Mach 1 with 460 takes too much gasoline!

[johan_89]

Hello!
My Mach 1 1972 with 460 takes way too much gasoline. When i checked the consumption it took 4 litres/10km in 50mph on careful country road driving. This is my engine specs:
Pistons:trw 2443
Intake manifold: offenhauser portosonic
Carb: Holley 850dp. Main nozzle 71 in front and 75 in back with 2,5 powervalves. Engine vac 4.0 in hg.
Heads: processed oem 460 heads with stainless valves 2.25-1.71.
Camshaft:crane hydraulic dur 248° 0,050 lift 0,599".
Whats the problem? It cant be normal, its so expensive to drive the car.

 

[Hemikiller]

I'm assuming this is your camshaft



If the car is set up to take advantage of the cam, I wouldn't expect it to be good on fuel at all. Converted to US units, that's 6 miles per gallon. A stock 429 or 460 with highway gears driven carefully will return 15 mpg.

You may be able to increase your total timing, or swap to a smaller carb, or change the intake to a Blue Thunder to gain back some low end to increase efficiency. Lots of ways to go about getting more out of it, but at what cost vs just enjoying the car as is?

 

[johan_89]

I was afraid to get that answer. I have to think about it. I drove 75km on country roads and it cost me 80$. But it is a wery funny car with a great sound

 

[will e]

I do a bit better than you with my built 351C. In America we are getting to 'enjoy' the high cost of gas as well.

 

[turtle5353]

With my built 408 Cleveland I was getting 6-8 miles per gallon. I did a cam swap and more tuning and have been able to get 12.
If you wanna play , you gotta pay.

 

[DAverkamp]

Hello!
My Mach 1 1972 with 460 takes way too much gasoline. When i checked the consumption it took 4 litres/10km in 50mph on careful country road driving. This is my engine specs:
Pistons:trw 2443
Intake manifold: offenhauser portosonic
Carb: Holley 850dp. Main nozzle 71 in front and 75 in back with 2,5 powervalves. Engine vac 4.0 in hg.
Heads: processed oem 460 heads with stainless valves 2.25-1.71.
Camshaft:crane hydraulic dur 248° 0,050 lift 0,599".
Whats the problem? It cant be normal, its so expensive to drive the car.

Well then there.. as we say over here mate. You can’t take it ($) with you when you go!

I love my 8.5 smiles/gal set up on my droptop. Its on the edge of being too hot to handle so I could enjoy the curves on my backroads and never complain about the cost of gas. Even if I would have known brandon was going to be the world’s leading indicator while being the worlds leading follower on blowing up FF costs, I wouldn’t have changed my mind towards my build. There is a thread on our site about going electric with about 25 replies so maybe you can get some pro tips from those folks.

 

[basstrix]

What rear end are you running and is it manual or automatic? If it's automatic, you would likely have a high stall converter with that cam.

I ran a 429 with similar specs...same carb and intake, cam was 237/240 @.050 and stock size valves on ported D0VE heads. With a 3.25:1 rear and 2800 RPM converter, I was getting 14-16 MPG. I leaned out the mains for cruise and used the power valves for wide open enrichment. If you go this route, you have always use good fuel and keep your ears open for detonation.

 

[Kilgon]

I have a 460 and I'm averaging around 7 mpg. I just changed over from a Holley 850 dp to a Holley Sniper and really didn't notice any change in mileage. Have a C6 with a 3.73 rear end which doesn't help. You might be able to tweak the timing and get a good tuner to adjust your carb but I wouldn't expect any big difference unless you start modifying the engine and changing things such a tranny and rear end. Depending on how much you drive the car, I guess the main question is will the saving in gas offset the cost to make the needed changes to increase mpg? Something to think about.

 

[73inNH]

While not a super-easy project, I'd bet fuel injection from a 90s 460 would double your mileage.

 

[adrose]

1973 351 Cleveland 2bbl, on non-ethanol gas, around 9mpg in Florida, $5.15/gal at Costco.

But hey,its a 73 red convertible so $20 for a couple weeks joy riding ...

 

[cjstaci]

I think that’s a lot of cam for street cruising, I wouldn’t expect much in regards to fuel economy. Idling around town it’s probably pushing more fuel out the exhaust then it’s burning.
My convertible has a 460 crank, oem cj heads, a lunati 233 intake duration at .050 and I get around 8-10 mpg depending on how I drive it.
In my younger days, I had a BOSS 429 with many nascar parts in the engine. 12.5:1 compression, 2.4 intake valves, a healthy Holman-Moody cam, and 3.91 gears, I got about 80 miles per tank of gas. It was a fun 80 miles!

 

[c9zx]

Without knowing what the static compression is (ie. head chamber volume, piston to deck clearance, and some other things) it is hard to say how to proceed. A shorter duration (and less overlap) cam and slightly more lobe separation would improve mileage, but the domed pistons may cause detonation (too much dynamic compression). Calibrating the carb will likely help some. Access to an AFR meter makes it easier to do but, you still must read the spark plugs and tune for smooth driving at idle, cruise, and wide open throttle. Good Luck. Chuck

 

[DAverkamp]

Day 2 of Memorial Day weekend and topped her off again. 68 miles / 8.5 gals at 5.24 = $41.92 & 8 smiles/gal but I had way more than that. To make it feel less painful and put on a positive patriot spin, invert the $ to 1492 when wind was actually the cutting edge of survival by attempting to take over societies.
Gunpowder (fossil fuel) took care of that strategy pretty quick like

 

[johan_89]

What rear end are you running and is it manual or automatic? If it's automatic, you would likely have a high stall converter with that cam.

I ran a 429 with similar specs...same carb and intake, cam was 237/240 @.050 and stock size valves on ported D0VE heads. With a 3.25:1 rear and 2800 RPM converter, I was getting 14-16 MPG. I leaned out the mains for cruise and used the power valves for wide open enrichment. If you go this route, you have always use good fuel and keep your ears open for detonation.

I'm not sure about the rear end but I think it is 2:73. I have a c6 with 10" converter with a little higher stall speed. I'm restoring my carb so maybe it will make it a little better too.

 

[Duncan Mach72]

Your vacuum seems very low?
Do you have a Lambda (Oxygen)sensor and Air-Fuel Ratio gauge on the car?
Driving at low speed you are not actually on the main jets, you are running on the idle mixture from the transition slots. Perhaps this is way too rich?
I have noticed on my 351C with a Proform-bodied 850 while tuning that it uses transition up to 60mph on a light load!
The original style Holley 850s have a very rich idle circuit. I swapped a primary metering block from a 750 vac sec on to mine & it was much easier to tune. I have some pieces of 26 gauge (measures 0.017") in the secondary idle jets as the secondary block is still stock 850.

 

[mrgmhale]

Even though I agree with any of the comments in this thread indicating what you are getting as becoming more the norm, there are still some thing you ought to be checking in an effort to get as much fuel mileage as you reasonably can. First, I look at the tire sidewall to get the maximum inflation pressure, and use that for all tires. The manufacturer recommended PSI is lower than the tire maximum as the interest is in having a smooth ride. The higher tire pressure will give you a noticeably harsher rise. But, it also reduced the rolling resistance of the tire, which in turn required the engine to work harder to move the vehicle - which in turns burns more fuel.

Next, using a vacuum tester check the distributor vacuum advance diaphragm. It is leaking or ruptured it will clobber your fuel mileage, and adversely impact your low end partial throttle response. I have some videos on vacuum systems that show how to test the vacuum advance diaphragm. While you are there anyway, test to ma certain Ported Vacuum is making it to the vacuum advance diaphragm Although testing the vacuum advance diaphragm is the central theme, these links do have a section showing how to test it (and testing to make certain Ported Vacuum id getting to the vacuum advance diaphragm.

https://studio.youtube.com/video/0yRh_m7TvxE/edit (05:13 mm:ss)

(08:03 mm:ss)

As for another area where fuel mileage can be improved, our right foot. The more you have to press the throttle the lower your fuel mileage, but not for just the reason(s) you are likely thinking On carburetors, any time to press on the accelerator you are causing the accelerator pump to inject a shot of gasoline into the carburetor venturis, whether is is really need it or not. If you can attain the target speed you want, then not force the car to maintain that speed by continually goosing the accelerator frequently, you will be burning fuel needlessly. I suggest establishing a longer following distance than you may normal us, then just keep up with the average speed of other cars using as little throttle position change as you can. Also, every time you press the brake pedal you are changing costly kinetic energy you have built up into heat energy though the brake pads/lining. Not only do i suggest extending your following distance to minimize the use of not only the changing of throttle opening, but the use of the brakes as well. When needing to decelerate for an upcoming stop light/sign, begin to coast down your speed sooner than normal, and minimize the use of your braking - especially with a traffic light as hitting the intersection just after the light turns green means less fuel used reaccelerating while rolling vs accelerating from a dead stop. And when accelerating, a longer, smoother rate of acceleration with minimal adjusting of the throttle opening once your target speed is attained, the better for fuel mileage.

So, how much do the preceding changes in driving style help? In our 1969 Shelby GT500 2with its 428 engine I am able to get 13 MPG as opposed to the 8 MPG (or less) when driving with my "happy feet." It makes enough of a difference to have caused me to change my driving stye all the time, as this works with any vehicle. But, there are other times when I ay the hell with the rate of fuel consumption Damn it I am going to enjoy my driving. And that, boy and girls, is where my MPG drops!

 

[giantpune]

Even though I agree with any of the comments in this thread indicating what you are getting as becoming more the norm, there are still some thing you ought to be checking in an effort to get as much fuel mileage as you reasonably can. First, I look at the tire sidewall to get the maximum inflation pressure, and use that for all tires. The manufacturer recommended PSI is lower than the tire maximum as the interest is in having a smooth ride. The higher tire pressure will give you a noticeably harsher rise. But, it also reduced the rolling resistance of the tire, which in turn required the engine to work harder to move the vehicle - which in turns burns more fuel.

Next, using a vacuum tester check the distributor vacuum advance diaphragm. It is leaking or ruptured it will clobber your fuel mileage, and adversely impact your low end partial throttle response. I have some videos on vacuum systems that show how to test the vacuum advance diaphragm. While you are there anyway, test to ma certain Ported Vacuum is making it to the vacuum advance diaphragm Although testing the vacuum advance diaphragm is the central theme, these links do have a section showing how to test it (and testing to make certain Ported Vacuum id getting to the vacuum advance diaphragm.

https://studio.youtube.com/video/0yRh_m7TvxE/edit (05:13 mm:ss)

(08:03 mm:ss)

As for another area where fuel mileage can be improved, our right foot. The more you have to press the throttle the lower your fuel mileage, but not for just the reason(s) you are likely thinking On carburetors, any time to press on the accelerator you are causing the accelerator pump to inject a shot of gasoline into the carburetor venturis, whether is is really need it or not. If you can attain the target speed you want, then not force the car to maintain that speed by continually goosing the accelerator frequently, you will be burning fuel needlessly. I suggest establishing a longer following distance than you may normal us, then just keep up with the average speed of other cars using as little throttle position change as you can. Also, every time you press the brake pedal you are changing costly kinetic energy you have built up into heat energy though the brake pads/lining. Not only do i suggest extending your following distance to minimize the use of not only the changing of throttle opening, but the use of the brakes as well. When needing to decelerate for an upcoming stop light/sign, begin to coast down your speed sooner than normal, and minimize the use of your braking - especially with a traffic light as hitting the intersection just after the light turns green means less fuel used reaccelerating while rolling vs accelerating from a dead stop. And when accelerating, a longer, smoother rate of acceleration with minimal adjusting of the throttle opening once your target speed is attained, the better for fuel mileage.

So, how much do the preceding changes in driving style help? In our 1969 Shelby GT500 2with its 428 engine I am able to get 13 MPG as opposed to the 8 MPG (or less) when driving with my "happy feet." It makes enough of a difference to have caused me to change my driving stye all the time, as this works with any vehicle. But, there are other times when I ay the hell with the rate of fuel consumption Damn it I am going to enjoy my driving. And that, boy and girls, is where my MPG drops!

To piggyback on this, you can use that same vacuum gauge to help learn to drive for more better fuel economy. Run the vacuum hose out from under the hood and into the cockpit so you can watch the vacuum as you drive. Watch as you step on the gas pedal, the vacuum drops down. Practice driving while keeping the vacuum gauge as high as you can.