748R Another One, This Time Without Porting

As the title suggests, this job was another 748R, but without the porting of the previous job. Everything else was replicated (apart from grinding the welds on the exhaust headers), so it gave us a fairly good indication of how much the port work was worth.

We also got to dyno this bike before hand. It was fitted with the usual Termi 45/50mm x-over and muffler kit, and a DP chip. Compared to the run I used in the previous report as a sample for a "std with pipes" 748R, it is almost identical. Which is good.

We reset the cam timing, bored the throttle bodies, made up the trumpet extensions and put it all back together. For a chip, we started with the same chip as the last bike ran – in real terms, the porting would make minor differences to the fuel requirement, and they would be easily accounted for (or so I thought). The biggest influence for the new chip, compared to the std DP or UM241, would be the throttle body boring at low/mid throttle, and the cam timing and inlet trumpet extension at higher throttle openings. Given both bikes were the same in these areas, the rest was pretty easy.

I took some photos while this one was in progress. First up are the trumpets. We machine up spacer sleeves to lift the trumpets, with little spacers for the securing bolts to go through. On the left is std, extended on the right.

The throttle body bore changes are shown below. Std is on the left, bored on the right. The photo on the right is a little blurry, but you get the idea. The removed trumpet amplifies the lack of clarity my photographic ineptitude has produced.

You can see the sharp taper in the std throttle body quite clearly – as required by FIM Supersport regulations – and the straight section below. Once machined, the throttle bodies have a nice smooth taper to the bottom, which is the same size as std (44mm). This isn’t a "bigger is better" mod, simply removing the sharp taper that is std. Sharp angles of more than approximately 25 degrees (I don’t remember the exact angle) in a surface causes problems with air flow, as the air flowing over the surface can’t "bend" that quickly. What you get is the air flow lifting off the surface, and a turbulent, low pressure swirling pocket of air forming in the area just after the sharp change. This is called an eddy, and most people would have seen it in water flows at some time or another.

The main effect of this, in a pipe, is to reduce the unrestricted diameter the air has to flow through, effectively acting as a choke. This reduces the overall maximum airflow, and gets worse as the air speed rises and more air tries to flow. By machining out this sharp edge, the air follows the sides of the throttle body bore without interruption, leading to a smooth, gently accelerating flow as the taper reduces in diameter. We purposely don’t machine this to a mirror finish, as small grooves in the surface produce very small eddies in the air flow next to the bore surface. These eddies reduce the boundary layer thickness and drag of the airflow in much the same way as marbles on the floor reduce your ability to stand up (or increase your speed across it). A good thing for air flow, probably not so good for you. Unless you’re young and the idea of crashing headlong into the kitchen cupboards at speed is appealing.

Once complete, the idea was to head to the dyno and tune as required. Unfortunately, on the particular day it all came together, the dyno got transported into the Melbourne Motorcycle Expo for a "run your bike" display. So, we gave it back to the owner with just some feel changes to the previously used custom chip. It was obvious that this bike didn’t go like the ported one did, however, and I was a bit disappointed with the outcome. It also needed some more fuel down low and less up top, according to my seat of the pants dyno. I was quite surprised – I’ve ridden a few bikes over the years that have been ported, and I must say the variation between these two was the most apparent I could remember.

The owner’s feedback was my next gauge. He mentioned several times that it was very different, and that it really went once above 7,000 RPM. Something his partner (a 748Strada owner) also commented on after she’d had a go. So he was obviously impressed with that bit.

The way it rode at lower speeds was another thing he was happy with. We sorted out a couple of issues with the clutch operation at the same time, and the bike was much (believe me, much) nicer to ride in traffic after the job was done. The mods we made to the fuel and ignition advance maps also seem to have helped here. Much less bogging down off the line in that typical 748R fashion, and easier to ride slowly.

He had told me that he didn’t rev the bike that hard, and this somewhat lead me to think that maybe he wasn’t overly happy with the result. I had an idea of where to go to give him what he was after, but first wanted to dyno the bike just to see how we’d gone with the changes as they stood at that point.

So, I organised for him to bring it to the dyno on a Saturday morning, just for some WOT tuning runs to see what the outcome was. The result is shown below. Power first, then torque. The baseline run before the mods – Termi 45/50mm muffler kit and DP chip - is shown in green. The after – cams dialled, throttle bodies bored, trumpets extended and custom Ultimap chip – is shown in red. The previous bike with the porting is shown as a reference in blue. The only difference between the blue and red lines is inlet port work.


Dynograph courtesy of DYNOBIKE (03) 9553 0018


Dynograph courtesy of DYNOBIKE (03) 9553 0018

It is fairly obvious why the ported bike felt better, although the improvement from green to red is still good. I emailed Doug some run files, and his reply was "I told you the ports were bad!" I can only agree (not that I doubted him originally), now with dyno proof.

For the work we’d done, however, I was still pretty happy with the result. The bike goes much harder (especially over 7,000 RPM) with a noticeably smoother delivery and is nicer to ride in any situation than it was before. Not pulling the heads saved quite a bit of money, but in the 748R’s case, that money appears to be well spent if you’re after some real power. If we repeated the previous bike’s porting, the labour and parts involved would cost more than the port work – it really isn’t that much. Which is unfortunate, but obviously the way it is.

You can see from the above torque curve that the torque peaks have moved down the RPM range, just like the previous ported bike. This is due to the inlet length change, a nice idea well worth stealing from Doug. On this bike without the porting though, boring the throttles is something I’m not totally convinced is justified. It seems to be a case of the weakest link, in this case the inlet ports, holding it back. I would be surprised if boring the throttles showed up in a before/after test on this bike, whereas on the ported bike I’d expect the effect to be clear. With porting, I’d expect all the 748R features – 54mm throttles, big airbox, short duration, high lift inlet cam, long duration, high lift exhaust cam – to come together and allow the 748 to make some serious power over a wide rev range. Without the porting, the features are to some extent just along for the ride.

Of course, not boring the throttles makes the custom chip we’ve done somewhat worthless, as the variations at quite low throttle come from the increased air flow the bored throttles allow at those points. As you get closer to WOT, the changes due to the boring alone are much less pronounced. Doing just the cam timing and inlet extensions will cut the cost a bit though, and we can always do another chip for that (as long as someone is willing to pay for the dyno time). Even zoning the base Ultimap chip (UM241) would work well.

The next graphs show a bit of the variation between the ported and un-ported bikes’ air fuel ratio, with both running the same chip. Just to show the difference in air/fuel you get with port differences. Red is the un-ported bike, green the ported one. The chip shown is the one I was starting from. First up is WOT torque, with a very dramatic difference all through the rev range. The second is at 55 degrees throttle, where the difference is much less, but still clear, especially as the two torque curves move further apart.


Dynograph courtesy of DYNOBIKE (03) 9553 0018


Dynograph courtesy of DYNOBIKE (03) 9553 0018

The final graphs compare output to the 748Strada with cams dialed and 45/50mm half exhaust. Red is the un-ported 748R, blue the ported 748R and green the 748Strada. The red and green curves are pretty much equal up to 9,500 RPM, at which point the Strada torque curve continues to fall away steadily, and the R torque curve plateaus for another near 1, 000 RPM, which is where the extra power comes from. Power first, then torque. The 5,000 RPM hole is consistant, if nothing else. The ported bike just whips them both.


Dynograph courtesy of DYNOBIKE (03) 9553 0018


Dynograph courtesy of DYNOBIKE (03) 9553 0018

Without porting, I’m not convinced the 748R inlet cam’s extra lift would be of any benefit – in fact, its almost identical duration to the Strada cam would have them behave much the same I’d expect. For a short duration, high lift cam like that to work well, it needs very good flowing ports and valves large enough to use the lift. Then, it will make good high RPM power, along with good low RPM power from the short duration. I wouldn’t expect it to work so well in a bigger engine however, where the demands on the inlet ports are much greater.

I’ve pretty much convinced myself the extra power – especially in the un-ported R over the Strada – comes from the longer duration exhaust cam. I’ve so far managed, with one bike or another, to discount all the other 748R features. I’d like to try this exhaust profile in a bigger engine (Vee Two duplicate it with some small profile revisions on the non 748R valve spacing), along with the longer duration 996SPS inlet (or the even longer duration 748SPS inlet). These cams don’t have as much lift as the R inlet (10.8mm vs 12.5mm), but the extra duration will work well in a std port /valve 996 I think. Although the 748SPS may not work with Strada or SPS typical combinations of valve lengths and available shims, due to base circle variations.

Of course, I could be wrong. Maybe I’ll get to try it in the 944 for the 851.

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