650 or 750
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
The smaller carb isn't going to give up any power with those CID/HP numbers, and will provide better throttle response in all scenarios except zero-to-WOT.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
http://www.nastyz28.com/~ericf/tech/htune.pdf
Last edited by 63mako; Dec 12, 2009 at 04:00 PM.
Instructor
Joined: Apr 2006
Posts: 212
Likes: 0
From: Woodland Park Colorado
I run a 770 sa on my 383 440hp 480 ftlb and I had a off idle stumble also. What cured mine was to change the power valve from a 6.5 to a 4.5. I had low vac at idle and the pv was kicking in to soon lower the powervalve cheap fix.
have fun with new motor
have fun with new motor
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Venturi effect basically means that you'll get slower intake charges out of a carb pulls less CFM than your engine needs. This is really important because it slows down how much fuel actually makes it into the motor. Even if you tune a 750 perfectly, you're getting less intake charge overall as the motor simply can't pull as much mixture from the carb as it could with a proper size carb.
This is negated with FI setups, which is why we have giant throttle bodies on lower CFM cars.
Go with the 650. You won't regret it.
This is negated with FI setups, which is why we have giant throttle bodies on lower CFM cars.
Go with the 650. You won't regret it.
Racer
Joined: Jan 2008
Posts: 338
Likes: 0
Trust the Holley Tech, they know their products... most people over carb their engines and think its going to be better, then they recommend a larger carb to other people and tell them it will be better. Like everyone said, 650 will help with low end performance due to the higher velocity of the air coming in (venturi effect) which will atomize the gas better. Only at very high RPM's will the 650 fall off because of restriction, but 90% of people here don't run at high enough RPM's for long enough for this to be a factor. (I know I don't) The 750 will also require a more significant jetting adjustment to tune... so the 650 will prob run better out of the box.
Last edited by DaJesterMan; Dec 17, 2009 at 06:23 AM.
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
The smaller carb isn't going to give up any power with those CID/HP numbers, and will provide better throttle response in all scenarios except zero-to-WOT.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
We have done some mild 383 builds 450 horse and 500 FT pounds of torque and on onewe used an HP 600 and tried a 750 results were about the same.
The 650 may give you more throttle response or the 750 carb.
You really have to have some big heads and cam to make a 750 carb work to its full potential on 383 CID
Burning Brakes
Joined: Jul 2009
Posts: 1,132
Likes: 14
From: A town SC
Well find you a good carb builder and have a custom carb built with 650 main body and 750 base plate. have the air bleeds tuned to your combo along with all the other advanced adjustments thats needed. No off the shelf carb is going to boltup an run perfectly. The 650 body /750 base will up the air flow and give you a happy medium. I have done this, it makes an awsome carb. I never buy a new carb i build all mine from the holley carb and parts collection i have...
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
Trust the Holley Tech, they know their products... most people over carb their engines and think its going to be better, then they recommend a larger carb to other people and tell them it will be better. Like everyone said, 650 will help with low end performance due to the higher velocity of the air coming in (venturi effect) which will atomize the gas better. Only at very high RPM's will the 650 fall off because of restriction, but 90% of people here don't run at high enough RPM's for long enough for this to be a factor. (I know I don't) The 750 will also require a more significant jetting adjustment to tune... so the 650 will prob run better out of the box.
Both carbs will work if they are correctly tuned, but the 650 will probably be very close to perfection right out of the box.
It can be a little more complicated but still feasible with the 750.
If I had to make the choice I would try the 650 except if the car is mostly for a drag/strip use.
Last edited by 73StreetRace; Dec 17, 2009 at 10:46 AM.
Racer
Joined: Oct 2006
Posts: 435
Likes: 18
From: Florida
What manifold do you have? I have seen a 650 only be down 7-8 hp on a 500 hp 355 @ 6400 with a Performer RPM. What was lost below 5000rpm (20 hp & 35 ft/lb @4200) was very noticeable though. Your engine will run differently in the car as opposed to what a dyno showed, dynos have unrestricted inlet air and exhaust, usually not run with accessories or water pump or the same advance you will need in your dizzy. Their is reasoning that a dual plane can be overcarbed because you only feed from half the carb, but where the engine runs mostly at you will feel the difference between a 750 and a 650 if they are both tuned correctly.
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Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
The 670 that you already own can be tuned to work properly for you. A double pumper is never required just to cure a stumble.
If you do decide to buy a new carb, I would buy the 750 if you have a dual plane intake.
If your 383 is lazy at low RPM with a 750, either your tune is off or you have a bad engine combo.
Remember, Chevy put 780s on the LT1 cars.
Burning Brakes
Joined: Jul 2009
Posts: 1,132
Likes: 14
From: A town SC
The 650 and 750 have the same base plate. That would be a bit of a waste to swap.
The 670 that you already own can be tuned to work properly for you. A double pumper is never required just to cure a stumble.
If you do decide to buy a new carb, I would buy the 750 if you have a dual plane intake.
If your 383 is lazy at low RPM with a 750, either your tune is off or you have a bad engine combo.
Remember, Chevy put 780s on the LT1 cars.
The 670 that you already own can be tuned to work properly for you. A double pumper is never required just to cure a stumble.
If you do decide to buy a new carb, I would buy the 750 if you have a dual plane intake.
If your 383 is lazy at low RPM with a 750, either your tune is off or you have a bad engine combo.
Remember, Chevy put 780s on the LT1 cars.
This is done to improve the flow restriction at the throttle blades and improve airflow on the 650 without losing the air velosity of the 750.Hence keeping crisp throttle response and supplying the air fuel the engine needs.
Last edited by forvicjr; Dec 17, 2009 at 03:10 PM.
