Intake manifolds

[Birol]

One of my (Weber) intake manifolds has a crack.
I have a replacement,
but the bottom end of it has a smaller cross section.

As known, the intake manifolds are not all the same.
Compared to the later ones,
the first versions had larger diameters at the bottom.

Now my question, what will happen when I use the manifold
with smaller diameters ?
Less torque in mid section ?
Less power in upper revs ?
Starting problems ?
I would prefer to use the original manifold again,
but the problem is the uncertain repair success of these cracks.
Engine is a tuned, high rev 2,2.

Thanks in advance for any ideas.

[Steve@Rennsport]

Replace the cracked one with another that has the same ID.

Conversely, find someone who can weld magnesium & machine it to effect a good repair.

[Birol]

Thx Steve !

The part number is always 901 108 321, no matter if small or large diameter.
There are only additional markings with 1R or 0R,
whatever they mean.
'67' or '70' should stand for the year of production.

If I would be convinced that magnesium welding will be successful in this case,
I would go for it.

Are the early manifolds made of magnesium ?

Here a picture with the difference.
On top a '70' manifold, below the '67' with larger cross section.

[James Brown]

i used a high speed die grinder to enlarge a replacement. takes about 15 min. to match the old one, easy peasy. bonus to keep the shavings and make a nice pyrotechnic display!

[1QuickS]

The "0R" manifolds were for 911S and had 35mm exit diameters, the "1R" were for the other engines and had 32mm exit diameters. Your manifolds were taper bored to a somewhat larger exit diameter.

Magnesium welding & machining will cost approximately the same as buying new PMO manifolds.

A summary of intake manifolds:

I have reviewed a selection of OEM intake manifolds to help answer some questions regarding individual variations between the years from early 1966 through 1970. I reviewed samples of these manifolds for each production year so I feel confident the gamut of variations is pretty well covered. Each of the manifolds has a date of production cast into the body except for the sand cast version. Surely there are manifolds with 1967 date codes and conical outer runners to coincide with the design change. I assume the oblong outer runner cross section was an attempt to accelerate air flow to help make up for the longer runner length. Also, early 1967 cast manifolds did not have the “Sportomatic”ports of those cast later in the year for the 1968 year model. This type of change within the year of casting is to be assumed in general regarding design changes over the years.

Info presented in no particular order:
• All manifolds are magnesium; earliest is sand cast and all others are die cast and have date codes
• sand cast, w/o mfg date; manifold part number: 901.108.321.01 with oblong outer runners; 34mm
• die cast, 1966, 911S manifold has part number: 901.108.321.0R with oblong outer runners; 35mm
• die cast, 1967, 911S manifold has part number: 901.108.321.0R with oblong outer runners; 35mm
• die cast, 1967, 911S manifold has part number: 901.108.321.0R with conical outer runners; 35mm
• die cast, 1968, 911S manifold has part number: 901.108.321.0R with conical outer runners; 35mm
• die cast, 1970, 911S manifold has part number: 901.108.321.0R with conical outer runners; 35mm
• die cast, 1966, 911N manifold has part number: 901.108.321.1R with oblong outer runners; 34mm
• die cast, 1967, 911N manifold has part number: 901.108.321.1R with oblong outer runners; 32mm
• die cast, 1968, 911N manifold has part number: 901.108.321.1R with conical outer runners; 32mm
• die cast, 1969, 911N manifold has part number: 901.108.321.1R with conical outer runners; 32mm
• die cast, 1970, 911N manifold has part number: 901.108.321.1R with conical outer runners; 31.5mm
• starting in 1967, reinforcing ribs extend from the bottom flange up the exterior of the runners; 1968 and later manifolds have larger and longer reinforcing ribs
• starting in 1968 the lug for the 8mm shaft for the bell crank has an added reinforcing web
• 1967 and later have ports for Sportomatic and/or other vacuum accessories; early castings in 1967 did not have these ports
• The sand cast manifold was for the first few 911s supplied with Webers which began to appear in Feb 1966 with engine #907001 as the Solex carbs were phased out.
• The early sand cast manifolds and those die cast in 1966 were finished the same as the early Solex manifolds with silver/gold magnesium chromate
• By the time the 911S appeared in July66 the sand cast manifolds were most likely replaced with the die-cast manifolds of two designations having either the .1R suffix for the Normal 911 or the .0R suffix for the 911S

[Birol]

Thanks a lot Paul for bringing some light in the manifold darkness.

Do you think the manifold was bored ?
Ok, maybe that was the reason for the crack...








Even if I would find a 67 manifold, it will be a difficult to
reproduce the same borings,
two different manifolds won't work.

I tend to use the 'smaller' outlet manifolds,
what do you think,
how much would they affect the engine characteristics ?

[1QuickS]

You could weld but new crack would appear with time, wall is too thin due to boring. I assume the engine is a large displacement with a performance cam. This would explain the large port in the head and the manifold. You can use a smaller exit port in the manifold than the port in the head and unless you need the benefits of matched ports (for racing performance) then you would be OK; if you had large manifold ports and small ports in the heads then that would not be good.

I recommend a new set of PMO manifolds. If you use an OEM air cleaner then there might be some clearance issue since with the sound mat in the ceiling of the engine bay; the "short" PMO manifolds are taller than OEM Porsche equivalents.

Another option would be to clean the area and apply a generous amount of high quality epoxy around the area (surround the runner) and embed some mesh into the patch. JB Weld is probably OK but I like Devcon Plastic Steel Putty for non-fuel immersion applications. I believe there is an aluminum formulation as well that would match the thermal expansion properties of the magnesium. The crack is not a typical crack which would be in the flange at the bolt hole, I am glad to see where the crack is to better understand the issue.

Epoxy is not elegant but is adequate.

Buy some PMO manifolds and put this issue to rest, there are quite a few configurations available to match your Webers to your ports.

[Birol]

Thank you very much, Paul !

The crack is on the bottom end at the mid cylinder on the left engine side.



Difficult to see on this picture.