1275cc Sportster conversion, cams, head work, Power Vision dyno tune

Fast Lane

Fast Lane

Club Member
I've been busy with 1275cc Sportster conversions for a few weeks now, this is the first one on the dyno. I'm very pleased with the results as it has the original 883 heads which in stock form are very poor in terms of air flow. This will be a real handful in a lightweight Sportster - 87 ft/lbs of torque and 100 HP!
Brett - Copy - Copy.jpg
 
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Re: 1275cc Sportster conversion, cams, head work, Power Vision dyno tune

Nice work Alex. I think there will be a lot of interest in the new 1275 kit Hammer are using as their de facto standard now.

Interesting choice of cams. What swung the decision for the SE?s? What timing tables were used?

Not sure what exhaust that is. Please can you clarify.

Really must get mine on a dyno someday. Need to get a lot better first so probably a few months away yet
 
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Re: 1275cc Sportster conversion, cams, head work, Power Vision dyno tune

For "bit of a handful" read fun Alex ;-)
 
Re: 1275cc Sportster conversion, cams, head work, Power Vision dyno tune

Very impressive, should keep a few people happy!

Rod
 
Re: 1275cc Sportster conversion, cams, head work, Power Vision dyno tune

Simon said:
Interesting choice of cams. What swung the decision for the SE?s? What timing tables were used?
Click to expand...

This build was an 883 in an earlier life so the cam choice came about by experimenting with the available 1275 pistons that would work with the stock 883 heads. I use the Andrews N4 more than anything else in Sportster builds and they always work well at around 10.5:1 which is about 195 CCP. Those CP pistons in the 1275 kits come in either a flat top version which is -1.83cc dome volume or the reverse dome which is -13.47cc. The stock 883 head chamber is only about 49cc so in stock form the reverse dome pistons and a 0.030" gasket will give 10.37:1 which is not too bad for the N4's. However the conversion headwork uses bigger valves so once they're in place and the chamber unshrouded it ends up at around 55cc which is only about 9.6:1/175 CCP and is leaving some power behind. I went for the flat tops which with some extra chamber work to get them to 57.5cc and a 0.030" gasket comes out to 10.85:1 / 195 CCP with the 52 degree intake close of the SE 575's. The N4's would have been a tad high at 204 CCP. I thought about the Crush 600 cams but they would be too pricey by the time they got here and the TDC lift is pretty high for the limited distance the valves can be sunk in those 883 chambers. I'm pleased with the results, hopefully the customer will be too. It took nearly three hours to tune this bike on the Dyno. The operator said it was a slow painful process probably due to the exhaust being difficult at lower RPM. I'll copy the timing table from the PV and post it so you can have a look ...
 
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