Looking for M90 engine rebuild info

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yellowbullet

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Hi all,

I recently purchased an M90 engine for my CS which needs a good rebuild. I don't even know where to start looking for rebuild information because I don't know if I can use the M30B35 specs or if the M90s are totally different (in regards to tolerances, torque specs, etc.). Can someone please point me in the right direction?
 
Malc, thanks for the links, but I've been there before. I know the engines are different (how different, I don't know) and here is the info to prove it:

M90

The M90 was the first road going BMW engine with 3.5 liters displacement. Originally this engine was used to test if the M88 engine was reliable with the narrow web spacing of 6,6mm. This means that this engine has exactly the same bore and stroke as the M88 and is directly related. However instead of a DOHC cylinder head with 24 valves, BMW used the SOHC cylinder head with 12 valves. The use of an electronically controlled fuel management system (Bosch LE-Jetronic) allowed a slightly higher compression of 9,3:1. Strictly, the M90 engine is not an M production engine although the E12 ///M535I in which it is used is a real M car. Until 1981, the M90 engine was used for the early 635CSI's and the very rare and delectable E12 ///M535I from 1980.

M30B35

To improve the reliability for road use, BMW redesigned the M90 engine in 1981. This engine, called M30B35 was slightly different. Compared to the M90, the M30B35 has a larger stroke (86,0mm), but a smaller bore (92,5mm). The use of a digital controlled fuel management system (DME) allowed to increase the compression ratio from 9,1:1 to 10,0:1. Although the engine capacity decreased slightly (3430cc vs. 3453cc), power and torque remained almost the same. The M30B35 remained in production for almost 13 years and in the late 1980's, catalyst versions where developed and offered for sale also. The M30B35 has been used in the E23 735I, the E24/1 635CSI, the E28 535I, the E28 M535I, the E32 735I and the E34 535I.

Basically, all I need to know is if the rebuild specs are compatible between the two engines.
 
Malc said:
Being somewhat "light" on engine specs, why not get your M90 engine rebuilt to "standard" spec

Just a thought
Malc
Click to expand...

Malc,

I'm doing the rebuild myself so I need to know which specs to follow. The problem I'm having is I don't know if the M30B35 specs (which are everywhere) are the same for the M90 engine (which is nowhere to be found). Things such as tolerances, torque specs and important stuff like that is what I'm worried about.

I thought a lot of people used the early euro 3.5L engines which is why I thought it'd be a straight answer, but it seems like nobody knows.
 
Was it just a change in designation number?

My 1986 M535 engine is a 3.5 litre and as nobody wants it I could take measurments if I rip it apart for curiosty (spelling)

Malc
 
The M90 is, as described above, an M88 block with a conventional M30 head. The engines are essentially the same except for the bore and stroke, and some differences in the coolant flow structures.

Since the head is the same, including the bolt pattern, use the M30 head torquing specs (I did this on my M90 nad it has been fine). Since the bottom end is M88, use those specs (or use the S38 specs).

You must be sure to get a proper M90 head gasket as the coolant and oil holes are different (although the head is the same as the M30.. odd).

The cam is also somewhat different, so be careful there.

HTH
S
 
I concur with Ephraim.

My M90 635 was able to blow away two Dinan M6's on the bay area's fabled Highway 9-Skyline-84 to the beach run. The S38 is a fire breather with 60+HP more than the M90, but all that juice comes above 3500 RPM, below that it's a pretty pedestrian engine; The M90, on the other hand, develops its power earlier in the rpm range and has a MUCH flatter torque curve. This (and, ahem, (sotto vocce) my superior driving skills) allowed me to really toast these guys through the twisties. The M90 just grunts you out of tight turns with so much thrust you need to be ready for it. I had to totally re-think my driving approach with this engine. I run a 3.07 LSD with a Getrag 265 (very tall gears), and the M90 has enough grunt to break both rear tires loose if you hit the juice too early on the exit. The car taught me to really (really!!) rely on the late apex. That way it is more or less going straight when you hit the afterburner. The weird gearing means you are often in 2nd and 3rd (the car will go 85+ in third with this gearing), and this simply adds to the corner exit excitement (read: danger, if done wrong...).

I have a line on a CR trans, but I think I am going to get theat for the E9 with its much more mild lump...

The stock M90 in good tune with a properly set up L-Jet is a kick-ass machine.

IMHO not worth the extra time and money to get a few more ponies...it is perfect as is...

(OK, OK, I HAVE thought about supercharging her...)

S
 
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