compression raito - how much to skim off?
Turbo Mini Forums

hi, i will be modifying a head for my mpi cooper. hope to run at 10.5:1

assuming head has never been skimmed, and using standard pistons, how much do i need to get skimmed off to raise the cr to what i want?


cheers

Already answered
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=267024

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

not really. i want to keep the chambers standard. dont have the time or facilites to alter them. need to tell the machine shop how much to take off the head.

Well if you need your hand held:
Standard volume of a 940 is 21.4cc.
Use a CR calculator to find out what head volume you need to achieve 10.5:1
work out the area of the chamber face and determine the height of the volume you need to remove. Or make up spacers the same shape as the chamber with a known thickness. Use these and a syringe to determine the height you need to remove

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

1cc is approx .010"

0.0438 or aprox 1.1mm to be skimmed then?

does that sound right? cheers

Edited by welshdan on 22nd Jan, 2009.

Sounds a bit much to me if you already have 10:1

0.010" (10 thou) is 0.25mm

No one will be able to tell you how much to take off, thats why you have not been getting a response. You need to calculate the compression ratio with all the bits you have, and do not leave anything out, take into consideration the ring land volume and gasket volume. Once you have the chamber volume for your current CR calculate the chamber volume that will give you the CR you want, subtract one from the other, giving you how much cc you need toremove by skiming the head. To calculate how much you need to remove, you need to calculate the area of the combustion chamber. You can do this roughly by measuring the depth of the chamber

Edited by Sprocket on 22nd Jan, 2009.

be carefull as i had a mpi head which had 23.4 cc chambers had to skim a shit load of to get my chambers to 199 cc for my 1380.

best to measure chambers first.

just incase she has the estra thickness.

always looking for them bigger bunches of bannanas

hi, if standard cr is 10:1

to reach 10.5:1, a correction of 1.86cc has to be removed.

1.86 cc x 0.25mm = .465mm

does this look right? .465 mm to be skimmed to reach 10.5:1?

assuming chambers are 21.4cc (i will check them later today)

what cc are mpi pistons?

will probably go for 0.5mm skim if these figures look about correct?

many thanks for your help

Edited by welshdan on 24th Jan, 2009.

where does the 0.25mm come from?

It all depends on the area of the combustion chamber.

0.5mm off the top of a narrow but deep volume is small in comparison to 0.5mm off the top of a wide but shallow volume.

21.4cc divided by the depth multiplied by the amount you will skim off, will give you the cc that will be removed

So for example (21.4cc / 10mm ) x 0.5mm = 1.07cc removed by skimming 0.5mm off the face.

if you change the depth of the chamber (21.4cc/ 8mm)x 0.5mm = 1.34cc removed by skimming 0.5mm off the face.

You need to measure the depth of the chamber as a rough guide!!

the head has never been modified before, and never skimmed.

chamber depth around 7mm. does about .5mm to be skimmed off look around right or miles off? not got time to do math at the moment, just poping out. will have a look later. cheers

have you measured the chambers with a syringe?

hi, sorry to bring this up again. chambers are standard. measured them today. its a standard head.

re calculated, worked out that around 0.5 mm is what i need to raise the cr slightly above 10.5:1

this seems right to me. does this amound sound about right for a standard head?

does anyone know what compression a standard mpi will run without ignition issues? thanks

I've got the approximate area of a chamber calculated somewhere for this very situation. I'll take a look later.

thanks

Below is copied from an email I sent MiniMark 5 years ago when he asked me this same question. Sure, the model is not 100% accurate, but it's better than anyone else had done back then - and clearly no-one really knows the answer still today...

It'll at least get you in the ball-park.


XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
These dims are for a chamber shape i roughed out. As you can see, for 1mm thick, the shape you see is 2.7cc.
We know that using the formula [ pi*(Diameter^2) ] / 4
that 7cm diameter circle, 1mm thick is 3.8mm thick. Hence, the difference is 1.1cc / mm in the remaining section, of which i reckon 0.9cc is the 'beak'
hence if the beak is 7mm high, it = 6.3cc if removed totally...

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX


http://www.turbominis.co.uk/uploads/i/wh12...bodave-1263.jpg

http://www.turbominis.co.uk/uploads/i/wh86...bodave-1264.jpg

Still think that if you divide the depth of the chamber in mm, by the chamber volume, then multiply that by the cc you want, will give as acurate a figure as you would need.

So 7mm divided by 21.4 = 0.327mm per CC if you are looking for 1.5CC simply multiply 0.327 by 1.5 = 0.491

So yes, 0.5mm will get you close to where you want to be.

Calculating compression ratio is never an exact science unless you have the calibrated measuring equipment to do it to the nth of a CC

Going off Daves figures, its not far off right too

Edited by Sprocket on 17th Feb, 2009.

thanks for the input. dave, diagrams and calculus interesting. sprocket, thanks for confirming what i made the amount to be skimmed off as. cheers#

sorry to drag this up again... will mems mpi injection run at around 10.65:1 cr with 1.5 roller rockers? will raising the cr by this much effect emmisions at mot time? many thanks

the 1.5:1 rockers might give you problems, depending on the cam

the compression ratio should not cause a problem with emissions, but if you start to get any sort of pinking, use 97 unleaded. Infact, use V Power what ever and that in itself will improve the emissions a little

am using standard cam. cheers