900 MONSTER CARB : BRAD’S EXHAUST RAVE

The second part of my 900 monster series, I was going to have a bit of a rave about exhausts. I have lost a little motivation to act on this I think, however. This has been due to trying some other mufflers and the result they gave.

First though, I shall head off on the tangent I had originally planned. This is mainly because I spent some time making a wacky graph and as such just have to show it to someone.

The graph below is a very simple representation of the exhaust requirements of various engines. In this case, a 650 single ( blue ), a 996 twin (red ), a 950 triple (green ) and a 998 four ( black ). It assumes so many things that attaching any real significance to it is unrealistic, but as a picture it can tell a useful story.

It represents the flow through the exhaust outlet over the 720 degrees of crank rotation that make up a full cycle of a four stroke engine. Given the way this graph has been created, the volume of exhaust gas is based on the capacity of the engine. From it we can see a reason why my ARROW TWIN TUBE may not work at the top end. The average flow through an exhaust is not overly relevant to the required size of the exhaust, especially the collector and muffler. If the average flow for each engine was calculated using the graph, it would be the capacity of the engine over a certain time scale. The fact is that the less the cylinders, the less average the flow becomes. The single will have lots of flow for maybe half of its 720 degree cycle, then very little ( or even backflow in reality ). It will need a large system to cope with this short term, high flow condition, much larger than the four cylinder bike would need at the same engine speed. Due to the even spacing of the four cylinder engine’s firing over the 720 degrees, the exhaust flow is much closer to constant.

Of course, the fact that the four cylinder bike will go on to make maybe three times as much power as the single will mean that in reality, its collector and muffler size will probably be close to that of the single. The four cylinder would also probably make do with an inappropriate system better than the single.

Anyway, all this is supposed to show that a large twin needs lots of muffler volume to make power. Although I have seen lots of 2 into 1 systems over the years for twins, most serious race efforts for larger engines have used two mufflers. The TT2 Ducati engines of the early 80’s are an exception here, but they were all around 600 cc, so may have been able to get away with it. Even going back to the Ducati NCR bikes used in the 70’s ( of which Mike Hailwoods TT bike was one ), all these bikes used twin pipes. It may be the uneven firing interval, as the BMW’s seem to be happy with a 2 into 1 system. I would one day like to build a system for a Ducati with unequal length headers based on this difference in firing angle spacing. There is every chance that this could be a backward step, and that the current, even length header pipes are a far better compromise for the real world we ride in. If I spent the time and money to build a system I’m sure there would be a reason why it wouldn’t work, and it would add its mass to the ever expanding scrap box of ideas that didn’t work. Not that there’s anything wrong with that, but the time and money are in too short a supply already.

The two following photos are to show what I am talking about in terms of exhaust. The first is the standard 900 Monster exhaust assembled on a workbench. The x-over looks like it is – a stamped and welded piece with the in pipes and out pipes at 90 degree spacings. This is the same style that Ducati use on every current model except the 748/916/996, and on past models like the 851/888 SP series. Given they are claiming 117 hp for the ST4S ( 996 engined ST4 ) it obviously does flow some serious power.

The second photo is the ARROW TWIN TUBE, fitted to a different bike ( a 600 Monster ).

Getting back to the bike in question, I feel that if I could use both tubes in the ARROW TWIN TUBE muffler for each cylinder, my problem would be solved. I sent Doug Lofgren an email on this subject and his reply said all the things I should have expected him to say, mainly "why bother". Doug has had plenty of 2 valve 900/944/984 engines make 90 or more rear wheel hp through standard headers. He has also seen quite a few big dollar, after market "spaghetti" style systems on his dyno that make not a bit of difference. He usually warns people of this before they hand over the readies, but some people just need to spend, often so others can see it. If power is not the aim, then it’s not always that important. Still don’t like being the one doing the selling though.

This is not meant to be offensive to anyone, as the reason I am keeping the ARROW TWIN TUBE muffler is simply because I like the way it looks. The performance I am giving up is shown below. I sold the incredibly loud STAINTUNEs that came on my bike to a man who came into work. His recently purchased Monster 900 Special had come with a set of gutted std mufflers. Nothing unusual about this, but something about them told me that they had been done properly. This was due to the fact that they weren’t painfully loud or barky on the overrun. I mentioned to this man ( John ) that I wouldn’t mind giving his original mufflers a run on the dyno. When he had fitted my ex STAINTUNEs, he bought them in and I refitted the std system. At this point I asked him what he thought of the STAINTUNE. He said the noise was great, but he couldn’t pick any performance difference. Pretty much as I had expected.

John’s original mufflers turned out to be pretty good. They had a very small hole ( good ) perforated core through them and hadn’t been damaged externally in the mod. One of them had been on the ground though, and the small marks painted over and then cleared. Not very obvious. The dyno graph is shown below. The ARROW TWIN TUBE is shown in green, John’s mufflers in red. These mufflers are probably for sale too, so if anyone is interested contact me and I can put you onto John. Location is Melbourne, Australia.


Dynograph courtesy of DYNOBIKE (03) 9553 0018

As you can see, I’m giving up around 3 – 4 hp at and after the power peak. I rode around with these mufflers for a couple of days before going back to the ARROW TWIN TUBE system. The difference, although small, is noticeable, but this bike is so responsive and goes hard enough that I didn’t really feel I needed it. I realise this sounds like I’m going soft, but, as Trev says every time I comment on one of his custom ( wacky ) paint schemes, it’s all personal. Correct. My next planned mod is high comp pistons anyway, which should put a big lift under the power curve. My plan to put a x-over in the ARROW TWIN TUBE headers may eventuate, as I am still quite a bit ahead money wise after the sale of the STAINTUNEs. It just depends on the cost, as stainless is not cheap stuff to get worked on.

Moving on, the next graph is a comparison between John’s mufflers and the STAINTUNE mufflers ( and tuning ) that were on this bike when I bought it. More than anything, this graph will show the improvement gained through the jetting changes and the sharper drop off of the power curve after the max power point due to the cam settings. The drop off is not as bad as I had expected it to be. John’s mufflers are red, STAINTUNE green.


Dynograph courtesy of DYNOBIKE (03) 9553 0018

The final curve is of the bike fitted with John’s mufflers, as shown above, in red against the totally standard bike in green. A pretty impressive improvement, and more in line with that of the other 900 Monster page that I posted originally.


Dynograph courtesy of DYNOBIKE (03) 9553 0018

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