It’s taken me a while to get around to this, given all the other stuff ( leg repairing, R1100S, 900M ) that’s been distracting me and the fact that the 851 spent quite a few months in bits as the frame was painted. I then decided I’d rather have white wheels than black, so I pulled the wheels out and got them dipped and sprayed.

I then finally got to ride it. Hmmmmm. Just like an 851. Guess I should have expected that. What did surprise me was the way it accelerated. Just like an S4 Monster, the speed it builds is quite out of character with the way it feels. Compared to everything else I regularly ride ( R1100S, 900M ), it feels pretty gutless. Certainly won’t lift the front wheel on the throttle alone. Damn noisy too. Gutted standard mufflers are always particularly offensive in the noise stakes, and these were no exception. I found some standard mufflers for both 851 and 888 ( longer and fatter ) at work, so I thought I’d run a few tests to see how they went. Then I found the lack of an airbox lid was responsible for much of the noise. Neil Spalding’s ST4 article on the Sigma Performance site says pretty much the same thing. The injected bikes without a lid get a real wacky gurgling noise happening, unlike the carb bikes.

The only part of the std bike I couldn’t find was an airbox lid. Given my reluctance to pay for anything I don’t expressly have too, I settled instead for a genuine 851 SP3 air filter frame we have had in stock for years. Cheap, naturally. So the base tests are run with an open airbox. This adds up to around 5 Hp at the top end, as a std 851 usually makes 85 or so Hp on a dynojet. The first dyno chart shows the bike after its big service and cam timing setting. I thought I’d start with std cam timing, just to see where I might end up. As delivered, way back in 1989, my baby’s cam timing varied between 4 degrees retarded on both horizontal cams to 8 and 10 degrees retarded on the vertical. Which means they haven’t got any worse in the last 11 years, nor any better it would seem.

Graph one shows three different combinations. Red is the post service with the modified standard 851 mufflers. Blue is a set of carbon Termis I found laying around, left overs from crash jobs. Green is std 851 mufflers. The dip in the midrange is common to these mufflers, which where also used on all the carb 900SS from 89 to 97 (98?).

Dynograph courtesy of DYNOBIKE (03) 9553 0018

We had some std 888 mufflers also, which are longer and fatter than the 851/SS ones. I figured maybe these were a better design than the earlier pieces. The std 900M mufflers always seemed to work better than the SS ones, so I was a little intrigued. The next graph shows the std 851 mufflers in green, the 888 mufflers in blue. The red line is from my next step, being ST4 header pipes and mufflers. I took this route because the first three or so inches of the ST4 headers are far superior to the 851 headers. The system is also one size larger at 45mm nominal, up from 42mm. Just simply better all round for what I have in mind. And, as you no doubt have guessed, they were very cheap.

Dynograph courtesy of DYNOBIKE (03) 9553 0018

Although it took a little bashing to get the ST4 rear header pipe in, as it tries to occupy the same space as the swingarm cross piece. I may get Mark to modify it to fit properly at some stage, and maybe open up the stamped x-over. Anyway, the result for std mufflers is very good. I’m not sure how much some open mufflers will help, but as we don’t have any aftermarket ST4 mufflers laying around, I’ll have to wait until I get around to paying for some ( or I can con someone into a loan ). Once again, Neil’s recently added ST4 article on the Sigma Performance site has provided me with much info. The mufflers he used are very nice looking Leo Vinci. The importers didn’t have any in stock when I tried to scam a set for a dyno test, so back to plan A, which involves Mark Harris at MADAZ making some for me that will be part custom to get the look I want. Just waiting for some free time. Think I might have to loan the R1100S to Duane for his touring holiday just to get back to the 851. Too many bikes.

Another problem with the ST4 header pipes is the ’89 851 fairing, which has little extensions that go under and behind the bottom of the engine casings. Unfortunately, they occupy the same space as the muffler pipes coming out of the ST4 x-over. Modifying the panels as per the 851 SP series will see enough clearance. Just another hassle. Although running it without sides means I don’t have to remove them before working on it. The other option is to have the first few inches of the ST4 headers cut off and welded to the 851 headers. Simple huh?

The final graph shows what I am using as a guide for all this. It shows my 851w/ST4 exhaust in red, 851 with std headers and Termi slip ons in blue and standard ST4 in green. The gearing is not equivalent between the 851 and ST4. The real differences are fewer than the similarities, being capacity and valve size. I have some 748 heads that are intended for my yet to be financed 984, but may fit them in the short term to the 851 to get the 1mm bigger inlet and exhaust valves all the 748/888/916 engines have. The smaller chamber will also bump up the comp a fair bit. Not sure how much I will get, but over 100 Hp should be achievable. Anyway, the way the ST4 makes and holds its power at the top end is what is encouraging me at the moment. Although, the amount of time I will spend using the top end is something I am already viewing as irrelevant. Maybe this pursuit of top end is something of a waste of time. We’ll see.

Dynograph courtesy of DYNOBIKE (03) 9553 0018

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