Hi guys! I'm putting my brother's 1981 F100 step side back together. It has the 300i engine and C6 trans. He wants to ditch the single barrel carb and go to a four barrel. He wants to add a little more "pep" to the truck not burn the tires in one shot. He used one of those carburetor calculators and it recommendeds a 400 CFM carb. He's looking at Holley carbs and wants electric choke and vacuum secondaries. Holley has the 390, 450 marine, and 470. Will any of these work for him? Will he get his "pep" with these or should he go larger?

I will hear any and all recommendations!

(He drives like Miss Daisy most of the time and likes a bit of fuel economy)

 

Stick with the 390 if you can afford it. The smaller carb is better matched to the lower rpm capability of that engine.

Make sure he does something to get heat into the carb, or he will hate it in the winter(not sure where you live and if it gets cold). He will find aftermarket intakes and headers separate the carb/intake from the exhaust manifold, and that will make it more cold natured. I think clifford had a spacer that they ran coolant through that would warm the carb up.

 

Instead of going to a 4BBL swap to a 2BBL An Autolite/Motorcraft 2100/2015 from 302 will be perfect for what he wants. There are adapters available to do this.

 

I agree with Dave.

What the CFM calculators don't do is take into account the driving style of the owner. And as you pointed out, in this case he's very conservative, which translates to low RPM and that means low CFM through the engine. So you need to be very conservative with the carb choice. Otherwise you'll lose part-throttle response as well as economy.

 

I was going to take issue with this. However, on second thought it isn't a bad idea.

My issue was with how large the 2150 is in comparison to the primaries of the Holley since the 2150 is rated at 287 CFM and the Holley is rated at 390. And, if you assume the Holley's primaries are about half of that then the 2150 is much bigger. However, then I remembered that 1 & 2bbl carbs are rated at one vacuum level and 4bbls at another. The conversion factor is 1.414, which makes the 2150 a 203 CFM carb in 4bbl-speak. So, the 2150 is only a bit larger than the primaries of the Holley.

The upside to a 2150 is that it is a simple and reliable carb - in my mind moreso than the Holley. And, with the adaptor it is closer to a bolt-on than the Holley as you could retain the original intake manifold, including the heat that Dave spoke of. The down side is that it doesn't have the extra CFM of the Holley if that was ever desired.

 

The other advantage is the annular discharge boosters in the Autolite provide a better signal so minimal throttle application still gives a good fuel/booster signal something that has never been Holley's strong suit. And there are lots of 2100's to choose, from 190CFM all the way to 425CFM.

It is a 300 I6 a low revving engine that has marginal breathing in stock form 400CFM might actually be overkill on a stock 300 I6. Not sure you would ever use all 400CFM.

 

Good point about the Autolite's boosters, and I agree about the Holley's tip-in.

As for the # of 2100/2150's to choose from, that's an advantage in cost. They are, like Chicken Man, everywhere! Used on many, many different applications and under-appreciated so they are readily available. And I rarely, if ever, see 390 Holley's for sale used.

 

I don't like swapping stock 2bbl carbs where a 1bbl was. I have tried it. I always have issues with idle speed and mixture adjustments. If he were going to do that, then maybe a aftermarket 2bbl or better yet, a weber if you want to learn how to tune it.

It seems the aftermarket carbs have a wider range of adjustments, I guess to accommodate the many different engines they are installed on. Each time I swap a stock type carb, I run out of adjustment on the idle speed, I think the air bleeds are too large for the smaller engine. Or being it's emissions, the idle mixture screw adjustment it too limited.

 

Thanks guys!! This is great! He wasn't really prepared to pay $400+ for a four barrel Holley anyway.

So out of the 2100 or 2150? Which one should he go for? You (all) had good point for both. Now I need to narrow it down to one.

(I didn't mention this earlier, this is a freshly rebuild 0 mile stock engine.)

 

I think a lot of engine tuners always seem to suggest opening up the exhaust before going with a larger carb to get more power.

A lot of performance and mileage can be realized by re-curving the distributor ignition timing and installing lighter springs, probably more than bolting on a larger carb and intake.

 

Holley makes a 2300 that is a v2 and can replace the MC 2150 carbs.


The Holley comes in a 350 CFM and a 500 CFM, the 350 CFM would be good for the 300. I can tell you a 500 is too much for an AMC 258 six but is what we had on the shelf when we needed a carb.



Also know that with Holley's they do not use the same vacuum to test CFM for the v2 & v4 carbs so you cant say the 350 is too much carb going by the v4 charts.
I do not know if the same holds true with other carb Manf.?


If you ask me the 390 v4 is too big for a six but that is me.
If I do go for a different carb on my 300 six it would be a v2 most likely a Holley as I have all the needed tuning parts in my pit box from racing.


I know there is a Weber carb that others claim works good with a six (Jeep guys) but I have not had good luck with Weber carbs. That is what was on the AMC 258.
Dave ----

 

Those CFM charts are setup for flat out wide open throttle all-the-time drag racers, their engines will barely even idle.

Instead, plug in say 80% efficiency and your typical highway cruise RPM in high gear; (versus some insane number that you never, ever drive at for any length of time) and the CFM recommendation will closely match what Ford provided as OEM.

A bigger carb really only helps at the top end, and usually at the expense of idle quality, low end torque, and part-throttle driveability.

 

The 2100 was the pre emissions version, the 2150 is the emissions version of the carb. Similar to the 4300 and 4350.

You want a 1.08 bore 2100 for the 300 six. The 1.08 bore 2100 is 287 CFM
The original Carter YFA is 195 CFM so you are getting a 38% Increase in available CFM with the 1.08 Bore 2100.

In a stock 300 the 1.08 2100 is the best choice of the 2100's The biggest mistake people make on a stock 300's is over carbing them it leads to all kinds of tuning issues , can't adjust idle mixture, can't set idle, flat spots in acceleration etc etc.

The 1.08 will give you a good strong signal to the carb and have more than enough capacity for the restrictive stock intake and exhaust on the 300. Given the low volumetric efficiency of the 300 six and you are hard pressed to rev them much more than 4000 RPM the 1.08 Venturi 2100 is the ticket. The carb calculators peg the 300 six to 300 CFM at 4000 RPM, take in to accounts it's low volumetric efficiency and it drops to around 280.

When you over carb with 4BBL it is not really that noticeable (unless a double pumper) As you are running on the primary's most of the time and if over carbed you will never get full actuation of the secondary's any how.

On a 2bbl over carbing is much more noticeable and can lead to whole host of tuning/drivability issues.

With a 1.08 2100 on a 300 6 you will want to start with #46 jets and tune from there . I seem to recall that 45's were the sweet spot but it has been 25 years since I last did one of these swaps. Since yours is an older 300 it may like a bit more fuel.

 

Do you know the inches HG that Ford tested CFM on the 2100/2150 carbs?
I don't remember what Holley tested theirs at but I know it is different than the v4 carbs.


I cant see swapping to a larger carb with out changing the intake and exh manifolds to something that can flow a little better, more so the intake.
Dave ----

 

I mentioned that in post #5. It is explained here as: