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Technical_   Engine_  M20 E to I 2.8 Liter Engine Conversion_  FAQ

M20 E to I 2.8 Liter Engine Conversion
Fred Kim

I.   Frequently Asked Questions

1.) What is the E to I Conversion?
It's the concept of making a hybrid high horsepower, high torque motor with a flat powerband using the top half of a 325i and the bottom block of a 325e. The idea is to obtain the high end power of a 325i and the low end grunt of a 325e using mostly factory parts.

2.) What parts are needed?
For a complete and detailed list, see page 2. But generally speaking, for an 84-87 325e you need the entire fuel injection system from a 325i as well as the intake manifold, throttle body, and complete cylinder head (with cam, valves, and dual valve springs. However, the stock 325e pistons will not work with the 325i head because of the different design; the 325i has a domed cylinder chamber to match the domed pistons while the eta has a flat cylinder chamber to match the flat pistons. Throw the flat 325e pistons into a 325i head and you have a mismatch that causes a lower compression ratio, and this for this procedure, lowering is a bad thing. So that leaves you with 3 choices:

#1 is to use the pistons from a 1988 528e or 325 (Super Eta), which works fine but has a less than ideal compression ratio of 8.8:1. The good thing is that they are cheap if you can find them, but then again, they are used pistons and they aren't very commonplace since they were only available for one model year.

#2 is to use the 325i pistons and shave the top. 3mm is supposed to be the accepted tolerance with stock 325i rods, though I have also heard 2mm as well. Pete McHenry of Precision Performance Services in Winston-Salem, North Carolina (333) 761-0643 is the pioneer of this conversion and would know this information firsthand, as would Jim Rowe of Metric Mechanic in Richland, Missouri (573) 765-1269. There may be other BMW tuners, technicians, and machinists in your area who know this, so ask around.

#3 is to order a set of custom made pistons from companies such as JE, Arias, Kolbenschmidt, and Wiseco. While this is probably the most expensive route, you can customize exactly what type, compression, and size of piston you want. They can be ordered through a variety of vendors, including Ireland Engineering, Top End Performance, Metric Mechanic, and Redline Racing.

3.) If I need the entire top half of a 325i, wouldn't it be easier if I started off with a 325i and throw on an eta crankshaft and some custom pistons?
Yes, but what's the fun in that?

4.) How much is this going to cost me?
Again, for a detailed list see page 2, but assuming that you are doing the same thing I am figure at least $2,500 in parts alone. If you shop around and buy some of the parts used and in good condition, you can save yourself a lot of cash. I am doing much of the labor myself (except the machine work), but if you're handing your car over to a specialist then throw in at least another grand in labor. Also, if you have the entire engine out you may as well change other parts such as the head gasket, timing belt, tensioner, water pump, front/rear engine seals, etc, which is obviously going to cost a few more bucks. Altogether, including machine work, I spent around $3,600.

5.) That's a lot of money. Doesn't Ireland Engineering sell a stroker kit for less than $2,500?
Hey, nobody ever said power was cheap. Yes, Ireland does sell a 2.9 stroker kit for around $2 grand, but it is meant to be used with an existing 325i motor. It doesn't include things such as an intake manifold, fuel injectors, cylinder head, camshaft, wiring harness, or ECU. (Seems kind of expensive now, doesn't it?) But if you already have a 325i and/or want a kit relatively free of hassle, then it may be a good choice.

6.) What about the 88 325/528e "Super Eta"? I hear those are better for the conversion than the 84-87 etas.
That's true because the '88 325's and 528e's already have the 325i cylinder head (though it retains the eta cam and single valve springs), Motronic 1.1, and pistons that will work without any modification. Saves some time and money.

7.) An eta is 2.7 liters. How do I get a 2.8 out of that?
The stock 325e engine displacement is 2693cc and the stock piston bore size is 84 mm. Order custom pistons and specify them to be 1mm overbore (so they are 85 mm) and that will give you a displacement of 2758 cc, which rounds up to 2.8 liters. Going even larger to an 86mm bore would also net a 2.8 liter displacement (2823 cc), though some tuners say that 86mm is too much.

8.) How much power are we talking?
It all depends on the setup. It's tough to talk straight numbers because there is so much variance between engines in terms of modifications, but figure that a conservative 2.7 liter setup with '88 325 pistons and stock 325i fuel injection, cam, and exhaust may yield around 180 horsepower and 190 lb-ft of torque at the crankshaft. Something a little more aggressive, such as a 2.8 liter with custom high compression pistons, larger air flow meter, larger injectors, 524td crankshaft, and mild cam may put out around 205 crank horsepower. Engines that are wildly tuned with a 2.9/3.0/3.2 displacement, mass air conversion, hot cam, open headers, bored throttle body with ported manifold, ported cylinder head with oversize valves, etc will put out more than 220 horsepower, but don't expect to get any insane numbers out of the M20 motor. The important thing here is not so much the raw horsepower numbers but the horsepower and torque curves. On a stock 325i both curves are peaked towards higher RPM, while on a stroker they are not only higher but flatter.

9.) Will the car pass smog?
I did. I had to smog my car with only 300 miles on the new engine because my registration expired, and while the results were higher than the 2.7 eta's emissions, it still passed by a wide margin. The idle results may be higher than what they should be because I had a misadjusted throttle body that caused the idle to be around 1050 RPM instead of the normal 750. But if you have other mods, such as a hotter cam, higher flow injectors, etc then you may not have the same results. To see my smog results, click here. The 2.8i should also pass the visual inspection because it looks completely stock unless you have mods like open headers or an adjustable FPR.

10.) What about gas mileage?
Gas mileage usually drops, though exactly how much depends on how you drive. I used to average about 24.5 mpg in my eta and now I'm averaging about 22 mpg with the 2.8i. However, on a 300 mile all-highway trip, I averaged 28.5 mpg at a steady 75 mph. But if you're that concerned about fuel economy, then you wouldn't be reading this right now, would you?

11.) What else needs to be modified when doing the conversion?
A shorter, numerically higher, differential is also recommended since the end result will be a 325i stroker motor (and the stock 325i came with a 3.73:1 rear end). However, many eta owners who are so accustomed to low revs at highway speeds find that too short and use a 3.46:1 or even a 3.25:1 differential instead. Either a 318i or 325i 7000 RPM tachometer needs to be fitted (unless you like to shift by ear), and it also wouldn't hurt to throw on the 325i exhaust system. Furthermore, I would also invest in some sticky tires and good brakes, otherwise your car may end up like one of those 1960's American suicide machines with 400 horsepower and 4 wheel drums.

12.) Can I do this myself?
It seems that most E30 owners pride themselves as being do-it-yourselfers, but there is a tremendous amount of work involved. If you've never done anything like this before and you don't have a dad/brother/neighbor who's a BMW tech, I'd definitely want a professional handling much of the work. A lot of guys on the Internet will say stuff like, "everything bolts right in and that you can do it in a few hours," but that doesn't cover any of the little things (seals, brackets, sensors, wiring, hoses, lines, etc), and believe me, it takes a lot longer than a few hours. If you are capable and/or adventurous enough to build an engine yourself and you have the proper equipment to do so, be prepared to spend a lot of late nights working in your garage and to drain large amounts of cash from your bank account. Just remember where everything goes :).

13.) I have an automatic eta. Will this work for me?
I'm not too sure how strong the torque converters are on the ZF 4HP22 autoboxes. It can probably handle a mild 2.7 at the least.


Table of Contents

1.) FAQ

2.) Parts List/Price Guide

3.) Short Block

4.) Cylinder Head/Intake Manifold/Fuel Injection

5.) Engine Removal

6.) Engine Install/Wiring Harness

7.) Cooling/Belts

8.) Final Thoughts

9.) Tachometer Wiring

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