Intake backfire/popping over 5# of boost?

Finally got my S-Trim installed in my 90 LX Mustang with a 306 and took it out for a drive. Over 5# of boost under full throttle, it sounds like it's popping in the intake -- it's loud. Everything appears to be working correctly, fuel pressure increasing under boost to as high as 80psi with the FMU. Timing set at 8 degrees initial, rechecked plugs, wires, cap, rotor, coil -- MSD boost retard set on 3 degrees of retard per # of boost. Monkeyed with the fuel pressure from 39# to 42# initial. Car drives exactly as it did when N/A when not in the boost. Drives great. Once over 5#, starts to what appears to be the popping in the intake, like an intake backfire. Any ideas on what the check? Many thanks.

What are your plugs gapped at?

Sounds like spark blowout to me.

Gap your plugs at the .030 to .035 range or get yourself an MSD ignition box.

Thanks for the response. During the install I reduced the plug gaps from 0.040" when it was NA to 0.035" now that the S-trim is installed. Perhaps I will drop it down to 0.032" and see what happens. Ignition is stock with a remanufactured Ford distributor, MSD TFI coil, FMS 8mm plug wires (OHM check is within specs), and autolite 3924 new plugs.

I have been eyeing the Crane HI-6TRC ignition.

WOW dude set the retard to 1 or 1.5 degree and gap your plugs per chuck. @ 5psi the btm is pulling 15 degrees of timing and more than likely your headers will glow cherrie red if you keep up.

With 93 octane in the tank, you think 1 to 1.5 of retard is enough? The folks at Powered by Ford in Orlando suggested 8 - 12 initial timing with 93 octane and set the BTM at 3 for street. For open track (SCCA Solo I events) they suggested a equal mix of 110 unleaded with pump gas then back the BTM down to zero.

I'm new to this supercharger thing and I gotta tell ya I am very nervous about blowing a head gasket. Hence my overly conservative initial tune.

Again, very much appreciate the feedback.

Well dont be afraid to close up the spark plugs. Alternative Auto gaps there plugs between .024 and .026 and they have some fast cars. I had this problem with my car, I installed a MSD 6BTM and it cured all my probs.

I pretty sure you are safe as long as the fuel system is up to snuff....put the btm @ 2 and see if it gets a little better if your worried. It is best to sneak up on a good tune but i think you're being overly safe. Don't use me as an example but i ran 8* initial all year with 10 psi on sunoco 94 for about 50 dragstip passes-w-no ignition retard. 1.5 in my opinion is plenty safe with 8* initial. Do you have stock pullies? I noticed your using an FMU I'd be carful about doing an SCCA event and having 60-100 psi of fuel pressure all the time.

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Originally Posted by Chuck

Well dont be afraid to close up the spark plugs. Alternative Auto gaps there plugs between .024 and .026 and they have some fast cars. I had this problem with my car, I installed a MSD 6BTM and it cured all my probs.

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I agree...infact La Rocca's gaps their plugs at .20. I would definitely gap them down to at least the .25 - .30 range and see if that helps

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Originally Posted by Chuck

Well dont be afraid to close up the spark plugs. Alternative Auto gaps there plugs between .024 and .026 and they have some fast cars. I had this problem with my car, I installed a MSD 6BTM and it cured all my probs.

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Apologies in advance for rambling - but I really appreciate the feedback. Perhaps I mispoke -- I'm afraid of being aggressive on the TIMING and blowing a head gasket. I will definately tighten up the plug gap a bunch til I upgrade to a MSD or Crane ignition. Part of my concern is the engine was built 2 years and 2000 miles ago with light roadracing and weekend street driving in mind, not blowers. Stock .030 over block and cast .000 crank, aftermarket balancer, reconditioned rods, ARP main cap studs, ARP rod bolts, KB hyper pistons, balanced assembly, Edelbrock Perf heads w/ lite porting and 1.94 intake valves, ported GT-40 intake, 24# inj, 75mm MAF, 190lph intank pump, Kiban regulator, 1 5/8 longtubes, 2.5 Dr. Gas X pipe, Canton RR oil pan, windage tray, accusump, stock Melling oil pump. CR is about 9.8:1. Was not originally intended to be supercharged but it made right at 300 rwhp when NA and has been a blast to flog on the track. But as usual, I wanted/needed more but wanted to retain the street driveablity. After some research, supercharging appeared to be the ticket. I selected the 93 Cobra Mustang Vortech V2-SQ-S-trim kit because the FMU was already set for 24# and it included an underdrive crank pulley (4.75" serpentine / 6.87" blower). The supercharger pulley is stock (8 grove 3.33") while the WP and ALT are ASP underdrive, an ASP smog pump eliminator, no AC, and the PS pulley is a large diameter from a 1990 T-bird V6. Fuel system is a 190lph intank and a T-Rex auxillary. Stock fuel lines and fuel rail with 24# injectors. I'm planning on running on the dyno to check the air fuel once I fix this ignition issue. Some dyno tuners here in FLA have strongly recommended I purchase 42# injectors and recal the MAF, then ditch the FMU before they will tune the car with a chip. The folks at Powered by Ford have provided great set up advice to over the years and urged me to install the 190 intank pump (which I did) but keep the 24# and FMU and forget the chip -- based on their experience. Being new to blowers, you can see I'm still finding my way.

You are so down on timing your going to be negative..LOL. Your plug gap is fine your just pulling out so much timing the car can't run. your at 8 degrees with 16 degrees advance= 24 degrees then you pulling out 3 degrees for each 1 psi at 5psi your pulling 15 degress that is only 9 degrees total timing and you can be at 21 degrees with 93 octane if not more, but 19-21 is really safe at 5psi. You take care of that timing your going to gain something like 50hp if not more and the car will run much cooler and will save a head gasket because you need to be in the middle with your tune. Put your timing back at 14degrees and pull 2 degrees for every 1psi.

take the dyno tuners advise and get 42# injectors and recal the MAF. get rid of that damn FMU. after this get on a dyno and get a good tune done. you WONT be sorry!!! after you do some more research, you will find this is the way to go. i'm very happy with my set up and making close to 5 to the rear and driving my car EVERY day.

I'm pretty sure the problems you described were due to the excess of timing you were pulling out.

From all your responses it would appear my overly conservative approach is hindering my cause. I'll try again given all the suggestions later today and report back. Thanks very much.

Good call everyone. I tightened the plug gap down to 0.030" and the intake popping/backfiring is completely gone. Then I reduced the timing retard down from 3 to 2 and noticed my Nitto drags increasingly lost traction as the rpms climbed -- a significant increase in power. I'm sure the low 40 degree weather here in North Florida tonight contributed to lack of traction. This is gonna be a blast on an open track. I should have done this years ago.

Awesome! Glad it's running good. You'll really love putting race gas in it and putting the BTM knob to zero! :eek: :D

Once again......the SC bug has struck! You will soon be addicted. The only cure is more boost and bigger SC's.

I don't agree with the timing theory. Too much timing will cause detonation, not bucking. Too little timing will cause low boost as load decreases but will not cause bucking.

I also don't think that a gap of .035 will cause the problem at only 8 lbs of boost.

I think you are experiencing lean bucks and need more fuel. I agree with the people who said go to 42# injectors and make sure your fuel system is up to speed.

well if you think about it, he had the timing @ 8* initial, and he had the BTM pulling 3* for every one pound of boost. The BTM pulls no more than 15* but that is irrelevant.

He said @ 5psi it would back fire or pop through the intake. Assuming the EEC adds the 16* to the 8* initial, that puts him at 24 total. After the BTM pulls it's 15* out he's left with 9*total. I've never seen or heard of one run like that but i do know it will run like crap!

How would it run at 9 degrees total timing? I am sure I am not the only one who has done this.... Ever set your timing (removed the SPOUT connector) and forgot to put it back in? If you have when you drove you were running at or near 9 degrees total timing during the drive. You drop a lot of power but it doesn't run that bad!

9* of timing and a power draw of 50hp turning a supercharger and adding all that additional intake heat. I am sure it would run like crap.

Let me give you some more info. I made one change at a time and road tested. Reducing the plug gap alone eliminated the intake pop/backfire at 5# of boost. I do not have any issues with bucking or hesitation during cruise or at full throttle -- just the intake popping/occasional intake backfire if I held it too long. Here are the changes and outcome in order -- all run on 93 octane pump gas:

Pre-supercharger (ran this for 5 years):
18* timing, naturally aspirated, 0.040" plug gap = pulls clean and smooth to the factory rev limiter

Supercharger installed:
8* timing, 3* boost retard, 0.035" plug gap = intake popping @ 5# boost

18* timing, removed spout connector, disabled the boost retard, 0.035" plug gap, = intake popping and now an intake backfire, too @ 5# boost

8* timing, 3* boost retard, 0.030" plug gap = pulls clean to 10# boost but I did not rev it to the redline -- still warming up to this new blower

10* timing, 2* boost retard, 0.030 plug gap = pulls harder to 10# of boost but still not the the redline -- but getting more comfortable.

10* timing, 1.5* boost retard, 0.030 plug gap = pulls even harder to 10# of boost and this time peek at the fuel pressure @ 90psi and held it to a higher rpm just under 6000 or so -- this is great, no audible detonation.

10* timing, 1.5* boost retard, 0.030 plug gap = same as previous only this time I held it to just under the factory rev limiter and noticed the slightest intake popping at the top of the rpm pull -- 10# of boost and rpm just over 6000rpm.

Next test: replace cap and rotor. Retest plug wire resistance.

and then: tighten plug gaps to .028"

and theeen: install MSD6-BTM

I recall a mag article some time ago where Anderson Ford installed an s-trim on a 90 Mustang with a factory ignition and experienced a serious miss at low boost/rpm. So they replaced the cap, rotor, plugs and wires. The car then pulled to redline but had a slight miss at high rpm. They installed a crane HI6 and eliminated all the ignition misfire issues.

Sounds like you are going about this the right way. Keep doing what you are doing! I am suprised that the gap at .035 would do that but hey - every engine is different!

as long as it runs good thats what matters. I'm glad it's heading in the right direction.

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Originally Posted by itlkick

Sounds like you are going about this the right way. Keep doing what you are doing! I am suprised that the gap at .035 would do that but hey - every engine is different!

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I am also very surprised that the gap at .035 would do this. You might find out later that there is something else wrong causing the poping at a .035 gap.

I hear you guys loud and clear on the plugs. But I can say I've pulled plugs between tests and checked them with a magnifier, particularly #8, and haven't see any cause for concern yet. I'll continue testing and changing one thing at a time and plan to head over to the dyno very soon and get some air/fuel readings.

Should I be shooting for an 11.5:1 a/f ratio? It ran a solid 13.5 when it was NA.

I like 12:1 but for an scca car you might want it around 11.5:1.