the m20 developement thread. Some progress and a new lump.

[eta]

I thought this was supposed to be a recipie for a an M20 that anyone can build. All this machining work is certainly beyond what I can do.

Good work though I hope the finished engine put down power worthy of all this effort.

[HairyScreech]

while machining down the crown of the piston may be beyond moat people, im sure with accurate enough specs that a good machinist could do that for you.
the cutting down of the skirts is piece of piss, as is the polishing of the rods and the removal of material from the ports.

[e301988325i]

I thought this was supposed to be a recipie for a an M20 that anyone can build. All this machining work is certainly beyond what I can do.

Good work though I hope the finished engine put down power worthy of all this effort.

Giving away the specs, that this guy has clearly spent many hours on, that if you're not capable, can be taken to an engine builder to replicate a perfected M20 is very generous, stop complaining.

[eta]

I was not complaining. The head specs given to be honest confuse me a little. I am not entirley sure what I would need to give to a machinist regarding the cylinder head.

I had a thought. For those of us running an eta crank, would M52 rods and pistons work as well? Would there be much piston machining required or will the pistons just need pocketing?

As these are lighter it seems like an option over using 2.5 pistons and decking the block. I wonder what CR would result?

[HairyScreech]

at the moment it would be best to give a machinist nothing, as nothings truly finished yet, which is why theres no accurate specs on here yet.

you will just have to wait until its all done.

m52 pistons would only work ok if you used a 731 head, which is for flat top pistons, otherwise there going to be out done by the correctly shaped b25 pistons and 885 head every time.

if your going to run an eta crank then it would be even simpler to reduce the height of a 2.5 piston as theres less that needs to come off the top.

as has been said several times, forget flat top pistons, they will suck due to bad combustion chamber design.

infact the first thing in the last update was:
having done a bit of combustion chamber research i can say that a flat top will be **** compared to a similar compression b25 shaped piston with correct squish clearance

[eta]

Well as you have demonstrated the 731 head is overtaken in flow by the 885 head beyond about 6mm of lift on your charts I don't see the M52 piston option as way forward. It would be no better than using the eta pistons with a 731 head.

So using a eta crank, 135mm con rods and B25 piston I could shave the ~3mm necessary of the pistons and then they would work. In doing this, would I be not turning the B25 pistons into flat top ones though? This is what I have trouble with not having any B25 pistons to hand to measure.

Using the eta crank 130mm conds and B25 pistons would mean I would have to get the block decked, then a vernier pulley would be required. I would rather use 135mm conrods for 2.7i build.

I currently have an eta block with a 885 head. Eta pistons of course and less power than I would like -around 125 whp and 145 ft.lb wtorque - so all I got is power the eta made at the flywheel is now at the wheels, better than an eta though.

[HairyScreech]

only if you skim the piston perpendicular to the normal.
if you skim at 21-23 degrees as suggested here:

the next thing is by skimming the pistons will reduce the depth of the bowl, this will bump up the compression, atm with just a piston cut down and no pockets the compression is 12:1. to me this seems high, but ant has hinted in another thread that this has been run on an m20 just fine before.

his further input here would be appreciated, as although i have a lot of technical info and analysis hes the one with the real world empirical testing to draw on.


as said before the crowns are real thick and the top ring is a way down the piston, part of the reason they work fine under boost, plenty of meat and good thermal resistance.

iv measured the two angles of the piston dome and the head bowl, and the squish bands are a 23 degree angle for the piston and a 21 degree angle for the head, im not sure on why the variation, but i intend to find out,
for now i have just cut the pistons to 21 degrees to match the head, this has resulted in a squish band that almost exactly matches the head squish band. it almost feels like im taking it back to its original spec.

i still need to work out a good way to hold the pistons in order to cut the valve cut outs, which i have measured to be 4mm deep stock intake and 3mm deep stock exhaust. im thinking a jig of some form involving a tube and a hole for the piston pin, welded to a mount that puts it at the correct angle.

next thing i have been considering is the structural integrity of the piston. the centre of the piston is no thinner at its thinnest point, which is under the bowl near the centre, which will mean the middle will be no problem in use.
the ring lands are now 3.8mm from the top of the piston. from what i can gather this is more than what quite a few after market pistons have, and there are a hand full of stock pistons from other cars that are similar to this, namely the 70s aston martin engines and once again the small block chevy at 0.150" (3.81mm)

even still i have been considering using a thermal barrier coating on the piston to keep heat in the chamber and reduce the heat transferred to the piston.

i have so far been either considering a bake on coat or a hard anodize.
im swaying towards the hard anodizing for tow reasons, it hardens the surface of the material and holds lubricant, two thinks that the bake on coatings dont.
only trouble is the anodize layer is 0.0254mm to 0.1524mm and the piston to bore clearance on these motors is 0.02mm.

so just banging anno on is going to result in binding pistons.

next trouble is just boring the block a touch will open up the ring gaps, and to stay in spec only .50mm gap is allowable, which means a 0.095mm max bore increase before the rings get a bit gappy.

not sure on the solution to this yet but ill get there.

its entirely up to you though, as these are totally experimental and purely theoretical components at the moment.

[eta]

I'll wait and see then.

[hennie]

HairyScreech what is the combined cc of your piston and combustion chamber

[HairyScreech]

not sure off the top of my head, ill look it up, but around 37cc, its up at 12:1 atm but will go down with more work and valve pockets.

[phelix]

Excellent thread! Looks as though you used an M52TU crank - please measure the overall outside diameter - it would be useful to compare it to others.

Here's a photo of 3 cranks. Top to bottom they're M54B30, M52B28 and M20B25.



Their "outer diameters" are approximately 140, 134 and 132mm respectively.

And in case it's of any future use, the M54B30 crank has a casting number of 1403866 and the M52B28 crank has a casting number of 2242898.

[HairyScreech]

im not sure on the total outside radius but, i did have to use a 135 rod, which is 5mm longer than the 320 rod, and take 9mm off the bottom of the piston, so im thinking its about 10mm bigger radius or more than the no tu crank.

ill measure it when i get home.

[gooner1]

Not that i needed it, but that confirms the one i have is the M52B28 crank , with a casting number of 2242898.

[HairyScreech]

ill get the diameter of my crank and the casting number so that there is a record of all of them in one place.

still on for you 2.8 then rob?

[gooner1]

Oh yes screech, oh yes.
Any more Conrod info forthcoming

[HairyScreech]

iv really not been doing much recently

http://s159.photobucket.com/albums/t157 ... d60a70.mp4

[e301988325i]

Screech, this thread is saying an M54 b30 crank, rods and pistons fit straight into an M20 block, now the CR is way too high, and the pistons clearly won't have the offset combustion bowl, but I'm wondering if the M20 pistons might somehow be matched to M54 crank and rods to go for 3.0L !?

http://www.e30tech.com/forum/showthread.php?t=79551

[steve_k]

Screech, this thread is saying an M54 b30 crank, rods and pistons fit straight into an M20 block, now the CR is way too high, and the pistons clearly won't have the offset combustion bowl, but I'm wondering if the M20 pistons might somehow be matched to M54 crank and rods to go for 3.0L !?

http://www.e30tech.com/forum/showthread.php?t=79551

sounds very interesting i'll be having a read through that link later today & if like you say the m20 pistons (b20 or b25?) could be matched then it could be a goer.

if you look into it anymore then please post up your findings.

nearly forgot, the only drawback i can see at the moment is the lack of e46/e39 m54b30 engines in scrap yards so that might put some people of going down that route if it can be done.

[HairyScreech]

well its 7.3mm more stroke than normal. so i cant see why with a 130mm rod and 2mm skimmed off the piston why it couldnt work.

should put the piston at the same position as the original set up.

only way to know is to finish mine off and see how the pistons hold up, if mine hold up ok then thats the most skimming needed for any stroker.

the only other issue is clearing the crank webs, if thats ok then your good to go.

[HairyScreech]

what we need to look at doing possible is getting some properly shaped forged pistons made for the strokers, with a higher pin, 10:1 cr with the bowl in the right place, short skirt or even slipper skirt.

atm the only aftermarket pistons iv seen have either had concentric bowls or have been flat topped, you only have to look at the head and compare it to the 320/eta head to see why the bowl is needed.
couple that with the slightly disappointing results from a lot of the stroker kits with flat pistons.

i think if you showed the engines designer some of the aftermarket pistons then he would just face palm.

[e301988325i]

M20 b25 crank = 75mm stroke
M52 b28 crank = 84mm stroke
M54 b30 crank = 89.6mm stroke

So would there be much of the bowl left with a 2mm skim? What would that leave the c/r at with stock h/g? Quite high for sure, what would be required to get the cr down to 11:1ish, a rebuild spec thicker h/g or 2mm FI spacer plate?

[HairyScreech]

iv skimmed mine about 3.8mm. there would be more bowl than there is on mine. as for CR i wouldnt like to say without looking into it further.

get one and get going, what can go wrong?

[phelix]

what we need to look at doing possible is getting some properly shaped forged pistons made for the strokers, with a higher pin, 10:1 cr with the bowl in the right place, short skirt or even slipper skirt.

Metric Mechanic sells these - for $1200/set.

[HairyScreech]

link? or picture? the only mm ones iv seen have been imho sub-optimal.

[phelix]

I think they all look roughly the same. See pages 5 and 9 of http://www.metricmechanic.com/pdfs/metr ... ooklet.pdf

[HairyScreech]

see thats what i mean, they have a bowl, but its a concentric one, which is totally wrong for this head design.

the m20b25 pistons is specifically designed with the bowl offset in order to concentrate the combustion chamber around the sparkplug, which my nature of the large 2 valve design is offset.

by using a bowled piston like those or a flat topped piston you are not only loosing the effect of the squish band (Ford/cosworth experimented with open and closed chambers, closed having quench/squish areas and open having none. In an N/a aplication the closed chamber was always more powerful.) but also allowing the combustion to take place in parts of the head where it was never supposed to go.

now adding in there clearance engine valve pockets (imo a stupid idea belt breaking hasnt been a problem since the 60s, just change the fucking belts on time you tards.) and you have an engine with a very long flame path from the spark plug to the end of the combustion chamber (far side of the bowl) with a pair of dirty great cut outs to harbour end gas and promote detonation.

there just wrong in so many ways.

[eta]

The 89.6 mm stroke crank is more than 7.3 mm more than normal. Its 8.6 mm more than the eta crank.

My sums using 130mm conrods, M20B25 pistons and the 89.6 mm crank show the pistons would come 7.1 mm above the bores. So I do not understand how you got to 2mm skim of the pistons? Please explain.

for a 2.5 block
Block height 206.7mm
Pistion height from centre pin to top of piston 34.2mm
conrod length 135mm
Total conrod and piston height = 169.2mm and this with the 75mm crank brings the pistons to the top of the bores. Therefore total distance from crank centre line to piston top at TDC = 244.2mm.

For the M20B27
block height 206.2mm
Pistion height from centre pin to top of piston 35.7mm
Conrod length 130mm
Total conrod and piston height = 165.7mm but the is 81mm so the total distance from crank centre line to piston top at TDC = 246.7mm and this brings it to the top of the bores.

So using 2.5 pistons in an eta block (which is what I have) and 130mm conrod and the 89.6mm crank give a total stack height of 253.8 mm which is where the figure of 7.1mm above the bores comes from. Of course this assumes this data is correct.

Info from http://www.bmwccn.no/rogaland/Mahle_m20.html

I would love proper aftermarket pistons to be available to match the 885 head. As for CR if the stock 2.5 piston profile is maintained and the combustion chamber is unaltered the CR would be (using a stock HG) 11.39:1 so not to high. Pistons would need to be custom though wouldn't they.

If stock M20 pistons could be used to create a 3.0l then surely someone would have done it by now.

[whodwho]

for a 2.5 block
Block height 206.7mm

For the M20B27
block height 206.2mm

Careful using this as the block deck height as it is actually the stack height.



You need to use 1/2 the stroke

44.8mm (1/2 89.6 stroke)
130mm (rod length)
34.2mm (2.5 Piston CH)
---------
209 (stack height) - 206.7 (2.5 stack height) = 2.3mm over the 2.5 stack

[HairyScreech]

89.6-75 = 14.6 longer stroke. therefore 7.3mm higher and lower at each end of the stroke. i didnt specify that was what i ment

if a 75mm stroke uses the 135 rod,

and we are adding 7.3mm to the stack height then using a 130mm rod would then require a 2mm skim on the piston.

added 7.3mm, removed 5mm on the rod and 2mm on the piston.

net result piston 0.3mm higher than stock (not a bad thing for squish clearance.)

[hennie]

the only difference between the alpina 2.7 and the 325 piston is the pin height and the dish is shallower to raise the compression ratio the shape and position is the same
the alpina 2.7 piston is not the mahle high compression 325 slug

[HairyScreech]

the mahle HC slug is just a 2.5 piston with less bowl.


the alpina piston is around 2mm shorter isnt it?

b30 crank, 130 rod and alpina piston?

[eta]

Thanks for explaining. This sounds viable. Where would you get alpina pistons. Surely new they would be very expensive from an Alpina dealer is they still sell them. How easy would it be to get pistons made to alpina 2.7 spec.

As the alpina 2.7 used a 135mm con rod there must have been substanial differences between there 2.7 piston and the stock 2.5 piston.

[reggid]

probably cheaper to go custom pistons than get alpina ones.........lol.

Also custom pistons have following advantages
1) let you use whatever rod you want, not that i belive rod ratio affects performance a whole lot but it will affect oil consumption and durability/wear.
2) and you can get +2 overbore so more cc and then you can deshroud the valves and maybe go oversize valves with more success.
3) Different friction and thermal coatings available
4) Reduced mass
...
so if you want the 89.6mm crank get custom pistons its a much better choice IMO.

[hennie]

the alpina pistons were mahle pistons
the hc ones had a short skirt similar to the way Screech has cut his
in South Africa we got the 325 IS that was from bmw with the alpina 2.7 motors and the m 3 suspension there were 2 versions one had a 145kw motor the other made 155kw
when i inquired from bmw Sa the alpina pistons was about 600 pounds each
one could buy a full set of 325 Ks pistons for less than half then this is agents price aftermarket was less than a 3rd for the ks ones about 120 pounds then

[hennie]

back to topic the hole in the valve seat have you changed it in your flow testing experiments
and what is your take on making the hole bigger by 1 to 1,5mm