89 replies to this topic

[1275 SPRITE]

Thats what people are trying to find out, people are trying to get larger s/c pulleys made to slow it down as it runs too much boost on small bore engines. Not sure of the exact figure, but 12psi springs to mind on a 998 at 6000rpm??

James

[smj]

would it help knowing what boost the charger makes on a bmw mini, knowing the both pulley sizes and engine speed??? could it be calculated then??? or am i clutching at very small straws going the wrong way down a blind alley without any idea at all??

[1275 SPRITE]

I dont think it would help an the bini is 1600, and yours is 1100, so the boost created would be completely different.

[smj]

i just assumed that the supercharger, whatever drives it will make a set value of boost at a certain rpm

[Turbo Phil]

I must admit I've not done a lot of reading on supercharging, but from what I understand. The supercharger is like a pump and displaces a certain amount/volume of air per rotation.
Now if you try to displace that air into a small area it creates a higher pressure than if you displace it into a larger area. So on a 998 the boost would be higher than on a 1275 {as long as the supercharger is driven at the same ratio} as the bigger or smaller volumes create a pressure difference.
So on the 998 the supercharger needs to be slowed down to get the boost to a sensible & suitable level.
This is my grasp of it, I may be off the mark though ?

Phil.

[smj]

that does sound like its right though, i still reckon if you know all the bmw min's displacement etc etc etc you can calculate backwards to find out what small bore classic min would boost too

[minitomfoolery]

Just read this topic and now i'm inspired to go down the home build kit myself.

Just wandering what you are planning on doing with transmission and clutch? are you uprating them at all?


I have a 1275, if I was to supercharge it, surely the gearbox and clutch and differential will need to be upgraded?


What kind of power is produced aswell?

Thanks and good luck with your build hope it turns out good for you.
I will be keeping an eye on this thread.

[1987_ParkLane]

Just read this topic and now i'm inspired to go down the home build kit myself.

Just wandering what you are planning on doing with transmission and clutch? are you uprating them at all?


I have a 1275, if I was to supercharge it, surely the gearbox and clutch and differential will need to be upgraded?


What kind of power is produced aswell?

Thanks and good luck with your build hope it turns out good for you.
I will be keeping an eye on this thread.

Clutch set up will need to be upgraded, as will your diff. Gearbox can be upgraded but it is expensive to go for the straight cut box, but don't think you would really need it for a supercharged 1275.

Depends on your set up what power you will produce, what cam you have what head, etc.

[spiyda]

I must admit I've not done a lot of reading on supercharging, but from what I understand. The supercharger is like a pump and displaces a certain amount/volume of air per rotation.
Now if you try to displace that air into a small area it creates a higher pressure than if you displace it into a larger area. So on a 998 the boost would be higher than on a 1275 {as long as the supercharger is driven at the same ratio} as the bigger or smaller volumes create a pressure difference.
So on the 998 the supercharger needs to be slowed down to get the boost to a sensible & suitable level.
This is my grasp of it, I may be off the mark though ?

Phil.

I've done a bit of research too and just finishing fitting an Eaton M45 onto a Suxuki g13b engine in a mini..

Pretty much correct..

The Eaton M45 is a positive displacement pump of 0.75 litres of air per revolution..

The engine only draws air on two cylinders per rev.. eg a 998 draws 499cc per revolution

If both pulleys are the same diameter, the supercharger is running at the same speed as the crank,
the boost is a ratio of 750/499 * 1 = 1.5
a boost ratio of 1.5 at an atmospheric pressure of 15psi is (1.5 * 15) - 15 = 7.5 psi of boost

If the supercharger pulley is half the diameter of the crank pulley
the supercharger is running at twice crank speed
the boost ratio is 750/499 * 2 = 3.0
a boost ratio of 3 at atmospheric pressure of 15 is (3 * 15) - 15 = 30 psi of boost

[jameswildcooper]

you dont need a longer throttle cable its just the choke cable you need longer

[joe90]

Hi all
great thred going on here. i too am looking into doing a home build kit. can anyone tell me the make & part number of the drive belt + what tensiner is used. I have also found this web site which is great for working out compresion ratio & unswept volume.http://www.csgnetwork.com/compcalc.html.
joe

[1275 SPRITE]

Jameswildcooper had to change his tensioner and belt, and I think he said it was off an escort cosworth? and he took the belt to the local parts supplier and got one of the same length
I'm sure he will confirm.

[jameswildcooper]

yes the tensioner is ment to be off a cossy and just pop down to parkers for a belt. i had to get a slightly smaller belt when i got a smaller pulley for the charger

[joe90]

many thanks, was that originaly an alternater belt on the cossie? just out of interest what size was the original pulley & what did you change to?

[jameswildcooper]

i think so but not to sure. the pulley was just standard ive gone slightly smaller to run 10psi