Crude formula to calculate boost

... for (non-360 degree) twin engines and fours like VW-flat twins. In reality you have to figure in a lot more variables, but just to make a rough guess, whether the supercharger needs to be over- or underdriven or simply be reduced to a mechanical hairdryer, because you have to spin it so much faster than it was intended to.

... that's of course meant to say - 14.7

Now let's have a look at the Eaton M45 setup on the TR1 and use the formula above:

Supercharger displacement: 45 c.i. ~ 737cc
Engine displacement of my TR1.1 engine: 1063cc = 531.5cc
Ratio: 1:1 = 1

Result: 5.6PSI

There's quite a few reasons, why in real life you (most likely) won't see exactly this figure:
1) engine efficiency and blower-efficiency are blatantly ignored in this formula
2) blower efficiency varies with blower RPM
3) overall pressure has simply be assumed to be 14.7PSI and varies greatly with altitude

Assuming that we roughly hit the numbers calculated above, why can't we spin the blower faster and faster or why can't we simply use a much bigger blower.  As with every mechanical machine, there's friction and all sorts of losses involved. Friction being one of the main contributors (among the work of compressing air. Luckily supercharger-manufacturers usually provide spec sheets, outlining where the "efficiency island" is in the pressure map and even more importantly where the redline for a supercharger is. This redline basically marks the point, where your blower turns into a mechanical hairdryer and only pumps hot air into the engine. (Incidentially on an Eaton M45 this is 16,000RPM, just to give you an idea!) On the other hand, a blower needs a certain minimal RPM to actually pump air efficiently, which is why you can't simply fit an Eaton M90 and then only run it on half engine speed and expect it to work somewhat efficiently. And as if this weren't enough, add the fact, that you have to apply some power to actually drive the supercharger, which can actually go up into the two-digit horsepower figures with said Eaton M90.

I hope you find this post (mildly) useful and I'd love to see/hear/read about at least one supercharger build one day that was sparked by this post outlining that it is a lot less of a dark art as some professional companies try to make you believe...

Eaton M45 on a TR1, perhaps?

I am of course not (yet) admitting that there will be a new project, but a potential project has already been christianed "Project Super TR1".

This post is mainly meant to give you an idea on the overall dimensions of an Eaton M45. And the next post will give you a *VERY* crude formula to work out supercharger sizing for your engine and thereby outline, why an M45 is about the right size for an 1100cc V-twin, but too big for a 1000cc inline four.

Let's dive into it then: The Eaton M45 I use, is out of a early 2000s Mercedes C180/200 Kompressor and goes by the Mercedes part no: A 271 090 20 80 or Eaton 307961.

By the way this is what you'll usually find, when you buy one from a wreckers:

As a matter of fact, only the portion in the red box is the actual supercharger. It measures approx. 250 by 180 by 130mm (length, width, height). An M62 is the same width and height, but is substantially longer and the M62 has got an electro-magnetic clutch behind the pulley, which is also the easiest way to find out which model the supercharger is that you're looking at. The other thing to note: This particular Eaton blower spins clock-wise and the pulley is roughly 71.5mm in diameter.

The following pictures are merely to illustrate how I took the measurements.

In the next post, I'll outline a *VERY* crude formula for guesstimating (I won't go as far as call this a calculation) supercharger-sizing in relation to engine size and thereby calculate also the resulting boost.