why would the E36 head have a better ability to rev? The double springs are better and the valve diameter is 1mm larger and the rest is fundamentally the same IIRC. Even then, why would BMW set the rev limit almost 1k RPM below what the engine's capability was?
Double springs resist spring surge better and they are indeed stiffer on E30. But they have to be to counter the greater mass of valves, springs, lifter etc.
Valve diameter is exactly the same on E30 and E36, only the stem diameter changes. And smaller is better because it is smaller airflow obstruction on port and it is lighter.
Stock E30 7 mm stem valve, spring, lifter, keeper and retainer has mass of
219 grams.
Metric Mechanic setup with 6 mm parts (basically E36 M42) has mass of
166 grams. That is 25% lighter! :cool:
That is why BMW used softer springs on E36, it does not need to be as stiff as in E30 to rev to same limiter. But because the springs are softer you have less power loss to them even if you cannot rev any highger.
Ok, lets say e30 springs are 25% stiffer (Dbilas site lists E30 springs 115 kp against E36 86 kp). When you fit them to bone stock E36 M42 you can rev it safely up to 7850. Not sure if they are straight swap, can anyone confirm, but you get the idea.
To get E30 M42 to rev that high you would need need again 25% stiffer springs which starts to be in race engine category -> expensive and rare. And your power loss and camshaft wear would go up. And you would still have thick valve stems obstructing the air flow in intake ports. OR, you could swap the valves and lifters etc to lighter ones from E36
So E36 requires less parts to be changed to rev high. Lighter valvetrain is almost always better. It is better to lighten the valvetrain than to make springs stiffer.
