I have an extra exhaust manifold that I have been experimenting on and I have decided to acid port it. I have done the standard lip porting on the manifold that is on my engine now, but I am convinced that the 2g stock exhaust manifold can be ported out to FP casting size runners, if only you could get a porting tool INSIDE the runners... Nothing I have used worked very well, die grinder wise. I also tried a homemade "extrude honing" mix I whipped up with silca sand, clay, and old motor oil. It was effective, but it would have to be automated. It'd take many hours to get much done. I learned something from the older super stock racers though. Acid porting used to be the best way to port out intakes and exhausts because it'd get to places tools couldn't. Plus, it'd leave a stock looking cast like appearance, so you could cheat the super stock class rules some.

I'll post some pictures when I get started. Anyone have any prior experience with this? I'd love to get a tip or two, just so I'm not totally guessing.

I said I'd post pics when I started. It took a while, but I finally got around to it. This is my manifold I am porting.


It is currently bolted to a piece of steel plate thinner than the casting, and sealed with RTV. I just proped it up level, then started the acid treatments. The first was hydrochloric acid. I used it because it quickly disolves the exhaust deposits and cleans the surface of the steel. It looked brand new inside afterward. I then filled it with sulfuric acid. Sulfuric acid directly dissolves iron much faster than hydrochloric acid does. When I put the sulfuric acid into the manifold, it was water clear. This is what it looked like after the first wash, which it soaked for a few hours.



It is black because of the amount of dissolved iron in the solution now. It doesn't remove vast quantities at once, but it does remove it evenly, and does so in places you could not port by any other method except extrude honing... And that costs a hell of a lot more than this does. Grand total for what I expect to be about a 3/32" port job through the entire manifold was the cost of the manifold and ~$20 for the acids. I also don't have to watch this too closely, and can do other things while waiting for it to be done... Something you can't do with a die grinder. The process is slow, but effortless.

Where did you purchase your acids?

Off topic but to me, it is mind blowing that a acid strong enough to dissolve an entire human body is not a highly controlled substance.

Very nice , an old , very old hot rod trick, when you get it to almost where you want it, start dilluting a bit for smoother surface, it is going to take a good while,
The acids are readily available even at pool supply companies, if you can get it at a chemical distributor, cheaper and stronger.

Show pics,

Yes more pictures! More lighting!

Ok, next batch of pictures I will get some light and a good HD camera.

Arrowhead nailed it... This is old school trickery that is still a viable, yet rarely used method of porting. I learned about it from an older hotrodder and super stock racer who did it back in the 70's. It is a MUCH faster process with aluminum, which is primarily what they were porting. The intakes on big V8 engines back then were choked down, and in super stock, you can't port anything. It's against the rules. So to cheat a little, they would acid port the aluminum intakes because the acid left the appearance of the cast finish, thus sneaking the port job past tech.

I expect the process with my manifold to take several days. The reason is steel doesn't dissolve quickly in anything short of hydrofloric acid, which will dissolve glass, and everything else it comes in contact with... It is exceptionally dangerous and nasty stuff. The acids I am using are commercially available, but you will normally have to ask for the sulfuric acid at a Napa or O'Rileys. It's battery acid... They just don't keep it on the shelves anymore because it is also used for whipping up drugs that will have the DEA kicking in your door. It is NOT illegal to buy or possess, and if you are using it for porting, there is absolutely nothing to worry about.

Also, just so everyone still feels all warm, fuzzy, and safe... it isn't going to melt any bodies very quickly, unless it was a LOT more concentrated... And if it did, don't you think there'd be a whole lot more stories and CSI type stuff about it? There are much faster ways than acid, I'm sure. Fire is the first that comes to mind. Sure would be a lot cheaper than a 110 gallons of reagent grade sulfuric acid... Just saying, but, uh...... Not from personal experience or anything.

Can this be done on a stock 1g intake manifold?

Yes, but the process for aluminum is different. Aluminum dissolves a whole lot faster, and it is usually just done by short washes of acid, followed by immediate neutralization with baking soda. Do not use sodium hydroxide as a neutralizer, as it will also eat through aluminum. You have to be more careful with it, or you'll eat holes right through it if you forget about it for very long.

Strong acids will dissolve glass, but certain plastics can hold it. That's why most strong acids come in plastic containers.

If you would like to speed up the process, try applying heat using electrodes. That's what's used in creating porous surfaces before plating.

If you want to create an auto-timer, you can use a piece of metal the same depth you want to remove and of the same type of metal to seal in the acid. When it eats through that piece, that's the depth it has dissolved, and it will drain.

That is exactly what the plate it is bolted to is for. It's 1/8" thick a point just at the bottom of the manifold discharge hole. Once it eats through that, it will drain itself, and then I will neutralize it. Then I will run a ball hone down thru each of the runners to clean it up and get out any deposits of metal that are loose. I expect that to remove the final 1/32" of material, thus ending with a 3/32" total porting of the entire manifold.

Alrighty, as promised, more lighting and close up with a higher resolution camera. The manifold looks gross... Like something you shouldn't touch... The gummy looking stuff is Vasoline, used to protect the flange face.

There was a lot of material removed over the course of the day though. This is a 5 gallon bucket 2/3rds full of water... You can see how black it is from the dissolved iron.


Some sort of weird crystal formation has occured inside also... Never seen this before.


But.....THIS is how much material has been removed, just in retained solids not dumped out when I poured the acid down the crapper. This is the heavies that stayed in the bucket.


Acid porting is no joke. It may be old school, but sometimes old school is the best way.

this is a very interesting method. i have a few spare damaged manifolds and what not im going to play around with and see what happens. thanks for the input

I never did update this... I finished the acid port at 1/16" wall removal. I didn't want to go any further because I stuck a set of calipers in the runners and decided the risk of cracking would be too high if much more was removed. At completion, it was about the same size as the Evo manifold runners, maybe a tad larger. The whole process took a week total, with one day just being neutralizing the acid with a bunch of baking soda in water. I then ran a small wire wheel down the runners to clean off any loose scale still remaining. It retained the look of cast metal, but was slightly smoother. I then ported the mouths of each runner to match the gasket, and also ported out the discharge to a shoulderless 7cm size turbine inlet. I have been running it for a few weeks now, and it is a definite improvement over the stock 2g manifold according to my dynobutt, but not as noticeable as a tubular or FP cast would be. The discharge is still the stock hard right angle into the turbine, and no amount of porting will fix that. For a cheap upgrade though, it was worth the time and didn't require much actual labor.

Awesome thread man cool to see something "new".