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Plenum changes -- Transparent Aluminum -- or similar material needed.

bjdecker

bjdecker

Well Known Member
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Hello hive mind,

I am reworking my plenum cover and looking for material that's stiff enough to resist bowing/bending from the incoming air, yet flexible enough to span 4 cylinder heads and absorb their movement relative to each other. Pictures of the current plenum cover below.

I used Van's baffle sides, end walls, and front ramps. The cover is my own design and comprised of a single trapezoidal piece of 6061-T6 0.040" with 2024-T3 0.040" edge reinforcement to capture the rubber baffle material. The top edges are constructed of 2024-T3 0.040" 3/4 x 3/4" angle, with another 3/4" plate to capture the outside edges of the rubber baffle material. The whole affair is held in place with #8 screws front, back, and sides.

The rubber baffle material interfaces with the upper inlet ramps in a 3/4" wide, 3/32" deep step formed into the ramps. Operationally, I see CHT's in the high 200s - 300's and oil temps in the 185-190°F range. Plenty of margin for these hot Texas days, and extended holds at you know where. I am very happy with the cooling capability of this system. However, the flexing (ballooning) of the rubber side covers stresses the stiffeners along and under the top piece and causes them to crack. :(

I'm thinking about replacing the aft 3/4 or so of the rubber top pieces with either aluminum sheet or perhaps fiberglass in the hopes the ballooning issue will be resolved. (aft of the green lines in the pics)

Will 6061-T6 0.040" be stiff enough but not too stiff?
Would fiberglass work just as well?
Maybe someone has access to some transparent aluminum?


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Have you considered adding the angle stiffeners to the inside to stiff up the upper plenum panel as it is pressurized and to limit the billowing effect.
 
PhatRV said:
Have you considered adding the angle stiffeners to the inside to stiff up the upper plenum panel as it is pressurized and to limit the billowing effect.
Click to expand...
Yes - the top piece edge stiffeners are formed with a 3/16" angle on the inside edge.

In addition to changing the thickness of the material (from 0.032" to 0.040"), I have enough room to increase this angle to 1/4" to 7/32", which is next on my list.
 
Carbon is a great route as not that much more expensive than fiberglass. Just do something where it contacts aluminum (a layer of E glass works). I built a carbon Plenum and all the baffling out of carbon. The baffling in the highest temperature areas is holding up great and I don’t ever expect to have a cracking problem.

Make sure and use a high temperature resin for the plenum and post cure it if you go that route. A cardboard box,3 speed hairdryer, and thermometer has served me well for many years. I was post curing some v high temperature resin one time and accidentally got the temps up to almost 400 F before I noticed it. The duct tape had shriveled up quite a bit, but the cardboard didn’t burn. Although that’s getting very close to the flashpoint. That was with the heat gun, which I wouldn’t recommend. So that was an accidental test of the heat resistance of my box method as normally I post cure at 180 F or so max.

For the baffling, I use an extra high temp resin, but it was made for tooling and has to be vacuum bagged because of the viscosity.
PTM&W makes some quality stuff and their engineer would actually talk to me.
 
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Yes, carbon! Stick gloss vinyl to what you have as mould release and lay up 4 or 5 layers of 200g carbon. As mentioned above use a layer of glass (165g will do) where the carbon is attached to aluminium. Hi temp resin isn't really required, I used 285/286 and have not had any problems, and didn't vac bag either - too much like hard work! The heat of the engine will post cure the resin on its own!
 
I’ve been running a carbon plenum that I vacuum infused after making a plug then a mold. I concur that high temp resin or post curing isn’t required in this application. Mine has about 400h on it now with nil signs of degradation.
I’d definitely consider just laying up on top of what you know works.

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That’s how to do a beautiful part above. However, if I’m putting all that labor and dollars into a part, I prefer the correct spec resin. But, my parts were almost touching hot cylinders. (Attached)

(Richard C- what’s your mold made from? Very nice work).

I didn’t properly post cure a stressed (when installed) carbon part one time. It Did post cure after weeks in the Corpus Christi blazing summer heat.
However, it took a set at a new angle and I had to toss the part.
 

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bakercdb said:
(Richard C- what’s your mold made from? Very nice work).
Click to expand...

Thank you. :-)
The plug I formed up doing a layup on top of the cowl. Then made some edges out of foam sandwich. Set that at about 1/2 under the inside of the cowl.
Spent a bit of time making the oil filler recess.
Once it was all done I 2k the plug and polished up.
The mold was just polyester. Cheap and quick.
I did go down the rabbit hole on post curing and high temp resins but it was the middle of Covid and it all got too hard. I thought about making an oven too. I ended up talking to a well respected composite builder locally that convinced me to do the layup in vinylester. I got one locally that has a Tg of 135c so 275F.
Miraculously I pulled that part on the first attempt. It was also my first ever infusion. So beginners luck I guess!
After that sucess I made a set of engine mount bolt covers for the rear baffle. That was trivial by comparison.

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