Reading a number of other posts about poor gas mileage with 460's made me start thinking about it a bit. I have a pretty quiet exhaust on my truck so I can hear things happening. My motor is a blueprinted 1976 460, rebuilt with stock pieces, but a Competition Cams RV cam and Hedman headers. The motor was built to tow easily and forever.

Anyways, I am running a 600 Vac secondary Holley on it. The previous owner has the carb leaned out to meet B.C. Aircare emmisions and it is soooooo lean that it staggers almost all the time until it is very warm, when you tip into the throttles. Like everybody else I am trying to get better fuel economy out of it when I am towing.

What I have noticed is that when I am in steady state cruise and then hit a slope I can hear the increased drone as the secondaries open to meet the increased demand. When I am towing, I expect that I am on the secondaries a fair amount of the time, even with smaller primary throttle bores due to the increased loads . So what I was wondering is the effect of controlled throttle opening and uncontrolled secondary opening due to the 600 being too small for the 460.

Has anybody done back to back on vacuum 600 versus 700+ cfm square bore carbs. I would think that the larger primaries would allow the motor to stay on the primaries more and for longer. Just a thought and I understand that the motor will demand a certain amount of airflow based on vacuum and load, but I know that I tend to use the throttle harder to get the performance I require to tow. Is a slightly bigger vacuum secondary carb better to use. Maybe a spreadbore like a.... cough, cough Quadrajet, cough, cough may be a better solution between power and economy.

Just wondering........... Opinions?

Eric

 

Your therory works on larger torquey motors becouse it allows you to run longer on the primarys, and being able to run leaner till full power is needed.

There is also something called venturi effect. Torque motors with small carbs have a tendecy to run rich becouse they will pull extra fuel in.

I ran into this with a 302 roller with ported heads, and a B303 cam. I used a 350cfm 2 barrel, and the plugs were always blacker then I liked. Plus the mpgs were a bit lower then I would be expected, and compared to a similar motor/car a friend had. So when I got done racing the car for money I swapped on a 600 vac sec 4 barrel and got a few mpgs back.

All I have said kind of assumes you have the right bolt-ons for economy, and your gearing is right for the motor/workload.

 

Have you looked into any carb mods Holley offers? Changing a spring or some jets may get you the performance you're looking for. I currently run a 600 Holley on my 351m changed from a q-jet, both work well but i am now able to adjust easier.

 

You are correct in your thinking.. I use a modified 800 cfm quadrajet just for the reasons you stated... It takes less primary throttle angle to maintain steady speeds when loaded... The 800 cfm q-jets 1 7/32" Primaries = 287 cfm according to Doug Roes book... Boats and Towing applications benefit the most from the larger primary carbs... Suggestion: Use a vacuum gauge hooked to a manifold vacuum source so you can actually read the Hg value while driving. http://www.jcwhitney.com/miles-per-g...p2008961.jcwx#