While following another thread on ecus I was interested to read where Mazda Maniac talks about the "cat preservation " thing that the ECU does so I did some observations with scanalyser yesterday (on my way to the rotary drag meeting ) . I just wanted to see if my car did the same thing & see if I could work out a way to stop it happening with my piggyback.

Anyway I found that this does indeed occur - AFRs suddenly drop from 14.6 down to about 13.8 in 6th gear over around 3750 rpm . If you are cruising for a while below that and gradually increase your rpm you can see the sudden drop in AFR as you pass that rpm.
If you get to say 4000rpm quickly then cruise you start at 14.6 AFR then drop suddenly after several seconds .
In 5th gear at the same rpm I was surprised to find that this was not happening quite the same - will need to test more to see what exactly goes on.

So : this could be usefull to know as I imagine it will have quite an effect on your mileage if you were to keep just below the 3750 mark rather than just above it .

Edit : further testing with scanalyser has shown that it is not so much the 3750 mark as a combination of rpm + %throttle but at a steady cruise with no extra load on the car (ie headwind,gradient etc) the point is 3750.

 

I suppose it would help with mileage, but...

A man's doctor tells him he has six months to live.

Shocked, he says "Can I do anything?"

The doctor thinks a moment, then says "Yes. Stop smoking and drinking. Stop eating rich foods. Stop seeing women."

"Will that help me live longer?" he asked.

"No," replied the doctor. "But it will seem longer."

Ken

 

I thought everybody knew to cruise below 3750 for best mileage?

 

everybody ?
OK , I have heard it before that staying under 3750 was good for gas mileage but thought it was to do with the 2ndaries & ignored it because of what Ken said However knowing exactly what happens & why does make it more valuable ...... (to me anyway)

 

It is true, while on a long trip following an old u-haul that couldn't get above 60 I got 26mpg. 26!! I have never been anywhere around there before. At that speed I was under the 3750. On the way home I maintained a speed of 70 which is just over 3750 and got 21 mpg. Now I'm not pacient enough to maintain a speed that slow on a long haul unless I have to as I'm sure no one else is. Just food for thought.

 

Hey just to add to this thread, since we're on the topic of MPG. Something I have observed since I purchased and installed a drop in air filter (not the intake) from K & N air filter. I was averaging 16-16.5 mpg with babying the gas pedal and staying around 3750-4000 rpm with 80% highway and 20% street. After installing the air filter, I've noticed a nice increase in my MPG, now at 17-17.8, WITH high revs 6000-7000 rpm and driving like a bat out of hell. So I figure, if I baby it like i did Pre-air filter, I would probably be getting good 18-18.5 MPG. My gas should get better as my engine ages though, I'm only at 3000 right now.

 

i know exactly what you mean, soooo hard to keep the car under 70mph in 6th if there's open road ahead, the car wants to go fast!!

 

and to test out this AFR switchover theory, i'm gonna drive a tank with mostly under-3750rpm highway cruising, see how my gas mileage does

i'll probably offset the gas savings by excessive revving on street driving though lol

 

Fwiw, on the freeway I ALWAYS stay under 3750 and have never seen better than 20.1mpg

 

So, if the 3750 thing is true (and I imagine it probably is), I wonder why the OB computer can't be programmed to bring in the secondaries (with the apparent richer fuel mixture) very gradually?

1.3L

 

it really is not the 3750. i have been paying attention to this for a long time.. it is cat temp....

i guess my cat runs cooler, being larger... my afrs only drop to 14.2 when the fuel richens and the cat temp drops...

beers

 

Swoope : try this - cruise at 3500 in 6th then very gradually accelerate to 4000 watching the AFR (and the road) all the way .
think this is what you will find
*can go for ever at 3500 & AFR stays the same
*There is a definate "changeover point" between 3500-4000

The way it happens points to revs & time - not temp .

I don't think this happens in 5th (but still need to confirm this) which further suggests temp is not the trigger.

 

well first of all it takes about 30 seconds to happen.... and i have found that with a really strong head wind it happens sooner...

and the change over is constant with cat temp... happens on my car at about 1635deg f.



then it goes to ~14.2 afr then the cat temp drop to below 1600.

this leads me to believe it is temp. not rpm... i could be wrong. only the code would tell for sure..

beers

 

I'll do some logs to see if I can pin it down a bit more

 

yes,

i have wondered. i thought it was a speed thing. i noticed the change over at 75 to 76 mph. whatever that rpm =. but the whole head wind thing changed all the rules. i started watching cat temp...

aint the scanalizer cool. shameless plug!!!!

beers

 

I have a A/T and these are the MPG I got at differant speeds.
At 55-60mph I got 29.5mpg
60-65mph I got 27.5mpg
65-70mph I got 24.5mpg
These were all done with more then half a tank of gas. Its very hard to stay in the slow lane for 200+ miles with the big trucks at just over 2000rpm on your tach. It takes patience too do this twice to verify your results, but I was curious what this my seventh Rotary would do at best. At 3000 RPM my speedo says 73mph, but I believe its really aprox 70mph.

 

confused. these figs are from m/t, i have a a/t?

beers

 

Did the scans & have had a good look at the results . I am now inclined to say it is a combination of both . It definately happens when you go from closed loop to open loop - every time . However , the revs this happens at varies between 3450-3750 with cat temp variations of 934 degC to 943 deg C . Will post up some charts later which I think you will find interesting .
Another observation is that AFR climbs back up towards 14.4 gradually from as low as 13.5 after switching to open loop.

PS - my piggyback could be influencing that 14.4 figure also making it leaner than stock - not sure .

 

mine hovers in the 14.2 to 14.3 range in the cool cat mode. have heard many are in the high 13s.

this is with no ecu mods.

beers

 

The figures before me are all from a stick shift. I have a Auto 4spd and these are what I have seen on mine.

 

got it thanks.

that 4.1 final drive really helps..

beers

 

Would a simple resistor not raise the "percieved" temperature just enough to move the switch-over point up into the 6000's?

I am just a doughhead with no electrical background at all, but would it?

S

 

charts - 2nd one is over a longer time but both clearly show the switchover point .
Have three more results similar to this .

 

Am now convinced the trigger is a combination of RPM & %throttle . Really don't think cat temp comes into it . This would also explain the headwind thing Swoope . Drawn this conclusion from these + other tests I did some time ago to find the open/closed loop trigger in various gears .

 

plausible,

as i never have the open / closed gauge up in a window.. not something i have ever paid attention to...

beers