900SS CARB WITH 944 KIT

This report is based on the same bike as the "900SS CARB WITH SOME VEE TWO CAMS" report, and, as with that job, was done back before I knew a whole lot about these things.

The 900SS we had previously fitted the Vee Two cams to was traded on a 916, and put on the showroom floor for sale. When sold, the deal was done to include fitting a 944 kit. This was the first I had had anything to do with big bore kits, and one we ( myself and Rob, the other mechanic of the time ) weren’t too impressed with.

The reason for this was the kit itself. I have no idea whose kit it was, but I’m guessing now it was a DP. This is due to the fact that the pistons had a very high dome on them, which I now know the DP pistons have. It was rather obvious to Rob and myself that this was cause for concern, given we had very recently been through a too-high compression drama with a Darmah ( amazing what you remember when you start thinking back about things ). However, the kit was part of a done bike sales deal and we got the "just fit it" direction that I would never accept ( and they know better than to give ) now.

I should also mention a little about the 944 kits in general at this point. 944 kits have a reputation for really picking these bikes up, but the reasons why are not understood by many. Increasing the capacity to 944, achieved by increasing the bore from 92mm to 94mm, represents about 5%. Given a good std model puts out about 60 ft.lb of torque, this would translate into about 63 ft.lb peak after. In this case, however, we ended up with just under 70 ft.lb. The reason for this is that the 944 kits are high compression kits as well. The increase in compression accounts for much of the performance increase a 944 kit gives. One of the graphs below includes the high comp 900ss from the "900 CARB : HI COMP PISTONS" report for comparison, and you can see from that just how much difference the higher compression makes. That bike pretty much equaled this bike through the midrange, although the detonation problems this bike was experiencing would not have helped its power output. If you fitted a 944 kit with the std 9.5ish compression ratio, the owner would probably be looking for a refund.

So, back to this report. The 944 kit went on, and off we went. The bike itself was a rocket. Want to fill a gap in the traffic? Bam, you were there. Although it pinged. Like nothing I’d heard before. We ended up using premium unleaded fuel and octane booster to tame it, although even this wasn’t totally satisfactory. Rob and I weren’t at all happy with this job, but we couldn’t get any advice on how to deal with it acceptably from the people we then considered to know more than us. The owner wasn’t too concerned, but I don’t remember seeing it again until spotting it for sale many years later, with minimal km added to the speedo. We did get to work on it later for a new owner, at which point it was obvious someone else had been inside, as it no longer pinged and had a Dynojet kit fitted. Stopping it pinging wouldn’t be all that hard these days, using a combination of retarded timing and some appropriate machining of the piston domes as required before it all went together. It was a real pity it left our workshop the way it did initially, as a badly done job really pisses me off.

Anyway, we did get some dyno runs. The chart below shows before in green, after in red. The before is the final, "after" configuration from the "900SS CARB WITH SOME VEE TWO CAMS" report. The after is with the 944 kit, without any other mods. The wavy nature of the curve through the 4,000 to 5,500 RPM range is probably due to the too much compression/ignition advance issue. The blue line is std with slip on mufflers, just to show how much more it had.

Although it was a pretty good result on the dyno, I certainly wouldn’t let one out like this again. High comp jobs we have done since have all gone out with retarded timing and run well on the premium unleaded fuels we have. I have also machined down piston domes in some cases to get the comp ratio we want, just to be sure. At around the time this job was done, Dynobike had tuned a high comp ( maybe 944 ) bike someone else had assembled. They gave it back to the owner with a clear warning that it had too much compression and the subsequent detonation was going to cause problems. Well it did, melting a hole in a piston. The problem was, they ended up wearing it, as the original assembler ( a local "expert" ) managed to bullshit his way out of the blame. Consequently, I have been pretty wary of high comp mods and the issues with them since.

The next graph compares this bike in red, to the "900 CARB : HI COMP PISTONS" bike in blue, this bike before the 944 kit from the "900SS CARB WITH SOME VEE TWO CAMS" report in green and a std 900SS with Termis and dialed cams in purple. The runs are not RPM based, so small variations in tyre size, etc make it not a totally accurate comparison. It will give you a good idea though. The extra compression and/or capacity add 8 to 10 Hp across much of the range. It just annoys me a little that none of these bikes ran the airbox/jetting mods I like to do.

Hopefully this will give you an idea of what the 944 kits offer, and some of the problems that can be encountered along the way. As I have said previously, I’m no expert. This report represents some of the learning ( and frustration ) that has given me the experience I have now. Of course, there’s lots more experience yet to be gotten. At the start of his old Motorcycle Performance Services web site, Doug Lofgren says

"My name is Doug Lofgren. I'm an idiot! Well, I used to think that, but it's clear that 'idiot' doesn't begin to describe the depths of my ignorance."

That’s pretty much how I feel too, but each day brings with it new knowledge, and frankly, that’s why I get out of bed.