Timing vs. AFR

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Daggar

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Oct 17, 2004

#1

• Oct 17, 2004

• #1

Anyone have an idea of what or how much a change in timing one way or the other should effect the air fuel ratio? For instance: How much do you think 2 degrees of timing advance should effect the Air Fuel Ratio if nothing else is changed?

 

ponycar17

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Nov 17, 2001

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#2

• Oct 17, 2004

• #2

That's going to be dependent on your fuel octane, flow through the engine, etc... I don't think it's as simple as one answer...

With a lower octane fuel, it won't matter as much. All of it will be burned up quickly... With a higher octane fuel, it may be a bit trickier because of the slower burn rate...

 

D

Daggar

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#3

• Oct 17, 2004

• #3

ponycar17 said:

That's going to be dependent on your fuel octane, flow through the engine, etc... I don't think it's as simple as one answer...

With a lower octane fuel, it won't matter as much. All of it will be burned up quickly... With a higher octane fuel, it may be a bit trickier because of the slower burn rate...

Click to expand...

I get you. I'm not looking for an exact quantity really. There are too many variables with unduction, motor size, fuel quantity etc. I guess I'm just wondering if it would have a significant effect.

 

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autoXr1

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#4

• Oct 17, 2004

• #4

Daggar said:

I get you. I'm not looking for an exact quantity really. There are too many variables with unduction, motor size, fuel quantity etc. I guess I'm just wondering if it would have a significant effect.

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The effect is minimal, like 10th's of points, and that's short term effect...like most fuel pressure adjustments. There's this little thing called "adaptive control" that the EEC has that may "fight" your tuning attempts. If you're truly having AFR issues on a speed density car after you've done mods....you may have to start looking for a programmable engine management system. As you know, the stock programming on a speed density computer is rather "tight" in what it allows.

 

Michael Yount

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Apr 10, 2002

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Oct 17, 2004

#5

• Oct 17, 2004

• #5

We played with total timing when tuning my chip - didn't see any a/f ratio changes while altering only total timing from 30 to 32 to 34.

 

D

Daggar

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#6

• Oct 17, 2004

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Ok... I think that helps a bit. I made a small series of changes today that included bumping up the timing. Prior to today my AFR stayed pretty much on the rich side during boost. That's with my boost-a-pump turned to it's lowest setting. I adjusted the tension on the blower, reset timing to roughly 12*, and reset the computer. When I test ran it on back country roads, I saw that I was considerably leaner under full boost (9lbs). I adjusted it out easily enough with the boost-a-pump but it got me thinking about what the big change could have been. I doubt that it was belt slippage because it wasn't that far out of whack. I just wanted it to be better. What you guys posted kinda rules out the timing change. So that leaves the computer reset as the primary culprit. Any thoughts?

Edit: Boost-a-pump is set roughly half way now where it used to be set to 0 giving me roughly 12.5 AFR under boost.

 

Michael Yount

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#7

• Oct 17, 2004

• #7

I'm naturally aspirated - you're boosted; perhaps those two situations don't react the same with increased/decrease timing. I'd guess if there were any significant effect, with more timing the burn would be more thorough -- so it would show leaner with more timing, richer with less timing. But it's only a guess - and for a few degrees change, I don't think there'd be much change.

 

D

Daggar

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Oct 18, 2004

#8

• Oct 18, 2004

• #8

Update:

After driving into work this morning, I notice allot more responsiveness and the Vac on my guage is being chased away considerably quicker than it used to be. The serpantine belt must have been slipping allot more than I thought it could have been. I was still seeing 9lbs of boost on my guage before repositioning that pulley but it did not spin up as quickly as it does now. Anyone know if there is a complete 10 rib pulley kit out there somewhere for Stangs?

 

FastGT94

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Oct 18, 2004

#9

• Oct 18, 2004

• #9

I'm also interested in this. My dyno guy suggested that for every 2 degrees of timing advance that I should add 2% more fuel in my PMS table for that RPM range.

But he said that there's really no definite number that you have to do and no way to truely know without getting it back on a dyno or running a wideband.

 

Michael Yount

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Oct 18, 2004

#10

• Oct 18, 2004

• #10

"I'm also interested in this. My dyno guy suggested that for every 2 degrees of timing advance that I should add 2% more fuel in my PMS table for that RPM range. "

In boosted combos this makes sense as you're using the cooling effect of fuel to stave off detonation with increasing boost and/or timing.

 

FastGT94

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#11

• Oct 18, 2004

• #11

Hmmm, I'm N/A so you're saying most likely I shouldn't need to worry about more fuel?

 

Tomato

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Oct 18, 2004

#12

• Oct 18, 2004

• #12

There really is not good rule....

for a given timing, there MAY WELL BE a more appropriate AFR. For instance, on a NA motor, 2 degrees timing wont have any impact on the afr that is "there", but 2 degrees timing may make more power with more or less fuel. Make sense?

On SC motors where 2 degrees can equal 10 - 15 lb ft at low rpm though, his changes things. If you are making significantly more power, you are using more fuel...

 

Michael Yount

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Oct 18, 2004

#13

• Oct 18, 2004

• #13

Tomato is spot on - with boost - more boost/timing usually means more power if you can stay out of detonation. More fuel helps that happen.

On my tuning naturally aspirated, adding timing increased power (up about 6HP from 32 to 34 degrees), and then we began to drop off. So we left it at 34. None of those pulls at different timing levels had any impact on the a/f ratio data. We didn't change fuel at all as we changed timing.

Work with your tuner - they should be experienced enough they'll know what to do - and if they/you don't have the experience, then I'd leave it alone.

 

Ranchero5.0

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Oct 18, 2004

#14

• Oct 18, 2004

• #14

Ditto the above. I'd love to see how your combo would react to a MAF conversion. SD being what it is works pretty good under vac conditions, but just seems too vague for boost related issues.

As said above, in my experience the extra timing down low works awesome with a torque cam, but I never saw a difference with the E cam on my friend A trimmed 88 Tcoupe. That cam just didn't do anything below 3k and the T coupe is heavy enough to need the extra low end grunt.

Jamie

 

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Daggar

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#15

• Oct 18, 2004

• #15

Ranchero5.0 said:

Ditto the above. I'd love to see how your combo would react to a MAF conversion. SD being what it is works pretty good under vac conditions, but just seems too vague for boost related issues.

As said above, in my experience the extra timing down low works awesome with a torque cam, but I never saw a difference with the E cam on my friend A trimmed 88 Tcoupe. That cam just didn't do anything below 3k and the T coupe is heavy enough to need the extra low end grunt.

Jamie

Click to expand...

The initial reaction would be a net loss of around 30hp due to the restriction caused by a mass air meter.