3.0L, 84mm bore, 89.6mm crank?

[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

[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]

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

[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

[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]

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.

[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]

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

[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]

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]

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]

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

[bmwmaster81]

Im also on the same route. Have got the whole m54b30 internals here since 2 years and would like to build it the cheap way with stock m54 pistons.

The pistons are about 7mm thick. My plan would be to shave them down about 1mm and cut 2mm pockets aditionaly. That would leave a thickness of about 4mm on the thinnest place.

Would this be enough?

I planned to modify the m20b25 chambers to make them hemispherical. Make them round and cut the squish away.

In this way i maybe could get the CR down to about 11.0.

Would this also leave the knock proned characteristic. I mean there are a lot of hemi engines and they run well.


Im not sure how much cc i could gain with this mod.
I think the original shape has about 40cc ? With mod maybe 43 cc?

What do you guys think?

[digger]

It’s difficult to machine a set m54 pistons to have the proper squish characteristics and CR.

On a 3L you basically need a 5 to 6cc net dish or so for 10.5:1, thats the same effective dish as the OEM 8.8:1 pistons (hemi dome and offset spherical dish). those ones done by IXER for a 2.8L m20 on e30tech will be too high CR (like 12.5-13:1) for mild 3L street engine on pump fuel.

ive got 3D cad designs including central dish or offset but id just buy a set of 4032 forged slugs rather than stuff around trying to implement something onto a 24V piston unless you own the necessary equipment to machine them and play around/experiment etc

you could do a flat top with central dish but its not gonna work that great, be prone to knock etc

[turboplzz]

what ever happened to this m54 rotating assembly measurements? I was super curious reading through the thread waiting to see the results of the piston material that needed to be machined and then it just ended like a bad movie. I'm super curious as to the m54 rotating assembly because I'm trying to get the most displacement without having to buy custom pistons. I would 24v swap it but just love the sound of a worked m20 theres nothing like it and the added weight of the 24 valve swap and already low oil pan I just want to keep it an m20 car. so please someone chime in on what happened here I know its been soooo long since this has been discussed but what happen to it????? I'm super bummed I got to end with no results I'm sitting here with blue balls