I have a 400 ci with a mild RV cam, flat top pistons, Wieand Intake with a Holley 600 cfm, stock heads with new valve job.
Has anyone used a combination of jets that reduces gas consumption by going smaller on the primaries, and larger on the secondaries which is when I need it anyway. The 4 bbl actually improved my mpg by 1.5 over the old motorcraft 2bbl.
I have put a 45,000 volt coil and gapped my plugs at .60 for more spark.
SO far I am getting 8.5 mpg with a stock c6 and stock differentials on 33inch AT tires.
I am also running electric fans to save horses from the belt driven fan.
Any sugestions?

 

Check your stock differential ratio. Gap your plugs back to 0.045. Use an Edelbrock #1406 which is tuned for mileage out of the box with the jets set up from the factory like you talked about. Get a stock or closer to stock cam like the Comp cams 255DEH which will reduce gas wasting overlap. Unless you have good fan controls the electric fans are not saving you any gas. Check out an overdrive and there are also things you can do with the transmission situation.

 

what sort of gear ratio are you running? What sort of driving are you doing? City? HIghway?

Keep in mind that jetting it leaner to get more mileage may result in drivability issues such as pinging and lugging.

 

What are the transmission options for improving mileage?

 

well there are 2 ways to look at the transmition one is its a big heavy truck transmition and takes a lot to turn it. if i remember right it takes 58hp to run the primary pump on it. but u can help this by making the trans. more efficient there was a post on this a long time ago about rollorizing a c6 in the trans forum. the other way to look at it is it is non overdrive transmition u can swap a od trans in (lot of money lot of time) or u can put a us gear od unit on it (lot of money) ither of which will gain u gas milage

 

I don't think you remember right on the pump. The pump does nothing but provide pilot pressure to operate the controls. The pump is actually quite small as hydraulic pumps go and not very high pressure at all. Otherwise the recommendations on the tranny are correct.

 

i could be wrong that could be in all the trans takes that out i cant remember its been a long time and its really late

 

Well, thanks for the replies on my part... I've heard the C-6 was an energy hog but lots of dollars does not equate to a go do... guess that's why designs change over the years.

 

Be sure to check the color on your plugs. It sounds like we have similar builds except my heads are ported and I have a bit more cam. I currently am running the Edelbrock 1406 but swapped the metering rods to go the richest you can going by their chart and it still wanted more fuel. There were slight driveability issues like a slight stumble off idle and when the secondary were suddenly opened plugs came out white after a 650 mile trip to N.C. I then replaced the primary jet to the .101's which is not on their chart but being it is larger it will alow more fuel. All driveability issues went away, motor runs much smoother and I gained .5 mpg on the return trip 12.5 mpg. I spoke to Edelbrock and they confirmed my idea on needing a bigger carb (750) They also said the 1406 out of the box is basicly tuned for appx a 302 inch motor. Most say a 600 is plenty for a 400 but mine is showing me otherwise with all indications saying I am to lean. Edelbrocks generally do better than Holley's for MPG and IMO are much less tempermental but Holleys seem to give better seat of the pants power. Anyway just remember to read the plugs, generally the motor will tell you what it wants. I also agree on reducing your gap to .045-.050

 

The main difference besides the electronic controls between the older units and the new units is bearings. The older units use thrust washers, the new use tapered needle bearings. The older units use bushings to support shafts and the new units use bearings. Otherwise parts from some of the new trannys swap into the old units and some would even swap back but why...

Carbs are designed to add a specific amount of fuel to the air as it flows past which is independent of engine "size". Off idle stumble and other issues are handled by circuits that can be tuned for engine size. A larger carb will not respond as well due to the reduced vacuum signal any movement of the throttle produces. Carb size is a compromise tho.

 

"Carbs are designed to add a specific amount of fuel to the air as it flows past which is independent of engine "size". Off idle stumble and other issues are handled by circuits that can be tuned for engine size. A larger carb will not respond as well due to the reduced vacuum signal any movement of the throttle produces. Carb size is a compromise tho."

Actually carbs are designed to deliver fuel based on how many CFM of air that will flow thru them while still maintaining a good A/F ratio. No aftermarket carb will fit every application so by using different jets, metering rods, step up springs ect the carb can be adjusted to fit many applications within limits. In my case the above parts were tried and some driveability issues either cleared or just moved to a different point but I could not get rid of all of them. A mildly built 400 + inch motor should be able to flow more than enough CFM to maintain a decent vacuum signal with a 750. Even on Edelbrocks site they say the 1406 is good for smallblocks built to their performer level and some bigblocks. I would assume a mildly built 400 would fall on the higher edge meaning the 600 CFM carb may be able to be tuned to run properly but if you are beyond its limits the 750 would be in order. Every motor will be different and no one combo will work on every 400, my info was only given as real world experience not some generic calculation. I also have a 78 super cab with a 400 running the performer package. That setup runs perfectly with the 1411 750 CFM carb out of the box. This carb is designed for 402 inch and larger motors and may be a better choice for a built 400. I myself would rather run a carb even if it is slightly larger or richer than necessary considering the severe damage that can occur from running to lean. I prob should have run that carb on the Bronco because that motor has ALOT more done to it but I had the 1406 600 CFM on the shelf new and thought with proper tuning may be able to get by with it. It is close but I am now beyond the mechanical limits of the carb and have to step up to the 750.

 

Talk to the guys over in the carb forum for more info. The 400 the original poster has would not need a 750. A 600 CFM carb will run a mild 400 past 6000 RPM which is way higher than we normally operate a mild 400.

 

i have a 351m that gets 11 w/a 600 holley that runs just fine. the only thing i have 2 do is keep the carb clean. also does anyone know anything about the holley truck avenger? i had an edelbrock on my old rig which was a 78 f150 2wd w/a 302 and i
hated the edelbrock i found it 2 be a piece of junk.

 

Check with the carb forum for carb information.

 

Wow, I can't believe the mileage you guys are getting. It sounds really low to me. With a fairly mild build and running the stock carb (2150 withough altitude comp), my 77 F-150 gets around 12 miles per gallon. I have a few buddies that put 650 cfm carbs on, woke up the engine a lot, and are getting 15-18 on the hwy. But, as it has already been mentioned, all motors will be different and respond differently.

W.D.