Hey guys. '87 930 with the following: K27, 1 bar, GSF IC, de-smog and cat, GHL headers and exhaust, Freeflow AF. Car has been running well. 70 degrees here in NJ today.

I'm getting a loud backfire at top of second gear while accelerating. Is this indicative of a rich or lean fuel condition? What could be causing this?

Yes, I know i need an A/F meter/logging set-up and possible adjustable WUR. My next steps...

But, I don't want to drive it if there could be a potential for disaster..

Thanks, Rob

 

Just a couple of thoughts come to mind:

First, are you getting backfire (boom) or misfire (stutter/power loss)?? Backfire seems most often to be the result of intermittent ignition issues. Could be as simple as one of the spark plugs momentarily fouling at high rpm/boost, then suddenly firing again and blasting all that unburned fuel. Or loose wires somewhere. Are your plugs in good condition?

Or, your overboost protection circuit/components are shutting off fuel flow at high boost, only to kick back on when the boost drops. The symptom for that, however, isn't really a backfire...but more a catastrophic off-on lurch of the motor. The first time I experienced it, it scared the crap outa me. I assume you have an accurate boost gauge and are not spiking past 1.2 bar (I think the factory overboost setting is something like 1.2 or 1.4, if memory serves).

Anyway, I would look at ignition first, making sure all connections are in good order, then move on from there.

 

A couple more random thoughts. All bolt-ons were added this past winter and car was tuned at that time. No detonation throughout rpm range. It was dialed-in rich. Car burbles and pops on deceleration greatly; shoots flames on highway. I suspect ambient temperature could be contributing to the backfire on acceleration now that its considerably warmer than when the shop added the mods?

 

Yeah, you're probably rich as a pig...and will only get richer as the ambient temp goes up and the air gets less dense. You may want to re-tune your AFR's and back off just a bit. I'm betting you have a plug fouled (or in the process of doing so).
If your shop dialed you in rich on purpose, that setting (AFR) would be established at idle, since there is no other way to adjust it on a stock engine. Let's say they put you at some eye-smokin' %CO. That would hold through low to middle RPM, but the RPM's move up the real mixture control at full boost is most affected by the WUR. The worse case scenario is that your WUR and/or fuel pressures are whacko.
Stay tuned, I'm sure others will chime in with their input.
I'm not a wizzard at this, but logic says to look there.

 

Thanks for your reply, Mark. I am getting backfire (boom). It's interesting that you bring up the ignition issue. Subsequent to all of the performance modifications I listed above, I did have stalling and re-start issues and a new Permatune ignition box was added, as well as, ignition coil and a new fuel cut-off switch. This fixed the problem.

However, I had problems starting the car today (didn't want to crank over and then would stall.) Then, 10 minutes into my drive when accelerating towards top of second gear is when the backfire occurred. It did this several times and I wasn't full-on boost.

I've experienced the overboost protection kicking in. Scared the crap out of me too. I'm pretty sure this is not the case here.

I have an extensive 7 page work order regarding the work done a few months ago by my shop here in NJ. Engine and transmission were out at that time. I don't see spark plugs on the parts itemization or description of work done. I'll have to check.

Regards, Rob

 

Oops, almost forgot how you guys are with pics. I'll take some nicer ones, but I just put it back in garage and snapped a quick shot. This is my first attempt at posting up a photo, so I hope it shows up below.

 

SWEET! A 930, a Viper and a four post lift!

 

...and carpet.

 

Looking forward to hearing some comparisons with the Viper.

The previous owner of my car had a friend with a Viper. The 930 was faster at speeds below about 115-120 mph. Then the Viper had longer legs.

My car was 300 pounds heavier at the time than it is now, with old fashion factory heat exchangers...

 

DDD, I'm not a talented track driver, so I can only offer my limited, real world street experience with both. A friend of mine (who owns a highly modified Viper) and I took the cars out last week to see how they compared. He drove my '97 B/W which has smooth tubes, A/F, no cats, B&B muffler, 1.7 rockers. I drove the 930 with above mods. On a highway pull (closed course, 3 a.m. etc. ) from 60-ish it was fender to fender with the Viper pulling slightly by the time the Porsche hit top of third. When switched to fourth and the looooong throw it was over...We were in the triple digits by then. I'll leave it at that.

I don't have your weight reduction on my 930, nor do I have mods to your extent. I'm sure you wouldn't have a problem if you came up against a slightly modified Viper. In the twisties would be another ball-game too. Though, Vipers do quite well at the track with a competent driver.

I think most 930 guys would appreciate a few things that the Viper and 930 have in common. 1) they both have that raw/visceral old-school quality to them 2) they're both fantastic platforms for modifications 3) they are both driver's cars that tend to reward those that can wring the most out of them (no techno gizmos) 4) they'll both bite you quick if you don't respect the car 5) people go nuts over them 6) both camps really seem to know the technical and mechanical aspects of the cars (I've learned volumes on these forums)
I'm not one to draw too many generalizations, but I grew a little tired of reading threads like "which wax do you use?" and "do you drive your Ferrari in the rain?" on some exotic car forums...

I'm ashamed to say that the 930 took a bit of a backseat to some of my other cars over the past few years. But I'm really happy with these new performance mods. They've renewed my interest and reminded me of what fantastic cars these 930's are.
I seemed to have hijacked my own thread...hehe

Regards, Rob