Holley 4175 vs. Rochester Q-Jet

[igasper10]

A few months back I replaced the stock Q-Jet carb in my 74 L48 with a new Holley 4175 650cfm I had. I don't think anything was wrong with the Q-Jet except for being dirty. There seems to be differing opinions on which carb is better. I know the Q-Jet has a bigger cfm rating than the Holley, but that doesn't mean it better. I'm only getting around 10 miles p/gal as is. The engine is pulling about 20in of vaccum, so I've got no leaks there. I jetted it to Holley's specs for a 74 L48 as well (#58 mains & #28 pump nozzle) If I would get better gas mileage with the Q-Jet I'd be willing to clean it up and reinstall it. Anyone done this before???

ian - ;help

 

[igasper10]

any thoughts???

 

[Tom73]

I would think the q-jet would get better mileage. It has the very small primaries which should help mileage. Also the engine was designed and tuned to run with a q-jet.

Performance wise, for the street, it is six of one or a half dozen of the other.


tom...

 

[vette-dude]

Is the 4175 Holley a square bore carb? If so did you use an adaptor or change intakes? An adaptor can have a negative effect with a Holley unless you are running at WOT all the time. I agree with Tom that the Q-jet is probably better for mileage and is just as potent for the street as the Holley. But, The Holley is more easily tuneable and can be modified with over the counter parts to be a better High Performance carb IMO. I have used both double pumpers and vacuum operated secondary Holleys on my street/drag cars and have been very satisfied with them. You do have to mess with them to get them dialed in but when you do they work great. The Qjets are pretty well "what you see is what you get" type carbs. They are okay but it depends what you want out of the engine performance.

Randy

 

[igasper10]

Is the 4175 Holley a square bore carb? If so did you use an adaptor or change intakes?
Randy

Its a spreadbore carb, no adapters needed. I'm just restoring the Vette to be a daily driver, so gas mileage is to me is #1. As I said, I'm getting about 10mpg with the Holley. Anyone know what the average mpg is for the Q-Jet? If its only couple more mpg I'll probably just stick with the Holley since its already installed & tuned.

ian

 

[JBWeaver]

Q-jet

I'm getting about 16mpg with my stock Q-jet, if I take it easy. Alum intake and heads, high flow cat and muffs.

 

[Tom73]

Anyone know what the average mpg is for the Q-Jet? If its only couple more mpg I'll probably just stick with the Holley since its already installed & tuned.

On my '73 L82 auto I get 13 mpg on the highway. Have never checked it around town. Don't think I really want to know

tom...

 

[Buckskin7T7V]

I also am using the 4175 Holley. I have a L-48 with 4-speed. I was wondering what color cam you are using on the accelerator pump and what hole position the screw is in? Also what color secondary vac spring? Did you stay with the 37 shooters on the primary side?