Last few questions before stroking/rebuild...

[Beavy_OZ]

Hi everyone, I'm new to the forums but think I know enough to start yet another m20 stroker thread. I have $3000 saved up, so this isn't a waste of time/piss in your pockets/daydream thread. I'm just after a couple of key questions before I start.

I currently have a 1983 323i, with a high comp (pre-cat) 2.5L engine with 288 schrick cam in it, ~230k on the clock. Currently at 130rwhp, but bottom end has not been touched. I also have the original 2.3L engine, which i am stripping down soon to keep the cam in case i need to pass smog etc.

I have my heart set on stroking the m20, I am not interested in an engine swap as I want to keep things 'original' and I don't like the 'on boost' manners of turbo cars I have driven. Any help with the following would be greatly appreciated!

Over-boring? How will this affect reliability? Is the $1200 for custom pistons 'worth it' over a setup using oem pistons? Can I over-bore a 323i block to fit 84mm pistons so as to have a spare block?

M52B28 crankshafts - Can someone please explain the difference between the standard vs. TU engine? I have read that the non-TU crankshafts fit more easily, but I don't know exactly why.

2.8 using 2.5i pistons? Can someone confirm this combo works:

M52B28 crankshaft 84mm,
M20B27 Rods 130mm (Are M20B23 Rods the same length??),
M20B25 Pistons.

I have seen this built on another forum but only once:

http://www.e30zone.net/modules.php?name=Forums&file=viewtopic&t=87909&highlight=

I understand the later low comp pistons are needed? Will there be clearance issues or just compressions issues? If this combo is possible, why isn't the following:

M20B27 crankshaft 81mm
M20B27 conrods 130mm
M20B25 Pistons

Every guide says there will be clearance issues, but wouldn't the shorter (81 vs 84mm) crank throw put the pistons lower?

M52/M54/S52 Cranks Most buildups using these either machine the block/pistons/crank... I am very reluctant to remove any metal from a crank due to balance issues, and I would hazard a guess that removing piston skirt can also lead to problems? Any opinions on this would be great!

323i vs 325i chamber/compression By how much does the 323i head increase compressions ratio? Using the above questions as a guide, is the following combination possible:

M20B23 Block, bored to 84mm
M52B28 (TU?) 84mm crank
M20B23 Conrods (130mm?)
M20B25 Pistons (Low Comp?)
and
M20B23 Cylinder Head? I will probably use my 2.5 head though.

What would be the compression ratio, expected power increase, and will it detonate on 98RON pump fuel? It would be great if this were possible, as I could build the engine outside the car and swap it over a weekend.

Thanks for reading!

Beavy

[jmc1590]

2.8 using 2.5i pistons? Can someone confirm this combo works:

M52B28 crankshaft 84mm,
M20B27 Rods 130mm (Are M20B23 Rods the same length??),
M20B25 Pistons.

If this combo is possible, why isn't the following:

M20B27 crankshaft 81mm
M20B27 conrods 130mm
M20B25 Pistons

Every guide says there will be clearance issues, but wouldn't the shorter (81 vs 84mm) crank throw put the pistons lower?

Keep in mind that the B23 and B27(non-super) pistons are flat / dished. The B25 pistons have a dome. The B27 crank and rods with B25 pistons will work, if you use the 885 head. They will sit about 2.5mm lower in the block than a stock B25. This will lower your compression (some say this is a good combo for FI as the compression ratio comes to ~8:1.) The pistons may interfere with a 200 or 731 head as the dome extends 3.2mm above the stock pistons. (Possible valve issues? I suppose I should go measure an extra head...)

323i vs 325i chamber/compression By how much does the 323i head increase compressions ratio? Using the above questions as a guide, is the following combination possible:

M20B23 Block, bored to 84mm
M52B28 (TU?) 84mm crank
M20B23 Conrods (130mm?)
M20B25 Pistons (Low Comp?)
and
M20B23 Cylinder Head? I will probably use my 2.5 head though.

What would be the compression ratio, expected power increase, and will it detonate on 98RON pump fuel?

I would expect a compression ratio of ~9.5 or so, with the B25 head. (If tests show no valve issues with the B23 head, expect ~11:1!) Power increase? No idea. I guess that would depend on how you decide to fuel it and control the spark. 98RON (which I'm guessing is about the same as US 93(R+M)/2) should be no problem at 9.5:1 compression.

Hope that helps...

James