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Air-to-fuel ratios, tuning, temp correction

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Old 10-12-17, 10:53 AM
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Air-to-fuel ratios, tuning, temp correction

For 2.5 years I was running a tune for 0.85 bar (c.12.5 PSI) with a Twin Power on the stock intercooler and air box but a full 3" downpipe, 3" high flow cat and 3" cat-back exhaust system. This tune was done by a rotary specialist that is known to tune quite conservatively and add quite a lot of fuel so the AFR's were apparently in the low 10's at WOT and about 11-12 at idle and low load. It certainly smelled rich at idle but that was fine because I knew the car was pretty safe.

This past weekend I had the tune updated by a different rotary specialist, whom is well regarded in the UK for their tuning capabilities (they seem to be the go-to tuner for about 75-80% of the market nowadays), for the addition of the Greddy SMIC and 250cc/min of water and we sought additional boost and ended up with what they said was the maximum level that they were willing to go to on my car's stock fuel system with a 12v feed. We ended up with 1.00-1.06 bar (14.5-15.4 PSI) and about 94.5% injector duty give or take. Intake air temps on water were consistently in the low to mid 30s C (86-97 F) when it was about 17 C (63 F) outside and sunny when doing 1-2 pulls at a time. I would guess on a track with 10-15 minutes of WOT the temps would drop a bit further and stabilise somewhere in 20s (70s F) and/or closer to ambient but not quite ambient given the small amount of water I am injecting.

It was road tuned and a wide band was hooked up to monitor AFRs at all times. According to them, the AFRs were approximately the following:

Idle: 13.5-14.0:1
Deceleration: 18.0:1
Cruise and part throttle: varies a lot so hard to say
WOT:
-- 0 to 2,000 RPM: Starts around 13.0:1 and moves towards 11.0:1
-- 2,000 RPM to c.6,500 RPM: Target is 11.0:1 +/- 0.2
-- 6,500 RPM to fuel cut 8,100 RPM: Leaning out slightly to 11.2-11.3:1

Naturally, they showed me all of the other settings in the Datalogit software for ignition timing, split, etc. but I don't really know what any of that means and I'm not going to touch it anyway. They did tell me though that they don't usually tune for aggressive timing on a relatively stock car as it doesn't really make any difference and just makes you more likely to detonate. Sounds fine as I just want the car to be within a safe range for street and track duty. Extracting the last hp out of the car isn't the point in this situation.

I am guessing the tune is within safe parameters if a bit leaner than what I had before, but I was wondering how this tune would work with temp correction when it starts to get cold outside and fuel requirements in the map increase? If my injectors are maxing out in 4th gear at c.93-95% at some point in the RPM range then will there still be enough injector to handle temp correction in the map and full boost when the temps drop from about 15-16 C (60 F) now to say 0 C (32 F)?

Car is running great for now other than one particular issue that I also have a question about. On deceleration when the fuel cut kicks in, as RPM's fall below 2,000 RPM and the air pump kicks back on I am getting back fire pops coming out of my exhaust due to the extra oxygen. They tuned for a decel AFR of 18.0:1 but how lean can they go with the decel AFR to ensure the backfiring stops at really low deceleration RPMs? How lean does the stock ECU go in this type of situation?

Last edited by cib24; 10-12-17 at 11:02 AM.
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Old 10-12-17, 01:30 PM
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Originally Posted by cib24 View Post
Idle: 13.5-14.0:1
Deceleration: 18.0:1
Cruise and part throttle: varies a lot so hard to say
WOT:
-- 0 to 2,000 RPM: Starts around 13.0:1 and moves towards 11.0:1
-- 2,000 RPM to c.6,500 RPM: Target is 11.0:1 +/- 0.2
-- 6,500 RPM to fuel cut 8,100 RPM: Leaning out slightly to 11.2-11.3:1
Sounds good. What's your idle speed?

I am guessing the tune is within safe parameters if a bit leaner than what I had before, but I was wondering how this tune would work with temp correction when it starts to get cold outside and fuel requirements in the map increase?
He most likely put in some preferred values for the table that he uses on all cars with similar level of mods. Whether it leans out or richens up depends on how the table was set and what temperature it was when he tuned. It also depends on heat soak. You can switch to a faster acting sensor or even re locate it, but I would not do that without a re tune. The characteristics of the stock sensor are baked into your tune.

You're not going to get consistent AFR across all weather. It will vary. Acceptable range in my opinion is about +/0 0.5:1 which is still a wide tolerance.

If my injectors are maxing out in 4th gear at c.93-95% at some point in the RPM range then will there still be enough injector to handle temp correction in the map and full boost when the temps drop from about 15-16 C (60 F) now to say 0 C (32 F)?
That depends what temperature we're talking about here, and how much your boost climbs, and how the compensation table is set up. Honestly, you're the one who wanted to push your current hardware to the limit. You're taking the risk. He thinks it will be fine, and I'm inclined to agree, but you're not leaving much of a safety margin.

Car is running great for now other than one particular issue that I also have a question about. On deceleration when the fuel cut kicks in, as RPM's fall below 2,000 RPM and the air pump kicks back on I am getting back fire pops coming out of my exhaust due to the extra oxygen. They tuned for a decel AFR of 18.0:1 but how lean can they go with the decel AFR to ensure the backfiring stops at really low deceleration RPMs? How lean does the stock ECU go in this type of situation?
The only FD that doesn't ever pop is a stock car on stock ECU with stock exhaust. Stock ECU has an algorithm that uses the idle speed control valve that pushes extra air through the system on deceleration. Previously on older rotaries they had what's called an "anti afterburn valve" which used the air pump. It looks like, as a cost saving measure, Mazda figured out how to get rid of that valve and use the ISC valve instead with a special algorithm.

The stock ECU also has a staged fuel cut, where it cuts fuel to one rotor at a time depending on speed and narrow range throttle position sensor. I don't think any aftermarket ECU can do this. Anything that might have some kind of emissions function (like decel fuel cut) is never going to be as good in the aftermarket as stock.

Last edited by arghx; 10-12-17 at 01:34 PM.
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Old 10-12-17, 07:23 PM
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Idle speed is 920 RPM with no electrical load. With electrical I think it's about 980 RPM and with A/C it's about 1,050 RPM.

Yes, I am pretty sure he started the tune by loading a simlarly modded car onto my PFC and going from there. It was about 14-15 C when he tuned it. I do have a fast reacting IAT (a Triumph temp sensor) that I installed 2.5 years ago at the time of the first tune for 0.85 bar.

As long as AFR's ranging from high 10's to high 11's is acceptable then I feel comfortable.

As for injector duty cycle, I guess what I'm trying to understand is how many percentage points the injectors can adjust depending on temp. If it only adjusts a few % from like 94% to 96% then who cares, I have enough room. But if it can adjust by literally 10% or something if the weather is too cold then that might be a concern. Not that I drive this car in the winter anyway because of salt on the road so the minimum temp I will usually ever boost in is about 5 C.

As for the popping on deceleration. My previous tune was done on a completely stock fuel system with no voltage modification to the pump. So, it still had the stepped voltage where at low loads it would be running like 6v. And with that I had no popping just like the stock ECU. Now, with 12v going to the pump at all times it would appear that where they currently set the car for 18.0:1 AFR on deceleration is too rich because it pops when the air pump kicks in, even though technically fuel cut is a feature on the PFC and is set to occur on my tune so there shouldn't be fuel in the car on decel which the air pump's extra oxygen is igniting as it exits the exhaust. I'm just wondering what is the leanest we can go and still be safe on deceleration. 19.0:1? 20.0:1? What does the stock ECU do?
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Old 10-12-17, 09:21 PM
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You can go as lean as you want on decel, down to the hard coded Power FC pulse width limit of 2.5 milliseconds that you will hit before deceleration fuel cut engages. You won't hurt the engine, but you might get a misfire. But I'm not clear about your air pump. You need to take a datalog showing the air pump activity bits. Basically you have to dig into the interaction of the whole system inclouding stock o2 sensor, decel fuel cut, air pump, and wideband reading.



For stock ECU you can see the staged fuel cut based on narrow range TPS. It it's either 1 rotor or 2. Piston engines do the same thing, cutting half the cylinders some times. I'm not sure if the PFC does. I doubt it.

this thread goes deep into how the air pump and anti afterburner function on the stock ECU. https://www.rx7club.com/3rd-generation-specific-1993-2002-16/why-engine-so-damn-complicated-part-2-emissions-controls-841963/ , which is performed with the ISC valve on an FD.

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Old 10-13-17, 06:20 PM
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Basing my knowledge on a different ECU... Doesn't fuel cut on PFC have a delay, i.e. it kicks in only after a certain amount of seconds have passed after you let off the throttle? If it does (and I think it should) this may explain why you get back fires when letting off the throttle.
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Old 10-13-17, 06:58 PM
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Hmm...I don't know how the fuel cut works on the PFC but I know that before hot-wiring the fuel pump and on my previous tune with the PFC for the past 2.5 years I never had any backfiring. So, my previous tune and I guess lower fuel from 6v on the primaries meant the car was leaner on decel prior to the fuel cut, I just don't know how much and what a safe level is.

I presume this would probably not occur if I didn't have the extra oxygen from the air pump?
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Old 10-14-17, 07:30 AM
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One other question. If you run lower AFRs, i.e. more fuel, it keeps combustion temps down and I guess will help prevent pre-ignition. Does water have the same effect as lowering combustion temps and preventing pre-ignition or bad lean conditions?
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Old 10-14-17, 03:15 PM
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I wouldn't worry that much about pre ignition with water. Methanol injection yes, water no. There's a bunch of details from SAE papers that we don't have to get into. Also, running colder plugs helps with preignition.
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Old 10-14-17, 05:18 PM
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i think if you gave me the car, and said 'tis yours' i would lower the boost on the secondary turbo by .5 of whatever units its in. so i'd go 1.0/9.5

your basic tune is ok, but lowering the boost on the secondary should give you a little more injector headroom. for what its worth i have an Re Amemyia chipped stock ECU, and its running quite a bit leaner than you are (12.5's), it is setup for a stock engine/turbos/main cat at .9/.8, so you're making more power. i would expect an RE-A ECU for a full exhaust at 1.0/1.0 to be somewhat richer.

for the decel fuel cut popping, you could probably mess with the RPM that the fuel turns back on, maybe the way its set now is too close to the RPM the air pump turns on. you can see on the stock table that Arghx posted the factory ecu comes back on about 1250rpm. i'd see what yours is set to, and then maybe look at the Apexi default and adjust as needed
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Old 10-14-17, 07:46 PM
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Water injection is the single best anti-detonation precaution you can take, although running AFR's of 10.5-11.5 is very safe. You can set it so that it only turns on at a specified boost level, say, 10 psi. Either in addition to that or rather than that, you could turn it on when your water or oil temps reach some predetermined temperature. A really sophisticated ECU could turn it on when this or this or that condition occurs. I have always been a big believer in WI, since even running lower AFGR's things can go wrong.
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