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Home > Help Needed / General Tech Chat > Supercharger Boost Calculation (can someone check my maths)

wil_h

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Member #: 123
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Betwix Harrogate and York

21st Jul, 2010 at 08:21:40am

A guy on TMF wants to calculate his pulley ratio for an M45 on a 998. As it's not something I've ever needed to do I've sctatted around and found this formula

PR(pulley ratio) = C(14.7+B+I)/14.7xV

C = 1/2 Engine capacity in cc.
B = boost in p.s.i.
V = blower volume in cc. (bear in mind that Roots blower volume is "displacement" and vane blower volume is "swept volume")
I = Allowance for valve overlap (ie for every 10 degrees on o'lap allow 5% of the boost. eg for 35 degrees, I = 3.5 x 0.05 x B ).

The answer I got for a 998 seems about right when you plonk it into the M45 compressor map, does this seem a reasonable guesstimation?

Fastest 998 mini in the world? 13.05 1/4 mile 106mph



On 2nd Jan, 2013 fastcarl said:
the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


wil_h

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9258 Posts
Member #: 123
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Betwix Harrogate and York

21st Jul, 2010 at 09:31:45am

Should maybe add some figures.

For a 998 running 7psi an MG cam and an M45

You get a pulley ratio of around 1:1

Fastest 998 mini in the world? 13.05 1/4 mile 106mph



On 2nd Jan, 2013 fastcarl said:
the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


Brett

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Member #: 1023
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Doncaster, South Yorkshire

21st Jul, 2010 at 10:52:38am

i have always used

displacement of charger x Pully ratio
____________________
(displacement of engine /2)
= Bara


tm45 =(45cuin or 737cc)

so-

737 x 1(ratio)
___________
998/2

=1.48 bara
(1.48 - 1)x14.7
=7.05psi

thats how i do it, but looking at your formulae its the same as mine just rearranged and im not fancy enough to account for valve overlap
*tongue*

Yes i moved to the darkside *happy*

Instagram @jdm_brett

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