View Full Version : Better gas mileage

03-04-2013, 10:56 PM
Well since no one is posting much lately here is a question. Im still running a 340 in my streetrod (newer rebuild with 2000 mi.) with a 5 speed Tremec TKO and a 389 ford 9" It has about 10.7 to 1 compression with edelbrock heads. Also running a Hughes 1928 flat tappet cam. My gas mileage sucks but then its a hot rod right. Well I am running a 700 double pumper Holley 4778 with mechanical secondaries. It runs great but mileage really sucks. Lucky if I get 10 mpg. Should I try a slightly smaller carb or one with vacuum secondaries for better mileage or just leave it alone? It really does scream for a 340 but thought I could maybe get a little better mileage out of it without sacrificing too much power.

03-05-2013, 03:47 PM
that holley dp carb will kill your milage,had a new 650 on my 340,great power but bad gas milage ,never hooked up choke in winter idled great at 30 deg.weather .switched to a Q-jet off a 350 1ton. pick up gas milage almost doubled and had more topend,but the holley was faster down the track,the fuel milage was so much better I thought the gas gauge was broke the first time out.have fun or just rejet it down.

Rich Kinsley
03-06-2013, 12:12 AM
Of course you have to watch your air fuel ratio or you'll fry the valves if you run it too lean. Running rich is safer. Getting it right is best. The vac sec's are given to better mpg. I am in the process of going from 2x4's, trick heads, 3:91 rear and nasty cam to a real gentle EFI set-up. I'm switching from rpm's to mpg's :-) Of course this requires all the above to be changed to make it work.

This is on my 318 poly - LeRoar Eventually to be Lemew

03-06-2013, 07:40 PM
Looks great Rich. Yes I am runnning a tad bit rich but figured that was better than too lean. Like I said its running well. I was thinking of trying a 670 street avenger. That probably won't help me with the gas mileage much though. I also was wondering if the Edelbrock AVS carbs are any good? I know they used a Carter AVS on 340's before they switched to Thermoquads. A friend bought a Dart GTS 4 speed from the factory with a AVS on it and it was awesome. It could give some 440's a run back then.

03-06-2013, 09:21 PM
If you decide on an AVS (good choice) get the larger one from a 440. I think they are rated at 750 cfm, and it will work great on your 340. You may have to open up the idle suction feeds to make it work with your cam, but they are attached to the booster nozzles and it's easy. Also, drilling a new hole in the accelerator pump lever, and enlarging the accelerator pump nozzle(drill) will give you the extra(length/travel/ shot) to make it really come alive when you mash it!

03-07-2013, 04:50 PM
Anyone had any experience with Edelbrocks AVS carbs?

03-07-2013, 06:30 PM
They (Edelbrock, Carter) require some work to get them matched nicely to any upgraded camshaft. And for that matter so does the Holley (for optimum performance, mileage/idle/off idle characteristics, etc.).

03-07-2013, 08:20 PM
How tall are your rear tires and what does RPM run at 60/ 70 mph?

"MPG" are you talking 10 idling around slow? Might be all you could expect

That cam doesn't sound very wild, though, what are you running for timing? You running a vacuum advance?

I used to like Carter, now Ed carbs, but this "new gas" is a challenge on hot shut--down.

What you REALLY need is a spread - bore design--really small primaries.

03-08-2013, 12:44 PM

You could always try redrilling your accelerator pump cams so they don't function below cruise rpm. If you do this you may have to feather it below cruise rpm or get a lean-out backfire, but it certainly teaches you how to go lightly on the mileage pedal.

It's a race car thing for keeping the pump shot available above the launch/staging rpm.

03-08-2013, 02:48 PM
I have an Edelbrock RPM Air Gap intake. (square bore intake) Edelbrock alluminum heads Rear tires are 28". No vacuum advance/mechanical secondaries. RPM at 70 is 2300.
I'm probably just dreaming about getting better than 10 mpg.

03-08-2013, 05:08 PM
I got 15+ with my six pack 357 at 60 - 65 mph about 2800 - 2900 rpm. You should be able to do WAY better!

03-13-2013, 01:10 AM
a vacuum carb and vacuum advance are the way to go on the street. every time you tickle the throttle, you have two pumps working. eliminating one will help, and at cruise, the more spark advance will help with gas mileage, ring wear, just an all around good deal. race setups are just that, and mileage is not a concern in those set ups.

03-20-2013, 08:49 PM
Larger diameter tires would give an itsy-bitsy improvement.
Higher gearing would help.
Modernizing with fuel injection.
Drive something else with high MPG.

03-24-2013, 08:40 PM
I'll be trying a thermo on my 340 this year. Should do better than my 750 DP.

03-24-2013, 11:25 PM
I tried a thermoquad once on the 340 and just could not get it to run right. I found a good used one and sent it to David Chavez (Demonsizzler) and he rebuilt it and set it up for my 340. I could not get it to idle down or run good. Sent it back to him to check and he said he couldn't find any problem with it so it must have been me. Had some friends try to get it right and they could not get it right either. I eventually sold it on Craigs list and bought the DP.

03-25-2013, 07:48 AM
Tq's can be very tricky. There are some passages that make tight bends that clog and are very hard to clean. Might take a couple of days soaking top and phenolic body in real carb cleaner. The idle passages are the worst. But, with the small primaries, mpg is much better than other carbs. Haven't heard anything from demonsizzler in a long time. Hope he is all right.

03-25-2013, 02:05 PM
I heard someone say he shut his business down. A lot of people used his sevices. I'm sure he will be missed.

03-25-2013, 07:56 PM
Yeah I got mine from him too. Runs great so I don't know.

Rich Kinsley
03-28-2013, 02:18 PM

I used to like Carter, now Ed carbs, but this "new gas" is a challenge on hot shut--down.
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Yes even the gas without ethanol is prone to quick evaporation it seems. Then when you try to start your car it has to crank forever to refill the fuel bowls.

There are two solutions that will at least help. Of course an electric fuel pump can be installed in addition to your stock mech unit. Put a toggle switch on it and use it to fill the carb bowls before you start it and if you are on the verge of vaporlock. This is a great idea for our old school cars with carbs.

Secondly reduce the temp in the carbs by installing a carb spacer made from an insulating material such as wood or phenolic. Forget the billet aluminum stuff as it's a great heat conductor. I lowered my carb temp from 180 deg to 110deg by making plywood spacers for my 2x4's. Also I did everything possible to keep the fuel cool before it got to the carb. In the one picture you can see how I rerouted the gas line and insulated it. I also made a heat shield for the fuel regulator. If you have a mech pump it's easy to build a heat shield for it and it's quite effective. With today's available fuels you have to try to restrict the heat or go EFI which is a sealed system.

03-28-2013, 11:01 PM
Fuel injection, no cold start/warm up issues better milage, better idle with low vac cams, I could never go back to a carb in my Dart Pulls way harder than carb ever did. Holley TBI 950 cfm on a 318 and it likes it. The 650 dp was a compromise to keep idle and driveability vs making power at the top end, would stop pulling at the top of second around 6000 and couldn't pull hardly to 5000 in third. The injection fixed all the issues.