250 racer engine set up advice - missing power?

[bendik]

Need some input from people w more expierence on these engines. Sorry for the slightly long post..
This is a racebike that I bought last year and that turned out to need a full refurbish on the engine. We have now done a dyno session and a track session on the engine. It runs good and revs freely w a really flat powerband but seems to be lacking some HP.
The result was approx 22hp at 8700rpm, but the curve is really flat from 6 to past 10k
When I bought the bike I thought that the exh valve and port was pretty big compared to the inlet. And I also had a cam reground by a specialist and it should have been like the Lacey 173 cam. We have all the other valve timing set up like it should be, but the exhaust opens almost 10 degrees early at 76 degrees BBDC instead of 67 like it should have been.
Valve size is 36mm exh and 38mm inlet
AFR is good and the timing is set at 37' now. We have also tried more advance without gaining anything.
Carb is a Mikuni 34mm but a switch to Amal MK2 32mm didnt make a difference so far. The carb responded well to a medium length stack which cleaned up the AFR and a dip in the powercurve.
Exhaust is a "traditional" 35mm pipe with a modern style end can at the end due to noise restrictions, but this is fairly straight thru
Piston is a 75.75 high dome piston. Squish is set at 1.2mm ish
So; is the exh timing, the big valve/port etc working against me here or should I look elsewhere? Would be happy to post pictures of the chamber/ports/piston if that helps?
I also have a "green/white" cam that I would like to check. Would that normally lose me much horsepower compared to the "173" grind?

[Andy Higham]

Advanced cam timing will hurt power.
You need to measure your opening and closing points accurately, from that you can work out your lobe center angles, a good starting point for any engine is symmetrical around TDC

[Duccout]

Cycle World obtained 22 bhp from a standard 250 Mach 1 at the rear wheel, so your engine is below par, although it seems to be producing the power at decently high revs. Nigel or Eldert are the experts, so hopefully they will see your post and offer advice.

Cheers,

Colin

[Andy Higham]

If your cam has been ground exactly as a 173 cam then your lobe centres will be exhaust 113.5 inlet 93.5
the correct figures should be exhaust 104.5 inlet 102.5

According to lacey Ducati site, moving the crank bevel 1 tooth one way and the cam bevel 1 tooth the opposite direction will give + or - 8.5 degrees which would bring your timing within 0.5 degree

[LaceyDucati]

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

[LaceyDucati]

Regarding the 173 cam, the cam profile is one of Phil Joy's Cam profiles the 173a, based originally on a green/white, mainly the changes are to the exhaust profile). The figures stated are those that have generally given the best "race" performance (figures originally stated by Phil). I have found this cam to be better in all ways to the Green/white which I never particularly liked in a 250 for race use. I have seen 31.5 BHP at the rear wheel from a 250 race bike using this cam using these figures. In a 250 from my experience you seldom get much better performance using other higher lift cams. A lot will of course depend on the head flow.

Regards Nigel

[Bevel bob]

Hi Nigel, Heading in the opposite direction !, what can i do to my ported Mach 1 motor ( 40/36 valves ) and 29 SSI to improve its mid range and make it better for pootling around? , Lower gearing also on the way. Thinking of doing some epoxy remodeling of the inlet and maybe a sleeve in the manifold and red cam.

[LaceyDucati]

Bob

It would seem the easiest/simplest answer to your requirements would be to fit a nice standard Monza head and carb. Alternatively if you wanted a little more, a sensibly ported Monza or SCR head and sensible size carb.. You could keep the Mach 1 piston if you are not experiencing any issues starting. As for Cams, the Monza cam would be ideal for a nice country lane pootle. No experience of the Red cam, but I would suspect that it is also "sports" orientated.

Put the Mach 1 head in a box with the SS1 carb to go with the bike when you are finished with it, then enjoy riding it for as long as you can!

Regards Nigel

[Bevel bob]

Hi Nigel, the red cam i have looks very similar to the monza cams i have and i think was fitted to the USA Dianna motors ,although the Mach 1 piston (Borgo ) only really achieves about 9 to 1 it may prove harder to kick start with the milder cam. I think the 29 mm SSi uses the same body as the 27 so it could be possible to choke it down. The big valves are an issue ,what were they thinking!.

[bendik]

If your cam has been ground exactly as a 173 cam then your lobe centres will be exhaust 113.5 inlet 93.5
the correct figures should be exhaust 104.5 inlet 102.5

According to lacey Ducati site, moving the crank bevel 1 tooth one way and the cam bevel 1 tooth the opposite direction will give + or - 8.5 degrees which would bring your timing within 0.5 degree

Hi Andy
I cannot really do this as all the other measurements are where I want them so that would bring those out by the same 8.5 degrees