2.9 stroker pinging

[eta]

We are talking about a 0.00508cm difference here, that equates to 0.28cc of chamber volume. Given the error of most laboratory measuring glassware - grade B I think (including burettes) - is +/- 1ml this is within the measurement error of the combustion chamber volume.

As to where this figure came from - the thickness of the crushed headgasket. May be yours came from a different supplier which was 50 microns thicker? Remember the error in measurment of a normal micrometer (the ones I have acces to anyway) is +/- 0.01mm or 10 microns. So at best our headgasket measurements are 30 microns out. I am not going to worry about that.

[CHR1S1990]

sorry, im aware of the difference, I was just wondering whether you had actually measured that using an old headgasket or if you had seen it published somewhere. Would be nice if you had actually measured it as thats damn close to what ive read published and at least nearly eliminates one of these descrepant values we are using to calculate CR's

[GeoffBob]

Thanks for that data Eta, that's valuable information there. Also answers one of my other questions - the volume of the 731 head is 37cc.

I only get the same value of 1.7cc for the piston because I have obviously performed the same calculation based upon a head volume of 42cc, and a total volume of 53.3cc.

Chris, you're our last hope - we need you to perform these measurements if possible please on your open engine. If you don't I swear I will rip open a perfectly good M20 for no other reason other than to perform these measurements

Sebastian - some confusion exists around the CR of the early M20. The Mahle piston is built for a 9.4:1 CR engine, but the original early high CR M20 was in fact (sfaik) 9.7:1. Following the data presented above by Eta, the 9.7:1 high CR piston should occupy a volume of +3.8cc. This means that the compressed volume would be (using Eta's numbers) 42cc + 9.6cc -3.8cc = 47.8cc. The displacement volume of the M20B25 (75mm crank, 84mm bore) is 415.6cc. Hence CR = (415.6cc + 47.8cc)/47.8cc = 9.7 .

The high CR b25 piston thus fills a volume of 3.8cc measured above the plane of the deck, where the plane of the deck is located at a distance equal to exactly 34.2mm ”“ 0.7mm = 33.5mm above the axis of the wrist pin. This is for the simple reason that the piston has a deck clearance of 0.7mm, meaning that the stack-height ends 0.7mm higher than the actual deck.

Unlike the low CR B25 piston which contributes volume in order to lower the CR, the high CR piston takes volume away from the head in order to raise the CR.

To effectively convert a high CR B25 piston to a low CR B25 piston you would need to reduce the crown and enlarge the bowl by exactly 3.8cc + 1.7cc = 5.5 cc. To account for the bore to 86mm you would then need to remove a further 1cc, bringing the total to 6.5cc removed. The piston will now occupy a volume off 3.8cc ”“ (3.8cc + 1.7cc + 1cc)=-2.7cc, resulting in a compressed volume of in the 86mm bore M20B29 of 42cc + 9.6cc + 2.7cc = 54.3cc.

The displacement volume of the M20B29 (84mm crank, 86mm bore) is 487.9cc. Hence CR = (487.9cc + 54.3cc)/54.3cc = 10:1 ”“ as requested.

I still maintain, as per my e-mail, that you should send them a sample of a low CR B25 piston to copy, not a high CR piston. Then all they need do is move the wrist-pin up by 4.5mm and remove 1.0cc from the bowl and the piston is guaranteed to give the correct CR..

[GeoffBob]

What I am no longer sure of is the 885 head volume is?

Overwhelming evidence now points to 42cc.

[Sebastian35]

Geoff I hae lost track of what measurements belong to what. I always had the stock 8.8:1 325i piston volume to 1.7cc so it would appear you have the same numbers that I have now. This is what I believe to be the correct data for the various M20 engine apart for the M20B20 for with I do not have piston data but could calculate this.


Independent confirmation of these numbers would be great.

I have been down to my local BMW dealer and i can say there were three different M20 B25 short ends they could sell to me? although they could not say what the differences were!
I may just buy a new bottom end as I can't seem to gather the info needed to get a correct piston made and i feel like giving up. But with all the bigger valves in the original head now i am not sure i can even do that?

I have a early M20B25 engine code 256E1 this engine has the high CR pistons was totally std.
All I need to find out is what what amount in cc's do I need to remove from the enlarged piston to suit an 86mm bore with a crank of 84mm and rods of 135mm to drop the CR close to 10:0:1 JE can enlarge the piston and change to pin height to meet factory spec. but the company I am using want me to tell them the difference in stack height to deck height, original head volume in cc (top end thought this to be 44 and i measured 42?) head gasket size and head volume: all of which I have conflicting info for!
Can anyone help!

can anyone work out what CR I would have using a enlarged early high CR factory piston, with the engine code 256E1 but to suit a bore of 86mm with the above rod and crank combo. Then I could caculate the extra volume needed to be remove from the new piston bowl to get the required CR. I have 4cc's in mind along with the extra volume taken out of the valve cuts of +1mm. Maybe i'm guessing here? As long as i fall between 9:5:1 and 10:5:1 i will be happy

[Sebastian35]

Geoff i could only send what piston I had!

I know someone else sent a piston via the same supplier but i can't rely on his info; as i would not want to end up with the wrong CR again. In my eyes it is best to use a copy of what came out the engine and lower the CR to suit. Besides I don't mind a CR of 10:5:1 but i want a target of 10:1 then i have scope to play with if heads have been skined ect.

[CHR1S1990]

Chris, you're our last hope - we need you to perform these measurements if possible please on your open engine. If you don't I swear I will rip open a perfectly good M20 for no other reason other than to perform these measurements

I'll get it done Geoff, don't worry

...the plane of the deck is located at a distance equal to exactly 34.2mm ”“ 0.7mm = 33.5mm above the axis of the wrist pin. This is for the simple reason that the piston has a deck clearance of 0.7mm, meaning that the stack-height ends 0.7mm higher than the actual deck.

Is this deck clearance definite Goeff? Just making sure that the b25 has a definite deck height of 206mm, and it is the eta which we are unsure about.

[CHR1S1990]

Surely someone has a scrap low comp piston to donate? Perhaps putting something up in the wanted section? Shame really, i recently purchased some low comp b25 pistons off some chump on ebay, described as "Good Condition" which were screwed. Would have been ideal had I not sent them back. Still waiting on refund though through paypal

[CHR1S1990]

As regards to measurments of my engine Sebastian, Im waiting on some perspex off ebay on which Ill confirm the volume of the 885 head, which is looking to be 42cc. Geoff has given you the info you need there for the high comp pistons, but it would be much simpler with low comp ones as a base line as stated.

[Sebastian35]

As regards to measurments of my engine Sebastian, Im waiting on some perspex off ebay on which Ill confirm the volume of the 885 head, which is looking to be 42cc. Geoff has given you the info you need there for the high comp pistons, but it would be much simpler with low comp ones as a base line as stated.

what is so wrong with the high CR piston it came out the engine and all i need to do is work out how much to lower the CR by.

Hopefully JE will not mind the request of the pre piston spec sheet and that will reveal the CR of my engine to be!

[reggid]

i will have an opportunity to measure the deck height of a B25 block in a few days. As already quoted the last B25 i measured was 206.2mm. I can't be certain that it was the original height but it was what i was expecting based on data i saw from a different wesite.

It is worth noting that it is near impossible to get the 1.75mm head gasket these days, they are the +0.3mm (2.05mm) ones that are being sold, atleast that is what the BMW dealer and another supplier told me and this was a few years ago.

[GeoffBob]

what is so wrong with the high CR piston it came out the engine and all i need to do is work out how much to lower the CR by.

Two things:

• 1) The dome is taller than the low CR piston. To lower the CR you can increase the volume of the bowl, decrease the height of the dome, or both. Without a low CR B25 piston as reference you really don’t know which to do and/or in what proportions to do either in order to achieve best combustion. You can count on the fact that BMW put a lot of R&D into this problem. FYI, the fact that both pistons have the same compression height (KH) does not mean that they have the same dome height. Inspection of the Mahle data indicates this.
  2) Getting the correct CR is reliant upon you (Cambridge/JE) having exactly the right volumetric data to hand, and as we have seen there is still a great deal of confusion in this regard. Whereas, if you start with a low CR B25 piston you are guaranteed to be starting out with a piston of the correct volume, regardless of its actual volume. You don’t even have to measure it’s volume. All you have to do is pull 1.00cc out of its crown to account for your 86mm bore, move the pin up by 4.5mm, and you’re done. It’s a fool proof recipe, which is why Ezagoods piston worked so well, and why TopEnds piston (for all their years of experience with BMW engines) failed!

Sebastian, my best suggestion to you: Send Cambridge a low CR M20B25 piston (buy one if you have to) to copy or your final CR will only be as accurate as the data that has been posted above.

EDIT: Please note that I offer the above as intended, as the best advice I personally can offer (I don't speak for others) under the circumstances. It is not intended in any way as a criticism. I know how frustrating the process of creating a custom piston can be, which is why your best option is always to choose the path of least risk.

[GeoffBob]

Chris, you're our last hope - we need you to perform these measurements if possible please on your open engine. If you don't I swear I will rip open a perfectly good M20 for no other reason other than to perform these measurements

I'll get it done Geoff, don't worry

Thanks Chris, much appreciated This will be a big help to a lot of people.

...the plane of the deck is located at a distance equal to exactly 34.2mm ”“ 0.7mm = 33.5mm above the axis of the wrist pin. This is for the simple reason that the piston has a deck clearance of 0.7mm, meaning that the stack-height ends 0.7mm higher than the actual deck.

Is this deck clearance definite Geoff? Just making sure that the b25 has a definite deck height of 206mm, and it is the eta which we are unsure about.

Well, since Reggid has kindly offered to measure the deck-height (thanks Reggid!) I suggest we delay casting this value in concrete. If it turns out to be 206.2mm then the deck clearance reduces from 0.7mm to 0.5mm. I would have thought that the German engineers would have settled upon a nice round number like 206.0mm, but there is no rule that says they had to. Also, Ant seems pretty confident in his value of 206.0mm, but then who knows.

FYI, those digital dial-gauges are expensive. Unless you have your heart set on one, an analogue is more than adequate, and they are more freely available.

[GeoffBob]

It is worth noting that it is near impossible to get the 1.75mm head gasket these days, they are the +0.3mm (2.05mm) ones that are being sold, atleast that is what the BMW dealer and another supplier told me and this was a few years ago.

This is good to know, thanks!

[GeoffBob]

[CHR1S1990]

would something like this not suffice Geoff?

http://cgi.ebay.co.uk/Digital-Probe-Ind ... 4aab6309ed

[GeoffBob]

would something like this not suffice Geoff?

http://cgi.ebay.co.uk/Digital-Probe-Ind ... 4aab6309ed

Should work fine Chris, looks very nice. Locally, if I want digital I have to order. For analogue I can just pop down the shops and buy one.

[Sebastian35]

It is worth noting that it is near impossible to get the 1.75mm head gasket these days, they are the +0.3mm (2.05mm) ones that are being sold, atleast that is what the BMW dealer and another supplier told me and this was a few years ago.

This is good to know, thanks!

my local dealer will supply both sizes although i have a MLS 86mm in std 0.70in on order.

[Sebastian35]

Can i just say i am the only here that ends it with criticism. I really do not know what to say! how do i know easagood had a low comp piston thats all hear say too! as i have not measured it have you? i measured my head then you told me i must have done it wrong! now everyone is suggesting 42cc is right! one thing is for sure I give up................!

[reggid]

i got 206.2mm again on my B25 block. I believe this engine has not been rebuilt so am confident that is the original number.

The way i measured was from the machined top deck surface to the machined face that the main caps are bolted to. I assumed these were accuratetly machined on the crank centreline since the caps are half round and it would be difficult to assume the bearings if not. If the cap/block interface are not quite on the C/L then the measurement is wrong.


This disagrees with this


which says 206.7mm

[GeoffBob]

Can i just say i am the only here that ends it with criticism. I really do not know what to say!

I have said this before Sebastian, I’ll say it again. None of what I have said is intended as a personal criticism of yourself. If I have criticised your work it has been intended as a constructive criticism. That is to say, with the purpose of identifying mistakes in order that you might avoid the pitfalls that both myself and others have already fallen into. Nothing personal was ever stated or implied. You have already fallen victim to one set of dud pistons, I am trying (with increasingly reduced patience) to help you avoid a second set.

FYI Sebastian, I have endeavoured to offer you the best advice I can. If you choose not to follow that advise then it is indeed your prerogative to do so (I am not the only person posting here) and your money to spend. However, please do not confuse my advise with criticism. I can assure you it is not intended that way.

how do i know easagood had a low comp piston thats all hear say too! as i have not measured it have you?

Ezagood posted pictures of his JE pistons which appear to be modelled on the low CR M20B25. He also stated as much, and we have no reason to doubt his statement.

He also compression tested his engine after fitting his new pistons and posted his recorded pressure, which is in-line with that which would be expected of a 10:1 CR engine.

did it today. 10 puffs brought it up to 225psi. i can see where your calcs went wrong though. im 85.5mm bore.

All of this was posted here in your thread.

More importantly, however, I am not suggesting that you model your piston on the low CR B25 piston simply because Ezagood did the same. I am telling you that you need to model your piston on a low CR B25 piston because when you stroke a B25 engine from 2494cc to 2894cc with an 8.8:1 crowned piston, the CR changes from 8.8:1 to 10:1 - That’s the reason why And we know it changes from 8.8:1 to 10:1 without having to have intimate knowledge of stuff like deck-heights, compression-heights, head-volumes etc, all of which we don’t yet know accurately (and apparently, neither do TopEnd or JE).

Having said that, I have twice now answered your question with regard to modifying the crown of a high CR B25 piston if that is the route you choose to follow. The choice is ultimately yours.

i measured my head then you told me i must have done it wrong! now everyone is suggesting 42cc is right! one thing is for sure I give up................!

If you did not place a piece of clear Perspex or polycarbonate over the head when you measured the volume then you did not measure correctly. You need to follow the correct measurement procedure in order to measure the correct value. Getting the correct number doesn’t justify your measurement procedure. For all you know your head has been skimmed multiple times during its life and you’ll never know unless you measure properly. FYI, I posted up detailed info on how to measure head volume correctly for you in your thread Sebastian. This wasn’t a criticism, this was intended to teach you how to measure the correct value in order to aid you in diagnosing your problem with high compression.

[whodwho]

i got 206.2mm again on my B25 block. I believe this engine has not been rebuilt so am confident that is the original number.

The way i measured was from the machined top deck surface to the machined face that the main caps are bolted to. I assumed these were accuratetly machined on the crank centreline since the caps are half round and it would be difficult to assume the bearings if not. If the cap/block interface are not quite on the C/L then the measurement is wrong.


This disagrees with this


which says 206.7mm

This document seems to be the root of much of the confusion on the block height. While this is great info on pistons it has nothing to do with the block dimensions.

That measurement is in reference to the combined height of the crank, rod and piston which is all documented there for you to confirm.

1/2 stroke + rod length + Piston Compression height

40.5 + 130 + 35.7 = 206.2 for a B27 stack
37.5 + 135 + 34.2 = 206.7 for a B25 stack

Forget all the fancy tools... If anybody has the two blocks and an accurate level, sit them side by side level them up put the level across both of them and if not level(equal height) then shim the low side until level and measure the shim.

Sounds like it would work?

Won't give you the measurement but at least the confusion there is a difference in the blocks. It seems that the 206 is an accepted dimension for the B25 block at least.

[CHR1S1990]

i have to agree. I think those are stack heights not deck heights

[GeoffBob]

Agreed, definitely stack heights. I think someone put that table together using data gathered from the Mahle website. Those are definitely screen-shots of the Mahle pistons that have been pasted into the table.

I guess it's official then - there is a huge amount of confusion out in the world between to deck-height and stack-height. The situation cannot be helped by the fact that the same terms are not employed universally. I have run into exactly the same problem with gearboxes. What I know as the "spigot" and "lay" shafts are now apparently called the "input" and "counter" shafts. Funny old world this.

I haven't yet bought this book, but if anyone’s interested I suspect it would help put to bed some of the confusion. I plan to buy this as soon as its on my local shelves: Performance Automotive Engine Math by John Baechtel, published by Cartech. See here. Only just been released, so hot off the press. I doubt it goes into as much detail as my Uni text books did, but it’ll be a shorter and easier read, and likely present only that knowledge which is required to get to the desired result. None of the usual gumpf that one has to cut through to get to the juicy need-to-know stuff. Well that’s my experience with the Cartech series of books anyway.

Chris, FYI, have a look at this sample PDF of chapter 7 here with regard to porting of heads. Right up your street RE your 731 head idea me thinks?

[GeoffBob]

i got 206.2mm again on my B25 block. I believe this engine has not been rebuilt so am confident that is the original number.

The way i measured was from the machined top deck surface to the machined face that the main caps are bolted to. I assumed these were accuratetly machined on the crank centreline since the caps are half round and it would be difficult to assume the bearings if not. If the cap/block interface are not quite on the C/L then the measurement is wrong.

Thanks for measuring Reggid, much appreciated. The way you measured is exactly how I understand it would be done in a machine-shop. The block would typically be upended deck-down on an engineers table and measured with a sufficiently long vernier calliper, which I assume is exactly how you did it?

I agree that if the interface (where the caps meet the block) is not in line with the centre of the crank then the measurement will be in error. I cannot imagine, however, that this would deliberately be the case, due, as you say, to complications with fitting the bearings.

Just to be on the safe side, however, it might pay to pop a straight-edge (such as an engineers steel ruler, sharp edge down) over the underside of the block and measure with a vernier calliper to the deepest point of the bearing surface (bearing shells removed of course). Then do the same on the opposite cap. Regardless of the exact measurement, which will of course include the width of the ruler, the two values should be identical if the interface is in-line with the crank axis.

Not the most accurate method, I agree, but sufficiently accurate to pick up a 0.2mm difference if that were to be the case (which I doubt).

[CHR1S1990]

Thanks for that Geoff, right up my street indeed and very to the point. Ill get it on order as soon as

[reggid]

i got 206.2mm again on my B25 block. I believe this engine has not been rebuilt so am confident that is the original number.

The way i measured was from the machined top deck surface to the machined face that the main caps are bolted to. I assumed these were accuratetly machined on the crank centreline since the caps are half round and it would be difficult to assume the bearings if not. If the cap/block interface are not quite on the C/L then the measurement is wrong.

Thanks for measuring Reggid, much appreciated. The way you measured is exactly how I understand it would be done in a machine-shop. The block would typically be upended deck-down on an engineers table and measured with a sufficiently long vernier calliper, which I assume is exactly how you did it?

I agree that if the interface (where the caps meet the block) is not in line with the centre of the crank then the measurement will be in error. I cannot imagine, however, that this would deliberately be the case, due, as you say, to complications with fitting the bearings.

Just to be on the safe side, however, it might pay to pop a straight-edge (such as an engineers steel ruler, sharp edge down) over the underside of the block and measure with a vernier calliper to the deepest point of the bearing surface (bearing shells removed of course). Then do the same on the opposite cap. Regardless of the exact measurement, which will of course include the width of the ruler, the two values should be identical if the interface is in-line with the crank axis.

Not the most accurate method, I agree, but sufficiently accurate to pick up a 0.2mm difference if that were to be the case (which I doubt).

i have a stock b25 bottom end available the moment and will check. I also have a 885 head getting a head job and will get them to cc the chamber

[Sebastian35]

Agreed, definitely stack heights. I think someone put that table together using data gathered from the Mahle website. Those are definitely screen-shots of the Mahle pistons that have been pasted into the table.

I guess it's official then - there is a huge amount of confusion out in the world between to deck-height and stack-height. The situation cannot be helped by the fact that the same terms are not employed universally. I have run into exactly the same problem with gearboxes. What I know as the "spigot" and "lay" shafts are now apparently called the "input" and "counter" shafts. Funny old world this.

I haven't yet bought this book, but if anyone’s interested I suspect it would help put to bed some of the confusion. I plan to buy this as soon as its on my local shelves: Performance Automotive Engine Math by John Baechtel, published by Cartech. See here. Only just been released, so hot off the press. I doubt it goes into as much detail as my Uni text books did, but it’ll be a shorter and easier read, and likely present only that knowledge which is required to get to the desired result. None of the usual gumpf that one has to cut through to get to the juicy need-to-know stuff. Well that’s my experience with the Cartech series of books anyway.

Chris, FYI, have a look at this sample PDF of chapter 7 here with regard to porting of heads. Right up your street RE your 731 head idea me thinks?

Thanks Geoff I have placed an order for the boodk!

[reggid]

i rechecked the deck height and got 206.15mm.

I ran a depth gauge from deck to the cranksuface where the oil seal sits then used a vernier for the crank some simple maths and hey presto.....

182.1 + 48.1/2 = 206.15mm (Deck to Crank C/L)

This is for a B25 (256k1) block from a 89 model.

[HairyScreech]

im going to have to read back through this thread and make a few notes, as there was a few things i was going to pick up on and either confirm or deny but having a short term memory thats next to useless iv already forgotten what they were.

when i measured my 885 head i got 41cc and in this case i was erring on the side of caution as under estimation by 1cc is better than over estimating by 1cc, thus i think the ~42cc measurement is the right number.

the 1.7mm head gasket that i measured was a fairly recently fitted elring gasket that was purchased from gsf about 2 years ago, so im working on the theory if you want the proper 1.7mm thickness then go for an elring.
if anyone knows a similar priced 2mm with similar quality it would be handy to know as that extra .3 of a mm will help with the development engines clearance. (obviously this is pointless if the gaskets are 3x the price)

as far as the compression ratio of the engine in the first place goes i would put money the detonation not being caused by the actual compression ratio but more the design of the piston.
the flat piston will have created a void where the squish band used to be, and this being as far from the plug as is possible in an m20 will have been the last part to burn, thus at that compression ratio that area of the cylinder was just too hot and pressurized for the fuel to handle.
i would put money on the engine having been fine at 11:1 with the stock low comp piston crown shape, as i have had conversations in the past with ant and he has run them at 11.5:1 on normal fuel with no detonation problems.
it has infact been stated that the 2.5 m20 is a very detonation resistant engine and some frankly stupid ignition advances have been tried in a "what if?" experiment.

"Page 172 - advanced engine technology.
To prevent detonation at these elevated compression ratios:
1. increase turbulence and thus the speed of flame travel therefore preventing the end mixture overheating
2. reduce the distance the flame front has to travel and increasing the volume to surface area in the region furthest from the sparkplug.
3. raising the octane value of the fuel used. "


now the fact the piston you got were over 12:1 is a complete fecking joke, if that was my company you would be getting a set of 86mm pistons with the correct shape foc out of shear embarrassment that we made such a large assumption (with out asking for confirmation from the customer i might add) on such a critical component.
seb i would say you are well within your rights to demand either a set of pistons to the correct specification or a full refund, as what you got there were completely different to your original ask.
(and one other thing about the spec sheet they sent back, "open design" i hope there not referring to the chamber design there as if they are then i question if they have ever seen the inside of the m20 engine in any form, open my fuking arse, but ill get onto that later)

a couple of points on the two heads, the 731 head is indeed designed to work with a flat topped piston, this is why a 731 head wont work on a 2.5 bottom end as the raised squish band portion of the piston WILL clash .

now i say that with one major exception, the engine im using as a base for the 2.8 was at one point running a 2.5 low comp bottom end with a 731 head.
my enterprising fellow spanner had done some interesting adjustment to the chambers.

where the dremmel marks are is where he has had to adjust the 731 head to account for the squish band of the 2.5 pistons, crude in execution yes but seriously effective, the engine ran sweet as a nut.
the areas he has taken material from are the bits that make the difference in the design from flat topped to profiled piston.
when those areas had more meat on them they behaved in the same manor as the squish band on the 2.5, thus creating a "closed" chamber.

im using the term "closed" to dictate a combustion chamber design that has a portion where the piston and head are in close proximity thus squishing the fuel air mixture out of the area closing off that area of the chamber.

these areas are really important as they create swirl and tumble, and help close off areas of the chamber where detonation is most likely to occur. (like the area furthest from the plug which your flat tops opened up again )

it was proven many years ago by ford that a closed design will ALWAYS make more power than an open design all things being equal.

[HairyScreech]

as for head choice for the new engine i see no reason to go with a stock 731 head for a number of reasons:
1. valves are smaller and even putting the 42mm inlets from a 885 into the head and working the ports to suit i didnt get enough improvement to justify the work or cost.
2. the ports carry the bulk of their flow lower down making them less of a down draught port.
3. there is not the material available in the head to correct situation 2
4. the 731 head has a smaller combustion chamber than the 885 which for a stroker is only going to whack up the cr even more
5. the offset dish of the 2.5 combustion chamber is theoretically superior in terms of placing the spark plug as close to the center of a spherical combustion chamber as possible, with this being the thermodynamic ideal.
6. 885 heads crack and we all know that cracked heads are for winners. just ask Charley Sheen.

[HairyScreech]

with regard to the deck height of the block, i think it may be a nominal 206mm measurement, in that there is a variance in the height within a certain specification and blocks falling between 206mm and 206.2mm were considered acceptable.
considering these blocks are castings with there associated loose tolerances and then have to be ground or skimmed to level perhaps someone made the decision to say over 206mm is fine as long as its not under and isnt so far over it buggers up the squish band.
this is only speculation of course but would make a lot of sense.

when i have been working with measurements like this on my project i have just been using the stack height of the 2.5 set up as my base line, knowing full well that those more precisely machined components will work in all cast 2.5 blocks.
which means as long as the new set up matches the stack height of the old set up then you cant go far wrong.

as for the 0.5mm difference when building 2.8s with 320 rods im thinking this is THE main cause of them making less power, i have done the CR math with the two different piston designs and then accounted for the 0.5mm skim and it gives the results below:


now im thinking when andyboy wrote the article he forgot to take into account the 0.5mm and the values in the wiki are true for a block that has been decked 0.5mm, an easy mistake to make.

the addition of 0.5mm extra clearance opens up another issue in the form of a slack squish band, (ooh err mrs), excuse my quick digression into yankee units.
"chamber cavities ranging from 60 to 120 thou are most likely to cause detonation"
now the piston is 0.045" from the head on a stock motor and an extra 0.5mm is another 0.019" leaving us with a cavity of 0.064" or 64 thou.
can you see where im heading with this......

i think the only reason this has not been a problem and we havent had 2.8 detonating all over the shop is that your still looking at a relatively low compression ratio at 9.3:1 and the pressure build up in these motors with slack squish bands isnt enough to light off the fuel in these areas.
if we were to somehow increase the compression ratio without tightening the squish band (by removing the bowl perhaps) then i expect we would start to see more problems with detonation in this area.

as for the motors that are running about at 10.5:1 and above i can only assume these are using custom pistons and these pistons are not the extra 0.5mm further down the bore. this will mean that the squish area quenches and squishes the mixture better and the detonation problem in that area goes away.

[HairyScreech]

just to clarify the piston does protrude somewhere in the region of 0.5mm from the surface of the 2.5.

and as for removing material from the head, you can gain 5cc by making it into a hemi head as i tried out, but by all accounts this is not worth doing.

oh and one other thing, those pistons look like the ones metric mechanic supply, and from looking at their numbers there getting unimpressive outputs for the capacities of the engines, they also seem to have the frankly laughable for a perfomance motor full clearance design in the valve cut outs, as for why thats bad see my point about cavities in the combustion chamber and then work out the size of the cut outs. (and lets face it, the only time you would need a clearance design is if your going to neglect your belt changes, if your doing that and risking a £2000 motor for the sake of a £30 belt ever 30k then you need your head read anyway)


now all of the above could well be the deranged hungover rambling of a 25 year old student and have no foundation in reality, but ill leave you to spot the holes (as that way it saves me a job )

[eta]

So to get a M20B28 to perform properly you are saying all that is needed is the blocked to be decked 0.5mm?

[HairyScreech]

well no, theres still the issues of, well everything in this thread

http://www.e30zone.net/modules.php?name ... c&t=170822

there is no quick easy answer but im confident as are a handful on this forum that there is still plenty more to come from the m20.