748R  45mm X-Over and Mufflers and Inlet Extensions

I recently had the opportunity to try something I’d been hoping to with a 748R, being the 45/50mm half exhaust system thing.  I wasn’t sure if the mid range hole the Strada models showed with a 45/50mm half system would relate to the 748R, which itself has quite a mid range hole.  Or if the mid range hole was a general symptom caused by the bigger exhaust cams that many of the hotter SP models over the years have had, compounded by the fact that many of these models had also been fitted with 45/50m half systems.

In contrast to the short duration, high lift inlet cams the 748R has, the exhaust cams are about half way between Strada and old SP in terms of duration, with more lift than any previous cams.  So they’re certainly pretty big.  Whether or not this is what gives the 748R its high RPM torque I’ve not yet been able to test for, but I was wondering if they also contributed to or were responsible for the 748R mid range hole.

We had a 748R traded on another bike, so I took the chance to do some runs with it.  With 23,000 km on the clock, it was certainly well run in.  What has been done to it in the past I’m not sure, and it turned out to be the most powerful “std” 748R I’d dyno’d.  Why, I have no idea.  But, it is what it is, and that’s how it goes.  By std I mean as delivered and fitted with the 45/50mm half system Termi kit that was offered at a good price when the bikes were new.

We fitted one of the UM241B12 eprom's we developed and gave the bike a 20,000 km service.  Once all that was done I took it to the dyno to see what it made.  I tried leaning off the fuelling a bit, as this usually tells me if the Ultimap eprom is doing as I expect.  In this case, it did.  The graph below shows the power and air/fuel curves.  Red is base fuel, green is -5.4%, blue is -10%, yellow is -15%.  It makes max power in the -5.4 to -10% range (around 13:1), and the base fuel runs through the mid 12:1 range.  Which is as I expect with Ultimap eproms.  In high load situations on the road or track you can feel the difference between these two settings – the “best dyno power” fuel setting will be a touch weaker than the Ultimap setting with the richer fuelling targets Duane uses.  The big rich spot at the bottom is somewhat due to the acceleration enrichment and what 748R seem to like – get rid of it and they often fall into a big hole on the road.  The -15% line also shows how quickly power can fall away with bad fuelling - in this case, too lean.

Next I fitted a 45mm x-over and mufflers I’d found at work.  The x-over was a std part, the mufflers Arrow.  You can see on the following graph that the mixture was much richer overall with the 45mm system, and that it also made max power on the richer side of 13:1.  It may be richer because it’s moving a little less air, a concept that I’m not sure has any basis in reality, but I’ve seen things before that give this impression.  Not noticeable in power terms, but in air/fuel.  Also I remember the bike was running a touch cooler for these runs, and the 748R chips have wacky coolant trim tables.  Not that this should affect max power fuel.

To compare the two set ups, we’ll take the best power run of each.  This gives us the graph below, green is the 45/50mm kit system, red the 45mm std x-over and mufflers.

Really not much difference.  The bigger system gives a smoother curve, but there’s very little in it.  I did have the feeling that the 45/50mm kit gave better performance on the road when riding it with the 45mm system, but after refitting the 45/50mm kit I realized that wasn’t true.  I did compare the on dyno acceleration though, just to see what that showed.  On the graph below you can see the green curve (45/50) slightly edge ahead of the red curve (45).  You’d have to be packing a very sensitive seat of the pants to pick them on the road though in my opinion.

So the conclusion I came to is that the dip you get in the power curve from 4,000 to 7,000 RPM on some models is due to general over exhausting, whether it be cams or system.  Although, to confuse this, I’ve found bikes with 50mm headers to often be quite strong in this range.  Maybe the bigger headers themselves don’t give the same dynamic affect as the longer cams or the large diameter (or volume) change halfway down the system.  At least the 45mm system didn’t prove to be lacking.  And I like them because they’re cheaper and I tend to be somewhat keen to avoid spending money I don’t need to.  Except when there’s a perceived bargain to be had, when all sense goes out the window and I hurl cash round like confetti.  Ask me about my cheap Carillos (that suit neither my pistons nor crank!) – what a bargain.

Anyway, back to reality.  After I’d refitted the 45/50mm kit I moved on to the inlet trumpet extensions, as shown in the previous report.  As I had the opportunity I machined some up and fitted them for some runs.  Just to see what change they made on their own.  You can see from the graph below how the 17mm extensions both lift the torque curve and move the peaks down the range.  Moving the torque peaks down also affects the air/fuel trace quite noticeably, creating peaks and troughs of sorts as the tuning points move.  Although the changes this causes are not that great, and fall within the range you’d get on the usual cross section of bikes.  Green is before (with the 45/50mm kit), red is with the trumpet extensions added (no other changes, base fuelling).  It certainly bought the bike to life on the road – maybe just exaggerating the torque rise around 6,500 RPM.  It’s amazing how fast a 748 can stand up on the back wheel when you hit it in the right spot, and this one would hold the front up with ease through second.  Certainly a great mod and one of the cheapest real value ones going for these things.  Another good idea from Doug.  Power first, then torque and air/fuel.

And that’s it for this report.

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