On your bike, the cam is directly actuating on the valve/"lifter" bucket. On a SBC, you have pushrods and rockers in between them. The rockers don't actuate at a 1:1 ratio, more like 1.5:1 so there is much more friction between the cam and the lifter.
So say you have 300lb valve springs in your bike, since the cam is acting directly on the valve it only sees 300lbs of pressure. The same 300lb valve springs in a pushrod engine with 1.5:1 rockers would be seeing 450lbs of pressure between the cam and lifters.
bwh998
New Reader
7/31/16 11:13 a.m.
Run_Away wrote:
On your bike, the cam is directly actuating on the valve/"lifter" bucket. On a SBC, you have pushrods and rockers in between them. The rockers don't actuate at a 1:1 ratio, more like 1.5:1 so there is much more friction between the cam and the lifter.
So say you have 300lb valve springs in your bike, since the cam is acting directly on the valve it only sees 300lbs of pressure. The same 300lb valve springs in a pushrod engine with 1.5:1 rockers would be seeing 450lbs of pressure between the cam and lifters.
Thank you, makes perfect sense now.
I'll hazard a guess that it's not just straight up spring pressure, the mass of the rocker, pushrod (full of oil!), and big lifter probably also comes into play, trying to move all that mass a distance of whatever the cam lift is in less than 1/6000 of a minute probably comes into play as much as if not more than the spring pressure. Whereas anything OHC is moving just the bucket or a rocker.
bwh998
New Reader
7/31/16 12:46 p.m.
In reply to BrokenYugo:
And the bikes typically use 4 or 5 tiny valves per cylinder, where the sbc has 2 large valves with much more mass.
BrokenYugo wrote:
I'll hazard a guess that it's not just straight up spring pressure, the mass of the rocker, pushrod (full of oil!), and big lifter probably also comes into play, trying to move all that mass a distance of whatever the cam lift is in less than 1/6000 of a minute probably comes into play as much as if not more than the spring pressure. Whereas anything OHC is moving just the bucket or a rocker.
wouldn't that be 1/1500 of a minute due to it being a 4 cycle?
In reply to mad_machine:
Assuming 6000 rpm the cam is doing 3000, the opening half of the valve movement that's going to have the most force on the lifter is half that, on the closing side of the lobe it's just spring pressure.
I thought cams ran at 1/4th engine speed?
Use whatever flavour of oil is your favourite and add a bottle of this for break-in:
http://www.acdelcochemicalcatalogue.com/view_picture_description.php?prod_id=15&start=0&page=1&search_frn=Lubrifiants&search=Lubricants
mad_machine wrote:
I thought cams ran at 1/4th engine speed?
nope, cams run half of crank speed
FYI---- The Joe Gibbs brand oil is now called Driven Racing Oil. It's the same stuff--- they just changed their name recently. It's top-quality oil.
