M20 Cylinder HeadsThis is an article that will cover the various types of cylinder heads avalable on the small six engine. This will be of benefit to owners who want to modify their small six (E30 or E21) or simply background reading for the technically inclined.
When the small six was launched in 1977 its cylinder head came with small combustion chambers. This followed BMW's philosophy of small chambers for high thermodynamic efficiency. The small chambers allowed shorter flame paths and more resistance to detonation and pinking (or pinging to all you US folk). Now I'll vouch for that - when I first bought my 1982 323i I ran the ignition 22 degrees advanced above the recommended setting, by mistake, for one day. All that was there was a slight metallic rustling noise at low revs/high loads. Now I would certainly not recommend over-advancing the ignition to such a magnitude as, on efficient engines this is detrimental to performance and will wreck the crank, bearings, etc, after prolonged use but it does go to show.
In general design, the light alloy head followed normal BMW practice i.e. a cross flow head with the valves in V formation operated by a single, central camshaft pushing up on short rockers (that look like snails!). The same head casting was used for all initial models including, the 320/6, 520/6 (E12) and the 323i. The combustion chambers have a volume of 37cc. This gave the 2.0 litre cars a compression ratio of 9.2:1 and the 323i one of 9.5:1.
This compactness was achieved by adopting a valve angle of 2 x 22 degrees as opposed to the earlier big six/four cylinder 2 x 26 degrees. In fact the small six's combustion chamber was more of a wedge-shape than the classic part hemisphere of past BMW's. A lot of stiffness was provided at the cam by running it with seven bearings (the big six only runs in four). This allowed the high valve lift of 10mm and squarer profiled cams (quite a common trend in engine design during late 70's/early 80's) i.e. steeper camshafts with higher valve accelerations made it possible to reduce the valve opening duration now being 260 deg compared with 264 deg and 272 deg for other BMW engines. This meant that high rev volumetric efficiency was not effected by the reduced duration with the added benfit of slightly better flexibility (claimed). Now I write a 'cynical' 'claimed' because this flexibility was not reflected in road test comparing the 320/4 and 320/6. To be fair the 320/6 was marginally heavier and pulling a taller rear end ratio. The inlet ports on this early head are small and circular and of about 32mm diameter. The casting number to look for is 1264200 on the inlet side. The 323i used tuned induction pipes of 454mm length, all drawing from a common small plenum. According to BMW the plenum chamber volume is kept small to improve throttle response.
The pistons were flat topped or slightly domed in the case of the 320/6. At top dead centre the crown of the piston is below the level of the cylinder block (how much so I forget, but I can look it up) which is not good in terms of squish. A piston to head clearance of 1mm or just under is desirable otherwise the squish zones become flame front quench zones and tend to extinguish the flame front. This has been adressed in later versions of this engine such as the 325i, 325e, E30 323i and E30 320i/6. This is a shame because the combustion chamber shape has a nice squish zone opposite the spark plug. When compared to other engines such as the Alfa Romeo 75/Milano V-6 the BMW chamber is a much more thermodynamically optimised affair.
The E30 320i/6 and 323i had a redesign for their cylinder heads. Again the 320i/6 and the 323i used the same head casting (1264731) with exactly the same combustion chamber shape but this time the inlet ports were enlargened to a 36.5 mm diameter. The compression ratios on both models was now 9.8:1. I have reason to believe that the cam timing duration was lowered but figures I do have conflict (I intend to get hold of a cam and measure it myself soon). I know the relatively large 110 deg Lobe Centerline Angle was retained. None of the above BMWs were officially sold in the USA. However the small six based 'Eta' engined was. This (as fitted to the 325e plus 528e-US and 525e-Europe) used the earler small port head casting (1264200) and a compression ratio of 8.0:1 in the USA (due to their lower octane fuels) and 11:1 in the UK. Three of the cam bearings were made redundant to reduce friction. The cams Lobe Centerline Angle was reduced to just 102deg with around a 236degree duration (don't quote me on that!) and low overlap. The inlet tracts of this motor are of considerable length-553mm.
However the milestone of the small- six engines was yet to come: the 325i unit (325is in the states). This head (casting number 1705885) has large U-shaped ports (about 37mm by 37mm) and a slightly larger combustion chamber volume of 40cc but follows a new combustion chamber philosophy. Several studies have shown that an open chamber design (basically spherical in shape) has a faster burn rate (desirable in terms of avoiding detonation and good for efficiency) than either a classical hemi chamber (such as that used on tha Alfa V-6) or a Heron chamber(bowl in piston such as that used in VW Golfs/Rabbits). On the 1705885 head there is a dish in the piston within a dome! This is hard to explain unless it has been seen. Basically BMW engineers probably reasoned out that it would not be possible to move the spark plugs to a more central position (desirable in terms on thermodynamic efficiency) and keep the cylinder configuration the same so they moved the bulk of the chamber volume around the spark plug (which consisted of the offset dish in the piston and the hemi in the head) all the rest became squish zones. This is an outstandingly efficient package. The stroke was made shorter than the old 323i while the con rods were lengthened to increase rod to stroke ratio to reduce friction/increase top end poke. The compression ratio was 9.7:1 and 8.8:1 for the US market/later European spec. The lobe centerline angle of the camshaft was reduced again to 108 degrees but so was the duration. The late Eta used the large port "885 head (often called the 'supereta'). I have no knowledge on what kind of piston or cam timing that has been used.
Early M20 head
This article courtesy of R K PAUL (1982 Kaschmir gold E21 323i)
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