So for clarity's sake, I just happened to think while writing on another thread that I wasn't sure if my car had the B34 or B35 motor. According to the online vin decoder, I have the B34, meaning that it is not the 10:1 ratio that the P/O told me, correct? (I took his word for it since he claimed to be a BMW mechanic and hadn't thought anything on it till now.)

Along with that, he told me that since it was the high compression engine, it required premium gasoline. So if I have the B34, and if the B34 isn't 10:1, do I need to run premium? The car runs fine as-is, but you shouldn't run premium in an engine that doesn't need it.

Thanks,
Alex

I think one way to tell is if you have an 052 ECU that came with the M30B34. There might be an easier way but I'm not sure what that is besides VIN decoding. It doesn't hurt to run premium in a lower compression engine as far as I know. I run 91 octane fuel most of the time and my 85' M30B34 only has 8.0:1 compression. The M6 always gets premium. It's not good to run low octane fuel in a high compression engine though. You could suffer detonation issues. Your 1989 635 should have the M30B35 with 9.0:1 compression, 208hp/225ftq.

Dwight

the '89 is my E30. The 6er is an '85.

Gotcha. Then you have a B34 head like mine unless your mechanic swapped it out.

Dwight

Cool. Thanks!

But technically, the B34 doesn't require high octane fuel does it? I'm honestly a bit sketchy of running high octane in an engine that wasn't designed for it. Longer burn=force down on the pison as it is being forced back up by the explosion from the next cylinder in the firing order. This just seems bad for the engine as far as I can tell.

Alex

no it doesn't i got the manual for an american 85 635 and it recommended nothin lower than 87 which is 'normal'. whos the P/O and what does that stand for?

PO = Previous Owner

Nope, the engine model has nothing to do with the compression ratio. Does the VIN return "ECE" under Model? If so, it is the Euro and has the 10:1 compression M30B34 engine. The U.S. version of the M30B34 got different pistons for the lower compression.

Here is an engine spec chart, the "635 CSi Kat" is the U.S. version:

http://www.bmwcoupe.nu/e24/engines.html

Using a higher then required octane won't hurt an engine, but it is a complete waste of money. In no way will it increase power, as some think.

Where are you guys getting the M30 ever having a 10:1 CR? The highest stock CR M30 I've ever heard of is the Euro M30 B35, which had a CR of 9.3:1. Heck, even the M90 only had a CR of 9:1, and that was the hottest factory M30. Until knock sensors came along, a CR of 10:1 was reserved for M-engines only, as far as I am aware.

Until the M30B35, all US M30s wre 8:1. (well... ok, the US 2800CS-es had a CR of 9:1, but that ended in 1969 or so)

Yeah, after looking around, I get mixed CR numbers that float between 9:1 and 10:1. Regardless of actual CR, ECE B34s had higher CR than US B34s, is what I get from here, and I should follow the advice and run premium since its the higher CR engine, right?


Alex

A 9:1 CR would really only need mid-grade. Higher than that, yeah, premium would be needed. Honestly, though, it is a matter of perhaps 200$ a year between premium and base, and the miniscule timing difference between mid and premium in a working engine doesn't have a negative effect. Like CW6er said, it won't hurt, but it won't help, aside from keeping the chamber a bit cleaner.

Where are you guys getting the M30 ever having a 10:1 CR? The highest stock CR M30 I've ever heard of is the Euro M30 B35, which had a CR of 9.3:1. Heck, even the M90 only had a CR of 9:1, and that was the hottest factory M30. Until knock sensors came along, a CR of 10:1 was reserved for M-engines only, as far as I am aware.

Until the M30B35, all US M30s wre 8:1. (well... ok, the US 2800CS-es had a CR of 9:1, but that ended in 1969 or so)

Check out this chart. I can't vouch for it's accuracy, but I've seen the 10:1 mentioned elsewhere also. It shows the Euro 633 (M30) from '70 to '82 and the Euro 635 (M90) from'79 to '81 as both having the 9.3:1 compression with the DME I. But It shows that with the introduction of the Euro 635 M30 with the DME II in '82 as a 10:1 compression until mid '87 and the M30B35 at 9.2:1
http://www.bmwcoupe.nu/e24/engines.html

Jeremy Walton in his book "6 Series Enthusiast's Companion" also gives the Euro as having a 10:1 ratio also.

Also check this page out:
http://www.bmwcoupe.nu/e24/82/hist82.html

The 10:1 is also mentioned here, under the heading "1985-87: 635CSi, L6":
http://www.europeancarweb.com/tech/0401ec_bmw_e24_6_series/index.html

Does anyone have the specific numbers other than bore and stroke? It would be easier to figure out what the real numbers are if we knew specific volumes within the cylinders of these engines.

CR = Maximum Volume of the Cylinder / Clearance Volume of the Cylinder

The max volume of the cylinder is the displacement volume + clearance volume

Displacement = PI/4(bore)^2(stroke); For the DME II '82 engine: [(3.14/4)*(92/10)^2]*86/10 = 571.7 cm^3 (all 6 cylinders = 3430.2 cm^3)

So basically, if we knew the clearance volume, we could check to see if it is correct. Im sure i could take the time to figure out the theoretical clearance volume for 10:1 to be possible, but i dont feel like it haha.

-Jeff

Ive got a rough estimate for that engine here:

If the clearance volume (n) is approximately 63.5 cm^3 (go figure haha), then the compression ratio will be damn close to 10:1 even.

My math:

CR = Max volume / clearance volume; [571.7 + 63.5] / 63.5 = 10.003

-Jeff

hmm...yes...I have seen those formulas before in my intro engineering classes. I remember everyone else sleeping through that portion of the class as the professor was explaining the volume ratios for BDC and TDC to get the CR. Strange though as most of us were mech engr.

math checks out, though I thought he told us something else in class for some reason. Anyway, the math makes sense...now all we need is the actual clearance values. If we knew the shape of the piston head, we could probably use geometry to find out how close the piston gets to the valves at TDC, of course, you'd also need to know the information about the inside of the head, etc. But seeing as that info will be just as hard to find, we might as well not worry about it.

Alex

pretty much...ill ask over on roadfly and see if anyone knows.

-Jeff

The clearance volume of the heads is a known factor. The problem is the minute differences in the piston's shapes.

http://www.m535i.org/officers/ra/article.html#motor

This would seem to back up the 10:1 claim, but I've also read about 9.3:1 in other places for the Euro B34s. So, I don't know what to think. Anybody got the E24 Euro manual?

My friend has an 84 euro 5spd...im going to get in touch with him

-Jeff

manual as in the book that comes with the car? Because I have that in german and I can fetch it this evening.

Alex

manual as in the book that comes with the car? Because I have that in german and I can fetch it this evening.

Alex

Should be good enough to read numbers.

Doesn't the head and block have identifying numbers? Alex, what #ecu do you have? Mines an 061. I had been talking w/ Mark D'Sylva up in Toronto about a possible chip. He said that my motor and ecu was tricky to chip as it was a 9.3:1cr and Premium was required all the time. I've been running nothing but Premium so far with good results.

PO = Previous Owner

Nope, the engine model has nothing to do with the compression ratio. Does the VIN return "ECE" under Model? If so, it is the Euro and has the 10:1 compression M30B34 engine. The U.S. version of the M30B34 got different pistons for the lower compression.

Here is an engine spec chart, the "635 CSi Kat" is the U.S. version:

http://www.bmwcoupe.nu/e24/engines.html

Using a higher then required octane won't hurt an engine, but it is a complete waste of money. In no way will it increase power, as some think.

it could still say ECE and have the 8:1 CR engine. go to this webite...seriously.
http://www.bigcoupe.com/buying/vinmumbers.html
if it says ECE and it doesn't say kat then u got the 10:1 CR. if it says ECE and kat ( like me :( ) then 8:1 CR

Don't know about my ecu #...lol...just literally got back in from going to get some dinner and fetch the manual from the glovebox.

*commence decoding of german text*

If it goes by whether or not its listed as kat, I can assure you that mine didn't come with a kat when it was manufactured. It wouldn't pass emissions when I first got it because the PO had unplugged the single wire O2 sensor that linked to the computer. IIRC, these normally have 2 or 3 wire sensors, right?

I'll keep decoding, mostly in-between study breaks...hahaha


Alex

Eureka! -
From the Technische Daten in the vehicle's manual:

Verdichtungsverhältnis (CR) = 10,0
Hub/Bohrung (stroke/bore) = 86/92 mm


Above these two are two listings for engine displacements in cc's. BabelFish shot out something weird for the first one, so I'm throwing it out for you German readers: (Babel translation in parenthesis)


Hubraum nach Steuerformel (capacity according to steering formula) = 3406
effektiv (effectively) = 3430


The 3430 corresponds to the stroke/bore listing on down the page, but not sure on the 3406cc number.


Ideas?


Alex

after reading all these links posted above it seems pretty safe to say the '85 euro is 10:1cr. I like it!

Our 9/82 built 635 appears to have the hi comp M30 (vin 8171276). The owner's manual in the car lists the 635 compression ratio as 10.0 and the M635 as 10.5.

Other specs shown in the manual for the 635:

218 Hp @ 5200
228.6 Nm @ 4000
6100 rpm limit
86/92 Stroke/Bore

Fuel consumption with 5 speed od
40.3 mi/gal @ 56 mi/hr :eek:
32.1 mi/gal @ 75 mi/hr
18.5 mi/gal in urban driving

We seem to average 26-28 mpg in the car. I've never held 56 mph for long so don't know about 40 mpg. :dunno There must be some light footed testers over in Germany.

Joe M in WV

nach Steuerformel (capacity according to steering formula) = 3406

Our manual is in English. Under "Displacement" there are also two numbers:

fiscal - 3406 cm3
effective - 3430 cm3

Perhaps the lower number has something to do with taxes?

Joe M in WV

Fuel consumption with 5 speed od
40.3 mi/gal @ 56 mi/hr :eek:
32.1 mi/gal @ 75 mi/hr
18.5 mi/gal in urban driving

Joe M in WV

Hmmm...mine doesn't show that. Best it has is 32.6MPG @ 56mph, but it has the auto trans getting 36MPG at the same speed rating.

Also shows the 218HP rating...must've gotten new fuel economy testers between 82 and 84


Alex