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#1 |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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86 GMC PU with new crate 350 getting 8 MPG.
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.
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"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 |
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#2 |
Registered User
Join Date: Jul 2012
Location: Central Oklahoma
Posts: 147
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
My experience has been the more miles a new motor gets. The better the economy gets. 500 miles isn't even broke in yet..
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"Sucks to be you" Suxtobu65 ![]() |
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#3 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Timing!!
What's your timing set at? |
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#4 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,356
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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?
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#5 |
Registered User
Join Date: Jun 2012
Location: Minnesota
Posts: 509
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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?
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#6 |
Registered User
Join Date: Sep 2013
Location: Richland, WA
Posts: 28
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
3.73:1 gears and 35" tires? Wow that's gotta be gutless especially on the hills.
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#7 |
Registered User
Join Date: Dec 2010
Location: Pittsburgh, PA
Posts: 1,009
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Bump that up to 4.10's and you'll see results.
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Steve Member of the Foot Pedal Club '83 Chevy K20: 6" Rough Country, 3" Performance Accessories BL, Bushwacker Cutouts, 38x15.50 Mickey Thompson Baja Claws, 350 HP 350 crate engine from Jegs, Turbo 400 ![]() |
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#8 |
Registered User
Join Date: Jan 2009
Location: Louisville,Ky
Posts: 5,811
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Yeah with 35s you need a 4.10 or maybe a 4.88.
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#9 |
Registered User
Join Date: Jul 2014
Location: Pacific Grove, CA
Posts: 105
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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.
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Robert 1975 C10 - Huckleberry 2007 Jeep JK - John Wayne Jeep Smittybilt front/rear bumper, 10K winch Pacific Grove, CA |
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#10 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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
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Rich Weyand 1978 K10 RCSB DD. |
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#11 |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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.
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"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 Last edited by dyno; 09-08-2014 at 09:29 PM. |
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#12 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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?
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Rich Weyand 1978 K10 RCSB DD. |
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#13 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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'!! |
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#14 |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
It's the 350/260hp. I don't have an A/FR meter, never used one. Maybe it's time to pick one up.
__________________
"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 |
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#15 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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.
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Rich Weyand 1978 K10 RCSB DD. |
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#16 | |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Quote:
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
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Rich Weyand 1978 K10 RCSB DD. |
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#17 |
Registered User
Join Date: Sep 2014
Location: Richfield, Oh
Posts: 112
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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.
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#18 |
Registered User
Join Date: Jun 2008
Location: Lubbock, TX
Posts: 2,047
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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. |
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#19 |
BAD BOW-Silverado XST
Join Date: May 2000
Location: Senior Member from Austin, TX
Posts: 6,431
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Have you used a vacuum guage to tune the carb?
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#20 |
Registered User
![]() Join Date: Aug 2014
Location: Pittsburgh, PA
Posts: 627
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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#21 |
Registered User
Join Date: Oct 2011
Location: hudson,wi
Posts: 805
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
4:56 gears would be best for your 35" tires.
RIZ |
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#22 | |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
Quote:
__________________
"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 |
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#23 |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
No, I don't have a gauge. A/F screws are about 1 1/2 turns out.
__________________
"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 |
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#24 |
BAD BOW-Silverado XST
Join Date: May 2000
Location: Senior Member from Austin, TX
Posts: 6,431
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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. |
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#25 |
Registered User
Join Date: Sep 2008
Location: Middleton, Idaho
Posts: 352
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Re: 86 GMC PU with new crate 350 getting 8 MPG.
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?
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"Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675 |
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