Optimum AFR for DI?

[dadasracecar]

I posted this in the Speed6 area but the MS3 pages get more traffic. Anyway...

So I'm very new at tuning, modding, and this whole forum thing. This is the first forum I've ever subscribed to, the speed6 is my first turbo car, car that I've tuned and modded. Crazy, I know, considering what I've done with this car. Anyway, I'm perplexed about the optimum afr for our cars so I've been poking around and found a couple of things but I know that other with more experience on forums will be able to find more information regarding the proper afr for DI engines. Please help. I think it would also be VERY interesting for someone with a stock speed6 and a dashhawk to log the afr in a WOT run in fourth. Maybe we're supposed to be in the 12-13 range at low rpms...

Here's a crazy high afr from a stock 335i: http://www.e90post.com/forums/showthread.php?t=29708&highlight=afr

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

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

on 91 octane http://www.e90post.com/forums/showthread.php?t=104489

 

[Fritch]

all the paperwork on DI that I've seen claims it can be run leaner than port injected cars and make more power/torque more efficiently.

 

[desperado-c]

Someone on mazda 3 forums mentioned in a thread on "running too rich" that the manufacturers in general run turbo GDI's rich to keep the exhaust valves, turbo seals, and cat from burning up prematurely. I then suggested it might be important to get more info about this given that a number of people appear to have had issues with turbo seals. Azms3 then countered that he was looking at some other turbo GDI's and they don't nearly as rich as the MS3. I'll try to get him to be more specific.

 

[Haltech]

Well, i for one wont run my street car any higher than 11.4... Race tune with high octane fuel or methanol, no higher than 12.2. DI or not, i dont like to run on the ragged edge and i sure as hell dont plan on being the test mule on proving lean works on a DI motor.

 

[driver311]

i tune cars for 11.2 to 11.8 on pump and 12.0-12.5 on race gas. Ill tune the putter wagon to 12.0ish once I have the capabilities, and 12.5ish on race gas.

 

[AzMz3]

The way GDI are designed we are just wasting gas dumping fuel to satisfy what we know about other turbo cars.

I have been watching the Solstice and BMW 3series for a while as I might be getting one. Also been on the listening end of a good converstaion between some good tuners about the GM GDI turbo car and was stated it was one of the few turbo cars tuned perfectly from the factory. This was after monitoring knock, egt's and timing on the stock car.

One benefit from GDI is it acts as a cooling effect to lower combustion temps to reduce knock.

I look to add a J&S safeguard to the MS3 and test some limits if the BMW doesn't replace it.

There are some tuners richening up the afr on the BMW and not seeing any gains at all. So why add fuel if it is not knocking or producing high dangerous egt's.

 

[matsuda]

all the paperwork on DI that I've seen claims it can be run leaner than port injected cars and make more power/torque more efficiently.

This is not true of homogeneous DI (as used in the speed 3).

 

[matsuda]

I posted this in the Speed6 area but the MS3 pages get more traffic. Anyway...

So I'm very new at tuning, modding, and this whole forum thing. This is the first forum I've ever subscribed to, the speed6 is my first turbo car, car that I've tuned and modded. Crazy, I know, considering what I've done with this car. Anyway, I'm perplexed about the optimum afr for our cars so I've been poking around and found a couple of things but I know that other with more experience on forums will be able to find more information regarding the proper afr for DI engines. Please help. I think it would also be VERY interesting for someone with a stock speed6 and a dashhawk to log the afr in a WOT run in fourth. Maybe we're supposed to be in the 12-13 range at low rpms...

The "proper" AFR wouldn't really be any different if the engine had port injection.

 

[matsuda]

Someone on mazda 3 forums mentioned in a thread on "running too rich" that the manufacturers in general run turbo GDI's rich to keep the exhaust valves, turbo seals, and cat from burning up prematurely.

I also read that nonsense.

People read something about DI and then confuse homogeneous DI with stratified charge ("lean mode") DI.

 

[3.slow]

A lean car will blow up faster than a rich car will. That is what he is saying and it is true.

 

[matsuda]

A lean car will blow up faster than a rich car will. That is what he is saying and it is true.

What is said was "Turbo DGI engines produce very high exhaust gas temps at high load compared to port injected motors."

That is total nonsense.

 

[AzMz3]

The "proper" AFR wouldn't really be any different if the engine had port injection.

I might not be understanding but saying the MS3 is homogeneous and should have the same AFR as a port injection 2.3L is kinda contradicting.

At full power the afr should only need to be slightly richer then stoichiometric.
I ran 12.1afr on my turbo Mazda3 with no deto unless I got crazy and ran 14psi on wonderful Cali 91oct. But at 7psi making 267whp I was good.

I'm seeing 12.1 afr being good, so in away I agree but think the MS3 can run slightly leaner and be safe like the other vehicles with GDI and turbos.

 

[matsuda]

I might not be understanding but saying the MS3 is homogeneous and should have the same AFR as a port injection 2.3L is kinda contradicting.

At full power the afr should only need to be slightly richer then stoichiometric.
I ran 12.1afr on my turbo Mazda3 with no deto unless I got crazy and ran 14psi on wonderful Cali 91oct. But at 7psi making 267whp I was good.

I'm seeing 12.1 afr being good, so in away I agree but think the MS3 can run slightly leaner and be safe like the other vehicles with GDI and turbos.

Because of the cooling effect, you could get away with a slightly leaner AFR or you could advance the timing a bit or you could use slightly lower octane fuel but in general the optimum AFR won't be drastically different between DI and port injection. You should expect maximum power at approx. 12.5:1 with either system.

 

[desperado-c]

Because of the cooling effect, you could get away with a slightly leaner AFR or you could advance the timing a bit or you could use slightly lower octane fuel but in general the optimum AFR won't be drastically different between DI and port injection. You should expect maximum power at approx. 12.5:1 with either system.

The Oracle of Mazda has spoken! Now we all need to go and review/learn about the differences b/t stratified and homogenous GDI engines and who uses which.

 

[redspeed]

http://en.wikipedia.org/wiki/Gasoline_direct_injection

"The engine management system continually chooses among three combustion modes: ultra lean burn, stoichiometric, and full power output. Each mode is characterized by the air-fuel ratio. The stoichiometric air-fuel ratio for petrol (gasoline) is 14.7 to 1 by weight, but ultra lean mode can involve ratios as high as 65:1. These leaner mixtures, much leaner than in a conventional engine, reduce fuel consumption."

 

[desperado-c]

http://en.wikipedia.org/wiki/Gasoline_direct_injection

"The engine management system continually chooses among three combustion modes: ultra lean burn, stoichiometric, and full power output. Each mode is characterized by the air-fuel ratio. The stoichiometric air-fuel ratio for petrol (gasoline) is 14.7 to 1 by weight, but ultra lean mode can involve ratios as high as 65:1. These leaner mixtures, much leaner than in a conventional engine, reduce fuel consumption."

Okay, now get some info on stratified vs. homogenous GDI charge throw a turbo into the mix.

 

[SpdFreak]

I read elsewhere that DI engines are designed to run a higher AFR at WOT near redline. IIRC it was ~12.5:1.

 

[nig3]

my car makes the most torque at around 12-13:1 afr, so thats a pretty good indicator there i guess.

 

[matsuda]

The MS3 doesn't use any kind of lean burn (stratified) mode.

Here are examples of injectors designed for DI. One is for homogeneous applications (like the MS3) and the other is for stratified charge.

https://delphi.com/shared/pdf/ppd/pwrtrn/gas_multecdig_hom.pdf

https://delphi.com/shared/pdf/ppd/pwrtrn/gas_multec_gdifsc.pdf