quick fuel 750s

[Stangman]

Well bit the bullit and ordered the quick fuel 750s from Barry. So my question is I have holley 1850s 600cfm currently on there
and was wondering if there will be a performance gain. I understand it might not be a huge gain but I was hoping for something.
I know now I will have alot more adjustability to dial it in. The car ran well with the 600s on there but I wasnt sure if the motor wanted alittle more. Anyway alittle info 485 with 10.75 survival heads out of the box low riser 2x4 intake 67 mustang fastback
c-6 with 3000 stall 4.11 with 26 inch drag radials. Best 1/8 mile time 7.13 at 95.60-- Best 1/4 mile time 11.40 at 117 but wasnt running that well although I did get some things squared away at the reunion. Would love to get a BT intake to match the heads but until they are around will have to wait. 

[Drew Pojedinec]

To make a blanket statement, less pumping losses is better.

The question is does your engine require the added airflow?  Go to the track and find out
My gut feeling is you won't see much difference unless one set of carbs was really out of whack.

[Stangman]

I understand what your saying what Im saying is some motors run great with a certain carb lets say a 650 double pumper for instance, now throw a 750 double pumper and it might pick up 10-15 horsepower. I am also thinking ahead when I get a different manifold and possibly a bigger cam. But I just thought maybe these carbs would flow differently and maybe give me alittle extra. I dont think my carbs have a problem now.

[Barry_R]

I'll bet that it will pick up....

We will see soon enough.

 

[Posi67]

A pair of QF 600's on my 390 with a stock BT Intake and I doubt I need more carb. I've been tempted try my 660's but suspect they will just kill the plugs and not go any faster. You have more Cubes although I think the Intake may negate any gains the bigger carbs would otherwise show. Like Barry said though, you'll find out soon enough.

[cjshaker]

Wouldn't a vacuum check under WOT show if he needed more carb? If it's at or near zero vacuum, would bigger carbs show an improvement?

[Stangman]

Hey posi 67 are your 600s  stock out of the box or are they set up nice to your motor, mine are right out of the box, look my car runs well no hesitations at any throttle points this is something that I have wanted to do was just hoping I would get a little performance out of it. I think it will make more of a difference when I finally get my manifold and a little bump in cam.

[Drew Pojedinec]

Wouldn't a vacuum check under WOT show if he needed more carb? If it's at or near zero vacuum, would bigger carbs show an improvement?

Is that the only indicator for a carburetor doing it's job    

[cjshaker]

Wouldn't a vacuum check under WOT show if he needed more carb? If it's at or near zero vacuum, would bigger carbs show an improvement?

Is that the only indicator for a carburetor doing it's job

I just asked a question to learn. It does indicate whether or not there's enough airflow to feed the engine, but doesn't say if there's enough fuel getting through. I'm just as interested to see if it does make a difference, but I'm also interested if anyone has an answer/explanation to my question. Do you? I think the LR manifold is holding it back, as it is. Switching to a MR 2x4, like he mentioned he wanted to do int he future, would probably show a marked difference, and then the bigger carbs would probably come more into play, but I don't have the experience to say for sure.

[Stangman]

I’m thinking the same way you are Doug. I don’t have the intake and I don’t want to take the chance now to break in another solid cam with it running fine. I believe that the motor will only pull what it can out of the carbs being they are vacuum secondaries so I was able to get these for now. There were some rumors that some manifolds might be made so I can only keep my fingers crossed. Not sure myself how to figure out the proper CFM but I think that’s what’s good about the vacuum secondaries. I could be wrong wouldn’t be the first time just ask my wife.

[Drew Pojedinec]

For sure intake swap would be great.....  I'm of the mind that carb calibration and fuel curve for a particular setup is so much more important to the overall power made than the size of the carb or the vacuum at WOT.

I think about my friend/mentor who built circle track carbs of very small size, or SS cars that run 735cfm carbs.

If Stangman was spending a huge amount of time experimenting with the calibration of his 600's to get them absolutely perfect and then upgraded to the 750's it would be a better experiment.
He says 1850's were the old carbs, I can say from personal experience that those do ok, but without some serious work you cannot get the best from them, ie jets and power valves aren't sufficient to get the most out of them.

[Posi67]

My QF 600's are just old Holley's they converted and removed the Holley name. . Came with billet base plates, a jet plate in the secondary's and I was surprised that they left the power valve in the primary side. They were seriously over jetted but other than that I haven't changed or adjusted anything on them. If I was running on the street then I probably would need to do something but at this point it's all foot to the floor stuff. That and the fact I'm no Carb Guru it's best I leave well enough alone.

I think you'll be happy with the 750's. As mentioned, the Vac secondary will make your life easy. I've never understood the need/want for Double Pumpers on the street. My know it all neighbour thinks everything should get an 850 DP no matter the engine size.   

[Barry_R]

They have moved away from the Holley center sections since then....but they also do a lot of tweaking on the emulsion and bleed side of the equation.  Its not just the airflow and enough fuel - its also about the quality of the fuel and air mix being delivered, along with the linearity of the fuel curve through the RPM band.

When carbs are on the small side they tend to roll rich due the the stronger signal as the engine pulls harder on the carb.  QFT seems to be better at jet guesses now, but they still err on the rich side - probably safer than way.

The neighbor is wrong.  Every car needs a Dominator.  Many need two of them.

[Drew Pojedinec]

but they also do a lot of tweaking on the emulsion and bleed side of the equation.  Its not just the airflow and enough fuel - its also about the quality of the fuel and air mix being delivered, along with the linearity of the fuel curve through the RPM band.

Dang it.... again Mr R said it so much better than I, your eloquence is astounding as always

The dynamic I didn't want to see was Stangman going from untuned 1850's to custom Qf 750's and go "wow, 750's are so much better than 600's!"  when the problem was the delivery the whole time.  Kinda like folks that go from worn out steering in a Galaxie to rack and pinion and insist that it's the best.... having never experienced properly built factory steering.

[Barry_R]

Definitely agree.  Folks that need to work with small carbs due to rules, checkbooks, or application can get really, really good results out of them if they work the package.  Like so many things, it looks simple on the outside and folks tend to fixate on the easy to understand numbers such as airflow ratings.  Carbs are rated at 1.5" of vacuum drop below the plates - not because it reflects any particular operating condition, but because of the limits of airflow testing equipment back in the proverbial day.  A carb rated at a particular cfm will flow more if the pressure drop across the carb is higher than the rating value - but the fuel handling calibration needs to be altered to reflect the operational situation, and may sacrifice performance at lower airflow levels (part throttle...)