calculating boost from a positive displacement blower? (SC14/SC12)

[brett_celicacoupe]

im trying to calculate the effective boost of running a SC14 on a 3SGTZE engine (yes more twincharge research )

im looking at running a belt ratio of 1.2:1, overdriven

so one rev of the crank forces the SC to displace 1.7litres of air (1.2*1.4)

since the engine consumes a fixed volume, i thought (suggested by someone else) it must be possible to use the formula P1V1 = P2V2,

the pressures are in absolute so;
P1 = 14.7 psi (atmo pressure)
P2 = unknown
V1 = 1.7 litres (amount consumed by SC)
V2 = 1.0 litre (volume consumed by 2.0L engine @100VE)

P2 = (P1*V1)/V2
= 14.7*1.7/1
= 25psi (abs)
= 10.3 psi boost

this figure sounds ballpark but i havent looked at any system losses. if the VE for the SC and engine is the same (probably not) , then they cancel as a fraction because the V1 would multiply by a number eg 0.9 and V2 would also multiply by a number ie 0.9..... therefore, 0.9 would be on numerator and denom so would cancel.


its seems so simple but ive read that calculating PD SC boost is complicated and not accurate at all?


i would like anyone with a SC engine (prefer SC14 or SC12) to provide me with engine size, SC type and SC pulley ratio so i can validate this formula


just something ive dug up about "mic*'s" setup

he has a 1g (2 litre), ~1.7pulley ratio and claims 16psi boost

putting the formula to the test;

P2 = (14.7* (1.4*1.7))/1.0
=34.986psi (abs)
= 20psi boost

close!

from that formula, the only way that value can be lowered is if the denominator is bigger (meaning the engine consumes more than 1.0L air/rev - should be opposite) - so prob not that case.

or, the numerator is smaller. this makes sense because its highly likely the efficiency of the SC deteriorates with RPM. since the atmosphere pressure shouldnt budge from 14.7psi and the pulley ratio cant change..... the calculated displacement of the SC is;

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/1

actual SC disp = 31/(14.7*1.7)
= 1.24L/rev

this equated to 88% VE

seems ballpark also

so now to redo this with engine VE of 95% (realistic?)

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/(1*0.95)

actual SC disp = 29.45/(14.7*1.7)
= 1.178L/rev

equals 84% efficiency

now i shall use mic*s efficiency figure for my calculated boost

P2 = (P1*V1*SC VE)/V2
= 14.7*1.7*0.85/1
= 21psi (abs)
= 6.5 psi boost

maybe the SC VE is better when less boost is run?



sounds about right to me but seems too simple to be accurate. anyone like to check my math?

any ideas how thos SC VE's equate in real life?


ideas and thoughts welcome


cheers

brett

[Ben Wilson]

The engine VE is going to be the killer - Unless my 4AG was heatsoaked, I never say more than 10psi with a 175mm pulley, but it was still making big power...

[brett_celicacoupe]

what ratio corresponds to a 175mm pulley?