Working out compression ratio's

Post Mon Sep 11, 2006 10:01 pm

Hi all,

Maybe a simple answer to this, but I really cannot get my head around trying to work out the Compression Ratio in my Sport Evo.

Anyone have a calculation??

I know what it should be, but I have a shorter stroke crank fitted (not by me but a previous owner - Dont ask!!) So I don't need to know that, I just want to work out what MY compression ratio is.

Thanks in advance.

Post Mon Sep 11, 2006 10:22 pm

Do you know things like the CC's of the combustion chamber/squish height/etc ?

It's a bit too technical for me

Post Mon Sep 11, 2006 10:25 pm

Well, I have tried to get the specs for the 2.5 S14 engine, but nothing forthcoming.
All I know is my 2.3 crank offers 3mm less stroke than the proper 2.5 crank.
I understand that the CR is 10.2:1 for the 2.5 SE. nut with the 2.3 crank with 3mm less stroke??? All I know is that its probably about right for turbocharging! lol

Post Mon Sep 11, 2006 10:26 pm

95 mm bore
84mm stroke then don't you have to know the headgasket thick ness??

Post Mon Sep 11, 2006 10:29 pm

I wonder if it's got the standard piston pin height ?

Do you know what pistons are in it ?

Post Mon Sep 11, 2006 10:31 pm

they are the 2.5 ones, which are decked in comparrison to the 2.3

Post Mon Sep 11, 2006 10:33 pm

The head gasket is 1.8mm compressed thickness according to my ETK

Post Mon Sep 11, 2006 10:33 pm

So, assuming everything is 2.5 S14 spec, with the only change being a 2.3 crank, whats the compression ratio?

Standard CR is 10.2:1
Bore is 95mm
Stroke is 84mm (instead of 87mm)

Post Mon Sep 11, 2006 10:39 pm

ShepsEvo3 wrote:they are the 2.5 ones, which are decked in comparrison to the 2.3

so the 2.5 pistons are lower than 2.3 in the bore at TDC ?

So your 3mm shorter because of the crank, how much shorter are you because of the pistons ?

Post Mon Sep 11, 2006 10:41 pm

Yes, the 2.5 pistons are lower in the bore than the 2.3 pistons when using the same crank.

I believe it is another 1.5 mm?? but the CR is 10.5:1 on the 2.3. Gets even more confusing! lol

Post Mon Sep 11, 2006 10:42 pm

2mm
2.3 93mm
2.5 95

Post Mon Sep 11, 2006 10:44 pm

maxfield, thats the bore sizes? I am sure the 2.5 pistons are shorter than the 2.3 too (from gudgeon pin to top of piston)

Post Mon Sep 11, 2006 10:45 pm

oh right give me a minute internerts a big place

Post Mon Sep 11, 2006 10:46 pm

LOL, am I just being lazy then!

Post Mon Sep 11, 2006 10:47 pm

I think you might want to take the head off to measure the squish height to know how far out of spec it is ?

How does it run ? Does it make decent power ?

What do they say on S14.net ?

Post Mon Sep 11, 2006 10:55 pm

Its not bad to be honest, just a little flat now and then at low RPM, but it has 226/228 BHP, 187 BHP ATW's so it really isn't bad.

The engine is being pulled apart anyway, but I am only doing that when I put the new 2.5 crank in that I have.

I was intregued as the car only had about 167 BHP (FW) when I bought it, I actually got it upto just over 200 just by changing the exhaust (put on a genuine BMW system with cats!) and fiddled with the AFM. I put the Unichip on and re-mapped it as obviously the ignition and fueling would be way, way out, and it was. Power came up another 25 BHP. Made the world of difference. But considering the Sport Evo usually only makes arround 225 BHP anyway (238 standard) mine isnt doing too bad. I reakon it will make all of the 238 with the new crank fitted.

I have been tempted to supercharge the thing just to see what it would be like, before changing the carnk and putting back to standard.

I posted on S14.net and BMWCC sites, but no real concusion was set in stone.

Post Mon Sep 11, 2006 11:01 pm

Sounds pretty healthy, have you done a compression test on it ? May be you could compare your numbers with another s14 ?

Obviously the compression test isnt a definative test but i'd have thought yours would be noticably low compaired to standard ?

Post Mon Sep 11, 2006 11:04 pm

Yes, this is what I thought. I had to buy a new adaptor for my compression tester from Snap-On as the S14 has smaller plugs than normal, but havn't tested it yet. You should see the ignition map on the Unichip, its nearly 20 degrees more advanced in some places, and this is ontop of the Superchip's chip already fitted!

Post Mon Sep 11, 2006 11:18 pm

paul, not sure about the decking the pistons thing. if running the same rods and block, the evo sport pistons will have a smaller pin height than the 2.3 ones surely?
anyway
if the 2.5 EVO3 has a CR of 10.2:1, and you're running the same pistons, rods and block with a crank with 3mm less stroke, then you'll have a REALLY low CR. the pistons will be 1.5mm from the deck, not to mention that the shorter stroke will give you a lower CR anyway. assuming chamber vols are equal (before taking into account the 1.5mm below the deck that the pistons will be), then you'd be running 8.8:1. The block may have been skimmed down to get rid of the deck clearance issues in which case you'd have 9.9:1
HTH
tim

Post Mon Sep 11, 2006 11:23 pm

tim_s wrote:8.8:1

Thats low.

Post Mon Sep 11, 2006 11:26 pm

Tim, thanks for the info, the 2.5 pistons are shorter from the gudgeon pin to the top for sure! John in Germany confirmed this.

The chap who replaced the crank wouldn't have had the block decked. He obviously didn't know what he was doing!

Everything is standard Sport Evo, except the crank. It would be nice to get some blueprints of the sprot evo engine......ANYONE?

Post Mon Sep 11, 2006 11:46 pm

8.8:1 sounds perfect for a supercharger