Big Six Turbo

BMW called M30 engines as "Big Six" (M20, just like mine is "Little Six"). This page is about an M30B34 turbo engine, originally in a 745i from 1985.


It's not a weak car even it's stock form (3.5 Turbo, automatic, see pic),

but this engine is now in a light chassis (e30):


This page is about GenBoard-ing it!

Original setup

Car outlines:

Buylist at MembersPage/GergelyLezsak/BuyList


Electronically the M30 engine seems very similar to the M20 little brother (from the same era: motronic 1.3). Even their original ECU can be swapped, and the engine will start up! (mapping is not the same, so don't use this as a common practice)

After i've received[first part of the order] I made a new harness, and ordered a new 3.3 GenBoard. Now, the car is running stable with GenBoard.



I've put together a testbench-setup for the old GenBoard (#378) (with silly-map) on the desk. I gave it trigger signal from soundcard. There were no suspicious signs until I connected an injector. There's a strange behavior when I stop the signal; sometimes (it's about 70% of the cases) it keeps the injector open (with pwm actually), no matter the signal is stopped, and rpm is 0000 also. /Note that LCD is strangely freezed in this case./

When I restarted the trigger it just operated normally again (LCD also).

- In the case (and only in the case) it was stopped with injector leaved open I saw constant lower MAP values than normal (eg. 85kPa instead of 100kPa). Battery voltage was almost identical (12.2-12.1V), MAP voltage was constant 4.92V in both cases.

Follow the grounding instructions on MembersPage/PhatBob/UserGuide. Bad grounding is the only likely cause. Without PWM-ed injectors, it is often possible to get away with bad grounding, but with PWMing it's not. Also, adjust priming pulse=0, and don't switch several injector simultaneuosly: use 6 entries in the h[0] injector sequence, each should switch exactly 1 injector.


The car's got new 500cc injectors now. I've replaced it's original GenBoard with a new one, as we couldn't find the reason for silly MAP values for a long time now. (External sensor were replaced, MAP +5V voltage is measured with DVM, still got much lower values than reality -but only when the engine is working.)

Unfortunately we couldn't map the car with the new injectors. Details at MembersPage/GergelyLezsak/BigSixTurbo/InjectorsTwo


The car is stable now, had an 1/4 mile drag strip with 12.88s best time. Stock 300cc injectors were not able to keep up high rpm @high boost (up to 1.4 bars).

Had some problems with ignition coils (this is the second one which gives the spark across its housing with high humidity -the newer type with plastic housing). Interesting: there's a strange behavior of LCD when spark is in the engine compartment: the characters are falling apart; sometimes it's a bit misaligned but sometimes totally unreadable. If you see something similar check your ignition system! :)

There's no change to silly map values, but we'll settle with it for a while, because behavior, power, fuel consumption is acceptable. Soon we need to upgrade injectors and then remap VE, so that's a good time to cure this problem also.


Some news. We had serious problems with the car's power and consumption, it turned out to be an ignition retard problem, which was caused by new ALS function accidentally enabled. (AFAIK newest MegaTune doesn't show the config variable ign_balance which was FF and caused the trouble. Thanks for Marcell's support!)

After setting it right, ign. adv. get normal and the car gained ~100HP, +50% fuel economy :) -Interesetingly boost pressure falled back (from 1.2/1.5bar to 0.9-1.2bar)with good ignition and power. We didn't deal with knock and boost control this time, since the car dealed with the turbo's max boost level well.

But MAP values are still silly. I checked values with engine not running. At ambient air pressure it's fine ~100kPa, with connecting GND to MAP input shows 000kPa, Connecting +5V (which was actually 4.93V) to MAP input shows 249kPa.

With the car running it showed inaccurate values like before. Replaced the MAP sensor to another similar one, but values remained the same. When engine running I see ~70kPa at normal ambient pressure (0 bar on analog boost gauge, double checked). When engine is off, MAP goes back to 100kPa@ambient pressure.

I think two sensors can't be wrong similarly, so I consider MAP sensors good. Displayed values at 0/100/250kPa are also fine so I guess analog input->GenBoard internals->LCD with firmware are also OK. What else could be wrong? Maybe +5V reference voltage is changing while engine running? Any other idea?



After a lot of mechanical failures (cracked head, etc), car's running again, but mapping is still poor. I'll definitely look for correct MAP values first (still not investigated).

. . .


At least we changed to the latest firmware. (1.0.38)

BMW-style dual IAC valve is fine now. And car had been mapped nice.

One strange problem with MAP: on boost, MAP values are less than real (probably the half of real) values.

I'm using 250kPa MAP sensor, and double checked the values with two (known good) analogue gauges.

I probably misconfigured something with the new firmware.

Config and tables are here:

By quick look, I couldn't spot anything to justify bad MAP reading. Preferrably with pressurized MAP connection (eg. using a compressor) Do the voltage measurements, or at least read the sensor_filt[] with TerminalProgram in hex-view mode with mdx23 command. mdx.. like mdx23 spits out 32 bytes from 0x230 SRAM address: since sensor_filt[] is a little-endian array of 11 bit sensor ADC readings starting from 0x23f in the 1.0.38 firmware, positions in adc.h 2=>TPS, 3=>CLT, 4=>MAT, 5=>MAP so MAP will be at 0x23f + 5*2 = 0x249 showing 0xff 0x07 (little endian 2047) when MAP input is close to 5V

After this, we will know where to look: in hardware (eg. MAP connection, supply) or firmware (eg. configuration).

In any case, check/measure the GND connections very carefully !!!



Car is running with GenBoard :)

We'have removed the whole factory harness. (Took a while to reproduce everything) Some experience: TPS wire's and IAC open/close wire's direction is always replaced :)

Had some difficulties with digital outs; EC36pin15 (Which is P259/5) didn't do anything (with the fuelpump). Tried to move it to the next pin (P259/6), resulted the same. Finally chose a different chip: S259/5, EC18pin5. It's working. Couldn't imagine what could be wrong, at the test time fuelpump out was working.

After we've got fuel I made it flooded. I don't know how but my previous calculations worthed nothing. I thought fuel_req at the 12ms range, but I needed to half that, and maybe we'll half it again. Idle (which is horrible now, but that's mainly pid-related) is around 1ms pw. Don't know why, these are ~300ccm injectors. Maybe too slow to close? Will try to lower pwm value of 30%, and PowerFlyback? if necessary...

Since we've got 25cm of snow the day the car started, couldn't tune it yet...

Some pics:


thumbnails_analog%20muszerek%20010.jpg [in big]

Nice work with the LCD mounting.

The first day at the backyard (no snow at all):

thumbnails_udvar2.jpg [in big]


and a [movie clip] for motivation

//Emil Larsson