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

 

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.

 

I think you are being a little annal. Do you think you are going to notice the difference between 100 or 101 lbtq? Think about it!

 

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.

 

When you pull the heads off to have them worked over, swap out the flat top pistons with high compression domed pistons.

 

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.

 

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

 

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

 

Did you consider the squish and adjust for it? Makes a good difference in torque and pinging.

 

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