Intake backfire/popping over 5# of boost?

[HazMat]

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

[itlkick]

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.

[regattacoupe]

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!

[itlkick]

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!

[speedytang]

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.

[IPPO]

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.

[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!

[regattacoupe]

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

[ray95]

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!

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.

[IPPO]

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.

[regattacoupe]

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