Front and rear timing at idle different

[07heri]

I've read in forums to keep timing pretty smooth, if not the same, from 750 to 1250, up to 40 kpa.  Recently I've seen cals that have the rear cylinder advanced 5 or 6 degrees more than the front.  Can someone explain the theory behind this 5 degree spread? 

[KE5RBD]

The M8's are like that.  They have different timing on the cams front to rear.  I suspect it is to help EPA requirements.  The M8's also run less timing all over.  Seems to work well for them.  I have bumped it up a little on a Stage 1 Street glide and encountered spark Knock on the Data Log I ran.  It actually needed little more fuel and little less timing.  The M8's Stage bikes are different primarily due to more compression and cams.  Buddy has 17 CVO SG and did Stage 4 with 117 inches, CNC heads, 11 to 1 compression, larger Throttle body and Injectors and SE8 515 cams and a Screaming eagle exhaust street cannons I think.  It had the odd timing between Front and rear cylinders too.  It ran fairly well.  Put T-Mann 238 cams in and made big difference and the tuning went back to normal as the cams are same front and rear.  The 515's have 8 degrees less intake duration on the rear than the front.  The stock cams for M8's are -21 over lap. Stock 16 to 14 cams had 12 degrees over lap.  That is lots of difference.  The 4 valve heads just flow so much better that I think they can design the cams to give much better emmisions and still perform well.  The M8's have lots of potential and respond well to mods. 

[07heri]

This is on a 2016 Heritage.

I see the same in the stock cal.  10* on the front and 15* on the rear.  So guessing it has something to do along the lines of EPA or heat in the rear cylinder. 

The current tune has 17* on the front and 22* on the rear.  Isn't this going to run hotter than the OEM timing?

1)  All EPA considerations aside, should I run the timing at the same advance front and rear,  or leave it set up with more advance on the rear?

2)  As a DIY tune, how should I go about determining the idle timing?  I'm trying to get it nice and smooth and also time it for best heat reduction or at least slow the heat build up from rising so fast at lights.  I know there's only so much I can do to control heat but would like to do as much as possible.

[Gordon61]

I asked the same question on the other forum.  Consensus was it was something to do with heat but I've never seen an informed from the engine designers explanation as to why they did this in the softail 617 config.

Is it to ensure the front O2 sensor (possibly a little bit more exposed than the rear) is hot enough ...although recent O2 are heated??  Or is it to ensure the catalytic converter is hot enough to do it's job?? Or, given closed-loop operation at idle - would lower timing reduce the amount of fuel in that cylinder??

Question becomes, should you keep that same strategy or as you ask, go back to the same front and rear? ...and, how do you determine idle timing ...will be interesting to see any replies

[07heri]

Come on pro tuners!  I know you guys have seen this.......right?

[joe_lyons]

The cylinders run different from one another so they both have different needs.

[07heri]

The cylinders run different from one another so they both have different needs.

I'm guessing you tune for a living.  What do you set timing at idle at?  I think it's obvious that both cylinders will have different needs.  I'm talking at idle.  I'm not going to see any knocks, at idle, to play with. 

Anyone else that tunes for a living please jump in.

[rbabos]

Come on pro tuners!  I know you guys have seen this.......right?

Not a tuner but usually managed to bs my way through it somehow. Study the firing degrees and you will see the cyls do not share equal loading at low rpm especially at idle and less noticable at higher rpms. To smooth the idle , the lazy cyl effect can be fooled with timing. If you have your bike idling, pull one plug wire and then the other. You will notice one struggles to hold rpm, the other not as much. On a 45* v twin one cyl always does more work then the other due to staggered firing. As Joe mentioned each cyl has it's own demands for fuel and spark.
Ron