Air Box Pop Off Valve

[Jase77]

Hi Fellas, I am nearing the end (hopefully) of my engine rebuild and I would like to hear your opinions on the Pop Off Valve for the airbox. I have to replace my old air box as it had a badly repaired crack in it. I have the new airbox and it has a kind of diffuser feature in it which I believe was a factory update from around 1980. From what I have read this is just another reason why a Pop Off Valve shouldn't be needed. My old air box had a valve fitted and the engine never back fired in 3 years of ownership, so I guess that the C.I.S was calibrated fairly well? I have read recently that if your C.I.S motor is running correctly then it shouldn't need a Pop Off Valve fitted at all. There are also lots of stories about valves failing etc. So, should I fit a new valve into my new airbox for added piece of mind, or should I concentrate on setting up my C.I.S properly and keeping the valve on the bench? This will all be fitted onto a freshly rebuilt 1977 2.7. To say that I am crapping it about that first start up procedure is an understatement and I would rather not have to be worried about the airbox being blown to pieces as well. As always, your comments, opinions and advice is always welcome. Cheers, Jason.

[Jase77]

Thought I would give my thread a bump, this time without the waffle.........

Airbox 'Pop-Off Valve', Opinions please.

Cheers.

[Flat6pac]

It’s an insurance item, if you were to have a backfire would the expense of the valve out weigh the cost of the air box and install on the engine?
Bruce

[Jase77]

Originally Posted by Flat6pac

It’s an insurance item, if you were to have a backfire would the expense of the valve out weigh the cost of the air box and install on the engine?
Bruce

Fair point Bruce.
Cheers

[boyt911sc]

Jason,

If you ever decide to install a POV on your car, don’t make the mistake of installing it without testing it. Some of these POV’s made by URO are defective or leaking not all but some. Regardless of brand name, test before installation to save you from some aggravation. There is a simple way to test to do it:







Below is the picture of a brand new POV fresh from its packaging that does not seal. It failed the test and should never be sold. The POV (good) with yellow sticker is an old used POV and could still hold vacuum.



This is about the POV itself. Installation is another story. The instruction sheet that cones with the new POV says to drill a 2” hole and glue it. This will work but not the best option for installing the POV. There is another option to improve the installation and promote stronger adhesion between the air box and POV. I will post the pictures later. My wife is calling me and it’s dinner time.

Tony

[Jase77]

Tony,

Thank you very much for the info and advice. You're right that the installation is one thing but the choice of bonding agent and method of curing it is a whole different kettle of fish. I look forward to seeing your recommendations about the installation.

Hope you enjoyed your dinner?
Cheers.
Jason

[boyt911sc]

Jason,

Sorry for the delayed response. Been busy with the Engine Rebuilding Sessions preparation. I have attached some pictures regarding the revised POV installation procedure.

Below are the conventional POV installation procedure:








Below are the pictures of the Revised POV installation procedure:








Aside from drilling the 2” hole for the conventional POV installation, trimming some small section of the ribs of the airbox allows the POV to sit lower and producing more surface contact area between airbox and POV. The extra surface contact area produces stronger and better adhesion.

The shoulder or collar of the POV comes in contact with the airbox at the rib contacts and at a small circular ring using the conventional method. The contact area is minimal at best. So to obtain a stronger and better adhesion between the POV and the airbox, additional surface area is needed. I have been doing this installation for a long long time and been keeping it to myself. Time to share this experience with fellow CIS enthusiasts.

For those people who have the time and interest into this trivial subject, measure and calculate the surface contact made by the conventional procedure versus the revised procedure. The difference is more than 4X.

Finally, POV’s should be tested prior to installation. I have encountered several new from the box POV’s that we’re defective (not sealing). Whether you do the conventional installation or not, check the POV before installation.

Tony

[Jase77]

Hi Tony,

Many thanks for the great info and recommendations. I can see how this method would help to give a better bonding area and I will be doing the same thing on my airbox.

Would you specify a high temp bonding epoxy? My POV didn't come with any and I am planning on using some Scotchweld DP 490 which is good up to 120 degrees C. Would you think this is enough?

Cheers,
Jason.

[Jase77]

I drilled and prepared the airbox to Tony's specifications today and it's very obvious how much more of a bonding area will be available. Grinding down the fins on the airbox is pretty quick when using a Dremel.
This was done with the airbox off the engine so access to these areas is very easy. Attempting this with the airbox still fixed to the engine and in the car would be possible but it wouldn't be very comfortable, plus the grinding creates a lot of plastic swarf so keeping things clean would be a hassle.

Cheers, Jason.