I recently replaced the worn out Quadrajet and stock manifold on my 78 T/A with an Edelbrock peformer Manifold and a Edelbrock 1406 carb. My car idles better and has better throttle responce now, BUT.............. At WOT it just doesnt have the power it used to. Is this because the 1406 is too small? If so what would be recomended? Maybe a 750 to 800cfm?

Thanks

 

I doubt you'll notice a big difference going to a bigger carb. Although a bigger carb can make more power, it's more about the type of power curve.

I'd look at tuning the carb first. Out of the box settings are only a baseline.

 

If you look at you quad it will have HUGE secondary throttle plates compared to the edelbrock. I did the same switch you did on my 77 z28 and I noticed the same thing on the top end. I plan to rebuild the quad and use it again.

 

is the 1406 vacuum or mech secondary? if your secondaries arent opening completely you'll just die up top, and your quad was a 750 cfm carb just fyi

EDIT : also with the 6.6 i'd look between 650cfm (economy) and 750 cfm (to make it move) carbs

holley equation
to determine carb size take engines cubic inch number (6.6L is about 400 some were 402 s) multiply by max rpm, and divide by 3456

so if you have a 400ci 6.6L that revs 6000 rpm....
(400 * 6000) / 3456 = ~700 cfm carb

also with you 600 cfm carb 3456 * 600 / 400 = 5184 rpm which is about where you'll die off much higher will really only happen without much engine load

 

I think that about everyone in the world knows that people pick carbs that are too big much more often than too small. But they keep doing it. The design of the carb also influences how much pressure drop it will need to operate right at low speed.

I don't deal much with carbs anymore (thank God) but I just looked at Edelbrock's web site and they say that the 1406 is "Designed and calibrated for good street performance and fuel economy in small-block and small displacement big-block engines. Match with an Edelbrock Performer or Performer EPS manifold and other brands of similar design. Calibrated 2% leaner than #1405". To me, this implies it may be lean. You could richen it up and see if that solves the problem. But from that description, it sounds like the wrong calibration out of the box.

And frankly, even with the above, 600cfm seems kinda small. How high does that thing rev? My memory for that era is fading fast, and again, I don't miss it a bit! It was such a pleasure getting rid of the carb on my alcohol BBC race motor. Also picked up two tenths and a couple hundred rpm in the power band - not much intake restriction from a 2000cfm TB!

Rich

 

just some thoughts...

The performer manifold flows well to around 4500rpms..(maybe less with the 6.6). if you want more than that, you may have to go with the performer RPM or the torker....(i suspect you may have a hood clearance problem with this)

I've seen some folks run a 2 inch open penelum spacer plate and pick up a little more rpm's (In a dual plan manifold, half the cylinders draw from one side of the carb. the spacer plate straightens out the air and lets all the cylinders sorta pulse feed across the mainfolds divided runner thereby letting each cylinder draw through the whole carb...)

the 1406 (600cfm) is probably too small. I run 750cfms on mild 350 chevy's using performer rpm or torker manifolds up to 6500 or so... you probably need the 750-800 cfm unit for the 6.6 if you want more rpm's..

I'd look for a 750-800 cfm and try that, if still not enough RPM's then try the the spacer or rpm manifold or the torker..

Also what are you running as a distributer? If it's HEI, then you may need to change to a better coil...

other issues could also be fuel supply, (volume and pressure), restrictive air filters. tune timing and spark issues...

Also FYI there is a company here in Las Cruces Called Pony Carbs www.ponycarburetors.com they specialize in rochester carbs and are very good with them. they do remanufactures for collector cars as well. there rochester guy is a wizard with the things.


Greg

 

and a side note, the Q-Jet you removed was 800cfm

 

holy wow (lol that was an 800 quad? nice, not too many of the upper cfm quads), i'd tune the quad, better gas mileage and just as good if done right

 

the quadrajet is a great carb when done right. i try to buy old 400 and 403 olds carbs for my cars. i run 12's with a quadrajet and get 18 mpg. you can tune a quad to run like a dream but it takes a little time to tune. i would suggest rebuilding it with one of the new quad books out and picking up some used carbs from the swap meets. the hanger and jets are different in a lot of carbs. you can get a combo that rocks. the performer intake is about right. the 6.6 isn't a huge rpm fan.

 

how do you tune a quad. i took mine off my car because it kept on trying to die, and stall. well i threw mine away and got a holley 750dp. i wanted to rebuild my quad, but a bunch of guys said its not worth the hassle and its almost impossible to tune them.but then again i think they are a bunch of idiots to. i had a bad temper that day i threw away the carb. ( frustrated with it)