2.8 390 cfm 4bl

cobrasplash · Nov 22, 2008

has anyone used the 390 cfm holley on the stock manifold with a 4bl addapter? the stock carb is broken, the smog stuff is long gone other than the air pump and still have stock ignition

Nipplechops · Nov 23, 2008

I pondered it when I built my last 2.8L, but decided against it. The stock manifold isn't the greatest design to begin with and adding a four barrel won't make it any better.

I would strongly recommend sourcing out an Offenhauser intake or going with a Holley 350 two barrel carb. Unless your engine is uncorked with a decent cam, headers and some head work, adding a 390 won't give you any benefit other than increased fuel consumption.

Even assuming 100% volumetric efficiency a 2.8L will only require 272 cfm of air at 5500rpm. Keep in mind that is very crude math, but still, it illustrates that the 390 is overkill in a stock (or even a moderately modified) application.

Hope this helps.

enginepaul · Nov 24, 2008

The stock ignition is a problem. In order for the TFI to work, you need all the stuff that goes with it. Do a DuraSpark.

And the carb info above is correct.

OldJunk · Nov 24, 2008

x2 stockish 2.8 will never be able to utilize all that cfm.

Matt Schall · Dec 16, 2008

you can use a 2bbl off a bigger motor,I have a 360 cfm motorcraft on mine.trick is to use the boosters from the smaller 2bbl.an engine is a big air pump,just because there is a 390 cfm carb on it dosent make it too big.the engine will only take what it needs ,you use the smaller boosters to speed the air velocity in order to properly atomize the fuel.Im running a comp cam ,ported intake and heads and aerostar headers,4.10 gears 4spd and 33s the trucks rips!.

skippy · Dec 17, 2008

Nipplechops said:
I pondered it when I built my last 2.8L, but decided against it. The stock manifold isn't the greatest design to begin with and adding a four barrel won't make it any better.

I would strongly recommend sourcing out an Offenhauser intake or going with a Holley 350 two barrel carb. Unless your engine is uncorked with a decent cam, headers and some head work, adding a 390 won't give you any benefit other than increased fuel consumption.

Even assuming 100% volumetric efficiency a 2.8L will only require 272 cfm of air at 5500rpm. Keep in mind that is very crude math, but still, it illustrates that the 390 is overkill in a stock (or even a moderately modified) application.

Hope this helps.

I talked with holley tech assistance today about fuel economy for my 350 cfm carb and the man told me it was wrong carb,get a offy and a 390.He said it gives better fuel economy due to smaller primary openings.Just keep your foot out of it.

Nipplechops · Dec 19, 2008

skippy said:
I talked with holley tech assistance today about fuel economy for my 350 cfm carb and the man told me it was wrong carb,get a offy and a 390.He said it gives better fuel economy due to smaller primary openings.Just keep your foot out of it.

True, the smaller primaries will help with fuel economy at low throttle positions, but that all goes to hell when you push the pedal to the floor. The 2.8L simply soes not have the displacement to take advantage of the 390cfm that the Holley offers. It may allow the secondaries to open at WFO, but the poor air flow through the carb won't allow decent atomization of the fuel, thus killing both economy and power output.

In the end you could get the same effect you are illustrating by simply using a smaller carb to begin with. The Offenhauser intake is a nice unit for sure, but unless you are building a 2.8L to scream (7500 RPM plus), you simply won't be taking advantage of the flow capacity that the Holley 390 offers. Hell, even at 7500RPM the 2.8L is still only chugging 371cfm of air. Again, that is crude math assuming 100% volumetric efficiency and doesn't take into account things like barometric pressure and such but it illustrates my point all the same.

A 2.8L that is built to utilize the Offy intake to its full potential wouldn't be suitable for use in a truck. Hell, in my area you could perform a 4.0L swap for the same price and have much better results.

skippy · Dec 19, 2008

Nipplechops said:
True, the smaller primaries will help with fuel economy at low throttle positions, but that all goes to hell when you push the pedal to the floor. The 2.8L simply soes not have the displacement to take advantage of the 390cfm that the Holley offers. It may allow the secondaries to open at WFO, but the poor air flow through the carb won't allow decent atomization of the fuel, thus killing both economy and power output.

In the end you could get the same effect you are illustrating by simply using a smaller carb to begin with. The Offenhauser intake is a nice unit for sure, but unless you are building a 2.8L to scream (7500 RPM plus), you simply won't be taking advantage of the flow capacity that the Holley 390 offers. Hell, even at 7500RPM the 2.8L is still only chugging 371cfm of air. Again, that is crude math assuming 100% volumetric efficiency and doesn't take into account things like barometric pressure and such but it illustrates my point all the same.

A 2.8L that is built to utilize the Offy intake to its full potential wouldn't be suitable for use in a truck. Hell, in my area you could perform a 4.0L swap for the same price and have much better results.

I agree with you on this,and when the tech rep was telling me about the Offy/390 swap,I was already thinking "good luck with that",cheaper to get a 2150 non feedback from napa,sell the Holley or keep it for a 302 swap.

speedysprocket · Dec 23, 2008

I've done the Offy intake with 390 Holley. Not as much power as I expected, maybe more tuning still needed. But I have been able to get as high as 20 mpg towing. That was with some water injection and my Mallory dual points. Next step is lower gears, maybe that will help the power issue.

small ranger · Dec 23, 2008

I have a 390cfm carb and an Offy intake on my rig. I thinks it is a little bit better then the 350 cfm and stock intake. I would only do the 390 cfm holley if you have a Offy 4 barrel intake. With that said, my truck has plenty of power to open the secondaries at least 3/4 of the way open. I get 18 mpg on the highway with 4.10, 33 tires and no overdrive(2800 rpm). I can also chirp the tires off the line with a welded rear end.

My advice. If you can get an Offy intake cheap and a holly 390 cfm carb cheap then do it, if not just run a 350 cfm carb. Both setups will kick the stock feedback TFI set up as$ all over the road/trail. Just remember the Dura spark ignition swap is the best upgrade for these old trucks.

gerko · Dec 30, 2008

390 4 barrel on original 2.8 will work
don't know about fuel but at 4300 rpm it's like a turbo boost you get with it

fastpakr · Apr 19, 2009

If anybody's looking for the Offy/390 combo, I've got a set for sale in the For Sale forum here. They're just taking up space in my garage at this point - I haven't owned a 2.8 in several years now. I'd love it if somebody else could get some use out of them.

Boletivore · Jun 14, 2009

You can tune the 4-barrel to open the secondaries very slow and very late, making the carb around 195cfm for most of your RPM range if you wanted.

The carb should be tuned to access the secondaries when needed and in the amount needed to keep the air velocity up.

They don't come that way, and often what seems cool to drive would give you a much slower quarter mile elapsed time with full throttle from a stop.

Tune, tune, tune.

Fastpakr, do you still have the carb and intake? How much including shipping to Portland, Oregon?

2.8 390 cfm 4bl

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