Having recently rebuilt a BMW M30 engine, I reached some interesting conclusions that I would like to share.
Note that the information below may be particularly useful for the markets where the 3.0l M30-powered E32 730i and E34 530i were available. As far as I know, these models were not officially sold in the US.
First, let me say a few words about the rebuilt. It was a 3.5l late M30 engine, with the block used on E34 535i and E32 735i. I tried to stay as stock as possible, so I used the 3.5l head casting# 1708843, which came with the M30B35 (the one having bigger intake ports, 1mm larger intake valves, and largest combustion chamber volume, compared to all other M30 heads). Stock camshaft, of course. The only deviation from stock was that the head height was shaved 0.3mm. Thus, the compression ratio was about 9.2:1.
The result was a healthy M30 engine, with beautiful high-end power. However, I felt that the low end to mid range torque was kind of disappointing, when I compared it to the Euro M30B34 engine I had years ago on my E28 535i.
So I decided to swap out the stock M30B35 head (casting# 1708843) and replace it with a stock late M30B30 head casting# 1708497. In theory, the narrower runners of the B30 intake manifold and intake cylinder head should promote better volumetric efficiency on low-end to mid-range, for the cost of some high-end power. It is a good trade, that suits better the character and heavier chassis of the E32.
At the first sight, the M30B30 casting 1708497 head seems about the same as the ubiquitous M30B34 head casting 1277358, as it has the same intake port size (and therefore gasket), and same valves size.
After reading somewhere that the ports were slightly modified compared to 1277358 casting, I decided to measure the combustion chamber. Unfortunately, the head I had was already skimmed 0.3mm, but I concluded that a stock 3.0l head casting 1708497 has a combustion chamber volume of about 63.6 - 64.0 ccm. That's quite different than the reported ~58 ccm of the M30 casting 1277358, and very close to the B35 head casting 1708843, which has ~65 ccm.
By shaving 0.3mm off the M30B30 head casting 1708497, the combustion chamber volume is reduced by about 1.60 ccm.
With this occasion I also determined that:
- the volume of the dome on a stock late M30B30 9:1 compression piston is ~10.4 ccm, dome height = 2.91mm;
- the volume of the dome on a stock M30B35 9:1 compression piston is ~2.0 ccm, dome height = 1.85mm;
- the volume of the some of a stock Euro M30B34 10:1 compression piston is ~5.6 ccm, dome height = 1.85mm.
Maybe the above will prove useful for someone, in the future.
So, I ended up with a hybrid M30 engine, having the M30B35 block and M30B30 head casting 1708497, head skimmed 0.3mm. Of course, I used the stock M30B30 intake manifold and I gasket matched the intake manifold runners to the head ports (which is something I strongly recommend). Everything else is stock. The compression ratio is within the range of 9.35:1 to 9.4:1. I run a M30B35 Motronic 179 chip.
I am very pleased with this hybrid engine.
Roughly, the hybrid B35/B30 engine feels better than the stock B35 from about 1500rpm to about 4500rpm, performs about the same from 4500rpm to 5000rpm, and then lags behind above 5000rpm.
The performance gap is quite narrow, but good enough to be noticed.
The M30B30 head casting 1708497 seems to be an evolution over the earlier M30 casting 1277358, despite that the valves and intake port shape are the same. Most likely, it got some of the intake runner shape and combustion chamber improvements of the B35 head. It was a pity that I didn't have a 1277358 head available, to perform an in-depth comparison of the two.