safe cruising AFRs?
 

First let me start by saying my Power FC was dyno tuned by Steve Kan about 3 months ago and I haven't messed with his maps yet. It seems lately now that I have a different 5th gear (Jspec transmission) my cruising AFRs seem to be higher.
When in 5th gear cruising the highway at 80mph or so. I will see High 14s to Low-Mid 15s. I used to see to High 13s Low 14s. But I really don't have a good previous run to base this on. Because before the Jspec my other transmission was missing 5th gear completely.

Am I now seeing leaner AFRs because of the cold weather recently or do I need another tune because of the transmission gear change?

And when should I start to worry, basically what is a safe cruising AFR (no load -15 vacuum)?

Sorry if I rambled a bit their.

14.7 to 15.3 is realy good for cruising speed at -15vacuum

Both of you are misinformed. You can safely run cruise as lean as your engine can take and not buck. Lower speeds require richer than higher speeds.
Mine go from about 15AFR at 1600 rpm to 17AFR above 2400 rpm. 17 AFR is about the leanest to go for beest mileage.

Richer is easier to tune as some cruise cells are also used for acceleration.
If you fine tune your complete cruise map, you will have the exact cruise cells lean with the cell above them (more vacuum) the same AFR. Cells below them (less vacuum) should get progresssively richer.
When I went from the stock 5th to the shorter JDM 5th, I had to retune cruise due to the P row shift caused by now using more rpms and more vacuum.

leanest i was able to get my t04s powered cosmo re was 16:1 with about 40deg of advance, take any more fuel away and it would break up

Your ignition system has much influance over this.
Plug type, gap size, firing voltage, and single or multi-spark all are important.

A hotter, longer, and multi-strike all help to burn leaner AFRs.

Yep I have seen +/- 17 AFR on my Half Bridgeport, I have a Multiple Spark CD Ignition

slightly richer AFR's are good for a bit more power, but thats not really important in cruise situations. Don't worry about the higher afr's, as others have said you could run them even leaner then that. I run 15.5 - 16.5 on the highway. You'll get less carbon buildup and better fuel economy running leaner under cruise.

If you're tuning for mileage, just watch your IDC/pulse width, the lowest spots will be the best mileage.

As others have said, in cruise/low load situations you can run the engine as lean as it will actually run (the bucking from over leaning is the engine struggling to run on so little fuel)

Also, I've found beyond a certain AFR (which seems to differ from setup to setup) the EGT's will cool off after a certain point. Going leaner would raise them up and up, but after a certain point (which on my car was about 15.5) the EGT's would slightly cool off.

EDIT: Also, its quite likely that since you were without 5th gear during your tune that steve wasn't able to nail down those cells in use during a 80mph crusie so the accuracy may be limited to the accuracy of his base map in those areas.

Best fuel economy is not based on just AFR's alone. AFR is a ratio and not the actual volume of fuel being used. To determin the best economy you got to look at 'injector pulse' vs rpm. You can cruise at 4000rpm at 14:1 with a 1.9ms injector pulse or 16:1 with a 2.5ms. Which one do you think will give better fuel economy?
Best economy is normally acheived with the lowest throtle position, highest vacume reading combined with the lowest injector pulse width. Ignition timing plays an important role and when using sequential injection the 'end of injection' angle also.
Always remember O2 sensors do not measure the amount of fuel in the mixture. They measure the O2 content and quite often a lean reading on an AFR gauge will not show all the wasted unburnt fuel from the result of incorrect ignition and injector timing.
For example on heavily ported motors due to high overlap quite often an AFR gauge would read much leaner than what is the norm for say a stock motor when in actuallity it's the opposite that's occuring because the AFR gauge cannot read all the unburt fuel that's exiting the exhaust port.

how do you tune ignition timing under cruise? Would a ported engine need more or less timing under cruise areas? Under full throttle it seems people run lower timing for ported engines, but at idle they seem to like more timing, mine does anyways.

Thanks guys for all the info.

Wish you could share more here, FD and PowerFC specific. When I was doing my lean cruise tuning ~7 yrs ago I took the simple approach because the feedback on AFR change after making feul map changes confirmed the approach. Take fuel out, it got leaner, map cells the car cruised in did not change, take out more fuel, it got leaner, take out more fuel until the car would buck, add back a little fuel, do more all around driveability tuning, add back fuel to portions of the map used during light accel to eliminate lean bucking, recheck cruise, repeat cycle till all driving conditions are better than factory driveability with much leaner AFRs. Perhaps I was taking too simple of an approach and left something on the table. This was on an average street port.

The .806 5th compared to the .719 uses 12% more revs.
I also have the short 5th. At 65 mph the car gets 20.9 mpg.

This is exactly how i did it too. The rpm's arent going to change for a given speed by taking fuel out, so that leaves the vertical vacuum rows.... mine also used the same vacuum cells as before. So taking fuel out had no effect on how much vacuum the engine pulled at a given speed..

I have looked at the injector MS value on the commander while cruising, but it seems like it would be hard to tune that way, because slight pedal inputs and hills greatly effect how much fuel is injected. Personally it seems that once you're at a certain speed, maintaining that speed with the highest AFR's would be most fuel efficient.

As mentioned i'm sure timing also plays a roll. i'm running the same timing the base pfc map comes with in my cruise areas, so im sure there could be an improvement considering my engine is ported and running single turbo. I just don't know how i would go about tuning that area correctly. There is very little information on cruise timing. I'm guessing i would need an EGT gauge to correctly tune that area.

I tried tuning and logging changes in AFRS and timing split for one speed on a very long and flat road. The results were mixed and inconsistant. It is imposible to maintane a constant rpm by foot in the real world.

Maybe cruise control could do it better and then look at TPS voltage or inj duty for lowest valuses for the same speed.

Thats what I used! And people call me grandpa for keeping my cruise control on the FC :lol: It has more purposes then just being lazy on the highway!! haha