[blau911]

Hi John,
I am using 34mm venturis.
I was reading Wayne's 911 rebuild book, and in the section on cylinder heads on page 89 he writes:

"So why not just enlarge the port diameter for improved airflow? Well, you can. This is exactly what the Porsche factory did on their 911S model and racing engines. For example, the intake port diameter of a 2.0 liter 1969 911T is 32mm. The 1969 911S has a 36mm intake port, and the racing 906 engine (also a 2.0 liter) has a 388mm port. Power ratings for those engines are 110, 170, and 210DIN hp respectively. The problem here is that the large ports slow down the incoming air-fuel charge, so the cylinders fill less efficiently at low rpm's, resulting in poor torque at low engine speeds. At high rpm, the intake has greater momentum, and the open ports offer an advantage in reduced resriction. The result is higher horse power at high engine speeds. The 906 engine produces maximum torque at 6,200, a full 2,000 rpm above the 911 T's torque peak."

Despite the above comments, I must emphasize that the engine has very good mid range power as well as top end power once I get rolling in 1st gear. In fact, I have had the pleasure of owning and driving my share of early 911S's, and this engine would easily run with the best of them. Furthermore, since it is not my grocery getter, the low end torque problem only becomes and issue when I get caught in heavy city driving or highway gridlock, which is rarely. I am merely hoping to make more of a minor adjustment, rather than rebuilding my top end with smaller ports. Of course switching to milder cams would be my prefered option if it helped out on the low end without giving away much of the power up top. Another possibilty might be to go with smaller intake manifolds. If I reduced air flow at the intake manifold to 37mm, wouldn't it effectively reduce airflow into the head intake ports in the same way as closing up the intake ports themselves? Just a thought.