On a twin cam with a Andrews 37G cam, how high can one go on the compression while running 93 octane before needing the compression reliefs? I see most BB kits offer 10:5:1 or higher.

 

The magic number seems to fall around 10 to 1. Over that and you shorten the starter life.

 

Thanks for the quick reply. I've been searching for a setup to install into my Street Glide. I am wanting a strong torque curve from 2000-5000 rpms. I'm interested in a 95-98" BB, gear drive cams and porting the heads. Any recomendations here?

I've been on HDWrench's site and seen a few dyno graphs. I'm a bit confused since one of them have the tq/hp crossing each other at 4k's instead of the tipical 5250 rpm range. I'm wondering if the run was done in a different gear than 4th. Should a dyno pull be done in a 1 to 1 gear ratio or 5th gear for the HD's? I'm familar with Mustangs being dynoed and we ran them in this ratio to get an acurate pull.

 

I stay away from recommending different options for builds for a few reasons. I feel it is like women, what I like and you like are both different completely, besides I've been out of the business for quite a while and I am not up on the latest in cams and wouldn't want to give you bad advice.. HDWrench could make some suggestions here for you. He's done many builds here on the board for people and they seem to be very satisfied with the results.

There's a few posts in the DIY about cams and the way they work in theory which may give you some direction...

https://www.hdforums.com/forumid_248/tt.htm

 

Thanks for the link. Good info there.

 

What you're really asking is not how much static compression, but how much effective cylinder pressure. A major factor in that formula is altitude. I live at 6,500' ASL. I can use more static compression than someone who lives at 10' ASL. Actually, to obtain similar performance, I must use more compression.

The power differance between 10 and 10.5 is so low that it's really not worth talking about for most riders.

 

True but I am considering pistons that don't exceed 9:7:1 comp. I'll be near sea level (east coast) but just don't want to have to bother with the comp releases upon startup.

 

That cam closes the intake valve at 42*.

Bore: 3.875
Stroke: 4.000
Rod length: 7.660
Static compression: 9.7:1
Altitude: 0'

Cylinder pressure: 180 psi.

For comparison, a stock motor at sea level is 169 psi.


How much pressure is too much? I don't know exactly. I'v read anything >200 needs a pressure relief, and I'v also read anything over 190. At 180psi, I wouldn't think you'de need it. But do some more research and see if you can get a solid answer for that one.

 

I'll let you know what I find out.