We're cool.

 

It's all about the Vette's baby!

Jebby

 

All this nonsense goes back to that worthless sizing guide Holley published a hundred years ago. A 750 is a good size for a 350.

 

The original should be an 800 cfm modified 4150. Should have center floats.
If it is the gm installed holley with the gm numbers i would definitely rebuild it.

 

Thats based on that silly thing called math based on the engine size and flow.

We all know a 750 cfm carb will allow more air and fuel flow at full throttle than a 600 or 650cfm carb and give the engine better top end performance and if the engine is built to be able to utilize that additional flow that then yeah a bigger carb will perform well at all rpms... Some bigger carbs were tuned and jetted down to perform like a smaller one... Simply bolting a 750 to a stock 350 will normally give poor mushy throttle response and poor drivability/ and mileage for maybe 5-10 hp at full throttle. (the exception being a carb jetted and tuned to the engine like the quadrajet)

A poster earlier in this thread mentioned he was able to get good performance out of an out of the box avanger 670 with #68 primary jets and a #31 squirter on his lt1 ... This was WAAY too lean for my mild 355 build (and many performance 350 engines for that matter)...HMMM makes one wonder how much cfm those stock headed lt1 engines really need? the 670cfm avenger is jetted very lean from holley. I had a horrible stumble and white plugs until I rejetted.

 

I think I agree with most of the above.

I have a 670 Street Avenger on my 400hp 383. Out of the box the carb was way too lean.

I went up 4 sizes on primary and secondary, and put in 37 squirter with a hollow screw and nozzles.

What a difference!!!

You really have to decide what the car is going to be driven most for. Very few of us ever drive at WOT.

The average 350 chevy engine, even modified, will rarely, if ever, require more than 650 cfm. And in some cases more will degrade performance. Lower vacuum, less draw of fuel resulting in lean conditions, stumbles and more.

The LT1 motor was designed as a race motor and only lasted a couple years in original form. It was just too much small block for the average Joe on the street. And then, of course, uncle sam, gas wars, insurance, etc etc.

Fact is, 800 cfm is probably way too big for joe businessman, but maybe not for a car destined for the track.

The LT1 is a great original classic and as such I would keep that baby bone stock.

I like vacuum secondaries on my street car as the motor will only take as much as it needs.

Mechanical double pumpers are wonderful carbs, just a bit much for me. I'm a weekend warrior and drive the car regularly.

You can definitely, and most guys do, over carb your motor.

 

a 750 will allow more air to flow, IF the engine requires it. it will only allow additional fuel to flow if larger jets are installed. a 750 is rated for air flow with a certain size venturi. the fuel aspect is metered separately, which is why a 650 is a better match. sure you can make a 750 work with proper jetting and tuning, but the ratio and overlap of the circuits is narrower in a 650 and easier to tune for most street 350's.

Holley's guide is just that, a guide, to help you get started. it is not worthless, it's just not a bolt and go type of thing. carbs have characteristics for different driving situations and make compromises in many areas.

(you guys should work on motorcycles, they have the same arguments)

a carb has to draw enough fuel for low rpm starting where air velocity is low. then it has to idle smoothly without bogging. then it has to transition into part-throttle. then mid-throttle cruising, then wide open, then back again, low vacuum, high vacuum, all without going to lean, too rich, bogging, popping, stalling, etc. the fact that carburetors have achieved any efficiency at all is amazing in and of itself.

so if you understand how they work, and that a larger cab will take longer to gain enough flow through the venturi to get the proper fuel mix, you see why guys have trouble with these, and end up going to great lengths to eliminate the bog, but destroying their low rpm and idle ability, and end up fouling plugs, hard starts, etc...

 

This correct Holley carb was on my '72 LT-1 with A/C. After I had it rebuilt and finished it ran perfect on that original motor.



1972 LT-1 with factory A/C (1 of 240)

 

Curious, with what size jets??

 

I also went 4 sizes up in jetting... and installed a #37 squirter...

 

bigger jets will supply more fuel regardless... and larger carbs can result in less atomized fuel suspended in the air since the velocity is decreased... basically the same issues as too larger of runners on the heads for an engine or exhaust for that matter... things work bet if the match up for the correct range of both air and fuel flow. Some only pay attention to the better WOT power output and some just dont comprehend how it all works so the bigger WOT numbers = better choice... Just like over camming a motor to make its power at 6800rpms when it may never utilize it due to the gearing of the car... all boils down to bigger power numbers= always better for some.

 

That sur is pretty!!!!!

 

I forget what size jets it had. But I kept it stock original. I sold it just before the d-i-v-o-r-c-e. It went from NJ to Phoenix, AZ, where the new owner bought an identical twin to this one. I restored that engine compartment to show quality, and just finished the one on my '66 427 convertible that my brother bought new. So I hang out in the C2 area now! But I've owned two LT-1's and love 'em.





1972 Corvette LT-1 with air conditioning. Fully loaded. Medium saddle interior. Matching numbers.

 

Whether it be selecting cams or carburetors or pistons or cylinder heads its always better to err on the side of caution. Street engines typically run between 550 rpm and 3500 rpm and everything in between so its the smart engine builder who takes that all into account.