- Joined
- Apr 24, 2020
- Messages
- 1,229
- Reaction score
- 1,290
- Location
- Pittsford, NY
- My Car
- My all time favorite vehicle is our 1969 Shelby GT500
This has been a fun thread to read. Now that several folks have had their fun, I have some fairly serious info to share (any surprises there?). We have a few fairly thirsty pony cars. The firs tis our venerable 1969 Shelby GT500 with its very powerful 428CJ engine. It is in pure stock form, to include the A.I.R. injection system with a still functioning A.I.R. pump. Usually, when driving in a sane manner, I can get 8 MPG. If I am cruising at 60 - 65 on an open highway, with no stops other than to refuel, I can get up to 13 MPG. But, if I begin to have some fun with Wide Open Throttle runs here and there, the fuel gauge and tachometer race each other to see which one's needle can reach its opposing extreme position first. I have no doubt that once that begins to happen I am running fractional Miles Per Gallon, and a more practical way of measuring fuel consumption would be Gallons Per Mile. An aside, Ford rated the 69 428CJ at 335 HP, but at only 5,400 RPM or so. When it is run to its "real" maximum HP rating it reportedly puts out closer to 415 HP - a fairly big number back in the day.
As if that is not "bad" enough, we also have a 2020 Shelby GT500. Its SuperCharged 5.2 liter engine is a lot smaller than the 1969 428CJ. But, between the computer controlled fuel and ignition, and the SuperCharger, that engine puts out up to 760 HP. So, more power is good news. The other good news is the ignition and fuel systems are both computer controlled, so there are some inherent efficiencies that engine has. With me driving in a same, calm way I usually get about 13 MPG. I am certain if I drive even more civilly I could get 15 or more MPG. But, when I am driving in a spirited manner I can count on 8 MPG. However, like the 69 428, if I begin to mash on the throttle I have little doubt I am achieving a 1 MPG rate of consumption. I dare not run it at wide open throttle long enough to get an accurate mesaurement however, as by the time the on-board computer calculated the MPG at WOT I will have hit Warp Speed, and may never come back to reality.
So, the two GT500s represent one extreme of both performance and fuel consumption under varying driving conditions. We also have two 1973 Mustangs. The 73 Mach 1 has a transplanted 1994 351W (not C) that replaced the original 302 2v engine. It is built for moderate street/strip performance, and sports (amongst other things) a Holley 735CFM Vacuum secondary carburetor. Many folks would think, and perhaps even say, the 735 CFM carb is too much for the 351W. But, the Vacuum Secondary controlled throttle blades make it a perfect carb for street use. I know when I open it up at road speeds of under 50 MPH, and the tranny in 2nd gear at Wide Open Throttle (WOT) I can both hear and feel when the secondary throttle blades are beginning to open, and given how much harder the engine is pulling it is obvious the secondaries are coming open at an appropriate time as the added volume of air/fuel coming into the intake manifold is not overwhelming the engine when it is at WOT and the engine RPM is climbing to and past 5,000 RPM. Normally, with tame driving the 73 Mach 1 runs at about 15 MPG. When it is opened up it sucks fuel very quickly. Likely not as fast as the 428 CJ, or even the supercharged 5.2L engines. But, it is a fast enough level of consumption to cause me to feel I ought not be behaving like that too often.
Then we have our docile 1973 Mustang Convertible with its 302 2v, C4, engine/tranny quipped vehicle. With its 2.79:1 rear axle ratio gear set there is not much in this particular Mustang that roars "high performance." But, it is an otherwise good performing machine, no complaints. I have not yet tried to measure its fuel economy, but I am guessing, based on prior Mustangs I have owned, it is getting an average of 17 MPG, and can be persuaded to tun at highway speeds at 19 - 20 MPG if I am very judicious in how I am driving.
All that said, the ONLY reason I am even the slightest bit concerned about fuel economy is for driving range considerations. This is not an issue for the 2020 GT500, as it is designed to tun with ethanol mixed gasoline. But, for the vintage pony cars I use only non-ethanol gasoline, despite its low 90 octane rating. Not every community has a non-ethanol fuel pump available. So, I have to plan my trips carefully to make certain I can continue to avoid using ethanol gasoline. In a pinch I know I can use ethanol gasoline, but I prefer to not do so with the older Mustags and Shelby - just because.
There are two considerations with all 4 pony cars that can help me gain higher MPG performance, and one more thing I can do with the vintage pony cars to help ensure the best possible fuel economy. For all 4 pony cars I can keep the tire inflation at the highest rated PSI level. I know a lot of folks will say it is not good to run tires at their maximum rated PSI level, but I do it to reduce rolling resistance, which increases MPG. And after all these years I have not had tires wear out their center tread from running at 32 PSI (or so for the older pony cars).
And, for the older pony cars it is good to be running the ignition timing as far advanced as possible without causing pinging. With me using 90 octane non-ethanol gasoline that means I am running the engines at the factory 6 degrees BTDC. If I had decent octane levels available I could move that to 12 degrees BTDC and have far more efficient and powerful engine performance. Along with the initial ignition timing being set to a level that allows good performance without significant pinging it is important to check the Vacuum Advance Diaphragm on the distributor to make certain it is not leaking or ruptured, and to make certain Ported Vacuum is provided off idle. Having a non-functional Vacuum Advance Diaphragm will cause the engine to run warmer than needed/normal, and will reduce both engine performance and fuel economy.
As if that is not "bad" enough, we also have a 2020 Shelby GT500. Its SuperCharged 5.2 liter engine is a lot smaller than the 1969 428CJ. But, between the computer controlled fuel and ignition, and the SuperCharger, that engine puts out up to 760 HP. So, more power is good news. The other good news is the ignition and fuel systems are both computer controlled, so there are some inherent efficiencies that engine has. With me driving in a same, calm way I usually get about 13 MPG. I am certain if I drive even more civilly I could get 15 or more MPG. But, when I am driving in a spirited manner I can count on 8 MPG. However, like the 69 428, if I begin to mash on the throttle I have little doubt I am achieving a 1 MPG rate of consumption. I dare not run it at wide open throttle long enough to get an accurate mesaurement however, as by the time the on-board computer calculated the MPG at WOT I will have hit Warp Speed, and may never come back to reality.
So, the two GT500s represent one extreme of both performance and fuel consumption under varying driving conditions. We also have two 1973 Mustangs. The 73 Mach 1 has a transplanted 1994 351W (not C) that replaced the original 302 2v engine. It is built for moderate street/strip performance, and sports (amongst other things) a Holley 735CFM Vacuum secondary carburetor. Many folks would think, and perhaps even say, the 735 CFM carb is too much for the 351W. But, the Vacuum Secondary controlled throttle blades make it a perfect carb for street use. I know when I open it up at road speeds of under 50 MPH, and the tranny in 2nd gear at Wide Open Throttle (WOT) I can both hear and feel when the secondary throttle blades are beginning to open, and given how much harder the engine is pulling it is obvious the secondaries are coming open at an appropriate time as the added volume of air/fuel coming into the intake manifold is not overwhelming the engine when it is at WOT and the engine RPM is climbing to and past 5,000 RPM. Normally, with tame driving the 73 Mach 1 runs at about 15 MPG. When it is opened up it sucks fuel very quickly. Likely not as fast as the 428 CJ, or even the supercharged 5.2L engines. But, it is a fast enough level of consumption to cause me to feel I ought not be behaving like that too often.
Then we have our docile 1973 Mustang Convertible with its 302 2v, C4, engine/tranny quipped vehicle. With its 2.79:1 rear axle ratio gear set there is not much in this particular Mustang that roars "high performance." But, it is an otherwise good performing machine, no complaints. I have not yet tried to measure its fuel economy, but I am guessing, based on prior Mustangs I have owned, it is getting an average of 17 MPG, and can be persuaded to tun at highway speeds at 19 - 20 MPG if I am very judicious in how I am driving.
All that said, the ONLY reason I am even the slightest bit concerned about fuel economy is for driving range considerations. This is not an issue for the 2020 GT500, as it is designed to tun with ethanol mixed gasoline. But, for the vintage pony cars I use only non-ethanol gasoline, despite its low 90 octane rating. Not every community has a non-ethanol fuel pump available. So, I have to plan my trips carefully to make certain I can continue to avoid using ethanol gasoline. In a pinch I know I can use ethanol gasoline, but I prefer to not do so with the older Mustags and Shelby - just because.
There are two considerations with all 4 pony cars that can help me gain higher MPG performance, and one more thing I can do with the vintage pony cars to help ensure the best possible fuel economy. For all 4 pony cars I can keep the tire inflation at the highest rated PSI level. I know a lot of folks will say it is not good to run tires at their maximum rated PSI level, but I do it to reduce rolling resistance, which increases MPG. And after all these years I have not had tires wear out their center tread from running at 32 PSI (or so for the older pony cars).
And, for the older pony cars it is good to be running the ignition timing as far advanced as possible without causing pinging. With me using 90 octane non-ethanol gasoline that means I am running the engines at the factory 6 degrees BTDC. If I had decent octane levels available I could move that to 12 degrees BTDC and have far more efficient and powerful engine performance. Along with the initial ignition timing being set to a level that allows good performance without significant pinging it is important to check the Vacuum Advance Diaphragm on the distributor to make certain it is not leaking or ruptured, and to make certain Ported Vacuum is provided off idle. Having a non-functional Vacuum Advance Diaphragm will cause the engine to run warmer than needed/normal, and will reduce both engine performance and fuel economy.
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