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Author Topic: m20b25 Straight 6  (Read 3744 times)

Offline Dirtracr95

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m20b25 Straight 6
« on: 10:30:06 PM / 23-Oct-10 »
Has anybody done any BMW straight 6 swaps?  I just bought a M20b25 with trans from an 87 325i 50k miles on the motor 100k on the trans and an equal length T3 turbo manifold for $450. Getting the motor in isn't going to be a problem should only take a day.  The problem is going to be making the factory A/C work because I dont want to lose it. Yes my factory R12 A/C still works.  The powersteering might also put up some problems too.  I just put a rack in the car not too long ago. Driveshaft wont be too hard just some cutting and welding.

There is lots of potential with this motor.  I hope to push around 400hp on the stock internals with a Schrick 288 cam. Should make a pretty crazy street car.

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Offline Jay

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Re: m20b25 Straight 6
« Reply #1 on: 10:51:02 PM / 23-Oct-10 »
It HAS been done, over in bulgaria or nigeria or some country there were photos of one that popped up
I've been pondering/planning this for a while now and have been drawing schematics...
(You should do an M50 instead)

Power steering is non issue (Adapters)
AC shouldn't be an issue either (Adapters)
Driveshaft is gonna be your biggest problem, and it's not just cutting and welding. Needs balancing too. The Getrag 260 is VERY succeptable to output bearing failure from the shaft being unbalanced and/or angled.

While the stock internals are well and plenty good for 400 ponies, the head and mating surface is not. You'll need to get it properly milled and decked, and O-ringed for your headgasket to survive any amount of time.

But, it's easy enough

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Offline Dirtracr95

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Re: m20b25 Straight 6
« Reply #2 on: 12:10:46 AM / 24-Oct-10 »
M50s are more expensive with little be gained over the M20 for what I want. And how often can you come across a motor/trans/ecu for $350 with 50k on the motor?  On E30tech people were saying the stock headgasket was good to around 1 bar but that doesnt mean much because theres tons of factors that effect that.  I wouldnt try pushing more than 1 bar until I had ARP head studs, mls headgasket and a schrick 288 or 304 cam (if i got enough coin for pistons) with springs.

Getting the driveshaft balanced isnt an issue. Theres places for getting that done.

« Last Edit: 12:19:15 AM / 24-Oct-10 by Dirtracr95 »

Offline Jay

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Re: m20b25 Straight 6
« Reply #3 on: 12:15:27 AM / 24-Oct-10 »
No need for ARP's, just get metric blues, repace them one at a time never cracking the head apart, you shouldn't have any issues with the head gasket then
Regular old Fel-Pro is MLS
Just run SETA pistons (Not much coin), and an ETA or SETA Crank (Make it a stroker)
Valve springs are a big concern too, as are the rockers.
The cam is unnecessary, but it helps.

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Offline Dirtracr95

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Re: m20b25 Straight 6
« Reply #4 on: 12:49:16 AM / 24-Oct-10 »
The cam is definitely necessary.  I dont go easy on my cars.  My present CA20e never sees less than 3k during normal driving so a hi-rpm cam is a must especially with the t3/t4 turbo I am putting on.

Offline Arro

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Re: m20b25 Straight 6
« Reply #5 on: 12:58:40 AM / 24-Oct-10 »
If you do this it will be epicness.
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Offline Jay

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Re: m20b25 Straight 6
« Reply #6 on: 01:01:29 AM / 24-Oct-10 »
There are several things that M20's don't mix well with. Consistent high RPM is definitely one of them. Rockers like to snap, and unless you have the IE HD ones, plastic deformation is a constant concern with them...
Also, you really, really don't need a cam until 450hp.... This is well documented on E30tech. It is 100% not a "necessary" part. It certainly doesn't hurt, but not needed...


Taken from the E30tech Turbo E30 FAQ

Q: What camshaft will work best for my turbocharged E30?
A: There is no universal M20 turbo cam shaft. What cam shafts that work in the engine is determined basically by three things:
- Manifold design
- Turbo charger size
- Pressure (boost and back pressure from the turbo+manifold)

This affects the duration of intake/exhaust valve opening and how much overlap is possible without affecting cylinder scavenging and filling.
Ultimately, the factory cam shaft works very well on a turbocharged car as long as all parameters seem normal, like increasing boost results in the expected power increase. If that is not the case, many things could be wrong:
- Too small turbine (excessive back pressure)
- Intake or exhaust leaks
- Malfunctioning wastegate or boost control

If you're looking for insane power increases, up to 500 crank hp out of what was originally a 170 hp N/A engine, then you better do some research and talk to a cam grinder. You should not expect low-end grunt with a turbo that flows enough for 500 crank hp on a 170 hp N/A engine.


AND backed up by a STOCK CAM M20 Dyno figure, up to 7200 RPM, which is a lot farther than I would consider the safe zone for stock rockers, This guy had IE HD's



Also, Found the photo

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Offline Dirtracr95

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Re: m20b25 Straight 6
« Reply #7 on: 06:02:18 AM / 24-Oct-10 »
Listen I am not trying to be arrogant, but this wouldn't be my first turbo build. I understand cam designs. The cam is the brain of the motor it determines where the power is. While changing the turbine housing A/R and trim will yield similar results.  The E30tech Turbo FAQ is geared towards people who dont know anything.  A good engine build starts with selecting a cam that suits its driver. I dont use the lower end of my rpm band hardly ever. I like high RPMs, therefore a cam with lots of duration and a low LSA is more suited for my driving style. Since I am planning on using a turbo I want the lowest valve overlap possible because the exhaust pressure will be so high when the turbo is at full spool.  You build the motor around the cam.  You do things to compliment the cam.

Offline EternalSwap

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Re: m20b25 Straight 6
« Reply #8 on: 02:28:47 AM / 29-Oct-10 »
just do a s85b50
-Alex

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Offline Dirtracr95

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Re: m20b25 Straight 6
« Reply #9 on: 03:06:25 PM / 31-Oct-10 »
if you can find me a running one with a trans, harness, ecu, turbo manifolds for $450 then sure. lol

When the BMW guys say "IE" I keep thinking Intergrated Engineering (VAG) not Ireland Engineering.