Compression info

gazzol

gazzol

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Hi guys, I was wondering if anyone can tell me the compression figures (as a ratio) for the M30 engine range (Euro specs please). What I'm after is building a screamer motor with decent headers mad cam and seperate throttle bodies etc etc the trouble is raising the compression ratio without resorting to custom pistons. I know that on the E28 model range the smaller the engine the lower the compression ratio was indicating only one combustion chamber size was used I hope that this isn't the case with the earlier engines.
Any info would be appreciated

Gazzol
 
Um, I'm a little confused by your post. Are you asking what clearance volumes the various heads have?

The M30 had a fairly constant CR of ~9:1 during the entire production run for the Euro motors. Sometimes they bumped it up to about 9.3:1, but I think that was the max.

What are your power goals? A bored, stroked, 11:1 CR, crazy cammed, ITB-ed, P&P, and using headers M30 won't break 280rwhp. It has been done, countless tens of thousands of dollars invested, and all that money points to one way, and one way only to make big power with the M30:
Forced Induction.

Turbocharging Dynamics makes a kit currently for the M30 motor which makes about 350rwhp and ~400rwft-lbs. That is E39 M5 beating power right there.
 
If you want big horse power then you need to do more than just boost the compression and put in a wild cam. Increasing the compression means that the valves are going to be closer to the pistons, it just works that way. So now you have a valve to piston clearance issue. Stick in a big-arse cam, and that increases the valve/piston interference issue. So even if you keep the stock pistons, you'll likely have to cut notches in 'em...not advisable...so now you're talkin' custom pistons. And with more compression and a bigger cam you'll be spooling up that bad boy pretty tight so the necessity for better rods, or at the least modified/improved oiling for same is another thing on the list. I gotta agree that the best way for big reliable horse power is the Turbo Dynamics system (no affiliation). I think the system with intercooler (you gotta have the intercooler) is around $6K USD. And now that you have the big ponies, what about the brakes. Don't forget the CVs too. When was the last time they were greased? I don't mean to be a dick but to keep you and your car fast and *safe* takes more that just kickin' up the horses.
 
There were only two combustion chamber versions -- Everything up to the M30B35, and the M30B35. In simple terms, that's one CC for everything up to about 1988, and one after. For the record, the older chamber volume is something like 59.5 cc's and the newer is 65 cc's.

If you want more than about 9.3:1, you'll need non-BMW pistons. There are aircooled VW pistons that will give you close to 10:1 with a pre-M30B35 head. Otherwise you will need custom.

Running separate throttle bodies on an M30 means you'll need an aftermarket engine management system.

If you want a high-output naturally aspirated engine, it makes more sense to find a Motorsport S38 or M88 than it does to try to get big HP from an M30. The generally accepted wisdom is that the M30 is good for about 300HP in naturally aspirated form. Otherwise, I too must endorse the forced induction bandwagon. There's no less-expensive (note I didn't say "cheaper") way than boost to make big power.
 
Thanks for your replies, I am well aware of the issues associated with any form of tuning an engine i.e. brakes and drive line the car is currently undergoing restoration in my workshop and these issues would be taken care of during the rebuild. I agree that forced induction is the way forward for large power gains relative to money spent.
According to a BMW book I used to have in the E28 model range the CR was different on each engine size 2.5ltr was lower than 2.8ltr which in turn was lower than 3.5ltr. My idea was this; say the EARLIER M30 range did all have the same CR i.e. 9.3:1 then I would use the head from a 2.8ltr engine on my 3.5ltr to give me a CR of arround 11:1 assuming a chamber volume of 56 cc on the 2.8ltr.
I have to dissagree with the statement that I would need different rods as the M30 engine (according to BMW literature) allready has steel crank and rods and not cast iron. This would mean (in theory at least) I should only need to shot pean/polish, a set of stronger rod bolts and a balance.
The problem I have with forced induction (belive me I would love to do it) is one of installation. I'm in the U.K. which means Right Hand Drive which means the steering box is on the same side as the exhaust, I've thought about this and I'd need to bring the exhaust headers to the front of the engine to be able to mount a turbo. The next thing is I'd have to cut large holes into the front panel (where the radiator mounts) to feed the intercooler pipes through. I have seen this done on a E12 M535, it was a home brew affair and not very pretty at all but it did work. The other option would be to junk the steering box and fit a steering rack. This in itself is no mean feat as it would require trawling the breaker yards with tape measure in hand trying to find a rack the right width and then the right throw to ensure I had enough lock.
Non of this is impossible but at the end of the day (can't you tell I'm nearly 40) you have to sit down and work out what you really want and why do you want it. It's all well and good having 450 horse power (I'm not knocking those who have) but if you never really get to use it whats the point of having it? I'd be quite happy with 280-300 forced or natrually aspirated.
For the time being I'll leave it to brew and concentrate on the restoration.

Regards Gazzol
 
If your goal is about 300, then the only logical solution is to find an S38 or M88 somewhere. It will not be cheap, but it will get you there. It will still be cheaper than the mods to the M30, though.

Heck, if you can find a 3.8L S38, those had 340hp stock, without any sort of mods. They would be far easier to find for you, since we never got them here, over the pond. That motor was in the later (93+) E34 M5s. If going that route, I'd suggest getting a parts car or the front clip. Plus, if you did that, you could have all the other stff, like ABS, Traction control, you could make the car much more modern.
 
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