1973 Upgrade intake and carburetor 351C 2V

Cougar Community Classic Cougar Maintenance Restoration and Repair
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Greetings, All. New here so forgive my ignorance.

I have a '73 XR7 Convertible with 351C 2V (code H) that I need to replace the carburetor on. No interest in racing, just want a nice driver. Car is completely stock except for mildly open exhaust by previous owner. Figured as long as I’m doing the carb, might as well do the intake to get a 4 barrel on there. I’ve pretty much narrowed the manifold down to an Edelbrock 2750 but am wondering about the carb. The more I research, the more confused I get. If you use the CFM formula @ 85% you get about 475, or a 500 CFM unit, however everybody seems to want 600.

I’m also concerned about clearance under the hood. Has anyone used this manifold on a 71-73 car, and with what carb?

Finally, I notice warnings for Summit M08600VS about using with AOD (“Severe Damage”), which I’d also like to do someday. Not sold on that particular carburetor but wanted to know if the same problem would be common to other units. Thanks in advance for any help.

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Most of the performance lost in '73 comes from reduced compression and changes to the cam shaft. I don’t think an intake and carb is really going to do much for you. The Ford 2100 carb is an excellent performer, I know it is not a sexy as a new intake and 4 barrel carb but it will be a lot easier to do and to maintain.

The big worry with an AOD is that you must have linkage to the transmission to correctly operate it. This will be true regardless of carb choice. My experience is that a lower rear axle ratio and an AOD will make a ton of seat of the pants difference in daily driving and still let you cruise comfortably on the freeway.

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Just to echo what xr7g428 said, and add a bit:

The “severe damage” business has nothing to do with the carb. It’s really just a warning to AOD users - if you don’t properly set up your TV cable, it’ll burn up the transmission. One of several reasons I’m not fond of that particular design. You can make a Summit carb work with one though, same as any other carburetor.

The other part of your question was much more interesting.

The 2V manifolds are pretty garbage for fuel/air distribution, so even if you just want better gas mileage, going with a mild 4V intake is a great idea.

You’re correct about your CFM calculations as well. Having an “oversized” carburetor makes it more challenging to draw fuel, because the bigger throttle openings mean slower airflow past the venturi. With a typical Holley style carb, the boosters use a very basic and cheap design (straight leg) that does not atomize fuel well at low flow either. It needs more velocity to smash up the dribbles of gasoline, and help turn them into small droplets.

Liquid gasoline does not burn. Only vapor burns. If the droplets are small enough though, the turbulence of being shoved into the combustion chamber, the latent heat of the chamber itself, and the piston smashing the mix together does a pretty good job of vaporizing small droplets. Bigger droplets tend to become carbon deposits, or get blown out the exhaust.

As a result, most “oversized” carburetors can perform fine at WOT on the big end, but have problems with low throttle, low RPMs, and during cruise. Throttle response and economy suffer greatly. There are exceptions, but not at an inexpensive price point.

The Summit carbs have a huge advantage though: Annular boosters. They allow for excellent atomization and fuel delivery even at low flow.

Most people would say that 600CFM is perfect for a typical mild 351, and frankly, even a Holley can work pretty well on one. But the Summit M2008VS would be superior in every way. Easier to tune, better economy, better throttle response, and better top end, compared to a cheap Holley (or Edelbrock) carburetor.

Their fit and finish is not as good as an Autolite 4100, but the Summit carbs are still an excellent design, and as far as bang-for-the-buck, they are very hard to beat.

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Edelbrock 1403 carb

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Thanks, Gentlemen, for the comments.

XR7g428, interesting thoughts on just changing the gears. I was thinking new carb as am on 2nd Ford 2100 in 2 years. Bought a reman from Summit a year ago and it just started (like 1st one) leaking gas onto the intake manifold. Makes life exciting!

Grim, appreciate your insight. I was initially wondering if everybody wants a 600 CFM because it’s more efficient at “cruise” (not working as hard as a 500 at same RPM) or it just sounds sexier? Your point about the better airflow & vaporization at lower loads makes perfect sense. I know what too rich of a mixture does; the aftercooler on my boat went bad last summer and you should have seen the cloud of black smoke! . And I’m not driving this beast to get good gas mileage but certainly wouldn’t object. I was also looking at the Edelbrock AVS2. Has the annular boosters but they’re a little pricey compared to the Summit brand. Although what’s a $100 in the grand scheme of things?

LSR_Mike suggests the 1403, which description states you can use with an AOD if you get the LOKAR bracket. So does anybody know if the same kit would work with an AVS2 or the M2008VS?

Obviously I have some decisions to make.

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Having tried a lot of carbs, I have some strong preferences. If you can spend the money, some of the high end Holley type carbs have nice air bleeds and a lot of tuneability.

To put this in perspective a bit, the Dominator carb (at I think 750CFM originally?) was designed for the Boss 302. And you can even make them streetable. But that’s not a cheap carb or intake!

For street, the old Autolite 4100s and Summit carbs are mighty hard to beat. I’ve tried AVS/2 as well, and while it’s better than the usual Carter knockoffs, they’re not as easy to tune as the M2008VS is by a longshot, as well as being more expensive.

If you already have an AOD, well, good on you; overdrive and some higher numeric rear gears are always fun! But the AOD has a lot of shortcomings. Its two piece input shaft will either bind or shear on full throttle 2-3 and 3-4 shifts. The stock shift quality is as as weak and slow as your average politician, which doesn’t endear this transmission to me either. All of its shortcomings can be overcome with the application of very large amounts of money, but often eliminates the torque converter lockup feature.

If you want overdrive, and don’t mind changing your rear gears, IMHO the 4R70W is a much better choice. It has lower 1st and 2nd gears, vastly improved reliability and shift quality, and you can easily tune how it shifts.

Most of my friends with AODs spent over 2500 on the transmission alone.

My 4R70W cost me less than $500 total if I recall correctly (including some mods to make it shift harder and handle 400+ horsepower). Even if you add the cost of a transmission controller (It requires one), that’s only another $400-600. Since I needed a computer for my 8 stack EFI, I just went to the Terminator X Max, which has a transmission controller included anyway.

In the end, spending less and getting more is a big win in my book. The two transmissions are closely related, but the newer 4R70W has a lot of advantages, including lower total cost of ownership. Get one from a '98-01 V6 Mustang for cheap, and it will be in great shape even if the car was high mileage. My own showed virtually no wear at all, and had never been opened.

I do feel I would be remiss in adding that the 2100 is NOT a bad carb, though. You may have had some leaks, but this is likely a result of ethanol in the gas you’re running, and outdated internals that weren’t intended for it. It tends to eat the accelerator pump and power valves. If you overtighten the covers to “stop the leak”, that creates more problems and makes them leak more. The 2100 is actually a fantastic carb. The 4100, its big brother (and the Summit carb) are superior in terms of peak performance, and even economy if you can keep your foot out of it, but the manifold itself is going to be a significant improvement in power as well.

With modern fuel, all carbs tend to have more problems than EFI, especially if you drive them and then leave them to sit for a week or more. The fuel will separate as the alcohol absorbs moisture from the air, and then chemical interactions between the water, fuel, additives, and carb metals will create crystals and corrosion.

Lastly, do not worry about whether these carbs are “compatible” with AOD, even if you go that route. It is 100% a nonissue. If you have any mechanical ability whatsoever, you can figure out how to hook up a TV cable, and create a way to connect that cable to the throttle to ensure that it works properly. It is not very complicated. =)

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The Summit is a fantastic driver carb. I have used several over the years and have been very happy. They have annular boosters like the Motorcraft 2100 you currently have, so it’ll retain the responsiveness you’re used to. Ran one on my 71 Mach, currently have one on a 71 Country Squire.

Edelbrock 2750 is only 1/8" higher than the stock, and the carb will sit be a bit lower since you’ll be eliminating the EGR spacer. I recommend a 1/2" phenolic spacer to help insulate the carburetor from manifold heat with today’s gas.

Lokar makes a kickdown cable for the AOD that is “corrected” for the various carburetor types. I have one installed on a 71 XR7 with an Edelbrock carb and it works well.

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The 2100 is an excellent carb. When they leak onto the manifold it is usually the accelerator pump diaphragm. I consider them a maintenance item. When I notice a strong gas smell after putting it in the garage, that is the first place I look. I end up changing it every 2 or 3 years. The other thing to keep an eye on is the Nitrophyl float. I replaced that in the mid 90s. I would keep one on hand while they are still available.

Your easiest answer is likely the diaphragm, and if you want more performance, put on a 72 or earlier timing set. In 73 Ford retarded the cam a couple degrees and did it with the sprocket.

Mine still has all the original stuff. I call it my decompressed Cleveland. When I need to replace the original water pump, I’ll revert to the older timing sprockets too. I’ve been told that will really wake that engine up. If I still want more, then I’ll worry about going to a 4V carb. If I really want more, someone (I don’t remember who, but that’s what Google is for) makes aluminum Aussie heads for a reasonable sum.

I have a 69 with a 351W M-code. That has the original 480 CFM carb. Like you, I’m interested in cruising, driving, not racing. However, it is very satisfying when the secondaries open. Everybody gripes about the bog when you go WOT with the 4300 carbs. If your over 2000 RPM, I don’t get much of a bog, and I adjusted the kickdown so the downshift happens during the bog. Easier on the trans.

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Well, thanks to everybody again for lots of interesting thoughts. Suspect I’ll be going with the 2750 manifold and probably the Summit carburetor. I may also have to rethink the tranny swap, although that’s a ways down the road anyway. More of a “might be nice” than any kind of priority. Really just want to get the car running decently before I worry about anything else.

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Exactly. Get a known good starting point before major modifications. That makes debugging much easier.

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