I am working on a stock 1968 318 in a dart. Its been a project for my son the last couple years and hoping to get finished before he graduates this spring
The original 2 barrel needs rebuilt and i think we will put about a 600 holley 4 barrel on it instead. I have not touched the one on my dart in almost 20 years.
I have an edelbrock intake for a spread bore but never had a spread bore. What is the benifit? Better fuel mileage? i know holley has both styles and i only want to stay with a holley because i can tune them allready.
Its not getting raced only street driven. But i am sure he would still like to have a little fun going through the 4 speed,i know i do.
Just need some carb advice. I have a fresh 750cfm 4160 holley. And also an old 570 cfm holley from a 68? 440 i rebuilt a few years back that i am not sure works right yet.
Of course i could look around for a different one if needed.

I suspect you'll get lots of varied opinions on this question. I can provide my own thoughts: the advantage of a spread bore is crisp throttle response when running on the primaries plus a bit better mileage. Sometimes it takes some tuning to get the secondary transition to come in smoothly.

I'm partial to the square bore 600 cfm Holley's myself, particularly for street driven cars with near stock engine. As you probably know they're pretty simple to work on and for myself, pretty idiot proof.

Having said that though, I would recommend you try one of the Holley spread bores. I've run them on a couple small block Chevies over the years and been very happy with the all around performance. I'm going through one right now for a 360 in a 74 Roadrunner, replacing a thermoquad that's beyond saving.

As an alternative, the Holley economaster carbs are nice running, economical carbs. I've had 2 of those over the years and just loved the crisp response. They're not as plentiful as the other spread bores in my experience, and a bit more difficult to work on.

Personally I think the 750 cfm you said you had on hand would be more than you need. Good luck!

I have to wonder if a stock 318 will handle a 4-bbl at all? At minimum, you'll need to dump the restrictive single exhaust or it'll choke the thing to death. Honestly, the 4-bbl is the final concern with a 318. You gotta be able to breathe deeply first.

If you have a 68 motor, don't expect the exhaust valve seal to last long on unleaded fuel either unless it's already been hardseated at some point. What's your budget? If it's low, any set of junkyard 360 heads, while not as good as Magnum heads will flow well on a 318 for cheap. Buy a Fel-Pro top end gasket set, add a double roller T-chain, a medium cam, and headers too. Then add a 600cfm spreabore carb for crisp response. Anything more is too much on that little thing. You'll eventuallly have cam bearing problems, but that's about as low budget as it gets to have gains from your efforts on a small engine. 318's do make significant gains from those little things tho.

I'd get the thin plate that covers the bowed out secondary flanges on the spreadbore intake (it's cheap) & run the old holley & see how it goes. I think the bowl vent was no longer vented out to the atmosphere due to '68 emission regs & instead used a nipple that ran with a hose to a charcoal canister which might help you ID the year just FYI. As said I think the 750 might be a bit too much even for a lead footed teen but if it's in good shape it would get him up & running if the other one needs rebuilt

SC, I call BS.

I am running a 570 Street Avenger on my '64Dog 318 and before that ran a R-1850 600cfm holley. Both were tunable and gave me much better throttle response and mileage than the stock 2-barrel.
A vacuum secondary 600 is a really nice fit.

On my next 318 I am going to use a '74 or so 360 intake, which is a spreadbore, with an Edelbrock Quadrajet. The reason for that is the superior metering of the Quadrajet over say a Holley squarebore. A Thermoquad would also be great but I have three Edelbrock Q-jets on the shelf and also a half-dozen real quadrajets with High Altitude jetting.

As far as exhaust goes I am running stock log exhaust into 2 1/2" pipes with Dynomax Hemi Super Turbos. Yes, headers would be better but the trnsmission crossmember of my '64 is directly in the path of the collector outlet.

So it can be done.

If the OP wants to stay in the Holley comfort zone the 80555 is a modern 650 spreadbore, the old standard 6210 with two metering blocks would be my next choice.

R.

600CFM is plenty for a 318, so I'd lean towards your small Holley unless you want to spend some money. Me, I'd prefer a Carter AVS or an AFB, or best would be a smaller TQ for what you're doing. I like the small primary deal on the street for street-RPM throttle response & MPG.

a squarebore 600cfm edelbrock on a LD4B intake rocks on a small port head 318.(thats what it is for)

a stock 4-v spredbore intake with a striped and tuned q-jet is hard to beat on a 318.

the holley street dominator is a spredbore/small port/low rise/open plenum that is for a small port 318 head.it works better than one would think it does as an open plenum on a stock engine.( I used a holley 650cfm sperdbore DP with mine on a stock 318.

I like the q-jet on a 318 for ease of dialing in the tune.

I have a holley street dominator intake that i found in the trunk of an old car over 20 years ago. I never hear anyone mention that intake. Its on a 360 right now.

I wouldnt hesitate for one second to use a quadrajet on a 318, one of the most underated carbureters around, hell Iv got one on my trucks 350 engine and wont change to a holley, no way, no sir.

When I was in high school back in the 80's, I put a 340 cam and a Quadrajet off a 350 Buick on my otherwise stock 318 Duster. I had no experienced gear heads around to help me tune the spread bore carb. From what I remember, I lost off-the-line torque when swapping from the tiny Carter 2bbl, but hang on when those big secondaries kick in! Love that ROAR! Tiny Carter 2bbl would roast one tire on the open rear, but not so much after the spread bore was added. This was probably a tuning issue that I didn't know how to correct, or possibly too much carb for the 318 at low rpms. Intake was a OEM cast iron unit from a '71 340.

SC, it was your first two sentences, the 600 holley made my 318 a much better engine from idle on up.

Now to carb sizing: A Q-jet or T-quad or AVS or, for that matter, AFB or vac secondary Holley size the carb for the engine. My 169 cubic inch Mercedes hemi 6 runs a German version of a Quadrajet. The air door cures all.

Now to the boggy 318 woes: They are so very easy to overcam! A 340 cam is about two sizes too big for a garden variety 318. A stock 360 cam will probably be all right, one step up is even better, like the 204/204 @50 lift 0.425 valve lift 110 LSA cam you used to be able to buy from Comp.

Now to finalize the head scene: Yes, 318 LA heads are woefully puny, good for max 270 hp stock. The latest LA 318 heads, like the 302s, are worth using especially if you can port them yourself. Use the thin composition head gaskets. But if you are starting out from scratch the Mag heads or their EQ counterparts are much more worthwhile. 360 heads flow well but their big chambers make milling necessary and that $200 or so could be better spent on some Mag heads.

I've been running my 318 on unleaded since 1984 when I bought the truck and the valve clearances haven't closed (mechanical cam) in around 70,000 miles.

To the OP: Get your engine going so you can drive it, THEN worry about additions. Otherwise, you'll get caught in the "as long as I've got it apart I might as well....." trap and $1000 later it still won't be running. I am living proof of that. Best to make a simple change and GET IT ON THE ROAD!

Happy New Year!
R.

I'm amazed when I read how "small" 318's are. There were the 302 Chevy and ford motors that were Dz an boss that spanked 340's in their day and they all ran big 4 barrels.... 327's are not that much "bigger",,,, Just sayin. Just cause it ain't a 340 don't mean it can't run. I ran a big cam x heads milled and a six pack with a 4 speed and took my share of 340 pride in my rear view mirror.

360 heads suck on a 318! any power gain from the bigger ports is lost due to what it does to the compression and crevice volume. The factory did it to reduce NOX emissions and as a result the 4bbl motors only make a few more hp than the 2bbls. If you are gonna do the 360 heads you need to do something drastic to increase compression. 300 hp is not hard to do with any 318 heads, the 920 is the best small port head for hp, the 302 is best for MPG.

Boss 302s and Z28s smoking 340s? Not in my neighborhood. The Swinger 340 with 4-speed and 3.91s was king of the smallblocks. When the 351Cs and 350 LT1s came out it was more even.

The 302s were built for one reason only, to homologate them for racing that had a 5 liter limit.

They were cranky to drive on the street, needed 4.11s to be much of anything.

I was in high school at the time, I remember.

R.

Yeah me too.