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Sometimes well meaning enthusiasts ask us "how to tune an engine."
That can be considered a question with few boundaries if one is going to attempt an accurate answer.
What we are going to discuss here is going to be limited to the basics of optimizing fuel & spark timing.
I would like to present one strategy that is well regarded, think of it is a tuning process.
Let's agree that there is more than one way to skin a cat(fish) ,the process that follows was arrived at by the influence of theory learned in engineering university,
experience in tuning everything from dirtbikes to Twincharged Evo's and my colleague's experiences.
On our downloads page I may expand on this process further.
[Please email me if you see errors or have useful additions to this information]
We will refer to these graphs later.
Note 1: E.Q stands for "Equivalvence Ratio" and is merely the reciprocal of Lambda Ratio
Note 2: The Torque graph on the right is the mirror image of the Burn speed graph on the left because the fuel mixture scale on the bottom is simply increasing rather than decreasing.
Note 3: Faster Burn speeds (flame front travel propgation) allow the engine to be tuned for higher TQ output. I will expand more on this later.
Note 4: Each engine has it's own specific AFR curve, but alll engines are very close to these graphs high turbulence and centralized spark plugs (or even twin plug) enable leaner target
AFR's for max torque. i.e a 2 valve enine may like 12.5 AFR for best TQ and a center plug (4 valve) may like 13:1. We will discuss why it another article.
All other things being equal (never true ;-) ), the more air an engine can consume the more power it can make.
We are not going to discuss (in this writing) the aspect of tuning which enhances air consumption (volumetric Efficiency) by an engine, for sure that is perhaps the biggest aspect of making power .....cams ,turbo,porting etc etc .
What we are going to cover is optimising fuel and spark timing for a given engine configuration.
A few definitions before we begin:
MBT : Maximum Break Torque Timing
This is the theoretical and sometimes laboratory tested ignition advance (timing) that yields the best Torque output for a given set of conditions (RPM,throttle angle etc).
It is the timing that yields highest output assuming no knock limit. see Knock limit definition
Knock Limited : An engine is considered to be knock limited (limited by knock), if it begins to knock before MBT (is reached. That is to say it could make more power if we advanced the
timing, but if we did that it would start to knock. An engine that can reach MBT timing (optimal timing) without knocking is considered not knock limited (duh ? )
Knock : a.k.a detonation
Unknown to many when the fuel & air mix burns in your engine, it really is burning....NOT exploding. If your look at he Laminar burning chart above (left) 30 cm/second is a lot slower
than one might think the events in your engine are occuring. Normal combustion begins at the spark plug and a flame front travels across the comustion chamber creating heat and
expanding the combustion gasses as it travels across until it burns all mixture . This expansion of the gasses pushesdown on the piston relatively gently...compared to
During a knock cycle something different happens; The spark ignites the mixture normally, the flame front begins to travel across the chamber, temperatures rise and gas expands
perhaps as normal, the "end gasses" / the mixture charge at the far reaches under heat and pressure and it "spontaneusly" explodes from this heat and pressure before the flame
front even reaches it! So why did the end charge not ignite from the heat in pressure in normal combustion versus knocking? That could be for many reasons but one could be
lower octane fuel, low octane fuel can not withstand as much heat and pressure as higher octane fuel. Now we can think of another way to avoid knock...increase the flame travel
speed if it burns all the mixture before it can detonate that is another way to avoid knock. This deserves it's own write up though.
Let's Begin !
Let's assume you have a turbocharged engine, this can be both the safest and most dangerous type of engine to tune, it will become clear why.
You need to monitor T.I.T/EGT's (turbine inlet temps/Exhaust gas temp) and also Knock (this is a different article however), and stay below their limits.
I am glazing over this now, but these are the most important things to monitor!!!!!
Step 1 : Disconnect your wastegate ! The Goal is to take advantage of the situation and run Zero boost and also take advantage of the turbo engines relatively low compression ratio.
This is ideal for learning and the easiest way to map some load cells that are difficult to reach with the wastegate connected.
By disconnecting the wastegate I don't mead pulling the vacume lines off, this often leads to very high boost. One want to disconnect the wastegate actuator from the
wastegate flapper/valve so that the wastegate stays open. Other ways to do this especialy with external wastegates is to pump air pressure in to the w.g and pinch the hose
shut. But be careful as this may slowly leak down and yield boost.
The nice thing about tuning turbo engines in "zero boost mode" is that it is nearly impossible to get a low compression engine to knock at zero boost, so it is very safe.
Step 2: We assume you know your system and can get the idle tuned etc and can disable closed loop operation while mapping.
Set the timing to a good conservative value where you can expect decent running and no chance of knock. At zero boost most engines can take 10 degrees in all load sites, as a
Step 3: Adjust your fuel mixture at light loads (< 75% load/ 80kpa at sea level ) and idle to about 13.5:1. This will help you get in the ballpark for the rest of the tuning.
Step 4: Take an educated guess (all software is different so I can not be more specific), use the values from the cells that you have achieved your target of 13.5:1 in and extrapolate values
in to the full load cells (wide open throttle/zero boost?/100% load/100kpa ) trying to target 11.8"1 AFR for now.
Step 5: Once you have achieved a 11.5:1 at 100%l load . I like to connect the wastegate,but adjust it to the lowest boost possible (say 3-5 psig).
Then also map the 3-5 psi range at 11.5:1. Now disconnect the w.g to run zero boost again. This way you have some safe headroom , many cars will be able to make a liittle boost at
high rpm even with the wastegate port open.
Step 6: <the safer pre MBT zone>
Start optimizing finally! (at zero boost 100% load)
Optimise the fuel mixture
Now we can optimise the 100kpa line, If you are on a dyno you can tune and test for fine increases at specific rpms, if not you have to use prior knowledge to get close.
Optimise the timing last.
Start with the air fuel ratio, the best AFR is more nearly a known constant than timing. If you look at the graphs above you can see the a fairly flat peak of max Torque from
about 12.5 - 13.5. however at high rpms and continuous WOT (wide open throttle) the chamber begins to heat up . So most tuners set AFR to the rich end of this flat spot
because it does not cost any power but is safer andd may gain power under continuous high load (heating) conditions. Also looking at the curve the loss in power increase more
quickly from being slightly leaner than slightly richer than ideal. It is also gives you a wider safety margin. The price is slightly higher fuel consumption...
Since you are tuning at zero boost on a turbo engine you are not in much danger in this mapping region so I would choose about 13:1 as my Target. If you are on a dyno
your job is to experiment and find the ideal for your engine!
Optimise the timing (also at zero boost 100% load)
Now that you have an "ideal" AFR set, you can change the timing to find it's optimum.
Since we are using tuning a low compression engine with zero boost we are not going to be knock limited and so will be able to find MBT (mean best torque timing).
Caution !!!! this does not mean we will be able to run this much timing as we get into higher boost ranges!
What I normally do is make several dyno pulls at say 10 degrees advance eveywhere . Then I make sure that all TQ graphs are the same for each run,so that I know everything is
consistent. Including the Dyno itself.
Then because I'm cautious I actually remove 2 degrees from everywhere at the 100 % load line and do a pull. Probably I will see a drop it TQ at all rpm's.
If TQ dropped everywhere I start increase advance 1t 100% load at all rpms. Then I do another pull. Compare TQ pulls. Keep increasing timing until you notice some areas of the
TQ curve are not improving as much as others. This means you are getting near optimum on these areas. You should be monitoring for knock during all this , but that is another
Assuming we are indeed not knock liimited you will get to a point where advancing timing further does not increase power and then actually loses power. Since the engine is not
knock limited set the timing to the minimum amount that makes peak TQ at each RPM range.
Double Check AFR
Know with your timing optimized and likely you have found MBT. You can try to fine tune your AFR's to see if a little more power can be had....usually not much is gained at this
point. It is best to stay in the 13:1 -12.5 AFR range under non knock limited boost levels.
Step 7: Start working up the boost level. And repeat the process with caution and new things to keep in mind.
There is some boost level usually around 3-7 psig. Where the boost pressure just compensates for the lower compression ratio (C.R) and you have a "normally aspirated engine" level
of power.This is also about the limit where the engine is going start being knock limited and you will not be able to run MBT . Typically ideal timing curve the same from zero boost
on up to whatevr boost it starts to become knock limited. So let's say you are starting to be knock limited at 7 psi. The optimum timing curves may be very close at 0,3,5,6 psi.
Because it's not knock limited it can run MBT everywhere up to there.
Once you have reached the piont that knock is detected at less than MBT...
You have now entered <the less safe post MBT zone> Congratulations (uncongratulations?) you are now knock limited! LOL Don't be ashamed nearly all high
output tubo cars are knock limited, it's just the nature of high power density engines. ;-)
From this point the strategy is different you are no longer trying to find the timing that gives best power, you are trying to get as close to your MBT advance without knocking.
So your are advancing until incipient knock and then backing off 2 degrees (a typicall #).
Now here is a fact: Once knock occurs it needs to be retarded several degrees past the point where it initially started ,to get it to stop. For example if knock starts at 23 degrees and
knock occurs a knock detecting spark advance sytem actually has to retard back to say 20 degrees. There are many Engine management systems out there so use that information as
you can. This is the reason that ECU's with adaptive knock detection don't feels as smooth as non-adaptive spark systems . The Ecu is hunting the timing around and reaching knock
and then having to retard. Most tuners in knock limited situations advance to (earliest detectable) knock and then back off 2 degrees.
Because when you were not knock limited you were set to an AFR optimized only for power, it is normal to target slightly richer to maybe 11.5:1 as one is knock limited -to try to
stave off detonation. As you can see from the graphs TQ starts to drop rapidly at excessively rich mixtures. Also excessive fuel leads to rapid oil dilution. It's a balancing act.
There are two effects here with the slightly richer mixture, one is that the specific fuel charge is slightly more knock resistant ,and two, that over time the extra fuel will keep head
Changes in retarding timing have much more effect in avoiding knock than does richening the mixture. Retarding timing increases EGT's. It's a balancing act.
In the knock limited boost ranges it is best to target the rich end of ideal , about 11.8 to 11.5 under max boost. And retard the timing from MBT as needed to avoid knock. Of
course you will reach a limit where adding fuel and retarded timing will still not prevent knock, or high EGT's . At that point fuel & spark can not be optimized further and you will just
have to reduce the boost to a safe level or use Higher octane fuel or change "hard parts" different intercoolers.turbos etc.......or BOTH !
Some endurance engines, engines that are under high load/heat for long periods of time will run 11:1 AFR or so to keep temps down, they may make up for this loss of TQ production
(efficiency) by running more boost. Also some vehicles leave the factory with 10:1 fuel mixtures under full load in order to keep catalyst temps down.
Under max boost conditions there is a fight between avoiding knock and high Egt's, these will converge and limit the amount of boost that be run at some point, usually well before
the engine is mechanicaly stressed from elemental power production.
oh, did I mention .......
have fun, be patient,cautious and careful
it's a balancing act
iterative balancing act
P.S There are many more intersting things to discuss regarding the graphs ,knock,egt's , E85 etc.....stay tuned !