So the day had come – after all this work not to mention a chunk of loose change, had these engine modifications actually improved anything ? Well, for one thing you will note there was a lot less smoke this time (see the original post here) !
After running in the engine and putting a few hundred miles on it, I took it back to RPM Services south of Calgary for another dyno day. As well, I took along a second exhaust system made by Gibson Exhausts in the UK. I bought these a few years ago when living in Europe as I just liked the look of them, but I also always wondered whether they actually did anything to improve the performance other then weighing a fraction of the stock exhaust system.
After several runs, the results were in – the chart below shows three curves – the lowest one is the result from the bench mark run before the engine rebuild. The middle line shows the result from the rebuilt engine fitted with a stock exhaust system and also with stock BS40 carburettors and airbox. The carburettor jetting is also factory stock, which for our altitude here in Calgary means the engine runs a bit rich which is OK as a starting point. I had forgotten to mention that I also had changed the points ignition to an electronic one made by Accent Electronics in Germany which I normally install as standard when I do rebuilds. The top curve shows the rebuilt engine fitted with the Gibson exhausts swapped in for the stock exhaust system, but with no other changes (so stock jetting on the BS40 carburettors, etc.).
You can click on the photo above to see a larger version, but to recap – the numbers are 55 HP at the wheel, dropping off to 53 HP in the original engine as it approached 7000 RPM. The rebuilt engine with the lift plate and clean-up (but no porting) and stock exhaust, BS40 carburettors with stock jetting, airbox etc., managed a clean performance increase right across the rev range topping out at just under 63 HP. That is an increase of roughly 15% which is respectable. The top curve shows that while slightly less than the stock exhaust below 4000 RPM, the Gibson’s come on pipe at about 4000 RPM and then peak at 70.2 HP at 7000 RPM, for a performance increase of roughly 27% over the base case, which is quite respectable.
So what’s next ? I have a couple of other exhaust systems I plan to test with this engine in its current iteration, and will eventually post the results here. At some point in the next year or two I’ll pull the engine and conservatively modify the porting, just widening the exhaust and intake ports, possibly dropping the exhaust port floor slightly but not much else. Then I will do the dyno runs again with the same sets of exhausts and see what changes.
And why bother doing any of this ? No reason really – having long winters allowing lots of shop time I suppose is a part of it, but truthfully ? It just makes me smile !