High Compression, High Torque Eta options?

[LateFan]

Just trying to learn more here from you guys. At some point down the road as your eta finally gets too tired, can you do a budget rebuild, leaving most of it stock, but bumping the compression? Low revving, keep the valve train the same, gain torque? New pistons would be an expensive part of it, but the rest is stock bearings, rings, etc. I've read a lot of 2.7 and 2.8 builds with new heads and cams and that would be fun, but I'm thinking of an affordable basic rebuild, with a bit of improvement. I may be missing some things here in that compression is only one component of better breathing and flow.

Then just yesterday I saw a link posted by digger, about Eta build options:
https://www.e30zone.net/e30wiki/inde...Building_a_2.7

"To build a 2.7 you have three options:

• 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 are not likely to get past 180 bhp but it will be pretty torquey."

And above that text was an interesting couple of sentences......

"Early Eta's 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!"

So according to that, the Eta had decent compression stock.

So what happened between Sept 1985 and Sept 1986? What was the stock compression in there? Ours is a 5/86 build so I'm curious what we have. It's out of town so I can't check those block numbers right now. A recent compression test showed 180 psi on all but two cylinders which were 160. Oil bumped those up to 190.

[downforce22]

That information is for european market cars without cats.

The us came with with m20b25 (i) 8.8:1 or m20b27 (eta) 9.0:1 and m20b27 (supereta) 8.5:1 (87-88 only)


Yours is likely the 9.0:1 low revving regular eta. If you can find the parts, I would consider the 2.7i swap as it involves replacing the head, intake, wiring harness and ecu and sensor but you can leave the engine in the car. The other option requires removal of the crank and bottom end. Not sure it is worth it for the increase in rpm. It turns out these parts are not as readily available as they once were so it could be an expensive upgrade to do any of these upgrades.

[Swanny]

Piston head shapes differed between the years and power remained somewhat consistent. The Super ETA is the most desired to do a 2.7i as is basically had an I head on it, just with a different cam and single valve springs. The ETA heads have smaller combustion domes which also gave the higher compression numbers. That’s why just throwing an I head on an ETA bottom doesn’t = better. In more instances its actually worse since you’ve almost always lowered compression. Its a great platform for forced injection.

[LateFan]

Oh, good chart. So I see, we got completely different pistons here. 9.0:1. Right Swanny, the i head on a stock eta bottom end doesn't really work well.

I realized I was asking a silly question if our etas already had the high compression pistons. Would the cost of new custom pistons make it stupid to not go with the 731 head and i intake etc per the E30zone article? That sounds like an interesting engine. Or are the Euro pistons out there for sale used? What were those like to drive?

On the other side of that coin, is it pointless to do a stock rebuild of a US eta?

My questions are because our '86es has 195k miles. Compression decent at mostly 180, with either some wear or stuck rings on two cylinders at 160. We were told the previous owner's daughter (10 years ago) ran it out of coolant, overheated it, cracked the head. A local mechanic put a new used head on it - I'm sure without rebuilding it. New radiator, water pump, TB, etc. We recently cleaned the block well so we could watch leaks, and there's a significant head gasket oil leak coming down the block at about cylinders 3-4 exhaust side. Minor annoyance for now.

My son is just starting out, without much money and with the crazy job situation out there he hates to spend anything. He really loves his eta and the easy torque and the decent mileage. He doesn't rod it around. The 5 speed swap has made a huge difference in enjoyment.

So he could go several years probably with it as is - just top off your oil. Or he could save up for some type of economical rebuild. Or the head gasket could go and force a decision. I don't think he wants me to build a sweet Alpina-style 2.8!

I was reading some old posts and BMW bulletins about buying the updated head bolts and careful replacing them one at a time and torquing down to spec. Don't know if that'll solve an oil leak.

[Swanny]

Stock rebuild the eta! It’s torquey and fun with the 5 speed swap. All the parts to tear down and start fresh are MUCH cheaper than hunting down the ever shrinking used parts market.

if there’s already a leak its safe to say the gasket integrity is compromised so new bolts most likely will not fix that. Its a great opportunity to tear down a relatively simple motor and get a lot of learning experience. If you rebuild now the future for forced air is as easy since you know the bottom end is solid and you can take your time gathering new parts.

[LateFan]

Interesting! If you were to go stock could you shave the head to bump up the compression (cam gear timing will be off?) or are the ports and breathing the weak points?

[ForcedFirebird]

The valves are smaller, cam is anemic, ports are smaller, runners are smaller, throttle body is smaller, the ECU is dumber lol.

The 200 head casting is the issue. There's only so much that can be done with it, without spending a fortune, anyways. The 731 head with +1mm valves and some light port work (from a qualified individual) will wake the eta right up.

Rama from RHD used a 731 head and flat pistons in his race car. ;)

[LateFan]

Interesting that anything on German ebay says "May not ship to the United States." Is that a grey market rule or similar? I remember that you can't order euro-only parts through BMW.

So the recipe for a "budget" 731 engine is:

731 Head
Stock valves?
Some port work
Which cam if it doesn't come with stock one?
Rockers if worn
325i intake and throttle body
325i AFM?
325i ECU? (and harness?)
Which injectors?
Which FPR?

Would you use the stock e pistons? (since the euro 731 eta had much higher compression) That would blow the budget, wouldn't it.

[LateFan]

Skool an old guy on the details of these heads please.

Eta 200 head.....


So, on the 731 head, those two small passages aren't there. Maybe I labeled them wrong above, because how do the block oil galleries match up with this head?



eta block...

[digger]

What is labelled as head bolts are oil return
What is labelled as oil return is oil feed
What is labelled as oil feed are steam holes

[LateFan]

Oh, thanks for that - I F'd up!

I'll attach a corrected pic for the record.
The steam holes are connected to what?..the coolant passages around the combustion chamber?
And then what happens in the 731 head with none to the block?

So there's oil pressure right near the side of the head where the leaking is.

[ForcedFirebird]

It's common for the m20 heads to leak on the passenger side, specially if using a crap gasket. The good gaskets separate the head bolt hole from the oil feed, the cheap ones combine the oil supply and head bolt into one boss.

Since the 731 head came off an m20 that used that same cooling system as the rest of the m20's, then you would suspect that the passages would line up. ;)

The recipe for a budget-friendly 731 head is to use 885 valves ground/faced down 1mm, seats/throats opened to match. This gets you larger valves without having to change the seats. Use the 885 spring package, and 885 cam (or aftermarket). We got an order in for this very service, and I will make sure it gets thrown on the flow bench for data.

I flowed a 731 a while back, but the work was done else where:




The first test was randomly selected stock 885 intake, second is an AMC, the next 2 are different ports on the 731 head. As you can see by the "percent of" the 731 potentially out flows the AMC 885 clone. I bet with a little flow testing, there's more in there (random dude port job on this head may have even hurt flow).

I have secretly been saving 200 heads from swaps/part-outs etc over the years. Also been saving all the valves from the 885's, and have drilled the 200 heads for 7 bearing cams before. Figure maybe some day I will get bored and make a budget-friendly performance 200 head, but there's just been so many projects, they just sit there for now, holding the shelf down.

[LateFan]

That's interesting info.

Yes on the leak - it was a small town shop with whatever parts they could get locally, is my guess. He's put a bit over 20,000 miles on it since.

I've done a little reading to not sound so stupid, since I've never seen one of these apart. It seems you sometimes get air pockets in high corners, so no coolant there and hot spots can form. These are the steam vents at the top of the block between cylinder castings? Other engines, like the LS, have steam tubes. I still don't understand what happens to these vents if blocked off in the 731 head.

I learned AMC in Spain has been building BMW head copies for a long time, and it appears you can get a 731 from them.
https://web.tecalliance.net/amc/en/p...77;pf:2;page:0

More M20 head info.....

http://e21.tricord.be/e21/about/m20/cylinder-heads.php

Pics are all gone here, but some discussion of the steam vents....
https://www.r3vlimited.com/board/for...o-the-m20-head

"The Comprehensive 731 Head + Eta Block Thread"... Info on components and details for that build.
https://www.r3vlimited.com/board/for...a-block-thread

Here digger has some comments about flow and 731 vs 885...
https://www.r3vlimited.com/board/for...ad-build/page2

So, my questions are mostly to push my learning curve up so I can converse about these engines. It may be way down the road before we need to do anything.

Seems to me that some options are:

1. Rebuild a stock eta with bearings and rings and valve job. Reliable, torquey, mileage, maybe boring - but much better after the manual swap!

2. Spend a bunch of money on a 731 with modifications and all related i intake components. Stock eta pistons?? I assume a big part of this is the Motronic 1.3 swap? I ran across a NOS 731, untouched.

3. Spend a bunch of money on an 885 with all related i intake components and custom pistons for good compression. Motronic 1.3 swap.

4. Spend a bunch of money on a Bimmerheads or MM head. Need all i intake components and expensive pistons.


If you design the engine for a lot of torque lower down, can you keep the valve train assembly of the eta or 731? Low drag, low rpm as a reliable daily that breathes? Or does the 731 have double valve springs and more cam journals?

[LateFan]

As you can see by the "percent of" the 731 potentially out flows the AMC 885 clone.

Am I reading that right then, that the AMC clone is not outflowing a stock 885, except at very low lift?

[ForcedFirebird]

Yes, the AMC heads have been available for some time. I actually have two here at the shop. They flow horribly, though (test #2 in my post above).

If you are concerned about the steam holes, they can easily be drilled. There are some 24v heads that do/don't have steam holes as well, so who knows (some are even same casting #). But, honestly, if BMW was OK with leaving them out, not sure why they would need to be there if you slide a b27 rotating assembly under it.

You don't have to modify the 731 head at all, really, it's still an upgrade over the 200....

https://www.speed-talk.com/forum/viewtopic.php?p=83856

Just look at the flow coefficient (Alpha-K) of the 731 vs 200. The 731 is on par with ls1 for flow vs valve diameter up until higher lifts. Then the 731 smokes the ls (and the 885 dominates both). So, just putting the 731 head is an upgrade, specially if you paired it with a good cam. This way, the build can be done in stages. First swap the 731 and leave everything else alone. Then you could piece-meal the rest and enjoy the upgrades as they happen and budget allows.