3310 downleg 780 vs 3310-6 straight leg 750 - Chevelle Tech
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post #1 of 28 (permalink) Old Feb 17th, 20, 4:18 PM Thread Starter
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Daniel
 
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3310 downleg 780 vs 3310-6 straight leg 750

Hey guys, so I recently picked up a 3310 (1968) date replacement carb with downleg boosters, 780 cfm and its in clean shape. Needs gone through but its all there and doesn't appear anything really messed with.

I currently have on the car a 3310-6 modern carb that I added a secondard metering block, and quick change vacuum secondary pod. The combo is in my signature.


Would there be any tangible benefit going with the older style carb with downleg boosters vs straight in the one I have now? Other than being rated a few CFM higher, the carbs are very very similar. The 3310 (1968) has the choke flapper removed which is fine, I never use the choke anyway. Only other difference I really see on it is a hole that looks factory cast on the choke side of the secondaries. On some pictures I've seen online there is a tube that sticks out at an angle. This tube has been removed and the hole has been plugged with RTV. It also has a larger hole on the dead middle of the other side that is threaded, that on my more modern version its cast but a blind hole. It was plugged also.

I can post pictures if necessary, but wanted to see if in any opinions its really worth swapping out to the older style for the extra 30cfm rating, and downleg boosters.

Thanks!

1970 Chevelle - Best time at the WCCG 12.749 @ 108.606, 2.011 60' on street tires!

427 BBC, 215 heads, 2.19/1.88 valves with some port work, 60212 Lunati Hyd Roller, 3310 Holley 750, Performer RPM intake, TH350 with shift kit, 3.55 gears 12 bolt posi, Pypes 3 inch kit, it's alive... again!!!
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post #2 of 28 (permalink) Old Feb 17th, 20, 5:24 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

I have done back to back testing with the 3310-1 and -2 Holleys. What I found was that the -2 was a tenth or 2 quicker than the -1. I personally think that the -2 has a better signal than the down leg boosters but this was with my combo on the past. Your test show differently.


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post #3 of 28 (permalink) Old Feb 17th, 20, 5:57 PM
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Downleg boosters are used on Holley’s up grade carbs. ?
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post #4 of 28 (permalink) Old Feb 17th, 20, 8:37 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Test and see.
I test and see all kinds of carbs and intakes on my personal stuff.

Here is one for you.
Local speed shop has a great set up and you can eat off the floors ..they have a dyno and my buddy took his engine there (327 small journal) to have it rebuilt and different heads (afr 180) and cam (Roller 276XE).
I sent him with his 600 Holley and a 750 edelbrock I built and tuned for the old cam XE268 flat tapper)and heads (Ported 492).
It was stuck on the dyno after being built and broke in with their Dyno carb Holley 750.
It was then ran with that dyno carb and jetted for best power.

Then the 600 was stuck on and very little difference.
My buddy said stick the 1407 Edelbrock on there and test it.
They said they never seen any gains with the Edelbrocks and did not want to do it.
He said i have time and money and will probably run the Edelbrock anyway because it ran best with the other build.

They stuck it on there and said it made more power everywhere 421 HP and 419 TQ.


My 4779 that is very tweaked made 10 more HP than my 750 Q jet that is worked over.
That was on a Performer RPM intake.
I swapped the intake to a ported Q jet Intake and the Q jet made 10 more HP than the 4779 750.

You never know until you try.
Low speed driving around not any difference in them once tuned in.. it is only at the upper mid and top end that I notice anything.
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post #5 of 28 (permalink) Old Feb 18th, 20, 12:31 AM Thread Starter
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Daniel
 
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Makes sense Jeff, I actually picked up a 4779 also with the stuff I got. The 3310 and 4779 were in good shape, and grabbed a 4777 just for parts and linkages since stuff was missing off the 4777.

So I will have an original 1968 3310 to play with, and a 4779 if I wanted to try a double pumper.

Thanks!
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1970 Chevelle - Best time at the WCCG 12.749 @ 108.606, 2.011 60' on street tires!

427 BBC, 215 heads, 2.19/1.88 valves with some port work, 60212 Lunati Hyd Roller, 3310 Holley 750, Performer RPM intake, TH350 with shift kit, 3.55 gears 12 bolt posi, Pypes 3 inch kit, it's alive... again!!!
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post #6 of 28 (permalink) Old Feb 18th, 20, 1:21 AM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

I use whatever makes the best ET and MPH at the track, does not matter what size the carb is. On the street the one that idles and has the best drivability is the one I use.
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post #7 of 28 (permalink) Old Feb 18th, 20, 12:50 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Carburetors and intakes are like wives..different circumstances, different outcomes....and I'l leave it there
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post #8 of 28 (permalink) Old Feb 18th, 20, 1:05 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

I run a 780 cfm vacuum Holley that works like a charm on my combo. It has a straight leg booster body and an older base plate with the thin secondary throttle shafts to achieve 780 cfm.
It has instant throttle response and a very simple calibration: 74/82 jets,6.5" primary power valve,purple secondary spring and #31 squirter.
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post #9 of 28 (permalink) Old Feb 18th, 20, 7:52 PM Thread Starter
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Daniel
 
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Quote:
Originally Posted by mr 4 speed View Post
I run a 780 cfm vacuum Holley that works like a charm on my combo. It has a straight leg booster body and an older base plate with the thin secondary throttle shafts to achieve 780 cfm.
It has instant throttle response and a very simple calibration: 74/82 jets,6.5" primary power valve,purple secondary spring and #31 squirter.
Now that is interesting, I could just swap the base plate on my and get the extra 30 out of it? That would basically give me the exact same setup as yours. I already have I believe similar jetting, purple spring. I don't remember the squirter.

1970 Chevelle - Best time at the WCCG 12.749 @ 108.606, 2.011 60' on street tires!

427 BBC, 215 heads, 2.19/1.88 valves with some port work, 60212 Lunati Hyd Roller, 3310 Holley 750, Performer RPM intake, TH350 with shift kit, 3.55 gears 12 bolt posi, Pypes 3 inch kit, it's alive... again!!!
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post #10 of 28 (permalink) Old Feb 18th, 20, 7:59 PM
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“ So I will have an original 1968 3310 to play with, and a 4779 if I wanted to try a double pumper.”


Put the DP on if you want instant acceleration !

(If you have a 4 speed, low geared car)
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post #11 of 28 (permalink) Old Feb 19th, 20, 7:22 AM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Quote:
Originally Posted by Stikman33 View Post
I could just swap the base plate on my and get the extra 30 out of it?
Yes. Pictured below are the differences in the throttle plates.
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post #12 of 28 (permalink) Old Feb 19th, 20, 7:37 AM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Thanks for the pic Mr. 4speed, I'm going to take a look down my 3310-1
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post #13 of 28 (permalink) Old Feb 19th, 20, 11:46 AM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Measure the throttle bores to be sure.
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post #14 of 28 (permalink) Old Feb 19th, 20, 12:16 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Straight leg booster can create more vacuum then the down leg booster when it comes to air velocity. A straight leg type carb as a better throttle response when it comes to transition from the idle circuit to the booster. 750 will give you better pull.
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Louis
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11:1 TR406sbc/AFR227/Comp cam 280HFT/3500 RPM Stall conv./TH 350/12 bolt 410 Gears
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post #15 of 28 (permalink) Old Feb 19th, 20, 12:42 PM
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Re: 3310 downleg 780 vs 3310-6 straight leg 750

Quote:
Originally Posted by pockets View Post
Measure the throttle bores to be sure.
bores are the same size (750 versus 780)


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