3.0L, 84mm bore, 89.6mm crank?

[bmwmaster81]

you are much better off starting with a 731 nicely ported head and bigger valves, which will work excellent with flat top piston

Sure but combustion chamber is much smaller, this rises the CR to high.
Im trying to lower CR by using b25 head

[digger]

Sure but combustion chamber is much smaller, this rises the CR to high.
Im trying to lower CR by using b25 head

but its easy to cut a central dish in the pistons no? if the pistons are 7mm thick like you say in middle then you can cut 3mm central dish. D50mm dish 3mm deep is almost 6cc plus new valve reliefs for performance cam getting close to where it needs to be. the rest you can remove from chamber without affecting squish

[bmwmaster81]

Im not sure if the piston will be strong enough after cutting the dish......dont have any experiance or comparison to other pistons.
It seems for me that its getting to thin.

[digger]

Im not sure if the piston will be strong enough after cutting the dish......dont have any experiance or comparison to other pistons.
It seems for me that its getting to thin.

cutting it 2mm deep and D60mm still has about same same volume if youre worried, 5mm will be plenty for NA engine if its tuned correctly

one thing for sure is i would not screw up the OEM head by grinding all the squish away it needs all the help it can to work efficiently

[Trnc]

What was the verdict on the m54 piston? I have this rotating assembly myself now, and i want to build a good compression engine to use my Jenvey ITBs on :)

[digger]

Not sure what aspect you are referring to but why buy expensive itb but go povo on pistons. Oem dish dome pistons ftw without squish combustion is sub optimal.

[hasa]

you are much better off starting with a 731 nicely ported head and bigger valves, which will work excellent with flat top piston

why is 885 different? Arent they both hemispherical heads?

[digger]

why is 885 different? Arent they both hemispherical heads?

No none are true hemi, they are all semi hemi quite a bit of difference . The squish band on 200/731 is flat (I.e. the head face) as it is a closed type design.

On 885 the squish band it is a chamfer of sorts (it’s actually an annular strip of a sphere) so you get negligible squish with flat piston used with 885 as the surfaces don’t come close enough to generate additional mixture motion

[digger]

why is 885 different? Arent they both hemispherical heads?

No none are true hemi, they are all semi hemi quite a bit of difference . The squish band on 200/731 is flat (I.e. it is the head face) as it is a closed type design.

On 885 the squish band is a chamfer of sorts (it’s actually an annular strip of a sphere) so you get negligible squish with flat piston used with 885 as the surfaces don’t come close enough to generate additional mixture motion. This is why the 325i and seta pistons are shaped with An odd dome. Not only do The stock pistons generate squish but they centre the combustion volume around the plug

[whodwho]

Seems like the flat top and 885 head combo 'could' be advantageous for boosted/nitrous engines, would be an interesting dyno/drivability test for the m20 on both combos.

a couple relevant videos - intentionally creating this combo 'chamber softening'

https://www.youtube.com/watch?v=lr86J9g0PWA

https://www.youtube.com/watch?v=3tLjxD37K3g

[digger]

Seems like the flat top and 885 head combo 'could' be advantageous for boosted/nitrous engines, would be an interesting dyno/drivability test for the m20 on both combos.

a couple relevant videos - intentionally creating this combo 'chamber softening'

https://www.youtube.com/watch?v=lr86J9g0PWA

https://www.youtube.com/watch?v=3tLjxD37K3g

One difference between these kinds of head and the m20 stuff is the domestic stuff especially on the shallow valve angles have a huge % of the bore area as squish area so they can generate too much mixture motion. You'll notice they still retain some flat squish but are just not wanting too much. i also think that the "angle" of the M20 squish band on 885 is a fair bit steeper.
I believe FF has done several builds and still favours the OE dish dome but im sure he will eventually chime in

one thing is how would you even know if it’s better or worse ? This is something you probably need cylinder pressure trace to understand imo. Of it takes more timing or makes more power it will be impossible to understand the mechanisms at work without sophisticated measurement systems