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Hi all. I am currently doing a top end rebuild on my 91 FXR with some performance upgrades . Here is a sort list of components That I have in place: Mikuni 42mm, Thunderheader and Dyna ignition. What I am adding to this happy horsepower compilation is: Woods w-6 cam, Tappets (forgot name, on order), ported/polished heads. Now the question is, with stock heads shaved .050" and Cometic .030 head gasket kit, how do I get to 10:1 compression from a stock 8.5:1? What size piston do I need and have any of you ran this type of setup (what to expect?)? Thank you in advance for any info you can give.
first you need the pistons I use wiseco 10:1 they have a 12.9 cc dome. after you have the cylinders bored you have to set the deck height by either turning the cylinders or using proper base gaskets you are shooting for .003 in the hole with a .030 head gasket it will give you .033 of quench that is the the minimum. after all that is done then you can determine how much you need to take off the heads.
by cc's not thousandths optimum compression on pump gas is 9.6:1 corrected.
so your build would be something like this.
wiseco 10:1 bored .005
pistons .003 in the hole
.030 head gaskets
heads milled to 79 cc's stock is between 82-85
woods W6 cam 40o ABCD
will give you 10.38:1 static compression
and a corrected compression ratio of 9.56:1
depending on your port job with that cam and getting the compression up to where it needs to be ive seen 90hp and 101tq
here is a dyno sheet from woods website
I had that exact setup in my 98. A real good combination. no stock harley could touch it. Not even a 110. I sent my heads to Hillside and had Scott match the pistons to the heads and bang 94 torque and 85 hp. and still maintained 42 mpg a real good all around setup for low mid and top ends.. and still streetable and very dependable..and my Dyno run was in 5th not 4th. I have seen (depending what gear its in) were that makes a difference in the numbers. You'll really like that setup, especially in a lighter FXR
Last edited by 98hotrodfatboy; Dec 5, 2014 at 06:14 AM.
Thanks for the replies 98hotrodfatboy and D_gyver. I already have the .050 shaved heads or I would go with the 10:1 pistons but I picked these up cheap some years ago and what to put them to use. Is there a piston combo that will work with my setup that will help me get to 10:1 or are these head going to need to go back on the shelf? Thanks again
Thanks for the replies 98hotrodfatboy and D_gyver. I already have the .050 shaved heads or I would go with the 10:1 pistons but I picked these up cheap some years ago and what to put them to use. Is there a piston combo that will work with my setup that will help me get to 10:1 or are these head going to need to go back on the shelf? Thanks again
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Getting to 10:1
