M30 and M20 Compression Ratio Calculator - Updated

[Brad D.]

My latest addition to the e28 knowledge base is a simple m30 compression ratio calculator. There are constantly questions regarding compression ratios of various m30s and resultant compression ratios when various mods are performed.

The spreadsheet calculates the compression ratio of all of the common m30 variants as well as these variants with the ubiquitous b35 head installed. It also allows the user to input the head gasket thickness as well as any cylinder head decking to determine the resultant CR. Additionally, from calculations I determined the theoretical piston ccs for various m30 pistons, whether dished or "pop up".

I hope this information is as correct as possible and that it may benefit the m30 enthusiast. Please let me know if you have any questions or comments regarding the calculator.
Enjoy!

M30 Compression Ratio Calculator

M20 Compression Ratio Calculator

[Bimmerguy2002]

i want one for the m20

DO IT!

[Brad D.]

i want one for the m20

DO IT!

Don't worry, it's coming!

[Bimmerguy2002]

[M. Holtmeier]

Awesome man! Thank you.

[Mike W.]

Great chart, it's saved already! Thanks

[wkohler]

You should right click and save as. Just thought I'd share.


Oh, thanks!

[Brad D.]

M20 calculator is now added. It includes the most common m20 head castings. I suppose I could have combined them into one file but oh well. Enjoy.

[pdx 528e]

Thanks for this!

[Bimmerguy2002]

what program do i need to open?

im new to this internets thing

[Mike W.]

what program do i need to open?

im new to this internets thing

Matt, Matt, Matt. While I think you're yanking chains here, if not, Excel is the preferred one, Works spreadsheet will work, or download Open Office and their calc one will work also. The latter one is a free download.

[Bimmerguy2002]

haha thanks Mike
i dont have neither, i dont computer much to be honest, its mainly for my shenanigans and m20 info!

[pdx 528e]

what program do i need to open?

im new to this internets thing

Matt, Matt, Matt. While I think you're yanking chains here, if not, Excel is the preferred one, Works spreadsheet will work, or download Open Office and their calc one will work also. The latter one is a free download.

I have found that the Open Office.org suite is quickly approaching the level of polish that allows one move over from Microsoft Office. Its fully compatible with MS file types including the new 2007 versions, yet the proprietary Open Office formats tend to take up significantly less space on disk than the MS counterparts. Free is a good price also.

[bm0p700f]

I second the comments about open office. However I did find it had probles with some tepleted document created in word as some all of the text was placed out of the boxes it was meant to in. It is free though and that alone makes it brillant.

[Devinder]

Brad,

I have something similar I made up for the S38. I'll see if I can find it and clean it up.

Devinder

Here it is:

S38_bore_stroke_comp.xls

Most of the data comes from my own measurements and I was using it to select piston compression height and dish volume to go with Honda rods.

Devinder

[Brad D.]

Brad,

I have something similar I made up for the S38. I'll see if I can find it and clean it up.

Devinder

That would be great to add that to the knowledge base as well! You've got a PM.

[Mike W.]

Most of the data comes from my own measurements and I was using it to select piston compression height and dish volume to go with Honda rods.

Honda rods? Ok, I'll bite, please tell me more, knowing you, there has to be a reason.

[Brad D.]

Most of the data comes from my own measurements and I was using it to select piston compression height and dish volume to go with Honda rods.

Honda rods? Ok, I'll bite, please tell me more, knowing you, there has to be a reason.

Well, NSX rods were titanium....do tell more.

[2ndeagle]

Interesting you mention Honda Rods. A buddy of min ementioned the same thing as a way to reduce the cost of rebuild. Seems the Rods are almost identical, but the cost variance is huge. Please correct me if I'm wrong. I thought he was just talking.

[babisbabou]

BBR


Thank you again!


btw can you edit the M30, with M30B28?

[Tusker]

I'm straying into Shawn's territory here but...

i dont have neither

= I have both

[Devinder]

Honda rods? Ok, I'll bite, please tell me more, knowing you, there has to be a reason.

Mike,

Honda rods are 143 mm and BMW rods are 144. All other dimensions are the same. I use them with Honda bearings and they fit without any problems. The 1mm difference in length doesn't matter since most of us get custom pistons for the S38 (especially if you're changing rods).

The real benefit is cost. Eagle, Carillo, Scat all sell 4340 H-beam BMW 144mm rods for about $1500. They sell the same rods in 143mm for Hondas for about $160. There is no reason to buy new rods unless you're building a performance engine. The Scat rods are rated to something ridiculous like 500 HP per cylinder. I got these to go with my 4.0L S38 because it was actually cheaper than rebuilding the stock rods.

I worked with Scat for several months to make sure these were going to fit. The sales guy there helped me get 6 matched rods (as opposed to the 4 for Honda engines) and was very interested in the outcome. They were going to do some marketing to their clients and it seems to have worked. The latest Metric Mechanic catalog lists 143mm rods on their S38 stroker engine.

Devinder

picture of the Scat Honda rods I bought:

[Brad D.]

I know this is an old thread but wanted to bump it as the links had been broken for a while and I finally remembered to update them so that these two files are downloadable again for everyone's reference. A few mods have been made as well.

[ThreeM30s]

Thanks for sharing... . I was wondering how it'd be if I put my M102 head on a B34 or a B35 euro.

Answer:
10:1 (b34)... e.g. the same
9.87:1 (b35)... e.g. increase

Forget the snail or a log going on with it... unless I was to use a thick head gasket. Would this be a bad idea in comparison to going with a b35 head:

M102 head

Pros:

• Smaller valves (for all round running, economy).
  Better valves (for turbo applications)
  Lumpier cam?

• Cons:
  May be just be plain silly to swap to a small chamber... perhaps worth opening up the M102 head's chambers and cc'ing them to get ideal compression ratio. Is it expensive?

M30B35 head

• Pros
  Stronger, better casting (?)
  Bigger valves (powaaaaugh!)
  Large chamber

Cons:

• Bigger valves (thirstier on fuel)
  b35 cam may not as tall as the M102 one
  (is it like b32 cam?)

Maybe I am wrong somewhere.

Comments, suggestions, abuse- all and anything welcome...

[Spaz!]

Please correct me if I'm wrong but this seemed a likely place to post this:

M20B27:
B/S:84/81mm
CC Vol.:37cc (200 Casting)
CR:9.0:1 (82-87 Eta)200 casting, 8.5:1 (87-88 Super Eta)
Piston>Deck: 0.2mm
Deck Height: 206mm
Piston Displacement: ???
Piston Comp Height:35.7mm (Eta) 36.2mm S-Eta)
Head gasket thickness: 1.651mm
Rod Length: 130mm
Rod Big End Bore:45mm
Crank Rod Journal:21.8mm

M20B25:
B/S:84/75
CC Vol.: 40cc (885 casting)
CR: 8.8:1 (US 87-90) 9.7:1 (euro spec 85-89)
Piston>Deck: 0.7mm
Deck Height: 206mm
Piston Displacement: ???
Piston comp height: 34.2 mm / 1.346 in
Head gasket thickness: 1.651mm
Rod Length: 135mm
Rod Big End Bore:
Crank Rod Journal:

Can anyone fill in the blanks and correct any errors please?

I am really curious about the eta dimensions. I ran the calculations for piston displacement numerous times on several CR calc's and figured it to be ~ -11cc (-10.85 by my own math) But the eta block I have sitting in the garage definitely has flat tops in it. To the best of my knowledge it is a stock engine, '84 model IIRC.

TIA,
Jon

[Yar]

So I got a pretty rusty late B35 block.
I can just transfer the B35 pistons to my B34 motor(block and head) and get 9.87-1?

[12valves]

The M30 excel sheet might be a little off. I don't think the head gasket is as thick as 2mm. And the gasket bore isn't exactly 89 or 92mm because you can bore .5mm over and still run a stock gasket. M30B32 pistons are flat and come out of the block .2mm which works out to 1.2cc not 2.9.

The M20 excel sheet shows a "CR for b27 eta block with '731' head" which doesn't really make sense because you can't put an 80mm bore head gasket on the 2.7 and otherwise the CR would be the same as with the 200 head. There is mention of a US b23 block (typo?)

I can just transfer the B35 pistons to my B34

I seem to recall reading that the B35 piston domes will hit the B34 head. You might want to do a search on that, because you might need to get the domes machined a bit for the combo to work.

M20B25:
B/S:84/75
CC Vol.: 40cc (885 casting)
CR: 8.8:1 (US 87-90) 9.7:1 (euro spec 85-89)
Piston>Deck: 0.7mm
Deck Height: 206mm
Piston Displacement: ???
Piston comp height: 34.2 mm / 1.346 in
Head gasket thickness: 1.651mm
Rod Length: 135mm
Rod Big End Bore:
Crank Rod Journal:

Can anyone fill in the blanks and correct any errors please?

I believe the b25 block and super eta have a slightly higher deck of 206.5mm, so the pistons don't actually come out by .7mm, rather .2mm like the others.

[Mike W.]

The M30 excel sheet might be a little off. I don't think the head gasket is as thick as 2mm.

Correct, more like 1.67MM

And the gasket bore isn't exactly 89 or 92mm because you can bore .5mm over and still run a stock gasket.

Correct

M30B32 pistons are flat and come out of the block .2mm which works out to 1.2cc not 2.9.

IIRC I measured more like .4MM but it was a while ago. All M30 pistons were definitely not at deck height, I've seen some a few thou below. That's for late B32s, early ones were 8:1 CR and dished.

[tig]

I noticed a lot of M30B35 tunes for the Miller WAR chip indicated CRs or 10.5:1 and 10.8:1. I understood my engine is 10:1, but I never really calculated anything; just went with what Paul Burke told me in one phone call. I found this thread kinda by random and downloaded the sheet to see what's up.

I think I'm missing something, because I don't think this sheet actually works for a high-comp B35.

My block is a B34 block bored out to 92.5mm. The pistons are B35 dome pistons (86mm stroke, 135mm rod length, 39.83 "comp.dist.", whatever that is). The head is a B35 head; no decking was required. I have a standard HG (0.167cm).

What is "Head CC". In the sheet it shows '65' for the B35. What is that measuring? CC's in the dome for each piston with the valves closed?

How do I actually calculate the CR for my engine using this data?

[2 5's and a 7]

So a ETA Block with a I head isn't as bad as everyone says.........