Ok, time is limited so I’ll give the short version.

The 745 came from the factory w/ ~260 hp and the waste gate limited boost to about .45 bar about 6 psi.). The post mid-1983 and later Motronic models had several very effective and technologically advanced safety features such oil sprayed on the piston underside for cooling, a loop back waste to reduce turbo spool time, dual knock sensors to retard timing at knock onset and finally a manifold pressure sensor that cut the fuel pump if it sensed a run-away / over boost condition above about 10 psi.

The m-102 fuel system is normally modulated to a max of 42 psi at 6psi boost, but is capable of providing 80 psi to the fuel rail

So if all you did was boost the manifold pressure you hit the 10 psi shut down safety feature. In late 1998 time frame Jarmo Tuoma (in Finland) found the ‘over boost’ part Motronic code and re-wrote Motronic EPROM. (also eliminating the “ultimate last resort” no sh*t safety feature) and the rest is history. At one point, his web site showed dyno charts w/ 650+ hp.

The rest of the story… The unmodified m102 fuel system goes dangerously lean at about 7.5 psi and near catastrophic lean at 11 psi, hence the 10 psi shutdown safety feature.

So with the m102 engine, if you want to go over 10 psi, you need something like the CARTECH FMU to boost the fuel delivery during higher than normal boost conditions. For about 7 years I regularly modulated my boost to about 16 psi, (and used it FREQUENTLY !!!! ) but could support (and saw spikes to 25 psi or about 1.8 bar. (~29 psi = 2.0 bar at standard day sea level)

Bottom line… with a 3.91 LSD at 80 mph on wet pavement.. If you hammer it.. you get wheel spin… Some think the 3.46 lsd is better.. and maybe so.. but since we are limited in this country to ‘legal speeds’ of 70, sometimes 80 mph, I chose a diff. ratio that puts me in the 3k rpm band at my cruise speed as the Germans would see at their normal autobahn speeds of 140 mph. So the stock 2.93 diff at 80 mph gave me 1680 rpm which was way out of the boost and torque range giving very slow response on the freeway because the tranny had to kick down, turbo spool and then you get some performance. The 3.91 gives me 3k rpm at 80 and instant performance. 80 –> 120 is a BLINK !!!!!

So there it. The ‘big six’ series engine is very strong and supports boost very well. The secret is keeping enough fuel in the motor to avoid detonation. In 10 years of running the 745 on high boost, I’m the only one I know of that did not grenade their motor, some did it several times to include things as mild as head blown gaskets to the extremes of grenaded pistons and rods through the side of the block…

If you want to go ‘boost’ start with TDC. Turbo has many volumetric advantages over supercharging, but is more expensive.

At this point, the 745 has about 250 kmiles and is down for a well deserved rebuild, but not because of a grenaded engine, I just want her freshened up a bit.

Note to Steve H: I can't wait to get'er back... (-: js...

Regards

John Stein
edgewater24.com

• It has to go... (url) - johnhstein, 85-M6CSi, 85 745i & 84 633CSi 2006-11-20 18:12:10 (498 views)
  • Leaving the Shark culture? :-)) (nt) - bmw635 or Peter/E24 on RF. 2007-05-08 02:13:36 (164 views)
    • No, not that easy to get rid of me.. more... :-) - johnhstein, 85-M6CSi, 85 745i & 84 633CSi 2007-05-08 12:26:43 (193 views)
      • Re: No, not that easy to get rid of me.. more... :-) - ram635csi 2007-05-08 20:42:11 (134 views)
        • So much to say.. so little time... more... - johnhstein, 85-M6CSi, 85 745i & 84 633CSi 2007-05-09 07:38:45 (130 views)
          • The not so short ... short version... - johnhstein, 85-M6CSi, 85 745i & 84 633CSi 2007-05-10 10:08:15 (141 views)
        • Look up TCD/ Turbo Charging Dynamic to see basic info. (url) - bmw635 2007-05-08 20:47:18 (206 views)
          • Re: Look up TCD/ Turbo Charging Dynamic to see basic info. - ram635csi 2007-05-08 21:05:31 (206 views)
            • Done research on turbo& M motor before I built the custom 3.7L normal aspirated. - bmw635 2007-05-08 22:03:16 (191 views)
              • Re: Done research on turbo& M motor before I built the custom 3.7L normal aspirated. - ram635csi 2007-05-09 12:24:47 (140 views)
                • There is 1 custom SC E24 using rotary style on RF. My idea is to use - bmw635 2007-05-09 20:27:01 (175 views)
              • Re: Done research on turbo& M motor before I built the custom 3.7L normal aspirated. - ram635csi 2007-05-09 12:15:56 (155 views)
      • Good to know since I know you've been around since BCG? Know what you - bmw635 2007-05-08 20:42:06 (150 views)
  • been sold... more.. - johnhstein, 85-M6CSi, 85 745i & 84 633CSi 2007-05-07 08:08:40 (169 views)