Compression ratio values not adding up [UPDATED WITH CALCULATOR]

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szacsi72
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Compression ratio values not adding up [UPDATED WITH CALCULATOR]

Post by szacsi72 »

Hi all,

So, I have a slight suspicion that my car has a head gasket fault. It runs pretty crap and has a really bad noise to it... ever since I cooked it once (bad t.stat).. I will measure compression in a couple of weeks, but in the meantime I'm just trying to see what options I have. Anyway, I have a complete cylinder head that I'd like to rebuild, so that the repair won't take multiple weeks with the engine open. While at it, I'm adding a new cam (284-292-ish), as I'll be adding ITB's, a new exhaust system and standalone management. And of course, the last element to this - I'd like to raise the compression ratio. I'd like to do that via shaving the head. I know, it's not the best option, but the engine block will stay in place so it is my only option.

The engine is an M30B35 from a '92 735i, europe (so I believe it is the 211PS version with cats).
I wanted to calculate the compression ratio, however when I did that with the stock values, I go the result of 8.7:1, instead of 9:1. Is this normal? I know that on M50 engines, the compression ratio is always lower than advertised.
These are the values I've used:
Bore 92mm
Stroke 86m
Head gasket thickness 1.72mm
Head gasket bore 93.5mm
Cylinder head CC 65cm3
Piston sticking out 0.35mm (compression height is 39.85mm according to Mahle, engine block I believe is 217.5mm)
Piston CC 0cm3 (in reality it is around -3cm3)

I'd like to shave the head around 0.75mm, which should result in a decrease in the combustion chamber of 4.5cm3, resulting in a compression ratio of 9.15:1, so a raise of ~0.45.
Last edited by szacsi72 on May 31, 2023 2:47 AM, edited 1 time in total.
Mike W.
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Re: Compression ratio values not adding up

Post by Mike W. »

The problem is trying to figure out the exact volume of that odd shaped piston in B35s. Don't know if you have seen it, but there is this thread from long ago and the link still works! viewtopic.php?t=51939

Yes, cutting the head will increase compression, I did one long ago what was about .025" or .6MM reduced for a US spec 2.8 engine with 8.2 CR and no problems, but that was because it was shaped like a banana, not for an increase in compression. Valve timing is affected too, I think retarded which seems to slightly reduce torque and increase power at a slightly higher RPM. Don't go off what I'm saying, do your own research.
szacsi72
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Re: Compression ratio values not adding up

Post by szacsi72 »

Hi,

the excel sheet also states 65cc for the B35 head.
The only thing I can think of is piston CC/compression height.
Mahle states the CH for the stock B35 piano tops is 39.85mm. But where exactly is this measured? Is this measured at the side of the piston, or at the "piano" part? Unfortunately I don't have access to unused M30 pistons as of now.

If the compression height is for the "base" height, and the piano part is a plus over that (7cc), then I get 9.472:1.
If the compression height is for the whole part, then I get a compression ratio of 8.675:1 if I use piston CC of 0.

I've found this link that says dome height of 2mm.
https://www.bmw2002faq.com/forums/topic ... 0-pistons/
If I calculate with a full height of 39.85+2mm, then subrstract 7cc, I get 9.38:1.

If I calculate with a piston CC of (9.2/2 ^2 * PI * 0.2)-7 = 13.29 - 7 = 6.29cc, I can get values of: 9.47:1, 9.38:1... still not close enough.

On a second thought, how much do head gaskets compress? On forums meant for old V8s I've heard around 0.005" or 0.127mm for a head gasket with a thickness of ~0.8mm. So based on that, let's say it would compress 0.2mm.
With that, I'd get a value of 8.82:1 if I don't take piston CC into account.

Nope, no combination or logic seems to get me the stock numbers.

Damn. It's getting hard to tune M30's. Half the links are not working anymore and most just don't bother with big sixes anymore. Even M50s are getting old. I'm also getting old... *sigh*
tschultz
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Re: Compression ratio values not adding up

Post by tschultz »

Based on Brad D's M30 compression ratio calculator, stock M30B35 is 9.0:1 with a .08" (20.3mm) headgasket and 65mm CC head value. I believe this assumes this the compressed height is .08in to make that compression ratio though they may be measured uncompressed. Still,with .02" (.5mm) compression on a gasket it would change your compression ratio to about 9.04:1 (or 8.03:1 for B34 engines).

I am assuming it is correct, but he accounts for the volume of the headgasket, area x height in each cylinder along with the piston dome of 7.05cc's.
(CylVol + (HeadVol + HGvol - PistonDome)) / (HeadVol + HGvol - PistonDome) = 9.0 for B35 and 8.0 for B34 (dish of 10.16cc).

His calculation includes a head skim field which, with a .75mm cut (~.03"), brings that ratio up to a calculated 9.64:1 or corresponding 8.46:1 for B34. Usually these cuts (for a trueing of the head) are less than that, so I'd lean towards less cutting and would instead go with a thicker MLS headgasket. They can be purchased in thicker sizes if desired. This leaves the head safe for future skimming if ever needed.

Note there are some thoughts about adding a B34 head onto B35 but some say the small piston protrusions may interfere with the head or valves. Still Paul Burke and some here have reported successfully mating it up to achieve the 9.87:1 compression and 'budget' euro 10:1 engine--Just slightly less compression than the true euro 10:1 piano top version parts. Sounds like you have a full B35, so this won't make sense you you, but still interesting to see the different possibilities.

Not sure if this helps or not, but the number seem reasonable.
Mike W.
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Re: Compression ratio values not adding up

Post by Mike W. »

I have measured used head gaskets and IIRC they came in at .067" or 1.67MM. At the fire ring, but they are pretty flat.

Also surfacing the head won't be as effective as you might think since ~ half of it is flush with the surface of the head so the only part that would increase compression is the combustion chamber portion.

I do like Toms idea of using a thinner HG if you want to go that route to increase compression and it would decrease the volume of the entire area, not just half of it.
szacsi72
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Location: Hungary

Re: Compression ratio values not adding up

Post by szacsi72 »

tschultz wrote: May 25, 2023 1:10 PM Based on Brad D's M30 compression ratio calculator, stock M30B35 is 9.0:1 with a .08" (20.3mm) headgasket and 65mm CC head value. I believe this assumes this the compressed height is .08in to make that compression ratio though they may be measured uncompressed. Still,with .02" (.5mm) compression on a gasket it would change your compression ratio to about 9.04:1 (or 8.03:1 for B34 engines).

I am assuming it is correct, but he accounts for the volume of the headgasket, area x height in each cylinder along with the piston dome of 7.05cc's.
(CylVol + (HeadVol + HGvol - PistonDome)) / (HeadVol + HGvol - PistonDome) = 9.0 for B35 and 8.0 for B34 (dish of 10.16cc).

His calculation includes a head skim field which, with a .75mm cut (~.03"), brings that ratio up to a calculated 9.64:1 or corresponding 8.46:1 for B34. Usually these cuts (for a trueing of the head) are less than that, so I'd lean towards less cutting and would instead go with a thicker MLS headgasket. They can be purchased in thicker sizes if desired. This leaves the head safe for future skimming if ever needed.

Note there are some thoughts about adding a B34 head onto B35 but some say the small piston protrusions may interfere with the head or valves. Still Paul Burke and some here have reported successfully mating it up to achieve the 9.87:1 compression and 'budget' euro 10:1 engine--Just slightly less compression than the true euro 10:1 piano top version parts. Sounds like you have a full B35, so this won't make sense you you, but still interesting to see the different possibilities.

Not sure if this helps or not, but the number seem reasonable.
Hi! Thanks for your input.
The stock head gasket is 1.72mm, so I never quite understood why it's 2.03mm. And if it was calculated using 2.03, the compression ratio would be, according to his data (65cc, 7cc, -0.35mm, 92-86mm, 2.03) 9.2:1 or 7.8:1, depending on how the piston CC was measured (or what exactly that means). I suppose I will need to find someone who has some M30 pistons that are trash or I can borrow them for measurement. And I do also account for the head gasket volume.

I will make my own excel sheet when I have some free time and link it here.

Regarding the head swap, I believe the head on the B35 is better (if you don't port them) so swapping to a B34 head does not make sense. I'd rather swap in some piano pistons if needed, but I just can't be bothered to pull the complete engine AGAIN.. so that's why I'm looking at decking.
Mike W. wrote: May 25, 2023 4:31 PM I have measured used head gaskets and IIRC they came in at .067" or 1.67MM. At the fire ring, but they are pretty flat.

Also surfacing the head won't be as effective as you might think since ~ half of it is flush with the surface of the head so the only part that would increase compression is the combustion chamber portion.

I do like Toms idea of using a thinner HG if you want to go that route to increase compression and it would decrease the volume of the entire area, not just half of it.
Thinner head gaskets are available, however, I don't like MLS gaskets. I have not had great luck with them, I prefer stock, regular composite (paper??) gaskets. Hell, my car has an MLS HG right now. And it's damaged. AGAIN. I've never had a HG failure with a stock head gasket even though I cooked it like 5 times.
I know a local guy who could make a 1.3mm head gasket for me, but I just don't trust MLS anymore. It is also 200$ compared to 30$ for an elring in my local shop.
Also, if I'd do it I'd need to pay quite some attention because if I go too thin, and the block has been already shaved some I believe, the piston might get too close to the head. I know this can be measured, but it does limit me.
I will need to use some clay anyway, because I will run a KM Cams Stage 2 cam + shaving. I like my valves straight.

"Also surfacing the head won't be as effective as you might think since ~ half of it is flush with the surface of the head so the only part that would increase compression is the combustion chamber portion."
Yes, I have 2D scanned the head surface and used CAD to measure the area/volume to calculate it exactly.
tschultz
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Re: Compression ratio values not adding up

Post by tschultz »

Good info, glad you were going through it in depth.

From RobertRO here on the forum: viewtopic.php?t=148202
With this occasion I also determined that:
- the volume of the dome on a stock late M30B30 9:1 compression piston is ~10.4 ccm, dome height = 2.91mm;
- the volume of the dome on a stock M30B35 9:1 compression piston is ~2.0 ccm, dome height = 1.85mm;
- the volume of the some of a stock Euro M30B34 10:1 compression piston is ~5.6 ccm, dome height = 1.85mm.
Mike W.
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Re: Compression ratio values not adding up

Post by Mike W. »

szacsi72 wrote: May 26, 2023 5:21 AM
Thinner head gaskets are available, however, I don't like MLS gaskets. I have not had great luck with them, I prefer stock, regular composite (paper??) gaskets. Hell, my car has an MLS HG right now. And it's damaged. AGAIN. I've never had a HG failure with a stock head gasket even though I cooked it like 5 times.
I know a local guy who could make a 1.3mm head gasket for me, but I just don't trust MLS anymore. It is also 200$ compared to 30$ for an elring in my local shop.
I've got to say I agree with you, although I've not had any failures. I've seen too many failures on other brands from them.
szacsi72
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Joined: Nov 13, 2022 3:41 AM
Location: Hungary

Re: Compression ratio values not adding up

Post by szacsi72 »

tschultz wrote: May 26, 2023 10:42 AM Good info, glad you were going through it in depth.

From RobertRO here on the forum: viewtopic.php?t=148202
With this occasion I also determined that:
- the volume of the dome on a stock late M30B30 9:1 compression piston is ~10.4 ccm, dome height = 2.91mm;
- the volume of the dome on a stock M30B35 9:1 compression piston is ~2.0 ccm, dome height = 1.85mm;
- the volume of the some of a stock Euro M30B34 10:1 compression piston is ~5.6 ccm, dome height = 1.85mm.
Thanks for the data. Good info.
Mike W. wrote: May 26, 2023 1:11 PM
szacsi72 wrote: May 26, 2023 5:21 AM
Thinner head gaskets are available, however, I don't like MLS gaskets. I have not had great luck with them, I prefer stock, regular composite (paper??) gaskets. Hell, my car has an MLS HG right now. And it's damaged. AGAIN. I've never had a HG failure with a stock head gasket even though I cooked it like 5 times.
I know a local guy who could make a 1.3mm head gasket for me, but I just don't trust MLS anymore. It is also 200$ compared to 30$ for an elring in my local shop.
I've got to say I agree with you, although I've not had any failures. I've seen too many failures on other brands from them.
Also, for an MLS I would definitely need to shave the block. Even if it would not be needed normally.. as they are extra sensitive.


I've made a calculator and got 8.89:1 in the end for a stock M30B35. This sounds better!
You can access it here:
https://docs.google.com/spreadsheets/d/ ... ue&sd=true

Image
tschultz
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Re: Compression ratio values not adding up [UPDATED WITH CALCULATOR]

Post by tschultz »

Something here isn't right, as i copied the formula and put in 10:1 B34 which as a different piston dome/cc and the 58cm3 head cc but am seeing 10.24:1.

Unfortunately I am not sure what the calculation is to truly determine this, though I am interested in the result.

Also, should piston dome height affect the calculation? Right now it doesn't
szacsi72
Posts: 224
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Re: Compression ratio values not adding up [UPDATED WITH CALCULATOR]

Post by szacsi72 »

tschultz wrote: May 31, 2023 1:51 PM Something here isn't right, as i copied the formula and put in 10:1 B34 which as a different piston dome/cc and the 58cm3 head cc but am seeing 10.24:1.

Unfortunately I am not sure what the calculation is to truly determine this, though I am interested in the result.

Also, should piston dome height affect the calculation? Right now it doesn't
Hi,

I got the same results for M30B34 10:1. This could just be due to inaccurate data. Some say 5.6cm3, some (such as the original M30 calculator) says 7ccm for B34.. also for the head gasket, the original one says 13.55cm3, but in reality is is 11.433cm3. (1.72mm).
If you put in 2.04mm as HG thickness into my calculator you will get 9.91:1.
Honestly at this point I don't trust any online source anymore. I will just 3D scan the pistons and the head once its off.

The calculation is simple: (Swept volume + combustion volume) / combustion volume.

Piston dome height is just there for me to snap pictures of the results for later. :)
szacsi72
Posts: 224
Joined: Nov 13, 2022 3:41 AM
Location: Hungary

Re: Compression ratio values not adding up [UPDATED WITH CALCULATOR]

Post by szacsi72 »

Hi, I've updated the calculator with an effective/dynamic compression ratio calculator. You may find it here:
https://docs.google.com/spreadsheets/d/ ... ue&sd=true

So now it's easier for me to explain why I want to shave the head.
As you know, adding a milder came, which in my case is the KM Cams Steg 2, generally means a different ABDC value, reducing the effective compression ratio. This is already quite low in a stock B35 at 8.12, so I'd like not to go under that. It will kill the performance down low, and the engine will feel sluggish below 3000-ish.
I used 43° ABDC for the stock cam and 53° ABDC for the Steg 2, however this is just according to my experience, I will ask KM Cams to verify this.
This results in a drop to 7.72:1. Quite dramatic.

Shaving the head 0.5mm, and 0.1mm from my block (previous rebuild) and using a 1.15mm thickness HG (called the guy making the custom head gaskets) will bump my static compression ratio up to 9.78:1, resulting in a dynamic compression ratio of 8.43:1.
That sounds perfect to me. A. Graham Bell recommends a maximum of 8.5:1 for 95RON, in one of his books - as it's a street car, I really don't see the point of building this engine for 100RON (I believe that would be 95.5 in the US). The fuel costs 25% more and I'd get like 5 more horses in return. I'll rather spend that on a nice interior upgrade. :)
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