From E30ZONE.
The 2.7 ETA ENGINE
These arrived in 1983 to power the 525e and 325e (not UK) although the US had them in 1982 for the 528e.
Eta is the Greek symbol for efficiency and it’s a low revving engine rich in low speed torque but low on power. It doesn’t rev much past 4500 and uses an 84mm bore and an 81mm stroke crank to give 2693cc. The crank is made from spheroidal grey cast iron and the connecting rods are the same 130mm end-to-end items as the 320, 320i, 520, 520i and 323i.
The cylinder head is derived from the E21 320/323i and E12 520 small port head known as the 200 from its casting number. It has a special four bearing cam designed for low speed torque and it is useless. Throw the entire Eta head in a skip, it is of no use at all for a performance application. The same goes for the inlet manifold and throttle body, injectors, ECU and AFM. You just need are the block, crank, rods and possibly pistons (more of pistons later).
Early Etas up until Sept. 1985 had a compression ratio of 11:1. The blocks of these engines is stamped 27 6KA. From 9/86 the compression ratio was dropped to 10.2:1 and these engines are stamped 27 6EB. These numbers are stamped above the main engine number. BMW dropped the CR due to detonation issues on 95RON - no knock sensors back then!
To build a 2.7 you have three options:
1.) Retain the standard Eta bottom end untouched and fit a head from an E30 320i or 323i. This is the ‘731’ head which has much bigger inlet ports than the 200/Eta. It has the same sized valves as the Eta which are 40mm inlet and 34mm exhaust. The combustion chambers are identical the Eta 200 head so by fitting a 731 head, you will retain the Eta compression ratio.
From here you can open up the tops of the inlet ports using a 325i inlet manifold gasket as a template to accept a 325i inlet manifold, or you can use the 320i/323i manifold with or without a 325i throttle body. It will be a bit flat at the top end without one though. You’re not likely to get past 180 bhp but it will be pretty torquey.
2.) Use the standard Eta bottom end with a 325i head, the ‘885’ unit. The 325i has strange pistons with a deep dish but a raised section at the front of the combustion chamber to accommodate it. Despite what ‘experts’ say, the 325i head will NOT give a horribly low compression ratio. The difference in chamber volume is quite small. The 325i head has a chamber volume of @ 42.5cc and the 731 and 200 heads have a chamber volume of @38cc. It all depends on whether the head has been skimmed. But 4.5cc difference isn’t much at all and much of the increased size of the 325i chamber is made up for by the bigger valves and surrounding raised areas around them. 325i valve sizes are 42mm inlet and 36mm exhaust.
A lot has been said about Eta pistons being incompatible with the 325i head. Most of it is nonsense as this has been done many times and it works okay. Of course it won’t give results quite as good as using the matching 325i pistons but it still has a good compression ratio which is most of the battle won.
Again, depending on which gasket is used and whether the head has been skimmed, using a 325i head on a later 10.2:1 Eta short engine will give a CR of 9.4:1 which is plenty. If you can even find an early 11:1 Eta engine, the CR would be 10:1 which is even better.
Because you are using a 325i head, just bolt a 325i inlet manifold straight on.
I converted a 1986 525e Auto to this spec and it was very lively - 0-80 in 21 seconds which is just 1 second slower than an auto 528i which has both 184 bhp and a lower final drive. Then I pulled the engine out and it went into an E30 Touring
3.) Using 325i pistons with a 325i head. This is the best way of doing it but requires a complete engine strip down.
Doing this, the pistons of course match the head. Compression ratios will be 9.4:1 using post 9/87 (plastic bumper) 8.8:1 CR pistons and 10.4:1 using the early pre 9/87 (steel bumper) 9.75:1 CR pistons. Why the higher CR in a 2.7? Because it has an extra 49.8cc of fuel/air mixture to compress.
To keep the con rod angle correct, BMW made the 325i conrods 5mm longer. So if you fitted a 2.7 81mm crank into a 325i block (basically the same as the Eta block), the pistons would stick out of the top of the bore by 3mm. To get around this, you need to use 130mm Eta/2 litre rods. Then the pistons don’t come to the top of the bore - about 2mm short in fact. It’s no good guessing how much to take off the block face because early Eta blocks were very slightly taller (we’re talking half a mm maybe) so you need to fit a 325i piston and 130mm rod to the Eta crank and block, take it to a machine shop and get it measured up properly to get an exact figure. It won’t cost any more, and it’ll be bang on.
1.5 - 2mm is just a ballpark figure.
The next problem is that with @2mm off the block, the crank and the cam are now 2mm closer together which will mean the cam timing will be wrong. For this you will need a vernier (adjustable) cam pulley to adjust the cam timing, particularly if you’re using an aftermarket cam.
Early 325i pistons (9.75:1) have deep skirts and need 12mm machining off the skirts because they’ll foul the Eta’s crank webs. Later pistons clear fine though.
Other stuff:
ECU etc: You will need a complete 325i Motronic set up. ECU, inlet manifold with injectors, air flow meter. Good news if you already have a 325i.
The ECU will need remapping. A 2.7 engine will have faster piston speed meaning it’s gets to the top of the bore and point of ignition faster - over a 325i, the piston has an extra 6mm to travel in the same amount of time (one crank revolution), requiring a change in ignition timing. It’ll need more fuelling too.
Head Gasket: Eta and 325i are the same. You can buy standard or thicker gaskets which will lower the compression.
Radiator: Is the standard 325i radiator okay? Yes, it’s fine.
Spark Plugs: Which spark plugs? Bosch W7DCR or it’s NGK equivalent. Plastic bumper 325i’s use W8’s which aren’t right for a 2.7.
2.7’s are higher compression engines and higher compression ratios equal more heat and you need a plug to dissipate the heat faster. The W7 is known to work.
Crank: UK 525e’s were autos and you need to fit a spigot bearing in the flywheel end of the crank.
How to tell an Eta crank from a steel diesel crank? Eta cranks will have either GG or GGG cast into a web meaning cast iron. Both will have ‘Hub.81’ cast in denoting the stroke. 81 is Eta/diesel, 75 is 325i, 66 is 2 litre and 76.8 is 323i.
Cylinder Heads: 731 or Eta heads don’t work with 325i pistons because the raised crown would smack into the head and lock it up.
Sump: The E30 sump fits straight to an Eta block but you need 25 longer bolts because an alloy sump is thicker than a pressed steel one. The BMW part number is 07119915031.
Eta steel sumps will not fit an E30. It’s too long and will foul the rack.
TDC Sensor: Eta blocks have the two threaded holes to take the later Motronic front TDC sensor.
