LaceyDucati wrote:Horsepower figures are only relevant to compatible tests, historic factory figures are crankshaft HP and invariably optimistic. 22 rear wheel horsepower although low in "race" terms for a 250, it is probably better than any original 250 model. However a 75.75mm piston will give well over 250cc.
Firstly is it a race piston and more importantly did you measure the CR, maybe a photo of the piston would help identify it. You could be losing power by either too low CR or possibly by too high CR with poor piston shape. 1.2mm squish is large and you will be losing some of the benefit of a squish type head. Mind you, you will need to ensure that the squish is consistent and angled correctly before lowering it. With everything else correct, you can run down to 0.6mm safely and I have gone tighter. With a good squish and piston shape I've not found it necessary to use more than 34 to 35 degrees BTDC using a single plug and have run around as 31 to 32 degrees using a twin plug (for a 250).
Cam timing can only be analyzed and compared if you have all the opening and closing figures. Either initial lift or better still 1mm lift, with the clearance used when taking those figures. Lobe center figures can then be calculated from a full set of figures and will indicate whether you are significantly out. Otherwise you are not comparing like with like and you can make any number of incorrect assumptions. I must say that over the years I have been surprised how many "race" cams are similar in performance and seldom will you achieve wildly significant changes by just swapping a cam. Also I have found in tests, the cam generally needs to be around 6 degrees out from "stated figures" before you start to see significant change in HP on the dyno. That doesn't mean small losses don't matter, as all those small losses eventually add up.
Poor cylinder head flow could also be to blame....There are many elements of a race engine that add up to good performance and all you can do is go through and check everything and experiment with one thing at a time. However from my experience if the head doesn't flow well you will be ultimately wasting your time with all the rest. Of course other things may improve power but flow is fundamental.
Of course poor exhaust design can kill an otherwise good engine. Testing your engine with a known open race design will help identify where the issue lies. Subsequently silencing the bike and maintaining HP is more difficult, but at least you would then be aware where the losses lie.
Your poor horsepower figure is probably as a result of many small things. Check the basic parameters and then if no luck get the head flow tested and do more dyno testing.
Building a fast reliable race engine is not easy and takes a lot of time and eventually experience. A tick list of settings and components will only get you so far and ultimately the engine build is more important than the performance....you need to finish to win, as they say.
Regards Nigel
Thank You so much for taking the time to make thorough reply.
Re power; This bench is set up w pretty strict parameteres so readings are conservative. This will also be mostly a parade machine so to get a good solid base set up is more important than the last hp. But we still felt this was a bit low.
Re piston; /¤./% is the correct dia, sorry. I have enclosed a picture. I know the squish is "generous", and I ordered some alu shims from You to rectify this. But my head has had some welding so the squish are is not that even or accurate so something to look at this winter. We did have to use some more timing the the figures You are qouting. I think we are at about 37 degrees now with dual plugs.
Re cam: This is a regrind on a cam I had from before. In hindsight it might have been better to buy a new one from You
We measured the cam at 1mm of lift and got these figures;IO 35 degrees BTDC
IC 63 degrees ABDC
EO 76 degrees BBDC (This is the one we are concerned "bleeds" of cylinder pressure esp with the big exh valve etc)
EC 34 degrees ATDC
Lift figures measured out at 9,5mm intake and 9mm exh
I am concerned that the ports are a bit too big, but would have expected to see some of the loss come back at the top end if that was the case? Mid range, where I would have expected to see the loss seems "strong" and whitout a big dip or loss of power. I have a std small port head also but even that seems to have a really big exh port and largish exh valve. Is there a reason these were designed that way from the start? Hope to get the the head flow tested this winter as I do believe that the ports are larger than what this engine needs.
Re the exhaust; This is a downpipe from Swarbrick and then just extended to where I "needed" the muffler to be. Do You have a recommendation for a exhaust design or maker? Should we have had more of a megaphone style design before the muffler itself? Most of the designs I have seen seems to use the same dia the whole downpipe. Is this ok or should I look at having different dia to promote exh speed and pressure wave timing?