Hi all,

I have a 2009 street bob and currently have a 103 kit with worked heads and woods tw 8-6 cams. I just put a new Thrashin AR exhaust on and now looking to upgrade to a 110 kit. I spoke to a couple guys who are experts with heads / builds and suggested I go 10.7 compression to get fullest out of those cams. I have hildreth heads that are ported and I used big boyz head calculator and also spoke to the main guy Bean who said to get to 10.7 I wouldnt have to machine heads but just put in a cometic .030 gasket.

Any feedback on this or recommendations would be great! Thanks

 

I just did an AMS (Ft Worth, TX) 105 kit. KB Forged pistons with stock heads gave me a 10.5:1 CR with SE-255 cams. A little slow cranking over and VERY slow on hot starts. Not wanting to upgrade the stater, I went with James 0.047 head gaskets and knocked the CR down to 9.9:1. Starts like stock cold and hot and runs like a screaming banshee. 10.7:1 seems a little high for a street engine. The more aggressive ya go on the engine go-fast goodies, the slower the advance curve to control detonation. Good luck with that 10.7:1 thing.

 

That cam should be good @ 10.7 to 11 since it has a 47* intake close, that's what you use to determine your CR. To find your actual CR you need to know the combustion chamber size of your heads, you should have gotten that info from whoever ported your heads. You then need to check your pistons after assembly to see if they come up even to the top of the cylinders, this is zero deck height, quite often the pistons will be below the top of the cylinders. If you have zero deck & 85cc chambers, they can range from 85-86.5, & run a .030 HG as you checked on the Big Boyz site you should have 10.7. If you have chambers that run to the high side @ 86.5cc & your pistons are down in the hole .006, which is quite common, you would only have 10.4, so you need to check all these factors when doing your build. Both of these factors can be corrected as you're doing the build, that's why good engine builders get paid for their work & come up with great results.

 

Awesome info! Thanks so much. Im by no means technical so appreciate the feedback. I am wanting to buy S&S big bore 110 kit. and trying to figure out if I should go with 10.7 for pistons.

So I did get info back from the guy who did my heads. And he provided the following:

"Here's a copy of the flow numbers for the heads you have. My computer modeling program indicates that even with the increase in engine displacement, these heads (#060901) with TW-8-6 cams should be more than adequate to handle demands of the motor to 6200rpms, but peak horsepower should occur at 5700rpms.
The compression ratio on a 110" motor with these heads will be 10.5:1. This calculation is based on using flat-top pistons with a piston-to-head clearance of .050". That equals a quench area of 10.3cc's, and with the combustion chamber of 85cc's, you get a total of 95.3cc's and the compression ratio of 10.5:1.

The springs used when the heads left the shop were VThunder by Comp Cams, part # 9700-kit, good to 600 lift. I'm assuming they haven't been changed.

Port velocity should be excellent for a really wide power band provided you don't get too carried away with your throttle body size and exhaust system. I don't have specific numbers for the port velocity since those numbers would be a result of the total package. The heads and the cams are just two pieces to the puzzle -- you also have to take into consideration the air cleaner, air cleaner housing, throttle body, and exhaust systems. They all contribute to the end results."

I attached the doc he sent for flow numbers.

 

Are you doing the build yourself or having a shop do it?

 

 

Well he should know what I'm talking about just make sure he knows that you want to be as close to 10.7 as possible + or - .1 or less. I looked on the S&S site for the 110 bolt on kit # 910-0651 & it lists the CR @ 10.5 with 85cc heads, so I'm thinking they're figuring that with a thicker HG, .040-.045. You should call & verify what they used to get those figures & tell them that you're wanting to have 10.7 with 85cc heads & see what they say because the kit comes with gaskets. You did well to use the BB site to check your CR, toward the bottom of the specs it asks for piston dome value, S&S says that figure is +.7 instead of the listed -1.5, you can see how that raises by .22 so that's why they must be using a thicker head gasket. I used .030 HG 4.05 HGT 85cc HGV .01 DH frt & rr & .7 PDV, that gives you approx 10.72, so hopefully they can provide you with a .030 HG. If not Cometic makes .030 .036 .040 & can custom make other sizes, I know they can make .027 which could be used if the pistons were further down from zero DH. If you have questions feel free to ask, it seems as though you have the basics down, such as using the BB site & I think you can figure out all my abbreviations if you use that site. Good luck, should make for one fast Dyna. My son just started riding & got a 09 FXDC a couple of months ago, he needs to get more riding time in before I put that much power under him. LOL

 

 

You're very close & doing a good job with the BB calculator, but you forgot to add in 2 values. The 2nd yellow box down is head gasket bore, that figure has to be larger than your 4" piston, I'm not sure what the actual is but it's somewhere between 4.0 & 4.1 so I just used 4.05, then below the head CCs box is the deck height value for each piston. I checked on some other builds & they reported the actual amount the piston are from zero deck, being perfectly even with the top of the cylinder, is .0015 to .0025 so I added .002 in for each piston & with a .040 HG that gives you 10.67 CR. It was late last night & I put in .01 in for the deck height value in stead of .002, that's how I came up using a .030 HG, OOPS. Anywhere from 10.65 to 10.75 will be fine, you could try to get some @ .038 but the difference wouldn't be noticeable, you might be able to get S&S to substitute ..040 for the .045 gaskets, .045=10.56 .038=10.72 .036=10.76 so you could fine tune if the piston deck height is different than the .002, but a .040 HG should be close enough. Have fun it should RIP after you get done, hopefully you have a good tuner in your area to get all of the power for you that's the last part of the puzzle & very important