possible cure for the 3B 'spark hop' issue?

[a4kquattro]

This information is 2nd hand, but it makes sense to me and thought I'd post it here to provoke discussion.

Most of you know about the issues with pre-ignition melting pistons on the 3B engine due to spark inside the distributor cap arcing to the wrong cylinder under high boost (where electrical resistance is greatest inside the cylinder).

The solution: Take a 10vt ignition rotor (which at 14mm is *wider* than the correct 3B rotor, bear with me here..) and trim off the leading edge of the tip making the new overall width 9mm. To visualize what I mean, here's a pic of an OE dizzy rotor where this is done:



The idea here is to make it so that if indeed the spark does fail to ignite in the intended cylider, it will instead go to the next least path of resistance, which will be the PREVIOUS cylinder which has ALREADY FIRED, eg the exhaust valves will be open and there is no risk of pre-ignition...

discuss.

[denniselbows]

sounds a pretty good idea to me.

[S2central.net]

Like the thinking - but do the dimensions inside the dissy cap stack up to this idea ?

What I mean is, lets say at 6000rpm that rotor arm is spinning at 50 revs per second and has just fired cyl #1 and is en route to the next in line of fire (1-2-4-5-3)... Is the distance from plug-past (#1 here) to the trailing tip of the rotor always closer than the distance from the leading tip to plug-next (#2 in this case) by the time that cyl next is well up the compression stroke ?

Someone needs to do a lil drawing to figure this out. I like the theory... I can do the drawing and geometry if someone gets me dimensions for the 3B dissy cap and the rotor arm - stock version and this tweaked one.

Paul

[Rusty]

Nice idea, not seen that before and it looks like a simple solution and simple is usually best I see no reason that it would do any harm but as to whether it will solve the problem completely would require investigation, perhaps some bench testing could be done using a spare dizzy, coil etc. ?

[Jonas_R]

Paul what do you need measurements of? I think I might have a broken dizzy lying around somewhere

[EspenW]

Nice thinking
This could possibly be a simple and inexpensive way of solving the root cause of the problem, rather than performing the expensive and time consuming band aid solution (rods).

[JRedig]

I completely fail to see how rods have anything to do with "solving" the spark jump issue. They are a repaired item that is a result of another issue. Stronger rods do not fix the problem of spark jumping ahead. They might endure the problem longer but certainly are not a solution to it.

[EspenW]

Easy: Spark hop causes bent rods on 3B. Curing spark hop is a lot better solution than installing stronger rods w/o solving the cause of the bent rods.

[JRedig]

Easy: Spark hop causes bent rods on 3B. Curing spark hop is a lot better solution than installing stronger rods w/o solving the cause of the bent rods.

Right, but first you said this:

rather than performing the expensive and time consuming band aid solution (rods)

Which is incorrect since the rods aren't the problem in the first place. ;-)

[EspenW]

Right, but first you said this:



Which is incorrect since the rods aren't the problem in the first place. ;-)

I totally agree with you, but the consensus has been that even sub 380hp 3B's bend their rods due to too much power/torque, while the real reason has been spark hop. Stronger rods sorta cure the symptoms, but not the cause.

[madwolf_Uk]

Interesting read this, 2 things im wondering though, what sort of power levels does spark hop become an issue, and i know a few are doing/have done it, but would it be safer just to remove the distributor in the first place?

[S2central.net]

Gary - this isn't something caused at any specific power level.

It is a function of pressure in the combustion chamber such that during high boost levels the high voltage that is being prepared to emerge from the spark plug discovers there is a path of lower resistance by jumping the gap (at ambient pressure) within the dissy cap.

Trouble is the next nearest location for that HT to jump to is the NEXT cylinder which has a piston racing past half way up its compression stroke... So that mixture gets pre-ignited and the catastrophic shock-wave causes the rod to bend.

This idea could be a belter.

[S2central.net]

To Gary's 2nd point - sure you could delete the dissy on the 3B - you wouldn't be the 1st, but its a pretty big undertaking for most people as the right loom is needed (to be bought or get into deep mods of 3B engine loom) along with 5 x coils (OEM or 034) and an ECU swap.

Admittedly the multi-coil DI setup is superior and can never suffer from this spark-hop phenomenon, but its a big undertaking and this idea has the potential to save a lot of hassle and prevent (or significantly reduce chances of) this happening.

I'd need to take some measurements of a dissy and rotor arm to get further with this one.


Paul

[madwolf_Uk]

Cheers for those answers Paul, so its more a case of higher than normal boost levels that are the cause then. I agree its a neat little fix if it works, especially now i understand how much work it is to remove the dizzy as well!

[a4kquattro]

To Gary's 2nd point - sure you could delete the dissy on the 3B - you wouldn't be the 1st, but its a pretty big undertaking for most people as the right loom is needed (to be bought or get into deep mods of 3B engine loom) along with 5 x coils (OEM or 034) and an ECU swap.

Admittedly the multi-coil DI setup is superior and can never suffer from this spark-hop phenomenon, but its a big undertaking and this idea has the potential to save a lot of hassle and prevent (or significantly reduce chances of) this happening.

I'd need to take some measurements of a dissy and rotor arm to get further with this one.


Paul

This is probably a good time to mention that I sell VEMS boxes configured to easily allow for a coil on plug upgrade (via flying lead harness out of the ECU) in a plug and play system