Metric Mechanic 2900 Sport

[nando]

they are usually 10:1

FWIW when I pulled my head after about 70,000 miles on the MM pistons, they didn't look anywhere near as bad as the ones above.

[zaq123]

the chamber is barely modified, except for grooving them, so the squish area on the head remains.

i assumed they designed the piston to have circumferential squish and they added a central dish to make it easier to machine but i have actually never checked it properly.

So if there is potential for squish you should be setting it up so you can use it which also would also optimize compression.

i never checked this properly on mine way back when

If chamber isn't modified, it looks like there is a barely any squish since the head chamber has very narrow band on the right (pic) and virtually no band near EX/IN piston area. If you look at OEM piston crown and compare it to 885 chamber shape, crown's shape makes sense.
What's CR with those MM pistons? Maybe they are turbo pistons

[digger]

typically, if the squish area is not too great, engine runs dirty since the fuel doesn't burn too efficiently.
I'm not an expert but I'm not sure I understand MM piston profiles correctly. I get their design to avoid P to V contact. What about the actual squish area? M20 head has that front stepped wide area to achieve proper squish that corresponds with piston's squish band hence oem piston's design has that wide area on the band (pic). Had to tell from the pic of MM head above.....Does MM machines the head to set proper squish and reshapes the combustion chamber to match those pistons?

the chamber is barely modified, except for grooving them, so the squish area on the head remains.

i assumed they designed the piston to have circumferential squish and they added a central dish to make it easier to machine but i have actually never checked it properly.

So if there is potential for squish you should be setting it up so you can use it which also would also optimize compression.

i never checked this properly on mine way back when

[nando]

You have to learn who to listen to and who to take with a grain of salt.

Megasquirt is more than enough for any m20.

no way dude! you need a $20,000 Bosch Motorsports DME to run an M20 properly. Gosh!

Nevermind that Motronic 1.3 can run it just fine and is basically as powerful as a dollar store solar powered calculator..

[zaq123]

typically, if the squish area is not too great, engine runs dirty since the fuel doesn't burn too efficiently.
I'm not an expert but I'm not sure I understand MM piston profiles correctly. I get their design to avoid P to V contact. What about the actual squish area? M20 head has that front stepped wide area to achieve proper squish that corresponds with piston's squish band hence oem piston's design has that wide area on the band (pic). Had to tell from the pic of MM head above.....Does MM machines the head to set proper squish and reshapes the combustion chamber to match those pistons?

[jneumss]

What I mean is the piston dome has a chamfered part about 5mm wide around the circumference, is it at the same “angle” as OE pistons ? Im wondering if you get the head and piston dome close enough like 1mm you can get some squish happening the surfaces need to be approx parallel to get a proper squeeze

I'll get some modeling clay and check out what the clearance is at the chamfer.

[MrBurgundy]

You have to learn who to listen to and who to take with a grain of salt.

Megasquirt is more than enough for any m20.

off topic, your sig is great. LOL

tis indeed a silly place

[digger]

The pistons don't have any "Surface Turbulence" steps in them and the chamfer is pretty close to stock, but it is consistent all the way around with a flat step down instead of an inverted dome.

"Surface Turbulence" grooves in head and valves.

I understand wanting to create turbulent flow in the combustion chamber, so as to limit knock at this CR, but I cannot quantify how effective this stuff all is. My guess is that the different boundary layers interacting would have quite nonlinear turbulent behavior especially at high rpms. Since the stock combustion chamber has many boundary layers interacting and that is largely turbulent already, adding grooves and machine marks would possibly create more turbulent flow.

What I mean is the piston dome has a chamfered part about 5mm wide around the circumference, is it at the same “angle” as OE pistons ? Im wondering if you get the head and piston dome close enough like 1mm you can get some squish happening the surfaces need to be approx parallel to get a proper squeeze

[Kershaw]

You have to learn who to listen to and who to take with a grain of salt.

Megasquirt is more than enough for any m20.

[MrBurgundy]

What's with the hate towards MS? That would be more than enough for his setup.

[jneumss]

Lol, of course mate.

Can the op tell us if those piston generate any squish ? Is the circumference of the piston dome somewhat similar to oem or is the chamfer at a different angle ?

The pistons don't have any "Surface Turbulence" steps in them and the chamfer is pretty close to stock, but it is consistent all the way around with a flat step down instead of an inverted dome.

Did they perform their famous "surface turbulence" black magic on that head?

"Surface Turbulence" grooves in head and valves.

I understand wanting to create turbulent flow in the combustion chamber, so as to limit knock at this CR, but I cannot quantify how effective this stuff all is. My guess is that the different boundary layers interacting would have quite nonlinear turbulent behavior especially at high rpms. Since the stock combustion chamber has many boundary layers interacting and that is largely turbulent already, adding grooves and machine marks would possibly create more turbulent flow.

[jneumss]

what color was that junk? Could be anything...coolant sludge, oil, overuse of the gasket maker etc

It was a dark gray color.

[digger]

Did they perform their famous "surface turbulence" black magic on that head?

Lol, of course mate.

Can the op tell us if those piston generate any squish ? Is the circumference of the piston dome somewhat similar to oem or is the chamfer at a different angle ?

[Bearmw]

Did they perform their famous "surface turbulence" black magic on that head?

[zaq123]

what color was that junk? Could be anything...coolant sludge, oil, overuse of the gasket maker etc