peejay wrote: ↑Mon Jan 15, 2018 6:09 am
Vee engines have a smaller package for the same power output.
Tht's pretty much it.
Notice that even as GM eliminated all pushrod inline engines years ago, they still are married to pushrod V8 engines, and they only stopped doing pushrod V6s for marketing reasons. The 3.6 is a significantly larger engine than the 60 degree or Buick V6 engines.
Like I said, if the 4 valves were given equal footing in rules as the 2-valve with the Corvette C7R, then GM would have no choice but to go to a 4 valve head. The difference really can be that much.
As far as Top Fuel goes - rules. A OHC would go a long way to mitigating the fact they have to deal with pushrod flexing - especially on the exhaust side.
The Illmor pushrod engine only had the advantage of a rules given increase in displacement and boost over 4 valve/ cyl engines (0.8L increase and 10 inches of boost) . Take that away and there is a reason we haven’t seen another one.
Shoebox car designs are typically driven by safety standards, particularly the pedestrian safety standards. Also, side impact standards have played a role as well. The Camaro and Challenger are just as if not more boxy than the Mustang. Old chassis that have had Mod engine swaps don’t need taller hoods. All of this is easily refutable. Single carburetor V engines are what lead to high hood heights.
The EMC in 2013 showed what 4 valve heads (and their increase in valve area for a given bore size) can do. The Mod motors that year used stock blocks, stock head castings, stock rocker arms, and a limited production OEM manifold. The pushrod engine that came in 4th behind them was a LSX (GM race block) with aftermarket heads and just about everything else.
Finger followers close and surpass the gap of a high rocker ratio pushrod. At some point you’re limited by the valve acceleration (which at the highest levels are maxed out in both configurations). That said the inherent stiffness in a OHC package as opposed to a pushrod system is MUCH greater and that allows you to do other things. OHC is more reliable for high rpm running due less moving parts.
From a strict engine performance perspective, OHC / 4 valve per cylinder engines have higher performance potential than pushrods. Formula One and motorcycle builders are not deterred by the higher COG / space component because they’re just that much better. If there is an application where the size would be a hindersnce it’s there, and it just hasn’t materialized.
OHC engines can do more with the port locations since the pushrod is not in the way.
As for rpm, no matter the displacment (from a lawn mower to a 1000ci engine), if you’re racing, you need to keep finding ways to find RPM. OHC simplifies that.
Some applications use a SOHC with finger followers to actuate 4 valve cylinders (like the Honda CRF450R) DOHC allows more tuning options - either on an engine stand or real time by the computer as the events can be changed independently.