Turbo M20 2.8L Stroker w/ 135mm rods

[digger]

Any updates or changes in thinking since you guys put this together?

on what in particular? with a 2.7i your hands are tied

[McGyver]

on what in particular? with a 2.7i your hands are tied

I get that most of my decisions are already made since I intend to reuse an 81mm crank with 130mm rods and b25 pistons. I suppose I'd like to know if the following still holds true:
- For piston rings, use a conventional top and a napier second.
- When upgrading main cap fasteners, this requires an align hone.
- When increasing to the next piston overbore, a torque plate hone is required. Is this required when keeping the stock 84mm bore?
- Piston-to-head clearance target should be 0.037".

I guess I'm a little unclear on how quench relates to compression ratios when choosing components. The stack height of the crank, rod, and piston is known, which defines the deck clearance. Machining the head will adjust the dimensions of the combustion chamber. From what I've read, aluminum rods generally require 0.060" of quench, do you shoot for 0.037" in your builds because the angled band on the 885 combustion chamber provides a little extra clearance? Do you shoot for 0.037" quench and arrive at a SCR, or do you have a target SCR that defines your quench? The target numbers are achieved by a combination of machining the head/block and head gasket thickness?

If someone were to go with forged pistons for an 885 head, is still the thought that the 885 crown is superior to a flat or dished piston in almost all situations?

[ForcedFirebird]

I get that most of my decisions are already made since I intend to reuse an 81mm crank with 130mm rods and b25 pistons. I suppose I'd like to know if the following still holds true:
- For piston rings, use a conventional top and a napier second. Correct, napier second is OEM
- When upgrading main cap fasteners, this requires an align hone. Correct, the main caps distort with increased clamping
- When increasing to the next piston overbore, a torque plate hone is required. Is this required when keeping the stock 84mm bore? Not if you are just deglazing
- Piston-to-head clearance target should be 0.037". Correct.I found .037" works great with m20 rods, .040-.045" is definitely acceptable.

I guess I'm a little unclear on how quench relates to compression ratios when choosing components. The stack height of the crank, rod, and piston is known, which defines the deck clearance. Machining the head will adjust the dimensions of the combustion chamber. From what I've read, aluminum rods generally require 0.060" of quench, do you shoot for 0.037" in your builds because the angled band on the 885 combustion chamber provides a little extra clearance? Do you shoot for 0.037" quench and arrive at a SCR, or do you have a target SCR that defines your quench? The target numbers are achieved by a combination of machining the head/block and head gasket thickness?

If someone were to go with forged pistons for an 885 head, is still the thought that the 885 crown is superior to a flat or dished piston in almost all situations?

Quench and compression aren't one in the same. Quench/squish is the distance between the head and piston at TDC full tilt. Rods stretch during use, so ideally you want the piston VERY close to the head TDC and max RPM. This causes the gasses top be expelled rapidly into the center of the piston where it can be better controlled by the ignition event. Aluminum rods stretch more than ferrous rods, hence the increased quench height.

SCR is just that. The area of the cylinder when piston is at TDC, divided by the area when at BDC, written as a ratio xx:xx (100cc at BTC and 10cc at TDC = 10:1 etc).

When using the 885 head, you are wise to take advantage of the benefits of said chamber design. If you want to use flat pistons, us a 731 or 200 head with the heart fast burn chambers suited to said pistons.

[digger]

I guess I'm a little unclear on how quench relates to compression ratios when choosing components. The stack height of the crank, rod, and piston is known, which defines the deck clearance. Machining the head will adjust the dimensions of the combustion chamber. From what I've read, aluminum rods generally require 0.060" of quench, do you shoot for 0.037" in your builds because the angled band on the 885 combustion chamber provides a little extra clearance? Do you shoot for 0.037" quench and arrive at a SCR, or do you have a target SCR that defines your quench? The target numbers are achieved by a combination of machining the head/block and head gasket thickness?

If someone were to go with forged pistons for an 885 head, is still the thought that the 885 crown is superior to a flat or dished piston in almost all situations?

The crank, piston and rod are defined. The block deck and head gasket thickness are not, so you can adjust the squish clearance to a sweet spot that way. With light pistons or lower rpm you can run it even closer like 0.030 to 0.035 but i probably wouldn't bother with FI getting it that tight.

Tighten up the squish clearance to the required amount and let the SCR end up where it does.... if you're ordering custom pistons and want a specific SCR you need to factor in the squish when doing so the piston dome is adjusted to arrive at the required SCR. The squish is basically directly related to the piston to deck clearance in that case. On the 885 the clearance is slightly trickier as the squish band is a convex dome on the piston.

[McGyver]

Ok, I think I get it now. These are probably pretty obvious questions, but I want to make sure I understand it so I can sanity-check what my machinist needs to do.

In my application, with m20b25 pistons and an 885 head, I will mess with my deck height, milling the head, and HG thickness to achieve a squish of 0.037" to 0.045". The final SCR will be whatever it is, probably in the 9.x:1 for my application. Good squish will result in a better combustion, and achieving good squish won't give me a crazy high SCR, so I should let my squish drive my SCR.

If I were ordering custom pistons, then I would have the option to get my squish where I want it AND modify the piston dish the get the SCR where I want it.

Next question, how do I measure the existing parts?

• Is there a second datum to measure the cylinder head? The Bentley says to measure from the valve cover gasket side to the head gasket side (124.1mm for a new head), but what if the valve cover side was fly cut to remove a dent or gash? Would you measure from the cam journals to the head gasket surface?
• Do I measure the block's deck height by measuring from the crank journals?

To figure out how much to deck the block, I start by figuring out how much has been taken off the head (lowering the squish band on the head). Then add up the rotating assembly stack height and add in the thickness of the head gasket. I adjust the deck height to achieve my desired squish. Right?

[digger]

Ok, I think I get it now. These are probably pretty obvious questions, but I want to make sure I understand it so I can sanity-check what my machinist needs to do.

In my application, with m20b25 pistons and an 885 head, I will mess with my deck height, milling the head, and HG thickness to achieve a squish of 0.037" to 0.045". The final SCR will be whatever it is, probably in the 9.x:1 for my application. Good squish will result in a better combustion, and achieving good squish won't give me a crazy high SCR, so I should let my squish drive my SCR.

If I were ordering custom pistons, then I would have the option to get my squish where I want it AND modify the piston dish the get the SCR where I want it.

Next question, how do I measure the existing parts?

• Is there a second datum to measure the cylinder head? The Bentley says to measure from the valve cover gasket side to the head gasket side (124.1mm for a new head), but what if the valve cover side was fly cut to remove a dent or gash? Would you measure from the cam journals to the head gasket surface?
• Do I measure the block's deck height by measuring from the crank journals?

To figure out how much to deck the block, I start by figuring out how much has been taken off the head (lowering the squish band on the head). Then add up the rotating assembly stack height and add in the thickness of the head gasket. I adjust the deck height to achieve my desired squish. Right?

I wouldn't use the head to adjust the squish, make sure its not warped or twisted which needs to be straightened first then a light skim. Could do a decent deck on a NA engine if you want to bump up the CR but on FI its not what id do.

then you mock the rotating asm into the block with solder/clay on piston dome with bare head and gasket torqued up. squeeze some solder or clay by rotating the engine and measure the clearance ( clay can be hard to measure accurately) and that gives you your delta off the block or delta head gasket thickness which ever method you employ. Id probably check solder on 1 and 6 to cover the likely extremities.

[McGyver]

Ahhh, I assumed the solder/clay was to verify, I didn't realize it was to determine how much to deck the block.

So do you normally alight-hone the crank journals, then size bearings, then use the solder to measure the squish from the resulting stack-up, then mill the deck height? (requires the bock to go to the machine shop twice) Or do you assume the block is close enough for the initial measure and then fine tune later on with a head gasket? (one trip to the machine shop)

I disassembled the block before taking the parts to the machine shop. No I'm worried I did this out of order.

[digger]

Ahhh, I assumed the solder/clay was to verify, I didn't realize it was to determine how much to deck the block.

So do you normally alight-hone the crank journals, then size bearings, then use the solder to measure the squish from the resulting stack-up, then mill the deck height? (requires the bock to go to the machine shop twice) Or do you assume the block is close enough for the initial measure and then fine tune later on with a head gasket? (one trip to the machine shop)

I disassembled the block before taking the parts to the machine shop. No I'm worried I did this out of order.

it is for both, if you measure 1.5mm and you want 1mm then you need to remove ~0.5mm

the align hone difference would get lost in the noise (about a thou or so) considering you are looking for like 40thou +/-

you can just mock it up quickly, doesnt take long