Dumb Tuning Question - N/A vs FI AFR

So I've generally seen stated that, in broad terms, WOT AFR is ~12.5 for N/A and more like ~11.5 for FI... But a recent thought had me questioning, how Volumetric Efficiency (VE) plays into that.

It's my understanding that the VE table is the primary tuning table, but it seems to me that the combustion chamber shouldn't care how the air got there. A good N/A engine can achieve, and even slightly exceed 100% VE. My (likely ignorant) thinking is that if an AFR of ~12.5 is good up at 100% VE, then it wouldn't matter whether it's hitting that N/A or using a turbo to get there. And that going richer should generally not be be required until the VE noticeably exceeds 100%... Or is there something I'm missing here?

In other words, if you add F/I to a lower VE (2V) N/A engine and restrict boost  to not exceed 100% VE, should you generally be able to maintain an AFR range typical for N/A tuning or do you still have to richen it up? And if the latter, why?

Historically, reduced AFR in boost was to provide more safety margin for less than perfectly controlled tuning. As engine controls advance that margin is probably getting smaller for the reasons you mention. There is a side benefit of charge cooling with extra fuel in boost if the intercooling is not well sorted or does not exist. Again this is a crutch for less well developed systems.

I would not be surprised to find modern factory boosted engines to run even closer to stoichiometric. I am sure some of our resident factory calibration forum members will chime in with more relevant and correct information.

The extra AFR with forced induction is to avoid detonation. It's not just fear of one cylinder running lean, although if you can't do per cylinder tuning that is a real risk. It's also increased cylinder pressure. The combustion chamber may not care how the air got there, but it definitely matters how much you got in there.

The other thing besides knock that needs watched are exhaust temps. To prevent damage to the turbo, exhaust manifold, exhaust valves, etc.  Depending on your set up, and how much you retard the spark to prevent knock, 11:1 may not be rich enough. But it's a good target to start with. 
 

Modern turbos are capable of 1050C, which is really hot, but that does allow for largely stoich running. But that is also DI fuel, and some great temp models. 
 

If you can swing thermocouples, I'd just peak at 850C. 

Many modern turbo engines run stoich under boost.

Many older endurance racing turbo engines ran as rich as 9:1 under boost to keep the engine alive long enough to finish a race.

One more thing to add, this isn't a dumb question. Not what so ever. 

alfadriver said:

One more thing to add, this isn't a dumb question. Not what so ever. 

Totally, what he said. Good question and thanks for the good answers. 

Driven5 said:

It's my understanding that the VE table is the primary tuning table, but it seems to me that the combustion chamber shouldn't care how the air got there.

The combustion process may not care about "how the air got there", but it does care about the temperature and pressure of the air/fuel mixture before ignition.  Comparing a 2L turbo (at 1 bar of boost) vs a 4L NA motor, both temperature and pressure will be higher.  That means more tendency to detonation, and richening up the mixture is one way to help deal with this.

MadScientistMatt said:

The extra AFR with forced induction is to avoid detonation. It's not just fear of one cylinder running lean, although if you can't do per cylinder tuning that is a real risk. It's also increased cylinder pressure. The combustion chamber may not care how the air got there, but it definitely matters how much you got in there.

That's the point. It seems to me that 100% VE should have the same cylinder pressure (and thus chances of detonation) regardless of how it got there. It's the same amount of air.

alfadriver said:

The other thing besides knock that needs watched are exhaust temps. To prevent damage to the turbo, exhaust manifold, exhaust valves, etc.  Depending on your set up, and how much you retard the spark to prevent knock, 11:1 may not be rich enough. But it's a good target to start with.

This question isn't turbo specific, but even in that case I was under the impression that controlling EGT isn't overly difficult in 'low-boost' applications.

codrus (Forum Supporter) said:

The combustion process may not care about "how the air got there", but it does care about the temperature and pressure of the air/fuel mixture before ignition.  Comparing a 2L turbo (at 1 bar of boost) vs a 4L NA motor, both temperature and pressure will be higher.  That means more tendency to detonation, and richening up the mixture is one way to help deal with this.

If they both make 250hp @5500 RPM, the a 2L turbo has ~160% VE while the 4L N/A has ~80% VE. I'm looking at it the other way around.

Sticking to 5500 rpm for everything...

If the 2L was N/A at 100% (so it's a 4V) it would only be making 160hp. So boosting it up to 250hp, I would expect 11.5 (or richer) AFR tuning.

Now that 4L engine at 250hp is easy enough, even for a 2V engine, and I would expect 12.5 AFR tuning.

But what if it was a 3.2L 2V engine. Stock it makes 190hp at ~75% VE. Putting enough work ($$$) into it (N/A) to get it up to 100% VE would hit 250hp, and I'd expect to still be running 12.5 AFR tuning.

However, what if it was mildly boosted (~5psi) to get the 3.2L engine up to 100% VE instead? Generally speaking, would it still be safe to run 12.5 AFR tuning? If not, why?

I don't have answers, but perhaps i have pertinent questions?

 In terms of, as you say, caring how it got there; if the intake is restrictive enough to need significant boost to get to 100% VE, how much is the boost heating the air?

 And on that older, less efficient design, how much is combustion chamber shape eating into your safety margin?

 This reminds me of an interview i heard a little while ago talking about how the focus on engine design has shifted from making it easy to get the air in and toward jamming it in with a turbo and focusing on combustion chamber shape to deal with the result.

In reply to Driven5 :

The simple 2.0 booster vs 4.0l na misses the physics that compressing air makes it hotter. Which means more boost to offset the heat to make the volume comparable. And making spark knock far more sensitive. Even with intercooling, it's hotter to not really be equal. 
 

Can you tune boosted engines to run at 12.5:1?  Sure. Will it be the same as the na?  No. The physics are different enough to be hard to equalize for comparison. 
 

Edit- another factor that makes the comparison hard is backpressue and how it effects breathing. 

It depends on how you're measuring VE. Some tuners normalize it to intake pressure to keep the numbers in a smaller range. It the VE is calculated relative to sea level pressure, the turbo motor will have much higher numbers; a motor with an 90% normalized VE at 7.5 psi would be more like 135% VE relative to standard temperature and pressure.

In reply to Jesse Ransom :

Much like exhaust temps, I was under the impression that controlling IAT isn't overly difficult in 'low-boost' applications. Assuming the combustion chamber shape supports 100% VE at 12.5 AFR for N/A, shouldn't it for FI as well?

In reply to MadScientistMatt :

The physical number of 'air molecules' that will fit in the swept cylinder displacement at STP, and getting that amount into the cylinder via N/A vs FI.

In reply to alfadriver :

I do realize that there will be additional inefficiency in the FI system, so more realistically the 3.2L low-boost (<=100% VE) setup would be down a slightly on power vs the 3.2L N/A setup if both are running the same ~100% VE at 5500 rpm. But if IAT's and EGT's aren't much higher than N/A, and again only having as much air enter the cylinder as N/A is capable of anyway, why would the 3.2L FI setup be tuned with AFR similar to traditional 2.0L 'high-boost' (>100% VE) FI rather than similar to 3.2L or 4.0L (<=100% VE) N/A?

In reply to Driven5 :

Yes. Getting 100% na needs a lot of restrictions taken out, and is pretty hard- but you end up with a cool, dense mixture just to get there. Getting to 100% boosted needs some back pressure from spinning the turbo, and some heat from air compression. So at a bare min, the charge will be hotter to get to 100%. 
 

Here's where it get funny, though. From a pure physics standpoint, heating the charge is good for the heat differential for the Otto cycle. In reality, it's bad for spark knock. So in the end, it's a balance of the raw physics vs the realistic physics. 
 

Meaning, it depends.

But for that little of boost, 12.5:1 would be good for peak power unless it's a terribly restricted system  

Driven5 said:

MadScientistMatt said:

The extra AFR with forced induction is to avoid detonation. It's not just fear of one cylinder running lean, although if you can't do per cylinder tuning that is a real risk. It's also increased cylinder pressure. The combustion chamber may not care how the air got there, but it definitely matters how much you got in there.

That's the point. It seems to me that 100% VE should have the same cylinder pressure (and thus chances of detonation) regardless of how it got there. It's the same amount of air.

VE is only part of the picture.  you can have the same VE but wildly different cylinder pressures and temperatures depending on how much exhaust from the previous cycle is left behind.  The higher the exhaust manifold pressure, the worse that is.

Pete. (l33t FS) said:

VE is only part of the picture.  you can have the same VE but wildly different cylinder pressures and temperatures depending on how much exhaust from the previous cycle is left behind.  The higher the exhaust manifold pressure, the worse that is.

Admittedly, that one took me a minute to think through. I know that intake usually gets more attention than exhaust because pistons are pretty good at forcing the air out, but I can also now see how FI could get you in trouble here on a crappy enough breathing engine. For N/A to get 100% VE would require good cylinder evacuation, but FI can get the desired amount of air in in either way. I understand it's probably part of what alfadriver meant too, but managed to click better for me in this context. 

So that's more of the 'it depends' part. More generally though, if I'm keeping track of key characteristics, it still sounds like if an engine has decent combustion chamber shape, can keep IAT and EGT in check, and can get reasonable cylinder evacuation, the mild-FI setup should (safely) tune AFR's more like N/A than traditional FI. Anything else that needs to be added to the 'it depends' list to help align the way it works in my head to the way it works IRL?

The tradeoff, as mentioned already, is usually peak power vs life. Depending on the use case, running leaner will bring egts up and you need better parts to deal with it, but is very engine dependent. An engine that runs under 10s at the dragstrip has a different set of constraints than one that runs for hours on a road course. 

That being said, the afr targets you mentioned are not a bad starting point for tuning and likely won't hurt anything in the short term.

Also AFR in DI setups can be way different than port.. which I won't get into.

Driven5 said:

If they both make 250hp @5500 RPM, the a 2L turbo has ~160% VE while the 4L N/A has ~80% VE. I'm looking at it the other way around.

Sticking to 5500 rpm for everything...

If the 2L was N/A at 100% (so it's a 4V) it would only be making 160hp. So boosting it up to 250hp, I would expect 11.5 (or richer) AFR tuning.

Now that 4L engine at 250hp is easy enough, even for a 2V engine, and I would expect 12.5 AFR tuning.

A 2L naturally aspirated motor making 250 hp at 5500 RPM is fantasy, it doesn't exist no matter what compression ratio you have or AFR you tune it at.

Also, the "turbo engines run rich" rule of thumb is roughly set by the boost level.  If you're running it in vacuum then the AFR target is usually set to around the same level it would be for an NA engine, and for the same reasons.

You specifically asked about AFRs at WOT -- if a turbo engine isn't making boost at 5500 RPM and WOT then there's something seriously wrong with it. :)

In reply to Paul_VR6 (Forum Supporter) :

Think 'budget endurance racing engines' for the combination of WOT and 'safe' AFRs.

In reply to codrus (Forum Supporter) :

I specifically never mentioned the 2.0L making 250hp at 5500 rpm while N/A.

I also specifically never mentioned the turbo engine not making boost at 5500 rpm and WOT. Just because it isn't above 100% VE doesn't mean it's not in boost. If an engine only runs 50% VE N/A, then getting it to 75% VE with no other changes still requires additional manifold pressure.

Volume is not mass, though.  Mass is what matters, the way to move mass when constrained by atmospheric pressure is to increase VE.

Increasing VE helps all engines make power but turbo engines can just crank up the boost to increase mass flow, up to the limits of the turbo to move air without adding a lot of heat or increasing exhaust backpressure to a high degree.

There were a lot of interesting discussions on eng-tips about twincharged engines.  Positive displacement supercharger, that a turbo blows into.  If the turbo has a 2:1 pressure ratio and the supercharger is 1.5:1, that is multiplicative not additive, so you have a turbo making 15psi boost and a supercharger making "7psi boost", for an end result of about 30psi manifold pressure (2:1 is 30psi absolute, times 1.5:1 is 45psi absolute, minus atmospheric is about 30psi boost).

But the kicker is the supercharger gives instant response, so there is "7psi boost" worth of additional exhaust flow to get the turbo party started.  And the exhaust backpressure is a LOT lower than a purely turbocharged engine would require to make 30psi since the turbo only has to spin hard enough to make a 2:1 pressure ratio.  This means there is very little in the way of exhaust residuals, so the engine becomes extremely detonation resistant.  Tales of 700hp on pump gas from small displacement engines...  the biggest issue is having an engine bay large enough for all the plumbing.

^ I kinda want to throw a turbo on my cooper s due to that exact line of thinking 

In reply to Paul_VR6 (Forum Supporter) :

I bought a Corrado G60 engine to start a project like this.  I still have a small selection of Eaton superchargers.

Where I hang up is engine management.  You'd almost have to run a MAF, or a MAP sensor and an exhaust backpressure sensor, and I worry it would be beyond my ability to tune well.

There is some craziness with boosting- I saw what I thought was nuts- just pile up boost even with horrible combustion, and it will make power. I know this isn't what is being asked, but on an experimental engine, right at the rich limit (like 9.5:1) with super retarded spark, and as much boost as possible, this engine made huge power. For less thank half the 5.0, more than 5.0 power.