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      01-05-2008, 07:34 AM   #1
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Optimum AFR for Direct Injection?

So forgive me for intruding here but I'm curious about the best AFR for direct injection. I've lurked and found several examples of what appear to be very lean afrs on dyno plots. I drive a direct injected mazdaspeed6 and everyone in mazdaland is trying to hit 11.5:1 or thereabouts with a couple of outliers tuning to 12.5:1. Can anyone speak with authority about the optimum afr for DI? Shiv?

Examples:
http://www.e90post.com/forums/showthread.php?t=29708

http://www.e90post.com/forums/showthread.php?t=105342

http://www.e90post.com/forums/showthread.php?t=105080

http://www.e90post.com/forums/showthread.php?t=104489
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      01-05-2008, 06:22 PM   #2
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I thought that you should keep the AFR in the high mid to high 11s at high rpms for FI engines. The SSTT goes to approx. 11.0 at high rpm. Three of the posts cited are Procede and are running lean. Could that account for the increased number of CEL and limp modes with the Procede?
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      01-05-2008, 07:20 PM   #3
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The afr you want to be at is around 11.5 at wot.That is just about perfect,at least for my Evo,s they were.
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      01-05-2008, 08:07 PM   #4
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For an evo probably but thats a port fuel injected car and 11-11.5:1 is backed by lots of experience with that technology. I'm new to DI and not really sure about the optimum afr. The benefits of DI are cooler temps due to vaporization of fuel and more efficient and homogenous fuel/air mixture. Because of this I'm speculating that they can be run leaner that pfi cars. The afrs of the dynos I found would scare the sh!t out of an evo owner if they were from his car.
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      01-06-2008, 01:29 AM   #5
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Regardless of how the fuel gets in, its still a 4-stroke combustion cycle with essentially the same chamber limitations as cars of 50 years ago.

Yes, there are improvements, but mainly for economy and light throttle cruise with DFI. At WOT, its all about correct spark advance for given dynamic compression, fuel mixture and volumetric efficiency of combined intake, heads, cylinders/chambers and exhaust.

These rules will not change while we have petrol common internal combustion engines. Best power by AFR, as a general rule, is 12.5-13 for NA and 11.5-12 boost. The rest of the power gains are to be made in increasing volumetric efficiency and timing.
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      01-06-2008, 03:47 PM   #6
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Quote:
Originally Posted by MNR0 View Post
Regardless of how the fuel gets in, its still a 4-stroke combustion cycle with essentially the same chamber limitations as cars of 50 years ago.

Yes, there are improvements, but mainly for economy and light throttle cruise with DFI. At WOT, its all about correct spark advance for given dynamic compression, fuel mixture and volumetric efficiency of combined intake, heads, cylinders/chambers and exhaust.

These rules will not change while we have petrol common internal combustion engines. Best power by AFR, as a general rule, is 12.5-13 for NA and 11.5-12 boost. The rest of the power gains are to be made in increasing volumetric efficiency and timing.

I think his point maybe has to do with the cooling effect offered by DI. Whereas a port fuel injector atomizes fuel by spraying directly on the backside of the hot intake valve, DI injectors (as I'm sure you know) spray fuel directly into the cylinders. Injecting the fuel directly into the cylinder means heat from the cylinder, and not the intake valve, is now being used to evaporate the liquid fuel. Since cars need to run richer than stoich at WOT for cooling reasons, it would make sense that you'd be able to run a DI motor slightly leaner than an equivalently equipped PFI engine. Of course that's in theory, so I'm curious to hear more about this as well.
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      01-07-2008, 09:16 AM   #7
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If that is the case why, time and time again, have I seen dynos with afrs for the 335i with numbers over 14 until 4000 rpm? I have heard about some sort of intake for the exhaust to make the cats work better but this has not been confirmed. If it was there, how would one be able to tune the car? The latest high afr dyno I've seen is in the "safe AFR under high boost thread" here: http://www.e90post.com/forums/showthread.php?t=105362. There are two dynos with afrs over 14 until at least 4K rpm.


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Originally Posted by MNR0 View Post
Regardless of how the fuel gets in, its still a 4-stroke combustion cycle with essentially the same chamber limitations as cars of 50 years ago.

Yes, there are improvements, but mainly for economy and light throttle cruise with DFI. At WOT, its all about correct spark advance for given dynamic compression, fuel mixture and volumetric efficiency of combined intake, heads, cylinders/chambers and exhaust.

These rules will not change while we have petrol common internal combustion engines. Best power by AFR, as a general rule, is 12.5-13 for NA and 11.5-12 boost. The rest of the power gains are to be made in increasing volumetric efficiency and timing.

Last edited by dadasracecar; 01-07-2008 at 09:17 AM. Reason: proper quotation etiquette
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      01-14-2008, 08:49 AM   #8
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Bump for any input from tuners? Shiv? Bueller?
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      01-14-2008, 10:40 AM   #9
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Quote:
Originally Posted by MNR0 View Post
These rules will not change while we have petrol common internal combustion engines. Best power by AFR, as a general rule, is 12.5-13 for NA and 11.5-12 boost. The rest of the power gains are to be made in increasing volumetric efficiency and timing.
Obviously, the rules have changed with the N54 high pressure DI. This fuel delivery system seems to allow a slightly leaner AF, even at max boost.
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      01-14-2008, 12:09 PM   #10
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From what I have seen on other stock car dynos, the AFR goes from 15-16:1 and gradually go down.. and around 4000-5000 the AFR becomes aaround12.5:1 to 1:1... in between 2000rpm to 4000-5000 you will see 14:1 13:1. Just about any stock cars will show this.

For a tuned car the goal is to set it richer because you are pushing the car to its higher limits and therefore you want that extra room for safety.
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      01-14-2008, 12:11 PM   #11
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from an engineering standpoint, DI systems run much higher fuel pressure compared to other injection methods.

Comparatively speaking, DI injection system run from at least 3x the fuel pressure of other methods. Most cars on traditional fuel injection methods are seeing about 45-55PSI of fuel pressure.

Now, the key point is this... if you can full atomize fuel into vapor form out of the injector, you can run the motor at stoich under WOT and under boost. But this is not possible just yet with current technology implemented in cars. This was a theory and understand of Graham Bell.

Because of the higher fuel pressure with DI systems, you dont need to have an 11.5 AFR to be safe. 12.5 would even be considered safe. 11.5 on a direct injection motor running low to moderate boost is a waste of fuel often times and not maximizing the use of the DI.
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      01-14-2008, 05:22 PM   #12
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I'm aware of the benefits of DI and I know that you can run leaner. How much leaner is the question. Many of the dynos have afrs in the 16s at low rpm. I'm assuming that's b/c they can't develop the fuel pressure quickly enough to match the boost. I get afrs near 14 at 2500-3000 but then it quickly drops to 12.5-12-11.5 at redline. I was always worried tremendously about the early afr spike. I can't believe how lean some of these are.

I guess I'm looking for experiences with motors that have been pushed hard and what afr was chosen. If there are any experiences with blown motors due to lean afr, that would be very informative. I'm going to rework my tune to put it in to the 12s at redline.

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      01-14-2008, 07:41 PM   #13
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Quote:
Originally Posted by dadasracecar View Post
I'm aware of the benefits of DI and I know that you can run leaner. How much leaner is the question. Many of the dynos have afrs in the 16s at low rpm. I'm assuming that's b/c they can't develop the fuel pressure quickly enough to match the boost. I get afrs near 14 at 2500-3000 but then it quickly drops to 12.5-12-11.5 at redline. I was always worried tremendously about the early afr spike. I can't believe how lean some of these are.

I guess I'm looking for experiences with motors that have been pushed hard and what afr was chosen. If there are any experiences with blown motors due to lean afr, that would be very informative. I'm going to rework my tune to put it in to the 12s at redline.
Wait till I get my 335i/135i. I will be able to answer all of your questions.
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      01-14-2008, 07:44 PM   #14
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Quote:
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Bump for any input from tuners? Shiv? Bueller?
I think Malekreza11 hit it right on the head. There is really no reason to run a DI engine as rich as a convention engine. Just a waste of gas.

The usual knock tendency you see crop up with running a conventional moderately boost non-DI turbo engine in the 13-13.5:1 range does happy with the N54 engine. In fact, you can almost run it on either side of stoich and not see much difference in power or allowable advance. It's pretty neat.

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      01-14-2008, 07:49 PM   #15
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Quote:
Originally Posted by Malekreza11 View Post

Comparatively speaking, DI injection system run from at least 3x the fuel pressure of other methods. Most cars on traditional fuel injection methods are seeing about 45-55PSI of fuel pressure.

.
It runs a hell of a lot more than 3x that of standard. The Siemens system on the N54 runs up to 200 bar, or nearly 3000 psi.
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      01-14-2008, 07:52 PM   #16
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It runs a hell of a lot more than 3x that of standard. The Siemens system on the N54 runs up to 200 bar, or nearly 3000 psi.
Yep... I've seen as high as 3500psi.
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      01-14-2008, 07:54 PM   #17
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It runs a hell of a lot more than 3x that of standard. The Siemens system on the N54 runs up to 200 bar, or nearly 3000 psi.
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      01-14-2008, 11:41 PM   #18
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Yep... I've seen as high as 3500psi.
Shiv
I take it you can log rail pressure? Is that what you are using as your basis for the flow limitation of the HPFP in your testing?

Its cool that our particular design HPFP (similar to that of the diesel variety) isn't the limiting factor in low end tq. On most hitachi based fuel pump setups, even with the tri-lobe, there isn't sufficient pump rpm at low end rpm to sustain the low end tq that most of those small turbo mills are capable of.

The N64 has the "standard" hitachi pumps, they'll probably end up being the limiting factor at first. APR has upgraded pumps, but they are $$$$ to make and sell (over a grand last time I checked, each).
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      01-15-2008, 01:35 AM   #19
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+2 Malekreza11.

Keep it simple guys.
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      01-15-2008, 05:37 AM   #20
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BMW clearly doesn't feel the DI system needs an 11.5:1 AFR from dyno's of the stock car. I think that says a lot on how well they feel their DI fuel system atomizes the fuel.
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      01-15-2008, 11:43 AM   #21
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Quote:
Originally Posted by shiv@vishnu View Post
Yep... I've seen as high as 3500psi.
Shiv
I was just mentioning that DI systems run at least 3x the amount of traditional fuel injection systems.

If the N54 does indeed run about 3000 PSI of fuel pressure, then in my opinion, I would tune the car to about 13.0 AFR to about 13.7. I cant make a definitive claim on that, but based on my experiences, it would work.
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      01-15-2008, 06:54 PM   #22
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Quote:
Originally Posted by Malekreza11 View Post
I was just mentioning that DI systems run at least 3x the amount of traditional fuel injection systems.

If the N54 does indeed run about 3000 PSI of fuel pressure, then in my opinion, I would tune the car to about 13.0 AFR to about 13.7. I cant make a definitive claim on that, but based on my experiences, it would work.
So you thought that DI systems run at 200 psi, yet your "experience" with them teaches you to run in the 13s?

There is no "if" about the N54 and any other DI engine running well above 2000 psi of pressure. Think about it, the injector is injecting the fuel into the chamber at/near TDC. This means near/full compression of the mixture, surely 200+ psi @ the N54's CR of 10.x : 1. Now factor in the fact that the injectors ALSO inject fuel after the mixture has been ignited (actually 3 injection cycles during the power stroke while in warmup mode) where pressures easily exceed 1000 psi.

Fluids move based on pressure differentials. The pump must at least overcome the cylinder pressures in order for fuel to even leave the injector. In the case of a DI injector, it must leave with much, MUCH more than 55 psi of force for the dispersion and atomization of the needle valve to be correct.
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