Jolle wrote: ↑
Sat Jan 15, 2022 5:06 pm
Hoffman900 wrote: ↑
Sat Jan 15, 2022 4:54 pm
Jolle wrote: ↑
Sat Jan 15, 2022 4:44 pm
NASCAR engines can’t have the aggressive cam profiles from DOHC engines, especially on closing. This limits the space available and/or the time the valve is fully open.
Are you sure? Elaborate on your thoughts and I’ll answer from there.
They run those things very tight and they are at the limit of the metallurgy of the springs.
Very sure. Because it the extra weight of the push rods (and the bigger valves), timing has to be less aggressive then engines without this excess (moving) weight. Simply: the more moving parts and weight you have, the slower the acceleration must be.
It’s very well known that aggressive cam profiles, especially at higher RPM is one of the main advantages of dohc engines.
To a point, yes, but with rocker multiplication, the NASCAR engines are at the limit of what a valve spring can handle. They didn't gain anything going from flat tappet (velocity limited) to rollers (acceleration limited at the lobe) from being able to open the valve any differently, just friction (source is from someone I casually know at Hendricks). With the way these NASCAR engines are engineered, they are held back by the spring itself, not the system stiffness. Pnuematic buckets would allow them to do different things, but rules.
NASCAR engines typically run 290-288* @ .5mm on the intake and over 25.4mm of lift at the valve. Remember, what is happening at the valve is all that matters.
If you look at the lobe profile from Honda's Third Generation Formula 1 engine, you saw maximum intake valve velocity of 15mm/rad (.0103"in/deg) and acceleration of around 55mm/rad^2 (.03779in/deg^2). This doesn't account for multiplication through the finger follower, but it likely isn't much at that point in the curve (peaks around max lift). An older NASCAR application when multiplied through a 2.37:1 ratio rocker arm, might only be .01778in/deg (velocity) and .00209in/deg^2 (acceleration). Pneumatic "springs" are nice.
I will say DOHC can spin higher, easier, and be more compact overall.