2.9 stroker pinging

[CHR1S1990]

I guess i could mock build the eta block back together with a 130mm rod and eta piston and measure how much the piston protrudes (if any) from the deck @TDC, then swap the crank/rod/piston into the b25 block, do the same, and calculate the difference (if any).

I need to buy a new dial gauge

[GeoffBob]

Chris, I get the feeling that it was Ant that wrote the Wiki page here too, and I think we can trust what Ant had to say.

just to quote from one of many examples

206mm CL of mains to deck is the given OE spec Alex(B20B23B25, B27E are 206.5mm)...

which suggests the eta blocks are 0.5mm taller which is what I originally was lead to believe, but this obviously doesnt add up by the stack heights.

Which thus indicates different deck clearance between the two engines. Remember, the difference between deck-height and stack-height is your deck clearance! And if the Wiki is anything to go by then BMW experimented with differing deck clearance values on different models by varying the deck height. All the more reason to follow the Wiki advice and measure your deck height to be sure.

Deck height is measured from the axis of the crank to the face/deck of the block. Stack height is all the components added up. Deck height is best measured using the crank, rods, pistons etc all of known values since it’s a bit hard to run a vernier to the axis of the crank.

One thing is for sure though, if the stock deck height of the 325i is 206.0mm, then the stock m20B25 has a deck-clearance of 0.7mm. And that means one thing, the head must have 3.9cc more volume than I originally calculated (My bad ). That is to say, 44cc rather than 40cc. Now before anyone jumps down my throat, this doesn’t change the fact that the total compression volume (at TDC) of the low CR M20B25 is still 53.3cc. All it means is that the volume that has been taken off the block (by reducing its height) is replaced by volume on the head, they still add up to the same (I can’t emphasise this enough). The implication, however, is that with a head up-turned on the work-bench you should measure 44cc. 42cc would indicate that at least 0.35mm has been skimmed off the head. Alternatively, the stock head volume is 42cc, and the gasket accounts for 12cc (not 10cc as I have believed for some time). I am beginning to suspect the latter (since both Reggid and Sebastian have measured 42cc) and that I am now as much to blame for spreading misinformation over the internet as I am a victim of it.

Note that once the head is down on the block there is still 53.3cc of compressed volume over the M20B25 low CR piston at TDC. All that has changed is the location of the interface between the deck and head in relation to the crank axis due to a change in deck-height and hence a change in deck clearance. This doesn’t affect the calculations that I did for Sebastian, Chris, so long as his deck height remains unchanged from stock (to which the volume of 53.3cc applies). In your case, however, you need to take deck-clearance into account since it sounds to me as if the 325i and Eta engines had different deck-clearance values.

Sorry to complicate matters, but I thought I should mention this.

[CHR1S1990]

Its the "Building a 2.8" which I was questioning Geoff...

"The beauty of the 2.8 crank is that it’s 84mm stroke - with standard 325i pistons and 130mm 2 litre, 2.3 litre and 525e M20 ”˜short’ rods - allows the pistons to come to the top of the block. No machining of the block face is required although a 5 thou clean up skim is a very good idea."

..as the b25 stack height is 206.7mm and using the same pistons in the same block gives a stack height of 206.2mm. Regardless of how 'tall' the block is, theres still a 0.5mm difference in the same block. I know it's 'only' 0.5mm, but perhaps another contributary factor as to why 2.8 strokers dont make their expected power with a slightly lower than expected CR.

Only way to be sure is to measure. Would be nice having some hard evidence to back up proposed deck heights, even for just peace of mind (not for a moment disregarding Ant's given values).

I'll head off to B&Q and get some perspex and meth's and measure the head volume (multiple times for a nice average). Ill get a new dial indicator ordered and get both block deck heights measured up properly, and then take it from there.

[whodwho]

The blocks are the same it is the stack height that everyone gets confused with deck height.

[GeoffBob]

[GeoffBob]

I did a quick calculation on that 2.8 stroker Chris.

With 84mm stroke, 130mm rods and stock M20B25 pistons with KH=34.2mm, I get a stack-height (same as you) of 206.2mm. I agree that’s 0.5mm lower than stock.

That 0.5mm will contribute an extra 2.8cc to the compressed volume at TDC, bringing the total to 56.1cc with the low CR M20B25 piston. Since the displaced volume is 465.5cc (84mm bore with 84mm stroke), I determine a CR of exactly 9.3:1.

If, however, I ignore that extra 2.8cc due to the 0.5mm reduced stack height, I determine a static CR of 9.75:1, which is the value published on the 2.8 Wiki page, so I think we can clearly see the mistake that was made.

Is our own Wiki really espousing such gumpf Chris, or have we missed a trick here and we're the idiots?

More to the point, do I really have nothing better to do on a Friday evening?

[whodwho]

The blocks are the same it is the stack height that everyone gets confused with deck height.

To expand and clearify this a bit

You hear the block heights of 206.7 for an B25 block and 206.2 for an B27 block

If you add up the stack height:

1/2 stroke + rod length + Piston Compression heigth

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

That would make them sit flush and we know that is not the case, most evident on the B25

[CHR1S1990]

[CHR1S1990]

I did a quick calculation on that 2.8 stroker Chris.

With 84mm stroke, 130mm rods and stock M20B25 pistons with KH=34.2mm, I get a stack-height (same as you) of 206.2mm. I agree that’s 0.5mm lower than stock.

That 0.5mm will contribute an extra 2.8cc to the compressed volume at TDC, bringing the total to 56.1cc with the low CR M20B25 piston. Since the displaced volume is 465.5cc (84mm bore with 84mm stroke), I determine a CR of exactly 9.3:1.

If, however, I ignore that extra 2.8cc due to the 0.5mm reduced stack height, I determine a static CR of 9.75:1, which is the value published on the 2.8 Wiki page, so I think we can clearly see the mistake that was made.

Is our own Wiki really espousing such gumpf Chris, or have we missed a trick here and we're the idiots?

More to the point, do I really have nothing better to do on a Friday evening?

I am in total agreement - i have nothing better to do either

I cant see anything we have missed, maybe they didnt deem 0.5mm that relevant? I hold it quite important, seeing as though now Im arguing over two blocks with a proposed 0.5mm difference between them

[toby]

If I remember correctly, Andyboy wrote the 2.7 & 2.8 guide. Some do prefer to deck the block on a 2.8 build.

[CHR1S1990]

To expand and clearify this a bit

You hear the block heights of 206.7 for an B25 block and 206.2 for an B27 block

If you add up the stack height:

1/2 stroke + rod length + Piston Compression heigth

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

That would make them sit flush and we know that is not the case, most evident on the B25

Not according to others on the zone...

...Aswell I heard (unable to confirm) that E block is 0.5mm taller than I block. can somebody confirm ? how to measure tell them apart ?

Response..

the ETA block is 206.5mm from deck to crank centreline, B25 blocks are 206mm dead .

[CHR1S1990]

Alpina used TD crank, 325i rods ( @ 135mm ) and specially made versions of the M20B25 pistons with the wrist pin moved to allow the whole lot to run in a stock M20 @ 206mm deck height.

however..

no my b25 was 206.2mm which is the same for b27 aswell despite there being a website with mahle piston data showing otherwise

[GeoffBob]

Some do prefer to deck the block on a 2.8 build.

I can well believe it Toby. 2.8cc can make a substantial difference at the height of the compression stroke (or so my wife tells me ).

Given that I have never laid hands on a 2.7 Eta engine, if it had the same deck-height as the M20B25 (and I am not disputing this, I have no opinion either way) then why would the Eta engine warrant getting it's own special block? What made it different? I can understand the different head with fewer cam journals (reduced friction), and the restricted intake; not to mention the 81mm crank and the change in compression ratio. But why give it it's own block? Sounds like a stock M20B25 block to me with a different number stamped on the side.

What am I missing? I ask only out of curiosity. Unlikely that I'll ever lay hands on an Eta engine down here.

[eta]

I was talking to Ant once and he said that the B27 block and B25 block where the same. I took that to mean the deck height is the same as I think our conversation was about that. I agree that the all the stack numbers you are using are the published one so must be good and they give that 0.5mm diference. I also agree that I have seen 2,8 build threads which point towards a 0.5mm difference in the piston top and block top. As this build was in a 325i I presume that a B25 block was used.

With apparantly condradictory information about someone needs to measure up a B27 and B25 block. This is why I posted what I did as I would hate for Chris 1990 to build his engine on a wrong assumption. Better to know all the parameters first.

I would really like to know the answer to this one but I do not have spare blocks. Also how would you measure the bloxk height as 0.5mm is not nuch over 206ish mm.

[CHR1S1990]

thats strange considering every post about the b25 block being 206mm and the b27 being 206.5mm was from ant...Im sure he hasnt just dreamt up these numbers but theres no harm in checking.

No problem eta, i merely wanted to know if there was supposed difference between the two blocks so I could try and calculate some different compression ratios between two identical builds (blocks being the variant). As most people know on the zone, whilst most information is pretty solid (like which cranks are swappable), some is the result of chinese whispers. As stated, many people believe the stack height to be the actual deck height. Nevertheless, Ill measure the two anyway as even if there is a difference between the two blocks as a general rule, doesnt mean there are between my two blocks.

I'm assuming I could build the eta crank, 130mm rod and eta piston back into the eta block, Zero my dial gauge on the block face and then measure the deck clearance at TDC. Swap it all over into the b25 block and do the same and calculate the difference, if any.

[GeoffBob]

[CHR1S1990]

I'd like to measure it myself ideally, how would I go about doing this without a spare piston? I don't need the eta pistons anymore, but theyre in quite good condition so would be a shame to mill anything off them. Saying that, I do have some spare high comp b25 pistons with scored skirts

[GeoffBob]

I can think of a variety of ways off-hand Chris but all potentially not very accurate. I'll get my books out and see what they suggest and get back to you.

[GeoffBob]

No easy answer I'm afraid. Seems your best bet will indeed be to have an old piston milled flat to a known "reference" compression height.

FYI, I did discover some conflicting information between books with regard to the definition of "deck-height" though. I have always understood deck-height (sometimes called block-height) to be the measurement from crank axis to the face of the block or "deck". One book I opened used the term block-height as the obvious, and then used deck-height to refer to what I understand to be "deck-clearance". Anyone else come across this standard? Possibly it’s American? It’s not what I was taught but I could have learned wrong.

Of course, when building ones own engine all that matters is the accuracy of ones own measurements, it doesn't matter what you call them. Using the correct terminology is, however, expedient to good internet communication, so if I have it wrong would someone please kindly let me know.

[CHR1S1990]

No easy answer I'm afraid. Seems your best bet will indeed be to have an old piston milled flat to a known "reference" compression height.

Well, I can have an early high comp piston milled down as theyre scrap - do these have the same KH as the late pistons?

FYI, I did discover some conflicting information between books with regard to the definition of "deck-height" though. I have always understood deck-height (sometimes called block-height) to be the measurement from crank axis to the face of the block or "deck". One book I opened used the term block-height as the obvious, and then used deck-height to refer to what I understand to be "deck-clearance". Anyone else come across this standard? Possibly it’s American? It’s not what I was taught but I could have learned wrong.

ive come across this a few times actually geoff, on e30 tech actually i think? Perhaps it is an american thing

Of course, when building ones own engine all that matters is the accuracy of ones own measurements, it doesn't matter what you call them. Using the correct terminology is, however, expedient to good internet communication, so if I have it wrong would someone please kindly let me know.

Which brings me to ask, how accurate do I need to mill the piston down to? I.e. is mr grinder my friend here? Surely I only need to mill a flat part on top of the piston (parallel to the axis/perpendicular to the sidewall), measure from this point to the edge of the piston pin bore using a verneir caliper, add half of the bore to find the exact axis (and therefore the total distance), then measure the deck clearance in both blocks. Or is it not this simple?

[GeoffBob]

[CHR1S1990]

I think I can use the ETA pistons afterall, Geoff, without and milling down. Heres a few pictures. Note this is the early 11:1 piston, with a slight raised crown, small valve relief and a very small squish band.



Measured from the flat part of the piston...



To the edge of the pin bore...



=



Piston pin bore is 22mm according to Mahle, so 24.77 + (22/2) = 35.77mm. KH is 35.7mm according to Mahle, again, so a squish band of 0.7mm which looks about right.

P.s. what a nightmare trying to hold the vernier caliper whilst taking pictures with the other hand

[CHR1S1990]

[eta]

Interesting what you say Geoff about the early eta blocks being 0.5 mm taller. I cannot confirm or refute this but in September 1985 there where alot of changes to the E28's, that may have been one of them.

This is all very confusing.

Also I have the measured crushed head gasket thickness as 0.17272 cm and it "volume" as 9.570 cc. So I think you value of 10cc's for the HG volume is correct Geoff. I have also always believed the 885 head combustion chamber volume is 42cc's.

[eta]

Eta blocks were changed in September 1985.

[GeoffBob]

I think I can use the ETA pistons afterall, Geoff, without any milling down. Heres a few pictures. Note this is the early 11:1 piston, with a slight raised crown, small valve relief and a very small squish band.

That's the first time I have ever seen one of those pistons Chris. Looks doable to me. I'd mark a point with an ink pen and measure to that same point each time in case there is any inconsistency, but I doubt there would be. Will be interested to hear your results.

[GeoffBob]

Piston pin bore is 22mm according to Mahle, so 24.77 + (22/2) = 35.77mm. KH is 35.7mm according to Mahle, again, so a squish band of 0.7mm which looks about right.

Technically a deck-clearance of +0.7mm, Chris.

The squish band (or more of a mild chamfer in this case) would be 0.7mm tall only if the inteface (edge) where the cylindrical piston body meets the squish-band is at exactly the same height as the deck, which is likely but not guaranteed.

Just to get this clear in my mind (given that I know nothing about the Eta engine) the Eta engine used the 731 head, right? Looking at that piston then it looks to me as if the the 731 head was clearly designed for use with flat-top pistons. So long as you can port it properly (as you discussed earlier), so long as the valve angles and positions are compatible, and so long as its volume gives you the static CR you are after, then its profile seems infinitely more compatible with those flat-top JE's you got from Sebastian. So I think your 731 idea has merit Chris and is worth investigating at least to the point where you have all the relevant volumes measured and documented (so that you can make a yay or nay decision).

Do you have a 731 head that you can measure at the moment or are you still waiting for one?

[GeoffBob]

[CHR1S1990]

I think I can use the ETA pistons afterall, Geoff, without any milling down. Heres a few pictures. Note this is the early 11:1 piston, with a slight raised crown, small valve relief and a very small squish band.

That's the first time I have ever seen one of those pistons Chris. Looks doable to me. I'd mark a point with an ink pen and measure to that same point each time in case there is any inconsistency, but I doubt there would be. Will be interested to hear your results.

Duely noted Geoff, Ill get right on it (and order a digital dial gauge).

[CHR1S1990]

Piston pin bore is 22mm according to Mahle, so 24.77 + (22/2) = 35.77mm. KH is 35.7mm according to Mahle, again, so a squish band of 0.7mm which looks about right.

Technically a deck-clearance of +0.7mm, Chris.

The squish band (or more of a mild chamfer in this case) would be 0.7mm tall only if the inteface (edge) where the cylindrical piston body meets the squish-band is at exactly the same height as the deck, which is likely but not guaranteed.

Just to get this clear in my mind (given that I know nothing about the Eta engine) the Eta engine used the 731 head, right? Looking at that piston then it looks to me as if the the 731 head was clearly designed for use with flat-top pistons. So long as you can port it properly (as you discussed earlier), so long as the valve angles and positions are compatible, and so long as its volume gives you the static CR you are after, then its profile seems infinitely more compatible with those flat-top JE's you got from Sebastian. So I think your 731 idea has merit Chris and is worth investigating at least to the point where you have all the relevant volumes measured and documented (so that you can make a yay or nay decision).

Do you have a 731 head that you can measure at the moment or are you still waiting for one?

Sorry Geoff, you're right of course, a mis wording problem on my part.

The eta uses the 200 head, with an identical combistion chamber to the 731 head. Funny you should say that though Goeff, the 731 head arrived today at my Mother's house. Ill take some pictures tomorrow (off to finish machine polishing her e36 )

[Sebastian35]

Interesting what you say Geoff about the early eta blocks being 0.5 mm taller. I cannot confirm or refute this but in September 1985 there where alot of changes to the E28's, that may have been one of them.

This is all very confusing.

Also I have the measured crushed head gasket thickness as 0.17272 cm and it "volume" as 9.570 cc. So I think you value of 10cc's for the HG volume is correct Geoff. I have also always believed the 885 head combustion chamber volume is 42cc's.

When i measured my head i got 42cc although there was a lot of noise on the Zone about it?

[Sebastian35]

Hi guys I have to say my pistons are on hold as I am no longer sure of the data I have given to JE as being correct?

I am in a catch 22 and I can't measure my original engine as it no longer exsists in factory form!
It would be helpul if i knew what the piston volume was on the 1987 engine with the high comp pistons.

JE have one of my high comp pistons and will take a pattern then enlarge it to 86mm but with the larger bore is more volume. what I want to know is how much extra volume would i need to remove in cc's from the piston to drop the CR to 10:0:1? I think my engine has a std CR of 9.4

At the moment I have asked them to send me the data of a enlarge piston with +1mm valve cut outs enlarged to 86mm bore and and extra 4cc's removed to what was already there, before they make them.
when i receive this info i should be able to caculate the intended CR and make any changes if needed? Geoff i will call on you then if you don't mind.

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

[eta]

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.

[eta]

I kind of wish I had the cash to get my spare 731 head rebuild and ported with 325i valves and put that on my 2.7 bottom end. Maybe I should have a go my self....

[CHR1S1990]

where did you get the crushed headgasket thickness of 0.068"/0.17272cm eta? I always used .070"/1.778mm