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