M70B50 Forced Induction and Tuning


Carmarthen, Wales
750IL 300bhp/450Nm
First Id like to introduce myself, im Graham, im 28 and im at university.

As part of my course I intend to examine the tuning potential of this engine.

The engine is an '88 from a 750il, and when complete will be mated up with a tremec (sp?) manual 'box.

I would like to know other members views on this engine, from what I can tell its a big lazy lump (8.8:1 :amazed:). Saying that, as it has low compression it would seem that forced induction is the way to go.

The final application will be a drifty toy for the weekend (in a e36 shell - thats shell in the strictest sense of the word) so I am looking at developing low down torque, but extra ponies are always nice.

We have access to lots of gear (and people to operate it) at the uni, such as rolling roads, engine dynos, flow benches, etc as well as a fully equipped machine shop.

Where should I begin? what are the weaknesses of this particular engine, what can the crank handle? will I need to upgrade injectors? what about headwork? should the head be ported/polished etc, or just remove the rough bits and crap (people tell me polished heads LOOSE power in FI engines)

I think that will do for starters, so any comments, advice, etc is welcome.

I personally would go down the Rootes S/C route, but am open to suggestions on alternatives/turbos etc if they can be backed up with some reasons for choosing that particular method.


The 750's were notorious for electrical faults and problems so it is probably a good thing that it is just the engine you are taking out!

I too would suggest a supercharger as these seem to be the best option on large capacity blocks.

If you polish the intake you will lose power - the rough surface aids fuel vapourisation. The exhaust is another matter though and will benefit from a polish.

Porting is also a good ideas with a 3 or 5 angle valve job.
I have 2 of the 750's one is my daily driver, and the other was my daily driver till I killed the gearbox :(

My daily driver had a few little issues with the electricals, but as im an autospark - it wasnt long till all was working as herr BMW intended.

I think the headwork will involve a 3 angle valve seat and just ensuring that there are no obstructions like 'flash' from the casting process on the heads or intake runners.

As the m70b50 was built effectivly as 2x2.5L engines with a shared crank, any opinion on the superchargers - 1 huuuuuuuge charger feeding both sides of the engine, or 2 smaller ones, one for each half? My concern with using 2 smaller ones is that if the belt slips on one side, that side of the engine will be recieving less boost than the other, and after reading about s/c ripping the heads off big block chevvys (when a backfire happens) if you run on a toothed belt. but a toothed belt guarentees no slippage.

Ive been doing some figures in excel, to work out the gear ratios needed to keep the engine in its powerband during up-shifts and am quite happy with the out come of that - although I need to finish the engine first, as each mod i do will move the powerband.

I will take some photos as I get bits done :D
Ok heres my input!!

I build turbo and supercharger BMW's got kits and alot more,!!

Ok, So you will be running the 750 in a e36 shell! and you want more power,?

the only option with that ciompression ratio is FI, Now i would agree with the supercharger option but you are wanting to drift, You want power through the middle of the rev range,

So this is what i would do:

Run two medium/small turbos on each bank, this will give you good response and awesome power, with 8.8:1 i would run around 15 psi max, I would not worry about the crank strength to much but would be an idea if you have an unlimtied budget!!

I would change the rods and pistons for forged H beams to give you the strength on the overlap,

I would run a large air/air intercooler maybe a water/air again depending on limitations,

For drifting i find one of the very best features is progressive NOS, this allows that extra bit of umph,,, to get the rear wheels running!!

high flow, low resistance injectors, (calcualtion horsepower desiredx0.55/number of cylinder) I have got all different types of injectors in stock

Run a 2 Bar map sensor on it( got some of those as well :)D)

Throw the electronics away of the engine, This is were i think i can help you, I am a dealer for the Electromotive Engine Management Sytems that would be idea for this application

Two choices on boost control, Dump Valve or Boost control valves controlled by the ECU!!
What kind of engine management would you suggest? I looked at a few from mega squirt through to haltech and gems, and although everyone seems to have a setup which can run 12 cylinders- it seems that they struggle- by that I mean that if I run all 12 from the ECU then I lose a lot of other features which share the same pins. My thoughts were to run wasted spark using 2 of the aftermarket coil pack available for the mondeo V6/scorpio V6. Haltech, along with a few others do small coil pack modules for wasted spark- with 2 cylinders running off each module, which can be banked together to give the required amount of HT outputs. I still think a couple of ford style coil packs would be neater, with one module on each side.

Would the system that your thinking of require 1 or 2 crank sensor inputs? I have 2 just now but I don't think I need both.

For gear ratio I set up excel to calculate road speed to engine speed for 5 forward gears, including final drive etc, then I plotted the results over a dyno chart - from a standard M70B50 - then I played with the gear ratios till I had a set where an up-shift at the top end of the power band would not be below the start of the power band in the next gear. Hope that makes sense. Then I picked the gearbox that matched the calculated ratio's best. If doing this yourself remember that changing the final drive changes everything.
I would rcommend the one that i use!! i use and love the Electromotive ECU system, Brilliant handle anything! You can run one or two sensors, run any output, nos etc
Coil packs we have look brilliant i will post a pick, you can use the wasted sparks, give me a list of what you need and i will find what you need,

Many thanks

Dan Wilkie
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