KhanArtisT

iTrader: (2)

I'd like to hear everyone's opinions and experiences of dangerously rich AFRs. How rich does an engine have to run to cause rich misfires that can result in catastrophic failure?

I have a 6 port turbo car with the following setup:

550ccx4
FD pump hotwired
4-7psi
Streetport, TII rotor housings, 9.7:1 rotors
Stock TII ECU/timing

At WOT in 2nd gear and higher over 6000rpm the AFRs (AEM UGEO) dip into the 9s since the gauge reads 10 flat the whole time. The car feels OK but I'd like to hear some opinions and experiences.

Pele

iTrader: (28)

No such thing as a dangerously rich AFR as far as I know.

As far as I can tell, once you get too rich, it just won't fire and dump the unburned fuel out the exhaust... Unless you think the unburned fuel burning in the exhaust is a dangerous thing... I think it'll just blow up a muffler.

Frostycrowd

iTrader: (9)

Yes you can be dangerously rich, a miss fire is a miss fire and several people on this forum have broken dowels from it.

flaco

iTrader: (7)

just make sure your timing is ok

Aaron Cake

iTrader: (1)

I can't imagine that the car feels OK at those ratios. To me, that is undrivable.

9s are as low as most gauges read so it's likely you are even richer. 10 flat is basically as rich as the gauge can interpret. I think the AEM stops reading at 9.90 or something like that.

At high RPM with those ratios, you are well into the danger zone. Also the car is probably down 50HP.

KhanArtisT

iTrader: (2)

The AEM has a calibration dial thing in the back of the gauge and I believe it changes the range of readings but I'm not 100% sure, I'll try it tonight though. The car surprisingly does feel OK and accelerates smoothly.

Next mod for the car is definitely a PowerFC. Does anyone think having the injectors cleaned and flow tested would help this issue?

Evil Aviator

Ignoring detonation issues, the best power on unleaded pump gas is usually in the 12:1 range, plus or minus depending on your particular engine and AFR gauge. Anything much richer than this is trading power for detonation resistance. Unless your engine is in horrible shape, I don't see why you would need a lot of detonation resistance if only running 7 psi boost.

Yes, the engine can get damaged by running a very rich mixture. The exact breaking point varies due to many factors. Regardless of this, as indicated by Aaron Cake, there is no way your engine is running well if it is truly in the 9-10 AFR range. This is why I think that tuning solely with an AFR meter is a really bad idea, and since you posted this thread I think you are also seeing that point now.

I don't think it would help this issue, but it will make your tuning more accurate.

KhanArtisT

iTrader: (2)

The engine doesn't need the rich AFRs but that happens to be where they landed with my fuel mods and crappy engine management (or lack thereof)

I don't quite see why it is a bad idea to tune with a wideband other than maybe the sensor being inaccurate, what else do you suggest to monitor engine parameters? I've looked at the plugs and they're always black with slight brown deposits (from the oil?) which as far as I know is normal for these engines.

The engine doesn't actually run in the 9s at all engine speeds at WOT, it only hits 9s from roughly 6-8000rpm. Before that, its in the 11s.

beefhole

iTrader: (29)

This is odd to me, but then again I am no expert. My TII at full boost/throttle would hit 10.0 on the dot. The car was certainly driveable at these ratios and made good power (305-315 wheel). Maybe it could have been leaned a bit and tuned better, but it wasn't undriveable. Is there any danger running that rich up top? It does seem like overkill, but it's safe no?

Evil Aviator

- Good tuners use trends, not numbers. You can always tell a noob tuner because they are overly-focused on the number on the gauge.
- You can build two of the exact same engine and a different indicated AFR may be required to run each of them at their peak performance.
https://www.rx7club.com/showthread.p...hreadid=220817
- O2 sensors were designed to tune engines for economy, not performance. It just so happens that they help speed up tuning of all engine aspects, but they are not intended for fine-tuning performance. Using an AFR gauge to tune for performance is like using the handle of a screwdriver to hammer nails.
- Currently-produced O2 sensors are not good for tuning transients, and their accuracy decreases with exhaust reversions, exhaust leaks, cat aux air, and mounting position.

A good professional tuner.

Some typical methods used:
- Sight: Rich = increased gray exhaust smoke, lean = less/no smoke
- Sound: Rich = low tone, lean = high pitch tone
- Smell: Rich = thick petroleum smell that makes your eyes water, lean = burnt or sulfur smell
- Feel: Very rich = surging/bucking, rich = bogging, lean = hesitation, very lean = surging/bucking
- EGT
- Exhaust Gas Analyzer
- Dyno
- Datalogging

Yes, spark plugs are pretty difficult to read on a rotary engine unless there is a major problem.

If the engine is not pinging due to the overly-rich mixture, I am not aware of any problems other than spark plug fouling, carbon deposits, decreased gas mileage, and reduced engine performance. If you are unsure of your AFR meter's accuracy, just take your car to a local auto shop that has a nice Horiba gas analyzer to test your car's emissions.

KhanArtisT

iTrader: (2)

Good info, but this car is not being tuned, it is being monitored to determine whether it requires serious tuning to avoid engine damage. There is no doubt that 1000 other readings would make the monitoring a bit more accurate.

So to change the thread up a bit, does anyone actually have experience with engine damaged caused by an overly rich mixture?

BLUE TII

iTrader: (9)

LOL, you guys don't put widebands on many factory turbo cars do you?

They usually run in the mid/high 10s on the top end for a wide margin of safety.

I put my Innovate on my Stock class FD running 10-8-10 and it was 10s up top just like I tuned my TII.

Now, truthfully the FD doesn't pull very clean up top unless it has fresh plugs, but the TII with a HI-6 and LX92 coil per leading plug pulls very clean and hard through that ratio and only feels a bit flat when it dips into the 9s when tuning.

I have found with a strong ignition the rotaries can make decent power even running absurdly rich.

Yes, when you lean it out to the 11s you get more power and you can hear it start to shriek as peak combustion pressures rise, but even in 4th gear pulls on the street it happens so fast you are pretty much always in a transient state between throttle pump enrichment, 2ndary inj staging, rising boost and rpms that I would rather play it safe in the 10s instead of taking the chance there is a momentary dip as it transitions through the maps that I am missing.

I will save the 11s for when it is on an eddy current dyno under the watchful eye of a real tuner with lots of data logged inputs.

Then again I am running 12lbs boost, 8.5:1 rotors, 120f iat and mixed up to 97 octane because I have put in my time rebuilding these engines and I just wanna drive some...

hhn2002

ok, well lets think about it here. under normal a/f ratio combustion, even just below stoic, in the imperfect world we live in there's always unburnt oxygen left over. running super rich would probably leave little if any oxygen remaining. from that point on the rest of the fuel would fail to burn because you cant burn without oxygen, then you just end up with it burning out your exhaust pipe as it exits. thats my .02 anyways. also danger of fouling plugs and possibly thins the lubricating oil film causing accelerated wear.

Aaron Cake

iTrader: (1)

A wideband is a tuning aid, not the be all and end all of tuning. As Evil mentioned the key in tuning is to pay attention to trends, the wideband being only one of those trends. More then once I've been tuning a car while the wideband was not lining up with everything else (injector PW, boost level, fuel pressure) and had to insist that we stop and check up on things. Pulling the plugs showed crazy lean and as it turns out, the sensor was hammered from running excessively rich. This happens all the time.

Your plugs are black because the engine is running crazy rich. My plugs are always dark tan.

Mid 11s is a nice safe area to be in boost. At lower boost you can run 12:0 and then go into the mid 11s to about 16 PSI or so. After that it pays to keep it in the high 10s for safety.

Of course, this all depends on timing.

But running in the 9s from 6-8K is fairly horrible. You're leaving a lot of power on the table, risking popping the engine and accelerating engine wear.

The Mazda ECU just hammers fuel into the engine under boost. 10.0 on the dot probably means that it was a lot richer. Just imagine, bu going to 11.5 you could have picked up 50HP and improved your turbo response by an amazing amount.

Plenty. Only the RX-7s are into the 10s. Most of the other factory turbo piston cars I have widebanded hold mid to low 11s, some are low 12s. Mazdas tuning method in boost is...crude. Then again, we know why they did it. So crazy people can open up the exhaust/intake on a stock ECU and not blow the thing up.


They usually run in the mid/high 10s on the top end for a wide margin of safety.

It's amazing how the rotary can pull through poor fuel maps and keep making the power that it does. I've shocked some dyno operators before. Also when it blows 10 foot flames and shoots the exhaust vent pipe off of the back of the car!

A 400HP+ RX-7 is very fast, but lets face it, a 4th gear pull from 2K to redline lasts more then a few seconds. There's quite a bit of time to datalog the run and make some informed changes. Though "in the 10s" is a lot different from the "10 flat" being described earlier in this thread.

I'm running 13 PSI on a GT4088R on 9.4:1 rotors on a S4 6 port block on 91 octane and put down ~420 to the wheels on the dyno last time. I've been driving the car in this configuration for 3 years from spring to fall as a semi-daily driver. We clearly have different tuning styles.

KhanArtisT

iTrader: (2)

I feel much better now that I've read BLUE TII's post. Couple more questions though. I see that the stock ECU dumps fuel under boost, question is am I achieving the rich AFRs at higher than desired duty cycles or would it start running leaner at 90%+ from the injector not opening and closing properly? I ask because I'm not sure I have enough fuel for my setup, especially at 3rd gear and higher at 7psi. I was told that the streetport would be pushing it.

Another thing is, can someone explain exactly what would cause accelerated engine wear from dipping into the 9s (10.0 on the WB) momentarily or even staying in the low 10s?

btw the AEM wideband can only be calibrated to put out a different range of voltages for an EMS, not AFR readings oh well.

RotaryRocket88

iTrader: (7)

Besides running high injector D/C at high RPMs under boost, you should also be concerned about maxing out the primaries before the 3800 RPM transition. With an open intake and exhaust, a good boost controller, stock turbo, 8.5:1 rotors & 9-12psi of boost, I've logged 550cc primaries maxing out in the 3300 to 3700 RPM range. I've moved my injector transition point down to 3100 RPM w/ my Rtek to get around this, but you won't have that luxury.

BLUE TII

iTrader: (9)

question is am I achieving the rich AFRs at higher than desired duty cycles or would it start running leaner at 90%+ from the injector not opening and closing properly?

I would avoid seeing 90% duty cycle in the logs as you map because you know there is going to be a condition where at the same point one of the correction maps is bumping you richer at the same point.

That said, there are a couple of things you have to check regarding high duty cycles BEFORE you go out and grab some big injectors.

1)AFRs- like you said, if you are in the 9s and low 10s it is amazing how much injector headroom you will clear up by going to high 10s and low 11s. I would rather see high 10s around peak torque tuning on the street, but to each their own.

2)Fuel pump- This is a big one. I was maxing my 720/1600cc set up on a Walbro 255. Turns out some joker sold me the low pressure Wally.

Put in the high pressure 255 and though duty cycles were still higher than I would like I was out of 100% duty cycle. Now the car would hesitate at 6,800rpm with all parameters looking good (though didn't have fuel pressure as an input).

2 tuners told me it sounded like the fuel pump was dropping pressure from what I described so I put in a Bosch '044 and problem solved!

Another thing is, can someone explain exactly what would cause accelerated engine wear from dipping into the 9s (10.0 on the WB) momentarily or even staying in the low 10s?

Fuel is not as good a lubricant as the oil from your oil injectors, so unless you are premixing then more rich means less lubrication for all your seals. You will see accelerated wear.

If premixing, no problems.

Really, at 7psi peak boost and under 300rwhp I would feel safe tuning in the mid to low 11s as a 4th gear pull lasts a long time. This sounds like a more stock set up, so keep a careful eye on intake air temps.

BLUE TII

iTrader: (9)

First, let me say that my original post was influenced largely by a reaction against the term "dangerously rich" AFRs.

Plenty. Only the RX-7s are into the 10s. Most of the other factory turbo piston cars I have widebanded hold mid to low 11s, some are low 12s. Mazdas tuning method in boost is...crude. Then again, we know why they did it. So crazy people can open up the exhaust/intake on a stock ECU and not blow the thing up.

I first saw the trend of rich running OEM turbo cars looking at dyno plots from "power pages" and such in tuning magazines where they dyno stock cars and start adding parts and dynoing. Lancer Evolution 9 was running AFRs in the 10s stock at high rpm!

Lancer Evo X is in the 9s stock at high rpm.

Here is a log of stock Evo 10 overlayed with stock Evo 9 as dotted lines. Blue is boost pressure psi and black is AFR



Stock 2008 WRX STi in the low 10s



Yes, Mazdaspeed6 and Mazdaspeed3 will run mid to low 11s AFR barely hitting 10s, but they have direct injection allowing a leaner AFR by design and by the tuning precision needed to implement direct injection.

Now, with the rotary there are "cold spots" in the poor combustion chamber shape so there is more unburnt fuel in the resulting exhaust gas AFRs. You WILL have to tune more rich for the same safety margin as a more efficient piston engine...

BLUE TII

iTrader: (9)

Err, I just looked at your set up in your sig KhanArtisT.

If you are running a stock 90 NA 6 port rebuild with its 9.7:1 rotors with that stock S5 turbo stuff, I never gave you advice- lol.

I fear the high comp rotors, boost and pump gas combo...