[70GS455]

Ran across this article from David Vizard's "How to Build Horsepower" Vol 2. They discuss modifications on a Quadrajet to get to 1090 CFM, as measured by a flow bench.

[1968GTO421]

Interesting read. Wonder what Cliff's observation are on this. Thanks for posting the article!

[steve25]

Yes, it can be done but your standard 750 Q jet can produce 580 Hp with ease .

Simple tweaks to get it up to even just 800 cfm will allow over 600 Hp to be made, but even at the 600 Hp level the Q jet is hard pressed to deliver the needed amount of fuel for that 600 Hp level no less the power that could be made with over 1000 cfm.

[RocktimusPryme]

I know they can be supplied with larger needle and seat assemblies, but at a certain point of airflow I wonder how much any single needle/seat and fuel bowl will struggle with at WOT.

[65 Lamnas]

Originally Posted by RocktimusPryme

I know they can be supplied with larger needle and seat assemblies, but at a certain point of airflow I wonder how much any single needle/seat and fuel bowl will struggle with at WOT.

I'm not sure they "struggle" as long as you can keep the fuel bowl full. When you're increasing "flow area" of the N&S from .135" to .145" or more, that's a relatively huge increase in flow capability. One of the biggest challenges is to control the fuel "slosh" when you're hooking up in first gear during the initial launch.

[Skip Fix]

Originally Posted by 65 Lamnas

I'm not sure they "struggle" as long as you can keep the fuel bowl full. When you're increasing "flow area" of the N&S from .135" to .145" or more, that's a relatively huge increase in flow capability. One of the biggest challenges is to control the fuel "slosh" when you're hooking up in first gear during the initial launch.

or blowing the needle off the seat as the float leverage is hard to hold that much pressure with the larger needle.

[VCho455]

Back in the day GM and most others measured carburetor CFM at 24 inches of vacuum. That standard is pretty much lost in the sands of time today. I'm suprised Mr Visard didn't print the inches of vacuum used in the testing.

Years ago we modified an 800 CFM and tested it on Dave Bishops flow bench and if I remember correctly it flowed 980. I however do not recall at what vacuum we tested it at. But after all the work it didn't make the car faster than the 73 SD carburetor we were running. The car ran 11.20's and pulled 460 HP several times on chassis dynos.

[steve25]

With any motor the bottom line is that at full throttle , max rpm and under load a vacuum gauge will tell you if you need a higher flowing carb(s) or not.

Also too large of a carb can undue what was good fuel distribution to the cylinders.

[Half-Inch Stud]

Q-JET my favorite carb, until the 1050 DOM presented better results across the full performance spectrum due to 2 needles and seats.

I really tink someone ought re-pop the Q-JET Body for 2 needles and seats with a bigger better fuel bowl. Body with Simplified-reduced "features".

Re-popped Top and Base plates could be interesting but optional.

[steve25]

GM did not make the Q jet Rochester mainly did and then Carter for a while.

The 4 bbl test pressure has always been 1.5 hg which converts to 20.4 inches of water, not 24 inches of water.

When David Vizards residence was CA he had a super flow 300 bench and around 2016 David had a Flowlab 520, when he moved to the east coast at some point he got a SF600.

Note the none of these 3 flow benches have the capacity to test a good size carb directly at 20.4 inches or 1.5 hg, which ever you prefer.

Testing with these benches would have called for testing at a lower pressure drop and then making the math conversion up to 20.4 inches, or the carb could have been flow tested by means of 1 or 2 barrels at a time and then added up, which can be misleading as compared to all 4 barrels drawing in at the same time around the air horn of the carb.

Here are the specs for a SF 300 and 600, from what I was lucky enough to still dig up on line about the Flow Lab 520 it had a capacity of about 450 cfm@ 28”.

A SF 600 could come very close to testing out a 600 cfm rated carb @ 1.5 hg with the primary’s and the secondary’s wide open.

[VCho455]

Originally Posted by steve25

GM did not make the Q jet Rochester mainly did and then Carter for a while.

The 4 bbl test pressure has always been 1.5 hg which converts to 20.4 inches of water, not 24 inches of water.

When David Vizards residence was CA he had a super flow 300 bench and around 2016 David had a Flowlab 520, when he moved to the east coast at some point he got a SF600.

Note the none of these 3 flow benches have the capacity to test a good size carb directly at 20.4 inches or 1.5 hg, which ever you prefer.

Testing with these benches would have called for testing at a lower pressure drop and then making the math conversion up to 20.4 inches, or the carb could have been flow tested by means of 1 or 2 barrels at a time and then added up, which can be misleading as compared to all 4 barrels drawing in at the same time around the air horn of the carb.

Here are the specs for a SF 300 and 600, from what I was lucky enough to still dig up on line about the Flow Lab 520 it had a capacity of about 450 cfm@ 28”.

Dhoo! You are correct 20.4! Thanks for the correction.
I wanted to point out the devil is in the details with these kind of claims. But I did a poor job of it by not getting my details correct.

[65 Lamnas]

Originally Posted by Half-Inch Stud

Q-JET my favorite carb, until the 1050 DOM presented better results across the full performance spectrum due to 2 needles and seats.

I really tink someone ought re-pop the Q-JET Body for 2 needles and seats with a bigger better fuel bowl. Body with Simplified-reduced "features".

Re-popped Top and Base plates could be interesting but optional.

Stay tuned. There's someone working on optimizing this area of the Q-jet. He works on it in his spare time and knowing him...it'll be a proven and worthwhile upgrade when completed.

[Stan Weiss]

Originally Posted by steve25

GM did not make the Q jet Rochester mainly did and then Carter for a while.

The 4 bbl test pressure has always been 1.5 hg which converts to 20.4 inches of water, not 24 inches of water.

When David Vizards residence was CA he had a super flow 300 bench and around 2016 David had a Flowlab 520, when he moved to the east coast at some point he got a SF600.

Note the none of these 3 flow benches have the capacity to test a good size carb directly at 20.4 inches or 1.5 hg, which ever you prefer.

Testing with these benches would have called for testing at a lower pressure drop and then making the math conversion up to 20.4 inches, or the carb could have been flow tested by means of 1 or 2 barrels at a time and then added up, which can be misleading as compared to all 4 barrels drawing in at the same time around the air horn of the carb.

Here are the specs for a SF 300 and 600, from what I was lucky enough to still dig up on line about the Flow Lab 520 it had a capacity of about 450 cfm@ 28”.

A SF 600 could come very close to testing out a 600 cfm rated carb @ 1.5 hg with the primary’s and the secondary’s wide open.

Steve,
When I first contacted / meet David over 20 years ago, he was already living in NC.

Stan

[steve25]

Stan thanks for nailing it down better.
I was trying to judge a time line by pictures in articles and books and when they where printed and or published.




No problem VCho455, it’s just that it makes a big difference in accuracy.

[Pav8427]

Interesting read. I can see how the concept of mods could increase total flow.
Even just some would help and may apply to building a sleeper Quad.
Looks like that was a partial of the whole story.
Anyone know if there is any testing of these mods?
One thing crossed my mind. Some of the SD carbs dont have the outer booster and Cliffs book shows the mods to remove. And yes it helps but hurts so to speak.
Has anyone ever tried or seen where there was any attempt to remove the inner booster?
I wonder what positive effect(if any)that would have.

[Pav8427]

Interesting read. I can see how the concept of mods could increase total flow.
Even just some would help and may apply to building a sleeper Quad.
Looks like that was a partial of the whole story.
Anyone know if there is any testing of these mods?
One thing crossed my mind. Some of the SD carbs dont have the outer booster and Cliffs book shows the mods to remove. And yes it helps but hurts so to speak.
Has anyone ever tried or seen where there was any attempt to remove the inner booster?
I wonder what positive effect(if any)that would have.

[jonmachota78]

I could not get any richer than 13.2 at 6.5lbs of pressure no matter what i did. Thats when i tried an 850 double pump and got the AF down to 12.5 and picked up 3 tenths

[RocktimusPryme]

Originally Posted by jonmachota78

I could not get any richer than 13.2 at 6.5lbs of pressure no matter what i did. Thats when i tried an 850 double pump and got the AF down to 12.5 and picked up 3 tenths

I wonder how much of that is fuel system?

I had no issue getting 12.5-12.8 out of the box with an SMI built Qjet (which many people say is inferior but I had very positive results with). I did run more fuel pressure. I feel like I ran 7.5-8 psi and I may have even turned it up a smidge higher at the track. I don't recall for sure. I was told with the larger than stock needle/seat you can run a little more pressure.

That said I struggled to keep fuel to it all the way down the track no matter how I did my mechanical pump setup. Once I switched to a tanks inc in tank setup the problems went away.

[jonmachota78]

Sumped tank. Return style regulator. Pressure gage i can see through the windshield, never drops below 6.5lbs

[jonmachota78]

I just was not willing to turn the pressure up on a mostly street car. So self imposed limitation i guess.