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Fabricated intake manifold plenum material thickness?

Turbeam

New member
Joined
Jul 19, 2009
I'm starting to put together a fabricated intake manifold and having almost completed the runners I am looking to make a start on building the plenum before long.

The basic design and layout for it will be very similar or virtually identical to how most other fabricated aluminium sheet plenums are built on here. I had been planning on using a 4mm material thickness.

Before I start, I was just wondering if anyone who has built one of these has ever experienced any issues with it due to the internal pressure? Not cracking or anything like that so much as shape distortion?

I'm only thinking of up to about 1 bar of boost initially, and in the future perhaps up to about 20psi or so absolute maximum.

The plenum's shape will in part be dictated by the space and depth available for it, but rather than it be rectangular/ square edged I'm thinking of forming it more into a tubular shape as far as possible.

Has anyone encountered any such problems with a 'flatter' rectangular section plenum.... I imagine that even at those lowish internal pressures acting on such a large surface area this could be an issue?

Also, what kind of material thickness would be recommended in that case..... might it be a good idea to go a bit thicker on the aluminium plate?

Cheers.
 
The base plate that goes around the runners needs to be thick, we made the mistake of using thin and I kept breaking the welds. I can not remember the thickness we used but maybe later today I can remember.
 
Thanks for the advice guys.

I have actually searched out a few other manifold builds, and on ones such as Nathan's he said he used 0.1" (2.5mm) material for the plenum walls, but on that one the plenum itself looks not only to be beautiful, but fairly square/ round in its proportions, although it does have some quite wide flat sections on its upper and lower surfaces nearer the throttle body end.

On a manifold like mine that is a little bit flatter/ more rectangular by necessity I have some concerns about a thin wall plenum bulging outwards under boost, which won't be a problem with its cylindrical runners at least.

In that sense I wasn't really looking for a manifold spec, but some feedback on how well these sheet material plenums have stood up to the internal pressures over time, especially when subjected to the higher levels of boost that are generally in use when others have gone to the effort of fabricating a new manifold.

If there haven't been any issues reported of these typically slightly better plenum shapes bulging even at higher pressures, then I guess that going up to 0.15 / 0.16" (4mm) thickness should be adequate for the job in my case while giving just a little bit of extra insurance against deformation for only a very small weight penalty. In use this manifold will also be designed to come off and on easily for access reasons too, so a small wall thickness increase may be no bad thing.

I'll be trying to keep the plenum design as square/ round as possible to actually fit in the available space for it anyway.

Many thanks.
 
You should avoid larger flat surfaces as much as possible. Assuming you are planning to run over 1 bar of boost, oil-canning of the surfaces can lead to failures.

I don't know what you mean by "I'll be trying to keep the plenum design as square/ round as possible." Square and round are two different things. Round holds pressure better.
 
Hmmmmm, my memory is getting foggy. Was it you that talked in my intake thread about an inline Six you were going to do? I formed a plenum back then and think its still kikick in around.
 
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Pwschuh..... that is exactly my concern, but I have a restricted height available for locating the plenum and to get a decent volume it might have to be a bit wider, so its proportions as dictated by the installation could be less than ideal, and sheet material is by its nature more inclined to flex in this way.

By 'square' in this sense I actually meant with all four of its long faces of roughly equal width, but still made with square edged butt-welded corners.....as opposed to a more rectangular section in which the two wider faces will be far more prone to having their centres push outwards under the internal forces acting on them.

Obviously a completely round section/ cylindrical plenum will be far better at holding pressure without distortion, as are the true spherical shaped plenums on some very high boost engine plenums.

Nathan, I have been thinking of building a manifold of this type for the inline 6 B6304 engine for some time, and I'm pretty sure that I commented on your intake thread at the time. I'm not certain if it would have been me that you are thinking of or not though as my need would be for one to use on a left hand drive car with a very short engine bay and not much room at the bulkhead end (non-Volvo).

My engine also now has an Eaton supercharger positioned quite close to the underside of the runners and both the compressor intake and output manifolds and plumbing running in the area normally occupied by the plenum. The proximity of a strut brace and even the underside of the bonnet directly above the engine all ads to the limitatioins being placed on the plenum design.

I have a feint memory of you starting to make a whiteblock intake of a similar type, but I thought that you would be concentrating on building he more popular redblock version for which there would no doubt be much more demand.

Are you still producing these intakes? I'd be interested to see some photos of your version of the five or six cylinder version for the whiteblock.
 
Nathan did a nice T5 intake for me - based on a mock up I made to fit my application. too much plenum below the runners for your application , and the entire design would need mirroring for RWD orientation. I'd think whatever he cooked up for T6 would make more sense...

XRintake.jpg


IMG_20110723_085928.jpg

IMG_9251.jpg

IMG_9250.jpg
 
^ You have hit the nail squarely on the head there.

I can't go down on the plenum underside like that as the supercharger is in the way. I can go up a little, but not much. My runners are already being swept back intohaving their centres grouped closer together to let the total plenum length fit in ahead off the bulkhead. To keep the plenum volume up, the only alternative would be to make it fairly wide and shallow.

My basic design will be reversed for RWD, but yours looks to have the main section from the throttle body slightly seperated from the upper area where the runner entries are...... a little like the type where the two sections are separated by a long thin slot to aid even air distribution. It looks like your installation has the opposite problem, plenty of depth but not much width.

Making the manifold almost as two linked cylinders is something I hadn't thought about..... I've no idea how good or bad internal flow would be like that?

I was thinking more along the lines of having the front face of the plenum angled and with the throttle body pointing inwards to try to stop the intake air just flowing over the end of the runners on the first few cylinders. Nathan's other RWD manifolds often have this feature, I guess for the same reason.

I'm not planning to use any really high boost pressures... my calculations usually stop at an upper pressure ratio of only about 2.02:1, I was just curious if anyone else had experienced any problems where by necessity the plenum walls are of a larger dimension than would be ideal.

It sounds as if I'm probably not going to have any issues, especially if I use a 4mm wall thickness.

Nice manifold by the way. It looks like you have also pulled the No.1 runner forward to clear the thermostat housing. At least on mine I don't have any power steering pump to have to avoid.

You might have seen it already, but here is the early mock up stage of what I have in mind.....





Jim
 
I forgot to take a peek for that plenum. Its just an overlong, extra deep D shape. What size runners are you using? I may have some velocity stacks.
 
In my case setting the runners @ 5 degrees cleared the t/stat housing - the cap I had to cut / weld to change the hose offset. I did have to keep the plenum end short due to the PS pump - another reason I elected to go with a tapered "diffuser" style design.

Not much room fore/aft
IMG_3085_zps0d4c4c28.jpg


The lower chamber design does a great job of preventing uneven flow to the ports. This intake alone allows me to continue to make power over 8k rpm - it is a thing of beauty to experience.
I can't see you having any issues with 4mm wall plate.
 
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So whatever design you end up using, try to find an oven big enough to put it in after welding. Your t6 aluminum will go back into the t0 state after welding (which is very weak) but it will begin to naturally age harden up to a t4 state in room temperature over I believe a couple months. This process can be accelerated by throwing it into an oven If I remember right 350f for 8 hours.
 
I forgot to take a peek for that plenum. Its just an overlong, extra deep D shape. What size runners are you using? I may have some velocity stacks.

I'm planning to make the main plenum body with one rounded end, so also a D shape, but its proportions are likely to be a bit wider and shallower than most other ones, hence my slight concern over deformation/ distortion.

I guess ther is only one way to find out.

I'm planning to make some bellmoithing dies which will be used to form the opening curvature directly into the holes cut in the plenum runner-side wall, again because depth within the plenum is an issue. These should themselves help to stiffen the while thing.
 
In my case setting the runners @ 5 degrees cleared the t/stat housing - the cap I had to cut / weld to change the hose offset.

The easy way to do this is to use a thermostat housing outlet from a Plymouth Cricket (or Hillman Avenger) that points in the right direction on a whiteblock motor that doesn't use a P/S pump. My photo above shows one of these fitted.
 
So whatever design you end up using, try to find an oven big enough to put it in after welding. Your t6 aluminum will go back into the t0 state after welding (which is very weak) but it will begin to naturally age harden up to a t4 state in room temperature over I believe a couple months. This process can be accelerated by throwing it into an oven If I remember right 350f for 8 hours.

Thanks for that. I'm not sure that the completed 6-cylinder manifold will fit in a normal domestic oven, but it might be possible to do something similar by sitting it inside a closed barbeque or something?
 
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