m10 pistons

[Mk2zetec]

[Mk2zetec]

evening all

im trying to work out compression ratio/spec of the engine 

just posted a pic of the piston used in the rebuild of my M10 with m12 head. 

spec of the engine unknown but its putting out circa 100bhp on twin 40's 

looking at ti pistons they have a raised section to increase compression ratio so wondering if these are standard 2l or low compression pistons. any ideas?

regards

rich

 

[Mark92131]

Rich:

 

2 liter flat-top piston, universal for all heads, 8.5/1 compression ratio I believe, maybe a little more or less depending on the head gasket or head thickness.

 

Mark92131

[Mk2zetec]

13 minutes ago, Mark92131 said:

Rich:

 

2 liter flat-top piston, universal for all heads, 8.5/1 compression ratio I believe, maybe a little more or less depending on the head gasket or head thickness.

 

Mark92131

Thanks mark. Do you know if they are interchangeable for high compression ones? I suppose the real question is what’s the best route to extract more power from the standard 2l engine?! 

[Son of Marty]

As Mark said low c/r pistons will work with any head, if you go with domed higher c/r pistons the must match the head type your using IE 121 head uses bathtub dome pistons the e12 head uses grand piano dome and the e21 hear runs a kinda reverse grand piano dome you can't mismatch these heads and pistons with out heavy modifications to the head and/or pistons.

[tech71]

Cant quite tell... does it show 89.27 for size ? or is that just wishful thinking and it actually says 88.97?

[Mk2zetec]

8 hours ago, tech71 said:

Cant quite tell... does it show 89.27 for size ? or is that just wishful thinking and it actually says 88.97?

I think it’s 89 but difficult to tell from the pics. It’s an e12 head so need the grand piano pistons. If it’s 89.27 does that mean it’s at it’s max overbore?

8 hours ago, tech71 said:

Cant quite tell... does it show 89.27 for size ? or is that just wishful thinking and it actually says 88.9

[Conserv]

15 hours ago, Mark92131 said:

Rich:

 

2 liter flat-top piston, universal for all heads, 8.5/1 compression ratio I believe, maybe a little more or less depending on the head gasket or head thickness.

 

Mark92131

I suspect even less. Flat-top pistons with an E21 head get you approximately 8.3:1 (alternately, but also within the BMW literature, 8.1:1). With an E12 head, you get 8.3:1 with low piano tops (first photo), so I’d guess that flat tops are less than 8.3:1 with an E12 head. And I’ve recently seen even lower piano tops with a 8.1:1 ratio (second photo). I don’t know what the base bathtub top piston is for a 121 head, so I can’t comment.

 

Flat tops are definitely sub-optimal for a higher performance engine. If you’ve got an E12 head, look for the 9.0:1 or 9.5:1 piano tops.

 

Regards,

 

Steve

 

[Conserv]

Here’s a partial line-up of pistons designed to work with the E12 head, which appears to be what you have (more specifically, the distinctive “piano top” was designed to work with the E12’s combustion chamber and valves):

 

1. 8.1:1 (first photo)

 

2. 8.3:1 (second photo)

 

3. 9.0:1 (not shown)

 

4. 9.5:1 (third photo)

 

The three examples shown here appear to be factory OEM replacement pistons by Mahle. If you showed me additional ratios, e.g., 8.5 or 8.6, I would not be shocked... 
 

(I wonder if the mere existence of both 8.1 and 8.3 versions is source of the “confusion” I see in BMW literature. Could that 8.1 version — stamped “USA” — have been introduced in 1975, and continued on the California version of the 1976?)

 

 

 

[TobyB]

In the C/R world, if you want to be more than .5 accurate, ya really gotta measure volumes,

at this advanced age.

 

That said, there are several flattop varieties- the 320i got an even lower crown height (by something like a mm)

than the 121 flattop.

 

So yeah, for performance that's not artificially aspirated, it's DomeTime.  At least 5mm

 

t

 

[Mike Self]

5 hours ago, Conserv said:

I don’t know what the base bathtub top piston is for a 121 head, so I can’t comment.

Early US 2002s ('68 and '69) had 8.3:1 pistons; later US '02s were 8.0, IIRC--at least that's what my owners manuals say.

 

mike

[2002iii]

On 5/30/2021 at 6:14 AM, Conserv said:

Here’s a partial line-up of pistons designed to work with the E12 head (more specifically, the distinctive “piano top” was designed to work with the E12’s combustion chamber and valves):

 

1. 8.1:1 (first photo)

 

2. 8.3:1 (second photo)

 

3. 9.0:1 (not shown)

 

4. 9.5:1 (third photo)

 

The three examples shown here appear to be factory OEM replacement pistons by Mahle. If you showed me additional ratios, e.g., 8.5 or 8.6, I would not be shocked... 
 

(I wonder if the mere existence of both 8.1 and 8.3 versions is source of the “confusion” I see in BMW literature. Could that 8.1 version — stamped “USA” — have been introduced in 1975, and continued on the California version of the 1976?)

 

 

 

 

For a more complete listing of 2002 m10 pistons originally available from BMW:

 

1. 6.9:1 dished top 2002 turbo

 

2. 8.0:1 flat top 1968-71 2002, California 2002

 

3. 8.1:1  piano top federal 1975-76 2002?

 

4. 8.3:1 piano top federal 1972-74 2002

 

5. 9.0:1 piano top USA 2002 tii

 

6. 9.3:1 piano top 2002 ti

 

7. 9.5:1 piano top Euro 2002 tii

 

8. 10.0:1 bathtub top early Euro 2002 tii

 

For the later e21 2.0 from 1977-79:

 

1. 8.0:1 USA flat top e21 320i

 

2. 8.3:1 Euro dome top e21 320i

 

Then you've got a lot of different compression ratio aftermarket pistons, from 8.0:1 to 10.0:1.

 

Also multiple companies can make custom pistons in almost any ratio you want from 7.0:1 for high boost to 12.0:1 for your crazy itb race car.

Edited May 31, 2021 by 2002iii
Typo

[visionaut]

3 hours ago, 2002iii said:

10.0:1 piano top early Euro 2002 tii

These domes ain't piano shaped - they’re more lozenge-like… ?

 

[Mark92131]

1 hour ago, visionaut said:

These domes ain't piano shaped - they’re more lozenge-like… ?

 

Or "bathtub-like" for the 121 and 121 Ti heads.

 

Mark92131

 

[Conserv]

4 hours ago, visionaut said:

These domes ain't piano shaped - they’re more lozenge-like… ?

 

+1

 

The 121 heads take a bathtub-style piston while E12’s take piano top pistons. The third style of 2-liter head is the E21. But I don’t know how to describe E21 pistons with raised crowns. Below, a custom E21 piston with a compression target of 9.5:1 (in my ‘76).

 

Best regards,

 

Steve