Alky/Meth + Waste Spark = Backfires

[Kristopher]

I have a theory but I need some friendly bantering to see if it's sound.

I've always been curious as to why guys blow up their intakes (magnus mainly) and I've found, from my research anyway, these guys all run meth/alky injection. Given we run a waste-spark configuration it occurred to me that perhaps the meth is flowing into the chamber when the intake valve begins to open and the exhaust is still open, so when the plug fires it might make it back up stream into the intake manifold if the driver closes the throttle plate and creates a pressure differential enough to cause this.

The explosions/backfires I've witnessed or heard about have always happened upon a shift or on hard decel.

My questions are:
 - have guys seen the intakes pop open under load (middle of a pull)
 - is there merit to the theory at all? I've searched "waste spark" & "meth injection" and only found some Evo & turbo Buick guys discussing this.

I see no reason to run direct/sequential fire ignition systems on these cars but if it saves a guy from blowing up his intake and valves then I might consider it. I also wondered if when under load the meth ignites when the valves are open, that this might help drive the turbine harder (this has been an argument for guys running hella rich + lots of advance - afterburn in the exh manifold)


discuss

[John Hartman]

as for the sequential, there is a discussion about it over at DSMap, but no cars run sequential fire in open loop.  Once the car goes into open loop(heavy throttle foot) it goes to batch fire because there isn't enough time in the cycle, apparently.

As for your theory, I say it sounds plausible.

now, do some more research and find out if this has happened to anyone who runs half water/half methanol, or bug wash.  That won't burn on its own like straight methanol.

[Kristopher]

I've never had the issue that some guys do, and I ran a mix - from bug wash to ~80% meth. Two guys I know with 100% alky had their intakes backfire, one ran a SMIM. That said I do agree...I haven't come across a SMIM blown open on a mix which is what led me to wonder why that is.

I think the injectors need to batch fire to ensure enough open time, definitely.
The ignition time might be the opposite. The coils need time to charge so having a sequential or direct fire ignition should give them that extra 720*. However the stock ignition coils have more than enough power to fire off both plugs and still ignite the charge on half the time. That's due to the 'wasted cylinder' not requiring much energy and it firing with reversed polarity (side to centre vs centre to side electrode).

BTW that last comment is why platinum plugs work like ass in any waste spark design. It operates the plug in reverse of its design half of the time. Buying double-platinum works, or iridium IIRC.

I'd love to find out if there would be enough of a chance in having the meth;
1. go through the opening intake valve on the 'fresh' cylinder,
2. become ignited by the weak wasted spark during the intake event,
3. and not get carried through the exhaust scavenging but rather revert back up into the intake.

I think for htat to happen you'd need much more pressure on the exh side than on the intake which may require closing the throttle (back to my original question). I do know that if you had no overlap this might be more of a problem (exh valves closed, intake open & spark ignites the meth on the upstroke of the piston). In that scenario closing the throttle to get vacuum in the intake might pull that charge back up through the manifold.

again, just supposing...

[Kristopher]

OK, i've thought more about this. John your comment was centered more on fuel injection rather than spark ignition, but it brought me to thinking about this another way.

We do run batch fire injection, true, but only in certain circumstances (I had to cheat and look this up in the $2 tech manual).
1. Upon engine startup
2. large changes in TPS (dTPS; fires every 10ms)

Otherwise it fires the injectors sequentially and the spark in waste mode.

[John Hartman]

yes, until its in open loop again, such as when you put the boots to it.

[daniel Dee]

Backfiring through the Intake manifold is usually late firing plugs.
Can be cause by fouled plugs,,wet plugs,,,weak ignition ,,bad wires..
I would think a over rich condition might cause the plug to late fire with the intake valve opening.

I think the main cause of meth smin explosions is meth dripping (accumulating) into the intake when the engine is not under boost.

I run a 4 port meth system so I'm a prime candidate for meth explosion but I'm confident it won't happen on my car which has seem lots of boost in its life.