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Would a Carb Change Be Worth It

PanzerschreckLeopard

PanzerschreckLeopard

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Hello. I was wondering if a carb change on a 318 engine, with nothing else done (or at least not removing engine), would be worth doing, or would just seem like a waste of time. Purely street driving. A previous owner has already had a dual exhaust put in , but it still has the old BBD. I kinda like the idea of a spread bore 4v, but the TQs and QJs seem a bit....large...and not sure what I'd do about kickdown either. Should I just leave it as it is?
 
The good thing about vacuum secondary spread bores, is the primaries are small, which helps keep good MPG if you stay out of it, but the secondaries are big and due to the vacuum control will only dump as much fuel as there is air to match.

There are many cases where adding a 4 v actually increased MPG on a 2 v engine.

An Edelbrock performer 318 intake can be found used at swap meets for well under $100.

Same goes for their square bore AFB/AVS 500-625 carbs.
Those carbs are super easy to rebuild.

Spread bore and vacuum secondary carbs are a bit more technically complex, and the cheaper they are, the more likely they are to have issues, IMO.
 
Kick-down can be dealt with a few ways.

The easiest is to add a length of all-thread to the end.

Also a small length of bar stock can extend the throttle cable/linkage.
 
Why don't you go with a Holley 500 cfm 2 barrel carb. All you need is the carb abapter plate to mount on the stock two barrel intake and just lengthening the auto trans linkage by turning the clevis counter clock wise a few turns and that's it. You don't even need to lengthen the accelerator cable at all.
I've been running that setup in my 68 318 Coronet since 1984. Car runs great and is very responsive.
 
PanzerschreckLeopard said:
Hello. I was wondering if a carb change on a 318 engine, with nothing else done (or at least not removing engine), would be worth doing, or would just seem like a waste of time. Purely street driving. A previous owner has already had a dual exhaust put in , but it still has the old BBD. I kinda like the idea of a spread bore 4v, but the TQs and QJs seem a bit....large...and not sure what I'd do about kickdown either. Should I just leave it as it is?
Click to expand...

What is your ultimate goal, better drivability, better mileage, both? Budget? Do you want to keep it close to stock? I assume since you posted QuadraJet, maybe not. You might consider an EFI such as Holley's Sniper. Several members here have used them with happy results.
 
I'd say fairly low budget. It would likely be a few months off, right now I'm just speculating over options. That Sniper EFI is a bit more expense than I'd like to put in right now. I do not intend to go deeper than the intake manifold, MAYBE the heads. As noted before, a previous owner has put in a dual exhaust, though still with manifolds. I'd say better drivability, but improved mileage (or at least "not worse") would be a nice bonus. I can understand if both aren't possible, but that's why I wondered about spread bores.
 
If thinking heads, go the extra and do a cam. That way you can take full advantage of a good 4 barrel. Doesn't have to anything radical. Remember, it's a big air pump, and all of the pieces should work together to give the optimum performance.
 
Edelbrock Performer 318 intake + Edelbrock Performer 600 cfm carb OR Holley 4150 Street Avenger 570 cfm. Both carbs are tuneable.
 
Okay, that seems a good setup. I don't really plan on removing the engine for performance reasons unless I'm swapping it, and I doubt that will happen. (Even IF that happens, it's be a decade or more off!) So yeah, that seems just right, thanks. Any other recommendations or tips?

Edit: the manifolds listed on edelbrock site say "idle to 5500rpm" for the spread bore, but all the square bores say "1500" for the minimum. What effect might this have on street driving? Also, what about Holley 4175, in regard to spread bore?
 
Square bore carbs fit on the Performer intake, its drilled for both.
I think you'd prefer Idle to 5500 for drivability with your engine.
All spreadbores have big secondaries. I think a decent sized squarebore carb will provide more even perofrmance.
The TQ, QJ, and 4175 are all good carbs, and they have been used on lower performance engines for years. Your preference.
 
Right, thanks for clearing that up. I'll prob do the 4175 or 4160/5150. What's the actual venturi size ratio or cfm ratio between the primaries and secondaries on those spreadbores? All I can find is total cfm.
 
Don't know, you can inquire to Holley tech support, they're friendly.
 
As written in the MP books I think, approximately, the primary side flows in the low 200’s to 250 cfm depending on the exact model.
The secondary side flow in the 550 cfm range.

The small primary TQ (stock and electric choke versions) is about an actual 740 cfm carb. The larger TQ picks up approximately another 50 cfm for about a 790 cfm rating.
The gains are in the primary venturi as it becomes straighter and larger.

Carter also had two racing TQ’s rated at an actual 815 cfm for the small one and 1,000 cfm for the large one.
Gains are in the primaries and lack of OE choke parts over the secondary side.

There is a member here that rebuilds and restores them with excellence. Scott Harms.

TQ’s are very forgiving and flexible and will fit most any engine size. But they do need to be set up well before installing one. That’s where they get cranky. Real cranky.


The QJ is also a very good spreadbore carb. I do not like it as much as the TQ and is low on my list to use. (Personal preference)
The Holley 4175 I have used before and will not use again. But it is a Holley so turning parts are easy to get and tuning is relatively easy chore.
But I will not be using one again.
 
Last edited:
Having done what your doing, a Carter/Edelbrock 600 (An AVS is preferred) or the 1850/4160 performed excellent.

Almost any dual plane intake will work excellent. Factory iron, Performer, Action Plus, LD4B ... etc...

For a kick down, on stock carbs, not an aftermarket carb, I used a polar cable set up. But it also required a simple adapter I made from sheet metal available at Lowe’s, Home DePot, Ace Hardware. I’ll post a picture later of this simple cut out.
 
From FABO, a very old Duster project I was fooling around with. The carb is an old AVS that I used on a 318. Pictures in red of course, is the adapter plate I made to connect the Lokar cables, throttle and trans kick down.

It has similar diamentions to the typical carb throttle cable bracket you would use from/for a Edelbrock/Holley. I basically turned the bracket from a 3D part to a 2D part.

https://www.forabodiesonly.com/mopar/attachments/100_1312-jpg.6146/
 
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