[scott70]

Isnt there some machining that can be done on a quadrajet to make the primarys bigger? I would think the torquey feeling between stock size primarys on a 750 cfm would feel noticeably greater with one that had its primarys opened up . When im not into the back barrels, the car is torquey just on the primarys but could be better I would think opened up. Maybe like make a 850 cfm quadrajet?? Is that possible? Is there enough material there to do that?

[Butcher]

Scott,
I never heard of anyone opening the primaries like your thinking, but you can get a 75 or later q-jet and cut out the outer booster rings. Doing this should give you around 830 cfm. Buy cliff ruggels book and he shows how to do it.
Butcher

[i82much]

Sawzall?

Sorry, had to.

[Blued and Painted]

You can adjust the link to the secondaries and change where thy cum in. But all the blade angle must again be checked/adjusted at WOT.

[Skip Fix]

Years ago Brad Urban's Carb Shop would put bigger butterflies in the primaries-maybe some Holley ones. Had one they did that to never ran as good as another same model ('73 SD 455 SR). Had Cliff go through it but never on an engine since then other than his before he shipped it back.

[scott70]

Originally Posted by Skip Fix

Years ago Brad Urban's Carb Shop would put bigger butterflies in the primaries-maybe some Holley ones. Had one they did that to never ran as good as another same model ('73 SD 455 SR). Had Cliff go through it but never on an engine since then other than his before he shipped it back.

Interesting,,,I would think that kind of mod would be a good thing,,,no other quad experts have attempted this and maybe got it to be a worth while modification? Im sure it would take some testing to make it right and not just make it bigger with oversize butterflies.

[Skip Fix]

They were not a whole lot bigger but are larger. I can measure tonight.

[Schurkey]

Originally Posted by scott70

Isnt there some machining that can be done on a quadrajet to make the primarys bigger? I would think the torquey feeling between stock size primarys on a 750 cfm would feel noticeably greater with one that had its primarys opened up . When im not into the back barrels, the car is torquey just on the primarys but could be better I would think opened up. Maybe like make a 850 cfm quadrajet?? Is that possible? Is there enough material there to do that?

Just get an 800 CFM Q-Jet. Work's already done.

First Guess: Your carb isn't properly tuned. Once it's tuned, you'll probably discover that you don't need more carb.

[scott70]

The carb runs beautiful,,,no complaints. Just would be nice to have a little more punch when only in the primarys. I would think some larger primarys would provide that,,,,thats all. And maybe a little more HP for someone that wants to run a qjet and needs or wants maybe 850 or so cfms.

[Skip Fix]

Mine as done to make an 800 cfm even larger.

FWIW using a wide band O2 the primaries affect mixture more than you would think based on their size. Dialing primary WOT in with the wideband can help a bunch

[Larry Navarro]

Just like Skip, Brad Urban did the same primary mods on the 750cfm Qjet that I used on my old SD clone trans am. He did awesome work.

[torqjunki]

It's past your bed time Larry!! Just put a Holley style carb on it and you can get that big power without opening the secondaries. I actually like the q- jet and there are plenty good ones out there, but if you want strong primaries often, just do a Holley IMHO. If your gonna be tipping in the secondaries often on the q jet, the mileage savings aren't gonna be there anyway. The weak primaries are why I don't care for them. For a cruiser they are money.

[76 poncho]

http://smicarburetor.com/products/sfID1/7/sfID2/9
900 cfm quadzilla check it out

[carbking]

It is not difficult to install larger primary throttle plates. There are lots of different carburetors out there with different size plates (and in a pinch, a specific size plate may be turned on a lathe, once one has fabricated the necessary form). The throttle bodies can be milled oversize to accomodate the oversize plates (pay close attention to the area of the idle discharge!!!).

But why??????

The oversize throttle plate will add virtually nothing to the airflow; as the main venturi diameter, and the mass of the venturi booster is the major factor in the airflow of the carburetor.

On carburetors with removable venturi boosters(ie Rochester 2G series), it is fairly easy to reconfigure the main venturii to increase airflow, but as the Q-Jet has fixed boosters, this modification, while possible, is somewhat difficult. Much easier to start with a larger carb (unless this is to be a "cheater" carb)!

Jon.

[Skip Fix]

Sean was working at Carb Shop when I had moine done probably learned that there.

Jon I agree with you that the venturi is probably the main "cork" to more flow, but there are not bigger than 800 cfm Qjets. "950" Holleys are basically a 750 venturi with 850 throttles.

[carbking]

As an addition to the above post, there IS a relationship of the maximum volume passed by the venturii to the maximum volume that will then pass through the throttle body. I don't know exactly when this was discovered, but the Stromberg engineers knew of it by 1935. However, this relationship MAY (or may not) have changed on more recent carbs due to booster design (however, most current booster designs had been tried by 1935).

Some using some of the repop popular hotrod carburetors from the 1930's have discovered the results of ignoring this relationship (although they may not know the cause).

The only real reason to get into the mathematics/expense of this is if one has a "numbers matching cheater carburetor".

Jon.

[carbking]

Originally Posted by Skip Fix

Sean was working at Carb Shop when I had moine done probably learned that there.

Jon I agree with you that the venturi is probably the main "cork" to more flow, but there are not bigger than 800 cfm Qjets. "950" Holleys are basically a 750 venturi with 850 throttles.

Skip - somewhere (don't ask me where, I don't remember) I have a flow test of the single booster 1971 Q-Jet. Memory says 839 CFM (memory is not perfect ). Of course, these things are pretty scarce and not inexpensive.

Staying with the spread-bore format; Carter offered TQ's in 800, 850, and 1000 CFM ratings.

Jon.

[ponyakr]

"...there are not bigger than 800 cfm Qjets..."



I think the Edelbrock Performer RPM was rated at 850 CFM.

http://www.carburetion.com/Rochester.asp

And these still turn up for sale occasionally. So this one and the already mentioned Quadzilla are 2 Q's which are available over 800cfm.

http://www.ebay.com/itm/1910-Edelbro...-/221795294707

http://smicarburetor.com/products/sf...9/productID/76

[Bruce Meyer]

Originally Posted by torqjunki

It's past your bed time Larry!! Just put a Holley style carb on it and you can get that big power without opening the secondaries. I actually like the q- jet and there are plenty good ones out there, but if you want strong primaries often, just do a Holley IMHO. If your gonna be tipping in the secondaries often on the q jet, the mileage savings aren't gonna be there anyway. The weak primaries are why I don't care for them. For a cruiser they are money.

I couldn't agree more. There is nothing like the throttle response of a square bore 850 dp on a built 455.

[carbking]

Originally Posted by ponyakr

"...there are not bigger than 800 cfm Qjets..."



I think the Edelbrock Performer RPM was rated at 850 CFM.

http://www.carburetion.com/Rochester.asp

And these still turn up for sale occasionally. So this one and the already mentioned Quadzilla are 2 Q's which are available over 800cfm.

http://www.ebay.com/itm/1910-Edelbro...-/221795294707

http://smicarburetor.com/products/sf...9/productID/76

CFM ratings vary with the criteria used to produce the ratings.

Mark Twain explained this well over a hundred years ago. "Figures don't lie, but liars figure"!

As as example:

The Holley 600 CFM is one "standard" against which many others are compared. One "later" company had no 600 CFM, and didn't wish to spend the R & D to develop one. Thus, they took a 625 CFM, and with absolutely no changes other than an identification number .......PRESTO, EUREKA, it is now rated as a 600 CFM!

To compare CFM ratings, one needs TWO numbers.....CFM and rating vacuum.

Jon.