CR on a 2.8 build??

[BadDave]

Hi guys

Just a quick question

I'm taking delivery of an M52B28 bottom end in the next few days and planning on using it in a spare M20B25 block

I'm planning on using M20B20 rods with M20B25 pistons(from a facelifted motor)

What sort of CR should i be getting with this setup ?.

Bearing in mind,that i'm looking to add a turbo at some point and don't want to have to rebuild it again prior to fitting above mentioned turbo.
Will a decomp plate be required??

Cheers in advance

Dave

[march109]

Don't think anyone has bothered to work it out.

IIRC late 2.5 head combustion chambers are 53.25cc
the 2.8L crank is 84mm 2.5L crank is 75mm = 9mm difference
320i rods are 130mm 325i rods are 135mm = 5mm difference

meaning the piston will pertrude into the combustion chamber by an extra 4mm, bore is 84mm so
(Pi(42)^2)*4mm = 2.2cc (I think!)

= combustion chamber V of 51.05cc
= CR 10.1:1 IMO decomp plate needed

Even if you can maintain the 53.25cc combustion chamber the 325i head normally has you would still have a CR of 8.8:1 (with late 2.5L pistons)again IMO too much for high boost and decomp plate needed.

I seem to find every 1mm you add to the combustion chamber height you gain about + 0.5:0 to CR so to get a CR of around 8.0:1 you need to add 4mm-5mm which just seems to me to be too much to be safe,you could enf up blowing decomp plates all day long.

Custom pistons are the way forward,

But quite frankly your better off actually testing it properly since my maths is normally crap after this much beer.

And all IMO and the bestI can do before and after pub.

[Turbo-Brown]

Don't forget that while the stroke is 9mm greater than the M20B25 crank, that means you add 4.5mm to the TDC and subtract 4.5mm from the BDC so with 5mm shorter rods the piston would be 0.5mm further down the bore than standard.

Forget what the standard facelift CR is, thing it's 8.7:1 isn't it?

So to work out what the clearance volume (chamber and anything else that's left over):

CR=(disp+clear)/clear

CR-clear=disp/clear

clear(CR-1)=disp

clear=disp/(CR-1)

So we've got a CR of 8.7:1

A displacement of 2494/6=415.7cc per cyl

so putting our numbers back into the equation gives:

clear=415.7/(8.7-1)
=415.7/7.7
clear=53.90cc

So if the bore is 8.4cm and we move the TDC piston down by 0.5mm, we get an extra:

4.2^2*3.14=55.42cm^2
55.42*0.05=2.771cc

So going back to the original equation:

CR=(disp+clear)/clear
=(415.7+(53.9+2.771))/(53.9+2.771)
=(415.7+56.671)/56.671
=472.371/56.671
CR=8.34

There ya go

[BadDave]

Thanks for the input so far guys

Don't forget that while the stroke is 9mm greater than the M20B25 crank, that means you add 4.5mm to the TDC and subtract 4.5mm from the BDC so with 5mm shorter rods the piston would be 0.5mm further down the bore than standard.

So that should drop the CR down to 9. something??

[Turbo-Brown]

Sorry dude went back and edited with equations

[BadDave]

Thanks Alex
That figure looks better to me than 10.1:1

So that means it'll be safe enough to add the T4 without any additional strip down/decomp plate



Much appreciated

[Turbo-Brown]

It'll almost certainly be fine like that.

Guess the thing to do would be start off on low boost and work up progressively, listening for pinging all the way.

[BadDave]

It'll almost certainly be fine like that.

Guess the thing to do would be start off on low boost and work up progressively, listening for pinging all the way.

I'll run it N/A till i've got everything sorted out in a dummy build then swap it over.

Would i be best to run the DTA on TPS or Map ??

Or just use one of Ant's chips till the turbo's fitted?

[Turbo-Brown]

If you've got a DTA then you might as well use it from the off.

If it's like the Emerald, it'll be really geared up to use the TPS with MAP corrections for boost but if it's like MS, it'll be fine with just MAP.

In many ways, it makes more sense to map a boosted engine using just MAP (unless you have ITBs or something like that) so it's probably worth rigging it up to use MAP as main load sensing and TPS just for accel enrichment.

[BadDave]

I'll dig out the old instuction manual than came with it and see what it's saying
I'll have a look at an old TB i've got and see about the connection for a compatible TPS then

Thanks for all your input Alex

[Turbo-Brown]

Just double check that I'm not completely wrong starting off with a CR of 8.7:1 too!

[BadDave]

Just double check that I'm not completely wrong starting off with a CR of 8.7:1 too!

Just double checked and it appears you were close to the mark there
Seems as if the cr is 8.8:1 unless i'm looking at the wrong figure

[march109]

Seems fine to me, I forgot about TDC and BDC and just plugged the new combustion chamber figure into the normal 325i equation.

[march109]

Stock late M20 is 8.8:1 but thst is down to pistons so as long as you use late pistons the calcs are reasonably accurate.

[BadDave]

So it'll roughly end up as 8.4:1 or there abouts

That's fine
Not looking for a tempremental high comp motor at the moment

[reggid]

Standard B25 chamber size is CL = V/(CR-1) = 415.6/(8.8-1) = 53.3cc
B28 Vol = pi/4*8.4^2*8.4 = 465.5cc

Delta piston position = (84-75)/2 - (135-130) = -0.5mm.

B28 CL = 53.3cc + pi/4*8.4^2*0.05 = 56.1cc

CR = CL/(CL+V) = 56.1/(56.1+465.5) = 9.3:1

Turbo-Brown you used the B25 volume not the B28 one

it will vary depending on whether the head or block have been skimmed and whether you use a 1.75 or 2.05mm gasket.

[march109]

Standard B25 chamber size is CL = V/(CR-1) = 415.6/(8.8-1) = 53.3cc
B28 Vol = pi/4*8.4^2*8.4 = 465.5cc

Delta piston position = (84-75)/2 - (135-130) = -0.5mm.

B28 CL = 53.3cc + pi/4*8.4^2*0.05 = 56.1cc

CR = CL/(CL+V) = 56.1/(56.1+465.5) = 9.3:1

Turbo-Brown you used the B25 volume not the B28 one

it will vary depending on whether the head or block have been skimmed and whether you use a 1.75 or 2.05mm gasket.

How is B28 CL more than B25 CL??

[Turbo-Brown]

Ooh bugger! That's a very good point there Reggid!

Our M20B28 CL is more than the M20B25s because the piston will be 0.5mm down the bore at TDC when compared to the B25.

Whadda mistaka to maka!

Sorry Dave!

[march109]

ok cool, didn't make sence when I first read the proof. I thought the combustion chamber V was smaller.

Proabably why idiots should'nt be allowed the power of math.

[BadDave]

So it's 9.3:1


It's ok Alex
Just means i'll have to use a thin decomp plate or a thicker head gasket it gets the turbo