1275 Sportster, Andrews cams, carburated ...

[Fast Lane]

Nice to build a carbed engine for a change! This bike lives in Jersey so the heads were set up with a bias on torque rather than horsepower. The final numbers were 90.41 ft/lbs of torque and 84.38 HP - perfect for thrashing around little country roads ...

 

[howo]

Re: 1275 Sportster, Andrews cams, carburated ...

Are the figures at the top left before and after the work or before and after the tune when the works been completed ? I?m guessing the second. Nice flat tq line btw.
while we at it Alex do the 883 heads get bigger valves with the hammer kit while the 1200 heads just need the standard ports tided up a bit?
I have a 1200 which I think puts out around 71 hp standard engine, carbed 2005 bike. If I went Hammer kit plus cam and head work I?m guessing new clutch too. Would I need a spark timing electric thing too? Or dos that depend on the build/cam choice?

 

[Fast Lane]

Re: 1275 Sportster, Andrews cams, carburated ...

Are the figures at the top left before and after the work or before and after the tune when the works been completed?

Yeah they're upon arrival at the dyno and then after being tuned. With it being carbed I changed the main jet at home then tweaked the idle screw to get it somewhere near. Most of the dyno work was on the timing as it has an adjustable ignition unit.

The 883 heads have tiny valves and ports so they both need to be enlarged whereas the 2004 onwards 1200 heads already have perfect valve sizes for the 1275/3.6" bore. The stock valve seat profile needs improvement though.

I have a 1200 which I think puts out around 71 hp standard engine, carbed 2005 bike. If I went Hammer kit plus cam and head work I?m guessing new clutch too. Would I need a spark timing electric thing too? Or dos that depend on the build/cam choice?

As for HP - I never think about it as it's an imaginary figure and means nothing. Torque is what's measured on the dyno and HP = torque x RPM/5252 which is why HP has to be higher than torque above 5252 RPM and torque has to be higher than HP below 5252 RPM. If you look on the chart above you'll see that peak torque (acceleration) is reached at 4270 RPM. At this point there is about 72 HP. Peak HP is calculated to occur at 5530 RPM at which point acceleration is zero because it peaked over a thousand RPM earlier. I was at the dyno recently and they had just tuned a Honda 600 track bike - it put out 138HP and 50 ft/lbs of torque. That's why they have to be ridden at high RPM to maintain speed. Those type of engines are probably above 5252 RPM a second after letting the clutch out or else they don't go anywhere. Harleys have a vastly different bore/stroke ratio which creates piston speed limitations as the RPM increases so everything I build is designed around high torque production in the low to mid RPM range.

Most of the 1275 builds I do get the Andrews N4 cams as they are a good all rounder. They like about 10.5:1 static compression ratio and about 195-200 psi cranking pressure which does demand timing changes. I usually use the Daytona Twin Tec adjustable ignitions but if a later closing cam or lower compression build was chosen then the stock ignition could be kept. A new clutch is a good idea as it will only be a matter of time before the stock rivet plate collapses ...

 

[howo]

Re: 1275 Sportster, Andrews cams, carburated ...

Yeah they're upon arrival at the dyno and then after being tuned. With it being carbed I changed the main jet at home then tweaked the idle screw to get it somewhere near. Most of the dyno work was on the timing as it has an adjustable ignition unit.

The 883 heads have tiny valves and ports so they both need to be enlarged whereas the 2004 onwards 1200 heads already have perfect valve sizes for the 1275/3.6" bore. The stock valve seat profile needs improvement though.



As for HP - I never think about it as it's an imaginary figure and means nothing. Torque is what's measured on the dyno and HP = torque x RPM/5252 which is why HP has to be higher than torque above 5252 RPM and torque has to be higher than HP below 5252 RPM. If you look on the chart above you'll see that peak torque (acceleration) is reached at 4270 RPM. At this point there is about 72 HP. Peak HP is calculated to occur at 5530 RPM at which point acceleration is zero because it peaked over a thousand RPM earlier. I was at the dyno recently and they had just tuned a Honda 600 track bike - it put out 138HP and 50 ft/lbs of torque. That's why they have to be ridden at high RPM to maintain speed. Those type of engines are probably above 5252 RPM a second after letting the clutch out or else they don't go anywhere. Harleys have a vastly different bore/stroke ratio which creates piston speed limitations as the RPM increases so everything I build is designed around high torque production in the low to mid RPM range.

Most of the 1275 builds I do get the Andrews N4 cams as they are a good all rounder. They like about 10.5:1 static compression ratio and about 195-200 psi cranking pressure which does demand timing changes. I usually use the Daytona Twin Tec adjustable ignitions but if a later closing cam or lower compression build was chosen then the stock ignition could be kept. A new clutch is a good idea as it will only be a matter of time before the stock rivet plate collapses ...

Many thanks, good info. Busy man, appreciate your time..