Replacing the ar-box 911SC?? Anyone done it?

[Pan]

I've heard they are prone to exploding when the car backfires. Thought about going for a pop-off valve, but I think the airbox is a big ugly thing, so thought about replacing it.



Has anyone done it?
Any pics?

[Oh Haha]

I believe to get rid of the "airbox" youwould need to replace the entire CIS system with another form of fuel injection. TBitz has a great setup to do this although it is a few $$$$/

If you want to just replace the cover with the snorkel then there are a few different options out there, including running just a filter, holding it in place with the straps. This is how I run mine. No performance gain butit does cleanup the engine a bit.

if you keep the factory setup you MUST install a pop off valve ASAP. ifnot, you'll be searching for the entire box, which can get pricey.

Here is a good thread with pics to give you some ideas.
Show me your modified 911SC motors...

[James Brown]

There are several options to the CIS system you have now. Just about every option requires you to remove the engine (unless your a real good mechanic). the CIS system is not a good option to modify. you can clean it up and put a pop off valve in it but it still will be the same. Remember it's almost 30 years old and most of the hoses are leaking/cracked/brittle or heading that way.
Other options are back dating to carburetors, about 3-4 thousand (more in AU) for the change, go with a electronic fuel injection like one off a 3.2 engine. The look is cleaner and better fuel mileage than the carbs but it's not a "kit" so you must obtain all the parts and bits for the changeover and it's computer controlled. Yet another way is to go with a aftermarket fuel injection like Bitz Racing has to offer.
Any way you look at it, it's pricey and difficult. AND i'm not going into all the other mods (exhaust, engine mods, cams, ect.) They all add up.

[tazzieman]

I read the airbox/pop-off valve section in Peter Zimmerman's used 911 story book with interest. He is not a big fan of them. He mentions the improved airbox of the 81-83 models which prevents the problem. Apparently this newer style is identified by Philips head screws instead of flat bladed ones holding the airbox together (under the air filter). Mine has these and I presume indicate a newer style box has been installed - though perhaps just different screws were used during the engine rebuild?
Was wondering where the parts number is located on these newer airboxes?

Pan you might like to check your screws! In the meantime keep an extinguisher onboard!

[47silver]

i just replaced an air box and i had a pop off valve. If you car has a mild back-fire it may loosen the screws i think on any of the cis equipped engines.

Nothing beats a perfectly tuned car which should not back fire, but if you get a hose leak it may cause a back fire..
A pop-off valve is cheap insurance and i dont know a downside other than if it is not installed correctly it may leak or not open the correct way.

An air box new is over 500 bucks used are cheaper but there are a couple of different intake runner sizes so you have to match them up.

I would put in a pop off valve and not worry about it. Waynes 101 things to do on porsche gives a very good write up on how to do it....

x

[john walker's workshop]

if the screws are loose or 1/2 way out, that means the seam has separated during the backfire, and you are probably drawing unmetered air in. if you install a pop-off valve be sure to rough up the slick plastic around the outside so the glue gets a grip. plenty of them pop right out again if you don't. i use a wire wheel on the bench grinder. once over lightly.

[nineball]

i attempted to install a pop-off valve a week after i bought my car only to find that the airbox was already cracked. the most recent version of the 38mm contains the "update", lines run inside the box, but the original box i removed did not. our host sells what appears to be the updated version based on the pic displayed. i also did the while you are in there method and replaced a few grand in parts including a new pop-off valve since i had it all out. runs great now. well it did before winter...

[bxtech]

There was one company years ago that was making aluminum replacement airboxes. If I remember correctly, they were pretty expensive and they didn't sell exceptionally well. That would probably explain why they are fairly rare today. However, I have seen a few for sale every so often.

[Sabre]

I have a SS box that I got just in case I needed it when I decided to buy a 81SC. If interested, PM me. Sabre

[VFR750]

Originally Posted by tazzieman

I read the airbox/pop-off valve section in Peter Zimmerman's used 911 story book with interest. He is not a big fan of them. He mentions the improved airbox of the 81-83 models which prevents the problem. tazzieman

Actually, the change in 81 was to improve the cold start, by spraying the fuel into each pipe using a manifold in the intake housing, verse a single spray at the back of the inlet housing.

Unfortunately, this isn't always successful. Start, and cold backfires happen once in a great while.

Thankfully, the pop-off valve functions as designed. IMHO, I'd add the pop-off valve ASAP.

[Pan]

Thanks for the responses guys!
Will look into some type of replacement filter, I think...

[FinallyGotOne]

I'm just curious, if the PO (Previous Owner) did not install the Pop-off valve, did he at least do the carerra chain tensioner upgrade? Or is that not needed on an '81? isn't that a more important item to address if it's not done yet?

[nineball]

it's "needed" on all pre-84 models i believe. the update is a great thing to do, because it's not a question of IF your old style will go bad but rather WHEN. that being said if the old ones are still working there is nothing wrong with driving them.

[Richard Burns]

If you replace the airbox with another type of air filter you will need to make sure that you can fit the oil breather hose somewhere. Failure to do this will result in a very messy engine compartment as the fumes and some oil are usually sucked into the inlet system. Your current fitting is located on the drivers side of the airbox (ie for those of us who drive on the left hand side of the road).
Regards,

Richard.

[Pan]

Originally Posted by FinallyGotOne

I'm just curious, if the PO (Previous Owner) did not install the Pop-off valve, did he at least do the carerra chain tensioner upgrade? Or is that not needed on an '81? isn't that a more important item to address if it's not done yet?

Previous owner did it!
Thanks for the message though!