Carburetor Plenum for Automotive Induction System

Here’s the next chapter in my lost foam casting projects and of course, it involves more template guided foam machining on my pin router.

It’s a carburetor plenum and manifold adaptor for a vintage automotive induction system. The intake base is a contemporary EFI casting I’ll modify and adapt.



I’ll skip over the induction system design details but as a first step the features from the mating intake manifold and carburetor base were enlarged to allow for shrinkage at the respective mounting interfaces. Beyond that shrinkage wasn’t much of a consideration. Each half of the plenum shares common interior and exterior features that all need to align so I made a set of templates to make reproducible patterns for the individual foam pattern pieces. The router jigs pictured below are kind of interesting for this one with a few new twists. I’ll document them with another installment over at my machining foam thread, haven’t gotten around to writing that one up yet.



Interestingly enough, these two plenum parts are actually easier to cast and finish as a one-piece casting in lost foam. It would be just the opposite for conventional sand casting. The individual patterns were not very rigid and were difficult to keep flat after being removed from the router jig even when freestanding. I was going to hot-glue on a piece of sheet metal channel to stiffen the part for molding as a chaplet of sorts.

When I was handling the two pieces as they would be assembled together I noticed they were a lot straighter and rigid as a single part so I decided what the heck…..I’ll just glue them together, cast them as one, machine the mounting surfaces and holes, then cut them apart. To give myself some machine stock I cut a one-inch spacer from one of the templates and glued it between the upper and lower plenum halves. I hope to use the section I cut out as a clamping template for cutting plenum gaskets or plenum spacer. -Worked a treat. After the engine development and testing work is complete, I may not even need to cut them apart, but it does allow me to alter the plenum height/volume with spacers and gain access to the plenum interior for finishing if so desired.

Then comes the gating and flask. Since the plenum was designed to flow air from carb flange to the base, and since the carb flanges are more massive than the remaining typical ¼” wall, it seemed natural to gate into the carb flanges and fill the part top to bottom. Only thing I was concerned about was those thin ribs between the runners in the base not filling since they would see the coldest metal. It looks elaborate but I just made a runner to connect the gates to a central sprue. The space between the gates was machined away in a minute’s work on the spindle sander with a coarse sleeve, chased the perimeter with a round over bit on the router, and then just glued on.



I also mudded the halves and left them to dry sitting on a flat surface before sticking the two halves together. It would be a good candidate to be dipped as one piece.



For the flask, I just knocked together an mdf box with a couple studs to mount a turbine vibrator….a 20 minute job.



Time to Cast.

Best,
Kelly

 

Onto casting, same old routine



....….I’m on a roll! So in one whack I got the upper & lower plenum, a gasket cutting template and/or plenum spacer or for engine performance and dyno development work. -Worked out pretty slick.



Here they are de-gated and cut apart



…and nailed the shrink



Havin’ some fun now….



Best,
Kelly

 

Impressive...
What shrink factor did you use??
Ive been going with 1.4% for 356 in petrobond.

 

Next i want to see you reproduce those carb bodies... lol

 

.013" per inch.....1.3%. It's 356.

I bought the phenolic spacer when I bought the intake. The second photo is the shrink allowance; about .2" over 16".



Best,
Kelly

 

It will happen but may not be in LF. May as well. I've reproduced everything else for them.

Best,
K

 

Thanks Mark. There have been more than a few head scratching moments for me and my foundry and pattern maker friends on this lost foam journey. I feel like I started a cross country drive, got off on the lost foam exit ramp, and never got back on the highway. One other thing that's sort of amazing, although I've had defects, I'm yet to have a shrink defect in a lost foam casting. On this one, the carb flanges are way more massive than the rest of the casting....... but no problem whatsoever, and these things are poured hot, as in high 1500sF.

That pour was all my little ole A10 could do. It wasn't brimful but easily within 1" of being so. That little dribble in my ingot tray is all that was left. I figure the casting was probably ~6.5 -7lbs and the gating and cup another 4lbs. -Almost had to break out my A20.

Best,
Kelly

 

Sort of depends, but if you can make the foam pattern to your satisfaction and only want a small number of parts, it’s pretty tough to beat, especially for one-offs, just putting metal where you want it, and especially if those parts would have complex coring and/or parting lines. I think it’s probably the easiest and most economical point of entry to the casting hobby for the backyard gamer and a subject that actually warrants its own thread…..like here.

http://forums.thehomefoundry.org/index.php?threads/lost-foam-pros-cons.120/

Best,
Kelly

 

to

That's another one of the quirks of lost foam that defies conventional casting wisdom. Once you get the metal front settled down after the initial foam contact, it just sits on top the foam and devours its way through the pattern replacing space once occupied by foam with metal....while the sand magically just sits there. If it can flow continuously from top to bottom without separation and rejoining, top gating is for sure the way to go and gravity is your friend. You just need a sprue and gating system that can insure the part is never starved for metal. For complex parts that will always have to knit, a central feed minimize travel length is usually more advantageous. In all cases and to the extent possible, one must avoid undercuts that can become difficult to pack...."leakers" as Bonz calls'em.

Best,
Kelly

 

In general I'd say LF is a practical approach for such a one-off and although certainly not impossible, I'd say taking on an engine build from scratch is a pretty (too?) ambitious first casting project. Are you also an accomplished machinist and engine mechanic? If it was just making a set of finned cylinders to accept a bigger bore that's realistic. Is it a two stroke or overhead cam four stroke? Air cooled or water cooled?

The sand media will influence the casting finish. Finer sand generally means better finish and less venting capability. In lost foam, it's typical to coat the foam pattern with something like drywall mud and this provides good finishes while allowing the use of common inexpensive sand for mold media.

Probably would need to see the proposed part before comment on that.

Cant help ya there.

Best,
Kelly

 

Johnef, start a separate thread in this sub forum about the potential/feasibility for producing LF engine castings you envision and I'll comment. For starters, include the approximate size L x W x H and approximate weight of each casting you envision.

Best,
Kelly

 

Thanks Chancre, very kind of you to say.

That part of the casting is certainly not my property but also, the parts are not for sale...just my own personal tribute as a Ford motorsport enthusiast, so hopefully forgiven for that purpose.

Best,
K