Port matching PMO manifolds
 

After discovering that my prior set of PMO manifolds were not the correct ones for my application, I just picked up a pair of PMO's tall manifolds. The largest intake port size they make manifolds for is 42 mm (actual size 41.7 mm). Application is a race 3.5L running PMO 50's, intake ports are 44 mm. Richard says don't open up the manifolds, leave them at 42 mm. I know you want to be careful not to make the manifolds larger than where they mate to the heads, but I find it surprising that it wouldn't be beneficial to open them up to 43 or 43.5 mm.

Thoughts?

Following


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Port Velocity.
https://www.youtube.com/watch?v=mAa4SVYZ3WE

My 0.02. Yes, match them. Just make a smooth transition from the manifold to the port.

Now there are formulas you can use to calculate velocity. But what does that mean?
(yea I watched that video). Take a 911 motor that has the same sort of tune as what you are aiming for as an example. Calculate the maximum intake port velocity on that motor. Then compare that to your motor and the port velocity you have with both the un-enlarged manifold and the port matched manifold. Then decide if the manifold as supplied will be a restriction or not.

I have found, on a couple of motors I have built, that matching the manifold to the port, but not enlarging the diameter of the manifold will increase the velocity aiding midrange and response, and possibly slightly reducing top end power. You just want a smooth transition throughout the manifold and port.

For example the PMO manifolds I have on my 911 with a 3l measure 40mm at top and 39mm at the port, but are 37mm in the middle.

In the OPs case with a race motor you have to decide where you want the most power, so maybe opening up the manifolds so they taper evenly from top to port may give the best top end power. Depends on intended RPM. You may want to talk to your cam grinder.

I would not port them. a 2mm ID difference leaves a 1mm step around the perimeter. This help with reversion and will run cleaner at lower rpm. 42mm is large enough for flow of a 3.5 making 100 hp/l.

Now you may want to take a measure the ID bore above the manifold. Potential gains if they are too small and are opened up.

Thanks guys. Watched the video. Interesting info, but not sure it answers my question "given my intake port size of 44 mm, even though it's a bit large, which is better - opening the intake manifolds from 41.7 mm at the base to 44 mm, or leave them as is?"

Richard said leave them alone, that the 1+ mm step around the perimeter is actually desirable. I found that surprising, so wanted to ask here.

A little more info. about the new PMO intake manifolds as delivered:
- 50 mm at the top, tapering gradually to 41.7 mm at the base where they meet the head (they don't choke down any smaller in the middle like my old ones do). I did use snap gauges to confirm the gradual taper.
- I measure several runners (side and center runners) to confirm consistency of measurements. They do appear consistent.

To be clear, if I were to have the bottoms opened up closer to 44 mm, they'd need to be opened about 1" into the manifolds as well to maintain the taper.

I think it comes down to whether it's preferred to have this 1+ mm step around the perimeter at the intake manifold / head interface, or not.

I think the theory is that the sharp step will reduce reversion. Not sure if that is a thing and if its only for carbs.

Well, I'm getting a little education on reversion in the intake tract and ways to counter. I see why leaving the step on the manifold side can be desirable.

Some of the reading (including comments on this thread) suggests that reversion may be a bigger deal at lower RPM. Some say that for racing use removal of the step to favor flow at higher RPM is preferable. Seems like it can depend on a number of factors, and its value can vary depending on engine, intended use, and other factors.

Curious if anyone has any direct experience with racing 911 applications to share.

As well as reducing reversion, wouldn't the step aid vaporisation of the fuel attached to the manifold walls by disrupting the fuel sheet flow and reintroducing this fuel back into the air flow?

However the question is really is are either of these effects measurably beneficial?

I suspect that if you mocked up the carburettor, manifolds - both standard and ported - and cylinder head on a flow bench the difference in flow would be undetectable.

Since no one has bought any real data to the table, I think Richard Parr's advice/experience probably stands.

What will your redline be?

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Expected RPM range 5000-8000, maybe as low as 4500 sometimes.