Fellas,

Does this look like a piston that would produce more than 9.5 to 1 compression on a 90mm bore (does bore size matter?)?

A guy has an "unassembled" engine for sale, these pistons along with it. Are the 8:1 slugs in the stock 2.0 flat or dished?

Don't know snot about the elements that affect these things, and I don't remember the content of all those cool Hot Rod magazine articles I read on the subject many moon back....

http://images.craigslist.org/011500010200011614200803228606e5b52651c6aec000bfcd .jpg

Thanks Gents.

That's a piston from a 121 casting head, like a 2002. Looks to be high comp, though, but you would have to use a less than ideal head.
--dennis

stock is raised but nothing signifigant

That's a piston from a 121 casting head, like a 2002. Looks to be high comp, though, but you would have to use a less than ideal head.
--dennis

Good sir,

Can you define "high comp", 10:1 or greater or less than 10:1? I think the guy's build out included a 292 cam. My recollection is that more duration offset low speed cylinder pressure/gas problems, such that on pump gas you might be okay at 10:1, but no low end at all.

The other recollection I have is that more bore "helped" to increase the cc volume, the effect being less compression (fractionally).

The guy says it should spec out at 9.5 to 1

Indeed, I believe it is a 121 casting on the head (in his write up, I'd have to double check, but it was for a 2002).

Other thoughts/info/I'm an idiot on what I believe about compression/gas/cam specs?

Thanks guys.

ask for the compression height and/or height of the raised section of the crown, will tell you what you want to know.

The ONLY way to know for sure what that engines compression ratio will be is to assemble it, measure the piston dome displacement, the combustion chamber volume and do the math with the actual numbers.

The compression ratio for a specific piston configuration varies between engines-due to assembly variances, machining variances and parts dimension tolerances. A piston that makes 9.5-1 compression in one engine might make slightly more or less in another.

I put some numbers into an engine calculator and came up with this info:

The extra 1mm bore over stock will increase your compression ratio about .2-that is a 9.5-1 set up at 89mm bore will be about 9.7-1 with a 90mm bore with all else equal-this is due to the slightly larger swept volume.

Now for some speculation, I believe that piston dome in stock configuration is supposed to give about 9.5-1 compression at 89mm bore x 80mm stroke (2.0L) with the 121 casting head so with the extra 1mm bore it will likely be closer to 9.7-1. This is assuming the piston is set up flush with the block deck and a .040" compressed gasket thickness. That is NOT a stock piston and the dome may vary from stock so contact Ross Pistons with the part number and ask what the as designed piston dome volume is-that's a good start.

I don't know what a stock 121 casting combustion chamber measures as I have never had one in my hands. I would guess it's less than the 58cc my e21 casting head chambers due to the more closed chamber design-maybe 55cc or so-that does NOT take into account any machine work that may have been done to the head this engine was intended to run.

ken

I'll do some web searches, or some hit some 2002 sites, try and see what the cc volume would be.

I was thinking the 121 casting was the 8:1 2.0 motor, but it sounds like this will not be the case. If the stock configuration was a 9 or 9.5:1 that piston will be at least a point higher especially if the 121 head had a smaller cc, but, I'll see what I can find out.

With only 91 octane available here in the Old Pueblo, even 9.7:1 would probably mean less than optimal timing to keep it from pinging....

I was thinking the 121 casting was the 8:1 2.0 motor.

Most engines with a 121 head will have the flat pistons and 8 point something compression. (The same is true for both e12 and e21 castings as well) There is however a stock piston that Ken is talking about that looks like what you have and is about 9.5:1. (I know it was used in the early 2002tii's, not sure about any others)

Gentlemen,

I found the following:

http://www.e21bmw.net/faq/new.php - posted by Dave McIntyre, is this the guy with the race e21?
121
58cc 2002, 1972 and earlier

E12
61cc 2002, 1973 to 1975

E21
60cc 2002 & 320i, 76-79

18i
63cc 320i, 318i, 1980 on


http://www.bimmers.com/02/faq/engine.html#121_head
121 Cylinder head:

8.5:1 CR U.S. "Spec" piston on 1968-1972 2002's
42.3 mm pin-to-deck, flat top

9.0:1 CR U.S. "Spec" piston used on '72 US-model 2002tii's

9.3:1 CR European 2002TI (twin-carb model)
42.1 pin-to-deck, 5.2mm dome

10:1 CR Early euro-spec 2002tii
42.5 mm pin-to-deck, 4.4 mm dome


E12 Head:

6.9:1 CR Factory "Turbo" pistons (used on 1974 2002 Turbo)
42.0 mm pin-to-deck, 3.7mm dish

8.1:1 CR flat-topped piston (much like the 8.5:1 CR used on the 121 heads) used on '75 US-model 2002's, also on most US-model 3-liter cars of the era. (keep in mind that the 2-liter 4-cylinder with the E12 head and 3-liter 6-cylinder cars can interchange pistons)

8.3:1 CR U.S. "Spec" piston on mid 1972-1974 cars

9.0:1 CR U.S. tii pistons

9.5:1 CR European tii pistons


E21 2.0 Head:

8.3:1 CR flat-topped pistons (used on 49-state '76 2002's, US 2-liter 320i's)

9.3:1 CR domed pistons (used on european 4-cylinder 320i's)

Note: compression ratios given are with a stock head, and zero deck height.


Please note that Mahle is not the only OE piston manufacturer, Kolbenschmidt and Nural also supplied BMW with pistons. However, the majority of U.S. parts distributors usually supply Mahle products.


The pistons here are all high-silicon cast units. Mahle (one of the OE piston manufacturers for BMW) also offers a forged round top aluminum racing piston that yields a 12.5:1 compression ratio. These pistons, however, require clearance machining off ALL combustion chamber configurations to work. It should also be mentioned that along with these pistons available from Mahle, many American piston manufactures will make custom forged aluminum racing pistons for the 2002 with any compression ratio. Vendors such as Venolia, Arias, Ross and Wieseco have all produced excellent quality units at surprisingly reasonable cost. In addition, Cosworth also makes some excellent (though expensive) pistons for four cylinder BMW's.

Given this, it looks like it's possible for that piston to be 9.5 in the 121 head.

I'm going to take a look at this a bit more, and additional comments are still welcomed!

Thanks gentlemen.

I'm going to save that info to a document for future reference.

I'm surprised the 121 head is 58cc as cast, the pictures of them I have seen look smaller than that.

My e21 casting has been milled very slightly twice and is 58cc.

The Euro 2.0L e21 pistons have a 9cc dome-these are what I run in my '82.

http://img227.imageshack.us/img227/264/bmwpistondomehx6.th.jpg (http://img227.imageshack.us/my.php?image=bmwpistondomehx6.jpg)

ken