Specs:
'81 F100
'89 Roller, NON-HO 302
Performer 289 intake
stock cam
stock heads
stock truck manifolds with 2.25" true duals and thrush welded mufflers, no cats
3.25 gears
30x9.50 tires
c6 trans

Looking at swapping the current 600cfm edelbrock out to a new carb. I know the 600 is too big (can completely cut off one of hte idle mixture screws and it wont kill the motor). I'm looking for low end torque (yeah i know its a 302), and fuel economy. The motor doesnt spin past 5200rpm EVER, trans won't let it and nothing in my combo makes power any higher in the RPM range. So me question is I have 3 carbs to choose from and need some help picking one.

Holley 4160 (390cfm) - Annular boosters, should help with fuel economy and throttle response

Holley Truck Avenger (570cfm) - not much smaller then my 600cfm ede, but holley's carb selector says this one will work. Will i see any fuel economy improvements?

Edelbrock 1403 (500cfm) - little smaller then my current, but says calibrated for power where as my 600 says calibrated for fuel economy.

Currently, with the timing tweaks and hard tonneau cover and a light foot i've managed to maintain 12-12.5mpg AVG combined which is up from the 10mpg i was avging. I would like to see 15mpg avg with this truck. Any advice / opinions would be greatly appreciated.


Thanks,
Rob

 

Closing the idle screws has nothing whatsoever to do with carb sizing. It takes very little fuel to run an engine at idle. The 390 will give you the best throttle response for what you're looking for. You may need to rejet it as it's a wee bit small for a 302. The 570 is an excellant choice, I ran one on an Explorer 5.0 in an 89 Ranger, which had probly 50-75 more horses to feed than your motor. Ran the motor like it still had EFI.

 

A 600cfm carb is not too big for a 302. Vacuum secondary carbs only let as much air through as the engine can use. Keep the carb you've got.

Do you have the vacuum advance connected to the ported or manifold vacuum side of the carb? With no cats there's no reason to use ported vacuum - keeping the timing retarded at idle raises the exhaust temperature for emissions purposes. Set your base timing to factory specs and run the vacuum can off manifold vacuum. You'll probably have to back out the idle speed screw a bit (more timing = more rpm at idle). You will feel an improvement down low and the engine will run cooler.

Recurving the distributor should help as well. Scrap the factory exhaust manifolds and get some long tube headers with an h-pipe downstream, too.

 

A few years ago I had a 260-4V that I initially put a big block Autolite 4V on. I had a really poor throttle response off idle, so I replaced it with a tiny 370cfm (sic) Holley 4V carb. This carb gave a tremendous improvement in off idle power, and I didn't notice any loss of power at higher speed.

I checked one of those carb size calculators, and a "stock" 302 engine at 5200 rpm, volumetric efficiency on the high side (.85) requires 386 cfm.

So if you are going to buy a new carb, I suggest the 390 cfm.

 

an Edelbrock 500cfm would do the trick IMP

 

I have always been dissapointed to find the lack of annular discharge carburators avalable. Barry Grant Demon was better about this but did he go out of business? I would hate to settle for less than four annular discharge boosters. I would also hate to run more carburator than made the engine run good for its current application and setup. Nascars run restrictor plates reducing the total air flow capacity of the carburator. I like the Holley 390 CFM idea. David Vizard had said in a book, How to Build Horsepower, that 1.8-2 X the cubic inches = CFM rating of carburator. That would be 544 CFM. You could go larger with a vacume secondary assuming you restricted the secondary opening with a stiffer spring.

CID X (.85) peak horsepower RPM
____________________________ = CFM
3456
The (.85) is a variable. It could be higher or lower depending on the engine.

I have stood by for a long time that I would want an Edlebrock 500 or Holley 570 Street Avenger for a 302. But even those two are not being fully used on a stock 302. That is cylinder heads, camshaft, compression ratio and headers all optimized and matched. I like the Holley 390 on a stock 302.

 

Apparently you've never used a 570 on a 302 then. I ran one on an Explorer 5.0 long block. Perfect carb for that motor. Ran just like it still had EFI. That said, the CFM ratings are only a guideline to picking a carb, what the carb actually flows in service and the cfm rating are two different things. So you're not wasting anything. You're not using the secondaries 99% of the time anyway, so why choose a 4 bbl ? Using your logic, the secondaries are just wasted capacity.

 

I understand tuning the secondaries with a stiffer spring. But wouldn't a smaller set of primarys have better throttle responce?

Correct I have never put a Holley 570 on a 302.