2 replies to this topic

[JonnyAlpha]

Just a sanity check really, so if anyone can cast their experienced eyes over the following.

 

I have used Cooperman's excellent guide on calculating the CR here.

My engine details:

Block is 1310 (1275 + 0.040”)

BK450 Gasket

Pistons Hepolite P21253 +40 Pistons

Average Deck Height is 0.0045"

 

And these are my calculations:

 

Initial calculations for a standard 1275 head with 21.4cc chambers: 

 

A = 0.45cc (Average Piston Crown to Deck Height = 0.0045”)

B = 8.3cc (P21253)

C = 21.4cc (Standard 1275 Head)

D = 4cc (BK450 Head Gasket)

E = 0.75cc (Ring Landing Volume)

SV = Swept Volume 327.5cc (1310cc / 4)

UV = 34.9cc Unswept Volume (A+B+C+D+E) 

 

CR = SV+UV / UV

CR = 327.5 + 34.9 / 34.9

 

Current CR = 10.38:1

 

Calculations for target CR of 9.75:1

To calculate how much you need the chambers to be (UV) for a given CR

 

UV = SV / (CR-1)

UV = 327.5 / (9.75 - 1)

UV = 327.5 / 8.75

UV = 37.428 

 

A+B+D+E = 13.5cc

UV = UV - 13.5

UV = 37.428 - 13.5

UV = 23.929cc

 

So my target chamber volume = 23.929cc

Edited by JonnyAlpha, 25 September 2021 - 09:01 AM.

[Steve220]

https://www.classicm...al/compression/

Have a peek mate, the formulas have been done for you, just fill in the blanks

[JonnyAlpha]

https://www.classicm...al/compression/

Have a peek mate, the formulas have been done for you, just fill in the blanks

 

I have used that calculator before, punching in my figures it suggest a chamber volume of 24.9cc to achieve a CR of 9.75 as opposed to 23.9cc by doing it manually.

The difference is probably the head gasket. Cooperman allows 4cc whereas Classic Mini's calculator sets this at 3.4cc.

 

 

So I guess I'll need to increase the chambers by around 4cc to allow for skimming.

Most info I have read shows how to measure the chamber capacity after modding to work out how much to skim off of the head. In this case I need to do the opposite.

What I need to know is how much I'll lose in CC when skimming the head, I think it's 1cc for every 0.012" of a standard chamber shape? 

IIRC a skim is around 0.003" off of the face (depending how bad it is of course), so that would be about 0.25cc.

 

If I can clarify this it will help to set the chambers to the required cc before skimming to bring them back to the correct cc to ensure the correct CR is maintained.

Edited by JonnyAlpha, 25 September 2021 - 11:30 AM.