Compression
Thread Starter
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Road Warrior
Joined: Aug 2005
Posts: 1,956
Likes: 288
Compression
Hello all,
Got a question about achieving a certain compression ratio. Here's my set-up:
SE Flat Top Pistons and 95" bore
Street Port by Big Boyz (planned)
Converting to the hydrolic/roller chain set-up with SE billet cam plate/oil pump
Woods TW 6-6R cams with 4 degree advance
Cometic .03 gaskets
When I go to the Big Boyz web site to use their compression calculator and punch in the values (plus 600 feet above sea level) I come out with 9.8 to 9.9 compression and cold crank compression of about 188. Talking with Bob Woods, he indicated that his TW 6 cam likes 10:1 to 10.5:1 compression. Assuming my heads are spec 85 cc, that would mean milling heads down to 79 cc, which will likely cause fitment issues. With this set-up, is there any other way to bump compression just another tenth or so, or am I being too annal about this? Thanks for any and all input.
Ted
Got a question about achieving a certain compression ratio. Here's my set-up:
SE Flat Top Pistons and 95" bore
Street Port by Big Boyz (planned)
Converting to the hydrolic/roller chain set-up with SE billet cam plate/oil pump
Woods TW 6-6R cams with 4 degree advance
Cometic .03 gaskets
When I go to the Big Boyz web site to use their compression calculator and punch in the values (plus 600 feet above sea level) I come out with 9.8 to 9.9 compression and cold crank compression of about 188. Talking with Bob Woods, he indicated that his TW 6 cam likes 10:1 to 10.5:1 compression. Assuming my heads are spec 85 cc, that would mean milling heads down to 79 cc, which will likely cause fitment issues. With this set-up, is there any other way to bump compression just another tenth or so, or am I being too annal about this? Thanks for any and all input.
Ted
Tourer
Joined: Sep 2006
Posts: 435
Likes: 0
From: Beaver Dam, KY
I'd give Scott at hillside a call. He'd have an answer for you. Maybe the 4 degree gear?
Nevermind about the gear, I see you already have it. My bad.
Nevermind about the gear, I see you already have it. My bad.
Last edited by 99embrey; Feb 15, 2009 at 08:58 AM.
Advanced
Joined: Oct 2008
Posts: 65
Likes: 0
From: Winston - Salem Greensboro area North Carolina
Hello all,
Got a question about achieving a certain compression ratio. Here's my set-up:
SE Flat Top Pistons and 95" bore
Street Port by Big Boyz (planned)
Converting to the hydrolic/roller chain set-up with SE billet cam plate/oil pump
Woods TW 6-6R cams with 4 degree advance
Cometic .03 gaskets
When I go to the Big Boyz web site to use their compression calculator and punch in the values (plus 600 feet above sea level) I come out with 9.8 to 9.9 compression and cold crank compression of about 188. Talking with Bob Woods, he indicated that his TW 6 cam likes 10:1 to 10.5:1 compression. Assuming my heads are spec 85 cc, that would mean milling heads down to 79 cc, which will likely cause fitment issues. With this set-up, is there any other way to bump compression just another tenth or so, or am I being too annal about this? Thanks for any and all input.
Ted
Got a question about achieving a certain compression ratio. Here's my set-up:
SE Flat Top Pistons and 95" bore
Street Port by Big Boyz (planned)
Converting to the hydrolic/roller chain set-up with SE billet cam plate/oil pump
Woods TW 6-6R cams with 4 degree advance
Cometic .03 gaskets
When I go to the Big Boyz web site to use their compression calculator and punch in the values (plus 600 feet above sea level) I come out with 9.8 to 9.9 compression and cold crank compression of about 188. Talking with Bob Woods, he indicated that his TW 6 cam likes 10:1 to 10.5:1 compression. Assuming my heads are spec 85 cc, that would mean milling heads down to 79 cc, which will likely cause fitment issues. With this set-up, is there any other way to bump compression just another tenth or so, or am I being too annal about this? Thanks for any and all input.
Ted
Why don't you do a cranking compression test on the engine and work the figures on the calculator until it matches your real cranking pressure? You can at least know what the true compression ratio is. Then adjust the cc's of the combustion chamber on the calculator for your target compression. This will let you know how many cc's you need to take off the head.
Tourer
Joined: Aug 2008
Posts: 385
Likes: 0
From: Diamond bar, California
milling down the heads would require you to have adjustable pushrods, thats about it for fitment issues.
you could change to a C/P piston, they have an off the shelf 9.5 and a 10.0 for a 85cc head, plus you would get the benifit of having larger valve pockets you decide to run larger valves one day.
I run the 9.5 piston with a 4cc dome and 82cc heads to get 10.1. with a .030 gasket.
At that point you will not need the 4 deg advance gear, thats why people run it, its to add cylinder presure for lower compresion engines.
you could change to a C/P piston, they have an off the shelf 9.5 and a 10.0 for a 85cc head, plus you would get the benifit of having larger valve pockets you decide to run larger valves one day.
I run the 9.5 piston with a 4cc dome and 82cc heads to get 10.1. with a .030 gasket.
At that point you will not need the 4 deg advance gear, thats why people run it, its to add cylinder presure for lower compresion engines.
Thread Starter
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Road Warrior
Joined: Aug 2005
Posts: 1,956
Likes: 288
Thanks all. Forgot to mention that I do have the newer SE Tapered Adj. Push Rods. I understand the fitment issue has more to do with intake manifold (mine is a fuel injected bike)? Is this a concern. Bean at Big Boyz mentioned it so I'm just trying to get some other opinions. Thanks again.
Ted
Ted
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Road Warrior
Joined: Aug 2005
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Just spoke with Scott over at Hillside. He was generous with his time and very helpful. I think what I've settled on is to switch to the SE Cast 10.25 pistons and ditch the 4 degree advance. This will bring me to 10.5 static compression (9.6 corrected) and CCR of about 198, so I'll add compression releases. Won't have to mill heads, so the extra cost will only come from the pistons. I will post in the classifieds, but I now have a set of SE Cast Flat Tops new, but out of the box, for sale. They list on Chicago Harley at $120 or so, so I'll let them go for $80 plus shipping. Thanks all for your input and advice.
Ted
Ted
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Thread Starter
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Road Warrior
Joined: Aug 2005
Posts: 1,956
Likes: 288
Thanks, Randy. I did consider squish. I may actually, depending on how the heads cc out at, going with Cometic .04 to keep the static compression around 10.25-10.3. We also run 93 octane here in Michigan, which should also help, though I'm cognizant that many states have max 91.
Ted
Ted