454 LS6 Carburator help - Chevelle Tech
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  • 2 Post By mr 4 speed
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post #1 of 15 (permalink) Old Aug 15th, 19, 8:55 AM Thread Starter
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454 LS6 Carburator help

I am considering changing the current carburetor on my LS6: Holley list 4780-3 800cfm DP to a more street friendly-cruiser carb. Would a 650cfm meet the needs of the engine and be more friendly?
Heads are rectangular port, open chamber 166cc, 2.19 intake and 1.88 exhaust valves.
Trans is a M20 4 speed.
Intake manifold is a High Rise-duel plane for rectangular port heads. It also has a electric fuel pump Holley "Blue" set at 6 1/2 psi. With this set up I am getting less than 7 mpg. And performance "bogs" off the line...

If anyone has suggestions or insights.....I would appreciate the info. Thanks! Craig
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post #2 of 15 (permalink) Old Aug 15th, 19, 8:58 AM
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Re: 454 LS6 Carburator help

A stock out of the box 750 cfm vacuum secondary 3310 like this will be perfect:

https://www.jegs.com/i/Holley/510/0-...content=Holley

I use the same carb on my car.Pretty much stock other than jetting and a purple secondary spring.

If you are getting "bogs" off the line,I would check the timing first. Sounds like you don't have enough.
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post #3 of 15 (permalink) Old Aug 15th, 19, 9:00 AM
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Re: 454 LS6 Carburator help

Your current carb should run fine on an LS6. The factory carb was a 780 vacuum secondary which is very similar to your 800 mechanical, and they run beautifully.

What cam is in the engine, what does your entire ignition system consist of (base and total timing), how do the plugs look, and what rear axle ratio is in the car?

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post #4 of 15 (permalink) Old Aug 15th, 19, 2:16 PM
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Re: 454 LS6 Carburator help

I have a good friend who has been running a 3310 on his L88 since the late 70's. Car performs great and he knows it could go better with something bigger but for the street you can't beat it. New gaskets every once in a while and a clean up and they will run forever. I have 2 of the Demon VS, never tried them on a street car but maybe next year I will give one a shot.
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post #5 of 15 (permalink) Old Aug 15th, 19, 3:52 PM
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Re: 454 LS6 Carburator help

The L88 actually needs around 830 cfm, the LS6 dont.

A smaller carb like the 650 will feel more responsive for sure. HOWEVER, 7 MPG means something wrong with your carb. Or you drive like a madman!

Assume you do not want to tinker with the carb? Then the 3310 recommendation is spot on. ( sell me the 800DP!)

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post #6 of 15 (permalink) Old Aug 15th, 19, 9:55 PM
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Re: 454 LS6 Carburator help

www.customrebuiltcarbs.com Click on the link and give them a call, I know they can help!!

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post #7 of 15 (permalink) Old Aug 16th, 19, 9:02 AM
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Re: 454 LS6 Carburator help

Fix the carb you have now.
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post #8 of 15 (permalink) Old Aug 16th, 19, 9:32 AM
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Re: 454 LS6 Carburator help

FWIW, these engines came with 780 cfm from the factory and there would be no reason to install anything smaller than 750 cfm
It will be down on HP and torque without a doubt with anything smaller.
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post #9 of 15 (permalink) Old Aug 16th, 19, 10:08 AM
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Re: 454 LS6 Carburator help

and he may not ever notice it, an assumption. Another assumption is that he doesnt want to tinker with it and learn how to dial it in. Which is fine, but a limitation.

Step #1 is to set floats while HOT, and I would recommend you at the least do this.
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post #10 of 15 (permalink) Old Aug 17th, 19, 3:45 PM Thread Starter
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Re: 454 LS6 Carburator help

Thanks....for the info. I am going to try the timing solution.....first. But keep this carb info due to the responses received about it...and the performance. Thanks!
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post #11 of 15 (permalink) Old Aug 17th, 19, 3:51 PM Thread Starter
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Re: 454 LS6 Carburator help

Eric....thanks for the response and info. Timing is going to be checked and (hopefully) THAT will provide the solution. Thanks.
Craig

All good info ..thanks!!!! I will provide feedback once I "time" it.....and recheck it.
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post #12 of 15 (permalink) Old Aug 17th, 19, 8:13 PM
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Re: 454 LS6 Carburator help

That 830 is a race car carb, will get single digits gas mileage. they're pig rich in the idle transition because they were originally designed for racing use where there was no expectation of street use.

there's no adjustment for this, you have to take the carb completely apart and start drilling orifices.At minimum, to get this carb to a reasonable facsimile of a street car carb, you would have to change the Idle feed restriction, the idle air bleed, the accelerator pump squirter and restrict the fuel to the idle transition slot.

Truth is, you'd better buying a new Holley like the one linked to in Post #2. Beware of any local "Holley experts", they're not. Bigger is not better with carbs, especially with street cars. Chevy sold those engines in cars with a warranty, they had a 780CFM carb with vacuum secondary. The difference between 750 and 780 is piddling.
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post #13 of 15 (permalink) Old Aug 18th, 19, 9:38 AM
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Re: 454 LS6 Carburator help

Correct me if wrong Tom, but believe mr 4 speed indicated the difference is shaved butterfly screws, etc to get an additional 30 cfm out of that GM hipo carb. Also, the OP's carb is a standard 800 cfm DP with downleg boosters. A bit of a orphan, but not a race carb, is it?

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post #14 of 15 (permalink) Old Aug 18th, 19, 12:20 PM
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Re: 454 LS6 Carburator help

ALL GM "780" Holley vacuum secondary carbs:

Primary Venturi: 1 3/8"
Secondary Venturi: 1 7/16"
Throttle bore diameter: 1 11/16"

Holley 4780 (-XXX)

Primary Venturi: 1 3/8"
Secondary Venturi: 1 7/16"
Throttle bore diameter: 1 11/16"

No need to pick the fly sh** out of the pepper here.

Save for minor calibration differences and mechanical secondaries on the 4780, they're essentially the same carbs.

Save your $, fix the carb you have now.

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post #15 of 15 (permalink) Old Aug 25th, 19, 3:34 AM
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Re: 454 LS6 Carburator help

doesn't matter much about the details of the calibration, it's still a 60's DP. expect single digits gas mileage.

Last one I checked with an WBO2 meter was in the mid 12s AFR at cruise. Those carbs are good with drag racing or oval track usage though. the 830 was the NASCAR speedway carb of choice for many years.

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