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Old 03-06-2008, 04:58 AM   #1 (permalink)
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Using a carb thatís too big?

I have a stock í76 351W with a 2 bbl carb, stock cam, exhaust, etc in a heavy Gran Torino. Engine has 80k miles, good compression, no blow-by, or valve seal leakage. Iím going to start making some upgrades and have purchased a Performer RPM intake.

I have a 4780 Holley DP (800 cfm) that Iíd like to use Ė and I know itís way too much carb, but future plans call for a camshaft upgrade and headers which may make it a little less oversized.

I know I should just buy a 650cfm vacuum secondary Holley or Edelbrock carb, but Iím just stubborn enough to want to make this work. Any suggestions? How about adjustable linkage to delay the opening of the secondaries (Holley p/n 20-122)? What would be a good starting point on jetting?
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Old 03-06-2008, 05:08 AM   #2 (permalink)
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Re: Using a carb thatís too big?

...whew...

800 CFM DP on a smog 351W?

I would give it a try if you are trying to save the cost of a replacement carb. The secondary kit is a must (I did not know such a thing exists). You are going to have to experiment with jetting and proper power valve.

I would try it and see how it works. The car will most likely crap all over itself if you get on it (until proper modifications are made), but it will make it somewhat driveable.

Holley 20-122 - Holley Pro Series Secondary Linkage Kit - JEGS
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Last edited by KULTULZ; 03-06-2008 at 05:16 AM. Reason: ADD ILLUSTRATION of HOLLEY PN 20-122
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Old 03-06-2008, 05:30 AM   #3 (permalink)
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Re: Using a carb thatís too big?

Your 2 Barrel maybe a 550 CFM Carb. If it is, you should keep using it till you upgrade the rest of the motor. Go ahead and put that 4 barrel manifold on, and use a 4 barrel to 2 adaptor for the 2 barrel. You should see a little performance increase.

You won't see much more by going with a 4 barrel carb, maybe 10 - 20 HP tops with the new manifold.

Using a way-to-big carb will actually hurt you worse then keeping the 2 barrel on there.

PS: If you have a stock air-cleaner on there, for instant pleasure, turn the top cover of the aircleaner over (Upside down) and you will pick up a mini _G-gasum from that....
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Old 03-06-2008, 07:46 AM   #4 (permalink)
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Re: Using a carb thatís too big?

It will run with a 800 DP but your not going to like the Fuel Economy and manners..... Plus it will take some really good tuning skill to get it to run decent.

If you trying to save money just stick with your 2v untill you can get the right parts.
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Old 03-06-2008, 07:49 AM   #5 (permalink)
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Re: Using a carb thatís too big?

I have run 850 carbs on 283 engines with no problems at all. DO NOT make the mistake of going to smaller jets like most people do. The carb can compensate for the size of your motor./ NEVER go down more than two jet sizes. I have seen this done MANY times.

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Old 03-06-2008, 08:32 AM   #6 (permalink)
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Re: Using a carb thatís too big?

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Originally Posted by Mikes66 View Post
Your 2 Barrel maybe a 550 CFM Carb.
No way.

Aftermarket Holley carbs flow more than the stock Motorcraft on there, and they are rated at 350cfm and 500cfm... But, that's not all. 2bbl carbs aren't rated at the same vacuum as a 4bbl, so they actually flow LESS than their ratings when compared to a 4bbl.

I don't know what a stock Motorcraft flows, but it's nowhere near 550cfm.

As to the original question....

Is this car driven much? On a totally stock, smogger engine, the 800 Double Pumper is way more than the engine needs. You 'might' get it to work, but the fuel mileage will be HORRIBLE, and any increase in performance will be minimal. That big carb is calibrated for larger engines/higher compression ratios/headers, etc. The calibration will be WAY off. You'll be very lucky to get it to carburate and transition smoothly between circuits, let alone provide decent drivability and air/fuel ratios under different loads.

If all you are worried about is wide open throttle at higher rpm, you can probably tune for that, but the engine will probably run terribly everywhere else, and give HORRIBLE fuel mileage.

Of course, if you are going to install the intake, it can't hurt to try it out. I'd just wager that you'll eventually either use an adapter to switch back to the 2bbl until you can build the engine the rest of the way, or buy a different carb.

Good Luck!
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Old 03-06-2008, 09:02 AM   #7 (permalink)
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Re: Using a carb thatís too big?

Quote:
Originally Posted by n2omike View Post
No way.

Aftermarket Holley carbs flow more than the stock Motorcraft on there, and they are rated at 350cfm and 500cfm... But, that's not all. 2bbl carbs aren't rated at the same vacuum as a 4bbl, so they actually flow LESS than their ratings when compared to a 4bbl.
Ya 500 sounds about right..., just could not remember... I think it has been atleast 15 years since I have even held a 2 barrel in my hands.

Still, should be plenty for a stock 351W. Plus he will not have to tune it...save even more hassel...

I do own about (7) 4 barrels though.....
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Old 03-06-2008, 06:49 PM   #8 (permalink)
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Re: Using a carb thatís too big?

Again, Joe is right!

I've used 1050 Dominators on 302's and never had any issues. Didn't lose any low end but picked up a ton of top end power...basically stock built 302 with a factory intake.

Just don't go jetting it down. Most times you'd have to jet it up a little to compensate for the loss of some booster signal. Not always, but sometimes it happens that way.
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Old 03-07-2008, 10:12 PM   #9 (permalink)
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Re: Using a carb thatís too big?

800CFM is more than you need, thats a given, but ive seen people run similar sized carbs on similar builds without running into issues.try it out, you MIGHT be surprised.
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Old 03-08-2008, 05:25 AM   #10 (permalink)
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Re: Using a carb thatís too big?

I'd give it a shot. Especially on a dual plane intake.

Put it on there with no spacer, or a 1/2 in 4 hole. I bet it'll do just fine
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Old 03-09-2008, 12:44 PM   #11 (permalink)
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Re: Using a carb thatís too big?

my 306 ran high 10's with only a 650
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Old 03-12-2008, 02:19 PM   #12 (permalink)
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Re: Using a carb thatís too big?

Just adjust your throttle linkage to only open the carb 3/4-way - then you'll only have 600cfm....
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Old 03-12-2008, 09:33 PM   #13 (permalink)
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Re: Using a carb thatís too big?

There's no such thing as a carb that's too big.....

Think about it. What happens when a bone stock 302 is wide open throttle, and running about 2500 RPM? It only needs just a little bit of airflow. So if the carb is 500 CFM, then it's too big

there's a hell of a lot more to a carb than it's CFM size. Just as I stated before...I ran 1050's on 302's for years with great success. They sure looked weird though. The carb was as wide as the rest of the motor. You CAN tune the carb FOR the motor, but generally they're pretty close right out of the box. notice I said GENERALLY. I've seen/had some Demons among others that were so far off of a tune that they would run like poo on anything but a lawn mower.

I say bolt that sucker on there & go drive it. I think you'll be surprised how well it will work.
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Old 03-13-2008, 06:12 PM   #14 (permalink)
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Re: Using a carb thatís too big?

I remember talking to a guy who had a 302 built and dyno tested by Superior Automotive here in So Cal (a very good shop). They tried several different carbs from 600 to 830 cfm on the 302 which had Edelbrock RPM heads. The 830 resulted in the most peak power (413 HP or something like that) by some 15+ HP.

Take it for what it is worth...there may have been other factors involved such as jetting...not sure.
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Old 03-14-2008, 02:22 PM   #15 (permalink)
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Re: Using a carb thatís too big?

I've got a 780cfm on my 289... 800 on a 351 should be fine.

As the other said, you might run into driveability issues... and you might not, it just depends on the tune of the carb. You will most likely loose some mileage though, because most of the time the Holleys are rich on the idle and transition circuit, which is what you will be running on most of the time with an 800 on a mostly stock 351. I think they calibrate them rich because most engines will DRIVE just fine rich, no hesitation or surging... they'll just get crap for economy... but if they're lean, you can run into all sorts of driveability issues.
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