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914World.com _ 914World Garage _ 914/6 Intake Manifold Question

Posted by: Peeterabytes Mar 16 2024, 07:57 PM

I am in the process of getting my 914/6 back on the road after sitting in my garage for 24 years. Finished the fuel tank, suspension, and brakes rebuild so on to replacing the Webers with some 40mm PMOs. The car has a 911 3.0 SC engine.

The current intake manifolds are 901.108.321 which I believe my father got from a 911s back in the day. Believe those are 32mm ports.

I am thinking about replacing them with new PMO intake manifolds but on the PMB website, it suggests 36mm X 40mm or 37mm X 40mm for 3.0 engine. I am thinking the 36mm. Only because I believe that is the intake port size for SC engines. And I’m just looking to make it a fun casual driver, not any crazy performance or worrying about where my peak HP comes in.

Asking you experts for advice? Stick with the 32s? Get the 36s? Or?

Posted by: flyer86d Mar 17 2024, 04:15 AM

I would try to match the intake port size. You can do that by buying the new manifolds or you can enlarge the 32 mm outlets of your manifolds to match the heads. I did this on the 2.7 that I built for my 68 911 which has 35 mm ports.

Charlie

Posted by: Luke M Mar 17 2024, 08:03 AM

My brother has a 3.0 6 in his 914-6 conv. He's running Weber 40ida's with PMO manifolds. He also has the head to manifold heat insulators installed. We ran into a height issue w the tall manifolds and swapped out for a shorter set. The pmo filter covers where hitting the backside of the truck wall and lid springs. Once swapped out it was ok. This issue could be from installing the front firewall mount a little high as well but not too sure. As far as which size port to run we went with the same size as the head intake port. His 3.0 has the early large port heads so matched to those. 78-79 are large port and 80 -83 are small port. I feel the 40's are a good match for his engine combo which has been modified.

Posted by: nditiz1 Mar 17 2024, 08:31 PM

Ports for the 80-83 SC are 34mm, oddly PMO (now EMPI) do not make that one and you have to go with 36mm or open up the 32mm ones. I asked this question regarding my 2.7 and getting intakes and the suggestion was go with 36.

If you want to save money you will be OK with the 32s. There is also the camp that says it helps prevent "blow back" or something like that when gasses want to travel back up that ledge helps prevent it. Either choice it will run well.

Posted by: technicalninja Mar 17 2024, 09:17 PM

Usually reducing port diameter will increase port velocity and help cylinder filling at lower RPMS.

I have a 98 E36 M3 and it came with an intake manifold that is significantly smaller in port size than the runners in the head. Like 10-15% smaller.

This skinny port manifold (S52) made 5-10 lbs/ft more torque below 4k RPM.
It made the same HP as the earlier S50 manifold which had ports that matched the size in the heads.
This makes the car a little bit quicker.

Now, as soon as you add compression, cams, or any other VE improvers the skinny manifold is a restriction, and EVERYONE installs a S50 manifold which thankfully are common (all pre 96 3 series have one, not just M3s).

For what the OP intends I'd get the 32s and try them first before I ported them out.
They might work better "as is" for his wants...

Posted by: Peeterabytes Apr 1 2024, 09:38 AM

Thank you all for the replies. I've decided to stick with the 32s and see how they run with the PMO 40s. I am sure my father used these for a reason.

Because I now have the transmission throttle linkage that Rich makes, I have ordered a PMO Manifold Bell Crank Stud which hopefully will ship soon. However this weekend I found in a box of old 911 parts, a couple of intake manifolds and found a couple with the pin still in them.

Does anyone know if this pin is pressed or screwed in/out?

You all have been so helpful thanks!