Freds65

I need some info help!
Will a Holly 750 carb off a GTO
Work on my 327 Corvette engine?
Thanks
Freds65

warrenmj

A 750 is probably too much carb for that engine.

Freds65

It's not a stock 327, it has 400hp

Is the carb interchangable

Nowhere Man

A GTO never had that much of a Holley. What intake is on both. What model Holley. And even for a 400hp SB that's probbly still to much carb

erb64

Don't do it. Had one on my 327 with 425 hp, go a bit smaller.

ghostrider20

You will be much happier with the throttle response from a 600 CFM carb, even at 400 HP.

LouieM

That was too big for my 400hp 327 once upon a time.

Kerrmudgeon

It all depends if your small block is built or stock. I had a 750 double pumper on my built 331 years ago, and it was perfect for the job. Yeha!

If it's stock, yes, it's a bit too much for the motor. Even bigblocks came from the factory with 600s back in the day.

Easy Rhino

Even at 400hp, throttle tip in for a 750cfm on a 327 will likely make you unhappy, and forget stomping on it, it will almost surely bog badly. Too much flow, unless your heads are bench flow tested and are way up there, but 400hp says that they're probably not.

GUSTO14

Yes, it is interchangeable. With the possibility of some jet changes, it could even perform quite well on a modified 327. On the other hand it will not likely ever get good gas mileage.

Does it have mechanical or vacuum secondaries? That will play a major role in its suitability on a 327.

Good luck... GUSTO

Frankie the Fink

I don't know why folks want to overcarb engines so badly. More is not always better. I ran a 680 Quick Fuel on my SS-396 'built out' to 425 hp. It was a perfect street combo (with vacuum secondaries).

vettsplit 63

My 68 Chevelle SS396 had the L78 engine rated at 375 HP in a Chevelle. Same engine was rated at 425 HP in an Impala. It had a 780 Holley, vacuum secondaries. Performance was great. Gas mileage-- not so much.

65tripleblack

Some of you guys don't know what you're talking about.

To the OP: if you have a stock intake manifold on your 327, then the throttle plates on the 750 are too big for the bores in the stock 327 intake.

I have a 750 double pumper in my 327 and throttle response is instantaneous. No bog. I can floor it from a dead stop and all I get is wheelspin and tire smoke. If the carburetor is set up and jetted properly, and timing is also set properly, then your engine will be very happy.

The 750 carburetor with its downleg boosters is much more versatile (and responsive) than a 600 with crappy straight-leg boosters.

PS: Gas mileage has NOTHING to do with the flow capacity of your carburetor. It has EVERYTHING to do with the way it's jetted.

tbarb

cubic inch x top rpm divided by 3456 equals CFM demand at 100% volumetric efficiency.

example, 327x6000=1962000 divided by 3456 = 568CFM. Now that's not to say that the carburetor does not need to be tuned to the engine.

Frankie the Fink

That's the common "rule of thumb" CFM calculation and a LOT depends on what is assumed about volumetric efficiency. 100% is not realistic in a street car.

And I guess the people that build the carbs don't know what they are talking about either. After extensive talks with the Quick Fuel technical folks they steered me off their larger carbs for a multitude of reasons.

Having said that, my dual-quad 283ci has a combined CFM of 384cfm per carb (IIRC) X 2....so figure 768cfm at WOT. HOWEVER, the ingenious flapper secondaries control flow rate so its sort of a self-tuning set up. Works great.

vettsplit 63

Tripleback, your post reminded me-- I built a little flat top motor for my son's 66 Nova, 327- 750 double pumper with a Chevy aluminum intake, and a set of iron dart II heads. 280 Comp cams hydraulic. Nothin special, but it was as you say. Instantaneous! The only thing I remember was having to run a 60cc secondary pump instead of the 30.

65tripleblack

Theory does not always apply to real life. That equation is all well-and-good, but doesn't explain how and why torque and power both increase with a bigger carburetor.
Manifold vacuum at WOT is lower for a longer period with a larger carburetor, and contributes to higher power output.
Here is an excerpt which should clear it up for you. Pay particular attention to the sections that I have underlined:

Formulas Are Useless!”
Textbooks tell us you can accurately select the proper carburetor size based on a relatively straightforward formula that takes into account engine displacement, max rpm, and volumetric efficiency (VE).
Plugging some actual numbers into the equation, what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE ?
607.6 cfm?! Virtually no one —whether the original equipment manufacturer, aftermarket tuner, or racer— actually installs such a small carb on a high-performance 350. In the real world, everyone knows these engines make more power with larger carbs. Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs. The equation breaks down as a realistic carburetor size selection tool because carburetor flow ratings (in cfm, or cubic feet/minute) are taken at an arbitrary vacuum drop—3.0-inches Hg for two-barrels; 1.5-inches Hg for four-barrels —and there'’s no guarantee that, at max-rpm wide-open throttle (WOT), any given engine actually sees the theoretical vacuum drop for which the carburetor is rated!
Pulling Vacuum Sucks
On the other hand, if a carb really does pull 1.5-inch-Hg (or more) vacuum at WOT, it has become a restriction. The carb is actually too small to let the engine realize its maximum power potential. That’s because the greater the pressure drop (the higher the vacuum reading) across or through the carburetor, the lower the air density is inside the intake manifold and combustion chamber. Racers like to see no greater than 0.5 to 0.75-inch-Hg vacuum on the top end. But race cars have high-stall converters and steep rearend gears, and most of them are lighter than the average street car. Racers don’t mind recalibrating the carb on the spot for changing track conditions. They are unconcerned about low-end driveability. That’s why all-out racers, when not restricted by the rules, run huge carburetors—the bigger the carburetor, the lower the pressure drop across it at any given airflow.


As further proof that you theoretical equation is flawed: the fact that both the LT1 and the Z28 came factory equipped with a 750 CFM carb..........the most versatile carb in the Holley/Holley clone arsenal.

Finally: downleg boosters are much more sensitive to venturi signal (vacuum) than straight leg boosters because the downleg places them in a lower pressure region of the venturi. Thus, low speed throttle response is actually better on a 750 than on a 585, 600 or 650, all of which use a straight leg booster.

65tripleblack

Some people will never understand, and it's like talking to a wall. Especially the NCRS crowd and the " Oh God, sacrilege if you modify anything that was designed by the almighty engineers of the fifties and sixties" crowd.
The Q 750 that I installed on my 327 was very, very close........right out of the box. All I did was to drop 2 jet sizes on the pri and sec jets (this is a drag race carb, and was jetted on the rich side) from 74/84 to 72/82. The drop made a slight difference in responsiveness and fuel mileage. Everything else was perfect, including the pumps and nozzles.

Car gets well over 23 MPG with the 5 speed, and has virtually NO exhaust odor that so many people here complain about. Absolutely no gas fumes as well. Starts instantly in 30 to 100 degree weather, hot or cold engine. his carb has no choke or choke horn.
I'm done.
Knock yourselves out.

65tripleblack

He probably sensed that you were leaning toward the small carb. He's a salesman, after all, and didn't want to confuse you, but rather ease you into an easy sale.

Which 396 version did you have in your Chevelle?

Frankie the Fink

It was a 396ci 325hp modded to 425hp (estimate - never dynoed).
IIRC the first generation Z-28s had 780cfm Holleys.