Wednesday, 22 March 2017

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...


  1. Fascinating stuff Greg, I'm no maths expert so have to work though all the calculations myself. Are the stock Yamaha bearing/seals up to the job of dealing with the increased pressure of a 'charger? What about the heat generated by the 'charger set-up? Somewhere in the depths of my brain I seem to remember cooling problems with the stock TR1? I know the front cylinder is off-set when compared to a Hardly- Dangerous but what are the problems associated with running the supercharger? Do you have to add external oil coolers or perhaps even water cooled heads/cylinders to get the maximum performance from this set up?

    1. Hello your Loveless-ness!

      You may recall I have built a turbo-charged TR1 last year and yes the crank is easily up to the job. Conrod big end and small end for example are a bigger diameter than on a small block Chevy.

      Heat generally is an issue, even though less than on Harleys. (You actually run the same plugs and jetting on both cylinders...) It became a serious issue on the Turbo-TR1 as the oil quickly hit 130-140 degrees.

      With the currently planned low RPMs on the blower and low boost, heat will siginificantly be less of an issue than with the turbo, overall a max. of 20-30 degrees of temperature increase on the inlet is to be expected and as I mostly plan to run the bike down the dragstrip it shouldn't be too much of an issue.

      Water-cooled heads/cylinders would be awesome, but are waaaaay out of scope. I guess I'd rather go down the nitro-route, if things escalated that far. Truth be told neither of which is planned at the moment, because that would then mean building a new frame from scratch etc.

      An oil-cooler might be in the cards though, when I come to think of it...