OK, I know these are muscle cars and we can expect to use more fuel that the econoboxes, however I am currently at the annual PeachState Chevelle Club gathering and have been told that a 71 Velle with a 454 and a 700R4 gets in the 16 to 20 MPG range with 3.31 gears. I have a 69 SS with a 454, Holley Commander 950 TBI, a Performer series 2162 hydralic cam, headers, Pypes 2 1/2" exhaust with X pipe and a TH400. I am running stock 14 inch SS wheels with 235x70R14 tires. The speedo is calibrated and is confirmed to be accurate with a GPS. When carefully driving at 70 MPH at 3500 RPM I get 10.3 miles to the gallon. Is this typical and to be expected? Will overdrive make that big a difference with my setup or should I be looking closer at the timing etc? Any recommendations would be appreciated as I want to drive my Blue Beast as much as possible.

Engine Displacement - Obviously lower displacement is more efficient.
A/F Ratio for Cruising speeds - A bit lean is said to be the best 15.5? 16.0?
Cam Selection - Cam that is very efficient in the rpm you cruise at is best.
Ignition Timing - Proper advance under vaccum, no magic number, depends on engine.
Transmission Ratios - Ratios not only help cruising rpm stay low but also how easily the car can accelerate.

Those seem to be the most important things.
Of course my car still doesn't get good fuel economy so I'm no expert in the practical sense. Though my cam isn't very good for economy. Areodynamics is a factor too but really nothing simple can be done to solve that.
For your specific case, I would try and optamize A/F ratio during cruise, ignition timing under vac, and if you can afford it an overdrive transmission would help. Someone else will have to chime in on what they think ignition timing should be, I'm wondering the same thing about my own engine.

Thank you so much for the response. One advantage I do have is a fully elextronic distributor without a vacum advance which means I can program it wherever I need it to be. R%ight now the highest timing value in the cruising range is 44 degrees. Is it safe to try more advance?

With my old ProSystems HP1000, I could get 17-18 on the highway. 3.42 rear gears, TH400 with a GV overdrive. (yes, it helps significantly)

Keep in mind, that 16-20mpg range is on the highway, not overall. I spent a lot of time tuning in the cruise AFR on that carb. The SV1 I have now is already in the same AFR range if I take it easy.

Here's the single most important thing I've found - tuning in a nice lean IDLE returns the best fuel mileage, and in the case of a carb the slightest turn of the idle mixture screws can make a big difference. Every time you're NOT on the accelerator, you're in the idle circuit, so it has a big impact on overall cruise mileage.

Ditto on what Mark posted above. We spend most of our time on the idle and cruise sections to get the best economy. Adding the OD Trans+Lockup will pay a nice return aswell. 44 degrees is not too much timing during light load. I've been up to 48 without issue.

I'm all for both performance and economy. An efficient engine should be both. Where the problem lies is in where it is being asked to do so. An engine that is built to make good power in the upper rpms is not going to be efficient at cruising speeds.

Do some searching for swheaton's posts, he's got it nailed. I could repeat it, but he's said the same things over and over again already and I've found them to be true. Will take some trial and error to see what your engine likes, but general principles are the same.

Something I've been rolling about in my head for a while is the use of EGR. Not exactly a common idea, but if you'll follow along I'll explain. We are all proud of our engine displacements, right? As you drive down the road, even under light load it takes air and fuel to fill your cylinders. Bigger cylinders take more air and fuel. What if we were to partially fill it with an inert gas? It is not combustable, therefore does not need more air or fuel to compensate. Then it takes less air, free, and less fuel, expensive to make the car go down the road. GM used 2 types of EGR valves, one opened at low vacuum, the other high vacuum. I don't remember the specifics now on vacuum for the high rated one, but say you're cruising down the highway at 15 or better inches of vacuum, and it is open. Less than that, it closes, and you have full use of air and fuel in your cylinders just like now. Now this is all theory in my head for the time being, but I will be trying it eventually when I get some time unless I can find a good reason not to.

I'm running a 427 with a 1000 cfm carb, 260*@.050 cam, rectangular port heads, single plane intake and all the stuff that gets lousy mileage. It's backed with a TKO600 and 3.55 gears. I run down the road at 2300 rpm or so. I'm just barely above chugging doing so. At cruise my carb is dialed in to about 14.5-15.2 afr, and I run 50 degrees total timing. I average 12 mpg in regular driving. I'd like to see 20 out of the same setup, but a carb just can't compete with timed port injection for economy, since they have to deliver the most fuel when there is the least vacuum, and as more vacuum arises, more fuel is spent. Any injection system will cut fuel when vacuum is high and load is low, port set up properly will do it more effectively.

Devin

Are you guys kidding? he's running a 454 down the freeway at 3500, he's lucky to get 10.

don't bother fiddling with details of timing or carb, what you need is an OD tranny. A 200-4R would cut your RPM to like 2350, if everything else is right it might get you up to 14-15MPG. Take about 10 - 15% out of your rear gear and/or get some taller tires, get your 70MPH cruise RPM down to 2000 or so, then you'll pull some MPG. Lockup converters really help too. Once you get down to around 2000RPM cruise then you can fine tune the timing and carb.

And, with the low first gear of these trannys it doesn't really hurt your off the line performance around town.

lets see: 2000 / 3500 x 100% = 57% fix the RPM deal and I'll bet your mileage will improve by 57% immediately. Then you can buy/borrow an wideband O2 meter and dial in the carb. You can get close to 20 on the highway. It'll still be a little rough around town though.

Engine Displacement - Obviously lower displacement is more efficient.
Nope. Not true. Otherwise 5.7L Corvettes would not be pulling down high 20's in MPG. What IS more efficient is minimum air and thus minimum fuel. A big engine loafing along can pump less air than a small engine humming.
A/F Ratio for Cruising speeds - A bit lean is said to be the best 15.5? 16.0?
Yup only go leaner. Run that sucker out as lean as you can get it to fire. That will literally be the limit: how lean you can go before it misfires. With a decent BBC you can get into the mid-16's approaching 17:1. Of course you will need a good EFI intake with equal air distribution and multi-port injection. Shucks given that you might even be able to go leaner as I have noted stumbles there running a carb and RPM Air-Gap intake.

Cam Selection - Cam that is very efficient in the rpm you cruise at is best.
Ignition Timing - Proper advance under vaccum, no magic number, depends on engine.
Transmission Ratios - Ratios not only help cruising rpm stay low but also how easily the car can accelerate.

Those seem to be the most important things.
Of course my car still doesn't get good fuel economy so I'm no expert in the practical sense. Though my cam isn't very good for economy. Areodynamics is a factor too but really nothing simple can be done to solve that.
For your specific case, I would try and optamize A/F ratio during cruise, ignition timing under vac, and if you can afford it an overdrive transmission would help. Someone else will have to chime in on what they think ignition timing should be, I'm wondering the same thing about my own engine.
So I run a rather aggressive cam for my little 402 BBC. When I started out I figured on 11-12 MPG down the highway. With the carb tuned just this side of stumbling at cruise (a lot harder than it sounds) and the addition of an OD transmission I am now figuring 14-15 MPG highway. The OD transmission is huge here giving a good 2 MPG. I run 3.31 rear gears and 245/60-15 rear tires for a 70 MPH highway speed RPM of under 2000.

I do run full vacuum advance so cruise timing is in the 50-52* range.

With EFI I could probably get my highway MPG up into the claimed 16-20 range, after all only have to hit 16 to be in that range ;) If I follow a semi I could claim 20 :D

The OP really needs badly to get that cruise RPM down. Running down the road at 3500 RPM is just way too high. Next they should check that they have the ignition timing setup correctly. You need a lot more at cruise than you do WFO so a vacuum advance is a MUST.

Next the OP might need to research their EFI controller. If it is an older model and running a narrow band O2 for feedback it will be driving the AFR to the 14.7 point. This is richer than it needs to be for cruise.

Good information above, however I stand by the displacement statement. A big engine has to choke its self to run slow, a smaller engine doesn't (as much). Pumping loss is an issue with all gasoline engines, the larger the engine the more pumping loss. Breathing through a straw isn't fun or efficient. Not saying you can't get a large displacement engine to get good MPG just saying lower displacement does help.

My 8 injector EFI SBC 350 cruises at 16ish A/F 2000-2300rpm at 70-80mph. Ignition timing vac advance is +15 giving me 36-40 during cruise. I may change to +20 vac, was afraid would be too high but vrooom3440 may have convinced me.

I get only 14mpg doing 70-80mph freeway. Not happy with it to be honest.

Also higher compression ratio helps, as long as its not so high you're spending extra money on special fuel. I'm sure my 8.8CR doesn't help me. Newer engines are well above that, even running on 87 octane.

I went from a 350sbc with th400 using 3k stall. Went to a 6-speed manual with a 427sbc(aftermarket). Keeping in mind I had previously installed 410 rear gears. MPG in first combo was around 11, no matter city or highway. Highway was really annoying with such high rpm. Now getting around 16mpg city and mid to high 20s for highway. Reducing weight also helped.

Gear Vendor units will help with rpm, but leaves you stuck with usually a poor stall speed. But something newer offers rpm plus a locking converter. If you really want to go all out look into a 4L80E or some of the aftermarket transmissions.

you need a locking converter because the low cruise RPM will be below the stall speed. this results in lots of slippage and heat.

So with the talk about ignition timing... I recommend a read through the "ignition 101" thread. Good information that I gathered into one place that tells you why timing needs to be this here and that there. All of the Chevy engines are really quite similar in their top end requirements of 36-38* WFO and 50-52* cruise.

Good information above, however I stand by the displacement statement. A big engine has to choke its self to run slow, a smaller engine doesn't (as much). Pumping loss is an issue with all gasoline engines, the larger the engine the more pumping loss. Breathing through a straw isn't fun or efficient. Not saying you can't get a large displacement engine to get good MPG just saying lower displacement does help. ...
So it is not "choking" itself, it is merely reducing pressure in the intake which reduces cylinder fill and pressure. And the choking is not a function of size at all but more a function of power required versus power available.

Sure there are some efficiencies using smaller. But the fundamentals remain the same. It is still a matter of how much air you pump and that is proportional to RPM.

It is also true that it takes a certain amount of power to get the car down the road no matter what is under the hood. At a constant speed you will need the same constant power. And it will take about the same amount of air and fuel to generate it.

The magic equalizer between big and small is that the greater power of the bigger engine allows it to make that same cruise power at a lower RPM than the smaller engine thus pumping comparable amounts of air and fuel. And I suspect that internal engine losses are likely proportional to RPM as well, giving the bigger engine even more parity than commonly believed.

You have to stop and think when a car like a Corvette, which is definitely NOT designed/built as a economy car, is in the top 25% of ALL vehicles pulling down 28+ MPG (I am guessing at the percentage here... but not the MPG data). Corvette owners love to brag :beers:

OK, I know these are muscle cars and we can expect to use more fuel that the econoboxes, however I am currently at the annual PeachState Chevelle Club gathering and have been told that a 71 Velle with a 454 and a 700R4 gets in the 16 to 20 MPG range with 3.31 gears. I have a 69 SS with a 454, Holley Commander 950 TBI, a Performer series 2162 hydralic cam, headers, Pypes 2 1/2" exhaust with X pipe and a TH400. I am running stock 14 inch SS wheels with 235x70R14 tires. The speedo is calibrated and is confirmed to be accurate with a GPS. When carefully driving at 70 MPH at 3500 RPM I get 10.3 miles to the gallon. Is this typical and to be expected? Will overdrive make that big a difference with my setup or should I be looking closer at the timing etc? Any recommendations would be appreciated as I want to drive my Blue Beast as much as possible.

When the LS5 car was new it got around 16 with the 1:1 final drive of the Muncie, and 3.31 gears. That was bone stock. When the intake was replaced with a cast iron high rise, 650 Holley DP Spread Bore, headers, and LS6 cam it still got around 15 on the highway. I'm sure with OD that could be improved upon. That was back when there was good gas at the pumps though. Right now that same set up gets about 12-13 on the highway.

So it is not "choking" itself, it is merely reducing pressure in the intake which reduces cylinder fill and pressure. And the choking is not a function of size at all but more a function of power required versus power available.



It reduces the pressure in the intake by choking the engine, the engine can't draw in a natural full load of air so it creates a vaccum during the stroke down. Creating a vaccum takes energy and is wasteful. Larger engine the more vaccum because as you said takes the same amount of air and fuel no matter what the engine size. Diesel engines have no throttlebody and thus no pumping loss, and that is one reason why they are more efficient than gas engines. The leaner the better, means more air for the amount of fuel which means less vac and thus less pumping loss. In a perfect world we could just be like the diesels and have full load of air and just toss in what fuel we need but we all know it wouldn't burn right in real life. Anyways I take it the OP has already stuck to his 454 and won't be changing displacements, so I guess it doesn't really matter.

Just curious but what is your (vrooom3440) initial, total (in by ?? rpm), and vac advance. Thanks.

So whenever you look at timing on the low side you need to factor in what cam they are running. My setup is a HR296 with 236/242 @ 050 and .610/.635 lift.

I run initial in the 16-18* range. Any more and you can start getting kickback during cranking from firing off the mixture too soon.

I run about 11-12" of vacuum at an idle speed of around 900 RPM in neutral (MT). I have a vacuum advance that is all in at 9" vacuum connected up to manifold vacuum. It has been mechanically limited to start at 6" vacuum and provide 12* of advance. So I idle around 30* of advance. Way happier engine than it was with only 18* :yes: Since I know all the parameters of the setup, I adjust timing to 30* at idle with everything hooked up. Could not do that if I did not know all the specifics.

My advance is all in a bit lower than I might like, I think it may be as low as 2200 RPM. My total timing is set to 36-38*.

So at cruise I can see pretty close to 50* of advance between mechanical and vacuum.

BTW you can reduce that vacuum pretty well with a Hugh Jass cam ;) So I guess that must make my engine more efficient running down the road at 10" of vacuum plus or minus? :disco:

OOOOhhhh, that famous New York cam designer, Hugh Jass. ;) I've had a few of those.

Guys, I really appreciate all the comments and insight to my situation. I had thought about a Gear Vendors but as mentioned here the lack of a lock up torque convertor might cause problems. If I can figure out how to paste an image to this site I would show my timing table. I am running a Wide Band O2 sensor. The advantage of the Commander 950 system is that you can set the timing while cranking at a different value than when running. The MAP sensor provides input that substitutes for vacum advance and the timing is fully programmable. Right now I have the cruise timing at 3500 RPM set to 44* and the idle timing is set at 28 (base of 16* and 12* equivalent vacum) WOT is set at 36* I may have plenty of room to advance timing further but have been cautious to do so. Right now I have the O2 sensor goal set to 15.5 while cruising.

Guys, I really appreciate all the comments and feedback. I would like to go with a Gear Vendors at some point but as stated without a lockup torque convertor I might have issues. I am running a wideband O2 sensor. If I could figure out how to do it I would paste my full timing chart from the Commander 950 system on this thread. One advantage with this system is that the cranking timing can be set independently from the operating timing. I am running 36* at WOT and 44* at cruise. Idle is set at 28* (16* base with equivalenmt of 12* vacum). There is no vacum advance. The MAP sensor substitutes for vacum values and is used to adjust timing. I have my cruising A/F at 3500 RPM set to 15.5.

I have a 3000 stall converter on my setup, GV overdrive, and do most of my driving between 1500 and 2000 rpm's. All you need is a small tranny oil cooler, you'll be fine.

Some more info to add is that while cruising at 70 mph and doing 3500 RPM, I am pulling about 17" of vacum and the timing reads at 44 degrees. I have a vacum gauge mounted in the vehicle and can use the laptop to look at the timing while in motion. Since there is no vacum advance on the distributor, I am using the MAP sensor to send vacum info to the ECU. With this setup I have the ability to set the timing anywhere I want under different MAP readings vs RPM. I can also set the timing while cranking at a different value than when at idle, The timing while cranking is set at 10 degrees and at idle is set at 28 degrees (16 base line, 12 = vacum).

IMO you're wasting your time and effort fiddling with timing details on a 454 running at 3500. Get a 700 or 200 4R tranny w/lockup converter, get your cruise RPM down to 2000 or under, then start the fine tuning.

Of course it is educational.....

From an order of magnitude perspective Tom is entirely correct. For your situation you will see the biggest gains by getting the cruise RPMs down.

However optimizing the timing is really cheap. And it sounds like yours is a bit conservative. I would advance idle timing for example until vacuum stops increasing and maybe back off just a smidge from there. I would also put in some more timing at cruise based on your vacuum level. You may well find that vacuum increases there too as you get closer to optimum. But I would target about 50* for cruise timing.

These guys are correct. get the cruise rpm down to 2k or less. the only reason to run at a higher rpm is if you have alot of stall in the convertor or no lockup