compression ratio for ultra bagger

Reply Subscribe

Thread Tools

Search this Thread

Mar 13, 2016 | 07:49 PM

Lsc8

Thread Starter

Cruiser

Joined: Aug 2015

Posts: 117

Likes: 0

From: Il

compression ratio for ultra bagger

Doing alot of reading on big bore kits. Thinking about it as a winter project..

Curious as to what compression ratio is ideal for a daily ridden 09 ultra? I know that compression should be set for the cam selection.(not sure if im going at this backwards)

I ride 2 up 50% and take a lot of long trips, as well as ride to work almost daily weather permitting.

I ride in the 1800-4000 rpm range.

I don't want anything crazy with some reliability in mind. I'd be happy getting back the lost tq from my cfr mufflers.

Seems reading alot of guys have 10-10.75 cr on the 106/107 kits...which seems high to me on a street bike?

I was looking at something more along the lines of a s&S 106 kit, 89cc heads 9.7-1 cr, and a 570 cam(possibly cyclerama 575?)

Am i way off base?

Mar 13, 2016 | 09:35 PM

djl

HDF Community Team

Joined: Jun 2006

Posts: 12,776

Likes: 2,607

From: san antonio

Quote:

Originally Posted by Lsc8

Doing alot of reading on big bore kits. Thinking about it as a winter project..Curious as to what compression ratio is ideal for a daily ridden 09 ultra? I know that compression should be set for the cam selection.(not sure if im going at this backwards) I ride 2 up 50% and take a lot of long trips, as well as ride to work almost daily weather permitting. I ride in the 1800-4000 rpm range. I don't want anything crazy with some reliability in mind. I'd be happy getting back the lost tq from my cfr mufflers. Seems reading alot of guys have 10-10.75 cr on the 106/107 kits...which seems high to me on a street bike?

I was looking at something more along the lines of a s&S 106 kit, 89cc heads 9.7-1 cr, and a 570 cam(possibly cyclerama 575?)

Am i way off base?

Not off base but looking at static when you should be looking at corrected CR and CCP. The combination you outlined above might be spot on but you will need to know the dome volume of the S&S piston, head gasket thickness and deck height to accurately calculate corrected CR and CCP. Google the BigBoyz compression calculator and insert the data. Shoot for 9.3-9.6 corrected and/or 190-200psi for CCP. For your application, i.e., long trips and a daily drive 195psi for CCP and/or 9.4 corrected CR should be about right. Both functions of the timing of the intake valve close event. Static CR is just a number, the motor never really sees static as it is operating on dynamic/corrected compression.

http://www.bigboyzheadporting.com/TwinCamComp.htm

You will get varying opinions on this; this one is mine.

Mar 13, 2016 | 09:42 PM

Lsc8

Thread Starter

Cruiser

Joined: Aug 2015

Posts: 117

Likes: 0

From: Il

I've been to big boys website, just not knowledgeable enough to use it.....plugging in various stock numbers corrected cr is 9.45-1 on the cycle Rama 575 and 9.27 on the ss 570. However the Calc also states static comp at 10.06 vs s&s stated 9.7

Last edited by Lsc8; Mar 13, 2016 at 09:48 PM.

Mar 14, 2016 | 06:21 AM

Hillsidecycle.com

Sponsor

Joined: Dec 2007

Posts: 12,084

Likes: 829

Quote:

Originally Posted by Lsc8

The Cyclerama .575 would have much more in the rpm range your looking for.
10.0 cr and a set of our Stage II heads with t/body mods will have you producing HUGE useable torque.
See the thread in this section with that combo on a 110" cylinder kit.

Scott

Mar 14, 2016 | 06:48 AM

Lsc8

Thread Starter

Cruiser

Joined: Aug 2015

Posts: 117

Likes: 0

From: Il

Is 10-1 cr obtainable by decking the 89cc heads or .03 head gasket(think it comes with .045)?

i want to replace my heads vs porting because they are high mileage and would need re powdercoated as well as port and polish.

I'm open to other cams as long as I stay lower compression.

1. Its getting harder and harder to find good gas.

2. With the ss heads, its my understanding I need their rocker boxes(or ez start cams) to utilize compression releases.