BBC valve train
12-20-2017, 06:23 PM
#41
Registered
VIP MemberThread Starter
That is the Beauty of a 55mm cam core you can get a lot more aggressive cam shaft with out it being super rough on the vavle train. Its going to be very interesting to see just how much power these things are going to make.
12-20-2017, 08:45 PM
#42
Registered
The 55mm and 904/5 and Jessels awesome. I ran similar lift duaration ratio with no issues std core. Nice to see your sparing no expense. I love it What peak rpm/hp did nickens end up spec'ing the cams for. 7k. You gotta be excited seeing all the come together. As long as your melon can take a break when your head hits the pillow at night. Seems to be when my mind goes into hyper drive. Lol..
05-03-2019, 02:04 PM
#44
Registered
So back to something that this discussion has brought up.....
Is running a more mild cam with a 1.8 ratio rocker better than a larger/more aggressive lift lobe and a 1.7 ratio rocker in an endurance application? Consider an application where a mild cam with 1.8 rockers or an more aggressive lobe coupled to 1.7 rockers. Both setups net the same valve lift.
To me, having less lifter movement and a bit higher transmitted pushrod force to the lifter due to the ratio increase seems like it might cause fewer harmonics, or at least lower amplitude ones, than running a larger lobe and a lower ratio rocker. The slight increase in transmitted spring force would also help to keep the lifter solidly on the lobe with no bounce along with the slightly milder lobe profile further reducing lifter bouncing potential and the need for stiffer valve springs to control lifter bounce.
Discuss:
Is running a more mild cam with a 1.8 ratio rocker better than a larger/more aggressive lift lobe and a 1.7 ratio rocker in an endurance application? Consider an application where a mild cam with 1.8 rockers or an more aggressive lobe coupled to 1.7 rockers. Both setups net the same valve lift.
To me, having less lifter movement and a bit higher transmitted pushrod force to the lifter due to the ratio increase seems like it might cause fewer harmonics, or at least lower amplitude ones, than running a larger lobe and a lower ratio rocker. The slight increase in transmitted spring force would also help to keep the lifter solidly on the lobe with no bounce along with the slightly milder lobe profile further reducing lifter bouncing potential and the need for stiffer valve springs to control lifter bounce.
Discuss:
On the pushrod side of the arm, stiffness is of critical importance. When the lever ratio is increased, it decreases how stiff the components on the pushrod side of the arm "look" to the valve. To find that value, take the stiffness of the component, and divide it by the rocker ratio squared.
ex: for a 1.7 ratio arm, a 200,000 lb/in stiffness pushrod appears to be 69,200 lb/in. Change the ratio to 1.8, and that pushrod appears to be 61,700 lb/in. The same things go for the lifter and camshaft bending stiffnesses.
So why do BBC's run a 1.7 ratio rocker to begin with? It's because the camshaft diameter is on the small side, the cylinder volume is enormous, and the intake valve area is comparatively small. Small cam journal diameters drive small lobe base circle radii, and that limits the acceleration that can be ground into the profile.
So the better move is to fit as large of cam bearings as you can, and have your cam grinder offer you a lobe that is optimized for that bearing size. You'll see that the Pro-Stock engine developers have already done this. Of course there's a limit to how far you can go with a stock block. Some aftermarket blocks offer larger cam tunnels, raised cams, etc.
Oh, and if anyone wants to do any Spintron testing on a BBC, I'm interested in helping that effort.
Last edited by NautiSouth; 05-03-2019 at 03:28 PM.
