Maths / pulley problem - clever maths advice needed

[H8RRA 0]

right then.

 

with regard to supercharger pulley sizes and as there's more and more charged vr's around, I want to work out what pulley sizes give what psi output. With the data I have, can this be done?

 

for the driven pulley -

From stg 1 kits, a 3.125" pulley gives 6 psi, at 7000rpm, I think

From stg 2 kits, a 2.7" pulley gives 8 psi, at 7000 rpm

 

for the driving pulley -

The crank pulley looks to be about 132mm (anyone confir this or correct this figure? - vernier calipers are too small to read the diameter directly)

 

So if the rpm and driving pulley are constants, can an excel spreadsheet be done to indicate psi outputs and varied pulley sizes to be made????

[dinkus 10]

Surely it's just a correlation of circumference of the pulley?

 

2x pi x d = circumference (where d is the diameter of the pulley in cm)

[dr_mat 0]

Charger vane RPM is proportional to the circumference of the pulley, yes. The problem here is we're assuming that the PSI output from the charger is linearly porportional to the charger RPM.. I'm pretty sure it won't be. Well, maybe it is, but only over a fairly limited range.

[H8RRA 0]

this is the problem - i've been playng around with exel to see what relative ratio's are etc for known circumferences and outputs to see what correlations there are....

 

dink - i think thats 2 pi R OR PI d your thinking of for circumference - ;)

[Yandards 0]

this is the problem - i've been playng around with exel to see what relative ratio's are etc for known circumferences and outputs to see what correlations there are....

 

dink - i think thats 2 pi R OR PI d your thinking of for circumference - ;)

 

There is a hell of a lot maths/physics involved with working out what you would like.

 

The problem is the number of factors involved, as you stick a smaller pulley on you will still be using the same size inlet to the 'charger so the airflow has to increase in speed through the pipework to compensate (That venturi chap strikes again) This will increase tubulence, air temperature etc and so as Dr. Mat has already said this will result in a non-linear output for a set pulley size change. Then there are factors like back pressure in the system, inlet air temp increases (both pre and post charge) will all effect the level of boost achieveable.

 

The problem being is that it is difficult to calculate the point at which a reduction in pulley size will result in little boost being generated due to a lack of hard maths on the factors mentioned above. :?

[dr_mat 0]

It is probably easier to use the time-honoured suck-it-and-see method .. :)

[Yandards 0]

It is probably easier to use the time-honoured suck-it-and-see method .. :)

 

Yeah, get a boost gauge fitted and just try different size pulleys :)