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I am trying to understand how load and/or VE affect ignition timing. Specifically in situations where you are rescaling an ignition map for increased load. I am working on Subaru specifically, where the x axis of the ignition map is in g/rev. I am working on a car that exceeds factory load limits. From my studies, I have found that most turbo Subarus like ~12* advance at peak torque and ~20* at redline, peak load.
What I'm trying to understand is why the large increases in load do not necessitate retarding the timing?
It would seem intuitive to maintain the ignition values in the previous max load column, and retard the timing in the new higher load columns.
I feel like I must not totally grasp either the factors that affect flame speed, or VE, or both!
On an airflow based system the spark should still retard with more load. I'm not sure where you get those numbers, but they are likely general rules of thumbs that people post about on various forums.
Here is the stock ignition timing map on a 2015 WRX with direct injected FA20DIT engine. You can see that the spark retards as load increases in the areas where the engine actually runs. At very high rpm or very low rpm/high boost the values are just copied and pasted by the factory tuner because the engine doesn't run there (6800+ rpm). Rescaling the load axis without retarding timing is something that you "could get away with" but doesn't mean it's a good idea. Also, keep in mind on a Subaru the dynamic advance tables to see the total possible spark.
On normal pump fuel, you need to retard with more load to reduce knock. On E85, you should still retard spark to keep cylinder pressures down, unless you have a built motor. It's possible that you are looking at maps for built motors on E85, in which case it's not as critical to retard spark with more load/boost.
Thanks for the reply. What you are describing is how I would predict the timing would be affected, as load increases. Am I correct in saying a denser a/f mixture will burn faster? Requiring less advance?
But what I am seeing is along the lines of the timing values at peak load I previously mentioned, in the new higher load range (3.5g/rev for example), then the timing values are horizontally interpolated back to 1g/rev. This results in higher than stock advance values in most of the load ranges between (17* at 2.75g/rev where it had previously had 12*).
Can you post a screenshot of what you are looking at/working with so we can have better context?
Sometimes you can get away with richer A/F ratio + more spark making more power than leaner A/F + less spark.
Ohhh you're talking about this dynamic advance table that you've been looking at.
Look at the Primary ignition timing first. It's retarding timing with increased load (more airflow mass per engine revolution) as we would expect. The dynamic Advance table is the authority to advance timing according to the knock signal. It's basically saying that the knock system is allowed to advance timing (relative to the primary ignition table) up to ~5.5 degrees at higher boost. It's only going to advance those 5.5 degrees if the knock sensor doesn't pick up too much activity (look at knock feedback correction and knock count).
So this is really just simple math. If you put "5" in the entire dynamic table, well that effectively adds 5 degrees to the primary ignition table (forgetting about air temp compensation etc) assuming zero knocking. In that case you are still decreasing final calculated spark advance with load due to the way the primary ignition table is tuned.
Basically there's nothing wrong with the tables you posted unless you have datalogs that show otherwise (serious knocking and dynamic advance multiplier never getting to max).
Thanks again for the reply. I totally understand how DA is applied to the primary ignition.
My question is in regard the timing values at peak load.
When comparing maps between cars that hit load of 2.6 and cars that can achieve load of 4, they both seem to have similar timing values at peak load. This is what is counterintuitive to me.
The tables I posted are for a car that can achieve well over peak load.
Unfortunately we need to have a lot more specific information and accompanying datalogs to say why. Maybe one engine is running E85, one is running 91 octane. One could be running a big rotated turbo, one could be running a stock turbo. Maybe one has a stock intercooler and the other has an upgraded one. Also, the stock tune has some margin in it depending on octane. There are also valve timing and AFR that can affect spark.
Also, there is the simple fact of groupthink/doing what the internet says. You don't want to be guy who did something unusual and gets called out ("you should see such and such guy's tune, he's an idiot").
Let me give you a more specific example. the car above is a stage 2 sti. Basically upgraded down pipe, more boost. It has timing retarded over oem tune which makes sense, and it already exceeds stock load range in tables.
My car (not tuned by me), has a bigger turbo, injectors, and pulls 4g/rev.
Both cars are running on pump gas and both cars have similar peak load timing values, despite different peak loads.
I definitely don't want to just plug in values due to precedent. I'm posting to understand this since it doesn't make sense.
I still don't know the exact details of what's been done to each car in question, but I will give you a general list of items:
1. Lower charge air temps - a larger/more efficient compressor wheel and better intercooler will give you lower charge temps which are less prone to knocking.
2. Larger turbine A/R in your case - I don't know what turbo you have and how it compares to the stage 2 STi. However if the turbine A/R is larger, you end up with less backpressure in the up pipe/turbine inlet and less residual gas in the engine. This reduces knock.
3. Some fueling differences. California/West coast 91 octane premium fuel isn't so good for knock compared to a 93 octane tune.
4. Differences in AVCS maps. Specifically, if the intake valve closing timing is later (less intake advance), you can knock less, but you might not trap as much air. If the exhaust opening timing is changed, you may evacuate the exhaust gases better, but only in conjunction with the right turbo.
5. Change in the engine noise profile (knock sensor has less/more background noise triggering retard)
6. Less restriction in the air intake and catback/downpipe back exhaust reducing knock, or maybe a more efficient wastegate reducing inlet pressure.
7. Ambient conditions. The two cars were tuned in two different environments in terms of altitude and ambient temperature/humidity, but still rely on the knock system to get the spark correct as weather changes.
8. General tuning philosophy/risk tolerance - tuners can get into a rat race to advertise the most power/torque/drag times, even with super sketchy things like questionable correction factors.
This is good info! Thanks for posting.
If this is still a MAF car don't discount MAF scaling and even possibly boost leaks to account for the difference. In other words what is reported as 4g/rev on 1 car may be equivalent to 3g/rev or whatever on another car. Intake pipes and air filters can change the MAF reading significantly. A boost leak will report as high airflow through the MAF but the actual charge reaching to the cylinder will be much lower.
Hmmm, interesting points. Thanks for posting!
I am starting to retune my car from scratch and am interested to see what timing values I come up with. I know for sure that on the current tune, MAF scaling is off due to my long term fuel trims being off by about 7% at low loads and my AFR in open loop being a little rich in the highest load areas.