Re: square bore or spread bore carb for stock 318?
[Re: dirt]
#1359348
12/26/12 02:26 PM
12/26/12 02:26 PM
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Joined: Jan 2007
Posts: 2,212 Minnesota
peabodyracing
top fuel
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top fuel
Joined: Jan 2007
Posts: 2,212
Minnesota
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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!
Lead, follow or get the hell out of the way
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Re: square bore or spread bore carb for stock 318?
[Re: peabodyracing]
#1359349
12/26/12 04:27 PM
12/26/12 04:27 PM
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Joined: Nov 2012
Posts: 52 Ohio
Super Coupe
member
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member
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Posts: 52
Ohio
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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.
Last edited by Super Coupe; 12/26/12 04:29 PM.
The name's Bondo. Jams Bondo
Tonight I'm dining with Clia Torrez
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Re: square bore or spread bore carb for stock 318?
[Re: dirt]
#1359350
12/26/12 05:47 PM
12/26/12 05:47 PM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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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
live every 24 hour block of time like it's your last day on earth
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Re: square bore or spread bore carb for stock 318?
[Re: RapidRobert]
#1359351
12/26/12 07:23 PM
12/26/12 07:23 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,376
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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.
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Re: square bore or spread bore carb for stock 318?
[Re: topside]
#1359353
12/26/12 09:50 PM
12/26/12 09:50 PM
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Joined: May 2003
Posts: 15,487 Florida
scratchnfotraction
I Live Here
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I Live Here
Joined: May 2003
Posts: 15,487
Florida
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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.
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Re: square bore or spread bore carb for stock 318?
[Re: Super Coupe]
#1359358
12/27/12 12:07 AM
12/27/12 12:07 AM
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Joined: Jul 2003
Posts: 6,167 Maryland
GO_Fish
master
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master
Joined: Jul 2003
Posts: 6,167
Maryland
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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.
Scott B.
"I'm a self-made man... I started with nothing, and I still have most of it!"
68 360 rusty B'cuda 'vert (GO Fish)13.59@ 98.72 mph
69 340 GTS stock 14.18@ 95.60 mph
01 5.9L Ram 1500 Quad Cab 4x4
01 3.5L 300M 16.23@ 86.97 mph
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Re: square bore or spread bore carb for stock 318?
[Re: dirt]
#1359359
12/27/12 12:49 AM
12/27/12 12:49 AM
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Joined: May 2003
Posts: 15,487 Florida
scratchnfotraction
I Live Here
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I Live Here
Joined: May 2003
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Florida
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Quote:
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.
that intake with a q-jet would work really well on a stock 2-v cam 318.
it has small port runners for 318 heads and it is a spredbore flange
i have one on a 360 engine with a lunati 292/292 cam,i think it holds it back some on top. will be going LD340 or performer RPM airgap on it next.
but I have used the street dom with a q-jet/headers many times on stock JY 318 engines in my 68 dart.
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Re: square bore or spread bore carb for stock 318?
[Re: GO_Fish]
#1359360
12/27/12 12:57 AM
12/27/12 12:57 AM
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Joined: Jan 2003
Posts: 16,376
dogdays
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I Live Here
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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.
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Re: square bore or spread bore carb for stock 318?
[Re: mopfried]
#1359362
12/27/12 01:47 PM
12/27/12 01:47 PM
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Joined: Jan 2005
Posts: 12,419 Kalispell Mt.
HotRodDave
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Kalispell Mt.
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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.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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