Budget compression

[Lunner]

Hey gang, so I'm new to the board, and I know the best way to make everyone warmly welcome you is to post a question regarding rebuilding/stroking an m20, but my question should be specific/simple enough, and not something I could easily find with searching.

I'm in the talks to get a MM cylinder head for my otherwise mostly stock 2.5 engine. I'm currently running Miller's MAF+WAR with 19# injectors with great success.

My current head is warped/cracked (and it was inspected as part of my rebuild too ), and I usually try and upgrade while I'm in there anyway, so the MM head sounds like a good thing with this in mind.

Here's the specs on the head:
Intake: 286 deg 11.75mm lift
Exhaust: 276 deg 10.44mm lift

It's my understanding that a camshaft like this would greatly benefit from a higher compression ratio. However, I just rebuilt my bottom end and everything's doing really great. Cylinder walls are honed beautifully and the crankshaft is still brand new.

That being said my flywheel is irritatingly bad (chewed teeth on the ring gear), so if I'm pulling the head I wouldn't be too bothered to pull the block with it to address this issue.

On to my question finally,: Will this cylinder head be a waste at 8.6 C/R, and/or are the gains by bumping it up to 10-ish worth it/necessary?

If I am to bump compression ratio, what's the best budget minded route considering I'm very happy with my bottom end as is and I don't wanna spend gobs of money essentially fixing something that isn't broke?

The ideal answer for this would be some sort of connecting rod that mated to an 'i' crankshaft and piston for increased C/R, but I possess no techincal automotive engineering skills and I know I'm probably dreaming.

I assume I'm probably looking at custom pistons with my existing crank+connecting rods, but I'm just hoping there's some combination (that ideally doesn't involve boring) of eta/i/m5x/s5x parts that'd give me a decent C/R.

I'd also prefer not to stroke, but if that's the route necessary then I could be willing to do it.

If there isn't a specific answer to this question readily available, would someone be willing to kindly take my hand and show me how exactly to calculate compression ratio, what measurements are needed, and where to get those measurements on parts for our engines?

Thanks, and sorry for the long post, I usually try and be as detailed and clear as possible, which sometimes makes for long sentences (English Major).

-Brett

[jlevie]

For a higher compression ratio I think you are stuck with needing different pistons. Seems like I saw those on MM's site. If you bump the compression ratio you really should change the chip in the DME with one tuned to higher compression ratio.

While that head and cam would work better with higher compression, it will yield a power increase wit the stock bottom end.

[NC325iC]

no need to change the chip he has war, with Great success

[whodwho]

The thing with going with a larger cam is it lowers your dynamic compression so CR does come into play with the net results of adding the cam.

You wont be able to change the compression by rods unless you were to go with some crazy sized expensive set of custom rods.

You can mill the head, mill the block or change pistons to raise it. and running the thinest headgasket will help a touch.

I would talk with the MM guys and see if the head can be milled to get your where you need to be and still have piston to head and valve to piston clearance.

Here is a Compression calc that you can see how it can change - Enter the total amount the cylinder head has been milled and the block decked.

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.0,m75.00,40.00,-7.50,m34.2,m135.00,m1.75,m0.000,7000,0,0

[Lunner]

no need to change the chip he has war, with Great success

High five!

Whodwho, thanks for that link, it's exactly what I needed. I already contacted MM, waiting for a response at the moment.

According to that calculator milling/decking isn't going to give me enough CR to be worthwhile, so I think I gotta go with custom pistons. Either that or hold out for a set of euro pistons or 133/137mm connecting rods to magically fall in my lap.

Well alright, I guess I can save up another 900 or so for a set of custom pistons. I was just hoping it wasn't gonna come to that.

[whodwho]

Unless you were going FI or determined to stay with a 2.5 going with custom pistons defeats the budget part of this post :D

You could go with the 2.8 combo (84mm crank, b27 rods, b25 pistons) seems to be a successful combo and would go great with that MM head and would be well under half of the expense of custom pistons.

[Lunner]

Unless you were going FI or determined to stay with a 2.5 going with custom pistons defeats the budget part of this post :D

You could go with the 2.8 combo (84mm crank, b27 rods, b25 pistons) seems to be a successful combo and would go great with that MM head and would be well under half of the expense of custom pistons.

Yeah, budget is definitely something to keep in mind here.

Edit: This went through all sorts of edits while I did my homework, haha. I think I finally have an answer though. m52+etarods+my already in block i pistons = 9.3-9.7 CR, depending on who you ask. Either way it sounds ideal.

I'll do some more research in the mean time, but the m52 crankshaft just plops right in right? What about bearings?

On to do more research and source parts.

Thanks again by the way.

[Lunner]

After doing some research, I've discovered that an m52 crank requires some machining to fit into this engine. Unfortunately, I haven't found any clear / decisive answers as to what that machining is, or where I would go about doing it / what would I be asking the machinist.

Thoughts?

[Wh33lhop]

The M52 crank just needs a spacer machined to fit in an M20. Someone was selling some awhile back for $75 or so, but I think they're long gone by now. Pretty sure someone posted a template/blueprint for it somewhere though.

[Lunner]

The M52 crank just needs a spacer machined to fit in an M20. Someone was selling some awhile back for $75 or so, but I think they're long gone by now. Pretty sure someone posted a template/blueprint for it somewhere though.

I'm reading a lot of things like this:

"To achieve 2.8l you can either shave the crank counterweights / rebalance and use eta rods & M20B25 pistons with modified skirts or go custom pistons and rods to avoid shaving the crank."

and:

"Nothing done to the block, you have to machine down the crankshaft counterweights however so they do not contact the piston skirts."

The latter was taken from Danny's FS thread for these exact items.

http://www.r3vlimited.com/board/showthread.php?t=161566

Again, I possess no technical automotive engineering/machining skills so I'm not sure who to ask about 'machining down the counterweights'.

[Wh33lhop]

I'm reading a lot of things like this:

"To achieve 2.8l you can either shave the crank counterweights / rebalance and use eta rods & M20B25 pistons with modified skirts or go custom pistons and rods to avoid shaving the crank."

and:

"Nothing done to the block, you have to machine down the crankshaft counterweights however so they do not contact the piston skirts."

The latter was taken from Danny's FS thread for these exact items.

http://www.r3vlimited.com/board/showthread.php?t=161566

Again, I possess no technical automotive engineering/machining skills so I'm not sure who to ask about 'machining down the counterweights'.

My bad brobrocop, I was just regurgitating--only adding to the "forum problem." Sooner or later a few big dogs will probably see this thread and set the record straight, although it sounds like you've already found the jist of it.

[Lunner]

My bad brobrocop, I was just regurgitating--only adding to the "forum problem." Sooner or later a few big dogs will probably see this thread and set the record straight, although it sounds like you've already found the jist of it.

No worries, that's why I do this kind of research before buying shit. And I've given some bad, and definitely worse, advice in my time, so no hard feelings.

And honestly, yeah, this whole subject is far too muddled for me to wanna throw my hat and wallet into the ring figuring it out. So, I'm gonna kill this thread (at least for me, hopefully the information will still be useful to some).

I'll let the seller of the MM head know I changed my mind. Which I hate doing, but I'll see if I can't make it up to him. But honestly this whole thing started with the idea that if I'm gonna put a head on this block for the third time in a year, it should be an upgrade. But then I found out that you really can't just throw a hot cam on a stock bottom end, and so on...

But this is gonna turn into a very expensive and time consuming project, and I should just bite the bullet (did I mention I spent $170 on dealership head bolts and gasket last time, assuming that a VR head gasket was to blame for the first leak? I've literally driven those $170 parts 350 miles so far and they're dead to me); and buy another regular old 885 head and do the head gasket replacement - again. I seriously wanna know what happened to my cylinder head, cause it didn't leak this bad before the rebuild. But I digress.

Lord, beer me strength.

[nando]

firstly, your engine is 8.8:1, not 8.6:1

secondly, MM uses very, very conservative cams. I wouldn't be surprised if it didn't work just fine on a stock engine. They basically go for low overlap, shorter exhaust duration, and higher vacuum at idle. This is for emissions and smoothness. They aren't neccesarily good performance cams, most people with hotter motors swap them out for something from schrick or catcams.

also not sure about all this talk of grinding down crank counterweights etc. for a stroker.. you shouldn't need to with an M52 crank, probably not even with an S52/M54 crank. It depends on your particular block more than anything. personally I'd leave the crank alone and clearance the block. machining BMW cranks is generally a bad idea.

also the poopy rod ratio of the 84mm crank/130mm eta rods isn't really that ideal, but people do it all the time. IMO, either leave the bottom end stock with the MM head, or go all out with a 3.1 liter and forged 10:1 pistons. The middle ground is a waste of money for little performance benefit.

[Lunner]

MM called me today, great bunch of guys.

Their recommendation was to use an eta crank + i rods + their custom 87mm bore 10:1 C/R pistons with this head. Which sounds like it'd be an awesome plan...if I had thought of it prior to the first engine build.

I may yet just buy this head and sit on it for a while, but honestly with smog up in August I'm very tempted to just get a working stock head and tune the WAR with that and then pray everything works until at least after I leave for the Peace Corps and/or get my Master's.

Now I'm just really pissed that I've sunk around $700 (or more, don't wanna do more adding) so far in my cylinder head, and it's gotta go. I could care less about my labor, this shit's actually pretty fun, but I am as always the proverbial broke college student, and this whole incident is making me realize I have no way to not just throw money at this, whether I upgrade or fix it the third time.

I gotta figure out my next move here, regarding trying to go back to my machinist to get some answers and sourcing another head.

If nothing else, this was all very informative, and hopefully the next guy on a search will pull some useful info.

-Brett

[Evilsmith]

Im also wrestling with the idea of using the M52 crank but am having trouble finding info on it. Here is what I have found so far:

Guy that is making crank spacers (Instead of the MM $200 ones):

http://www.r3vlimited.com/board/showthread.php?t=153911

Here is a small and vague writeup I have referred to a couple times:

http://www.blackbmw.net/index.php?option=com_content&view=article&id=60:crankshaft&catid=46:parts-collection&Itemid=56

Thats pretty much it, I would love more info as well.