M30 OEM Turbo project

[CorvallisBMW]

O I know. My 2.5 on the late model drones like a mofo would just do it for the power gains. I'm gonna run this one first for a while.

Just for reference:

2" dual: 6.28 sq in cross-section
2.25" dual: 7.95 sq in
2.5" single: 4.91 sq in
3" single: 7.09 sq in

Stock on the 745i was 2" dual, so I'm actually loosing some flow with my setup. My plan is to get it on the dyno some day soon and be able to measure backpressure during some pulls to see how much efeect it's having.

[ThatOneEuroE30]

M30 OEM Turbo project

I'm just straight 2.5 inch pipe all the way back to a muffler no cats etc

[CorvallisBMW]

That should flow well. I had a resonator and cat on my n/a m30, so I'm keeping those for now (I have the keep the cat no matter what). Depending on the volume level, I may or may not remove the resonator down the road.

It's funny, the stock downpipe is 2.5", meaning it only has a 4.91in^2 area. But the factory exhaust has a 6.28in^2 area to accommodate for the extra flow that will come from the wastegate when it opens and starts dumping in to the stream as well. My exhaust is essentially the same size as only the downpipe, which means once the wastegate opens, I'll likely see an spike in exhaust backpressure. I wonder how that will affect the turbo? I need to do some research...

edit: Did some research. Higher backpressure will potentially slow the turbine wheel, which at higher RPMs will cause more pressure to build up in the manifold, and therefor also cylinders when the exhaust valves are open. This means that during the overlap period (around TDC on the exhaust/intake stroke when both valves are open) I will likely have a bit more exhaust gas hanging around in the combustion chamber instead of making it's way out. This means a lower VE (volumetric efficiency), slightly less power, and slightly more heat buildup. It's not going to kill the motor, but it will loose me some HP and limit my max reasonable boost. Down the line it would be best to go dual 2.25" if I can find the components to make it work.

[cypriotgeeza]

That should flow well. I had a resonator and cat on my n/a m30, so I'm keeping those for now (I have the keep the cat no matter what). Depending on the volume level, I may or may not remove the resonator down the road.

It's funny, the stock downpipe is 2.5", meaning it only has a 4.91in^2 area. But the factory exhaust has a 6.28in^2 area to accommodate for the extra flow that will come from the wastegate when it opens and starts dumping in to the stream as well. My exhaust is essentially the same size as only the downpipe, which means once the wastegate opens, I'll likely see an spike in exhaust backpressure. I wonder how that will affect the turbo? I need to do some research...

edit: Did some research. Higher backpressure will potentially slow the turbine wheel, which at higher RPMs will cause more pressure to build up in the manifold, and therefor also cylinders when the exhaust valves are open. This means that during the overlap period (around TDC on the exhaust/intake stroke when both valves are open) I will likely have a bit more exhaust gas hanging around in the combustion chamber instead of making it's way out. This means a lower VE (volumetric efficiency), slightly less power, and slightly more heat buildup. It's not going to kill the motor, but it will loose me some HP and limit my max reasonable boost. Down the line it would be best to go dual 2.25" if I can find the components to make it work.

Interesting! Its just a fundamental part of engine work that the flow has to be able to escape the engine smoothly and efficiently. Sounds the dual 2.25" is the best option as you said! :up:

[4ce30]

Just run 3in all the way! These motors flow a lot

[ak-]

And it won't drone or be as loud as you think considering the turbine hushes a lot of that already!

[ThatOneEuroE30]

I wonder how hard it would be to make my own 3 inch down pipe. Could I use the same flanges? If I could I can crank that bish out in a weekend.

[stanhayward]

I wonder how hard it would be to make my own 3 inch down pipe. Could I use the same flanges? If I could I can crank that bish out in a weekend.

Not hard at all if you're patient and have basic welding knowledge, and pretty fun and rewarding. Just grab a step up flange, I know that ATP turbo sells 3" adapters for 55mm tubing (~2.165") that you could use with your stock flange and then just either run an external wastegate dump or bring the wastegate back in. I will warn you that on turbo M30s, as proved by RagtopE30 in his turbo M30 vert thread, bringing the wastegate dump back into the exhaust, even in the least turbulent way possible, you can lose a serious percentage of power. He lost 50whp IIRC on his turbo M30.

[Mlarsen]

Tick tock! You got one week till the picnic!

[CorvallisBMW]

Just run 3in all the way! These motors flow a lot

Not disagreeing with you on that one, I just don't want to get a shitty exhaust note and unrelenting drone

And it won't drone or be as loud as you think considering the turbine hushes a lot of that already!

According to a lot of guys on the 745i section of bf.c, it still drones a ton. We'll see how this goes.

I wonder how hard it would be to make my own 3 inch down pipe. Could I use the same flanges? If I could I can crank that bish out in a weekend.

I don't know honestly. The stock turbo has a 4-bolt flange that, IIRC, is specific to the K27 turbo. Given that these have been out of production for 2+ decades, there's very little in the way of aftermarket parts.

Not hard at all if you're patient and have basic welding knowledge, and pretty fun and rewarding. Just grab a step up flange, I know that ATP turbo sells 3" adapters for 55mm tubing (~2.165") that you could use with your stock flange and then just either run an external wastegate dump or bring the wastegate back in. I will warn you that on turbo M30s, as proved by RagtopE30 in his turbo M30 vert thread, bringing the wastegate dump back into the exhaust, even in the least turbulent way possible, you can lose a serious percentage of power. He lost 50whp IIRC on his turbo M30.

Did he use the stock downpipe and wastegate dump when he lost that 50hp? Or was it something custom?

Tick tock! You got one week till the picnic!

Picking the car up tonight!

[stanhayward]

Did he use the stock downpipe and wastegate dump when he lost that 50hp? Or was it something custom?

Sorry should have made it more clear; his was a turbo M30 using one of Otis' manifolds, not an M106. So it was custom, and also he made 440whp I believe so it would be a relative percentage lost. On your car though it sounds like that wouldn't be a big issue and that you'd want it to be quieter.

[ThatOneEuroE30]

Please Uploads vids when shes done!

[CorvallisBMW]

Of course! My wife is driving me up to the shop to get the car, I'll put her in charge of video production :)

[ak-]

Well duh - only old people drive 745's :p
Anything other than stock is loud AF & drones. You're going to be surprised how surpressed it's going to be.

[ThatOneEuroE30]

Well duh - only old people drive 745's :p
Anything other than stock is loud AF & drones. You're going to be surprised how surpressed it's going to be.

I got a buddy with a few 745's you don't hear turbo like a top mount but you sure feel it. His exhaust are nice to. He's actually the guy I got my system from.