday. Should be fun to see what's possible on 93 octane. Will post up a dyno for comparison before dabbling with ethanol.

Fueling wise I'm not too sure but at 22-24 pounds of boost with the bigger turbo it held, however I'm sure you'll want to max it out at 30 if possible due to forged internals and the rating from Mitsubishi. This should be good information for you:

https://www.notion.so/N2x-B4x-HPFP-I...f2cd8b2c0b1038

Navardi has been awesome with everything, he has been doing very good with tuning my car as well. I'd give him a shout and get the car tuned by him from my experience.

Nice! Thanks for all the info! I think the TTE400 turbo might have a little more oomph in it than the MHI, but I’m hoping for similar numbers. I’m trying to decide now whether to just upgrade to the b58tu HPFP for piece of mind or just replace my old worn HPFP with a factory replacement. I don’t see my self going over 24 pounds of boost if my power goal is met, but I say that now lol…..

I’ve heard that going with ARP head studs after 25psi is probably a good idea to avoid head gasket problems as well? Also, did you go with colder plugs or are you still on stock plugs?

And so Navardi was the one who tuned the car when it made 330-340rwhp? He does seem to know his stuff and know it well, especially when it comes to forged N20 builds…

[SRHosein]

Currently on 397whp

BMW F30 330e Hybrid B48 Stage 2 PJT (Stock Turbo, Stock Internals)

The first B48 Custom tune with F80Paul, still in beta phases but here are the results thus far:
Stock 91 Octane - 265.77 whp, 317.70 wft-lbs
Stage 2 91 Octane with Meth - 397.83 whp, 460.26 wft-lbs
+132whp +142wft-lbs in gains

Modifications:
• ER Catless Downpipe
• FTP Charge pipe & Intake pipe
• Snow Performance Stage 2.5 kit
• Full 3" custom exhaust:
- PipeWerks Custom 3” Dual Titanium Exhaust Burnt Tips
- Borla 40943 XR-1 Sportsman Racing Muffler 3”
- Vibrant 1142 3” Ultra Quiet Resonator

[sqwinny]

Originally Posted by FinnF30

[

Nice! Thanks for all the info! I think the TTE400 turbo might have a little more oomph in it than the MHI, but I'm hoping for similar numbers. I'm trying to decide now whether to just upgrade to the b58tu HPFP for piece of mind or just replace my old worn HPFP with a factory replacement. I don't see my self going over 24 pounds of boost if my power goal is met, but I say that now lol…..

I've heard that going with ARP head studs after 25psi is probably a good idea to avoid head gasket problems as well? Also, did you go with colder plugs or are you still on stock plugs?

And so Navardi was the one who tuned the car when it made 330-340rwhp? He does seem to know his stuff and know it well, especially when it comes to forged N20 builds…

Tte400 is the same as a pure stage 2. Induser and excuser (spelling and fighting autocorrect lol). You can reach 330-340 on the mhi I'm sure with the higher boost level, but just to be on the safe side the upgrade for the HPFP makes sense. Also if you plan on running e85, you have to run the stock injectors at a higher psi than normal, to flow enough. You might want to upgrade injectors to s63tu ones so you don't have to or you'll have to use meth/port injection. It's up to you, but I went this direction. Stock 97605 or whatever it is plugs at .022

ARP head studs are a good thing, but I'm not too sure when the stock head bolts will lift and cause issues. Might as well upgrade as you'll most likely go to 30psi if possible and don't want to cuss yourself for skipping them only to open the block again.

How to accept the block for evo ARP head studs.

330-340whp at 21psi with Navardi. We stopped there as my fuel injectors showed clogging/dirty on logs, and with the s63tu injectors install, it has been working and tuning little by little to get power back. Last week found out the stock hpfp cannot handle these injectors over 18psi, thus the b58tu HPFP install is happening asap to continue. Previous person was able to run 27psi on this turbo with port injection (meth), so let's see what happens there with e85 down the road.

[Amazinjoey]

Originally Posted by SRHosein

Currently on 397whp

BMW F30 330e Hybrid B48 Stage 2 PJT (Stock Turbo, Stock Internals)

The first B48 Custom tune with F80Paul, still in beta phases but here are the results thus far:
Stock 91 Octane - 265.77 whp, 317.70 wft-lbs
Stage 2 91 Octane with Meth - 397.83 whp, 460.26 wft-lbs
+132whp +142wft-lbs in gains

Modifications:
• ER Catless Downpipe
• FTP Charge pipe & Intake pipe
• Snow Performance Stage 2.5 kit
• Full 3" custom exhaust:
- PipeWerks Custom 3” Dual Titanium Exhaust Burnt Tips
- Borla 40943 XR-1 Sportsman Racing Muffler 3”
- Vibrant 1142 3” Ultra Quiet Resonator

I would have gone with 3.5", you got some backpressure and could probably get a 30-40hp more with 3.5" exhaust

[FinnF30]

Some progress!

Rotating assembly in…new head, oil pump, counterbalance shaft, timing chain/guides etc. Excitement is mounting!

[SRHosein]

Originally Posted by Amazinjoey

I would have gone with 3.5", you got some backpressure and could probably get a 30-40hp more with 3.5" exhaust

I actually dropped my exhaust and dynoed with and without exhaust with no difference in hp. lol.

I have also tested numerous intakes and stock intake box with a drop in filter works best thus far for b48 applications.

[sqwinny]

Originally Posted by SRHosein

I actually dropped my exhaust and dynoed with and without exhaust with no difference in hp. lol.

I have also tested numerous intakes and stock intake box with a drop in filter works best thus far for b48 applications.

This is what I was finding out with my internet searches as well. Wanted to ask from physics friends but it wasn't necessary. It's not that big of a deal for us under 500-600whp or so. Most of the power we make is the downpipe and turbo exiting at 4", slowly turning into 3.5", then 3", then 2.80" or so for the n20/n26/b48.

If you really wanted to test that theory, it would be the standard downpipe into a 4" -> 3.5" burns reverse megaphone into a custom 3.5" exhaust all the way back. CTS turbo in my option does this the best for the price, but they go from 4 -> 3.5 -> 3 -> 2.8 or whatever it is (not 2.75" but not 3" either it seems).

A thread way back in the past talked about this and to get rid of the bottle neck was to cut off a part of the downpipe and weld a 3" v band to ensure you were 3" all the way through. This is something I'll do as I want to replace my stock midpipe section to angle the exhaust just right.

https://f30.bimmerpost.com/forums///....php?t=1589618

[Amazinjoey]

Originally Posted by SRHosein

I actually dropped my exhaust and dynoed with and without exhaust with no difference in hp. lol.

I have also tested numerous intakes and stock intake box with a drop in filter works best thus far for b48 applications.

You also have to tune for it...

Originally Posted by sqwinny

This is what I was finding out with my internet searches as well. Wanted to ask from physics friends but it wasn't necessary. It's not that big of a deal for us under 500-600whp or so. Most of the power we make is the downpipe and turbo exiting at 4", slowly turning into 3.5", then 3", then 2.80" or so for the n20/n26/b48.

If you really wanted to test that theory, it would be the standard downpipe into a 4" -> 3.5" burns reverse megaphone into a custom 3.5" exhaust all the way back. CTS turbo in my option does this the best for the price, but they go from 4 -> 3.5 -> 3 -> 2.8 or whatever it is (not 2.75" but not 3" either it seems).

A thread way back in the past talked about this and to get rid of the bottle neck was to cut off a part of the downpipe and weld a 3" v band to ensure you were 3" all the way through. This is something I'll do as I want to replace my stock midpipe section to angle the exhaust just right.

https://f30.bimmerpost.com/forums///....php?t=1589618

It's an old myth that 3" is good enough for 500-600hp but fun thing is, it's not This is due to backpressure, which forces you to run even bigger turbo and more boost just make more power. With lower back pressure you can run alot higher ignition and less boost while making the same power


Here's an explaination from a friend of mine that does saab tuning and kinda pioneering the idea of removing back pressure:

Understanding Backpressure and importance of exhaust size


Gonna start with saying.. Yes, your exhaust might function for the power you have, but do you really know how much backpressure you have or know if it is even near optimal? Something worth looking into if doing max effort setups.

Understanding backpressure and importance of exhaust dimensions, this is a very heated subject across all brands and many knows a very very little on the subject, even many professional tuners which is quite worrying.

Most might think their exhaust system flows enough, like these aftermarket 3'' and 3'' to 2.5'' systems. However it is far more complicated than that and most dont even suit the stage 5-6 tunes in reality without tuners having to lower torque peak a whole bunch to lower backpressures and reduce risk of choke out. There is this big myth trending worldwide in the car scene that 3'' works for everything and flows enough for 700hp or even as high as 1000hp which makes no sense.

Since mid 2019 until now I have probably done a well over 400 hours worth of CFD research on the subject established from a tone of physics calculations along with a few real-world scenario tests to get a perfect template to predict flow, velocity and backpressures for various setups. What I found most might find it very surprising and everything is leaning towards the same conclusion, bigger is always better. Anything behind the turbine is just waste gasses to no use.



Real-world scenario backpressure testing


The car: 2008 Saab 9-5 2.3t Biopower wagon.
The exhaust side setup consisted of: 3'' downpipe with race cat to 2.5'' cat back exhaust, running tubular manifold and ported TD04HL-15T 6cm2 turbine housing. I also fitted an 60mm wastegate to the downpipe on the first bend in the line of flow so it could work its magic, ran it as an boost activated exhaust cutout valve which opens at 0.7 bars of MAP.

Power figures were into the 350hp region and plenty of torque. The fuel that were used was E85.

I bought a backpressure testing kit which was based on using O2 bungs and there was this O2 plug with a threaded hole for a brake line and fitting to go in. In the bottom of it is there an calibrated hole(1.5mm) that the pressure can pass through and is small enough so it cancel out the cylinder pulses. EMAP and exhaust system backpressure were measured. I welded on a O2 bung on the manifold collector in such manner that it is not in the direct line of flow of any of the cylinders for highest accuracy hooked it to an analog 3 bar boost gauge. The exhaust system I took the existing 1st O2 sensor bung on the downpipe and ran the car in open loop during all tests. All pics attached in the end of the post.


First run measuring in the downpipe with the exhaust cutout closed it peaked at 0.5 bar of backpressure.

Second run measuring in the downpipe with the exhaust cutout open it peaked at 0.2 bar of backpressure.

Here is the interesting part on how it affects the EMAP(Exhaust manifold absolute pressure).

First run measuring in the manifold collector with exhaust cutout closed on the downpipe it peaked out at 3.3+ bars of EMAP, the 3 bar analog gauge has an mechanical stop at about 3.2-3.3 bar and the pressure continued to build past that for a little while so made the assumption of it atleast reaching 3.40bar of EMAP.

Second run measuring in the manifold collector with exhaust cutout open on the downpipe it peaked at 2.6 bars of EMAP.

The difference in EMAP by removing 0.25-0.30 bar backpressure post-turbine makes for an 0.8 bar difference in EMAP which is a quite drastic change.

Video of the gauges while testing here:

You might now ask yourself how the hell could the EMAP differ that much?! Well this is due to pressure ratio. The turbine and turbine housing works through pressure ratio in order to be able to get turbine drive, in other words an pressure release for the turbine to be able to spin/accelerate to those extreme RPMs and be able to have enough energy for the compressor side to draw in air and compress.
So this means if you have the scenario of a pressure ratio where it is 3.0:1, the pressure from the downpipe will triple into the EMAP automatically in order to overcome the restriction of the exhaust system. Also take note the more the wastegate flap is closed for example with an stiffer wastegate spring(or running excessive pwm% on boost controller without gaining anything) the more the pressure ratio while make itself known with backpressure so finding the correct balance with wastegate spring is essential especially on these small turbos to make them flow optimal.

You now see that slightest elevated pressure present in the downpipe can have drastic changes on the EMAP and the EMAP affect the overall engine flow a whole bunch, will come to that in a bit.

After all this my car felt so much more alive on the top end like it just wanted to go, before it was like hitting a brick wall and it would just stop accelerate even tho the airmass was almost the same. What's funny was that it was even noticable in the acceleration times. However my intake side of the turbo was not up to spec during this so didnt see any major increase in airmass. But I have since mounting that cutout been able to push it into 373hp on the hubs with a basic Saab OE 15T with cast wheel and 12 blade turbine combined with ported OE exhaust manifold, because it is basically free flowing and only needing to boost 1.49 bar at the most with this close to optimal ignition. The screampipe I previously had on the 60mm cutout during this test I removed and now I have no backpressure present at all in the downpipe so might have dropped the EMAP down to barely 2 bars now even on OE manifold.

Calculating the cross-section area of my exhaust and combined with that 60mm wastegate as cutout valve and taking into account of the pressure drop that early, my 3'' to 2.5'' now flows better than any regular 3''. CFDs were made on this aswell and it truly adds an whole new dimension of flow, more like in to the region of a slightly restrictive 3.5''.



How it affects the engine and turbos performance


So how does this backpressure and EMAP affect the engine and turbos performance?
Lets start with the turbine while keeping backpressure present in the exhaust system.

What happens is that you are working against the pressure ratio in the turbine and turbine housing from being able to function as it should so you decrease the drive of the turbine, basically loosing turbine efficiency as it is harder to make that pressure release. The exhaust manifold flow decreases due to velocity cannot be maintained so ending up choking out that's why you can see certain setups starting loose boost. But also this is something that can occur due to too high entry velocity aswell, because the energy transfer doesnt function. The exhaust gasses escapes over the turbine blades faster than what the energy can be transfered. Sometimes it is choking, sometimes it is too high velocity and sometimes it is an combination of both things happening at the same time. Backpressure mainly halts overall flow and velocity, the gasses take the fastest way out of least resistance like out the wastegate and ramming it open the flap on internal wastegate applications or escaping the turbine wheel too fast with pressure being too high closest to the blades and the inner turbine wheel core.

This excessive backpressure around the turbine does also increase the thrust bearing wear by a lot on many journal bearing turbos. The reason why so many TD04HL that has been pushed very hard end up with a tone of axial shaft play but the radial play is still just fine.

Now the EMAP(exhaust manifold absolute pressure) the higher the pressure and dependant on exhaust cam duration when the exhaust valve opens, just a little or a lot of pressure escapes in to the cylinder bore and chamber. The longer the duration of the camshaft the more pressure will get in and the pressure difference between bore and exhaust manifold will level out making the engine flow horrible if a lot of backpressure and high EMAP is present. Like if you would run a obnoxiously large exhaust cam like 300 degree duration you can end up almost having 1:1 pressure between cylinder bore/chamber and exhaust manifold. This is why it is so important in understanding what camshafts for what setup and the balance between exhaust cam and backpressure. Upwards 300 degree duration exhaust cam is rarely ever used on turbo setups even over 1000+hp, because most people end up loosing power.

The point here is if you have higher EMAP from backpressure when it is time to close that exhaust valve again the pressure remaining in the chamber is trapped. This pressure remaining starts immediately work against the new air and fuel trying to enter the chamber as intake valve starts to open. This is what causes us to gain boost pressure as we try to push more and more power on the same turbo. You now see how this can potentially make the power very limited with a lot of backpressure and high EMAP.

But there is more to it, this elevated pressure causes the disturbance of fuel atomization as the intake ports has to be over-sprayed with fuel to get fuel in efficiently against this backpressure leading to less efficient combustion etc. More heat gets trapped within the cylinder aswell which changes the ability of how much ignition timing advance you can run before knock on petrol or on E85 just that the optimal ignition point is lowered because the burn cannot get anymore efficient due to poor fuel atomization and many other factors.

Now you kind of see where this is heading, the compressor on the turbo is kind of self-explanatory of what happens, the whole chain is affected by this. The backpressure acts against the compressor wheels ability to flow and draw in air as you have to raise the boost more for the same work while it is getting hard to even get more airflow out of the compressor wheel. This can also cause the turbo to get heavy compressor surge in some cases.
This is why many get disappointed when their new fancy turbo specified for like 750hp only makes between 600-700hp before boost pressures and choke out starts occurring. Good example is the trend of internal wastegated gen 1 GTX3071/76Rs always choking out around 500-550hp for everyone while running 3'' turbo back(In the region of 0.8-1.1 bar backpressure in the downpipe).



External wastegated setups and exhaust system backpressure


There are many running an external wastegate with screampipe and still using their 3’’ turbo back exhaust system while still going up in power. There are a few complications with this, think of the turbine housing and pressure ratio that was mentioned earlier and how the backpressure affects the EMAP. While running an too small exhaust system you now get all the backpressure directly through the turbine housing and turbine causing it to get that more prenounced multiplication from present pressure ratio. This is much like what happens as mentioned earlier running too stiff wastegate spring/too high boost controller PWM% while not still getting anymore out of it on internal wastegated setup. The pressure on an internal gated housing shares the same outlet which levels out this phenomena a little bit.

I made an quick CFD analysis of this showing how the pressure changes in the exhaust manifold also how the flow and velocities pick up. Can be found in attachments aswell, you may notice the 3’’ lower bend of the downpipe is quite sharp. Left it as it is as I noticed that didnt do much difference as the restrictions were already so high. Like 0.02 bar decrease in the downpipe making a more sweeping bend.

Running an GTX30 turbine size and 700hp worth of flow combined with an 40mm external wastegate with screampipe. I was trying to mimic the og9-3 exhaust routing and during this test EGT was set to 800 degrees C. Only changes that where made was the turbo back exhaust system. 3’’ which causes backpressure vs 4’’ to 3.5’’ which has no backpressure present. Even due to the very design of the beginning 4’’-3.5’’ we get an little act of bernoulli’s principle where actually pressure drops below atmospheric pressure because velocity picks up so much.

What can be noticed is how the velocities picks up as backpressure decreases and something standing out significantly is that the wastegate gains an additional 100m/s worth of velocity meaning it can function properly. One thing we must understand is that wastegates are not meant to flow exhaust mass, they are meant to hold enough pressure to just stabilize turbine drive to get boost pressure. Therefore the theory of running excessive wastegate size to add flow is an poor way to actually add flow as you get very limited results in the end compared to just bump up the turbo back exhaust size.

An example on how much that can be gained. With these CFDs I helped my buddy design an properly flow exhaust system for his 700+hp 2.3L 9-5 build, running 46mm precision wastegate and 4’’ downpipe to 3.5’’. This was combined with an GTX3076R gen 2.
He accidentally put in a little bit too stiff spring in the external wastegate and reached about 1.3bar. Like no mans business during baseboost testing we exceeded 2000mg/c of airmass on E85 with only 1.3bar of boost. That is in to the 600hp region on E85, oh boy that car moved out on baseboost. It was faster than his old 580hp setup with and IRX600 turbo with internal wastegate which he had to boost 2 bars with and had him locked out due to backpressure so boost started dropping with high rpms.



Ignition timing and boost curve how it affects backpressure build up


How you setup your boost curve/power curve has an big effect on the backpressure how you much EMAP you end up building before reaching your RPM limiter.

If you go all out max boost/airmass in the low RPMs you start building up pressure a lot earlier and it continues to build much harder as you reach the top end of the powerband. As we already know by now that will interfere with the ability to get the most power out in the higher RPMs or even cause complete choke out where it just stops making power no matter what you do.

If you have trouble with backpressure you can reduce the early torque peak a bit and induce a slight ramp on the airmass/boost pressure, this will slow down the build up so you peak with a much lower EMAP in the higher RPMs. Can lead to an overall increase in power gain as mid to high RPM experience much less restrictions.

How does ignition have a roll in this? Well ignition timing advance is quite important timing the burn of the combustion to get an as complete burn as possible. If you have too low ignition advance you have an low temperature combustion occurring, but the setback is that you now can continue to burn the fuel as piston tries to push it out the exhaust port. This increases EGT significantly in some cases going out in to the exhaust manifold, with heat comes gas expansion so meaning you are reducing flow as it starts to build the EMAP higher. One more thing that reduces flow and velocity is that the piston doesnt get to push out the full exhaust volume at once, this is why spool up/response out of the turbo and setup gets so hurt by sluggish ignition.

Like running close to MBT/optimal ignition(maximum brake torque) we get an very energetic high temperature combustion which completes the burn much better. We get the full volume exhaust before the piston has pushed the exhaust gasses out the exhaust port and leaves us with much higher velocity/flow. The exhaust gas heat drops aswell reducing the expansion while you still gain more response and less pressure.

I did some testing on my own 15T setup on the daily 9-5 going down in ignition timing to what is more commonly used and then I went closer to the MBT ignition, I saw an difference in up to 0.2-0.3 bar EMAP from just going to better ignition. Also adding to this the MBT ignition makes so much more power that I could even lower the boost/airmass further down in low range and lower midrange and still maintain a good torque peak without causing build up. Ultimately having an very low EMAP peak and backpressure on the top end.
This is how I am able to push so much power with this small turbo and still maintain flow without choke out..




Engine displacement


Engine displacement have an effect on the exhaust sizing, EMAP and backpressure.
Many try to argue when this subject get brought up that the 2.0L 4 cyl engines doesn’t need larger exhaust because of the engine volume, but that is sort of incorrect in terms of 2.0L 4 cyl not needing larger, but correct that the larger engine can cause more build up.

Lets say you have an GTX3076R on a 3 liter engine and the same turbo on a 2 liter engine. Both engines will take the same amount of air to make the same power theoretically. In reality it differs from friction and more moving parts inducing losses etc.

What makes them both different even tho they make the same power with almost the same air is that the initial pressure build up is higher on the larger displacement engine as you pump much more volume with per combustion. Much like having an crazy torque peak with a lot of boost on our small saab engines. This makes the larger engine peak slightly higher in EMAP and backpressure as it chokes things out earlier. However the pressure can be very close to eachother on the top end as the 3 liter engine ends up outflowing the GTX3076R compressor faster meaning that it most likely end up with the EMAP peak building very very slowly towards the top end. While the smaller 2 liter engine is catching up further up in the RPM. This makes the larger engines more sensitive to small exhaust systems because it builds so much faster down low so more of their powerband gets affected by higher backpressure earlier. But as mentioned earlier boost curves and such have an drastic change in the behavior of pressure build up.


Exhaust sizes for optimal flow at different power levels, internal and external wastegate


Here are some guideline for you people who have desire for max efforts and want the backpressure staying at minimal or basically having zero backpressure. Based on the testing I have done from real-world scenario and CFD data, this is mainly for 1.8-2.3L engines. This is with taking the exhaust routing of our Saabs into account as piping length and where the bends are placed affects backpressure significantly.

If you manage to keep the EGTs down on petrol/pump gas you get sometimes a tiny bit more head room here on exhaust system sizing as the exhaust mass produced by E85 is 50.6% larger than petrol/pump gas.

External wastegate with screampipe setups.

340-400hp: 3’’ downpipe to 2.5’’
400-475hp: 3’’
475-550hp: 3.5’’ downpipe to 3’’
550-650hp: 3.5’’
650-750hp: 4’’ downpipe to 3.5’’
750-850hp: 4’’

One thing to address from what has been mentioned earlier here, even tho you have an external wastegate with screampipe drops EMAP and some exhaust mass to the exhaust system. You can still in some cases use larger than mentioned here and benefit from it as flow can still stagnate by poor pressure drop behind turbine outlet due to exhaust pipe routing still allowing for an slight pressure build up. Also as we know from before the full backpressure now goes directly through the turbine housing so even the slightest pressure increase will be able to stagnate flow to an certain degree. Once again external wastegate is only there for one thing and it is pressure control, not for great deals of exhaust mass flow.

Internally wastegated setups.

275-310: 3’’ downpipe to 2.5’’
310-360hp: 3’’
360-450hp: 3.5’’
450-550hp: 4’’ downpipe to 3.5''
550-650hp: 4’’(4.5'' downpipe can even be of use here.)
650+hp: 4.5''

Cross-section area/flow area of each pipe dimension.

Showing how every half inch increases the flow area.

2’’ = 3.14159 in2 / 2026.8282 mm2
2.5’’ = 4.90874 in2 / 3166.9227 mm2
3’’ = 7.06858 in2 / 4560.3651 mm2
3.5’’ = 9.62113 in2 / 6207.1682 mm2
4’’ = 12.56637 in2 / 8107.3193 mm2
4.5’’ = 15.9043 in2 / 10260.8182 mm2
5’’ = 19.63495 in2 / 12667.6843 mm2



Gonna end this post with saying if bottlenecks exists elsewhere in the setup while going up in exhaust size you might not see as much of a big difference, every setup if different and keep in mind on how you build so you dont get bottlenecks.

If anyone wanna see more CFD content just message me.

Incase someone missed while reading, video of the real-world backpressure testing showing the gauges here again:

As mentioned CFDs in attachment show 3'' vs 4''-3.5'' exhaust at 700hp combined with 40mm external wastegate.
Also take note that all CFDs are made with atmospheric pressure present so withdraw 100000 pascal off what's showing. 100000 pascal is 1 bar of pressure.

I hope you guys like this gigantic wall of information!

[sqwinny]

Originally Posted by Amazinjoey

You also have to tune for it...


It's an old myth that 3" is good enough for 500-600hp but fun thing is, it's not This is due to backpressure, which forces you to run even bigger turbo and more boost just make more power. With lower back pressure you can run alot higher ignition and less boost while making the same power


Here's an explaination from a friend of mine that does saab tuning and kinda pioneering the idea of removing back pressure:

Have you done this with results? I'm curious to know how it plays in the real world on our platform. It should be universal, but then again it hasn't been attempted or replicated.

[Amazinjoey]

Originally Posted by sqwinny

Have you done this with results? I'm curious to know how it plays in the real world on our platform. It should be universal, but then again it hasn't been attempted or replicated.

Not yet on the F30, looking for a 320i to try it on, but they are pretty rare here and expensive if you want one with good option

My 320D xdrive is fully loaded and finding a 320i with same specs and milage will cost me 10K USD more for that price I might as well get a F10 535i.


I've done this on a Saab, and the results were great gained around 40hp with bigger exhaust, same boost and higher igniton! For example this guy did 370whp on a stock TD04-15T, while people before only did 300-320hp on the crank with a similar setup. This due to back pressure