new guy with 92 mustang GT

[rob342]

just joined the site so i may keep track of my mileage over time as my car gets better.

i have a mustang with a bone stock 5.0 with a supercharger and a 4.10 gear i picked up recently. it's fun and quick, but wasn't getting the mileage i thought it should. driving like a little old lady i could muster up 17.25mpg, tank after tank.

i keep a file on what i do to the car and it's effects on performance and mileage, and since starting my quest to get the car to at least 20mpg, i have inched it up from 17.25 to an average of 24mpg. this is 50/50 city/highway. anyways i've decided i'll shoot for 27mpg, and when i reach it, maybe i'll raise the bar higher.

i am in no way willing to sacrifice performance for MPG, this is my fun toy summer car. i'm sure it'll end up getting worse mileage sooner or later. the 4.10 gear is staying in there. i'm planning to add more boost and bigger injectors, better heads, better headers, and a better intake manifold. the front tires i want are 1/2" narrower and 4lbs lighter, the back tires will be slightly narrower but much stickier(drag radials). i'm not sure how those mods will effect my mileage but i'm hoping it will lightly improve more do to being more efficient. the car will also get lighter as time goes on. i removed the 23lb front swaybar to help with straight line traction. i'm planning to buy a fiberglass hood and fiberglass hatch over the winter. those 2 items shave 68lbs. the cylinder heads will be 60lbs lighter. im getting a tubular K member and coil overs to shave more weight off the front and help the car launch.

anyways, that's me. nice to meet you. -rob

[rob342]

i would also like to add i am from Forest Park, IL and this is the first forum i've ever bothered to introduce myself in.

[rob342]

ok, i posted a thread in the new user welcome section, and i've been reading other "new user" posts for a while, and i have a question... how do you deal with all the crazy ideas and bad information? after reading the post with the "CARE guy", and the "chamber dimpling" guy, and a couple other nuts, i'm wondering if i(or rather they) should even be here. sheesh.

is there an "in crowd" here? who are the "good advice" people?

right to the point: i am interested in learning to adjust the EGR strategies in my tune to get better fuel economy under low engine load. right now my 1992 OBD1 antique electronics are set to inject a maximum of 11% egr gas at only 40% load or less, as seen in the table below. i would like to learn how to tune that so i can get better MPG while cruising. i know i could pick up some MPG here, trial and error on a supercharged car just doesn't seem like the best way to do it. is there is a way to calculate how much i can lean out the mixture, advance the timing, and compensate by adding more EGR to the mix to make this somewhat safe?

stock EGR and corresponding timing advance tables:


thanks in advance, and i hope to stick around as a valuable contributing member. -rob

[theholycow]

Welcome!

On wondering if you or others should be here, and who the "in crowd" is: The culture here is in a transitional state and that will sort itself out over time, but there is no clear answer at the moment. You can influence the direction.

This place was, until recently, two unrelated forum sites. One was GasSavers.org, a site about increasing fuel economy, mainly through hypermiling-style driving techniques (not necessarily the dangerous extreme ones that make the word "hypermiling" offensive though), though modifications were certainly given their due. The other was Fuelly, a site mainly for mainstream (i.e. average Joe) fuel economy logging that had an underutilized forum. They were bought by the same folk(s) and merged.

GS certainly attracted plenty of fringe folks, and I imagine the old Fuelly did too. I've noticed a few non-GS Fuelly users who seem that way to me.

Being from GS, of course I think myself and my favorite GS users are the people to whom you should listen. I like to think we are more thoroughly grounded in reality and practicality than others while knowing how to get the best results within those bounds.

So, having thoroughly patted myself on the back...here are my thoughts on your situation and your plans:

Fuel-saving driving is not always about driving like a bluehair. Low cruising speed isn't always helpful, it's just the simplest useful piece of advice to communicate to people who do not want to study and practice, so it's what is most well-publicized.

If you are willing to invest in equipment (a tiny fraction of the budget you have for upgrades), you can determine what is most effective. That's the most important thing, getting lots of good data. Without it you can't know which modifications and techniques actually, objectively worked. It takes more than one run's worth of instant and average fuel economy readings; gather lots of data. With a pre-OBDII car it's more difficult and there may be less data that you can gather, but OTOH I bet there's tons of stuff on the market specifically for your car and ECM...easier with a 1992 Mustang GT than something not popularly modified.

More important than electronically-collected data is your fuel log, and Fuelly specializes in those! Long-term trends and A-B-A-B-A testing can absorb all the millions of variables that are impossible to control and make short-term data have limited meaning.

[theholycow]

Here's where I get a little fringey...

On tires: Data, math, and logic that I've seen indicate that narrower tires may not be better. Contact patch is a function of pressure and weight. With narrower tires, your contact patch is narrower and will be longer. Sidewall hysteresis increases with a longer contact patch. I've seen data and arguments supporting both sides, but it is my belief that common knowledge is wrong.

Additionally, the further you get from the same model/size/compound/tread pattern/manufacturing run, the further you get from being able to accurately compare rolling resistance. There's very little data in existence, unfortunately.

On weight: Given a driver who is interested in efficiency, I find that simply removing weight does little or nothing for fuel economy. Unless you can remove a very large amount, like 20% or more, you will never be able to measure an increase in fuel economy. Smaller amounts of weight reduction at an OEM level work because they adjust gear ratios or engine management to save fuel by sacrificing acceleration, bringing it back down to the acceleration performance before the weight loss.

Related to that: The extra significance of rotating weight to fuel economy is a flat-out myth. You put energy in and it becomes inertia. That inertia is stored until you brake. Nothing more, nothing less. Same as static weight, for that matter.

[theholycow]

As far as modifications go, I agree with EGR experimentation. You should read up on potential side-effects and damage risks, but it hasn't seemed to harm my 1980 Buick for the past 70,000 miles.

I usually advocate increased tire pressure. It's easy, free, and includes a plethora of additional benefits (lateral traction through sidewall stiffness, longer wear, increased robustness) besides fuel economy. However, it sounds like you're into straight-line acceleration, and only enough lateral stability to keep it straight. I suspect that your needs are at odds with increased pressure. However, you certainly could reduce it when you're at the track and increase it when on the road.

Aerodynamic modifications are a huge help for highway driving but tend to either cost too much to ever pay for themselves or be horribly ugly. That's probably still true for your car, but it probably has better chances than most. You may be able to identify manufactured aftermarket parts that will actually lower drag (vs. ones that increase it for the sake of downforce or ones that are made entirely for visual effect), and you may be able to find those very inexpensively used.

Or, if your goal isn't to save money but to prove a point even at your own expense, do invest in things that increase fuel economy but never pay for themselves.

With money, willingness, and ability, I'd recommend upgrading your transmission. I know you want close ratios and a short rear end for drag racing, but how about more gears with your racing gears close and the remaining gears widely spaced? For example, if you only use 4 of your gears on the track and you get a 6 speed, you could put in a somewhat average overdrive and an ultra-tall highway loafer overdrive. One caveat, I have no clue how a supercharger figures in. A normally aspirated engine has the best BSFC at low RPM with the throttle as close to wide open as doesn't engage any enrichment (i.e. open loop power enrichment in the ECM, or in my mechanically carbureted car, the carburetor's vacuum-operated power piston), but I imagine that a supercharger can change that dynamic quite a lot.

[theholycow]

The most effective modification you can make is to adjust the nut behind the wheel -- the driver. As I said earlier, the simplest worthwhile advice to give, reduce speed and reduce acceleration, isn't necessarily the most effective; it's just the most effective that you can expect out of every non-studying speed demon grudgingly trying to reduce waste. It is quite effective for freeway driving, though. With logging, data collection, study, etc you can find what speeds and levels of acceleration produce the best fuel economy for your car and your needs.

That said, there are some things that are pretty universal.

It's natural to concentrate on the go-pedal, but there's a whole lot more you can do -- especially, avoiding the brake pedal. Every time you brake you discard energy that you recently extracted from fuel, and then you'll need to burn fuel again to re-accelerate. Look far ahead and plan ahead to avoid braking more than necessary and having to re-accelerate. The prototypical example is braking early and coasting through a fresh green light at 20mph instead of braking to stop at that same red light after reaching it at 40mph. Another is not braking for curves...it saves fuel to fly around turns and is fun! Of course, never exceed speeds that are safe and legal. There's nothing like a crash or a ticket to set you back further (in someone getting hurt or at least monetarily) than any amount of fuel economy will ever save you.

With a manual transmission, neutral coasting becomes convenient enough to be worthwhile. Beware that it is illegal in many places to coast downhill (but those laws do not specify level land). It can be controversial, and certainly if your driving conditions, confidence, or car would not be safe for it then don't do it. When you intend neither accelration nor braking, it is the most efficient option.

Some people think that DFCO (Deceleration Fuel Cut Off), where you leave it in gear but close the throttle, is superior, but it is less efficient unless you intend braking, and even then only if your ECM actually engages DFCO readily. It's nowhere near as dependable or instant as a simplified view that people like to believe, and that would be especially true in your car. Even if DFCO is engaged, it's not more efficient than coasting (unless you intend braking); while true that it uses no fuel and idling the engine uses some fuel, the braking effect (even if it doesn't feel like any braking) will require more fuel to re-accelerate than idling the engine would have cost.

On the highway there is less that you can do. One thing that works for some is drafting -- not like NASCAR, not even as close as most people follow, but finding a tractor trailer and keeping a 3 second distance (most people keep a .5 to 1 second distance, in my observation). Too close and, beside being dangerous, the turbulence wastes fuel. At the end of the draft is where you can be pulled, and that's about 3 seconds behind a standard 53 foot box trailer at 65mph.

[rob342]

thank you for the long and inspiring reply. it gives me ideas to pursue and ideas to avoid.

i did not think about the rotating mass the same before. when i said "narrower tires" i'm talking going back to the stock size of 225 in front vs. the 235s' that are on the car now. i did shop and study to find lighter ones in back i have 255's which are so wide they actually touch the exhaust. those will be downsized to a 245 width for that reason. i was hoping MPG would just be an added benefit. i bought the car like this, and haven't gotten to changing them out yet. i've been trying to get it as efficient as possible before i start adding performance goodies. i have the front tires at 40psi which game me NO increase in MPG, which kinda shocked me. the back tires i need to keep softer. even rolling on the gas in 2nd the tires just spin. I do have a street/strip drag suspension setup for the back i'm going to install pretty soon that will dramatically help with traction, so hopefully then i'l be able to keep more pressure in the back tires as well.

with the transmission.. if i were to upgrade my T5 5speed to a 6 speed, a T56 viper spec trans would be the common choice. My T5 has a .63 5th gear, the T56 is .50 in 6th. I really like the T5 i have though, as it only weighs 75lbs. the 6 speed weighs 149. they do make a beefed up gear set for my trans for less than half the cost of the T56, and it has an optional .59 5th gear. i may go that route.

i never heard of BSFC before today. that is something i will study up on to learn about. I do know the centrifugal supercharger I have has a bypass valve to vent boost when vacuum is present in the manifold. then again the bypass tube is only 1" in diameter. some people say their highway MPG goes up about 1mpg on the highway after they install it because the engine has to work less to fill the cylinders. i don't know if that is true, because the car came with the supercharger already installed and the internet is filled with bad information. when you do get into boost, the fuel management unit increases fuel pressure by 12psi per lb of boost. on a fun driving day, i've gotten as low as 13.8mpg(but IMO it's worth the added fuel cost.) when trying to get good MPG is when i see 24. driving normal, suburban driving enjoying the car from light to light but not driving like a jerk, i see around 17.6. i'm hoping that after adjusting the car and my style of driving to get 27mpg with max effort, i'll end up getting that 17.6 number to be a little over 20.

money is a big limiting factor in what i'm doing(or at least it's limiting the rate at which i can upgrade stuff). it's a hobby. fun is important, but mileage is also important because i drive my toy about 60 miles per day. i think i spent $6.50 on gas yesterday. not bad at all considering what i'm driving (:

-sorry if i wrote that a little scattered. i'm at work eating lunch. and thanks again for the lengthy response.

[rob342]

here's an interesting find. i was looking at the programming in my ECU and it seems that mustang GT's are set to wait 31 seconds before the fuel is cut when downshifting, which basically disables it. some people think they did that to help with emissions, as it helps keep the catalytic converters at a higher temperature.

here's a picture(i stole) showing the mustang GT(left) next to a better tuned Cobra ecu(right). seems that people assuming the fuel is cut while downshifting is not always accurate. in the pic below, DSFO = decel fuel shut off


my car didn't come with cats installed, so i will be adjusting mine to the values on the right so it is enabled. it may help my economy a hair, but it also gets rid of the burbling/rumbling/popping sound i hear when i coast and it also makes engine braking more effective.

here is a picture from my screen showing what is in there as far as EGR control, since i wanted to mess with that:


i had NO IDEA that would be so involved or complex. i mean adding more EGR mix should be fairly easy, but it may cause unexpected problems before i learn what's what. for example changing the EGR mix in the picture i posted a few posts above might cause problems once the computer factors in the other tables, like the four EGR multiplier ones.

another trend i'm finding reading about EGR and MPG, is that common consensus seems to be EGR-disabled helps city MPG, but EGR-enabled seems to help highway MPG. i will test that theory and if it's right, i'll see if there is a way to only activate EGR above a set speed, say 50mph

I'm not trying to sound like a know it all, i know nothing. the hardware i ordered that lets me change ECU stuff is on backorder until july 21st, so I don't really know anything about it yet. the software im using lets me look and learn in the meantime. it's very scary and confusing!! -but i'm learning (:

i'll start a thread in the project section once the hardware arrives.