General Tuning Question - AFR vs Timing

[oldskool]

Ok lets assume you have a car tuned to an AFR and the timing at that load/rpm is ideal. Now lets suppose you richen the mixture/lower the AFR. What should you do to the timing, if anything?

Some things, possibly misconceptions, that I've heard in the past:

- Advance timing until it knocks and then back off 1-2 degrees
- A the flame front in a lean mixture burns slower, so more timing advance is appropriate, (intuitively a rich mixture would need less advance)
- More timing advance + more fuel = more power
- Not enough as well as too much timing advance can cause knock
- Too much as well as not enough fuel can cause knock

Please feel free to dispel or confirm any of these statements with good scientific explainations. I've spent hours searching the net for the releationship between AFR and timing and have not found a good explaination.

Thanks!!

[dont_blink]

Not enough timing advance will only cause knock if you are already running too rich causing knock. So that kind of goes hand in hand. Bear in mind that while a leaner mixture can allow more timing, a leaner mixture also creates more heat, causing that extra timing to make some knock. Generally speaking the more "filler" in the cylinders, the slower the flame, the more timing. Higher octanes, water, exhaust gas, the more stuff left in the cylinders that require more energy, the more timing you can add. Obviously most motors have diminishing returns. On the LNF it seems like more than 13 degrees WOT gains no benefit other than heat and premature rod bearing wear. It's also pretty much a given that if you richen the mixture, you can add more timing as the extra fuel is cooling the cylinders more.

Here is a quick run down that goes a little more in depth:

http://www.max-boost.co.uk/max-boost...20metering.htm

[uberDoward]

Personally, I run minimum timing for best torque.

[oldskool]

Personally, I run minimum timing for best torque.

That can only be determined on a dyno...
Are you claiming that you street tuned timing for MBT?

[Terminator2]

Personally, I run minimum timing for best torque.

less timing means less torque ( and hence less hp) in most cases but you have to dyno it to see the results. I have used anywhere from stock advance to 19* and I can tell you my car on 93 octane seems to pull a lot harder on 18-19* than it does on 13-14* I get more wheel spin and more top end power it seems. It flows a little more air too on the logs I have.

[Terminator2]

Not enough timing advance will only cause knock if you are already running too rich causing knock. So that kind of goes hand in hand. Bear in mind that while a leaner mixture can allow more timing, a leaner mixture also creates more heat, causing that extra timing to make some knock. Generally speaking the more "filler" in the cylinders, the slower the flame, the more timing. Higher octanes, water, exhaust gas, the more stuff left in the cylinders that require more energy, the more timing you can add. Obviously most motors have diminishing returns. On the LNF it seems like more than 13 degrees WOT gains no benefit other than heat and premature rod bearing wear. It's also pretty much a given that if you richen the mixture, you can add more timing as the extra fuel is cooling the cylinders more.

Here is a quick run down that goes a little more in depth:

http://www.max-boost.co.uk/max-boost...20metering.htm

As long as you are not knocking more timing should not cause more rod bearing wear. There is a point at which more timing will not make more power but that point seems to be past 13* of advance as long as there is no knock. depends on fuel and mods it seems. More efficient IC cools intake charge off 30*-40* better than the stocker on my car so It seems like with the cooler air charge I can run more timing than I could before and still get no KR.

[Terminator2]

Ok lets assume you have a car tuned to an AFR and the timing at that load/rpm is ideal. Now lets suppose you richen the mixture/lower the AFR. What should you do to the timing, if anything?

Some things, possibly misconceptions, that I've heard in the past:

- Advance timing until it knocks and then back off 1-2 degrees
- A the flame front in a lean mixture burns slower, so more timing advance is appropriate, (intuitively a rich mixture would need less advance)
- More timing advance + more fuel = more power
- Not enough as well as too much timing advance can cause knock
- Too much as well as not enough fuel can cause knock

Please feel free to dispel or confirm any of these statements with good scientific explainations. I've spent hours searching the net for the releationship between AFR and timing and have not found a good explaination.

Thanks!!

Lean mixtures burn faster than rich mixtures and they tend to produce more heat. Richer mixture needs more timing to completely burn and they tend to produce less heat. Too rich will cause knock, too lean with too much timing will cause knock. 0.85-0.86 Lambda seems to make the most power on the LNF so find the appropriate amount of timing for the fuel you are using and mods. Ie. tune the A/F for maximum power and base you timing off of that and your fuel grade. If you are knocking pull out timing in those areas you are getting the KR.

[oldskool]

Lean mixtures burn faster than rich mixtures and they tend to produce more heat. Richer mixture needs more timing to completely burn and they tend to produce less heat. Too rich will cause knock, too lean with too much timing will cause knock. 0.85-0.86 Lambda seems to make the most power on the LNF so find the appropriate amount of timing for the fuel you are using and mods. Ie. tune the A/F for maximum power and base you timing off of that and your fuel grade. If you are knocking pull out timing in those areas you are getting the KR.

So if lean mixtures burn faster they require less timing than a richer mixture to be optimal right?

[Terminator2]

So if lean mixtures burn faster they require less timing than a richer mixture to be optimal right?

Correct. Too much burn time with a lean mixture will cause lots of KR.

[cobaltssoverbooster]

and increased timing does contribute to premature rod bearing failure do to you increaseing the cylinder ignition pressures by making it more exact thus more pressure on the rod bearings. the man is correct.

[uberDoward]

That can only be determined on a dyno...
Are you claiming that you street tuned timing for MBT?

No way - I do the afr on a street tune, do rough timing on the street, and fine tune it all on the dyno.

I'll work up timing until I no longer develop more torque, and back it 2-3 degrees from that point, depending on where the timing is at. Hence 'minimum timing for best torque'.

For example, I dyno tuned a friend's MS4 LS1 setup, and made no more power from 26 degrees to 29 degrees. Evidently, a lot on LS1 tech run around 29 degrees, I set his to 25.5 degrees and rolled.

Ended up running an 11.8 on exhaust + cam

[dont_blink]

As long as you are not knocking more timing should not cause more rod bearing wear. There is a point at which more timing will not make more power but that point seems to be past 13* of advance as long as there is no knock. depends on fuel and mods it seems. More efficient IC cools intake charge off 30*-40* better than the stocker on my car so It seems like with the cooler air charge I can run more timing than I could before and still get no KR.

Do you have any idea what timing advance means? It means that's how many degrees before top dead center that the plugs are fired. The farther away from top dead center the plugs are fired, the more stress is place on the moving parts. The rod bearings end up taking a bulk of that beating. As far 13 degrees advanced, I'm not sure what car you are referring to, but on most 4-cylinder engines (even boosted ones) that's less than stock WOT timing.

[oldskool]

I just read through the article linked in this thread - very useful!

optimum timing is a function of load and RPM right - on any engine? I don't understand why people say, "max timing at WOT". That value may be fine at 6300rpm, but what about 7300rpm? Timing advance increases/plateaus with RPM and decreases with "load" right?

[uberDoward]

That's a very rough way of putting it, but in a general sense - yes.

It has to do with the density of the airflow/fuel coming into the engine. A less dense mixture requires more timing, while a more dense mixture requires less. RPM comes into play, in relation to the amount of time available to make things happen

That said, the entire idea is that you want maximum cylinder pressure approximately 20 degrees AFTER top dead center. In my experience, the amount of power made is EXTREMELY similar with maximum cylinder pressure between about 15 and 20 degrees ATDC, so I set my timing to maximum torque, and retard 2-3 degrees from there.

Oldskool, a lot of people rely on knock data for part throttle - IMHO, a bad idea. Knock retard happens AFTER the fact

[oldskool]

Oldskool, a lot of people rely on knock data for part throttle - IMHO, a bad idea. Knock retard happens AFTER the fact

I totally agree - which is why i do not use the method of advancing to KR and then pulling a couple degrees.


I guess a general consensus is that factory tunes are conservative - leaving room for bad gas, weather conditions, ect... So if you add a couple degrees to a stock tune you will probably realize a gain. Going to KR...not smart IMO, but to each his own.

Is there a way to measure gains from spark advance excepting a dyno...

[EaglePerformance]

Lean mixtures burn faster than rich mixtures and they tend to produce more heat. Richer mixture needs more timing to completely burn and they tend to produce less heat. Too rich will cause knock, too lean with too much timing will cause knock. 0.85-0.86 Lambda seems to make the most power on the LNF so find the appropriate amount of timing for the fuel you are using and mods. Ie. tune the A/F for maximum power and base you timing off of that and your fuel grade. If you are knocking pull out timing in those areas you are getting the KR.

Just to clarify a bit, 0.85-0.86 Lambda, 12.5-12.6 A/F produces the fastest laminar flame speed, therefore at this point, the least amount of timing is needed, so generally speaking, any thing leaner or richer will require more timing. On a N/A engine, WOT, peak torque is peak efficiency, the cylinder is filled the most it can be depending on design, mods, etc., the more tightly packed a mixture is, the faster the flame front, so the least amount of timing is needed at this point to get maximum pressure exerted on the piston at a point approx 15* ATDC, earlier than this starts to fight the rising piston causing rod wear. More timing is needed past this point because of decreasing VE, also as rpms rise, the window, (time), to get maximum pressure at the right time is shorter because the time needed from spark to when the charge is completely burned is relatively constant, things happen much faster at 6,000 rpm than they do at 4,000.

[EaglePerformance]

Do you have any idea what timing advance means? It means that's how many degrees before top dead center that the plugs are fired. The farther away from top dead center the plugs are fired, the more stress is place on the moving parts. The rod bearings end up taking a bulk of that beating. As far 13 degrees advanced, I'm not sure what car you are referring to, but on most 4-cylinder engines (even boosted ones) that's less than stock WOT timing.

I'm new to this platform, just had a guy come into the shop with a GXP, he had some mods on it, dynoed at almost 270HP, 295TQ, (I'm impressed, didn't realize these things were out there running around!), timing was at about 11* WOT, which seemed real light to me, although he was pushing 23lbs boost. He MAY have had the ecu tweaked by someone up in Canada, I don't remember, but I just got an HPTuners beta that should allow some tuning work, does the fun ever stop?!