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Infidel
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Discussion Starter · #1 ·
What exactly do they do? I've read that they can increase compression. My local dealer says they install them with all the cams...why would they do that? If you have set up your heads for the compression you want these would be useless...right? Is there any other benefits? Does it move the powerband any? I'm getting ready to do my BB build with Andrews TW44's and need to know if I should use one...only want to tear into it once.
 

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2005 Road King Classic
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2,649 Posts
Gulfstream said:
What exactly do they do? I've read that they can increase compression. My local dealer says they install them with all the cams...why would they do that? If you have set up your heads for the compression you want these would be useless...right? Is there any other benefits? Does it move the powerband any? I'm getting ready to do my BB build with Andrews TW44's and need to know if I should use one...only want to tear into it once.
I think, but am not sure, that every 2+ degress of advance lowers the cam powerband by 200rpm. So if you had a cam that was 2400-6600rpm it would change to 2200-6400rpm.
 

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Addicted to American Iron
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460 Posts
I asked this same question of the guy who ground my cams and he gave me this answer about a 4 degree advance cam sprocket (copied from email):

They make the events that happen, happen slightly sooner in relation to valve opening and closing. Dave you are running a very agressive cam in your set up. The 4 degree advance sprockett will help cranking pressure. The more duration you have the longer the valves stay open so as a result the intake valve does not close (thus allowing the piston to start compressing fuel and air) untill the piston is further up the bore I.E. less cranking pressure.The reason we do this is because you want to take advantage of the stored energy in the intake tract.We bump the static comp raito then blead it off with the cam profile so that when the engine is running the dynamic comp raito will be higher .The more radical we get with this practice the critical the balance between cam , C.R. carb and heads becomes. As well as the engine develops more of specfic output nature (peakey) almost like a 2 stroke .
So advancing the cam will allow the intake valve close sooner thus building more cylinder pressure static and dynamically hopefully moving your hole torque band to a lower rpm range.
 

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Infidel
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1,428 Posts
Discussion Starter · #4 ·
Thanks Dave, that's exactly the type of answer I was looking for! I will be running an Andrews TW44 which has a powerband slightly on the high side for me (2400-6000+ RPM's). I'll install the +4 degree sprocket and hopefully lower the powerband down into the 2200 rpm range. Now that raises an interesting question...how will the TW44 with the +4 sprocket compare with a TW37B, any takers on that one?

TW44
21/41 242 .495 .182
49/17 246 .495 .158

TW37B
14/42 236 .510 .151
48/12 240 .510 .140
 
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