I have a 2006 Ultra Classic with 48k miles that has a CSC trike kit on it. Over the past couple years I've replaced the compensator and added an S&S 551 cam and cam chest kit with an S&S oil pump. I'm intending this winter to add a S&S big bore cylinder kit and wanted some input. I wanted to go with the 98ci kit but the 551 cams aren't recommended for compression ratios higher than 10:1 and the 98 kit comes in at 10.4. My thought was a thicker head gasket would reduce the ratio to where it would be ok.

 

The 10.4 static advertised for the 98" kit is, I am sure, based on the .045" OEM head gasket even if you could find a thicker one, that would be a mistake.

I don't know the specs of the 98" kit but with stock chamber volume I am pretty sure the pistons would be domed; maybe 6cc to 8cc so a little math and the piston domes could be machined to reduce compression. You would have to send the pistons to a shop like Hillside Cycles to assist with calculating how much to machine and, the machining.

JMHO but a better option and a better outcome would be to abandon the 551 cams, use a .040" or OEM head gasket and go with a cam set with a 40* to 45* intake close like the S&S 570 or TMan 590; some would like the Woods 555 but I haven't run them so I can't comment.

You would need to measure chamber volume and deck height to get precise data inputs for a compression calculator and shoot for say 9.6 corrected CR and/or 190-195CCP. Forget about static, the motor never sees static CR under operating conditions.

You did not mention fuel management but you will definitely need a dyno tune, even if you find a way to retain the 551 cams which IMHO, is not the way to go.

 

Thanks for the input djl. What are your thoughts about the 97inch kit from S&S? I lose one cubic inch but with stock heads the compression ration comes in at 9.7:1. I went with the 551 cams because it is a heavy trike and I generally shift and cruise in 3k to 3.5 k rpm range which is were this cam does well.

 

OK, I need to take a step back. I took a look at the S&S description for the 98" and 97" kits; the difference in bore diameter between the two kits is only .011"' 3.938" vs 3.927". The pistons in the 98" kit do have a 10cc dome which could be machined smaller to reduce compression that is compatible with the 551 cams. The pistons in the 97" kit, are flat tops that are designed to make 9.7 static with OEM heads.

I have been using OEM specs in the Big Boyz calculator to try and help decide how to retain the 551 cams but did not adjust for sea level at Arnold, Pennsylvania. After adjusting for sea level where you live, the cams suggested in my previous post are not suitable for your application with either the 98" or 97" kit; my bad for missing that detail.

I want to be very clear that I am making assumptions on input data to form an opinion on the viability of retaining the 551 cams with the S&S 97" kit. I am using the S&S bore of 3.927", OEM head gasket thickness of .045", chamber volume ('06) heads at 86cc, deck height of .005" and using a 4" dome volume to hit the 9.7:1 static compression for the 97"" kit.

Using that data and a baseline of 9.7:1 static CR and the 551 cams, corrected CR is 9.4 and CCP is 190psi which, IMHO, is very manageable with premium fuel and a good dyno tune. Considering that some of my assumptions may be over or under your actual dimensions, compression could also be over or under 9.4 and CCP could be over or under 190psi. CCP at 190psi is the starting point to consider compression releases which, IMHO, are a good idea to extend battery and starter life and I would have manual compression releases installed while the heads are off.

So I am providing information but you have to make the call on whether or not the S&S 97" kit is the suitable for you. Do I have an opinion, you might ask? Yeah, keep reading and consider another and I think a better way to skin the cat.

Send your heads and cylinders to Hillside Cycles for machining. Hillside can help you decide whether or not to deck the heads to hit a compression target for the 551 cams or maintain OEM chamber volume and use a 3cc domed piston to hit that compression target. Hillside will bore the cylinders to 3.938", fit the pistons to the new bore and gap the rings. I know there is some down time involved but, JMHO, a better option than an " off the shelf" kit regardless of the source. Scott might even work with you on an exchange basis where he does the work and sends you the parts but charges a "core" charge which is refunded when he receives you heads and cylinders in serviceable condition; no down time. I will suggest that you give Scott a call and see if you can come up with a plan.

Apologies for the long post but I don't want to have a forum member pissed because I "recommended" a modification that did not produce the expected results; not going there. I have done my best to provide you with the information I think you need to make an informed decision. Happy to keep answering any questions you might have.

I don't know what your goal is but you must be looking for stronger off idle and mid range TQ/HP. If it were me and funds were available, I would be going all out and rebuilding a bigger and badder motor. I think when you get the extra performance from your plan, you might have a taste for more and wish you had taken a bigger bite at the apple.

 

Thank you djl, I appreciate you and all the time and thought you put into the answer. I will definitely give Scott a call to see what he would recommend.

 

Glad I could help. Sometimes I have to decide to ignore a post because I know I will have to get into the weeds to provide a complete answer. In your case, I decided to get into the weeds. I could have just suggested Scott from the jump but thought if I didn't address your original questions, you would still be stuck on S&S as a solution. I am sure you and Scott can come up with a plan.