Get over yourself:
1:8.6
no EGR
wide sticky tires
Forget internals (efficiency losses due to blow by caused by larger bore margins)
Turbo restriction
lead foot
Cam timing ?

Not gonna make a Prius out of your track whore.

I'm only talking about cruise here.

I also used this logic on the dyno. We unloaded the roller, and I attempted to hold 70mph. I then adjusted for 15.1:1 AFR, then added spark until there was no increase in power. By doing this I dropped down 2-cells in KPA, closing the throttle, letting in less air.

I was able to get 28mpg out of my 9:1 compression turbo vr6 corrado on road trips. The EPA said 22mpg on that car from the factory.

At what point does a lean AFR lead to melt-down inside the cylinder (not through the exhaust-stroke)? I live by the low-EGT logic with spark advance, but how hot is that flame in the cylinder when I'm cruising on the highway? i had the old 1.6l test-dummy as lean as 18:1 and it felt OK while yeilding one tank at 38mpg, but I was a little worried by a number that lean.


I've been able to get 230-miles on a tank with mixed driving (like a bastard at times) on the last 2-tanks, but I expected more with the 3.63 ring and pinion on the highway. Thanks for the discussion everyone.

Lean it out in low boost some. My AFR decreases from 15ish in cruise to 13 up to like 5psi. Reason being, hilly areas. You will boost some even with light throttle on hills at speed because your wg pressure is higher. Even boosting around like crazy on on ramps etc... I still get 25mpg on a tank. If I were to go completely off boost as much as possible for a tank I have no doubt I would get >30mpg.

I spend a lot of time at part throttle in this car on the track, so I can sustain the 101-140kpa loads longer than most of you guys. However, I still stand by my experience that running fat, with lots of spark advance produced more output in those cells, and quicker spool below 4000.


edit: I may go leaner at 100kpa because I did not steady-state tune those cells because I'm dumb...and I was getting tired and cranky.

I'm not arguing that running fat makes more power in boost. I've had my car on a steady state too, I've watched it with my own eyes.

What I'm saying is that there's no reason for your boost transition (call it atmo to atmo +3psi) needs to be fat. Lean that area out a little and you'll see some economy back.

How lean is too lean in cruise? I dunno. Honda runs something ridiculous like 25:1. Other OEM's have run similar set ups. Reason why you don't see "lean burn" more often is high NOx output.

I could cruise in many load zones no problem at leaner than 19:1. Every once in a while, I'd get a misfire. I backed everything to about 17.5:1, just to make sure I'd never misfire, gave it some advance, and let it go. My EGT gauge reported about 1400° pre turbine. Which is about the same as it reported running stoich. I think it read high, because it would peg out in boost, and EGT probes are not known for being accurate. As I've said before, I use gauges not for absolute numbers, but to look for unexpected differences in readings.

The guy who bought my car drove it back to his home... It was Indiana I think. I'm in GA, pretty long drive. If it was going to melt something, it would have.

I think what y8s is getting at is that you ideally you would be at WOT making just enough power to maintain cruise.

Tuning for 70mph cruise on an unloaded dyno isn't terribly useful. If you aren't going to do it on the road then you should load the dyno so that you start tuning in the same cells that you cruise in. That way you can simulate the air resistance, which is significant on our cars.

The 3.63s aren't a magic bullet for mileage. I've tried 4.1,4.3,3.63 with the 5 and 6 speeds and I always get the same mileage. Driving style and tire choice have a more noticeable affect.

There is a function on the Mustang that simulates highway cruising so you can work on mileage. It loads the dyno very slightly, but does not lock roller speed. I incorrectly described the function.

I refuse to believe that lower KPA load somehow means I have the throttle open further. You guys keep telling me that running enough spark to run lower kpa at cruise is going to use more fuel, and I don't get that.

Maximizing output in every cell, and smoothing it out in a few places by removing spark angle is the difference between playschool shit and driving my car.

Race Recap

Hustler:
How do I get better economy?

Ben:
Lean out your a/f

Hustler:
But I tuned my spark map on the dyno. How do I get better economy?

Ben:
Lean out your a/f.

Hustler:
But I tuned my shit on teh dyno. Plus I live out of a suit case and smash bitches in the face. How do I get better economy?

Ben:
Lean out your a/f.

Hustler:
I tuned my spark on the dyno. How do I get better economy?

What do you think I'm going to say next?

That he needs to tune his spark?

No one has chimed in with any logic on a safe number, or presented any long-term success. If mazda can do it at 14.7, I should be able to get close.

Are you fucking stoned?
You want better economy? Squirt less fuel. That's all that's to it.

It's not pratical to remove your cool air box or reduce the cooling system's effectiveness, even though a hotter motor and hotter charge will reduce the density of air entering the chamber. You don't have EGR so you can't displace O2 with oxides. So what can you do? Oh yeah, that's right, squirt less fuel. I wish someone thought of that earlier.

Tuning for economy and tuning for power are different things. You do both, but their cells fall on different parts of the map.

Run the car up the street. See how lean you can go before it starts cutting out. Add a little fuel from there.
??????
Profit

Need "logic and a safe number?" Get some sensors and datalogging equipment. We'll need to keep track of CHT, EGT, TPS%, MAP, RPM, Injector Duty, Lambda, Vehicle Speed, Spark Angle. With data from those sensors and time to experiment and compile findings, I'm sure we can find ideal numbers for you.

This reminds me of "What turbo kit should I get?" all over again.... The nightmares..

I would imagine that it is the low compression. Two identical engines except for compression ratio. The higher compression engine should need less fuel to make same power as lower compression engine.

Since at hwy speed it takes a certain amount of energy to propel a vehicle along at a given speed. I would think that the lower compression engine would need more fuel to create the energy needed.

I'm interested in this because I believe the diypnp has the ability to run two different tunes at the flick of a switch.

It doesn't take a rocket scientist to jump on board with Ben.

Delivering less fuel is going to use less fuel.

I also think hustler's fear of running too lean is valid. Who the hell wants to dump thousands of dollars in an engine just to ruin it because you purposefully ran it too lean. Then your just an idiot.

Seriously though; You built a fucking race car and now want good fuel economy. You need to get laid hustler. Have your boyfriend give you some road head, you're not thinking straight.

I drive the miata and a toyota echo. I have never said to myself while driving the echo, "Man, I love driving this car, it gets such great gas mileage."

I'm sorry dude, but I refuse to believe that pulling spark and running a higher KPA with a constant AFR is going to yield superior fuel economy.

Who is telling you to remove spark advance? Lean ratios burn slower, and if anything will need more advance.

If you can't believe that running a leaner AFR is going to produce higher economy, then I weep for the dozens of bastard children you've fathered all over this country.

I absolutely want good fuel economy. Both qualities in the same tables.

Advance spark to MBT in cruise and extreme load.
add fuel for output in load cells
run as little fuel as possible in cruise to avoid mechanical melt-down

jesus fuck, I've been supporting this for 2 god damned pages.


Then, people suggested that max output per cell via spark angle wasn't the answer, and I still affirm that MBT yeilds more efficiency and output.

I have no idea why you're under the impression that you need to reduce timing advance. The leam mixture will needs lots of advance because it burns slow.

Squirt less fuel
Fuck bitches
????
Profit

Make the motor as efficient as possible, with the least amount of fuel required.

PS, as I told you earlier, this is what Honda does. 22:1 at cruise. They have concepts that are leaner than 60:1 for non US markets. They work great, just produce too much NOx to pass our emissions standards.

Keep your adv and take away fuel. Nothing will melt and die.

I would just try what Ben is telling you to do. Figure out what rpm your happy cruising and pull a couple of fuel points. Monitor for a tank of gas and repeat until you find something you are happy with. Let s face it, unless you are dumping tons of fuel because your ignition is to advanced you shouldn't have a problem when you pull fuel.

Eventually your car will stop running because it's not getting enough fuel, however I think we can all agree on that. I think the point of concern would be the transition into an area where you need more fuel and the spark advances rapidly. That should be easy enough to adjust for since cruise rpm should hover around the same spot for a given gear. By adjust, I mean start adding more fuel a couple hundred rpm before the max cruise rpm. I might be over analyzing now.

EGT shouldn't get out of hand if spark is acceptable ( I think)