Fun with engine rebuilds.

[John V]

Some of you know about this car and some don't. I bought this car a few years ago to be a competitive national-level autocross car that I could also use for a few track days. It has a long, storied but boring history that I'll spare you except to say I bought it with the understanding it had a professionally rebuilt engine maximized to a particular ruleset. Peter (my engine management tuning guru) and I have struggled to get good torque out of this car since the beginning and I've chased my tail trying to determine the problem. I eventually determined it had to be an engine problem.

It makes decent horsepower at the peak, but midrange torque production is pretty poor, indicating low peak cylinder pressure, meaning bad compression. Towards the end of the season I also struggled with high oil consumption and lots of oil getting sent to the catch can. After some less than stellar compression test and leakdown test numbers I decided to yank the motor and fix it. Originally the hope was I could just do a hone and re-ring, but that didn't end up being the case.



This engine was advertised by the previous owners as being rebuilt with BMW factory overbore pistons. A quick look inside the cylinders with a borescope showed that was indeed the case - I could see the markings indicating they were rare and expensive BMW factory +.2mm pistons. So what gives? After tearing down the motor, I got my first clue:



That's the top piston ring near TDC, measuring more than 0.5mm ring gap clearance. The BMW spec is 0.2 to 0.4mm, so out of spec but not too far off. But this was the best cylinder. Most were 0.6mm or greater. The cylinders themselves looked OK, good crosshatch and no scoring. I measured them with a bore gauge for size, roundness and taper and found that there was essentially no ovaling or tapering of the cylinders. That indicates to me that the engine had low hours on it, consistent with the story of it being rebuilt. Measuring the piston skirts and comparing them to the bores was the dead giveaway. The nominal spec for piston skirt to block clearance (how the bore size is determined when boring the engine) is 0.02mm. The maximum wear tolerance permitted is 0.15mm. My cylinders were 0.20 to 0.25mm. My guess is the previous shop that did the work just bored the cylinders too large before the final hone, or didn't use a torque plate, or some other fuck up.

So I sourced a new (used) engine block with a stock bore and had Abacus Racing in Virginia Beach do all the checks, machinework and cleanup. They also file fit the piston rings for me and installed them on the pistons. So all I had to do was do a final soap-and-water cleanup, wipedown and re-assemble.



I decided to re-use the bearings, but checked all of their clearances, of course. I am re-using my old crank, rods, and pistons as well and transferring them to the new block. New stretch bolts are used everywhere.



Bearing clearances were right in the middle of the spec throughout the engine.



I took this opportunity to get a baffled oil pan. This engine tends to oil starve in sustained turns, like every other BMW. This should help.



Installed the stock oil pickup and scraper.



Got all the pistons pushed in and the rod caps torqued. Engine spins over nice and smooth.



Picked up an upgraded water pump. The BMW water pumps are pretty notorious failure points.



Cylinder head and cams re-installed. I had the cylinder head checked and then decked to the factory minimum. We had been told that this was done when the engine was rebuilt the first time, but it measured out as being a stock thickness head. Just another in a long line of disappointments with this car.

[John V]

More assembly. This engine is an M54B30 (E46 330Ci) but you can see the intake port work done to accommodate the M50 (E36 325i) manifold we run, with its larger runners.



I took this opportunity to make better brackets to adapt the E46 fuel rail to the E36 manifold. My buddy has a pretty small end mill that you have to go slowly with, but it can crank out little parts like this.



And finally back in the car. I replaced all the cooling hoses and power steering lines since those were all leaking badly before the swap. I hate fluid leaks. I probably shouldn't own a BMW.





I haven't uploaded the video to Youtube yet, but suffice it to say it fired up quickly on the first crank and settled into a nice idle. I'm not claiming complete success yet - it has a date with the chassis dyno where we'll break in the rings and check the tune, but no bad noises and smooth running seem to indicate all is well.

[bren]

Originally Posted by John V

Just another in a long line of disappointments with this car.

"Doesn't need anything but a set of brake rotors." Yup, been there.

Hopefully when you drain the oil you don't find it full of anti-freeze like I did after the head replacement.

[Plaz]

Most impressive.

[blee]

I bought my M3 and discovered it had major issues only when copper flakes came spewing out during its first oil change, maybe 100 miles after I bought it. That was not an inexpensive rebuild, but neither was this, I imagine. Very nice photos.

[FC]

*slow clap*

[JST]

In those last couple of pictures, I think you need to tighten your strut mounting bolts.

[FC]

Originally Posted by JST

In those last couple of pictures, I think you need to tighten your strut mounting bolts.

Weight reduction.

[John V]

Originally Posted by JST

In those last couple of pictures, I think you need to tighten your strut mounting bolts.

You didn't know? I run double wishbones up front.

[Jeff_DML]

Originally Posted by Plaz

Most impressive.