Using a carb that’s too big?

I have a stock ’76 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. I’m going to start making some upgrades and have purchased a Performer RPM intake.

I have a 4780 Holley DP (800 cfm) that I’d like to use – and I know it’s 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 I’m 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?

...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

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....

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.

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.

JOE SHERMAN RACING ENGINES

Originally Posted by Mikes66

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!

Originally Posted by n2omike

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.....

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.

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.

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