UK Buell Enthusiasts Group

Venturi 2
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Author:  edd [ Fri May 26, 2017 4:09 pm ]
Post subject:  Venturi 2

For a slight continuation of my exhaustive tale! :hehe:

Recent ride outs in high pressure/dry conditions have caused me to think about continuing my experiments with narrow tubing exhaust inserts. A 180 mile round trip into Wales exposed the current venturi as possibly working a bit too enthusiastically. :?

Comparisons with previous rides in the cool damp air of winter and an intoxicatingly smooth roll on power, the motor was not as sweet. On the plus side it felt strong and I achieved 67mpg on one tank full.

The motor would probably benefit from a vapour injection system. I considered cobbling up such a device in the air box, but that would be too complex. I have now commissioned my local engineers to turn out another section of the 304 stainless tube I have by lathe to save my grinding wheels from being totally obliterated! ;)

This next piece will be wider at the choke to slightly reduce the venture effect for 'high n' dry' atmospheric conditions. I was hoping to have it fitted before my trip to Lincs, but that's not looking too certain as I type this. Still, the current forecast predicts that we may be in for some cooler conditions with a generous helping of water injection all round! lOl

Author:  edd [ Sun Jul 02, 2017 10:40 pm ]
Post subject:  Re: Venturi 2

Update on Venturi 2: Payoff?

It’s funny, but my own XB at least seems to need around 150 miles or be well into its second ride before the full effects of a change to the exhaust or induction system become apparent. It’s like the ECU suddenly saying ok I can dig this, lets party! 8-)

If you get bored easy you may want to skip to the last paragraph or so. lOl

Basically this went in 3 stages (also see thread ‘Exhaustive Tale -Part 47); Standard ’06 XB9SX header with 12R muffler – good upper mid to top end with indifferent low to midrange. 42mm ID venturi fitted inside collector – good low to midrange pull with rolled off top end (some loss of smoothness in high and dry conditions). 43mm ID venture fitted inside collector – apparently good throughout (accepting that I rarely go over 90 for more than a few secs or wish to.).

Some have doubts about the effects I am seeing, but many on here who frequently ride the ‘big power’ of a 12 may not appreciate the benefits. You would have to ride and compare a 9 with all the variations I have tried in place. However the more Eureka moments are not subtle.

I don’t claim to be an expert on this and welcome comments, but all other things being equal, the shorter stroke engine should be more sensitive to exhaust scavenging. Another factor up to 07 models at least may be that the 9 has only 38mm ID headers entering a 46mm collector and the internal join and weld is not well streamlined. The XB12 has 43-45mm ID (I’m guessing size there) headers entering the same 46mm collector, giving a 44% reduction in pipe as opposed to the 9’s 36% reduction. Given that the 12 also has a greater exhaust pumping action then in any given situation the 12’s exhaust gases at the collector are more likely to be fully charged and up to speed than the 9’s at low rpm. This may also explain why the factory servo exhaust valve is of little or no benefit to a 9 apart from when it’s running flat out.

If you blip the throttle on a standard 9 at tick over the gases seem to stall or they have no sense of urgency until up to speed a second or seconds later (you have to be going, before you can go, if you get my drift). This was still apparent when I fitted an open piped muffler to my 06’ 9SX. Maybe I was just dealt a duff header to start with. But inserting a bit of filler into the neck of my Stealth to form a flush joint to the header, and by accident creating a venturi shape downstream of the collector, immediately transformed my motor. Blipping the throttle from tick-over had the motor going RUMBAH, and for a few magic moments before the filler fell out, the 9SX would rip through the gears in the midrange (3500 – 5500rpm ) with an insane bark from the tailpipe, jumping passed cars during overtakes. I have not been able to recreate that effect in more permanent steel, but nor am I going to run an open pipe again in respect for my ears. A standard ’07 12R muffler serves me well enough now. This earlier experience with a ‘venturi effect’ got me thinking about the 9s pipework. :?

Of course, had I been richer I would have commissioned Albert for a complete set of pipes. One of his XB header pipe convergences I looked at recently looked remarkably like those I read about on RB Racing web pages. I can only assume that those lucky enough to have such a header on their XB9s will have noticed a more ‘crisp’ throttle response at low to mid revs. In the meantime I am continuing with my cheap experimental plug in ‘choker’ sections.

At the end of May my new ‘wider’ venturi was completed by KJE Engineering in Cheltenham. I specified a minimum of 0.5mm wall thickness at the start of the tapers because there was a risk of crushing the piece during turning. I finished the tapers down to zero with a flap wheel. This took an hour with regular dips in a tub of water to cool the piece down. The new section of 304 stainless is 62mm long and 46mm OD as before. The inside finish goes from 46mm down to 43mm ID over the first 13.5mm, then a straight 43mm ID for 21.5mm, then tapers out again to 46mm over the trailing 27mm. I also rounded off the point of changes in taper to avoid de-laminating the exhaust flow.

The new section weighs only 81 gm compared with the previous 42mm sections 127 gm. After cutting the longitudinal or vertical split to spring the tube section, the 43mm ID tube felt more flimsy than the 42, and didn’t feel so secure when it was slid into the XB header. To counter this I heated up the metal of the new section at a point where it fits just below the header with a blowtorch and peened the metal 1-2 mm outwards each side of the vertical split. This forms a stop to hold the section in place. There is just enough space in the muffler flange for this. The position is critical so that the entry taper meets the start of the curve inside the headers taper section. You can feel this curve with a finger if they are long enough (see previous thread sketch plan for fitting position).

My first ride with the 43mm choke was on Sunday 18th, which was the hottest day of the year up until then (34 C). I avoided the Gaydon Museum meeting due to the heat (mostly for the sake of my lungs) and headed west for higher and cooler ground. The new section appeared to be working well in spite of the heat, which is a good sign and just what I wanted. The choking-effect was noticeably less than the 42mm insert to start with, but there was still a beneficial ‘crispness’ in the throttle from low revs. A fast overtake on the motorway slip road showed that there was plenty of power for overtakes.

Fast forward to 1st July and a run to Bristol and back in cooler ambient conditions than before. A short stint on the M5 showed a sweet spot around 4200 – 4500, which was promising. It was not until the return trip that things sort of ‘took off’. The motor began to feel well strong in the midrange and the throttle had become like a switch to a perfectly set up potentiometer! It just felt stronger and stronger the more I wound it on and I was regularly seeing an extra 10mph than I usually reach in any situation with regular treading on the rear brake to curb my enthusiasm. I was having some serious fun and all without exceeding 5700! So a result it would seem. I can only conclude that there is a happy compromise at 43mm (I cannot be sure if it is now 43.5 as there is still a longitudinal spring gap after fitting, but the section was turned out to exactly 43mm). This may be the point at which the venturi effect ‘corrects’ or balances out the unwanted turbulence or ‘stalling’ of exhaust gases I suspect is happening in the headers confluence, while still allowing enough flow to give good power throughout the range I ride in. ;)

Author:  edd [ Fri Jul 07, 2017 9:37 am ]
Post subject:  Re: Venturi 2

I have received some negative 'waves' (ha!) about my claims here for the above device (offline), such as any gains are unlikely because a ‘venturi’ would create a loss. In my original thread I said that there would be a loss of output and I even explained how the normal torque peak had dropped away, but that I had found a ‘better’ throttle response elsewhere! It’s a question of how you prefer to ride. Wind it on and wait for the fluffy torque to waft you along until a big rush later on or get a more instant sensation on part throttle. No more having to blip the throttle and fan the clutch as you accelerate away from the traffic lights is an immediate bonus. Admittedly that may just be overcoming a weakness exposed at very low revs from using an XB12 muffler over a standard 9s, which was always good in town (easy wheelies off idle), but a bit crap during some overtakes unless you wound it up. :ill:

In theory then; the wider the ‘venturi’ choke at the collector, the closer you get to no effect, the narrower the ‘venturi’, the closer you get to a tractor with no speed (obviously). However, there is something else going on here at what may be the optimum shape by virtue of the smoothness combined with pickup through the throttle range. Whatever you believe, the funny little steel sleeve in my bike is unlikely to vanish in a puff of logic! lOl

I should correct some of my statements on the earlier Stealth pipe. That did in fact have a 'flare' not a ‘venturi’. As there was no flange fitting for the header, I created a taper inside the muffler inlet with filler so that it blended from 46mm ID out to the Stealth's wider inside dimensions and smoothed out any other steps and bumps I could reach. All I know is that this somehow created a change in flow and pressure for the better. The torque with the Stealth in this configuration was impressive in that it made 5th gear feel like 4th much of the time and a number of times I made the mistake of trying to change up when I was already in top. Only torque can have that effect!

The current set up does have less torque than a Stealth, but there is no space in my XB muffler to build in a similar ‘flare’. In my experience, a standard XB12 muffler on a 9 (with or without a ‘venturi’) requires more use of gears than the Stealth did anyway. This does not detract from the riding experience and it may save your hearing. That self-imposed rev limit of 5700 equates to an easy breeze up to 90 in forth, which is fast enough on the road for most situations. ;)

I suspect that the ECU is seeing something change at some point (at about 150 miles after fitting or removing any device) or I wouldn’t experience any change either unless I up the medication! lOl Need to log some data if I can find that lead! :?

Author:  pash [ Fri Jul 07, 2017 5:34 pm ]
Post subject:  Re: Venturi 2

People are baffled. I think you ought to organise a demo day where people can roll up on their 9, you fit a venturi and let them taste the difference.

Author:  Tadpole [ Fri Jul 07, 2017 6:01 pm ]
Post subject:  Re: Venturi 2

The veturi effect is used a lot in my business. The fact that your dealing with a compressible gas does make things different when it comes to scavenging and mixtures. I get all of our nozzles made by spark erosion in America. It is a definite science and a small change can make a huge difference. Interesting in the way that you are trying to apply it. All I can say is don't make it run to weak 😉 I use CFD to get a leg up to start with. 🖒

Author:  Adam [ Fri Jul 07, 2017 8:52 pm ]
Post subject:  Re: Venturi 2

A very good mate of mine played around with anti reversion cones on his Harley race bike, this is quite popular in the US. He also has his own Dyno. He found the cones gave the best spread of power at the top of the headers irrespective of the cam being used. I called in to see him the other day, and he was building a frame for his new project using a supercharged Ducati 1198 motor ( he blew his 750 Ducati to bits) He experiments a lot with exhaust cans made out of cardboard, which apparently last long enough on the Dyno to see if they're any good or not, before making them in aluminium.

Author:  edd [ Sat Jul 08, 2017 1:00 am ]
Post subject:  Re: Venturi 2

pash wrote:
People are baffled. I think you ought to organise a demo day where people can roll up on their 9, you fit a venturi and let them taste the difference.

That's an idea, but it would definitely be at their own risk. I have no idea what would happen if a less patient than I speed merchant wound their 9 beyond the envelope. :?

Tadpoles note about mixtures may also be valid. Venturi 1's sensitivity to the weather made me think that it was pushing the adaptive limits of my XBs systems at times. The 2nd less so (as it is less restrictive) but l am due to renew the plugs and have a look see anyway before l record any data. Who knows, maybe the plugs had just got oily, and finally had a good blow out last week? ;)

Author:  Graeme1203 [ Sat Jul 08, 2017 12:12 pm ]
Post subject:  Re: Venturi 2

Sounds like there's enough variables to keep you busy :D

Author:  edd [ Sun Jul 09, 2017 7:21 pm ]
Post subject:  Re: Venturi 2

Only jesting about oiled plugs. ;)

I now doubt that a demo day would work yet. I didn't do the 10 miles at 2500rpm reset if that works? It actually took 175 miles for the full benefits to show. Maybe a trial by post on receipt of a cheque deposit so that another owner of an identical 06/07 9SX could try it for 200 miles. :? But I would prefer to see what the fuelling implications are on my example first. ;)

Author:  edd [ Sun Jul 30, 2017 8:44 pm ]
Post subject:  Re: Venturi 2

Finally changed the plugs today. They looked normal for my motor. Maybe had been a little dark (rich) around the edges at some stage due to previous experiments, but looks like it later corrected itself. Electrode and ceramic were clean and appeared to have been running right. ;)

There can be little made by man that frustrates me more than the rear plug cap. :ill: The resistance of the electrode cap to engage the lead seems to exceed that of the rubber cap sliding along the lead. In the end l resorted to pulling the connector out of the cap (carefully as l have previously snapped a lead doing this), at which point it clicked onto the plug so easily :roll: then l slid the cap over the plug. Damned if l am going to rotate the motor to change the plugs. ;)

Now would you believe, my palm refuses to charge so l can record data! :headbang:

Author:  DaveH [ Sun Jul 30, 2017 9:52 pm ]
Post subject:  Re: Venturi 2

Having read most of this and realising that 6 of the 9 posts are from Edd I realise that's why I've given up visiting the forum and started reading war and peace. Edd just get out and thrash the fu**ing thing. For gods sake man. :roll:

Author:  edd [ Sun Jul 30, 2017 10:51 pm ]
Post subject:  Re: Venturi 2

I do.....but l thought we wrote our drivel on here to vent our frustration on days when we couldn't! lOl

Author:  edd [ Fri Aug 18, 2017 4:26 pm ]
Post subject:  Re: Venturi 2

Back again like a bad penny. :twisted:

I debated this subject over coffee and cake (or rather after cake lOl ) with pash recently, but we have yet to reach a unified theory of everything. As of yet I have still not been able to log any data as my palm showed a connection error on my last ride out. Thankfully pash came to my rescue here again! Maybe next time. :roll:

I read somewhere on here that Gunter had commented that the standard XB9 muffler gives the best power. I would agree with that at the top end assuming it's an 06, and at the bottom end, but not in the midrange even with the white wire mod (I still contend that this is due to the header :? ). Back then I was getting despondent about my 9 on overtakes with the standard muffler. The 12s item is better in this respect, but it's a bit soft to respond very low down without the venturi. Unfortunately I am currently unable to try the venturi with a standard 9 muffler as mine has previously been 'gutted'.

Theories come and go, then I found the following article:

At a guess I would say some of the above elements could be at work in my experiment and as I suspect may be countering a negative element in the XB9 header collector.
The 'loss' of flow aspect only appears to be significant at a peak flow if the restrictor is TOO narrow, but with the 'Venturi 2' at just over 43mm and a self imposed 5700 (104 in top) that does not seem to be a problem.

The trip to Devon on Sunday featured numerous motorway miles, and there was instant throttle response when rolling on from 70 in top. Leaving the M5 slip onto the A40, I countered a cheeky BMW estate and saw impressive D-R-I-V-E winding on the throttle in 4th. My immediate thought was; F*** that was pulling well! The best description is that the throttle feels more 'regulated' or 'connected' to the engine. Call it what you will I'm keeping this one! ;)

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