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Old 03-22-2017, 10:31 AM   #1
Jdurb729
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Engine displacement questions

I find the calculations to find the displacement and compression ratios for your engine pretty confusing. Was wondering if somebody could run some for me ? I'm doing the b25/b27 stroker swap, I'm leaving the eta block completely alone so all eta crank, rods, and pistons. Using the 885 head but I'm also using a .120 MLS gasket which is thicker than stock. I'm boosting the motor but just would like to know exactly the displacement of the motor and even the compression ratio. Thanks!
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Old 03-22-2017, 03:08 PM   #2
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This is how I would do the calculation without having the engine in front of me and explicitly measuring and calculating piston dish/dome cc, combustion chamber cc etc etc. It is an approximation as it assumes the original engine is a true 9:1 CR

Start with an Eta as a baseline then add on the extra chamber volume of the head and extra volume due to thicker gasket.

Standard Eta is 9:1 CR
Stroke = 8.1cm
Bore = 8.4cm

So swept volume per cylinder is
V = stroke * bore * bore * pi / 4
V = 8.1*8.4*8.4*3.1415/4 = 448.8cc

Total clearance volume v is
v = (V/CR-1)
v = 448.8 / (9-1) = 56.1cc

Additional volume due to 885 head vs 200 is typically 4cc (40cc-36cc)

Additional volume due to thicker 0.12Ē gasket is
= pi / 4* 8.5*8.5* (0.12*2.54-0.175) = 7.63cc
(this assumes it is 85mm bore gasket and original gasket is1.75mm thick)

So new total clearance volume is
v = 56.1 + 4 +7.63 = 67.7cc

Since swept volume V is the same

CR = (67.7+448.8) / 67.7 = 7.63:1

This is way too low, you donít need a thicker gasket
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Old 03-23-2017, 12:24 AM   #3
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Thicker gaskets aren't ideal for boost anyway
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Old 03-23-2017, 07:58 AM   #4
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I still have no clue why people think they need such low compression for boost. The m20 came with a great boost-friendly CR of 8.8:1, and yes, the eta pistons/885 head make it an awful 8.4:1. We have had great success with higher CR's and boost, a turbo will spool sooner, and the engine will make more power before spooling, it's a win-win.

The m20 is naturally detonation-resistant with the squish band, and in all stock form runs lean from the factory.
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Old 03-24-2017, 10:03 PM   #5
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So should I ditch the headgasket? Makes sense that it would give me better resoponse until boost kicks in and would also make boost come sooner. So why do people usually want lower CR anyways? And also what is my actual volume of the engine? Not just the CR. Should be around 2850 or so cc right?
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Old 03-24-2017, 11:44 PM   #6
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Swept volume is 2693cc same as an eta
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Old 03-25-2017, 08:25 AM   #7
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Quote:
Originally Posted by Jdurb729 View Post
So should I ditch the headgasket? Makes sense that it would give me better resoponse until boost kicks in and would also make boost come sooner. So why do people usually want lower CR anyways?
Many conversions involving putting a turbo on a non-turbo engine involve a higher compression engine than the M20, and in those cases lowering a bit sometimes makes it a bit easier to tune than a high strung high compression turbo engine, it often reduces the likelihood of detonation. It's also a bit of applying old conventional wisdom which is past its prime. A lot of it depends on head design, if you run a thick head gasket you run the risk of ruining the quench and losing efficiency and spool, good quench and a decent compression ratio makes for better off boost performance and faster spool. In any case, the stock B25 8.8:1 is a great compression ratio for a turbo conversion. You couldn't ask for any better.
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Old 03-25-2017, 12:42 PM   #8
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Swept volume is 2693cc same as an eta
How would it be the same volume with a 885 head and thicker gasket?
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Old 03-25-2017, 01:04 PM   #9
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How would it be the same volume with a 885 head and thicker gasket?
Combustion chamber volume does not affect swept volume, only bore and stroke. If you were to measure the volume of an entire cylinder and its combustion chamber, swept volume is the volume of the area above the piston when it is at BDC, minus the volume of the area above the piston at TDC, it is the area within the piston's "sweep" of movement. In math; Vs = π*(Bore/2)≤*Stroke

Combustion chamber volume does affect compression ratio though, as it is the ratio of the total volume (swept volume + combustion chamber volume) to the combustion chamber volume. In math; (Total volume)/(Combustion Chamber Volume)
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Last edited by varg; 03-25-2017 at 01:14 PM.
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Old 03-25-2017, 02:19 PM   #10
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Quote:
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Many conversions involving putting a turbo on a non-turbo engine involve a higher compression engine than the M20, and in those cases lowering a bit sometimes makes it a bit easier to tune than a high strung high compression turbo engine, it often reduces the likelihood of detonation. It's also a bit of applying old conventional wisdom which is past its prime. A lot of it depends on head design, if you run a thick head gasket you run the risk of ruining the quench and losing efficiency and spool, good quench and a decent compression ratio makes for better off boost performance and faster spool. In any case, the stock B25 8.8:1 is a great compression ratio for a turbo conversion. You couldn't ask for any better.
Quote:
Originally Posted by varg View Post
Combustion chamber volume does not affect swept volume, only bore and stroke. If you were to measure the volume of an entire cylinder and its combustion chamber, swept volume is the volume of the area above the piston when it is at BDC, minus the volume of the area above the piston at TDC, it is the area within the piston's "sweep" of movement. In math; Vs = π*(Bore/2)≤*Stroke

Combustion chamber volume does affect compression ratio though, as it is the ratio of the total volume (swept volume + combustion chamber volume) to the combustion chamber volume. In math; (Total volume)/(Combustion Chamber Volume)
In short, Varg hit the nail on the head.

Displacement is not affected by CR, and it is old thinking that less CR=more power. You have to take in account in the old days, people would put a turbo on a carb, and lower the CR to ridiculous levels since there was little control over timing.

More CR makes the tuning window smaller, yes, but with modern fuel injection and computers, you have control, better combustion chambers, better piston design etc - all of which BMW has been way ahead of the "masses". So, the m20 is a good candidate for a turbo, right as it came from the factory (many years before turbos were even common).
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Old 03-25-2017, 03:58 PM   #11
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the simplest way to think of it is this

CR = volume above the piston at BDC / volume above the piston at TDC

Swept volume = volume above the piston at BDC - volume above the piston at TDC
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Old 01-12-2019, 03:09 PM   #12
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Quick question for you digger.
My engine is a 8:8:1 compression JE piston M20 with 885 head. I am using a 0.080 Athena Cutring head gasket. I have had my head shaved twice, with not a whole lot taken off, and I plan to have it shaved once again when I swap to this stroker.

What would my new CR be? I know you cant give me an exact number, but a best guess would be good with me! Thanks!
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Old 01-12-2019, 04:20 PM   #13
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Quick question for you digger.
My engine is a 8:8:1 compression JE piston M20 with 885 head. I am using a 0.080 Athena Cutring head gasket. I have had my head shaved twice, with not a whole lot taken off, and I plan to have it shaved once again when I swap to this stroker.

What would my new CR be? I know you cant give me an exact number, but a best guess would be good with me! Thanks!
8.8:1 what bore and stroke is this based on?

what stroke is the stroker?

at some point you are changing pistons going to a stroker but its not worded very clear what you are doing

as reference shaving head 0.1mm increases CR about 0.1 on bone stock m20b25
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Old 01-12-2019, 05:50 PM   #14
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Sorry I should have known to be more specific. It's 85mm bore 84mm stroke. Dished JE pistons. 135mm rods. 0.080 Headgasket.

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Old 01-12-2019, 08:31 PM   #15
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So the piston is just the 8.8:1 style piston dome/dish profile (as opposed to actually being 8.8:1) and used on a 2.9L bottom end but the gasket is 0.080" and head has been skimmed a few times?

ballpark is around 10:1 give or take depending how much was skimmed
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