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The effect of ignition advance on AFR?
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kj16v



Joined: 08 Apr 2008
Posts: 19

PostPosted: Mon Jan 25, 2010 5:15 am    Post subject: The effect of ignition advance on AFR? Reply with quote

Hi, I am tuning my Fiat Coupe 16V Turbo. (rebuilt stock engine, fmic, 44-trim T34 turbo @ 18psi)

When I first tuned the engine with it's stock log-manifold, I found that, in all cases (cruise to full load) increasing ignition advance had the effect of leaning the AFR. This was the opposite of what I expected. I thought that lighting the fuel mix off earlier, thus allowing a more complete burn, would mean there would be less free o2 'kicking about' in the exhaust gasses.

Recently I changed to an equal-length tubular header. Besides that, everything else is identical, 18psi, etc. Now I find that ignition advance has the opposite effect, now ignition advance has the effect of richening the AFR.

Could anyone please explain the reasons for these findings?

On a side note; the stock Fiat Coupe log-manifold is known to be restrictive, even on the factory standard engine, and I could see this by comparing MAP datalogs before and after the change of manifold. With the stock manifold boost pressure would spike briefly,level out, and then drop off 3psi by the redline. With the tubular header the boost spikes less and stays steady right up to the redline.

Could the opposing findings be related to the restrictiveness of the manifold (exhaust gas reversion diluting the charge? I dunno)?

Thanks.
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gunnar



Joined: 30 Aug 2006
Posts: 513
Location: Cruisin for a bruisin

PostPosted: Mon Jan 25, 2010 5:28 am    Post subject: Reply with quote

Can you verify that pulsewidth has not changed at all.

How much variation in AFR are you seeing for how many degrees advance change?
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kj16v



Joined: 08 Apr 2008
Posts: 19

PostPosted: Mon Jan 25, 2010 5:39 am    Post subject: Reply with quote

Unfortunately I don't have a way of verifying pulse width. But I do runs at the the same engine temp and I can get repeatable consistent AFRs

With the tubular header 4 degrees advance changed AFR from 11.8 to 11.4.
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Gadgeroonie



Joined: 24 Aug 2004
Posts: 1726
Location: UK

PostPosted: Mon Jan 25, 2010 7:57 am    Post subject: Reply with quote

beware

a change in ign can effect EGT's

some wideband lambda sensors can be affected by EGT - typically you may see the wideband report a richer mixture when the egt increases

this may be what you ae witnessing
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kj16v



Joined: 08 Apr 2008
Posts: 19

PostPosted: Mon Jan 25, 2010 8:55 am    Post subject: Reply with quote

Hi, that's a good point, Gadgeroonie.

However, I just checked the Tech Edge website for my 2J1 WBo2 controller, and it looks like the sensor heater is PID controlled. So it should stay at a constant temperature?

Also, this wouldn't explain why the afr now richens with ignition advance when using the new tubular header (more advence=cooler egt).
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Gadgeroonie



Joined: 24 Aug 2004
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PostPosted: Mon Jan 25, 2010 8:58 am    Post subject: Reply with quote

in theory the afr should not change when the valves have closed and the spark ignites the mixture

the volume of air and fuel can NOT change

can you log egt's to see if they have changed

fit a heatsoak to the wideband sensor to see if you can stabilise it

( has your boost / fuel pressure changed between runs ?)
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Grocerius Maximus



Joined: 02 Apr 2006
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PostPosted: Mon Jan 25, 2010 9:50 am    Post subject: Reply with quote

Keep in mind a wideband doesn't measure what happens INSIDE the engine, just what is present in the exhaust stream.

For instance at a constant PW/load and fixed AFR of say 14:1, introduce a bunch of EGR, what happens? O2 goes rich.
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Gerry Gaffney



Joined: 30 May 2006
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PostPosted: Mon Jan 25, 2010 11:31 am    Post subject: Reply with quote

If you introduce EGR you displace the O2 to complete combustion so yes it will show rich.

You have to remember the sequence of events: Suck - Squeeze - Bang - Blow. once the air has been ingested & then the fuel added, the valves shut, then the time at which the Bang happens can have no effect on the actual Air to Fuel ratio (mass) but by looking at O2 only, things such as EGT, misfires, cam/ignition timing, can all affect the reported AFR as displayed by a typical wideband lambda meter.

A more accurate (yet still not perfect) method is to apply the Brettschneider formula to a 4 gas analyser (most decent 4 & 5 gas analysers do this automatically) but not many 4/5 gas units stand up to dyno testing, plus there is the lag of the measured gases.

The only true method is to measure the actual mass of air & mass of fuel used at that instant.

You would need to provide a little more data on things such as EGT, IAT, AFR for both conditions etc, to help aid further diagnosis, however you would normally find that with less advance for the same pulsewidth & mass airflow you would show a leaner AFR (assuming a good wideband system with proper sensor temp control/correction), as the exhaust gases are still burning when the ex. valve opens & you end up with residual O2

HTH

Gerry
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Turboivo



Joined: 09 Mar 2006
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PostPosted: Mon Jan 25, 2010 12:29 pm    Post subject: Reply with quote

Hi,
What is the cam overlap? Does that happens at MBT?
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baldur



Joined: 12 Oct 2007
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Location: Reykjavik, Iceland

PostPosted: Mon Jan 25, 2010 4:20 pm    Post subject: Reply with quote

Yes ignition timing can affect air-fuel ratio indirectly by changing the exhaust temperature.
A change in the exhaust temperature changes the pulse speed in the exhaust headers and the volume of the exhaust gas that needs to be evacuated from the cylinder, a change in the scavenging properties of the exhaust system will affect the air-fuel ratio inside the cylinder on the following cycles.
I would however make sure the lambda readings are correct before jumping to conclusions and making changes. I have on several occasions observed inaccurate numbers from lambda sensors.

Slightly related. Two stroke engines are so heavily affected by exhaust temperature when it comes to scavenging that modern EFI two strokes tend to have an exhaust temperature sensor located in the mid section (coldest part) of the tuned pipe for fuelling calculations.
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Grocerius Maximus



Joined: 02 Apr 2006
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PostPosted: Tue Jan 26, 2010 1:23 pm    Post subject: Reply with quote

Gerry Gaffney wrote:


The only true method is to measure the actual mass of air & mass of fuel used at that instant.

Gerry


If you have this data it certainly helps to rationalize La readings being 'close' or 'WTF?', but still you don't know for sure whether complete combustion is occurring. BSFC #'s can be helpful in this case.

Determining fuel mass is somewhat of a non-trivial pursuit for EFI, need to know injector characteristics exactly, fuel properties exactly, measure temps etc.

La numbers can lie like a cheap toupee- so skewed from reality its funny.
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mrx



Joined: 05 Nov 2005
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PostPosted: Wed Jan 27, 2010 1:11 pm    Post subject: Reply with quote

at my work we use some kind of device to measure the weight of the fuel which was taken from a small tank over a certain period of time. This is by far the most precise way to determine air mass. But is very time consumpting because it only works steady-state.

We mainly use this method for OEM base calibrations on engine dynos.
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RICE RACING



Joined: 25 Nov 2004
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PostPosted: Thu Jan 28, 2010 5:29 am    Post subject: Reply with quote

mrx wrote:
at my work we use some kind of device to measure the weight of the fuel which was taken from a small tank over a certain period of time. This is by far the most precise way to determine air mass. But is very time consumpting because it only works steady-state.

We mainly use this method for OEM base calibrations on engine dynos.


Done a similar thing on an engine dyno where the whole fuel tank was put on precision lab scales and thus the mass of fuel consumed was known and specific gravity was measured prior thus you have how much fuel was used.

We used a boat tank and fitted the EFI pump inside with return going back to it.

Simple and works great, turned out more accurate than $10000 fuel flow sensors!
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Max rpm



Joined: 06 Dec 2009
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PostPosted: Thu Jan 28, 2010 2:37 pm    Post subject: Reply with quote

Have any of you guys used one of these, looks to be more accurate then just a lambda meter.I mostly work on twin cylinder & multi cylinder motorcycles.

http://www.ecm-co.com/product.asp?5220
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Turboivo



Joined: 09 Mar 2006
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PostPosted: Fri Jan 29, 2010 3:00 am    Post subject: Reply with quote

ECM are technology leader. Their cheapest meter AFM1000 is abot 1500usd. The "Lambda pro" is about 3500. 5220 could easily go over 10K bucks. Whatever you buy from them you'll not feel sorry. The most entry level( but not about the quality) is the Powerdex AFX AFR meter (actually older AFM1600) sold by NGK. Even better with NTK WB sensor.
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StockieXR6T



Joined: 19 Feb 2010
Posts: 9

PostPosted: Mon Feb 22, 2010 1:03 am    Post subject: Reply with quote

Gadgeroonie wrote:
in theory the afr should not change when the valves have closed and the spark ignites the mixture

the volume of air and fuel can NOT change

can you log egt's to see if they have changed

fit a heatsoak to the wideband sensor to see if you can stabilise it

( has your boost / fuel pressure changed between runs ?)


I find generally as you add timing you get leaner mixtures. Advancing timing will mean more complete combustion and lower EGT, by the time the inlet valve opens there is less residual pressure (heat) which in fact will increase the volume of fresh charge entering the cylinder.
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kj16v



Joined: 08 Apr 2008
Posts: 19

PostPosted: Mon Feb 22, 2010 6:26 am    Post subject: Reply with quote

That makes a lot of sense Stockie

Thanks for commenting, and thanks to everyone else too.

It might get the chance to test this thoroughly on a dyno in the near future so I might be able to get a definitive answer.
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jasaircraft



Joined: 15 May 2008
Posts: 100

PostPosted: Wed Mar 03, 2010 9:13 pm    Post subject: Reply with quote

Actually if the combustion is "more" complete then you should have less residual O2s and HCs and if you have less O2 to measure it will show richer.
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marc99



Joined: 01 Nov 2018
Posts: 3

PostPosted: Thu Nov 01, 2018 1:27 pm    Post subject: Reply with quote

jasaircraft wrote:
Actually if the combustion is "more" complete then you should have less residual O2s and HCs and if you have less O2 to measure it will show richer.


A Google blast from the past!

Generally (but not 100% of the time) "retarded from Best" ignition timing tends to leave more residual o2 and more residual HC.
If you are only looking at residual o2, you would think that an area of "retarded from Best" ignition timing was lean and you would richen it.

You'd be fat and still have retarded timing.

Marc Salvisberg
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joe90



Joined: 19 Mar 2013
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PostPosted: Thu Nov 01, 2018 10:10 pm    Post subject: Reply with quote

Yet more internet BS.

Timing advance changes EGT.

EGT changes The output voltage of the WB.....because they're really not very good at all.


Timing advance never has and never will change AFR.



Again, contrary to popular belief WB o2 sensors don't detect o2 levels when on the rich side of lambda 1, only on the lean side.
On the rich side where everyone is really interested in, the WB actually detects the concentration of combustibles in the exhaust, ie leftover products of incomplete combustion as in CO, HC and H2.
With excessive temp you get a leaner output.
BThe Bosch data sheet says that the hex part of the sensor should never exceed 800 C
On a long long WOT pull the EGT WILL exceed 800 C, it will probably exceed 900C.
If you measure AFR by some other means like how you do on and engine dyno AFR remains the same.

EG. use fuel and air mass flow meters or 5 gas exhaust analyser. Or both, or better still, all 3 at the same time.


One of the biggest problems these days with kids and modified cars is instrumentation and their lack of understanding of how instruments read wrong under (many) certain conditions.
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