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 Rochester 4 GC for Super stock 
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Joined: Sun Mar 17, 2013 9:17 pm
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The super stock combination I race has the option of a 4 GC carb. [1965 SBC 327/250 horse] The carb has 1.437 throttle plates, 1.125 primary and 1.250 secondary.

Any experience with drag race prep for these carbs?


Tue Feb 16, 2016 11:10 pm
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What other carb is possible?


Wed Feb 17, 2016 11:05 am
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JMark,

the combination has three carb options but all carry different power factors, thus, different classes.

[1] the Rochester 4 GC

[2] Holley {Quick Fuel} 585 which i have now

[3] Q-jet

My reason for considering the 4 GC is because it allows a favorable power to weight factor.

The Super 10 is as light as I can practically make it at 3170 pounds with driver. This means it is 80 pounds over weight for "B"

With the 4 GC factor I can race in "C" and only be 30 pounds heavy. The "C" index is with 4 GC 11.10 while the "B" index with 585 Quick Fuel is 10.85

I know a number of Super Stock racers run the 4 GC combination and I plan to experiment with doing so.

Dean Oliver is a recognized expert with the super stock 4 GC and I will be talking with him. I was just check here to see if someone on this board has experience with race prep on the 4 GC

This link shows the Super 10 build http://classracer.com/classforum/showthread.php?t=54442


Wed Feb 17, 2016 8:17 pm
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this run is at "B" weight with the 585 carb

https://www.youtube.com/watch?v=U0IXR-5qZf4


Wed Feb 17, 2016 8:21 pm
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Tuner has probably been thru them, if I have it's been a long time. Certainly something you will have to test to see what runs best under the index.


Wed Feb 17, 2016 11:24 pm
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uncrated wrote:
The combination has three carb options but all carry different power factors, thus, different classes.

[1] the Rochester 4 GC

[2] Holley {Quick Fuel} 585 which i have now

[3] Q-jet

My reason for considering the 4 GC is because it allows a favorable power to weight factor.

The Super 10 is as light as I can practically make it at 3170 pounds with driver. This means it is 80 pounds over weight for "B"

With the 4 GC factor I can race in "C" and only be 30 pounds heavy. The "C" index is with 4 GC 11.10 while the "B" index with 585 Quick Fuel is 10.85

I know a number of Super Stock racers run the 4 GC combination and I plan to experiment with doing so.

Dean Oliver is a recognized expert with the super stock 4 GC and I will be talking with him. I was just check here to see if someone on this board has experience with race prep on the 4 GC

This link shows the Super 10 build <span class="skimlinks-unlinked">http://classracer.com/classforum/showthread.php?t=54442</span>
I see the other choice for the weight break is a Carter WCFB, but it has 1/16” smaller primary venturi (I think). Not much difference between the two in size and I haven’t played with them on a flow bench so ?? I can’t say much in that department. From tinkering with both 4Jet and WCFB in the occasional boat and back in the 7” tire days it was evident the float bucking springs and baffles around the inlet valve are important.

The brass floats are relatively heavy and have long levers, so controlling the reaction to g forces is important. On the other hand, the long lever gives mechanical advantage in controlling the inlet. The last time I tinkered with one of these on a race car it had 7” tires and I don’t think the 60’ was as quick as nowadays, though it did leave with the wheels up.

I haven’t done a 4G for anything other than restoration of a street car for several years (decades, LOL), but from a fair amount of experience with the 2G setup used in circle track it becomes obvious carbs generally benefit from a few common sense things.

I recall when the 4G carbs were new (and hadn’t been raped by a hothouse mass rebuilder) they all had float bucking springs, (Doug Roe called them “float assist” springs in the first edition of his Rochester carb book. You’ve seen that, haven’t you?). Also, some versions of the 4G had a baffle around the inlet valve, somewhat like the 2G, but those disappeared when the carbs became victims of the mass rebuilders sold through auto parts stores.

I expect if there is a float control issue on the launch something could be gained by adding some baffling in the float bowls to get some control of front to rear sloshing. For example compare the 2G and 4G. In circle track with a 2 G the float hinge is parallel to the crankshaft and the G force is sideways. Drag racing with the 4G the float hinge is perpendicular to the crankshaft and the G force is rearward. In both cases the G force sloshes fuel perpendicularly to the float hinge. The float displacement in the liquid may remain the same but the leverage changes as the liquid moves toward or away from the hinge. I can say this for sure … with a 2G, adding a baffle to isolate the float pontoon from the dead volume near the accelerator pump bore gives better control in cornering and I don’t see why it would be any different if the carb were turned 90°, which is the situation with the 4G float, hinge and leverage.

The asymmetrical float shape with more displacement closer to the float hinge is usually desired but that might not be the case primary vs. secondary in this case.

A baffle around the inlet needle, which in the 2G is the box retained by the float hinge pin and top gasket, prevents the fuel entering the bowl from impacting the float and affecting float function.

In the wayback days (50s) some 2G carbs had a flat sheet baffle insert across the bowl, retained by the ridges in the casting, which isolates the float pontoon in its own chamber separate from the accelerator pump well. For 2G circle track I make a baffle using thin brass sheet. Reducing the mass of fuel sloshing is always a good thing.


Sat Mar 05, 2016 12:28 pm
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Turner,

Thank You.

In looking over a core I have on hand I agree with your comments about float and slosh issues and realize some experienced modification will be required.

In addition, modifications to the booster clusters for idle fuel and air bleeds.

As of now I do not consider the time and experimentation a good investment.

Plus, beening a dye in the wool true hot rodder I do not like the idea of installing a smaller carb over the 585 Quick Fuel design.

My decision is to continue with the 585 carb and refine that package.

I may be back with you and Mark about the 585 refining once the truck returns to race service


Sun Mar 06, 2016 9:12 am
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