acroy

There’s good theory that *more* restriction in the exhaust will *help* cruise mileage!

At steady-state cruise, the car’s computer watches the O2 sensor and maintains stoichiometric ratio of 14.7:1 air/fuel.

Low-restriction intake & exhaust are designed to better fill & empty the combustion chambers. At steady-state cruise, the throttle plate is far & away the restriction in the air system; it’s almost closed. The amount of vacuum in the intake is measured by the MAP sensor.

A low-restriction intake ‘might’ ever so slightly reduce the airbox system vacuum relative to the ambient pressure. However the throttle will then stay 1 or 2 % more closed to restrict the airflow.

So – high-flow intake does absolutely squat to cruise mileage.

The stock exhaust restricts the amount of air leaving the engine. Say the stock exhaust allows the 5.7 liter to be 90% efficient at cruise (i.e. 90% of the exhaust gas leaves the cylinder on the exhaust stroke, or 5.13 liters).

Aftermarket exhaust might allow it to be 95% efficient (5.42 liter), just pulling these numbers out of my hat. At the same rpm, the car is then using 5% MORE gas to hit the stoichiometric ratio of 14.7:1 air/fuel, because the effective engine displacement just went up.

It also in theory makes 5% more power at the same rpm & stoichiometric ratio. So what happens – the throttle plate stays another 1-2% closed.

So how to improve mileage? Reduce the effective engine displacement.

Add restriction to the exhaust; make it say 80% exhaust scavenging effective. The 5.7 then ‘acts’ like a 4.56 liter. Some engines have EGR and other systems for this & other purposes.

Or increase the gear ratio, say from 3.42 to 2.73, so the engine spins over fewer times, throttle is more % open, burns less O2 & fuel for the same distance.

The engine (which is just a pump) is most efficient when the pumping loss is smallest. The throttle plate is just an enormous restriction on the pump. This is why a lot of hybrids, etc are designed to run their engines at a fairly high RMP/open throttle position, while maintaining stoichiometric ratio of 14.7:1: to reduce pumping loss. And part of the reason why diesels (no throttle plate) are more efficient.

Anyway…. Best o luck!

Corvette_Ed

I think most people here would agree that tire pressure should be set at 30 cold, not 32, dependent on weather conditions.

s'noJob

Acroy, it sounds like you're confirming that reducing restrictions with aftermarket intake and exhaust mods will improve HP by increasing the effective displacement, BUT it comes at the expense of gas mileage.
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acroy

Yep - For power production the limiting factor is how much O2 can get jammed in the cylinder. Thus high-flow intake & exhaust, cam, turbo, etc.

To maintain the closed-loop cruise 14.7:1 ratio, more O2 = more fuel required = less mpg...everything else being equal

I suspect some of the HP gains from the standard upgrades of intake, exhaust etc is that the engine computer operates in open-loop under heavy throttle. The factory calibration might be 12:1 under load (again just making this up). The new parts let more air in& out, ratio might go to 13:1.

This will make more power on the dyno but at the risk of higher combustion temp, detonation etc. So having the car competently tuned after any major mod is extremely important for the engine's longevity.

I would love to see dyno results before & after the standard upgrades (intake exhaust etc) after competent retuning and with AFR logging. Such data is vanishingly rare....

GoldenFlame

Psshh, If I drive 50mph I get 40mpg, 3.42/M6. At 75mph I'm not even at 2,000rpms

s'noJob

I wonder if anyone has studied whether there is a break-even point when the C5 starts to gain gas mileage because the cD causes the car to become more effective beyond a certain speed.
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paxmanz06

Thanks for the tips everyone. I'm keeping everything stock since I autocross. I got a new set of daily driving tires today (Continental DW's) since I was still on the same set that came with the car and were in need of replacement. I'm taking a trip to Houston this weekend so I'll report back. I haven't used a fuel system cleaner yet so I'll pick one up before the trip.

QCVette

I have never heard that before and it does not match to my experiences. On every car that I have added headers and/or low restriction exhaust, the gas mileage improved.

crazystevex

After I put on my LG long tube headers and got it tuned I lost some gas mileage....went from 34mpg to 31-32 on the highway at 70-75mph LMAO! I saw this as exceptable for 34 more rwhp

paxmanz06

I ran into some traffic on the way to Houston and cruised at 75 mph for most of the way averaging 21.6 mpg. But now I have bigger problems. My transmission is done. I started having issues with reverse last night and while driving today, it's grinding in every gear and making awful sounds. Waiting to get towed right now...

92GA

After I installed my AFE cai I noticed a slight dip in gas mileage, but increased throttle response. It was worth it. I also do get some lean codes sometimes when the air is real good in the spring and fall of the year. The car is much more responsive now. I also enclosed the frontal area around the cai with aluminum so the air is pulled from the bumper area.

scarecrowkc5

That sucks dude, I hope you are safe.

I hate breaking down on a long trip.

I have put 1100 miles on my LS1 driving around town with a few 100 mile round trips tossed in and over five tanks I am getting 25 mpg.

I'm going to run up to KC here in a few weeks so I will see what I get then.

paxmanz06

Thanks. Luckily I wasn't on the highway and I was able to pull into a parking lot. I'll get it fixed in the next few days.

luke87gt

I get 25mpg in my z06 cruising at 80mph. Bone stock

inauguralfbody

I clocked 33mpg coming back from the flat lands of Illinois, running 60 to 65 mph. I started running 80mph on I44, and it started dropping, but got home and still saw 32.3 on the dash. (stock '98 mn6 coupe) I made a trip at night from Springfield MO, headlights up, and only clocked 25.6 mpg. I assume the added aero drag killed my mileage. I would think even with the super slick aerodynamics of the C5, that a headwind would still negatively affect mpg also.... just an unmentioned variable.

s'noJob

I doubt if headlights would kill over 20% of one's gas mileage. They'd have to change the window sticker to show an asterisk next to the suggested mileage that states it's daytime figures only.
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Lee DeRaud

Something in the 5% range seems a bit more reasonable.

inauguralfbody

I agree. However, I was driving into a stout head wind that night also. As in, headlights + head wind = a considerable drop in efficiency. And it may have been 26.5, I cant really say for sure. But it was a big difference.

Lee DeRaud

The headwind certainly matters a lot more.
The thing is, raising the headlights doesn't increase the frontal area. They increase turbulence over the hood slightly, but they're out toward the edges where the flow isn't all that smooth to begin with.

inauguralfbody

Right. My whole point was, with the same car, mpg can vary greatly, based on driving conditions. And any increase in turbulence is negative in terms of efficiency. Perhaps my wording was not how I had intended. cheers