Wider Lobe Seperation...Higher CR?

TheBigO · 07-26-2009, 05:04 PM

I would like thank everyone on this forum for helping me and advising me on my 455 Olds turbo camshaft swap(JM-34 0.488 0.472 216° 210° 114° 1000-5000) and removing the very restrictive Crane Cam 0.484 0.496 222° 230° 110° 2500-5500

My question is: I orginally had a crane cam with a 110 lobe seperation and but now I have swap in a cam that has a 114 lobe seperation ....because of the wider lobe seperation....will this raise my CR from its current 9.5.1CR because it will have less overlap?

http://videos.streetfire.net/video/T...psi_649505.htm

P.S.

I will be running a air to air intercooler conjuction with my current water methonal injection system with only 8lbs of boost on my 461 BBO.

DoubleV · 07-26-2009, 11:30 PM

Your static CR of 9.5:1 will not change. Your dynamic CR however, may change.

Either way, 9.5 seems a bit high for a turbo car. I hope you got this all figured out!

83hurstguy · 07-27-2009, 06:05 AM

Quote:

Originally Posted by DoubleV

Your static CR of 9.5:1 will not change. Your dynamic CR however, will increase.

Either way, 9.5 seems a bit high for a turbo car. I hope you got this all figured out!

you can't answer the DCR question unless you know where the ICL is installed... if you have two identical cams other than LSA, both installed on the same ICL, you won't see a difference in the DCR calculator, due to it only using intake valve closing as its calculation reference.

how'd you pick this cam out?

TheBigO · 07-27-2009, 09:20 AM

Quote:

Originally Posted by 83hurstguy

you can't answer the DCR question unless you know where the ICL is installed... if you have two identical cams other than LSA, both installed on the same ICL, you won't see a difference in the DCR calculator, due to it only using intake valve closing as its calculation reference.

how'd you pick this cam out?

Thanks for your tech insight....I pick out this cam out by the fact that duration on most turbo cams that are using a log design turbo manifold, will usually have 6 degrees more intake duration than exhaust duration (ie:226/220).This is know as reverse split duration. This is mainly because a log manifold design will typically see higher then a 2:1 pressure ratio in the exhaust ( as high as 4:1 with some logs). By using a reverse split duration this will somewhat help prevent from getting exhaust gas reversion.

DoubleV · 07-27-2009, 12:31 PM

Post edited for accuraccy.

I figure if he's going to a smaller overall cam with a tighter LSA, his dynamic compression will increase. I guess you gotta crunch the numbers to know for sure if it increases and if so, how much.

83hurstguy · 07-27-2009, 01:22 PM

I would have personally ordered a custom cam from BTR Bill for this. Not many Olds vendors deal with blow through boost setups. As mentioned, the 9.5 CR is probably high, and that's a pretty small cam for a BBO. The alky should help. What intake centerline is the new cam ground on? 110? 112?

Vince, if you know the intake centerline, you can generally throw the cam specs into a calculator and get the intake valve closing point. The IVC point isn't the tell all story of what cranking compression will be, but it's a decent comparison point between cams. My IVC on the new cam is actually identical at the .050" duration numbers as the stock VIN 9 cam, and with the static CR bump, it cranks at 175 psi.