Hi guys.

First I'll list engine specs:

stock bottom end/cam 5.0 Mustang
Promaxx heads
Ported EFI Edelbrock RPM intake
Autolite 3923 plugs gapped at .026
Intercooled single turbo (Turbos Direct 62mm)
DUI Screaamin Demon coil
DUI Live Wires
Stock TFI
8-12psi boost

I'm experiencing spark blowout with a .026 gap at 8psi of boost. My tuner routinely recommends a .032 gap for low boost. I'm scratching my head trying to figure out why I'm experiencing this with a hotter coil. The coil helped blowout at around 3500rpm and now the blowout has shifted to around 5000rpm.

 

I have seen LS guys run gaps as low as 15. I would close them up some more. I don't remember what I gapped my 5.0 to but it i think it was around 20.

 

That's where I'm planning to gap them tomorrow. It's really strange how wide of a variance there is in gap used by folks with builds like mine.

 

Grounds affect dwell if they are poor. It can even cause early fire condition.
Make sure Coils are grounded to the same place as the ECU.

Look at "Star point"
https://www.haltech.com/news-events/ecu-grounding-the-dos-and-donts/


Gap should be as large as possible to maximize spark area unless using nitrous or dangerous fuels in high volume.
Too small of a gap may start influencing the timing needed, if the cylinder heats up the pressure will rise much more rapidly approaching TDC on compression with the early timing due to a small gap. In other words, needing a lot of timing is a sign of inefficiency somewhere, in this case the spark gap is too small, can lead to engine damage if things get hot. Better to find the source of ignition issue and coil strength issue than close up the gap.

Many use OEM 20 year old D585 successfully with iridium plugs heat range 8 for 60k to ~100k miles in gasoline applications 600 to 700rwhp .024" to .026" gap. Closing in on 1000rwhp with Alcohol fuels better safe than sorry .018" to .022". I never tried the rest of these coils, the D585 is very cheap I always use it.
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What is the spark timing when it blows out? Lots of retard increases voltage requirement.

 

I'll have to look back through my datalogs. That's something that I didn't know.

 

It looks to be 15°.

 

15* With how much boost?

As piston approaching TDC the pressure is rising, please observe
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Notice the later the spark, the more cylinder pressure, the harder to jump the spark gap.

What year is your engine's head design? There are two major types of head in the world, early and late. Early are the SBC style that take 32 to 36 btdc of timing. Late are 2jz/ls/rb/sr style heads which take max 22 to 26* btdc with natural aspiration, because they are far more efficient.

So when you say 15*, we would need to know, early or late design head, how much boost pressure, what is the temp of the IAT for reference adiabatic efficiency and combustion rate, type of fuel (gasoline/alcohol) any aux injections, and in some cases even rodxstroke ratio (dwell and rate of piston approaching tdc) and of course static compression ratio and overlap/cam profile plays a role... its a deep suite. A deep ocean of variables. So just throwing out 15* Isn't going to tell anyone anything really.... trying to help

If you gave me all that info I could write the entire ignition profile for you though from scratch and be pretty damn close. I've done almost a thousand vehicles by now I think.

 

His heads being sbf and aftermarket aluminum will want 34 ish deg. n/a

Besides low rpm cylinder pressure fighting igntion, distributors only give some much range for timing changes. As you add or pull timing from the base timing it moves the ignition event farther from the post on the cap. As its farther from the post on the cap it causes even more ignition load because of the gap it has to jump. One of the reasons (all though not the primary reason) why individual coils are so great.

 

Excellent point. This engine has a lot of retard happening (~20*?). The distributor-cap-rotor would be indexed to allow for vacuum advance, so rotor gap probably increases from the first degree of boost retard.

OP. Try retarding your rotor 20 crankshaft degrees (10 distributor degrees) on the distributor shaft - or advance your breaker plate the same amount.

 

I can't run your normal timing advance with boost. 16-18 degrees is pretty normal with these builds while using pump gas.. This is a '94 Mustang GT with modern ignition. The timing is adjusted in the tune, not at the distributor.

 

Does it have a distributor? If so:-
I'm not talking about changing the timing - just the phasing of the rotor-to-cap. If the car has electronic spark timing from the factory the phasing should be OK but you never know. To check, just turn the motor until the crank is 15* BTDC then take the distributor cap off and make sure the rotor tip is close to the post in the cap. If not . . . .

If the ignition trigger is NOT in the distributor, you can just rotate the distributor body - 10* in the same direction as the rotor rotates.

If the ignition trigger IS in the distributor, you can:
- move the rotor on the shaft - 10* opposite direction to rotation.
- move the cap on the body - 10* same direction as rotation
- move the trigger or trigger plate - 10* same direction as rotation (you will have to reset your timing if you choose this option)

 

It's a Ford distributor with internal trigger wheel and PIP. The phasing is pretty fool proof in this design. What am I trying to accomplish? I'm a bit lost. The ignition is working fine now with the narrowed plug gap.

 

Sounds good.
It wouldn't hurt to check rotor gap as I described. Set the crank pulley at 15* and make sure the rotor is close to a distributor cap terminal.

 

Well the spark blowout is back. I took fuel out of the tune at WOT and the car is most likely making more steam. It was fat up top. It's breaking up again at around 5k plus rpm. I know guys that run boosted completely stock 5.0's don't seem to have this issue. My car has higher flowing intake, TB and heads. Could this extra flow make a difference?

 

Without an ignition box of some sort, you'll be fighting this forever. I ran into the same issues 20+ years ago. Now every 5L with boost I use a box.

 

On gasoline , running rich wont usually cause a misfire. Ive played with cars that where in the 9s and spitting black and they just drove through it. Now ethanol and methanol will misfire when on the rich side if ignition isnt enough.

I will throw this out there. My buddy that used to have a 306 sbf had to run his gap down into the into the teens or it would misfire. Like around .018.
That was on 15psi at the time.

 

Do you have any suggestions on a dependable box? MSD seems to be a crap shoot.

 

I may not have worded my reply properly. I wasn't saying that running rich caused the misfire. I'm still fine tuning the car so I needed to pull some fuel up top. My thinking is that leaning the top out from 10.5 to 11.5 afr created more power. Is it possible that more power brought back the misfire?

 

well it possible. Cylinder pressure is power.