When I had my 352 rebuilt 5 years ago we put the Edelbrock Performer manifold and 1406 carb, with Hooker Supercomp headers on it. I have always disliked the performance of the truck. I could only get about 10 mpg out of it. Changed the 3.25 out for a 2.75, picked up mpg a little but when pulling a loaded 18 ft car hauler I felt it dogged it on take off. I changed out to a 3.00 and like the performance better. We started rejetting the carb. and got 13.9 mpg on one trip with the trailer.

We changed the carb off and installed a 600cfm Holley for a 390GT, man what a kick ice carb, could not believe the difference, even picked up the mpg a little, could get around 14 mixed driving but slower.

Yesterday we put a 390 cfm Holley 4 barrel that was used on a Pinto. It seems to perform about like the 1406 did, but no where like the 600 cfm GT.

Gonna fill it up and start testing for mpg today, if this carb works like I think it might, we could get past the 15 mpg mark.

John

 

Why did you take the Holley 600 off? The 390 is no where near enough air for that motor. I am running the Holley 600cfm 4bbl carb and it runs like a scalded ape. Unbelivable power.

 

I don't understand no where near enough air. Can you better define that?
I will agree, with the 390GT carb my truck ran strong.

How much out of a day of driving are you at "runs like a scalded ape"?

John

 

Well, if your priority is mileage, then the 390cfm carb should do a good job of that. You will lose power though, since 390 cfm's is not enough airflow for this motor. However, keep in mind that some factory 2 barrels were rated less than this, so you should be fine.

Read the plugs, jet accordingly, for best mileage.

 

Thanks Rusty, Better gas milage with gas over $2.00 per gallon, is the prezackly the reason for the smaller carb. I have put 150 or so miles on the truck and hope to fill it up later this morning for the first milage check.

I find that the smaller carb seems to perform about as well as the Edelbrock 1406 carb that I ran for about 5 years.

The biggest challenge for me so far is driving at 55 mph. I like 70-75 much better.

I have an absolute rev limit of 4500 rpms for my engine. With the fast shifting C6, it usually never gets close to that. When I was running a 2.75 rear gear I had used all of the numbers on the speedo at 3800. I just had it up around 105 mph on the first run after installing the new carb. but I didn't think to check that against the tach. I also feel that the smaller carb pulled much better between 85 & 100 mph, than the 1406 did. I used to think the engine just ran out of breath (like it's tongue was hanging out) past 85 mph with the 1406. It seemed to take way to long to go between 85 & 100.

My currently disassembled red 66 that had a 390 2 barrel COM would pull strong even at 100 mph. It always seemed to want more. I have always been disappointed with that aspect in this truck. The 390GT carb that we just took off probably would have done that. That is some carb.

Enough of my rambling.

John

 

Sounds good. Be sure to let us know how your mileage is. You may be on to something here.

Also, with your C6, you can adjust it. A lot of it has to do with how much force there is on the kickdown rod. The more force, the longer it holds off has been my experience. I've got mine to shift at 5000rpm, by setting the kickdown rod correctly.

 

Finally took the truck out for a refill. Ran a total of 169 miles of mixed driving, many stops & starts, soft peddeled it on take off, didn't let it over 2200 (about 60). Put in 12.0 gallons of 87. If my calc. is correct looks like 14.08 mpgs. I was hopeing for 15.

Rusty the C6 is shifting like it is supposed to, no problems. I have a self imposed 4500 max rev limit on my truck.

John

 

For a stock or close to stock motor, 4500 is about where it runs out of wind anyway, so that's probably a good number to limit it to. Remember, RPM = Ruins People's Motors.

Of course, there's still some things you can do for mileage. Run 10w30 instead of something heavier, make sure the brakes aren't dragging, inflate the tires to 35psi, advance the timing a little, etc. But I'm sure you know that.

How do your plugs read? Maybe you want to jet it down a bit?

 

ok now put the trailer on and try passing somebody at 60 mph accelerating to 70+ the stock 2bl on a 390 flows close to 500 cfm and they flatten out at about 75-80 if it works
you may be onto something I tried a 1400 edlebrock on my 390 wich is the 500cfm carb and it ran like poop put stock 600 holley on and it's a 500% improvement in all areas
but my truck has a np 435 tranny 4.10 gears and 35 inch tires so maybe its like comparing apples and oranges

 

PP, Hooking up the big trailer will tell the tale for sure. I do think it will handle it much better than you suggest. All I can compare it to is the 600 cfm 1406 that it replaced. I weighed one load with a 4X4 on the trailer 10,520 pounds total. I had no problems running 80-85 with the Edelbrock.

I had another 66 with a 390 two barrel and I couldn't get over how strong that truck pulled even over 100 mph. It was like it always wanted more.

We had the truck out yesterday tweeking on the carb, trying to get it better adapted to the big engine. We unhooked the secondarys and you could tell that it was a tiny carb, it really needs them to work well.

My truck is setup to be a crusior with grunt, I don't rev it, or race it, even though I still pull a lot of other trucks from the lights, speshly after it makes about 2600 rpms in second. When we were still trying to figure what was up (shop didn't put hardened seats in heads on rebuild and I had 0 compression on one cylinder), I ran a test in second gear. I ran it up to 4000 and it was running 85 mph. Thing is even with 7 cylinders it was as happy as a fat baby with a big nipple in it's mouth.

John

 

Thats all i do with my truck. My car is main transportation. I have found a holley puts a edelbrock to shame as for performance. But I notice no signs of faliing off at 85mph with the 1406. Stock 2bbls are 351 and 356cfm at 3 inch of merc so 390cfm at 1.5 inches is enough carb.

Pulling 5000rpm in gear is alot. I don't have my tach anymore but without the kickdown i have let my truck go 52mph in first and 82mph in 2nd with 3.50's and i have a 390.

 

From the 2 setups ive had the Edelbrock 600 cooked the Holley 600.

1974 F250 360 C6 3.50 35"

W/Edelbrock 600 boosted my mpg from 7->15
When pulling 7 tons of bobcat i had plenty of power to spare.

W/Holley 600 boosted my mpg from 7->12
While pulling 7 tons of bobcat it was a dog at 2700 RPM. My dad loves the holleys and he was disipointed with it.

Plus the Edelbrocks looks hella better then the holleys.
My .02

 

i get 8mpg driving slow and most of my driving falls between 4 to 6 miles per gallon but if you see how i drive it makes sense. I 'm telling my 670 street avnger with the light secondary spring proforms. And btw my holley is just as shiny! in fact it doesn't rust like the edlbrock where the name plate is attached. I will have to run some tests sometime but i am 475 miles away from my truck these days.

 

If the Holley was a dog it was set up wrong. I don't doubt the Edelbrock got better mileage, it's not really a preformance (any metering rod carb is an economycarb) carb campared to a Holley.

 

Both carbs where setup the exact same as the other. Same spacer, fuel line, type of fuel, ect. The only difference where the 2 different carbs. And i didnt adjust the carbs right away...they came pre-adjusted from the factory. It the only way my dad will buy them. After 2 weeks of testing both carbs my dad tuned them to see what they can do, and the Edelbrock came out on top. When he tuned it to max hp it spun all 4's on my truck. Right now the Edelbrock is on my truck and he stuck the holley on his 3500 and is trying to find out what exactly is wrong with it. Its a dog at 2900 RPM on his truck.