Blower Sizing

[Roger P.]

Does anyone have a general guideline as to how much displacement a supercharger should have for a given engine size? I see lots of small blowers from Japanese Kei cars on offer for little money, the Aisin AMR500.

They displace 500cc per revolution and are good to 16,000rpm, so with a 1.6 overdrive and keeping it to 10k rpm or less you should be good to go. I've run the numbers through an online calculator but i get only 4psi of boost for the answer. I'm not too sure how much confidence I should have in these answers. Any ideas?

[no class]

that thing is tiny ........ what do you plan on putting it on ?

[Danm54]

The bigger the better right?

[Arttu]

For rough estimations the blower displacement / rev and maximum rpm are enough. Like you already figured out the maximum rpms of the blower and engine determine maximum drive ratio that you can use. 10 000 rpm for engine and 16 000 for the blower will result maximum 1.6 ratio like you calculated.

For boost pressure estimation you need engine displacement, blower displacement and drive ratio. The formula is: blower displacement * drive ratio / engine displacement (per rev). So if you take that 500cc Aisin, drive it at 1.6 ratio on 1000cc four stroke engine it will look like this: 500cc * 1.6 / (1000cc / 2) = 1.6 pressure ratio = 0.6 bar = 9 psi. Engine displacement is divided by 2 because a four stroke engine has intake cycle on every second revolution.

In real life the result can be somewhat different due to various factors like engine volumetric efficiency, valve overlap, air heating and so on. But if you just want to figure out if the blower will suit to engine and what drive ratio to use this should be close enough.

[Roger P.]

Ok, so 500cc * 1.6 / (1157/2) = 1.38, so, 0.38* 14.7psi = 5psi on a Bandit 1200 motor. Probably not enough for a massive power gain, but I wonder if such a low figure would play nice with a higher compression ratio?

[Blower1]

Aisin 500 is ok for engines 750cc or smaller. For Bandit 1200 I would choose Eaton M45 (750cc/rev) or Eaton M62 (1000cc/rev).

If revving roots type supercharger to it´s given max rpm, it´s thermal efficiency drops = it´s making more heat than boost.

You can google "Eaton supercharger performance map" and check for the best suited supercharger size for your bike/need.

From my experience the engine needs about 300 m3/hr air flow / 100 rear wheel hp.

About the compression ratio, if using less than 7 - 8 psi boost no need to worry about compression ratio if it´s less than 12.

[MeanBean49]

You have to remember those figures are based on the blower spinning at its maximum and the engine at 10k revs

In reality it will be way lower, like at 5k revs it will be half (ish)

Almost not worth bothering with for the amount of power it will sap just to drive it