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Old 02-26-2010, 12:25 AM
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Do I need small base circle cam?

Hey guys, building a 383 and I am currently in the process of looking for a cam. I'm gonna go the hydraulic roller route and the block is a 1970 350 so I'll be getting the retrofit package. I have been told to opt for an LSA of approx. 108-110* to accomodate the larger stroke and still have decent idle and drivability. I am running AFR 195cc heads fully ported that will accomodate up to 0.600" lift and I plan on running close to that 0.600" because from the tests I've seen there is a gain in power across the the entire rpm range when running a larger rocker ratio if the heads haven't reached their max. flow potential. Anyways I have more or less narrowed it down to 2 choices,
the XR276 from comp cams- http://www.compcams.com/Cam_Specs/Ca...?csid=159&sb=2
or the XR282 also from comp-http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=160&sb=2

I'm not sure which one of these would be a better choice for my application but before I come to that decision I need to figure out if my rod bolts and lobes will clear. I am running 6" rods as well which, from what I understand, will most likely mean a small base circle cam. Do you think I will be needing one and if so what other components need to be special to match the small base circle cam (I know push rod length but what about timing set, distributor shaft length, etc?) Also any input on cam recommendations is appreciated. Below is a list of more detail of this build.

-350 block 0.040" over and 0 decked
-Wiesco flat-tops +5.4cc
-6 inch scat I-beam rods
-Scat 3.75" stroker crank
-AFR 195cc aluminum heads with 72cc chambers
-0.040 compressed gasket thickness
-1.6:1 rockers
-Edelbrock rpm air gap intake manifold
-10.1:1 SCR
-Going into a 1980 camaro with a manual trans.

Thanks,
Keith

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Old 02-26-2010, 12:47 AM
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Which Scat I-beam rod, the 3/8" capscrew or the 7/16"? Surprisingly the 7/16" size rod has more clearance.

Actually the rod length has nothing to do with the clearance at the cam, it is the big end of the rod, specifically in the area of the threaded end of the capscrew/rod beam shoulder that interferes with the cam lobe.
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Old 02-26-2010, 06:40 PM
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Quote:
Originally Posted by 383SBC
Hey guys, building a 383 and I am currently in the process of looking for a cam. I'm gonna go the hydraulic roller route and the block is a 1970 350 so I'll be getting the retrofit package. I have been told to opt for an LSA of approx. 108-110* to accomodate the larger stroke and still have decent idle and drivability. I am running AFR 195cc heads fully ported that will accomodate up to 0.600" lift and I plan on running close to that 0.600" because from the tests I've seen there is a gain in power across the the entire rpm range when running a larger rocker ratio if the heads haven't reached their max. flow potential. Anyways I have more or less narrowed it down to 2 choices,
the XR276 from comp cams- http://www.compcams.com/Cam_Specs/Ca...?csid=159&sb=2
or the XR282 also from comp-http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=160&sb=2

I'm not sure which one of these would be a better choice for my application but before I come to that decision I need to figure out if my rod bolts and lobes will clear. I am running 6" rods as well which, from what I understand, will most likely mean a small base circle cam. Do you think I will be needing one and if so what other components need to be special to match the small base circle cam (I know push rod length but what about timing set, distributor shaft length, etc?) Also any input on cam recommendations is appreciated. Below is a list of more detail of this build.

-350 block 0.040" over and 0 decked
-Wiesco flat-tops +5.4cc
-6 inch scat I-beam rods
-Scat 3.75" stroker crank
-AFR 195cc aluminum heads with 72cc chambers
-0.040 compressed gasket thickness
-1.6:1 rockers
-Edelbrock rpm air gap intake manifold
-10.1:1 SCR
-Going into a 1980 camaro with a manual trans.

Thanks,
Keith
Whose 6 inch rods by part number, some are made to clear some aren't?

Between zero decking, flat top pistons, and rod length with your cam and rocker ratio choice you need to check valve to piston clearance.

The block needs to be clearanced along the pan rail and the lower cylinder wall to accommodate the swing of the rod's big end on the longer stroke crank. Especially with these older blocks as they have more beef in the bottom end right where the rods is going to swing. This needs .050 to .060 inch of free clearance, less you'll get a collision, more you'll discover coolant.

You can use a smaller core diameter cam to get clearance. But as the overall diameter decreases, the load between the lifter and the lobe increases and the bending moments from stiff springs become measurable length wise deviations between bearings. No cam likes this, in race engines they at least last long enough for frequent rebuilds, for street engines they become the cause of rebuilds.

Balance becomes important especially as the revs go up. The SBC depends on cylinder offset to statically balance the middle of the crank. This puts a side to side couple where the engine is trying to twist away from the dynamic imbalance here. The number three is in the middle of this twisting while the 2 and 4 are trying to contain it. This makes 2 and 4 mains work pretty damn hard. What salvation there is to be found is in 4 bolt mains and a good balance job.

Bogie
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