Turbo M20 2.8L Stroker w/ 135mm rods

[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

[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]

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]

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]

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.

[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.

[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?

[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]

Fantastic thread! SLEEPYDUB , thanks for asking all the questions. ForcedFirebird and digger , thanks for providing such clear answers and theory.

Any updates or changes in thinking since you guys put this together? I'm embarking on my own turbo 2.7i build and this info has been invaluable.

[SLEEPYDUB]

My math was wrong, leave the .045" quench (in reality it will be more with the angled band on the head). The flat pistons work just fine, that's been proven over. Would be interesting to do a comparison some day.

Well thats reassuring that the pistons Im using will be fine. The 0.080 gasket should compress some and make the quench even closer, so I think it should work. As for the gapless ring, i saw hotrod mag do a comparison and they saw an increase across the entire power band when using gapless. This was in an NA engine, however I think the same would hold true to a FI engine as well, and may provide even better results

[ForcedFirebird]

As far as gapless, I haven't seen any test data, but have seen a ring gap test Hot Rodding magazine did years ago. They kept opening ring gaps and dyno testing the engine taking note of power and oil consumption. By the end of the article they ended up making the gaps ridiculously large. The power loss was minimal, but oil consumption did increase.

[ForcedFirebird]

My math was wrong, leave the .045" quench (in reality it will be more with the angled band on the head). The flat pistons work just fine, that's been proven over. Would be interesting to do a comparison some day.

[digger]

I must admit I've not seen evidence of large gains from gapless top posted anywhere and would be interested in seeing such data.
Sometimes people change from thick conventional to thinner gapless so is the gain the thinner ring or a smidge less end gap. I've heard people say that power gains are small when you compared rings if same thickness despite often showing much better leakdown numbers

There are mixed reviews on them mainly from an oil consumption point of view but this seems sketchy that the top ring has such a large effect on oil control

[SLEEPYDUB]

That will be fine. .080" gasket minus the .035" piston protrusion leaves you with .055" quench which is more than enough - shooting for ~.040" is best, so if you can get a .070" gasket, you will be even more gold.


EDIT: What shape pistons did you end with?

Wouldn't that actually net me a 0.045" Quench? Or is my math off?

I went with the dished 8:8:1 JEs with Top Seal gapless rings, anti detonation grooves, and HD pins. I debated the OE crown with Steve over and over, and he has recommended against the use of it for my application. I understand the benefits of the OE crown over the Dished, and I am still nervous that I may have made the wrong decision. Although, I do see the benefits of the dished as well especially at extremely high boost levels.

I think the Gapless rings will help to restore some of the loss of power from the dished pistons being that it will dramatically decrease leakdown. Ive seen some pretty amazing dyno results back to back with conventional rings vs gapless, and I must say that the amount of power that was picked up all across the curve was pretty phenomenal. Couple that with boost and I think it may make up for the power loss from the lack of oe crown.

[ForcedFirebird]

Would you happen to have any dyno results to compare the oe crown piston vs the dished? I am not doubting your findings, but I would like to see where and how great the power loss is.

I started a dyno thread. You can see there that a stock tired 2.5 will make 160whp and the 885 head on an eta bottom end made 130. While it doesn't sound like much in HP, when you look at percentage, the flat pistons made 80% the power the stock dome made.

I have had the chance to test it on a turbo car. The same client I mentioned earlier started with flat pistons, but his second setup had a different turbo and cam. But, I can say that I have tuned 2 of the same turbos on two different m20 cars (one having flat, other domed pistons). The flat pistons made 250hp/280tq and domed pistons made 290/304, both at 12psi. Hard to say if there was any other factors since it was 2 different cars after all.

Hey guys, my engine is at the machine shop, and after a few set backs Im on track to have the engine finished today. I need to verify some things first though. They said my pistons are sticking out of the block by 0.035. I will be using a cooper cutring 0.080 headgasket. Does this sound right?

That will be fine. .080" gasket minus the .035" piston protrusion leaves you with .055" quench which is more than enough - shooting for ~.040" is best, so if you can get a .070" gasket, you will be even more gold.


EDIT: What shape pistons did you end with?