Help chosing a carb size 650 or 750

[91hrdbdy]

i have a mild built 400 sbc it has motown heads and a medium sized cam (of unknown spcs) medium to rough idle. i slapped a old edelbrock 1406 on it and the car is need of replacement im looking at buying the summit 650 or 750 holley style carb. but im note sure what one i should get? any help would be great.

[Scott Liggett]

750 would be better for full horsepower. The 650 will have great low rpm throttle response, but expect it fall flat before you reach peak rpm possibly causing engine damaging detonation. Don't expect the smaller carb to get better mpg. I have seen too small of carbs get crap mpg. Ever seen a 350 with those Rochester 2 bbls get good mpg? The engine needs a certain amount of air to function. The smaller size venturis, the air moves faster which is what pulls fuel out of the boosters.

No real proof, just my theory and the fact I got the best mileage out of my 850 dp on both my 383 and 454 over smaller carbs.

[cstmwgn]

IF this is a primarily street driven car with what you described as a mild build then I would consider a vacuum secondary 750 over a double pumper.

[yellomalibu]

IF this is a primarily street driven car with what you described as a mild build then I would consider a vacuum secondary 750 over a double pumper.

He brings up an excellent point in that you must ask yourself what the intentions are going to be with the car. I'm a double pumper guy on my hot rods... but vac secondaries are known to get better gas mileage.

We were told about the engine... but what about the rest of the vehicle? It's always about the right combo of parts that optimizes the whole car. A heavy tow vehicle would need a completely different engine set up than a light weight street strip car.

In a street driven toy/ hot rod there are almost always compromises that have to be made.

[91hrdbdy]

okay thanks for the info this is for my wifes 70 c-10 turbo 400 307 rear end cruiser with the once in a while towing of my malibu to shows

[yellomalibu]

Let's see what Holley has to say about it:

https://www.holley.com/news/articles/the_world_s_first_interactive_carburetor_selector_/

or if that one doesn't work, try this simpler one:

http://www.holleycarbs.com.au/chooseCarb.htm

[redneckjoe69]

i vote 650 vacuum secondaries for that set up. especially if the motor never goes over 5,000 rpm. no point in the 750.

[anotheridiot]

yeah, those calculators always throw CFM's right out the window. Think this setup is calling for 463. I have heard vacuum for automatics and double pumper for manual transmission to help when you take your foot off the clutch to save that lag on street vehicles.

Just think if you go with the 750 you might end up needing to change jets and power valve and if you are not interested in tearing apart your brand new carb you will probly be closer with the 650 setup.

[Ron Ward]

Looking at the options the 91hrdbdy has listed, here...

" i slapped a old edelbrock 1406 on it and the car is need of replacement im looking at buying the summit 650 or 750 holley style carb. but im note sure what one i should get? any help would be great."

First, what is wrong with the 1406? Why replace it? It's a solid AMERICAN MADE piece from Edelbrock that can easily be rebuilt for less than 50 bucks.

Second, take a long look at those Summit carbs - especially where they are made - and check out the reviews on them.


It's only my opinion, but I think that 1406 would be the ideal carb for that application.

[greenjunk]

You should adjust the jets on any carb. None of them will be perfect for your specific application. If its an auto with that tall of a gear in that heavy of a truck I would go smaller. I'm assuming you're turning no more than 6000rpm. I bet it has a dual plane on it too. Just no need for a huge carb, you will need to change the spring on the vac secondaries, they all open too slow. As far as non name brand carbs. Think of it as buying it twice, because all off brand parts you'll generally have to buy at least 2 in the same time you would get one more expensive real one. Its worth the extra to buy the holley. Its not that much more.

[BigAL]

The 355 I just built and had dyno'd pulled 700cfm of air, so a 650 carb would have been to small. Since you have a 400 I would go with the 750.

[greenjunk]

Big, thats very vague. How many rpm, how big is the cam how big are the heads?
Same thing for the 400.
A 400ci at 6500 rpm w/ 100% VE is a max of 750cfm
A 400ci at 6500 rpm w/ 85% closer to a street application with a non perfect combo is 640cfm

Thats assuming it makes peak power at peak rpm. which is more than likely also not true. Assume it makes it at 5700-6000 the crab can be even smaller.

[BigAL]

Big, thats very vague. How many rpm, how big is the cam how big are the heads?
Same thing for the 400.
A 400ci at 6500 rpm w/ 100% VE is a max of 750cfm
A 400ci at 6500 rpm w/ 85% closer to a street application with a non perfect combo is 640cfm

That's assuming it makes peak power at peak rpm. which is more than likely also not true. Assume it makes it at 5700-6000 the crab can be even smaller.

Take these numbers and plug them into that Carb Calculator, 355ci, 7000rpm, 112.8% VE.

If you use the Wallace Racing Calculator, they say anywhere between 813-1056 cfm. I put a 830 cfm Holley on my 355.

http://www.wallaceracing.com/intakecfm.php

IMO without knowing the VE of the motor proper carb sizing is just a stab in the dark. And when stabbing in the dark bigger is always better, you can't go wrong with the 750.

[greenjunk]

I agree, for a street motor I always would err to the smaller. If you think about the small percentage of time a car driving down the road is at WOT vs a race car. The extra carb is just a waste. You can tune on the 650 and get generally better driveability. But I agree with your VE statement. i would highly doubt a street motor would hit 100%ve. especially a 23* sbc that probably doesn't have a lot of head work.

[Scott Liggett]

Edelbrocks 1406 was calibrated for a mild 350. It is way too small for a 400. And, that gets worse if you are towing where much more fuel is necessary. The company I work for builds thousands of performance engines. We dont use a 650 on anything bigger than a 350. If you want a great carb that you could pull ou tof the box and bolt on, get the Holley 0-3310S 750 cfm vacuum secondary. We use hundreds of them. I tested one on my car. They work great.