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a person who can pressure weld can make welds that are nearly flawless and are Extremely NON-Porious...

it's not a type of weld so much as a quality..

noone can truely understand PSI without having seen an intake manifold explode from shawdy welds...or something else blow up from a relatively low PSI... <30...

(the F&F reference was somewhat correct in that aspect)

Cy is very correct, be careful how your parts are welded, especially the intake components as they will rupture very violently under pressure if incorrectly designed or welded.

Pressure testing is what your refering to along with the porosity of the weld. Porosity can be contributed to contaminates and wrong filler metal selection along with the improper type of CFH flow of purge gas. To little purge gas (guys try to save money by cheating shielding gas) will allow the hot metal to oxide and become not only porus but very brittle as well. We weld with a Lincoln Precision TIG 185 and sheild with a little above average CFH flow of pure Argon. The TIG process is technically acknowleded as GTAW (Gas Tungsten Arc Welding) and is generally the stongest, cleanest weld possible. The best weld possible would be from GTAW PLC controlled robotics with an electron microscope equiped lab checking the intial welds before running production parts. Over the years they have found out that Pulse GTAW is the best way to weld yeilding the smallest most uniform metal grain structure possible by welding. Pulse GTAW welding is what I use and I absolutely LOVE it. It allows you to controll your weld speed so you can pay more attention to where your welding to instead of awwww crap the pools about to drop through. The Pulse is basically a circut that oscillates out put voltage base on a sign wave frequency. With out robotics this process is a hands on learing experiance. Lowes carries a good book for learning the basics about welding and the different welding technologies. Another really good place that also has technical data and instruction is http://www.lincolnelectric.com

My car has a TON of our GTAW welded parts that have been pushed to 30psi on a pretty big turbo repetitivly for over 10,000miles of torture, has spiked to nearly 40psi before due to AVCR control line blowing off and still no problems.

P.S.

Pressure testing is commonly done by capping off the ends of a welded product and the pressurizing with air or another gas to a predetermined pressure and then data logging the time it can sustain that given pressure with no loss.

Hey...I welded all kinds of stuff and I do that every day for work....
You just have to make sure you don't have any holes and you'll be fine
40psi isn't that much...do one drop at a time and make sure you melt it good befor you add filler