Ignition Timing Mechanical Advance

[AK260]

[mention=32052]AK260[/mention], you suggested two of them...

28 degrees @ 3500 rpm - your tuners numbers for your 10.5:1 motor. 

34 degrees @ 2800ish  - From "the Z world"

Also when I get a chance, I'll back calculate your mechanical advance degrees into time measurements and see how that looks on a curve.

 

 

Thanks captain, this is all very interesting stuff.

 

The 22 degs ATDC is the constant regardless of the head / chamber shape and compression. So it would be interesting to see if there is a formulae for taking into account the variables like VE, dynamic comp, chamber shapes, RPM, octane etc.

 

I know a couple of “geeky” engineers at McLaren Cars (which is the recruitment criteria for anyone there) - I will have to put this question to them when we next have a catch up.

 

 

 Edit: this is a VERY good read ...

 

https://ls1tech.com/forums/advanced-engineering-tech/586666-there-formula-calculate-ignition-timing.html

[240260280]

8 hours ago, Captain Obvious said:

@240260280  Thanks for your input too. All that stuff would certainly have an effect, as does VE changes with RPM.

With all your research... Have you come up with any curve that shows VE versus RPM for our motors? Any idea where it typically peaks?

There was a dyno run posted on zcar.com that had Ve. It was ~ 15 years ago however I can not find it yet. That is the only one I know of for an L engine. 

I recall Ve being low at lower rpms, then peaking, then falling off at higher rpms.

Edited March 13, 2020 by 240260280

[AK260]

Further to the link I posted above, there is an interesting discussion here ....

 

 

https://honda-tech.com/forums/engine-management-tuning-124/calculating-ignition-timing-2702491/

 

I’ve stolen the text of the juicy bit and pasted below:

 

 

Your questions means many things to many people but There are models (MATHLAB/PHD papers) but you also can estimate your timing curve bearing in some key principles and doing a "analytical review" if that is what you are after (or mean by "calculation")

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For a given engine, and IAT/ECT operating point, Ignition timing is principally a function of the burn rate & VE (torque)

 

Burn rate a linear function of rpm up of a certain break point and swirl/tumble (for a stock ITR is about 4,000rpm - the ignition curve flattens out (Swirl/tumble is proportional to rpm)

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Just to qualify the above is only talking about cruising to WOT loads (not talking about Timing under Decel, on/off throttle

 

 

 

VE is influenced by intake resonance, exhaust scavenging (you can hear your engines sweet spot)

 

The Ignition Curve is a "inverse of the torque curve" - assuming your engine burn rate rises with rpm, you should have minimum timing at peak torque ( I guess someone with a maths engineering expertise can write an inverse function)

 

Bearing the above in mind you can tune your ignition to be pretty close

 

Pay attention to shape of torque curve and tweak ignition accordingly

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Torque - u get this from the dyno

 

U then cant get a idea of the shape of burn rate vs rpm

 

U then tweak the peaks/valley and gradients of ignition curve by eye-balling the torque curve

 

On your DD you get a feel for your engine

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Look at the stock ITR and D16 maps - u can learn alot (on a D16 OEM timing drops after 4,000rpm)

 

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Tuning ignition is way more fun than tuning fuel

 

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Google is your friend.

 

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There are videos on youtube on MBT/Steady state, but they are always at 3,000rpm/WOT on a chassis dyno - Only on OEM Engine dynos they can hold 8,200rpm/WOT for 10 seconds - This is not not practical for the average Honda-tech forumers or tuner shop (who does no want to break their dyno) - Thus most tuner interpolate with gut feel (which is their un-articulated experience of tuning scores of engines - based on the above (and other) basic principles

 

Do a search and u will find published combustion models (Phd) that can predict MBT within 5%, that have been verified by dyno runs

 

If if u (not a OEM) tuned to MBT on a steady state dyno - your ECT/IAT compensation needs to be spot on for you to be getting MBT all the time (Tune in the day run at night) - and if u are a tuner u would know ECT/IAT move around alot in normal driving

 

 

 

 

[jonbill]

Here's my VE table. At the front is closed throttle, the back is full throttle.

 

 

[Captain Obvious]

Thanks for the input guys. I'll look through this a little and also see if I can find my old piston speed study.

[Zed Head]

6 hours ago, AK260 said:

Further to the link I posted above, there is an interesting discussion here ....

 

There are videos on youtube on MBT/Steady state, but they are always at 3,000rpm/WOT on a chassis dyno - Only on OEM Engine dynos they can hold 8,200rpm/WOT for 10 seconds - This is not not practical for the average Honda-tech forumers or tuner shop (who does no want to break their dyno) - Thus most tuner interpolate with gut feel (which is their un-articulated experience of tuning scores of engines - based on the above (and other) basic principles

 

Do a search and u will find published combustion models (Phd) that can predict MBT within 5%, that have been verified by dyno runs

 

If if u (not a OEM) tuned to MBT on a steady state dyno - your ECT/IAT compensation needs to be spot on for you to be getting MBT all the time (Tune in the day run at night) - and if u are a tuner u would know ECT/IAT move around alot in normal driving

I don't have anything to add, just have to point out really weird it is for a person to take the time and effort to write all of his complicated thoughts down.  But still cut the corners on the word "you".  Makes me think of a high school kid on his phone trolling a bunch of engineers.  It is on a Honda forum, but sill...just weird.  Just type in the y and the o.  The whole word is contained in four keys across the top of the keyboard.

Just one of those WTF things.

[AK260]

forum, but sill... just weird.

 Just sayin’ ;) :p

 

 

 

[Zed Head]

2 hours ago, Zed Head said:

just have to point out really weird

You missed this one, above.  I left out the word "how".  Those are mistakes though.  The Honda guy did his on purpose.  It diminishes the impact of what he's saying, dramatically.  Plus the fact that he's just talking from his own opinions, apparently.  Texting it out to the world on his phone.

"I saw a video, do a search and u will find,", etc.  He's just saying "I saw some stuff on the internet and here's what my opinion is".  It's just bench-racing.

Interesting reading but at the end you have to wonder, "why should I believe this guy?". 

This part might be my favorite - "and if u are a tuner u would know ECT/IAT move around alot in normal driving".   ECT/IAT move around alot (sic) when driving...WTF?  This is tuner-knowledge?

Just having fun, nothing personal.

[AK260]

Just having fun, nothing personal.

Likewise sir - just enjoying the fun with you! [emoji106]

[Captain Obvious]

Searching through my stack of old CD's was fruitless. I was able to start over though and here's what I was able to whip up so far. Using the 280Z stroke length and center to center rod length, I get the following:

Note that for my calculations, I used everything pointing straight up (TDC) as" zero degrees". So the maximum piston speeds occur at 75 degrees off vertical. About 75 degrees after TDC when going down, and about 75 degrees before TDC going back up.

[AK260]

Here's my VE table. At the front is closed throttle, the back is full throttle.

 

 

 

 

This is great Jon - it looks like your engine comes on song at 4K rpm. Does this lot correlate with your BHP curve?

 

How did you get this table? What are the parameters used to generate it? Does it take into consideration things like inlet tract / header length etc or is it purely what happens in the engine by itself?

 

[MENTION] Captain Obvious[/MENTION] - we need one of these ....

 

https://www.kistler.com/en/product/type-611xc/

 

[jonbill]

 

 

 

 

 

This is great Jon - it looks like your engine comes on song at 4K rpm. Does this lot correlate with your BHP curve?

 

How did you get this table? What are the parameters used to generate it? Does it take into consideration things like inlet tract / header length etc or is it purely what happens in the engine by itself?

 

 

It comes from Tuner Studio (I'm a tuner too, u get me bro?) It's the fuel table for my injection, so it's actually a pretty accurate reflection of the VE and includes all the bits and bobs from the airbox onward. Yes, it does correlate to where the power is well.

 

In a stock engine I guess the action starts around 2500, so I think the rising VE and likelihood of knocking is probably why the ignition curve stays flat.

Maybe in theory one could start adding more advance again after the the torque/VE peak is passed, but that's would be very hard with springs and weights!