Subscribed. This may help me track down why I have hesitation problems at cruise speeds.

 

3.9 gear third member. Standard on S4 Verts and all S4 automatics. They were installed specifically to help MPG. You'll want the speedo gear out of the tranny as well so yourspeedometer maintains calibration.

I'm planning on doing this as well. Just need the speedo gear.

 

Did the mirror delete over the weekend. All the car's I'd seen this done to on ecomodder just self-tapped the mirror bracket to the dashboard. I thought this was retarded, so I made it mount at the existing lower 2 mirror mount holes. The same 2 screws hold the delete plate on as well. Excuse the paint, it's a little chipped in some spots as installation was rather hurried

Hozzman, the 3.9 rear sounds awesome, I just don't want to loose the acceleration. If only there could be a way to change out just the 5th gear inside the transmission to effectively have a 3.9 in only 5th...

Vacuum gauge is now on my list of things to do!

 

Agreed.

There probably is a way, but it could involve custom machined gears, which I'm sure would NOT be cheap. You could always do some digging to see if something's already out there. However, you may not even feel the power loss going from what you have to the 3.9 diff.

 

 

I'll have to go back and look at the wiring diagram, but to maintain the idle high so you don't lose closed loop is pretty simple.

The stock BAC valve operates under pulse width modulation. So the more the ECU pulses the BAC during a specified time, the higher the idle is. If you hook up a switch going to it to apply constant power, you can raise the idle up at your discretion. Install a diode in the stock harness to prevent reverse-powering the ECU. If you find the idle is too high, you've only wasted a little wire and time.

The other option for that would be to do a ghetto rig like I did a while back when I couldn't get my car to stay running at idle with the AC on. I bought a fast idle solenoid for an old carbureted chevy and made up an adjustable bracket with the plunger aimed at the throttle wheel. It worked, but required constant adjustment. At least the BAC wiring job will provide consistent results.

By the way, I'm really not convinced the benefits of the mirror and headlight modifications outweigh the dangers. I'd put the aero mirrors back on and install some brighter driving lights recessed in the grill somewhere if it's drag you're worried about. And remember to not go overboard when fairing the bottom of the car. Hot air from the radiator has to escape out the transmission tunnel.

 

hahaha

 

Hmmm, both are great idle kick-up ideas! I actually may switch the the carb solenoid method. The one worry I have with using the BAC is, could it possibly go too lean during idle? and this lead to seal damage? I mean I guess it wouldn't... I dunno though.

I know what you mean about air having to escape. I plan to design it for it to exit after the sport aluminum under engine tray i have, and then flow under the car there.

 

When I took out the 3.9 in favor of a 4.1 LSD version, I honestly didn't feel much seat of the pants difference. Of course because a stock S4 Vert is a heavy underpowered pig, it could b e that it was simply too difficult to tell. Seat of the pants feel might be more noticable in a lighter coup.

I was primarily interested in the advantages of an LSD diff more than anything, so I was good anyway.

 

tripping over a dollar to pick up a dime.

 

Lol, or perhaps running over a dollar because you're squinting to see a dime in your convex mirrors....?

All jokes aside, I am most interested in this under body setup. I as it is probably the only aspect that is not dangerous and also applicable to a turbo car. Perhaps you can sell the blueprints if it makes a measurable difference.

 

The BAC draws in metered air. It just makes more or less bypass the throttle valve. So it won't lean out the engine in any way detrimental to apex seals. They both have the exact same effect, the tasks are just carried out by different valves.

 

Ohhhh right right! Ok, I gotcha now. What if you could make a switch to have the BAC not stay wide open, but instead flutter like it does, but at a changeable rate? Then the idle could be set wherever, and not just confined to wide open.

As for the mirrors- We'll see how much a difference they make, but it's all reversible if I feel it's too aggravating or dangerous to use.

 

Although I'm good with electronics, I'm by far an expert. So I really don't know off the top of my head right now if there is such a circuit to allow the user to specifically control duty cycle of the BAC. See if Aaron will chime in and offer a solution for this. There may be some way of modifying one of those "Extra Injector Drivers" to do this.

 

I've always been thinking about making an under tray for the whole car. I think this would be a good idea. Maybe try something like the RE-A diffuser as well (if you want to drop that much cash on it.) Or just include some vertical fins coming down from the diffuser to straighten out airflow.

Some other things you may want to consider is weight reduction.

If you have a rear wiper ditch it.
Sound deadening stuff in the interior. Like the foam under the carpet and the tar.
Aftermarket exhausts are MUCH lighter than stock. Especially at the header. But this will change your tune slightly. It may improve it - I don't know.
Brake rotors - maybe stay with the single piston brakes. I don't know how they compare to the turbo/4 piston brakes.
Tires. Try to find the lightest tires that you can. Then you can also get some that are slightly smaller, or even go as far as stretching them over the rim so that you have less rotating mass.
Aluminum driveshaft will cut back rotating mass as well.
Ditch the spare and jack.
Cut back on the crap inside the car.
Lexan windows?

I could keep going on for a long time about this, but you get the idea. I've gone to the extreme on my track FC with weight reduction. But yours is a daily so you probably won't want that. Keep up te good work this is an interesting thread to read.

 

the engine air still needs to escape from under the car as well as let everything else breath such as the transmission, exhaust and differential. drag from under the car is minimal. there is more lift than anything else as the air cushion tends to push up from under the car at speeds above 100mph, so unless you're building a bonneville racer, the added weight of the tray will supercede the .2-.5mpg you may have noticed.

similar concept to removing the mirrors, they really don't add much drag to the car.

if anything, getting the most from fuel mileage would be from gutting the hell out of the car, adding an SAFC, gutting the exhaust, tuning AFRs and performing all essential tune up related components to be sure the engine is running soundly. you can also advance the timing a few degrees to optimize the power from combustion cycle so that less fuel is wasted. these will show more noticable gains, but then again it will be a bit less of a street friendly car at that point.

 

Well, take that dime to pic a part and exercise the "a dime a dozen" truism.

But in my case, I've got one that has been sitting in the garage since I pulled it in favor of the S4 LSD diff.

 

so ur getting around 24 mpgs after ur mods? not bad.
i have an fb with RB header and custom exhaust, and a weber carb. getting around 20 or so mpgs. with the stock carb i was getting like 22 or 23. thats about 75-80% city driving too.

 

I would go for a manual steering rack, save weight and engine drag.

 

Check your speedometer calibration. I'll bet your reading faster than actual, so odometer saying you're going farther than actual. I've been driving RX7's as long as there have been RX7's, and I'm having a real hard time believing anywhere near 20 mpg with 75-80% city unless you're behind it pushing 75-80% of the time with the engine off.

 

Have you thought about meth injection?

 

is this the near impossible to track down front aero lip you speak of in your first post?

 

there is no economy is substituting the petrol for a liquid that often costs more and needs a higher fuel to air ratio to be stoic

the nail on the head is 'lean of peak' motor operation,, with water injection

i would think it needs to be set up with an exhaust temp sender to get it right


IE
a resister in the the lambda circuit when the water is enabled may make the ecu cut the fuel ratio and use water injection instead for boundary cooling to control exhaust and chamber temps ( and n20 production )

 

Bump, i think your going over his head. (but not mine )

Lean burn tech has been worked on in piston motors for quite some time. Requiring proper swirl to concentrate the fuel around the plug to get it to fire.
The temps should fall as you get leaner than stoic, the issue is going to be getting a mix of 15.5:1 or leaner to actually fire.


OP - Getting your drag Cd. lower is good for economy - but there is only so much you can do. I think you have passed the point of diminishing returns (in $$ terms, your going to have to start spending more and more to get less and less)
I would suggest shutting down one rotor under light throttle cruise conditions, but i honestly have no idea if thats going to upset the harmonics too badly (perhaps bumpstart can comment on this)
If its something you would consider, your going to have to either build a circuit to cut the injectors to the front rotor - or install a switch to do it manually () The issue here is that your going to be pumping air into the exhaust, which is going to drive the ECU nuts trying to prevent the lean condition (which doesnt exist) Perhaps installing long tube headers, and putting the O2 censor into the rear rotor tube could prevent this - there are dangers with only sampling from one rotor.

Have you gotten your injectors cleaned and flowed?

 

'active fuel management ' is an interesting fuel economy strategy used by a few OEM's

i have driven 25+ km on one rotor due to igniter fail and suffered no engine issues
( i have only dual igniter wasted leading DF ignition )
they don't shake massively like a blown rotor , but you are fully aware of the change in NVH

the one rotor operation was/is a feature of the decel /anti backfire/buck strategy in many production rotaries
however traditionally there is usually heavy enrichment on the rotor that runs
and you may find the result is almost counter productive to keep smooth while under load and not decel
you also will find a NA single rotor in a heavy car ( FC ! ) will struggle to maintain 90 kmh

it would be worthy of a look see at a workable result
but i think it may be a long path not in the realm of the stock ecu or with 2 rotors


but i think you gave mazda a great reason to build us all 3-4 rotor engines with active fuel management


------------

however lean burn tricks on an FC ECU are relatively easy
using the 5th gear switch
trip a pair of changeover relays to bypass the leading coil resister to hot the spark
and enable the lambda trim resister / rheostat
trip also a standard relay to enable the water injection

set up the water injection flow with a pill or valve and adjust the flow to control exhaust temps/ ignition beak-down point and adjust for the lambda trim ratio

am expecting the spark will give up after a point
and / or need a few tricks but you will get a reasonable result as there is much reserve in the FC NA ignition
( being the same ignition used on the turbo and capable of pushing reasonable boost )