Quadra jet CFM difference.

I'm trying to figure out the difference in the CFM between Quadra jet's from a 1968 350 compared to a 430. I have a 350 with no carb and a 430 carb with no engine. I know the later big block carbs Went up to 800 CFM but was not sure about the early ones

 

A few members should be chiming in soon.
I know the 1970 455 Q-jet was 750 cfm before they went 800 in 71.

 

I thought the 68 small and big block both used 750's. I think really the calibrations are different.

 

That's correct.. same carb, slightly different calibrations.

Stock engines, using the same fuel, require very little change when it comes to carburator jetting and air bleeds.

Not unlike a torque converter, the basic carburetor functions based on it's inputs... in a carb's case, it responds and meters fuel, based on airflow. It's dumb, it does not know if it's bolted onto a 350 or a 455.

Minor jetting and air bleed changes were made to "fine tune" the carb for load/ambient condition related issues, more than anything.

JW

 

Now, I'm no expert but I've tinkered with a few Q-Jets. The CFM rating was similar on a 350 and a 430 at 750CFM. I believe it was controlled by how much they allowed the rear flaps to open at WOT. I've seen some that weren't very close to vertical.

In addition, the 350 was jetted smaller and the secondary rods were larger (less fuel) since it didn't require as much fuel/air. I think the float was set slightly lower as well as internal passages being different sizes.

With all that, you should be able to tune a 430 Q-Jet to run on a 350 with some well-planned tweaks. Is the 350 warmed over any? Like headers or a mild cam? It might be pretty close as is.

Or, push the easy button and let one of the guys here rebuild it to your engine specs. There are at least two fine Q-Jet magicians here that do good work. CarManTX and Tech8

 

Just what makes the difference between the 750 and 800 The air horn internally only?
Both the primary's and secondary's are rated the same size.

 

The difference is on the primary side of the main body.



Devon

 

I've found that the 71 up 455 800s actually work great on 350s since the 455 calibration for emissions was quite lean it makes it just right for a stock or mildly modified 350 in terms of jetting and idle fuel availability. The 455s wanted more idle fuel than circuit could provide (even with screws all the way out never rich (for emissions) but enough for 350. You do have to tighten air valve some to avoid bog tho....

 

I'm always amazed at the wealth of knowledge on this board. Thank you all for the quick replies