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Old 04-21-2010, 06:47 PM   #1
tcrote5516
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Default What size carb???

I'm switching out my 600cfm Edelbrock to one of those new Summit carbs. My question is what size? They come as 600 and 750cfm. I have a decent breathing 302. High rise intake bolted to 351 port matched heads, headman headers, upgraded ignition ect...

Will I gain anything by going to the 750CFM??? Will I be able to lean it out enough to run right?

Thanks guys!
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Old 04-21-2010, 07:25 PM   #2
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Engines are basically air pumps.

Unless you are planning to run the engine above 8500 rpm, the 600 will be fine.

That 750 would run your 302 up to 10500 rpm. Of course, in normal street driving throttle response of the 750 would be suluggish compared to the 600.
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Old 04-21-2010, 07:34 PM   #3
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Does Summit make their own carbs or are they rebadged from another mfg? The Edelbrock Performer coincidently also only comes in 600 and 750. Maybe see if they look the same as Summit's.
The vacuum secondary carbs are much more forgiving of over-carbing. VS only
uses what it needs, whereas a mechanical secondary (double-pumper) just dumps gas down wheather it needs it or not.
I got an Edel Performer VS 750 on my stock 351C and it runs great. It increased the top end HP quite a bit over the old Motorcraft 735cfm and low end seems a little better too.
If you're drag racing, I think a 750vs would be better. For low end torque on the street, you may be better off with a 650. Tough call, it looks like you're setup for the drags, presuming you have a hot cam too. It probably
could use the bigger cfm. Maybe shoot a question over to Edelbrock or Holly.

Last edited by 1971mach1; 04-21-2010 at 07:50 PM.
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Old 04-21-2010, 07:58 PM   #4
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Quote:
Originally Posted by 1971mach1 View Post
Does Summit make their own carbs or are they rebadged from another mfg? The Edelbrock Performer coincidently also only comes in 600 and 750. Maybe see if they look the same as Summit's.
Summit is producing these carbs based on the Holley 4010 design, which Holley discontinued years ago.

Holley destroyed the tooling, and Summit claims to have made some improvements when they re-tooled the design.

The Summit uses features of the Autolite 4100, such as top cover, and bowls and mounting palet integral to the body. It also has the annular discharge, on the primary side.

However, it uses the meters, pumps and diaphrams of the traditional Holley.

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Originally Posted by 1971mach1 View Post
I got an Edel Performer VS 750 on my stock 351C and it runs great. It increased the top end HP quite a bit over the old Motorcraft 735cfm and low end seems a little better too.
No doubt. But your 351C is a much bigger air pump than a 302.

Last edited by 2+2GT; 04-21-2010 at 08:02 PM.
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Old 04-21-2010, 08:42 PM   #5
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Great advise guys, thank you.

It does have a pretty heavy cam, however its driven on the street 95% of the time. (50% of that time its driven like its on a track but that's besides the point

I'm replacing this block with a 347 stroker this coming winter, so my thought process was to go for the 750 if I can get it to run right to save me from having to swap it down the road. Sounds like I better go with the 600 though.

FYI, I have done alot of reasearch on the Summit's. Seen plenty of teardowns and bang for the buck it doesnt look like you can beat it. Its a little wrough around some of the castings, but nothing that 1hr of polishing cant fix (if it even needs it). For half the price of a comparable piece, you cant go wrong IMO.
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Old 04-21-2010, 10:43 PM   #6
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depending on the improvements they made over the 4010 I dont know if I'd use one with a really big cam, I used a 4010 years ago on a 351w, ported 351 heads and comp 268h cam and it was a nightmare to tune, I even had to put small strands of wire into the emulsion tubes to tune it, it was super rich idling and had a transition stumble I could never cure, I swapped it onto a milder 351w in another vehicle and it worked perfect.

With a big cam I lean towards a mighty demon or holley HP because of the high level of tunability, good thing with summit is they have a great return policy, the 600 is fine for a 302 other wise a 575 demon is a good choice.
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Old 04-22-2010, 05:05 AM   #7
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Quick Fuel if it's going on a serious engine.
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Old 04-22-2010, 11:26 AM   #8
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Or Pro Systems...

A rule of thumb is to double your peak horsepower and that is the cfm carb you need. So if you are going to have a 325hp small block you need a 650cfm carb. If you are going to build a 500hp big block or big small block, you need a 1050. That is the old school was of thinking. Another way is to use a carburetor calculator and with max rpm the engine will see and volumetric efficiency of the engine you will get the cfm the engine needs.

Speaking from experience, there is power to be had from over carbureting but you have to live with issues you run across on the street which is often over rich conditions at certain throttle settings and poor fuel atomization.
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Old 04-22-2010, 01:12 PM   #9
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leave the edelbrock on it until you have the stroker. Once you have the stroker might go as well on a more serious carb. what's wrong with the edlelbrock?
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Old 04-22-2010, 09:51 PM   #10
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The Edlebrock is finicky, it probibly just needs to be cleaned up or rebuilt. I'm just learning about carbs, so I was looking to get a cheap replacement while I tear down and learn how to rebuild my edelbrock since its such a popular carb.

One major issue with the Edlebrock is heat soak, the other is choke. Warmed up its finiky about starting. When its cold, the choke doesnt seem to do anything. (its electric and it does heat up) I have to keep cycleing the pedal for the first min to keep from stalling.

I also have a 68' Holley that came with the car to tool around with.
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Old 04-22-2010, 09:51 PM
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