headgasket for 86/86.5mm pistons?
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Author Topic: headgasket for 86/86.5mm pistons?  (Read 20308 times)

thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #15 on: October 14, 2013, 04:13:36 AM »
allright! very interresting once again!

then I will have to change my plan.
plan A was to use a crank from a M44, honda conrod and euro S50B32 pistons. around 12:1 CR NA. the whole idea was to keep machining to the minimum (boring) for cost reasons.

plan B will be M44 crank, M42/44 rods and S52B32 pistons and aim for a squish of 0.8/0.9 depending on pistons rocking. don't know yet how much CR to look for with that combo but I'd definitely want to be around 12:1 (E85)
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #16 on: October 14, 2013, 06:46:47 PM »
Just a thought. If you skimmed the block 0.65mm and kept everything else stock. You would get a squish of 0.8mm and 10.7:1 CR...

If my maths is right, it would be a relativly cheap rebuild option assuming you could get the chip to suit. Would require piston to valve checks but may yeild some HP otherwise missed out on.
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #17 on: October 15, 2013, 02:52:51 AM »
or use a 1mm thick cometic headgasket. that would be interresting to see how power we can get out of a M42 with its stock crank/rods/pistons...
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Vince
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #18 on: October 15, 2013, 09:13:22 AM »
or use M50TUB25 pistons which have 0.65mm higher compression height and are flat top...
I will calculate new CR with that combo later.

I've to re-think the whole project and decide which way to go...

EDIT: M50TUB25 pistons have 0.9mm taller KH. sorry... but tehy can still be shaved to the correct value...
« Last Edit: October 15, 2013, 05:10:13 PM by thebrelon »
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #19 on: October 16, 2013, 06:49:13 PM »
According to the excell spread sheet i just whipped up...

If .65 above deck and flat top you get ~ 12.1:1.  Squish distance on 1.7mm HG = 1.05mm
If .9mm above deck and flat top you get ~12.5:1. Squish distance on 1.7mm HG = 0.8mm

Machining to reduce compression would take a fair bit of meat off the piston.

Im was looking at a Nissan piston which gives 12:1. A squish distance on 1.3mm HG of .8mm  (86mm bore). To get 11:1, I need to gain 4cc in the chamber as i dont want to machine the piston.

Or

Stick with 1.7mm HG and resultant 1.2mm squish and 11:1.

Do I shoot for a better squish of .8 but have to machine somthing to drop CR to say 11:1 OR go the easy way of 11:1 CR and wide squish gap with no machining. What i have learnt so far im not sure if i want to trade off squish gap...

Takes alot of math and playing with ideas to get numbers which sound right...

 


« Last Edit: October 16, 2013, 10:26:41 PM by MLM »
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #20 on: October 17, 2013, 04:04:00 AM »
thank you very much for your reply MLM. I use the same kind of spreadsheet but it seems I don't use the right numbers for the M42...

from calculation the dead volume of a stock m42 is 49.8cc, of which  the chamber itself is 32cc, HG 9cc and piston dish around 11cc. is this right?
it seems that most BMW engines from that era (M50/52/42/44 and even S50/52) use a 32.5cc combustion chambers...

then I tried different combo of piston/rod/crank aiming for a squish somewhere between 0.8 and 1mm and a 12:1 CR -ish...
added to that I tried to get wrist pin diameter and rod small end diameter identical to avoid machining or bushing.
I also want to limit piston machining or deck machining to the minimum.

taking all that in account i ended up with a M44 crank, honda 138mm rod, euro S50B32 piston and a 1mm headgasket for a 1mm squish and a little over 12 CR.

I was very interrested by the M42 crank/rod with flat top pistons but my calculations showed a much high CR than yours. I must be wrong somewhere...
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #21 on: October 17, 2013, 04:21:23 PM »
From measurement I recorded.

Piston 7.5cc
Head 34.5cc
CR comes out at 9.87:1


Make sure your adding and subtracting volumes appropriatly for deck heights and dishes etc. Messed me around for a while. For my theoretical combo I use stock rods, pin diameter, HG, Crank, bored but not decked block and get 11.5:1 with the nissan piston and 1.2mm squish. Just trying to decide if I decrease the squish to .8mm or not with some block machining (or thin HG) and go for 12:1.

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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #22 on: October 18, 2013, 01:34:45 AM »
thank you for the numbers. I will see what I get with that.

for your engine, you may also have a look at your camshaft profile before deciding which way to go, as you may have 12:1 static but less dynamic.
you may also decide which petrol to use and which engine management (knock sensing and advance management associated) before cutting between the 2 options you have.
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #23 on: October 18, 2013, 02:19:34 AM »
Thanks, cams are another set of tradeoffs.. but eyeing up VAC 280/247 "hot" cams. Management is sorted so will be fine there and good fuel readily available here.
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #24 on: October 19, 2013, 11:56:44 AM »
I just spent 2 hours re-doing my math with the actual volume figures from MLM...

the M42 crank/rods + M50TUB25 pistons combo requires some pistons shaving to get a CR in the low 12.
the M44 crank + honda rods + S50B32 euro piston combo has a 11.3:1 CR  with a 1.25 HG to achieve a 1.2mm squish. looking for a lower squish would require even thinner HG... (0.7 squish, 0.75 HG and 12:1 CR)

so my last "idea":
M42 crank/rods + M52B28 pistons. no shaving required but a thinner HG (1.3mm) to achieve a squish of 1mm and 12.2:1 CR. this isn't a stroker, but it should make a nice, cheap hi-perf M42B18.
no machining is needed, only a thinner HG (cometic) and M52B28 pistons. looks easy!
« Last Edit: October 19, 2013, 12:19:41 PM by thebrelon »
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #25 on: November 06, 2013, 06:25:26 AM »
How about plan C?

M52B25Tu (euro) flat top pistons. Piston height is little bit bigger, 32,7mm vs. M50b25 32,55mm vs. M52B28 ~31,8mm. These pistons already have more then 3mm deep valve pockets intake and exhaust side. With thicker 2mm (stock)HG you get 0,8mm squish. I can't calculate exact CR but deep valve pockets will lower it.  You can also shave the pistons 0,2mm to get 1,0mm squish.
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #26 on: November 06, 2013, 07:38:05 AM »
it should be in the mid/hi 12:1. actually I did calculcations with M50TUB25 flat top pistons, they have a piston height of 31.55mm. you have a 0.7mm squish without any mods but a 12.7:1 CR (without taking account of valve pocket volume). same pistons with 0.3mm piston shaving give a 12.2:1 CR with a 1mm squish.

so M52TUB25 pistons with a 32.7 comp height should give a higher CR. I will check that option tomorrow...
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Vince
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #27 on: November 07, 2013, 04:41:03 AM »
I did a quick CR calculation using stock bottom end and M52TUB25 piston: 12.5:1 CR. this is without valve pocket volume as I don't have any volume measurement.
so this is the max theoritical CR with that combo, considering that each additional cc to the dead volume has a big impact on the final CR...

so yes, it may also be a good combo as it doesn't need any machining but a 2mm HG to achieve a 0.8mm squish.
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Vince
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Re: headgasket for 86/86.5mm pistons?
« Reply #28 on: November 07, 2013, 12:47:52 PM »
Quote from: thebrelon on November 07, 2013, 04:41:03 AM
I did a quick CR calculation using stock bottom end and M52TUB25 piston: 12.5:1 CR. this is without valve pocket volume as I don't have any volume measurement.
so this is the max theoritical CR with that combo, considering that each additional cc to the dead volume has a big impact on the final CR...

so yes, it may also be a good combo as it doesn't need any machining but a 2mm HG to achieve a 0.8mm squish.
And you can use quite wild cam with high valve lift without machining pistons.
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thebrelon

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Re: headgasket for 86/86.5mm pistons?
« Reply #29 on: November 07, 2013, 02:07:47 PM »
interesting... do you have any experience with such a setup?

what kind of power/torque should be expected with this setup and mild cams (between 260 and 280°) with E85 and MS3X standalone? 150hp at the wheels?
« Last Edit: November 07, 2013, 02:10:23 PM by thebrelon »
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Vince
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