[snowman]

The following is a "speed secret" for 356/912 race engines.

The intake stack length, from the back of the valve to the top of the intake horn determines the rpm range that the engine will make max power. There are multiple lengths that will work but the shortest one is the most beneficial and works over the largest rpm range. This is well known.

The secret is that if you have the valves closed and fill the intake with water all the way to the top of the horn, carbs and all it must be exactly 110% of the cylinder volume. (note for the mechanically challenged. DO NOT DO THIS WITH THE ENGINE IN THE CAR as you won’t be able to remove the water). The reason for doing this is that you can get about 110% VE with tuned intake and exhaust. The 110% volume is exactly what’s required to charge the intake. For example if your engine is 1720cc each cylinder is 430cc. The intake must hold 430cc plus 43cc or 473cc.

This means that if you port the intake and heads you must be careful. If you increase the diameter to much you may need to shorten the intake stack, increasing the rpm where the peak performance occurs. This is a problem that must be solved simultaneously for optimum performance.

The second part of the secret that is not so well known is that there are multiple intake lengths that work. Unfortunately the longer lengths will not produce as strong or as wide a power peak.