Gas mileage

[Max Power 74]

Hooking up vacuum advance to manifold is defeating the purpose of vacuum advance. It gives you full advance at idle, you might as well just do that centrifugally. Also, if you have a big cam and an automatic, it pulls out timing when you put the car in gear, and that is the opposite of what you want.

[kyle7713 10]

Hooking up vacuum advance to manifold is defeating the purpose of vacuum advance. It gives you full advance at idle, you might as well just do that centrifugally. Also, if you have a big cam and an automatic, it pulls out timing when you put the car in gear, and that is the opposite of what you want.

 

i know its not how you normally should hook it up but for my case it works and works well

[danno 128]

Danno, if you have a 302 and can't get 20 mpg on the freeway, my guess is there is something wrong with it. What carb do you have, and is it running too rich ? What kind of ignition do you have ? Do you have a vac advance ? hooked up to what ? What axle gears do you have ? Do the brakes drag ? Is there fuel leaking anywhere ? An AOD with a 302 ought to get 20+ with zero effort.......what heads did you start with, the 351W heads might be a step backwards, depending on what you had to start with......LSG

 

I have the factory Autolite carb, 2bbl, I think it is a 2100. Rebuilt 6 years ago. I have Ford Electronic ignition, standard Ford disti. I have the standard rear gears for a 302, maybe 2.78? I had the standard 302 heads, and came across some 351 heads, so I put them in. I noticed little difference in mpg. I also installed headers for the same reason, thinking they also might help mpg, but again, no effect. I have lifted the wheels to see if the brakes drag, and that is not the case. The 351 heads were completely reworked, so they are close to perfect. The engine was reworked 20,000 miles back, and is tight.

 

The rear end has not been touched in 44 years, I am wondering if it is creating friction? I guess I could lift one side, put the tranny in N, and see if I can get the lifted wheel to turn and how easy it does? I have done a rolling test, where I compare how far the Mustang rolls down a hill compared to my other cars, and it is just about the same, if not better.

 

How do I tell if it is running rich, and what is done about it? From what I remember, I turn the idle adjustment screws all the way in, and I cannot get it to stall the motor. So maybe there is some leak in the idle adjusters, but that should have little effect at 60mph?

 

You mentioned the 351 heads as being too large, I have thought of that. But how could larger valves and easier flow of air hurt mpg?

 

Any comments are welcome.

[69_Mustang 26]

Hey 69Mustang, I believe everything but no traffic in SoCal :)

 

Bob

 

I'm self employed, so I choose to drive when there is less traffic, stay near home, or work from home. I don't have the typical commutes most people put up with, haha.

[bnickel 10,004]

with the old 351w 4v I had in the stang I was able to get 26mpg on the highway from Lubbock to Austin and 24 on the return trip and this was with 275/50-15 tires out back 10.5-11" wide tread and right at 26" tall on an 8" rim. the motor was fresh and had stock replacement flat tops, .030 over bore, edelbrock performer, carter 600 AFB, Mallory unilite conversion in the stock dizzy, stock replacement cam from Sealed Power with like .440 lift and 270 adv. duration that was advanced 4 degrees. stock exhaust manifolds into 2-1/4" pipes with turbo muffs. I also used one of the old Purolator Pro-Fuel adjustable fuel pressure regulators adjusted down to around 2.5-3 psi for economy. initial timing was set at around 10-12 degrees and the vacuum advance was hooked up to ported vacuum. this was with 3.25 gears in the 9" rear and the speedo was accurate within about 2 mph at 60.

 

the key here was the mild cam advanced 4 degrees, the high compression which should have been close to the factory 10.7:1 rating, the AFB and the low fuel pressure along with the rear gears. my best friend also made the same trip in his mid 90's dodge Dakota 4x4 with the v6 and he kept having to stop for gas and I didn't have to stop nearly as much. in fact I have made this exact same trip from Lubbock to Austin and/or back at least 150-200 times over the last 25 or so years and that was the only trip where I only had to stop for gas once and that was only because I wasn't sure if 1/4 tank would make it all the way from Brady to Austin and it was the middle of the night on Sunday so which meant that most of the gas stations in the next 2 or 3 tiny towns would be closed.

 

the current engine with about 9:1 compression, a big lumpy speed-pro cam, hedman shorty headers with 2.5 exhaust, 625 road demon carb and the same 3.25 gears but with 245/40-17's that measure about 24" tall gets about 6 mpg in town and maybe 10 on the highway.

 

my next engine that I build is going in the cougar and will have Ford Racing GT-40x "turbo swirl" aluminum heads, around 10:1 compression, a mild roller cam and 1.7 Cobra roller rockers, fully curved distributor with an NOS Stinger ignition system with adjustable vacuum advance and an NOS Accel Super Stock coil that looks almost exactly like a stock yellow top coil, functional ram air and an NOS electronic adjustable Holley carb kit on either a 470 or 600 cfm Autolite/Holley carb or a 1.19 venture Autolite 4100 along with my vintage Hone-O-Drive unit and either 3.25 or 3.70 gears with 215/65-15 tires on 15x7 wheels. I'm expecting that it should get between 24-28 MPG on the highway and around 16-20 in town depending on what gears I end up using. the 3.70's would actually be better around town for short trips and the advantage of the Hone-O-Drive is that I can leave it engaged in Overdrive all the time if I want and it will overdrive all 3 gears not just 1:1 3rd which is pretty darn cool.

[Max Power 74]

I am interested in why you think lowering the fuel pressure helps fuel mileage.

[bnickel 10,004]

I am interested in why you think lowering the fuel pressure helps fuel mileage.

 

 

I don't that it was a major help or if it even helped at all, I'm just saying that's the way the car was set up at that time. the carb should only use as much as gas as the engine is pulling through it. I can only say that it did seem to help some.

[sleek98 12]

I get about 18.5 on the highway. Stock 69 302, carb and C4. The carb was rebuilt a couple times but the rest is how it came from the factory.

[LSG 15]

Max, you're misunderstanding how vac advance works. You can't replace it with centrifugal and, the way it works when connected to manifold vac is exactly what your engine wants.

Yes, fuel pressure can have an effect as well, but it isn't huge.

 

Danno, the reason the 351W heads may not help is because the might have larger chambers, which would lower compression a bit, and more compression is better for fuel economy. You mentioned that the engine has been rebuilt- do you know what pistons were used ? Some of the replacement pistons have a shorter pin height and thus lower the compression- which hurts power and economy. Your engine may also have been built with the wrong timing chain set. Back in the early 70s the OEMs were trying to reduce HC emissions at idle, so they made new timing sets that had the camshaft retarded 4-6 degrees, which burned lots of extra fuel and cost power and economy. They also had ported vac advance, which is also a mistake to use. manifold just plain works better.

 

You can look at the sparkplugs and see if the mixture is too rich or not. LSG

[kyle7713 10]

Max, you're misunderstanding how vac advance works. You can't replace it with centrifugal and, the way it works when connected to manifold vac is exactly what your engine wants.

Yes, fuel pressure can have an effect as well, but it isn't huge.

 

Danno, the reason the 351W heads may not help is because the might have larger chambers, which would lower compression a bit, and more compression is better for fuel economy. You mentioned that the engine has been rebuilt- do you know what pistons were used ? Some of the replacement pistons have a shorter pin height and thus lower the compression- which hurts power and economy. Your engine may also have been built with the wrong timing chain set. Back in the early 70s the OEMs were trying to reduce HC emissions at idle, so they made new timing sets that had the camshaft retarded 4-6 degrees, which burned lots of extra fuel and cost power and economy. They also had ported vac advance, which is also a mistake to use. manifold just plain works better.

 

You can look at the sparkplugs and see if the mixture is too rich or not. LSG

 

 

 

Great info!

 

I heard about the timing chains being off off a little after i built my engine, especially true with the double roller chains. Might be able to free up some horses and not hurt my fuel economy.

 

Plug reading is a HUGE HELP, that what i used to turning my car.

 

Here is the site i used http://www.4secondsflat.com/Spark_plug_reading.html

[danno 128]

Danno, the reason the 351W heads may not help is because the might have larger chambers, which would lower compression a bit, and more compression is better for fuel economy. You mentioned that the engine has been rebuilt- do you know what pistons were used ? Some of the replacement pistons have a shorter pin height and thus lower the compression- which hurts power and economy. Your engine may also have been built with the wrong timing chain set. Back in the early 70s the OEMs were trying to reduce HC emissions at idle, so they made new timing sets that had the camshaft retarded 4-6 degrees, which burned lots of extra fuel and cost power and economy. They also had ported vac advance, which is also a mistake to use. manifold just plain works better.

 

You can look at the sparkplugs and see if the mixture is too rich or not. LSG

 

It sounds like I need a good method to determine my compression ratio. I am not aware of what pistons were used by the shop, and at the time I was not aware of the proper questions to ask. I replaced the timing chain with last year when I had to do the oil pump, and I think I got it from one of the big Mustang parts catalog suppliers. I also need to review the difference between ported and manifold vacuum advance. Mine now is connected under the carb in the carb spacer. Isn't that manifold?

 

You have made great suggestions. A good compression test is definately next.

[Max Power 74]

Max, you're misunderstanding how vac advance works. You can't replace it with centrifugal and, the way it works when connected to manifold vac is exactly what your engine wants.

Yes, fuel pressure can have an effect as well, but it isn't huge.

 

 

Manifold vacuum vs ported vacuum is full on at idle. You can easily get to that with base timing settings and centrifugal. There isn't much point to it.

 

And your bowl of fuel if full or it isn't. Fuel pressure in carb applications should have zero effect on fuel economy.

[LSG 15]

Max, I'm thinking you don't understand how vacuum advance works. Yes, centrifugal can be set up to give full advance like vac at idle, but then how does the centrifugal relax the advance when you start moving, like the vac advance can and does ? You really do need both to work well. LSG

[danno 128]

There have been some discussions about the camshaft advance due to timing chains set differently. How would I go about checking this out? Maybe put my #1 cylinder top center for firing, take off the valve cover, and watch the valves to see when they start to open? Would that work?

 

Also, can someone tell me the difference between manifold and ported vacuum?

[LSG 15]

Dan, I'm thinking that looking at when the valves open won't be enough. My guess is that you'll have to pull the timing cover off and see what the chain and gears look like. Most timing sets only have one way to install. Some have three sets of slots, so that you can install the cam straight up, 4 degrees advanced, or 4 degrees retarded. Some sets have nine key ways. And then, to complicate matters further, there were some single key timing sets produced with a 4~6 degree retard- they were intended for emission regulations cars and trucks in the early seventies.

 

Manifold vacuum is exactly what it sounds like, full manifold applied to the vacuum can all of the time. Ported vacuum shuts off the vacuum to the can at idle. This was an emission era scheme to reduce unburned hydrocarbons at idle. Then there is a further complication, some ( many ) cars had a DVCV, a distributor vacuum control valve, that shuts the vacuum off to the advance until the engine begins to overheat. It really kills fuel economy, and you don't want it.

 

 

LSG

[danno 128]

Ahh, ok. My vacuum goes to the port under the carb, a slightly different location than were the power brakes hose is connected (on the back of the intake manifold). I have the temperature controlled vacuum valve (DVCV) disconnected. Does it make any difference which place the vacuum advance is connected to?

 

I will check on my timing chain. I might find the receipt from when I bought it and see who made it. Then contact them to see if it has different selections. From what I remember, there was only one way to connect it. If I did purchase the Ford Original ( there is a good chance I did) then there is a chance it is retarded? Can I see this on the cam and crank shafts? Is there a marking on them for position?

[Max Power 74]

Max, I'm thinking you don't understand how vacuum advance works. Yes, centrifugal can be set up to give full advance like vac at idle, but then how does the centrifugal relax the advance when you start moving, like the vac advance can and does ? You really do need both to work well. LSG

 

I am not suggesting running full centrifugal at idle.

 

I know exactly how vacuum advance works, both ported and manifold. Sure, the timing backs out a bit when applying load with manifold sources, but there was a reason the factory uses ported in just about every application. It usually relates to smooth transitions between idle and putting the car into gear or accelerating from a stop. The difference is essentially with a ported source you do not get the drop off in advance with a ported source like you do with a manifold source.

 

You can do a complete re-curve to run manifold instead of ported, but it just doesn't make sense to me. Plus if you are running a big cam with an automatic, the last thing you want to have happen is timing backing out when you put the car in gear. It will stumble and stall.

 

If you have managed to get the best possible results by backing out timing as you are applying load, more power to you. Manifold vacuum pulls timing out when applying load, ported does the opposite. You are stating that the "exactly what the engine wants" is the former. I disagree. So do most the manufacturers.

[LSG 15]

Max, don't think that i'm picking on you, cause I don't mean it that way, but you still don't understand how vac advance works.

 

Yes, manifold vacuum pulls timing out as you increase load. Ported does the same, not the opposite as you thought, but with one main difference. Ported vac is off at idle, Yes, lots of manufacturers made lots of cars that way. But it wasn't done to make tip in performance better, or because it worked well. It was done ( the ported thing ) to shut off vac advance at idle so that the engine would idle warmer, much warmer. This was done as a means to reduce unburned hydrocarbon emissions at idle. It did that. It also made cars idle much too warm and hurt fuel economy and driveability. The factories knew this and weren't happy about it, but the emissions rules from the FEDs trumped every other concern.

 

And, no, the manufacturers don't don't disagree with me, they disagree with you. Every car now produced has manifold vacuum advance. Yes, ALL of them. Look at where the MAP, the Manifold Absolute Presssure sensor is, you'll find it reading full manifold vacuum. The computer that now runs the timing in newer autos is reading the pressure in the intake itself, not someplace before the throttle blades.

 

If one wants the best economy and power available, manifold vacuum, properly set up, is how you get it. LSG

[Max Power 74]

Max, don't think that i'm picking on you, cause I don't mean it that way, but you still don't understand how vac advance works.

 

Yes, manifold vacuum pulls timing out as you increase load. Ported does the same, not the opposite as you thought, but with one main difference.

 

So, you are telling me with ported, once you crack the throttle and apply vacuum, that is is actually pulling timing out. Wow, did not know that.

 

Thanks.

[stierg 10]

At 90 on the highway use to 14-15 mpg (2500 rpm) . In town was around 9-10 in short terms

 

393w stroker

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