FAO Lee - CR calculations

[Doug_RS2]

Lee - this is where I got the 4.2cc (off each piston) from to lower CR.

I think you worked out 3.3cc.

Maybe Marco or someone else can confirm - to change form 9.3 to 8.7 CR on our cars

Let B be the compressed volume and A be the uncompressed volume

A / B = 9.3 (ratio of two volumes) (1)
and A - B = 445.2cc (volume of stroke) (2)

Rearranging (1) gives A = 9.3B
Substituting in (2) gives 9.3B - B = 445.2
Therefore, B = 445.2 / 8.3 = 53.64cc and so A = 498.84cc.

Now if we let C be the additional volume required to reduce the compression
ratio to 8.7,

then (A + C) / (B + C) = 8.7 (3)

Rearranging (3) gives A + C = 8.7B + 8.7C
so A - 8.7B = 7.7C
and so C = (A - 8.7B) / 7.7

Therefore, C = (498.84 - 8.7 x 53.64) / 7.7 = 4.18cc.

[Pisobiker]

Doug, your numbers should be correct. Following is the calculation my way, and result is just the same:

If OEM initial bore is 81.0mm then bore volume is 444.99 cc
If we say the metallic head gasket takes OEM engine to 9.0:1 CR then combustion chamber is 444.99 / (CR-1) = 55.62 cc

If final bore is 81.5mm then bore volume becomes 450.50 cc, while taking 0.2mm off the head and 0.1mm off the cylinder deck will reduce combustion chamber to about 54.30 cc

Your new numbers are:

VC=450.50
VCC=54.30

so being CR=(VC+VCC)/VCC then
VCC=VC/(CR-1)

For CR=8.7 then

VCC=58.51 which compared to the initial 54.30 cc makes

+ 4.21 cc.



Marco

[Doug_RS2]

Thanks Marco - I feel better after getting those numbers confirmed.

[Lee]

Yep, I agree too. I forgot to consider the chamber volume in the uncompressed charge. I redid the calcs last week as I couldn't figure why my answer was different and came up with what you have.

Anyway, I blame the Belgian keyboard, Dirty Sanchez and that funny cherry beer!!:wacked: :wacked:

Inexcusable mistake for such a simple calc really!! I live in shame and wonder why I did my Engineering Degree!!



Lee

[Audi Parts Guru]

Originally posted by Lee
Inexcusable mistake for such a simple calc really!! I live in shame and wonder why I did my Engineering Degree!!



Lee

Yeah shame on you Lee

[Mark Halligan]

Yeah shame on you Lee

LOL, keeping my eyes open for falling satellites now :eek2:

Mark.

[Lee]

I insist it was Mihneas dodgy fruity beer, honestly :eek2: :eek2: What other excuse can I find? Oh yeah working 12- 14 hour days for the last few months and then spending hours here is feckin with my brain!! :wacked: It was after a 'spirited' 5 hour drive, several beers and in the early hours of the morning. LOL! Just don't ask me for calcs these days!!

Lee

PS Don't worry Mark, we'll give you plenty of notice when not to look up! Just be more worried about all of those cold war nuclear propelled spacecraft that have been floating around for the last few decades.....they WILL be coming down to the earth's surface one day. Ours are so small they burn up completely after about 20 years in low earth orbit. You can now sleep at night! LOL!