comp ratio help

Reply Subscribe

Thread Tools

Search this Thread

Apr 24, 2013 | 04:05 PM

williamf57

Thread Starter

Cruiser

Joined: May 2009

Posts: 111

Likes: 0

From: kent, OH

comp ratio help

Hey all, I am a little challenged on the tech side of things so I have a question. If my jugs on a 1340 evo are bored .020" and I leave the heads stock and add 9.5:1 domed pistons will the ratio be higher then the 9.5:1 or below it. The reason I ask is Crane says the 9.5:1 ratio is the limit at which the 310-2 fireball should be. Thanks for your time, Bill.

Apr 24, 2013 | 04:11 PM

1984_FXRT

Road Captain

Joined: Jun 2012

Posts: 533

Likes: 2

From: Virginia

a .020" overbore isn't going to put you over the edge. Here's a set of formulae copied from RBRacing's website that'll help:

Displacement = (Bore ÷ 2)2 x 3.14 x Stroke x 16.387
Head Gasket space = (Bore ÷ 2)2 x 3.14 x Gasket thickness x 16.387064
Deck Height space = (Bore ÷ 2)2 x 3.14 x Deck Height x 16.387064
Compressed Volume = Head Gasket space + Deck Height space +
Piston Top volume + Combustion chamber volume
Uncompressed Volume = Compressed Volume + Displacement
Compression Ratio = Uncompressed volume ÷ Compressed volume

Apr 24, 2013 | 04:26 PM

ReapersWG

Cruiser

Joined: Jun 2012

Posts: 145

Likes: 0

From: Maine

Checkout the EVO calculator on the Big Boyz site. If you know most of the info it'll put you in the ballpark. You could also check your CCP and do some reverse math on that same calculator to get your CR. Hope this helps.

Apr 24, 2013 | 05:08 PM

williamf57

Thread Starter

Cruiser

Joined: May 2009

Posts: 111

Likes: 0

From: kent, OH

Thanks guys, got what I needed, piston set on it's way.

Apr 24, 2013 | 05:14 PM

aces&8s

Seasoned HDF Member

Joined: Sep 2012

Posts: 6,570

Likes: 590

From: almost there

310 is a pretty good cam , what about a .550 lift, how did it figure out on compression 9.5 or 10-1 and why?

Apr 24, 2013 | 07:10 PM

crazybob

Stage IV

Joined: Apr 2007

Posts: 4

Likes: 0

From: St. Louis, Mo

Aces&8's,
I beleive he is referring to the Crane Fireball 310-2. It is only 236/242 duration and .490 lift. The H310 is a completely different animal, 266/276 duration and .550 lift. A Strip/Street cam works best with Big Bore and 10.5 + compression. Must have hi-rev springs and have the head-valve clearance confirmed.

Apr 24, 2013 | 07:57 PM

aces&8s

Seasoned HDF Member

Joined: Sep 2012

Posts: 6,570

Likes: 590

From: almost there

Thanks Bob, Just looking for a little clarity. That helps. Wish companys would just list their cams by lift- duration would make it simpler.

7 Times Harley-Davidson Chucked Tradition Out the Window

Verdad Gallardo

7 Surprising Harley-Davidson Products that Are Not Motorcycles

Verdad Gallardo

8 Best Harley-Davidson Motorcycles Ever

Pouria Savadkouei

10 Worst Harley-Davidson Motorcycles Ever

Pouria Savadkouei

Killer Custom's Jail Break Is The Breakout That Refused to Blend In

Verdad Gallardo

Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?

Verdad Gallardo

Harley-Davidson Reveals Super Cool Cafe Racer Concept

Verdad Gallardo

Engraved Rebellion: Inside Bundnerbike's Glam Rock II

Verdad Gallardo

10 Motorcycles You Should Never Buy

Joe Kucinski

10 Things Harley-Davidson Needs to Fix in 2026

Verdad Gallardo

Apr 25, 2013 | 09:15 AM

aces&8s

Seasoned HDF Member

Joined: Sep 2012

Posts: 6,570

Likes: 590

From: almost there

Im far from an expert but I would think .02 over bore wouldn't change much of anything being so small a demention. Even cid would be about nothing and we are splittin hairs here.?