MY problem is poor gas mileage. I have a '76 vet with a ZZ4 crate engine(355 hp) hooked to a 700 R4 transmission with a 3.08 rear end. My current carburetor is a Holly 750 CFM. I've visited the Holly web site and their online calculator suggest carbs from a 600 CFM to a 670 avenger. One of their techs recommended the 670 avenger. I don't want to lose performance, but I would like to get better gas mileage. I would think the 600 would give better mileage, but wonder how much peformance would be reduced. Any suggestions would be appreciated.

PRIrish

 

The best and quickest way to do what you want is swap me for my
L-48.I have a turbo 400 3.08 gear with 164HP that should take care of it. the only other way would be to not drive it (no Way).
Seriously I dont think you will see that better of gas milage by going to just 600 or 670 possible.But not likley.Call a local speed shop. Jegs??

 

What kind of mileage are you getting?

 

Be careful with over-gassing. You run a chance of washing the rings too (like I did in my 383 stroker) in my 69. The previous owner was of the impression that bigger is better in all cases. I wondered why a brand new engine still smoked after about 3000 miles on it. My gas mileage was also horendous but I just figured it was due to the added performance mods. Only difference was that I had an adjustable fuel pump on it too that was set waaaaaay to high.

Fortunately I recieved the original 350/300 with the car which is now what I'm running.

During my searches for my problem, I was also told that re-jetting may help with the fuel dumping but I'm not familiar with that process and to be honest, I didn't want to mess with it because I knew I was going to the original motor again.

 

On my OEM/rebuilt mild cam L48 in my '72 Muncie 336 car....with the OEM Qjet that seemed to be running fine...the car got about 12-13 around town and maybe 14 on the highway.....

I put on a L98 TPI speed density setup....closed loop and the oval K/N cleaner for cheep in front...serp drive, electric fans....

went to 24 mph at 80 indicated (probably only 70) on the freeways from DC to Florida in early '97.....

here in Florida, having since moved here, I put in a cammed up 350 roller engine, and so went down to 18 mpg at most, even with overdrive lockup automatic....

watch that cam and the HP there is your killer....

 

Need to start with all the facts.

 

700r4

 

Get EFI.....

 

Bluthundr, I got 13 MPG driving from Chicago to Texas, and I'm getting about 10 MPG driving around town. Holly suggested I check the spark plugs and if black, change the jets from 71 to 69 on the 750 CFM. Going to a Holly 670 CFM Avenger might help performance, but I don't think it would help with MPG. I would think a Holly 600 CFM would improve GPM, but it might also hurt performance. One might have to choose between MPG and performance. I don't know if it is worth the money to find out. Does anyone have an Avenger or 650 on a similiar set up as mine?

 

Ditch the over the top carb and get a 650 CFM carb. Map your timing per the timing sticky at the top. Do a vacuum check and a leak down check to be sure all is well.

Then, take the car to a dyno and get your fuel/air ratio from idle to max hp. If the fuel/air falls within the acceptable range live with it. If not you can tweek the rods and jets to get it correct. This is from my Edelbrock carb manual:

A/F RATIO CHARACTERISTICS
5 RICH BURN LIMIT: Combustion is weak/erratic.
6-9 EXTREMELY RICH: Black smoke and low power.
10-11 VERY RICH: Some supercharged engines run in this range at full power as a
means of controlling detonation.
12-13 RICH: Best power A/F: Un-supercharged WOT.
14-15 CHEMICALLY IDEAL: At 14.6 the A/F is at the theoretical ideal ratio with no
excess fuel or oxygen after combustion. Good A/F for part throttle cruise and light to moderate acceleration.
16-17 LEAN: Best economy A/F ratio. Borderline for part throttle drivability (worse than borderline if EGR is used).
18-19 VERY LEAN: Usual lean limit (Driveability).
20-25 LEAN BURN LIMIT: Varies with engine and system.

Even though engines will run anywhere between 5 and 25 A/F, the usual target values for an unsupercharged engine are a fairly narrow range (Figure 1). A/F is about 12.5 for the WOT and 14.0- 15.5 at part-throttle cruise. An intermediate value of about 13.5-14.0 is usually used for mid-range power (non-WOT acceleration).

 

At the VERY LEAST get rid of the Holley. You'll never get ~any~ kinda mileage but bad, using a Holley carb.