Decided (finally) to go NA with my subaru.

Discussion in 'Technical' started by Mr Y, Feb 9, 2006.

  1. Ive decided not to Turbo or swap my 99 2.5 RS Impreza. I simply want to experiment with bolt ons and things like that. but this question is about electronics, but i'll start with my questions about A/F Ratio

    1. I want to buy an air fuel ratio guage and im assuming running perfect (or as close to ) is best for power.

    2. How can a cars a/f ratio be thrown off? When I get say a CAI, and Headers and a cat back, will my stock a/f ratio be different? or can it be?

    3. Do stock engines A/F ratio get thrown off over time?

  2. An A/F ratio guage on your car really wont give you an accurate reading of whats going on. All it will do is tell you the current reading of your O2 sensor. Since your OEM O2 sensor is not a wideband it will only give you 1 of 3 readings: lean, stoich, or rich. An Exhaust Gas temp. guage would be a better option.

    An ideal set up for A/F's would be a wideband O2 sensor, and a digital A/F guage.

  3. Well yea this is what im planning to go with. I know about the wideband sensor and guage, but can you answer the 3 questions above?
  4. 1. Having optimum A/F's is want you want for making power. On my 240SX I plan on running 12.1 to 12.5 to 1 at WOT and 14.7 at idle everywhere else 13 to 1. My fuel map will be extremely linear, Im not planning on putting 10 to 1 buffers in areas of the map that my ECU will never see.

    2. Headers and a cat back shouldnt throw your A/F's off. Bad spark, clogged injectors, intake leaks after your MAF, intake leaks in general, the list can go on.

    3. Yes. Poor maintenance comes to mind.
  5. When one is going to use a gauge for the air fuel ratio this gauge should not be connected to the oem sensor as that may result in incorrect readings for the ECU.

    For use with wide band sensors you can't use a simple A/F gauge but more expensive equipment is required. As the sensor has a limited life, with decreasing accuracy this type is sensor is only used when it's needed. Unless the ECU uses the signal.
    A good sensor and electronics for a wide band lambda typically cost around $1k.

    Max power with gasoline is usually at 12.5:1 but at high loads even more fuel can be required for cooling. Normally you try to use lambda 1, 14.7:1 for gasoline when possible except at high loads. For a low fuel consumption slightly leaner than lambda one can be used. This will result in high emissions but fuel consumption goes down. Lean mixtures are commonly used in motorsport to conserve fuel and the richer mixtures for maximum power.

    Like the extra lambda sensor exhaust temperature thermocouples are also only added when they are needed. Also they have a limited life with decreased accuracy. To use more than one thermocouple per exhaust manifold is usually overkill. If an EGT gauge should be added I recommend that one K type termocouple per manifold is used, mounted through the flange of the exhaust manifold after the collector. With multiple thermocouples it's very difficult to know if a difference in temperature is caused by inaccurate thermocouples or an actual difference. Multiple termocouples also won't give you much more information about the combustion inside the engine, for that cylinder pressure sensors are needed and they are expensive. Just the sensors alone cost $3-4k a piece, then signal amplifiers, a crankshaft position sensor and wheel, and computer equipment is required.
  6. Check out

    Their wideband can hook up to a digital or analog A/F guage. You just need to buy the optional equipment.

    I cant speak for its ease of use just yet, as its still in the box.
  7. #7 Mr Y, Feb 9, 2006
    Last edited by a moderator: Apr 25, 2016
    Okay, so say I get this here :

    Is this enough? Johan will i still need other equipment ?

    and I want to "change" my A/F ratio to correct it, how is that done? Can I buy something as simple as an Apexi SAFC and tweak around?

    There are also different chips and what not for subaru's specifically, and those can be tuned using windows. (there are versions for the 2.5 rs na engine)
  8. A/F ratio varies a lot from car to car. For 400whp 1.8T Golfs, people like to run high 11s at redline, where as a stock 2.0T FSi engine for the new VWs runs in the 10s stock.
  9. #10 Monkey, Feb 10, 2006
    Last edited by a moderator: Apr 25, 2016
    The AEM UEGO is an excellent unit. I've heard many, many good things about it. You won't need any additional hardware as it comes with the Bosch wide-band O2 sensor and has the electronics for reading the signal and powering the heater and such built into the gauge/controller. All in all, it's a pretty straightforward installation. If I remember correctly, it also has a narrow-band output for use on factory ECU's. Then you don't necessarily have to run two O2 sensors, you can just plug in the wideband in place of the factory narrow band and then run the narrow band signal from the AEM UEGO to the factory ECU and it doesn't know the difference. You'll have to look that one up for sure, but I swear I remember reading that on AEM's site...

    The only way to change your air/fuel ratio is to change the amount of fuel you are delivering to the engine, since the air is going to be dictated by throttle position and rpm for your given engine and intake and exhaust system configuration. If you reduce the engine's pumping losses by reducing the pressure drop in the intake system, then you'll flow more air at the same throttle position and rpm than you did before, so you add more fuel.

    Really, the best way to really fine-tune your air/fuel ratio is to be able to tune your specific car to its specific modifications, so the programmable chips, or a standalone ECU are really the best way to go, but the Apexi SAFC will work well too as witnessed by my friend's Supra that put down 430ish whp using the stock ECU, stock turbos, and the Apexi SAFC to add more fuel with the additional boost.
  10. Are you sure about the FSI? That's ridiculously rich... to the point of poisoning the cat and fouling the plugs. I wouldn't expect VW/Audi to go richer than around 12.5:1 since the 2.0T FSI has to meet stringent emissions requirements, and it's not pushing that much boost.
  11. That's what I read about the 2.0s. One post only, in the 1.8T forum. Haven't looked into it though.

    And I wouldn't get the AEM wideband. You can't log with it unless you have their standlone. It's only ~300 now, but another 100 or so you can get a unit that logs A/F for you, and might do throttle position as well.
  12. I don't know how rich mixtures they are using but they use a quite high compression ratio on that engine. If the compression ratio can be increased this can decrease the fuel consumption at part load but at the cost of some extra fuel at high load. However, I would assume that they prefer to retard the ignition instead of enriching the mixture at high loads, but this will eventually result in very high exhaust gas temperatures.

    Car makers are today trying to run lambda 1 as long as possible, hence avoiding enrichment even at high loads. EGT does however limit how long this can be taken.

    Emission requirements usually puts more demand at low load than high load, so even if the engine is very dirty at high load it can be vary clean at low load. To heat up the catalyst quickly, and to keeping it warm at low loads may also have the effect that the catalyst is a bit too small to handle the high flow at higher loads.
    Fuel consumption at low loads also have a greater effect on the fuel consumption of a car during daily driving thatn the consumption at high loads.
  13. So timing retard = high egts? What do high EGTs mean for the engine? How do EGTs kill
  14. I think he may be right. At redline an A/F ratio of 10 to 1 really isnt that uncommon. Nissan has 10 to 1 buffers all over the higher RPM area of its fuel maps(on their 1990's ECU's they did).

    If its 10 to 1 anywhere else in the power band then that just sucks.
  15. Retarded ignition timing means a lower thermal efficiency and thereby a higher exhaust temperature.

    A high exhaust temperature will increase the working temperature of exhaust valves, valve seats, the exhaust ports, exhaust manifold, turbochargers, catalysts and so on. Common stainless exhaust valves can for example handle a working temperature of 700 degC, for temperatures of about 800 degC for example Nimonic 80A may be used. To improve valve cooling, seats of for example beryllium-copper may be used. There are also other superalloys which can handle even higher valve temperatures. To handle high temperatures the exhaust manifold must be made of a better material, more costly heat resistant steels or superalloys like inconel.

    This does of course affect the cost of the engine. Usually you can get a set of exhaust valves for a 16 valve engine for a few hundred USD. The superalloy option will increase the cost to more than $1000 for the exhaust valves. Beryllium-copper valve seats aren't cheap either, same goes for a superalloy exhaust manifold.

    A higher EGT also calls for a better heat shielding under the hood. It may also require better heta shielding of the complete exhaust system.
  16. Only 800? I thought EGTs were like 1200+.

    And wouldn't higher EGTs mean faster spool up?
  17. Exactly.

    I guess I didn't expound on my last sentence enough. An air/fuel ratio of 10.0:1 is terrible for a catalytic converter, spark plugs, etc. because of the excess deposits... so too much time at 10.0:1 would be bad for the catalyst (from my reading, it doesn't take that much time either) and thus reduce the engine's emissions performance. And since most automakers are required in the U.S. to provide 10-year emissions compliance warranties, it just doesn't seem logical that VAG would risk poisoning the catalyst and then simply eat the warranty costs.
  18. Emissions regulations have changed since the 1990's though. Like I said in my post above, manufacturers are required to provide a 10-year emissions-compliance warranty, and very much time at 10.0:1 will kill the catalyst. 10.0:1 as "null" points in the fuel map would make sense if they were in areas of operation the engine should never see (e.g. zero boost at WOT and redline, full boost at zero throttle at idle, etc.), but anything near any point where the engine will be operated is just a bad idea from the warranty standpoint.

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