The basis for this question is engine management but I thought I'd post it here so more might see it and its not strictly just EM.

What is the reasoning behind having leading and trailing sparks and are there alternatives? in particular I'm talking turbocharded cars, not NA.

As I understand it the trailing is usually 12 ish degrees after the leading whilst on boost and in vacuum (or NA) many people dont have this split. What I'm trying to establish is if there are any disadvantage from having for instance both firing at the same time of just the leading spark whilst on high boost. Would this cause bad det? I know of drag cars that do it but I'm specifically interested in drift/circuit cars. Any thoughts much appreciated

 

Buuump, anyone? Would really like to know if anyone has any experience of just running the leading spark on a turbo'd Rex? Only person I can find is bcool who is also using the VEMS ecu I want to use, so he may have the answers I need, but I don't know if he's actually got it working at full boost yet.

 

reason its there
- petrol doesn't mix homogeneously, and can stratify under certain conditions in what is a rather long chamber
- which ( with stratified petrol )
normally wouldn't have the time period per combustion event at higher RPM
to allow a flame kernel to burn all the chamber fully with a single ignition location

its main advantages are considered to be lowering certain emissions
( i think at some cost to other emissions, and also at power cost to a single plug ideally centrally located Ala suzuki )



the trailing spark is to ignite the charge that "*****" to the trailing edge of the combustion chamber
and then ignite it as the area around the trailing plug achieves peak combustion pressure
-which is slightly after that measured at the leading hole


i have trialled with petrol and propane the effects of the changing trailing split and also its deletion
- and have also tuned several EFI rotors running dual fire, 2 igniter no split mode

my personal EFI setting where last set to emulate the older dizzy
- at idle , and loads my split is maximised, at part throttle, minimised
my start split 10 , load end split is around 8 , at part throttles 4 ish
nothing to note either way with its manipulation wider and closer at load
- seems the leading spark IS doing most for power, and the trailing incidental

my experience with primitive ECU's capable of no trailing split is to configure them as two igniters
and run one channel per rotor, firing 180 apart , to fire l1 / t1 and l2 / t2 paired
- as per standard leading timing map numbers
this also brings not a huge difference in the bum meter
( though appears a preferred method by some ECU 's )
- HOWEVER on pull down
one motor ran thus did show some housing marks opposite the combustion area
as indication this earlier than normal trailing timing was banging the rotor bearing a little unnaturally

( something that was predicted for the LPG, but didnt appear, but reared once in a dual fire with petrol instance )


however
with LPG ( propane blends )
the mixture is much more likely to be homogeneous
and to an extent that the emissions needs for trailing spark are negated
LPG likes an earlier part load leading spark due to a lower flame speed
and then burns happily throughout the chamber when lit
- despite conditions that make for poor petrol stratification and burn
( interestingly RPI seems also to have deleted the trailing spark in analysis of LNG and LPG )
- the trailing spark, while adding next to nil power installed a knock limit when seen with high inlet temps !

with LPG deleting the trailing ignition, with zero change to leading ignition amounted to more allowable boost and less pings !


i know that some racers also followed the trailing timing deletion method
they too see the "softer" ignition allowing more boost
( that may not be reflecting as higher peak chamber pressures due to the single flame-front )

-----------
where do i stand with it--

- petrol results not conclusive, 1 or 2 hp either way, and EPA loses
LPG, definite gains to be had in deletion rather than manipulation

PS


i drive an FC daily with leading only ignition, propane, and 1 bar boost , in 100 F Perth OZ

my petrol EFI rx2 ( under resto ) retains current trailing split config somewhat nearing a dizzy response
- also at 1 bar boost, 98 RON petrol

 

Realy like to know where you (bumpstart) got all this info, for it is unlike any thing I ever came across befor

 

^^Hey, what's with the McDonald's setup?

 

So I presume there's no real benefit to keeping trailing with a dual leading spark setup, a la DLIDFIS? (It's a popular SA/FB ignition mod utilizing two leading coils and igniters fired at the same time, causing a "late leading" spark.)

 

Well, just remember that you need to keep the trailing spark firing nonetheless, or else carbon buildup will occur in the hole there and then it will eventually free up and (to make a long explanation short) cause detonation.

 

A few simple facts on leading and trailing. #1 Leading goes frist and the flame front heads toward the trailing side of the rotor and would smash in to the apex seal and I think this is what leads to the seal failer. The hard engine knok knoks them out, I"ve been there. So the trailng pops off just a moment later this in turn lite off the gas charge and this acts like a shock asorber for the flame front that wants to smash in ti the trailing. #2 The trailing has a small hole this keeps the plug from fowling like on a COLD mornning start which in turns keeps the motor running when the leading has fowled out. But since it's running the plug quickly clears it's self and start fireing. A signe of this is when the engine feels like it's not hitting on all cly but then kicks in, mustly on carb models from being over chocked.

 

Thanks for the great info guys, really helpful in giving me some understanding.
Bumpstart: Are the things you talk about all on NA engines?