What intake size to run on '81 3.0

[justinp71]

I am putting carbs on my 81 3.0, and wondering what intake manifold size I should use, specs are:

34mm ports (36mm factory chamfer)
DC19 cam
CIS 9.3:1 pistons

Pmo offers the intakes in 34, 38, 39, 40 mm sizes....

I could run the 38mm and do a small port on head, but prefer to leave the heads stock.

[Steve@Rennsport]

Justin,

The only way your engine is going to run properly is by using manifolds that match your heads to your carbs,....

That means 40mm at the top (for 40mm carbs) and 34mm at the bottom for these later 3.0 heads, regardless of chamfer. The 38mm ones are too big for your application.

JMHO, so YMMV,

[justinp71]

Would it be worth it to port the heads? It would be a little of an inconvience but the motor is partially apart right now... But I'm thinking that smaller port heads should give more low end.

[Steve@Rennsport]

Originally Posted by justinp71

Would it be worth it to port the heads? It would be a little of an inconvience but the motor is partially apart right now... But I'm thinking that smaller port heads should give more low end.

Not unless you do it right. That means enlarging the whole throat from just below the guide boss to the port entrance and then checking your work with a flow bench to ensure they are all equal.

You're right though, the small port heads do make better low-end power and throttle response andthats why Porsche did that in 1980. The only time we use the large port ones is for large-displacement street engines and race motors.