how to get 10:7:1

[rash103]

mike i have a 2007 fxst with a 103 build cast flat top pistons with stock heads that have only been port and polished. not decks or larger valves added. also a T-man 625 cam. i know that this cam will do well with more compression. HD says that my compression is 10:1 but i think that is more like 9:5:1. i would like your thougths on what combo i need to get 10:5:1 to 10:7:1 compression. thank you for any help you can give me.

[Mike]

Typically less than advertised due to casting variance and addition of area above the top ring and larger diameter of the head gasket. The 625's like close to 11:1 and you'll probably need the HC pistons to achieve this, along with decking. Only way to know for sure is to cc the complete combo.

[rash103]

what brand of HC pistons do you recomend? im looking at the CP 10:5:1 or the HD 10:5:1 for the stock heads.
and how much should i have shaved off the heads to hit 11:1 with the pistons?
also can i still use pump gas with no issues?

[Mike]

I re-ran the numbers for the 103/625 combo and 10.5 may be the limit for pump gas. You can run higher but tuning will be critical and would not allow lower than 91 oct fuel. The exhaust and porting will have an affect on how far you can push this (heat retention). The H-D SE pistons (22144-08) would be my choice, no decking required.

[rash103]

as it is i never run lower than 93 oct and have played around with some 110 oct (good stuff). so maybe i chould go alittle higher. do you think that if i use the 10:5:1 SE pistons and a .030 head gasket ill been around that 10:7:1 i was shooting for?

[Mike]

Yes, that should get you there, but you'll want to cc the combo to be sure. With that combo, you may have to carry octane boost when on the road. Our defenition of a "street" motor is one that can be riden across the country on the pump gas available. With 10.7/625 cams, this line may be crossed.

[rash103]

thanks mike for your time and help. i think im going to give it a try with the HC pistons and the .030 head gasket and see what i got. hey if i dont like it its only another teardown. thanks again for the help.