ID of manifolds for PMO 50's

[stownsen914]

I'm looking to know the smallest ID for the manifold runners of PMO 50 mm carbs.

New manifolds are 50 mm at the top, and I believe they are 42 mm at the bottom. My question is how small do they get in the middle of the runners? I have an older pair that choke down to about 40-41 mm in the middle of each runner and then open up again at the head end. The outer and center runners on each manifold all have the about the same dimensions, top, middle, and bottom.

Do newer manifolds for PMO 50's also choke down to 40-41 mm in the middle before opening up to 42 mm at the head?

Edit - I did call PMO and wasn't able to get a clear answer.

[Gordo2]

I've assumed the manifolds are sized to the carb and intake ports, are they not?

[stownsen914]

Originally Posted by Gordo2

I've assumed the manifolds are sized to the carb and intake ports, are they not?

I was surprised by the measurements of mine. 50 mm at the top, choke down to 40-41 mm in the middle, and open back up to 44 mm at the bottom. They may have been opened up at the head end.

I'm curious what a new set look like. I'm told they are 50 mm at the top and 42 mm at the bottom. I'm wondering what they measure in the middle.

[Gordo2]

How are you measuring the opening / diameter at the middle (small bore gauge)?

I have a set of PMO manifolds in my garage - out of curiosity, I would like to measure and provide feedback, but I don't have a bore gauge and I'm not sure how I could accurately measure otherwise.

Thanks,

[stownsen914]

Thanks Gordo, really appreciate that. You have to promise though not to laugh at my method of measuring! I also don't have a bore gauge the right size to measure the manifold ID. So I found some different size bottle caps that were close to the ID of my manifold at the narrowest portion. I found that one cap that measures 39.5 mm on the OD, passes through the narrowest portion of the runners with roughly 1 mm to spare. Another cap that is 41.5 mm OD doesn't fit. Hence my manifolds are approximately 40-41 mm in the middle of each runner. Substantially smaller than the base/opening which is 44 mm.

[Gordo2]

Originally Posted by stownsen914

Thanks Gordo, really appreciate that. You have to promise though not to laugh at my method of measuring! ... different size bottle caps that were close to the ID of my manifold at the narrowest portion.

GTG, I got ****loads of beer bottle caps in my garage - under the workbench, toolbox, shelving etc. I will fish a few out with the broom and get to measuring

[H-viken]

Originally Posted by stownsen914

Thanks Gordo, really appreciate that. You have to promise though not to laugh at my method of measuring! I also don't have a bore gauge the right size to measure the manifold ID. So I found some different size bottle caps that were close to the ID of my manifold at the narrowest portion. I found that one cap that measures 39.5 mm on the OD, passes through the narrowest portion of the runners with roughly 1 mm to spare. Another cap that is 41.5 mm OD doesn't fit. Hence my manifolds are approximately 40-41 mm in the middle of each runner. Substantially smaller than the base/opening which is 44 mm.

Awesome! Such a pragmatic approach, surely accurate enough to warrant the question about the shift in size from top to bottom of the manifold

[RobFrost]

Originally Posted by Gordo2

How are you measuring the opening / diameter at the middle (small bore gauge)?



I have a set of PMO manifolds in my garage - out of curiosity, I would like to measure and provide feedback, but I don't have a bore gauge and I'm not sure how I could accurately measure otherwise.



Thanks,

With a burette you can repeatedly add a small volume of water, measuring how far up the sides of the manifold it rises each time and thereby calculate the cross-sectional area.

Sent from my SM-G988B using Tapatalk

[svcetiquette]

Just checked some fresh 50mm manifolds I recently received from Richard. The manifold bores are between my dial bore gauges so had to use snap gauges and calipers. I only checked one center bore (cyl #5). Results are rounded to the nearest .25mm, again snap gauges so this accuracy is approximate.

Manifolds are 100mm tall between gasket surfaces:
Height above cyl head Diameter
0mm 41.75mm
4.5mm 41.25mm
20mm 42.00mm
40mm 43.50mm
60mm 45.25mm
80mm 47.25mm
90mm 48.00mm
100mm 50.25mm

[faapgar]

Scott.Those manifolds were opened up from a set of 46mm PMO for your Dad.Of course they would be a little narrower in the middle.Hope that helps.Fred

[stownsen914]

Thanks for doing that svcetiquette. That's really helpful info.

Fred, I did wonder about that - should've asked you in the first place!

[stownsen914]

Well, turns out my manifolds are the shorter ones at about 8 cm tall. I was going to try to work with them until I realized that. I'll be calling PMO to order a new set. Thanks again @svcetiquette for measurements.

[Mark Henry]

Be careful, some have had issues fitting the tall PMO manifold with rain hats in the 914. Some drop the engine mount 3/4" to gain some clearance.
I have less than 1/4" clearance between the right hat and the body at the trunk firewall in a '74.

[stownsen914]

@Mark - good advice there, thanks. This will be in a 911 actually. I will also be contending with a clearance challenge due to a taller filter arrangement, but I've accepted having to deal with it.