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Discussion Starter · #1 · (Edited)
I'm about to pull my 916 engined 748 off the road for a light overhaul post IoM trip. There isn't really that much I can do to make it much faster as the rider is the limit here. However I WOULD like to make it a bit quicker as I could handle that. The first priority will be later, lighter wheels. After that will be some engine tweeks. It's already as light as I can afford.

The engine already has cleaned up heads and valves, adjustable cam pullies (cam timing set i.a.w Brad Black's article a lightly modified Senna map, Piper foam filters in the intakes, Termi silencers, standard but cleaned up internally exhaust (man, you should have seen some of the welds from the inside!!!). The focus, engine wise, has been on making the best use of what I have.

After some study and a great deal of though I have the beginnings of a plan. Amongst the books in my library is "Design and Simulation of Four Stroke Engines" by Dr Gordon P Blair. On p527 onwards ther is a study of the Ducati 955 racing engines. The study notes the pressure wave differences in the exhaust and inlet tract and suggests that this is due to the L-twin arrangement. The study does not comment on how to change/modify these differences to improve the engine.

Putting it simply the vertical cylinder is not optimally set up. The study suggests the exhaust arrangement is the root of the problem. I would suggest the throttle bodies also play a part. Most engines have the butterflies open so that the edge of the butterfly at the top of the port rotates towards the body intake. This helps direct flow over the bottom of the port at part throttle and slightly so at WOT. On the Ducati arrangement only the horizontal cylinder works like that. I have often wondered what the real world effect of this would be. I was tempted to think "not much if Ducati thought it was OK" but then I remembered that initially Ducati didn't use the offset map for the horizontal cylinder either.

As I have a spare set of throttle bodies I was thinking of cobbling up a set that made both open "the right way" to see if/what difference there is. Or I might go the whole hog and get a suitable set of bodies to modify in this way but use pico type injectors if my 900SS/Monster throttle body swap gives good results.

Together with this I have three different sets of bellmouths. 748/916, ST4 and 996. My preference is the ST4 bellmouths. It has occurred to me to experiment with an asymmetric arrangement using the good Dr Blair's book as a guide.

Before I jump down this particular rabbit hole I would just like to ask, "has anybody out there been here before and if so what were the results"? Don't worry, you are unlikely to meet me on a race track :) .
 

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Discussion Starter · #2 ·
Well this project is growing arms and legs! My injector body swap on my 900SS worked rather well so I took a long hard look at the injectors on the 916. I think it may be possible to modify the bodies to take pico injectors so I've ordered a rather shabby set of throttle bodies of Ebay from the UK (don't want to trash something decent) and some IWP 181 injectors off Ali Express.

Trying to find out the flow rate of the standard IW724 injector was a bit of a pain. The rates given by various sources go from 285cc/min, 300cc/min or 317cc/min. The median is 300cc/min and the 2 others are +/- 5% so I used that as a bench mark when ordering the IWP 181s. The have a flow rate of 317 cc/min.

While researching this little project I looked at the butterfly set up on the later bikes and they do it the way I'm planning so I think I may be on the something. Whether any benifits are worth the bother remains to be seen. At the same time I took a closer look at the plumbing for the injectors. I will use the top feed of a Monster of 999. The 999 plumbing is interesting as it is more technically correct. The 916 fuel feed goes from one injector to the next which means the down stream injector will get slightly less pressure, theoretically anyway. The 999, which is top fed, has a splitter that feeds both injectors from a 'Y'. The fuel return to the tank is Teed off before the 'Y' so this should supply the same pressure to bother injectors. Some people take great care to match injectors - they should note this.

This little project will probably be carried out in slow time but needs to be finished before the IoM Grand Prix next year. Hopefully this project will actually get finished!
 

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‘05 749R, ‘89 851/926, 749R/999RS project
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Well this project is growing arms and legs! My injector body swap on my 900SS worked rather well so I took a long hard look at the injectors on the 916. I think it may be possible to modify the bodies to take pico injectors so I've ordered a rather shabby set of throttle bodies of Ebay from the UK (don't want to trash something decent) and some IWP 181 injectors off Ali Express.

Trying to find out the flow rate of the standard IW724 injector was a bit of a pain. The rates given by various sources go from 285cc/min, 300cc/min or 317cc/min. The median is 300cc/min and the 2 others are +/- 5% so I used that as a bench mark when ordering the IWP 181s. The have a flow rate of 317 cc/min.

While researching this little project I looked at the butterfly set up on the later bikes and they do it the way I'm planning so I think I may be on the something. Whether any benifits are worth the bother remains to be seen. At the same time I took a closer look at the plumbing for the injectors. I will use the top feed of a Monster of 999. The 999 plumbing is interesting as it is more technically correct. The 916 fuel feed goes from one injector to the next which means the down stream injector will get slightly less pressure, theoretically anyway. The 999, which is top fed, has a splitter that feeds both injectors from a 'Y'. The fuel return to the tank is Teed off before the 'Y' so this should supply the same pressure to bother injectors. Some people take great care to match injectors - they should note this.

This little project will probably be carried out in slow time but needs to be finished before the IoM Grand Prix next year. Hopefully this project will actually get finished!
Not so sure any minor differences in fuel pressure are an issue as both cylinders should be mapped individually.
Even so it is not as though both injectors are firing together so realistically they will both see the same fuel pressure.
Happy to be corrected.
 

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Discussion Starter · #5 ·
If I remember right from the days when The Dark Wraith was around the injectors actually DO fire together but I'll try and find out for sure. Did the re-plumbing on the 900SS anyway. It didn't do any harm, if it did any good then it is at the level that in indistinguishable from my imagination. However I haven't had the bike fully warm and up to high speeds. If there is anything that I noticed it was that a lumpy spot around 4000 - 4500 appears to be gone. Further testing required. I need to check the CO2 as well as this may show up something.

I found a set of 748 bodies I forgot I had and have done a little stripping and measuring. Flipping the butterflies may not be that difficult. The hardest bit may be making a new spring for the butterflies. I stripped out the injectors an compared the pico injector I have with the resulting hole. I'm going to have to do some serious work with the high speed grinder but it looks do-able.

The tricky bit looks like it will be the mapping. It isn't just a case of removing X% right across the map (although that might be a good starting point) due to differing flow characteristics and a better spray pattern. Just waiting for all the bits to arrive.
 

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Discussion Starter · #6 · (Edited)
Now out of winter hibernation and returned from Planet Ural. My Isle of Man trip is cancelled :( but I'm going to continue with this project. I've decided to split it into 2 parts. The first involves rerouting the fuel pressure and return lines in line with theory to make the fuel flow and pressure fluctuation the same on both injectors. I have also modified the butterfly pushrod so that they both open in the same sense compared to the port floor. I

I've aleady tried the fuel lines on my 900SS. Did it do anything? - Yes! What did it do? Eh.....I don't know, it feels different and it certainly doesn't go any slower. It feels a bit smoother and is a bit better around 4000ish. What am expecting these mods to do on the 916? Better throttle response and better mid-throttle power. Maybe even a tadge more power at WOT but I doubt I will notice.

Once that is done and tested the I'll get round to the injector swap.

1005342
1005343
1005344
 

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Any chance of larger photos ?
I run S4RS throttle bodies on my 996 Bip.
When making the mounting brackets, it didn't occur to me that, I could flip the butterflies .
My concern is, with the modified linkage, do they open by the same amount at part throttle openings ?
 
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