86 GMC PU with new crate 350 getting 8 MPG.

[dyno]

So I got my boy his first truck, 86 GMC 1/2 ton 4x4. Just put a new crate 350 with Performer intake, Headers, Edelbrock carb. No matter what we do, we can't get any better than 8.2 MPG. It has a 4 speed with 3:73 gears and running 35" tires. Hell, my old Suburban with a worn out 350 running 35's got 13 mpg!

Do these crate motors get better MPG after they are broken in and loosened up? We only have about 500 miles on he motor.
BTW, it has the stock cam.

[suxtobu65]

My experience has been the more miles a new motor gets. The better the economy gets. 500 miles isn't even broke in yet..

[geezer#99]

Timing!!
What's your timing set at?

[cadillac_al]

I usually get 11 mpg with my K10. I'm pretty sure I could get 8 mpg if I drove like a teenager. Who is driving this truck?

[kalbert]

Originally Posted by cadillac_al

I usually get 11 mpg with my K10. I'm pretty sure I could get 8 mpg if I drove like a teenager. Who is driving this truck?

Ha ha yep! When I was a teenager I got 6-8 mpg. Same truck, same motor, same driver +15 years, now I get 12-13. Its not hard to get less than 8 again, but why would I want to?

[jlk]

3.73:1 gears and 35" tires? Wow that's gotta be gutless especially on the hills.

[Your41Plague12]

Bump that up to 4.10's and you'll see results.

[andrewmp6]

Yeah with 35s you need a 4.10 or maybe a 4.88.

[johnwaynejeep]

There's a couple things that jump out at me.

1. how far is it being driven...cold motors suck down gas, if your sone is only driving a couple miles to school/work/home then this will efefct your mileage. Let the engine warm up before driving it and you'll get better gas mileage.

2. Timing...I would imagine if your timing is off you would see other indicators, but I'm not really sure about this one.

3. catalytic converter...is it equiped with a converter? If it is then make sure it's not clogged. A semi clogged cat will cause your gas mileage to drop significantly. I'm not sure why, but it does.

4. Let it break in and see what it does down the road.

5. I had 4.10s and 31s on my 1959 1/2 ton with a GM performance 350 and I got 15mpg. With 35s you can expect less. The final gear ratio is more important then the axle ratio. a 3.73 axle ratio is great with a T18 with a granny low, it sucks with a high geared automatic and you'll see the effect on your gas mileage at slower speeds.

Just some things to consider when you try to find ways to improve your mileage.

[rich weyand]

Which 350 is it, the 350/260 or the 350/290 or some other?

Set the base timing to 16 BTDC. It shouldn't knock there, but you'll get more horsepower and torque as well as better mileage.

The vacuum advance can should be connected to the driver's side port on the front of the Edelbrock, not the passenger side port. This will hurt mileage as well.

3.73:1 on 35" tires works out to a 3.1:1 on 29" tires. That should actually get you better mileage, on the highway anyway.

Did you check the float levels on the Edelbrock before you put it on? They are notorious for being set wrong right from the factory. I think they get banged around in transit. Anyway, if the floats are set wrong, you can run rich and burn up a lot of gas. Not too hard to check them, but you will need to pick up a package of gaskets so you can put a new one in when you put the cover back on the carb.

Using an A/FR meter would tell you what your mixture is doing. Again, most carbs are set up a bit rich for that engine, because most people do not tune the carb, and a lean carb can burn the engine up. Much less problem running rich.

You could also copy my carb tune for the exact same setup. Read this post on another forum; I give the full tuning spec for what I am running. 12 mpg at 60, 11.5 mpg at 75, and 9.5 mpg on very short trips around town.
http://www.fullsizechevy.com/forum/g...ml#post5758323

BTW, you should also be running the thermostatic air cleaner, or you will have problems in the winter. Here's how to do that with headers.
http://www.fullsizechevy.com/forum/g...d-headers.html

[dyno]

Thanks for the replies boys...

The kid primarily drives around town and to school, says he keeps his foot out of it...

Timing is set at 12 degrees BTDC, no ping. I will try bumpin it up to 16.

No cat. Running headers and turbo mufflers.

Had a Q-jet on it, thinking those small primarys would help, but got the same with the edelbrock. Did not check the float levels, does not seem to be running very rich.

[rich weyand]

Careful. The difference between 8 mpg and 9.5 mpg around town isn't "very rich".

The A/FR (air/fuel ratio) should be about 12.5 on the secondaries, about 13.5 on the power circuit and idle circuit, and about 14.5 on the cruise circuit. If you are running 11 or 12 on the cruise circuit, say, and 10 or 11 on the power circuit, it will run fine, it won't act real rich, but you will get 8 mpg instead of 9.5 around town, and 9.5 to 10 mpg instead of 12 on the highway.

BTDT. I was initially getting about 8mpg as well. That's why I got an A/FR meter and fine-tuned the Edelbrock. The good thing for you is that with the same setup, you can skip getting the meter and just copy my tune.

Mileage also depends on which engine you have (though that tune will work for both -- BTDT as well). Is it the 350/260hp or the 350/290hp?

[geezer#99]

You know Rich, your carb tune won't help him at all.
He lives 1700 feet closer to the sun than you do.
Less air density affects his carb and his timing set up.
Just sayin'!!

[dyno]

It's the 350/260hp. I don't have an A/FR meter, never used one. Maybe it's time to pick one up.

[rich weyand]

Originally Posted by geezer#99

You know Rich, your carb tune won't help him at all.
He lives 1700 feet closer to the sun than you do.
Less air density affects his carb and his timing set up.
Just sayin'!!

Agreed that altitude affects the mixture requirement, but the mixture has to be set more lean, and my tune leaned out the stock Edelbrock. So my tune would be an improvement, but he could probably go further, with larger needles than I used. Still need to go smaller on the primary jets.

[rich weyand]

Originally Posted by dyno

It's the 350/260hp. I don't have an A/FR meter, never used one. Maybe it's time to pick one up.

Ah, good. The 350/290 is a mish-mash of parts, and will never get the mileage of the 350/260 engine.

An A/FR meter is the only way to get it spot on, but you need to get a good one. This is the one I used, and I recommend it. I didn't buy it from these people, but this looks like the lowest price currently.
http://ergode.com/index.php?route=pr...FaY-Mgodzl0AJg

[Zone47]

Yeah, a tight motor is going to suck gas. You'll need to log about $5,000 miles before you notice any improvement. Just break it in right and you'll be good.

[BigBlocksRule]

It's the foot on the accelerator pedal, mostly. Not pointing a finger at the kid or calling him a liar, but...
does anybody out there know any teenage boy that can resist mashing on the "go faster" pedal with headers and turbo mufflers? Or for that matter, a 50+ year old?

Big ol' tires with the wrong gearing will kill mileage every time. My '97 Tahoe with a 350 lost about 3mpg when I tried 'em.

[gchemist]

Have you used a vacuum guage to tune the carb?

[dfrank]

Originally Posted by Your41Plague12

Bump that up to 4.10's and you'll see results.

+1 I am running 4.10's on my 86 with 35's and it seems like the perfect gear ration with 35's. I think 4.88's would be a little too low to get the gas milage you are looking for.

[riz]

4:56 gears would be best for your 35" tires.

RIZ

[dyno]

Originally Posted by BigBlocksRule

It's the foot on the accelerator pedal, mostly. Not pointing a finger at the kid or calling him a liar, but...
does anybody out there know any teenage boy that can resist mashing on the "go faster" pedal with headers and turbo mufflers? Or for that matter, a 50+ year old?

Big ol' tires with the wrong gearing will kill mileage every time. My '97 Tahoe with a 350 lost about 3mpg when I tried 'em.

Yeah, I know it. I drove it myself though and checked the mileage and it was not much better. I'm sure he gets on it from time to time but he has to pay for his own gas and every dime he makes goes in the tank, As a matter of fact he just asked me if there were some chores to do because he needs gas money!

[dyno]

Originally Posted by gchemist

Have you used a vacuum guage to tune the carb?

No, I don't have a gauge. A/F screws are about 1 1/2 turns out.

[gchemist]

Get a $15 guage from Harbor Freight or parts store.
Read this on using a guage: http://www.centuryperformance.com/fo...a-Vacuum-Gauge

I use a guage on my 1406 carb/RPM air gap intake. My truck can manage 18.5 MPG at 70 MPH at peak conditions.

[dyno]

Thanks. I'll head over to Harbor Freight today.

Forgot to mention the Edelbrock is a 750 CFM. It was a newer carb that came on the old motor. I had 650 CFM Edelbrocks on 350's before. If metered right will there be a mileage difference between the 750 and a 650?