horrible mpg, gears + cam the issue?

sixt5 · 08-03-2009, 12:20 PM

3.25 gear with edelbrock performer cam (idle to about 5500). at 65 mph im doin right at 3k (fourth gear, 1:1). Do you think with me revving that high that my 4 bbls are opened up? is there any way to get around that?

currently have 600 holley vac. secondaries. thought about going 570 street avenger?

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kalli · 08-03-2009, 01:25 PM

first of all fill the tank to brim, go for highway drive at 65mph for half hour and fill again. from that calculate gas mileage. how much is it?
570/600 cfm won't be much of a difference, depends on your tuning (carb and ignition).
the secondary is usually operated with vacuum against a spring. you an swap that spring to a stronger one. but maybe the carb is jetted to heavy, maybe your ignition not up to scratch or simply loosing fuel?

when you floor it and just as the 2ndaries open, if you have a bog there, then the spring is likely to be too weak

what exactly is your gas mileage at highway?

1971mach1 · 08-03-2009, 01:25 PM

What do you concider "horrible" (in mpgs)?

These cars with v8s and carbs are not noted for getting good milage.
Your setup is ment for performance not milage. I doubt changing to a 570 will help much.
A tuneup would help more and setup your carb so it is not running too rich.
Changing gears or transmission to lower your hiway RPMs will help alot, and your engine will last longer too.

nassaubayman · 08-03-2009, 01:48 PM

I have the same issue. I have no idea how fast I'm actually going because the previous owner didn't worry about changing the speedo gear when he installed the 3.70 gears. But I'm turning 3200 RPM at freeway speed.

I found an AOD out of a Mustang LX, 5.0. I think it was a 91 or 92. That should get my RPMs down to a reasonable 2200 or so at freeway speed.

Starfury · 08-03-2009, 03:10 PM

If you know the tire size, gear ratio, and rpm, there are online calculators to figure out speed.

disposablehero27 · 08-03-2009, 03:22 PM

i just intsalled 4.10's at 75mph, i'm hittin 2900rmps. sounds like the trans to me.

MBDiagMan · 08-03-2009, 03:35 PM

Assuming that you have a vacuum secondary carburetor, put a stiffer spring in the secondary diagram if you're worrying about the secondaries coming in. I REALLY doubt that they are.

Unless something about a particular car is way off, fuel mileage is determined more by vehicle weight and aerodynamics than most other mechanical issues. Driving style is the most important variable in fuel consumption.

Make sure that it's not running too rich and has enough ignition timing advance. A properly set up vacuum advance distributor will help in this area. The vacuum advance is nothing but an economy device that doesn't hurt anything under performance conditions.

1971mach1 · 08-03-2009, 06:01 PM

Quote:

Originally Posted by MBDiagMan

Unless something about a particular car is way off, fuel mileage is determined more by vehicle weight and aerodynamics than most other mechanical issues.
.

Disagree with the "weight" part.
Weight plays some part but it has MUCH more to do with engine size, efficiancy and gearing.

I'll give you a more extreme example:
Look at EPA estimates for:
2009 Chevy Traverse (3.6L) WEIGHT 4790lbs MPG 17/24
2009 Dodge Charger SRT8 (6.1L) WEIGHT 4160lbs MPG 13/19

sixt5 · 08-03-2009, 06:16 PM

i have changed the springs before, i put one of the lightest ones i could find. may have to change it

65rangoon · 08-03-2009, 06:52 PM

what do you consider horrible? you never mentioned.

i filled my car up to the brim (could see the gas in the filler neck) road around and did prolly 50-60 miles and then filled up again. i was not able to go to the same pump though although idk how much of a difference it would have made.

My 1965, all original engine, trans, rear end with a 2 barrel carb got 13 to 13.5 mpg.

That 50-60 miles was a mix of in town & highway driving.