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Pop off valve on a turbocharged 911SC

I have a 1980 911SC Targa with an older BAE - Rajay Turbo kit installed. Like all SC's, it does backfire on a regular basis. The car has the standard airbox. I've been given different opinions on this but given the set-up, should I still install a pop-off valve to protect the airbox?
Old 06-08-2011, 12:38 PM
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No, it will open the first time you come on boost. I had a hard time finding a box without pop-off for my 2.4L project. Now I have one of the stainless aftermarket types. Still had to weld it's pop-off contraption closed.

Last edited by copbait73; 06-08-2011 at 05:14 PM..
Old 06-08-2011, 04:48 PM
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i used an expanding plumbers pipe plug, used for pressure testing they work perfect are cheap and easy to install.

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Old 06-08-2011, 06:30 PM
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You have to close it, but be aware you more than likely will loose a box. The last one I built I actually made a box out of sheet metal. Unless you can install some type of pressure valve that is above your boost levels but below the backfire pressure you are going to have issues. I was never successful in fully eliminating the issue and built a box that wouldn't split.
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Old 06-09-2011, 07:27 AM
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Gentlemen, thank you very much for your input!.....I am now more informed. IAMCHAPPY it sounds like you have a great solution to the problem. It would appear to me that if one were to experience a pretty hefty backfire, that your plug would "give" very much like a pressure-relief valve, before the airbox breaks.....has that been your experience?
Old 06-09-2011, 01:55 PM
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I have a stainless steel airbox. I blew up the factory plastic one on a backfire after I installed the turbo. This is because I put in a plumbers plug, as the boost pressure was opening the standard popoff valve, and cinched it down pretty good. It was in too tight.

I also blew two seams out of the SS airbox...same reason. So, finally, I welded the SS box back together, coated all of the seams with JB weld, and pressure tested it in a big tub of water. I put six plumbers plugs in the outlets for the manifold runners, plugged all of the other holes, and rigged up a fitting for my air hose from the compressor.

Test was this...put in air pressure at whatever low PSI (I only run around 6PSI boost). The plumbers plug (the IMChappy method) is not tightened down too hard during the test. So, for example, if I put in 5 PSI and the plug blows out, I tighten the wingnut incrementally. I repeated this until I could hold 15 PSI without blowing out the relief plug.

Now, I set my regulator to 50 PSI ( I had a hard time figuring out the equivalent pressure a backfire pressure wave would carry, and I don't remember what it was), with a ball valve in the system, such that I could simulate a pressure wave. You'd be surprised how far out of a tub of water and across the lawn you can launch a plumbers plug doing this. I think I was blasting the plug out at about 100 PSI after awhile.

Anyway, I now have my right hand calibrated to tighten the wingnut properly to contain the boost pressure, and still allow for popoff. If I was smart, I would have put on a regular hex nut and tightened the plug with a torque wrench.

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Old 06-11-2011, 06:52 PM
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Thank you very much PATKEEFE for the input and also the photo.....very nice!

From what I understand, you also started off with the BAE kit, including Rajay turbocharger. Though I have owned the SC for 20 years with no modifications done, now that I have more time, I would like to now enter into that domain...like so many other fellow Pelicans, I would like to "add stuff" to improve performance. Currently my set-up is the basic BAE, with Rajay turbo and simple BOV. Would it be possible for you to post a simple diagram outlining your turbocharger set-up, detailing the individual components (wastegate, boost controller, etc.) installed and a brief description of what each does? (I'm new to this Turbo stuff)......much appreciated!

Alex

1980 911SC Targa
2006 997 C4S Coupe
Old 06-13-2011, 12:12 PM
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One last question....does the BOV not also act like a pop-off valve when there is a backfire?
Old 06-20-2011, 08:48 AM
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Alex:
No, the BOV doesn't help in case of backfire.
Post some pictures of your installation, and we can tell you what you have.
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Old 06-20-2011, 10:10 AM
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Patrick:
Thank you......I will post some pictures this evening or tomorrow.
Old 06-20-2011, 11:21 AM
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Here are some photos of my 911SC Targa BAE/Rajay turbo set up:



BOV



Rajay turbo - from above



As I had mentioned before, this is the original set-up as it was installed in the early 80's......Rajay T3577 Turbo / BOV. What should I do to "improve performance" without breaking the bank. The SC is basically a Sunday drive....I don't intend on tracking or racing it.
Thanks!

Alex
Old 06-21-2011, 08:35 AM
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Add water injection, basic remote wastegate and what does it have for a muffler?

Last edited by copbait73; 06-21-2011 at 08:47 AM..
Old 06-21-2011, 08:45 AM
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Copbait73, it has the original BAE exhaust system...circa 1983......
Old 06-21-2011, 12:10 PM
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I assumed that to be the case.

Whatever boost level BAE set your engine, it tolerates it well. I wouldn't change that (what is the boost?). However controlling boost on the compressor side is very inefficient and only done because it was very cost effective back when.

Adding one of the many modern exhaust wastegates at the junction of your BAE exhaust will control your boost without imposing excessive backpressure. You net a more HP at the same boost level and possibly noticeable engine response in the higher RPMs.
Old 06-21-2011, 02:04 PM
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Good question Copbait73....as I wasn't around when the BAE kit was installed (I am the second owner), I am really not sure at what boost level it has been set at.

Can you recommend a wastegate set-up and I'll do some research? My understanding is that when you install a wastegate, you can then control boost? (Sorry for being a newbie but I am just getting into the whole turbo thing....I'm no mechanic but am eager to earn more DIY talent!)

Thanks again for your time!

Alex
Old 06-21-2011, 02:25 PM
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Alex-
Your honesty is refreshing and there are may specialist on this board to help you. You have a very unique engine to have lasted this long with an aftermarket turbo kit.
First, step back and understand what you have. Buy a VDO boost gage, install it and find out what boost your system is running. I suspect 5psi. Based on it's longevity I wouldn't recommend running higher boost whatever hardware changes you make in the future.
Equally important is to troubleshoot and find the cause(s) for it backfiring. This is not something you should just accept. You need to learn the basic systems so you can troubleshoot them yourself or understand your mechanic. I just went through my 73.5T cis and found my backfire problem to be a blown intake gasket on #6. Happens with boosting a system not designed for pressure.
Your engine is telling you something(s) need repair before you move on to increasing performance over the present level.
Old 06-21-2011, 05:41 PM
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Copbait73, you make a great point! I am constantly on this forum in the hunt for new knowledge with regards to turbocharging 911s and am often taken aback (OK, maybe more like superbly-lusty!) by what some guys (and maybe some girls) are doing to their cars in order to improve performance......and, let me tell you, it's highly contagious!!! BUT, yes, I need to, as you pointed out, step back and "smell the coffee", and...... go back to school! In short, I trully appreciate what advice/tips you and any other member passes on to me...btw please keep them coming.....hopefully, one day I will return the favour!

Cheers to a great forum!

Alex

1980 911SC Targa "Turbo"
2006 997 C4S Coupe
Old 06-22-2011, 01:28 PM
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A few of us have the BAE's, and what is left of them. I believe it is around 6PSI boost. The Rajay is a T04B in "V" trim. Search for the really long thread by Tsuter.
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Old 06-24-2011, 11:57 AM
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Patkeefe, I did follow Tsuter's old (911SC Turbo) thread and was able to obtain a pdf of the original BAE Turbocharger Kit installation instructions.....a big thanks to Iamchappy. In the instructions, it mentions to disconnect the vacuum hoses between the distributor and the throttle body and to cap the nipples on the throttle body....what does this do?....
Old 06-27-2011, 11:08 AM
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Alex, I would imagine that was BEA's way of assuring you don't advance to the point of detonation. Do a little search about advance and retard on this forum, there are many different theories and methods of advance and retard. I was running stock 78 SC advance. I still am, which I believe is about 5 degrees BTDC, but I also have an MSD Boost Retard unit.
Pat
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Old 06-27-2011, 05:02 PM
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