adding cubic inches effect on cam performance
HDF Community Team
Joined: Jun 2006
Posts: 12,776
Likes: 2,607
From: san antonio
You can use the BigBoyz compression calculator.
http://bigboyzcycles.com/TwinCamComp.htm
The introduction of Wiseco pistons is a new wrinkle as the dome volume, even if a flat top, will be different from the OEM flat top pistons. Look at corrected and cranking compression, not static as those should control your decision. There may be some debate on what those numbers should be but a general rule of thumb for a street build would be 9.3 corrected and/or 190psi for cranking. The critical inputs are chamber volume, piston dome volume and intake valve closing. Not saying your tech doesn't have this figured out but it's your bike and you should be in control of the outcome.
On the surface, with the information you have provided, I.e., 76 cc heads, 4.375" stroke, 4" bore and 211 cams, the compression will be too high but maybe that information is not accurate?
http://bigboyzcycles.com/TwinCamComp.htm
The introduction of Wiseco pistons is a new wrinkle as the dome volume, even if a flat top, will be different from the OEM flat top pistons. Look at corrected and cranking compression, not static as those should control your decision. There may be some debate on what those numbers should be but a general rule of thumb for a street build would be 9.3 corrected and/or 190psi for cranking. The critical inputs are chamber volume, piston dome volume and intake valve closing. Not saying your tech doesn't have this figured out but it's your bike and you should be in control of the outcome.
On the surface, with the information you have provided, I.e., 76 cc heads, 4.375" stroke, 4" bore and 211 cams, the compression will be too high but maybe that information is not accurate?
Thread Starter
|
Intermediate
Joined: Aug 2006
Posts: 31
Likes: 0
From: california
Ok, started researching and found Big Boyz compression calculator which led me to deck height and squish and pistom dome height and CHAMBER VOLUME !! i'm starting to understand why everything needs to be taken into consideration. i'm going to assume (i know, i know )....... deck height, squish and piston dome is a mandatory check for a pro builder but how about chamber volume. Is it standard to check this or should i ask the builder to check. I don't want to insult him , just want to know if it's also covered in engine building 101.
Thread Starter
|
Intermediate
Joined: Aug 2006
Posts: 31
Likes: 0
From: california
dkl my last post was added before i saw your last post so please excuse the repetition. The info i stated is as correct as i know. 211's , new 4.375 stroke, The 88 cylinders bored to 95 with s.e. flat tops and later fit with new pistons (keith black)and sized again with a top end rebuild . Not sure if this changes the math in figuring bore but i believe the bore is 3-7/8 not 4
So it sounds like i should talk to him again and express my concern.
So it sounds like i should talk to him again and express my concern.
Last edited by hogkrazy; Mar 27, 2012 at 11:12 PM.
HDF Community Team
Joined: Jun 2006
Posts: 12,776
Likes: 2,607
From: san antonio
dkl my last post was added before i saw your last post so please excuse the repetition. The info i stated is as correct as i know. 211's , new 4.375 stroke, The 88 cylinders bored to 95 with s.e. flat tops and later fit with new pistons (keith black)and sized again with a top end rebuild . Not sure if this changes the math in figuring bore but i believe the bore is 3-7/8 not 4
So it sounds like i should talk to him again and express my concern.
So it sounds like i should talk to him again and express my concern.
Last edited by djl; Mar 28, 2012 at 01:57 PM.
Thread
Thread Starter
Forum
Replies
Last Post
