Most of these wonderful-enough-guys have little more experience of tuning Harley-Davidsons than their customers and a good deal of their knowledge comes from reading the same magazines that you read. So how to find the true Grail?

Let's take a look at the bhp numbers we were talking about at the start ...

A stock XL1200 (74 cubic inch) will give around 52bhp at the rear wheel, that equates to 0.7bhp per cubic inch. An XL883 gives us around 0.78bhp/cu.inch. A US spec 1340cc bike (82 cu.inch) will show just under 50bhp at the rear wheel which is only 0.6bhp per cubic inch: a European 1340 manages a sad 42bhp.

It doesn't take an Einstein to see that the Evo Bigtwin must therefore be running even less efficiently than the Sportster. Now the argument goes like this; if the Sportster is happy at 0.7bhp per cubic inch then the Evo Bigtwin should also be quite okay doing the same. So the minimum that should be available from the 1340 – if it is as 'efficient' as the 1200 – is 57bhp. But 57bhp is 20% more than the Americans are being given, and 33% more than the Europeans get!

The Evo BigTwin is someway less efficient in it's combustion as the Sportster, so if the factory don't make the Evo BigTwin as efficient as they could it is unreasonable to believe that the Sportster itself may not be perfection-personified either? Of course it isn't – and latter output figures from the Buell twins prove it can be better.

Now just what sort of numbers should you expect from different engines and different sizes?

There is no magic forthcoming from bigger engines (strokers etc) beyond that which their extra size can develop, in other words if a 1200 cc engine produces 60bhp then a 25% larger 1500 cc engine, (providing, of course, that it's carb/cam/ignition/burn/exhaust etc. are all 25% more efficient too), will manage 75bhp. No magic – only simple mathematics. What does this tell us? A few years ago, in a British magazine called Heavy Duty, we ran a series called 'Bolt-On Benefits'. Changing various components on the 1340 cc Harley engine – in this case a 1994 Dyna -  we increased efficiency to the result that we had over 86bhp at the rear wheel. And this – due to time restrictions – was far from 'perfectly' set up.

Test case Basically, what we have here, is a little eye-opener. The lower reading is in fact two. It's from a stone-stock 1995 U.K.-spec Fat Boy. It says 33bhp at the rear wheel at 3000 rpm. 40bhp at 4000 rpm. 42bhp at 5000 rpm. Then the ignition module rev. limiter comes in. Some checking on the jetting gives the other line which virtually mirrors it - so the factory sent it out as near as perfect comes. Take a bow, boys! The lines which come down from the upper left to join it give the torque readings. More on those later. Our plan was to check and test the stock airfilter set-up against some of the claimed 'performance' & custom aircleaners on the market. The numbers we obtained may well only be good in respect of this particular bike - but you can judge for yourself how differences apply. To see if there might be any restrictions being applied by the stock system we took the crude but simple step of removing the airfilter altogether and re-jetting the carb to suit. Not a recommended path to take for street use - but a useful indicator of what might be available. We left the backplate on - so the bike would hold together - and the readings we obtained are shown on the upper line. That's right. No airfilter gave us 36bhp at 3000 rpm. 49bhp at 4000 rpm and 58bhp at 5000 rpm. then we hit the rev. limiter again. Now that has to tell us that there are gains to be had in changing the stock airfilter arrangement. Agreed? We had an increase of 3bhp at 3000 rpm - of 9bhp at 4000 rpm and of 16bhp at 5000 rpm!!!

This was matched nicely by another one at a later Dutch dyno shoot-out organised by Zodiac.

Now here we see 82 cubic inches producing 86bhp – that's 1.05bhp per cubic inch. Before anyone says that such a figure seems a mite high compared with the stock figures and therefor must be too 'highly tuned' let me say that this is actually very low 'tuned' still! Mid '50's Chevrolet run-about family cars put out 1bhp per cubic inch. That's right, forty years ago! We are talking low tuned here, low tuned indeed!

And we are still finding ways of improving that burn and that output number knowing there's even better yet to come.

Now accepting that a low 86bhp is okay then where should all the other engines be? Simple maths tells us that a 1200 Sportster (74 cubic inch) should find a good 78bhp at the rear wheel (76bhp is coming along quite regularly with our customers' bikes). A 96 cubic inch Big-Incher should have a minimum 101bhp and a 104-incher should really be giving 109bhp. These figures should also take into account that our 1340 was set up for low to mid-range power rather than top end. So much more would be on tap if we were looking for top-end only.

This would indicate that most of the bikes at the Dutch shoot-out were less than wonderful in the efficiency stakes. A couple show the way though with a 104-incher slap-bang on the ball and a 96-incher showing the rest just what proper parts and tuning are all about.

Let's look at those numbers again. That 96-incher in Holland kissed 110.5bhp which means he is developing 1.15bhp per cubic inch. Now if we scan that back through the range then we should be getting 85bhp from a 1200 Sportster, 94bhp from a 1340cc BigTwin and 120bhp from something of 1700 cc. Mmm ...

Now this is still only 1.15bhp per cubic inch remember. We know from experience that our 1340 is very smooth to ride with 86.5bhp – smoother than a stocker. We also have an 88 inch (1440 cc) Sportster which lays down a creditable 122bhp at the rear wheel (Thanks Axtell!!!) and is a joy to ride on the street – it's just that telephone poles move quicker than they used to!

It is little wonder that so many of you out there have your doubts about what (or who) is best when three 1700 engines at that Dutch event can result in outputs varying from 102bhp down to 86bhp! And to top it all they should be putting out nearer to 120bhp if a privateer's 1600 cc with over 110bhp is any measure!

So. Look to bhp/cubic inch to help you get a grasp of what exactly is being managed by others out there and if it is likely to be the sort of power that you actually want. It certainly means that is your 1200 Sportster is not allowing you 80 to 85bhp to play with, or your Evo Bigtwin 1340 cc isn't putting down 90-95bhp on the road then there is still a possibility of increasing your power output by improving efficiency.

Not everyone wants everything – but these figures hopefully give you some indication of where you can be should you want to.