What's the motivation? Either should produce the same power if properly selected for the application and tuned. In addition to the carb, you'll need a new intake manifold.
Personally I prefer Holley's because I'm most familiar with those. If you're putting on a mostly stock 400, a 750 vacuum secondary would be the best choice. For an intake, I'd suggest an Edelbrock RPM if it'll fit under the hood with the stock air cleaner setup.
My carburetor is bad. It's from a 77 trams am. So I was thinking a Holley is easy to adjust. I can't afford the correct carb for my car. I thought the Holley works with an adapter plate. Thanks for the help. I'm not sure what to do.
The Quadra jet has never worked right even with several timing adjustment. The mixing screws do not work. I don’t want to spend money in a rebuild kit if it needs more work. Its from a newer car with emission ports that I have to plug up.
I can answer your questions regarding carbs for your Firebird. I just completed the rebuild of our firebirds engine and I spent $600 on the dyno testing different combos. Everyone here will offer advise, very few have any DIRECT actual experience with their opinions they offer, as well as they are trying to be. Hardly any have dyno numbers and live testing to back up their recommendations. I do. It cost me some bucks, and I don't mind sharing.
I saw you were advised to run a RPM manifold instead of the stock iron manifold. I ran test on an engine dyno with a Pontiac 455. I only changed the intake, using the same carb, same day, same gas, same everything. The gasket matched RPM makes 3 hp and 3 ftlbs tq more than a gasket matched stock iron 1972 intake averaged from 2600rpm to 5600rpm. It won't clear the factory firebird hood without a dropped air cleaner, which will likely cost you more than 3 hp. So toss the performer, performer plus, torker 1 & 2 manifolds away if you are under 500HP.
I tested a 1968 RA2 Quadrajet (750) that had the shafts bushed, a complete rebuild with a Cliffs performance kit, and had the air valve, jets and rods optimized to allow more fuel. The carb was optimized using Cliffs book, and it ran great. The carb ran perfectly on my car, I had driven it about 600 miles after the rebuild with no issues. In fact, I though it was greatly improved over stock. If you recalibrate the idle bleeds, jetting, rods, align the blades for full throttle, and do all of the tricks in Cliff Ruggles book, it will run fantastic. However,...
The dyno test on the stock manifold had the Rochester at 394.63 hp. A 4165 Holley 800 CFM carb (mod 6212), out of the box stock in every way, made 490.3 hp on the same factory intake 2 minutes after the other run with nothing but the carb changed. I was shocked, as was the dyno operator.
The factory intake is a spread bore design. Use a spread bore carb (4165), as the adapter to a square bore carb has been known to cost 15hp.
That dyno taught us some amazing things. Some are thinking, well that wasn't fair, the Quadrajet is rated for 750cfm, and the Holley 800 cfm. While the engine was making 490.3 hp, it was consuming 605cfm of air per the dyno hat.
I agree it sounds crazy. Thank god for the dyno as the plan was to run the quadrajet! The even crazier thing is I am making 490hp 577 tq with no trick or fancy stuff. No porting, no trick anything. I haven't even rejetted the carb, or adjusted the pump shots. Idle is 750, very stocking, vacume 15 inches. 35 degrees of timing. Stock hei. It seems like it took a year of reading to sort the bs from what works. Mail order, assemble. Toss the aftermarket intakes, distributors, porting of heads, ect.
I mean absolutely no offense by this. So, if anybody takes it that way, your wrong !
BUT, I don't believe it !
Think about what you're saying. You're expecting us to believe that your power increased by 100hp, with nothing more than a change from a well prepared & tuned 750 Q-jet, to a box stock Holley spread bore 800.
There are lots of 750 Q-jets running low 9's & even high 8's in Super Stock legal drag cars. These cars are making well over 500hp. IF a 750 Q-jet was that restrictive, they would never run that quick.
I suspect a dyno results error. OR, something wasn't right about the Q-jet. If a spread bore Holley is that much better than a Q-jet, it is the best kept secret in the history of hot rodding. As long as they have been around, somebody would have discovered it, either on a dyno or a drag strip.
Did you report these dyno results to Cliff Ruggles ? I'd LOVE to hear his opinion !
I do know that some of the higher hp BBC engines came with a square bore Holley. I think the most popular one was a 3310-1, which was called a "780".
"...I am making 490hp 577 tq with no trick or fancy stuff. No porting, no trick anything..."
Just for kicks, list all the specs for your 490hp engine. A Pontiac engine with no porting or trick anything, don't make 490hp.
Here's a typical pump gas 455 build, that is said to make aprox 425hp. I suppose if it was to use your Holley carb, it would make well over 500hp, with no other changes ?
FYI: My pump gas, roller cam 455 (462) , with an OEM rotating assembly, but Edelbrock heads, plain Performer (not RPM) intake, and a E-brock 1901 Q-jet replacement carb made 489 hp on the dyno.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
I don't want to offend anyone either, but I agree, it's pretty unbelievable that carb swap could change the HP output that much. Sounds like the dyno went way out of calibration while the carbs were being swapped. I swapped a 4150 onto my 464 ci while on the dyno and gained about 7 HP over the EPS800, probably would have gained another 7 if I put on a Q-jet.
I have custom made pistons, forged H beam rods, steel crank, ported high flow aluminium heads. ported high rise, matched exhaust, roller cam and rockers, and I make just over what you have with a basic stock engine and a carb swap? I know one dyno differs from the next, could get 500 on dyno 'A' and 535 on dyno 'B', but 100HP change with a box stock holley? I hope when Oldskool wins the lottery and has lots of money, he sends me one of those carbs as well. I'd like an easy way to move up to 635HP.
Sounds like something got bumped or tweeked on the dyno calibration.
No offense intended, just my thoughts on those numbers.
Funny. These days I try not to tell somebody, at say a car show, that they are wrong about what they "know" about their car because in the end they just write me off [censored] an A-hole or somebody that just doesn't know what he's talking about. But I have to agree that something was way off on those Dyno numbers. Unless something was bad wrong with the qjet and something very right with the Holley there should not be that much of a difference. Did the OP say that he built the qjet to Cliff's recipe or that he had Cliff build it? I can't remember what intakes were used but the Holley would've required a spacer to mount on a spread bore intake. Might account for a few ponies. A lot of the time the first pull doesn't read as high as the second or third but usually not that much. I think a fairly well put together 455 with a good carb and good exhaust flow could possibly make 490 on a generous engine Dyno. I could be wrong.
I have to agree that something was way off on those Dyno numbers. Unless something was bad wrong with the qjet and something very right with the Holley there should not be that much of a difference. Did the OP say that he built the qjet to Cliff's recipe or that he had Cliff build it? I can't remember what intakes were used but the Holley would've required a spacer to mount on a spread bore intake. Might account for a few ponies. A lot of the time the first pull doesn't read as high as the second or third but usually not that much. I think a fairly well put together 455 with a good carb and good exhaust flow could possibly make 490 on a generous engine Dyno. I could be wrong.
The carb was a somewhat rare Holley spreadbore designed as a direct Qjet replacement (no longer available new from Holley), so no spacer. Pretty sure if they were worth 100hp, Holley wouldn't have discontinued them.
Carb ratings are based on 1.5in-Hg for 4bbl (3.0in-Hg 2bbl) pressure drop as a standard. It is possible on a dyno to see <1in-Hg for the engine. Which makes the effective flow rate for the carburetor smaller. You'd have to know the pressure drop during the run, to determine if the smaller Q-jet was a possible restriction. Still I wouldn't expect more than 10% between the two. And even if the tune was dead on for each, the most you would expect is roughly 10% difference (for exact same cfm carb) just due to lambda being off.
So, either it was: a combination of air/fuel + cfm difference; a dyno input value was fat-fingered between runs; the throttle linkage wasn't allowing 100% WOT; or (and I think the most likely) the airvalve on the Qjet wasn't opening properly.