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Old car guy.....
 
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Backfire

I went on my first Porsche rally Sunday with 21 other cars, most of them a lot newer than mine. My 1981 sc with a modified 930/51 twin plug k27 turbo 4 speed car ran very well out there in the 100 degree West Texas heat. We ran about 150 miles in a 2 hour period. The oil temp ran as high as 3/4 on the gage but not near the red.
The only problem was a constant backfire (Bang!) from the exhaust when shifting 2nd to 3rd and 3rd to 4th at .5 - .6 Bar. What would cause this? too rich? This is the first time I have run the car this hard or in this heat. We had a lot of fun and will do it again!
Old 05-07-2014, 07:56 AM
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There are so many things that can cause what you describe, and it is difficult to understand what someone means by "backfire" without hearing it in person.

When you say "backfire" do you mean that in the traditional sense - as in a large, sounds like a grenade going off, single "bang" out of the tailpipe, or is it more like "popping and crackling" out of the tailpipe when the throttle is closed between shifts?

Also, I can't remember what fuel system in on your car - is it the OEM, 930, K-Jetronic/CIS or an aftermarket electronic fuel injection system (need this info to help answer your question)?

PS - my brother told me he recently met you while he was in Lubbock - wow, small world!
Old 05-07-2014, 08:05 AM
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High temps and popping could be too lean.
Old 05-07-2014, 11:00 AM
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Originally Posted by Ronnie's.930 View Post
There are so many things that can cause what you describe, and it is difficult to understand what someone means by "backfire" without hearing it in person.

When you say "backfire" do you mean that in the traditional sense - as in a large, sounds like a grenade going off, single "bang" out of the tailpipe, or is it more like "popping and crackling" out of the tailpipe when the throttle is closed between shifts?

Also, I can't remember what fuel system in on your car - is it the OEM, 930, K-Jetronic/CIS or an aftermarket electronic fuel injection system (need this info to help answer your question)?

PS - my brother told me he recently met you while he was in Lubbock - wow, small world!
Yes, your brother seemed real nice and talked highly of you.

It was a single BANG like a grenade each time I shifted to 3rd or 4th with boost.
I thought the car behind me would be dodging fireballs but he said he didn't see anything. I am worried this could blow the muffler apart or worse.
I hear the "popping and crackling" always when shifting down with throttle closed.

Yes a CIS on 1976 930 with twin plug distributor 2 cd boxes and 2 coils in the 1981 sc body. It has the air pump and diverter valves, an O2 sensor but no frequency valve. There may be a modified fuel head and other mods.
I don't know the internal mods.
I doubt a picture will help but I'll send one anyway.

Old 05-07-2014, 11:26 AM
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^^^ wow...
Old 05-07-2014, 11:52 AM
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Quote:
Originally Posted by Hot Euro View Post
Yes, your brother seemed real nice and talked highly of you.
He must have had me confused with someone else - ha!

Quote:
Originally Posted by Hot Euro View Post
It was a single BANG like a grenade each time I shifted to 3rd or 4th with boost.
Ok, I am not an expert on backfires, but I know that typically they are caused by excess, unburned fuel in the exhaust system that ignites due to the heat in there. Do you have an onboard air/fuel exhaust meter (can't remember if you've mentioned that in the past)? It would be really interesting to see what your AFR readings are around the time of the backfires. I would bet that you are too rich at that point, but it's almost pointless to start making fuel pressure adjustments without first verifying what is going on with an AFR meter.

Last edited by Ronnie's.930; 05-07-2014 at 12:02 PM..
Old 05-07-2014, 11:59 AM
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Real Sweet Engine

The only time I heard a SERIOUS LOUD BANG when I was doing a little side by side (going all out in 3rd-4th gear) with a hooked up corvette in cool weather upper 40's-50degrees. I wasn't sure if his car or mine or both? I have an 911-SC-915- 3.3 turbo with goodies...Kinda scared the crap out of me.. My temps were low and my afrs looked good (afr-meter). Have not had a problem since. I do occasionally shoot flames.
Old 05-07-2014, 12:11 PM
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Originally Posted by Ronnie's.930 View Post
He must have had me confused with someone else - ha!



Ok, I am not an expert on backfires, but I know that typically they are caused by excess, unburned fuel in the exhaust system that ignites due to the heat in there. Do you have an onboard air/fuel exhaust meter (can't remember if you've mentioned that in the past)? It would be really interesting to see what your AFR readings are around the time of the backfires. I would bet that you are too rich at that point, but it's almost pointless to start making fuel pressure adjustments without first verifying what is going on with an AFR meter.
No AFR meter. Have looked at one on ebay for about $250. May be my next toy...

Innovate LM 2 LM2 afr Wideband Meter Basic Kit O2 Sensor Included | eBay
Old 05-07-2014, 12:49 PM
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It's likely you may be running a little rich. My car is also running slightly rich, which causes occasional backfire. I may have to get it retuned sometime.
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Old 05-07-2014, 02:09 PM
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My guess is on being too rich as well. You probably haven't noticed it until just now, doing 150 miles in 100 degree heat. I bet your exhaust temps are pretty high, high enough to initiate combustion in the muffler of all the excess gas. Could also be your WUR is acting up on boost and over-doing the enrichment. Or maybe you have a small air leak in the IC plumbing, enough to cause a richer than normal condition when on boost.

AFR gauge setup, great diagnostic tool, must have.
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Old 05-07-2014, 02:36 PM
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My guess is on being too rich as well. You probably haven't noticed it until just now, doing 150 miles in 100 degree heat. I bet your exhaust temps are pretty high, high enough to initiate combustion in the muffler of all the excess gas. Could also be your WUR is acting up on boost and over-doing the enrichment. Or maybe you have a small air leak in the IC plumbing, enough to cause a richer than normal condition when on boost.

AFR gage setup, great diagnostic tool, must have.
Can you recommend an AFR gage setup? Should exhaust temps be monitored?
Old 05-07-2014, 06:15 PM
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Someone put some time and money into that engine. Looks like it has aluminum injector blocks so the intake ports are probably enlarged.
I'll guess it has a modified fuel head and the single big pow from the exhaust is unburned fuel exploding in the exhaust system. Modified fuel heads can increase that like you've described.
If you had a zork I think a flame would be shooting out when that happens. A baffled muffler may hold it back.
Old 05-07-2014, 06:52 PM
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No AFR meter. Have looked at one on ebay for about $250. May be my next toy...

Innovate LM 2 LM2 afr Wideband Meter Basic Kit O2 Sensor Included | eBay
I have an LM2 - excellent piece with huge, easy to read display. I had a problem with it at first, and even though I had it in my possession for over 4 years (unbelievably, never got around to installing it in that time), Innovate exchanged it for free - so remarkable customer service, too.

That said, there have been plenty of quality complaints about Innovate throughout the years, so if you buy an Innovate product, make sure it is from a recognized dealer (I bought mine from Pelican) so that you won't have any problems if you need service from them.

Last edited by Ronnie's.930; 05-07-2014 at 07:03 PM..
Old 05-07-2014, 06:54 PM
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I had a faulty green wire on my distributor that would cause intermittant problems that i chased for a long time. towards the end i would do that when i shifted gears Loud bang and huge fireball behind the car. When i got to my destination the car wouldnt start and i had to flat bed it home. The connector where the green wire plugged into the distributor was cooked and pretty much fell apart when i disconnected it.
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Old 05-08-2014, 04:17 AM
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