Monday, November 2, 2020

[M3] Bad news, good news... 2.3L -> 2.5L

[note: this post, like an air-cooled 911, is back-dated to when these events occurred] 

It's time for some big fat updates. Please note: Josh is the technical expert and I am sharing the information as best as I understand it. If you have deeper technical questions I will have to defer to his expertise. 

At last check, Josh was fitting the Schrick cams. The cam sprockets had to be slotted to achieve the recommended timing.

Photo credit: Josh

When that was done, he was able to time everything to spec and check piston-valve clearance. Unfortunately it was found to be inadequate. So, options at this juncture: 

1. Modify the stock pistons valve reliefs to achieve required clearance 
2. Aftermarket pistons to achieve required clearance
3. Abandon the cams
4. Time the cams in a non-optimal way for the required p-v clearance

We discussed the options and went with #1. No sense coming this far and not going through with it, and no point timing the cams in such a way that they don't do what you installed them to do.

Photo credit: Josh

In the process of reinstalling the pistons, Josh noticed end play on the crankshaft. Excessive end play, 2-3x what it should be. This is an unpleasant surprise - obviously one of the big appeals of this car was that the motor was rebuilt about 6k miles ago. Josh sent me an email containing the phrase "ticking time bomb" with some images of my thrust and main bearings.

Photo credit: Josh

Photo credit: Josh 

Photo credit: Josh

Excessive wear on the thrust bearings and unhappy, premature wear on my main bearings. Josh's hypothesis is that the crankshaft, which was machined to first under (over?) size when the motor was rebuilt, may not have been done quite right. As a result we had another choice to make.

1. Weld material back on to my crank, have it re-machined and re-coated
2. Find a good used 2.3 crank
3. 2.5L crank

So at this point, motivated to turn lemons into lemonade, Josh and I agreed to increase the displacement. We are going to bore to 94.5mm to leave a bit of extra meat on the cylinder walls, and Josh has ordered a custom piston from JE with a little bit lower compression than usual 2.5 conversion spec (10.8 vs. 11.5:1) to help with the CA 91 octane gas. By his calculations I think this brings my 2.3 up to a 2.44L. Obviously it's a bummer that my motor was not healthy, but I am glad that we found this issue sooner than later, as it obviously would have started to cause huge problems. And while it was never in the cards for me to increase the displacement, since I was not expecting to have to touch the motor any time in the next decade, now that the opportunity has presented itself I think it's worth doing. The parts are in stock and the motor's in pieces anyway. I am hopeful that the net result will be a seriously pumped-up motor from my stock 2.3L, with more torque and power across the range and the throttle response and sound of the A-N setup. I think it's the right combination of ingredients for a fun street motor, without getting into the really expensive stuff.

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