Intake Manifold Question???

[garycox]

The intake manifold shown in the upper photo is on my 68 that was rebuilt to 2.7L RS specs according to the builder and PO. This manifold is larger and taller than the manifold on my 67 911S, shown in the second photo. So what do I have on the 68?

Second question, is there a factory type air filter that will fit this application? The 67 air filter housing will not fit (the manifold is too tall).

[Steve@Rennsport]

You have the normal aluminum PMO manifolds; the internal diameters do differ so thats something to check.

Obviously, the lower pic show the OEM magnesium ones,...

[andyjboy]

The PMO manifolds also come in different heights - both sizes being taller than std original Factory magnesium manifolds :

Original Porsche manifolds - 68mm tall

'Short' PMO manifolds - 82mm tall

'Tall' PMO manifolds - 98mm tall

Original Porsche 911R/906 manifolds - 98mm tall

[garycox]

What is the gain with the taller manifolds?

[andyjboy]

Essentially there are 2 benefits to using taller manifolds:

1) Fuel ‘Percolation’ – when the engine gets hot & then sits idling for any length of time or is required to run at low (road) speeds, heat soak conducted from the cylinder heads up the inlet manifold can lead to fuel boiling in the carbs. Therefore, moving the carbs further away from the heat source (heads) can minimize this problem.

This is why you should always install the Factory (or PMO) phenolic ‘isolators’ – basically a thick insulating spacer – between the manifold & the heads for the same reason.

2) Fuel reversion – engines with more aggressive cams like 906/RSR which have lots of overlap can suffer from fuel reversion at low-to-medium revs caused by pulses up the intake which hit a resonance and create a cloud of fuel droplets floating above the mouth of the carb.

By making the intake track longer the frequency of the intake no longer matches that resonance and so the cloud disappears - at higher revs this tends not to be a problem since the air/fuel mixture is traveling at a higher speed into the cylinders.

The main reason Porsche used the shorter manifolds for its production models was due to the need to incorporate an effective air filter (& more to the point intake noise silencing) assembly which, in turn, limited the available (vertical) space.

The above is a very simplified explanation & many other factors influence the intake pulses (& power/torque figures) – including header sizes, etc.

[garycox]

Thanks for the responses.

Gary

[1QuickS]

Check out the sectioned aluminum manifold (SWB cars); it shows an obvious change of section of the internal passageways for the outer two barrels. I can't vouch for IDS manifolds but the magnesium ones look pretty good compared to this.

Paul Abbott
www.PerformanceOriented.com

[H-viken]

Essentially there are 2 benefits to using taller manifolds:

1) Fuel ‘Percolation’ – when the engine gets hot & then sits idling for any length of time or is required to run at low (road) speeds, heat soak conducted from the cylinder heads up the inlet manifold can lead to fuel boiling in the carbs. Therefore, moving the carbs further away from the heat source (heads) can minimize this problem.

This is why you should always install the Factory (or PMO) phenolic ‘isolators’ – basically a thick insulating spacer – between the manifold & the heads for the same reason.

Reviving this thread - can anyone recommend a good source for insulation spacers (for the magnesium 911R/906 manifolds / Weber 46)?

Johan

[chiroracer]

FWIW I have found different internal profiles for the early mag manifolds one has round runners all the way through one(shown ) has the elliptical outer runners. I have used the shorter PMO manifolds with the phenolic spacers and stock airbox with no clearance issues. Do you have the AC condenser in the lid ?