Hi guys,

Daily driving the fd, I don't have a choice now. I don't need 50 mpg, but I need better than 7. (lol, that's what the 7 stands for in RX7 right?) Last tank was 7.2 mpg and I'm depressed. My goal is to get 15-19 mpg, and I'll be happy. Car is a bridgeported (brap brap brap) 5 spd, 3" turboback exhaust, and medium sized FMIC.

I know, get a Civic, but I am stuck in the fd for now. No one will buy it and I don't have the funds for another car. So, my plan is to run it as lean as safely possible, drive it lightly, and run a good cleaner or premix if that works well at cleaning. I don't want to blow my expensive motor, but I don't want to get 8 mpg this winter either!

I figure, if the cleaner or premix does the cleaning for me, then I don't have to redline it. Then I don't have to boost, and then I don't need to run it stupid rich. I am living in a cold place, and it's hard to redline the car anyway without killing myself.

So, my questions are..

Has anyone tried something like this?
Does a fuel injector cleaner or premix work well at keeping the carbon out?
How lean can I safely run the car?
Would the car be okay if I drove it lighter all the time?
How about water injection? Is there a way to run it at cruise for a "cleaning cycle"?

That's all for now, I know the RX isn't for mileage, and I am okay with 15 mpg, but 7 is just ridiculous. Thanks!

 

Do you have a programmable ECU?

Water injection steam cleans an engine. Premixing instead of using the stock metering system isn't really going to change much. Both are oil, and both will leave a little carbon behind. Why are you so concerned about carbon? If you're getting 8 mpg, the computer/fuel system is dumping in fuel, or you have a lead foot.

 

I do have a programmable ECU, and the 7 mpg comes from this kind of driving pattern.

Start car, idle for 10-15 min to warm up so I don't blow the coolant seals under boost.

Car is modified, so I run it rich to avoid detonation also. Once warmed up, I run it to redline to "clean it out" so I avoid detonation from carbon build up as well. I really don't want ruin my engine.

Drive to work 3 miles. Shut off.

After work, warm up car for 10-15 min and repeat when going home.

I am just afraid of blowing the engine, hence, the warm up for the coolant seals, redlining for the clean out and running rich to avoid detonation also. This combo I assume is murder on gas for obvious reasons.

 

Who tuned the car? You? Someone else? What injectors/pump/FPR? What fuel pressure?

A rich mixture is only necessary under boost, and the ratio depends on boost pressure. In very general terms, most turbo rotarys are running in the 11.0 to 12.0:1 AFR range under boost. With very high pressures or a lot of timing advance, some are tuned into the 10s, and at low boost pressures (ie 0-5 psi), you can get away mixtures in the low to mid 12s. It depends on the overall setup really. Timing, spark plugs, intercooling, etc... are all factors.

Do you have a wideband 02 sensor/gauge?

If tuned properly, at cruise you should be somewhere around the stoichiometric 14.7:1 ratio. At partial throttle, expect the mixture to drop down into 12-13 range, and then under boost you should basically see some solid 11s.

Dumping way too much fuel into the engine will lead to carbon deposits due to incomplete burn. There is such a thing as too rich.

 

It was "tuned" by a local speed shop who is no longer in business. I guess that might be a bad indication.

Fuel system is stock, it just has an exhaust, intake and FMIC, that's it. I was told with those kind of "mods" I'd need a fuel computer.

I don't have a wideband, but the car blows black out the back at idle, partial throttle and boost. I bought a new O2 sensor as I keep turning the fuel down on the FC but it doesn't even change anything. I would get a wideband and AFR gauge, but the whole money thing is an issue right now. I was hoping just to turn down the fuel "pot" and see how it ran afterwards. Taking it easy of course.

Sigh, sorry I didn't really sleep last night so I might be a bit loopy.

 

I think at least some of your poor mileage is the porting and there's nothing you can do about that. Regardless, I doubt you will get to your 15 mpg goal....around town.
IMO auxillary injection is for a safety margin and NOT for tunning. So if your suggesting an intention to tune it leaner with AI to improve mpg, your gambling your engine that the AI tank never runs low or the system malfunctions.
I know you've ruled out a winter beater, but IMHO that's the most practical option you have.

 

I was afraid of hearing about the porting causing mpg woes... Haha.

But either way, the AI idea was to run it to clean the engine out rather than compensate for overly lean mixtures. I just want to run it at cruise most of the time and not build up carbon while doing so. So I figured with a light boost run (upto 4000 rpm only, and an addition of a few "shots" of alcohol, I'd be able to keep it clean without redlining it all the time. As for money wise, I have one kit sitting around, so that's why I was asking.

The AFR's I wanted to keep leaner but not overly so. As close to stoich as possible.

 

Black smoke = way too freaking rich. That's 9-10 AFR territory. You should never be anywhere near that at idle or partial throttle, let alone under full boost. It sounds to me like the car was never tuned at all, or they half-assed it and left 90% of the fuel map with default settings.

You also shouldn't be playing with the settings without a wideband or EGT gauge. Idle and vacuum cells are fine, but any of the boost cells should be left alone until you know for sure where you're at. Is this a PowerFC or something else? There are subsections in the engine management section of the forum that can help teach you how to tune it on your own. Otherwise, you might try to find someone better than the guy that messed with the settings before.

 

It's this....

http://www.autocarparts.com/part/list/1883/ I really don't know much about it.

I am familiar with tuning as I used to run DSM's all the time, but this was just with DSM link and my laptop. This thing I have no software for, nor a plug, so I have no idea how to play with it except for the little "pot" dial thing it has on it. I've been told that I can borrow some software to map it, and then I've been told this thing is pre-programmed, so.... I've kind of given up on modding this thing further because I've already spent too much money on making it run. The novelty of having a built FD has long since worn off, and at this point I'd be happy with my old Dodge Neon more, just for the gas mileage and having a mechanic who knows then in and out and fixes it right the first time.


That's it so far, just want it to get "fair" fuel mileage without spending a lot more. In it over $10k already in paying people to work on it, and it's still seems like it wasn't done right...

 

I'm not too familiar with the HKS F-CON, but after a little searching, it's apparently a piggyback similar to an SAFC. Only instead of intercepting the AFM/MAF signal (which you don't have) to force the stock ECU into delivering more fuel, it sounds like it's altering the MAP signal. The only info I could find is vague, but there are apparently ROMs for various cars, and then you can adjust an internal dip switch with 10 settings (the dial you mentioned I'm assuming).

http://www.fd3s.net/fcon_install.html

The install guide for an FD ^. Dip switch setting 0 is "stock", and 1-9 are various "rich" stages. The "stock" setting should just be stock fuel levels, but I don't know for sure. Now do the 1-9 settings add fuel to the whole map, or only under boost? Got me. I'd probably contact HKS to find out more.

This is on page 7 of that instruction booklet:

A little better overall description here too:

http://www.horsepowerfreaks.com/part...PFC_F-CON/7587

So, what setting is the thing on (0-10)? How much boost are you making (stock FD twins have a 10-8-10psi boost pattern)?

Can you switch it down to 0, and see how the car runs at idle and low throttle only (no boost!)? If it's essentially turned to "off", and you still have black smoke pouring out the exhaust, you've got other problems.