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Old 02-19-2013, 10:38:22 PM   #1
luda
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Bad Gas Mileage - Carb Tuning?

Hey guys, we recently made some changes to my car hoping to get better gas mileage and went from 10 to 12mpg. I realize my carb is too big for my setup. I have a few questions with regards to this.

1) Can this Holley be tuned to give me better gas mileage?
2) If not, how hard is an Edelbrock to tune?
3) Could I expect a significantly better mpg with an Edelbrock 650?
4) How much performance can I expect to lose if I drop to a 650? I'm guessing none but figured it was worth asking.

If I can't tune this one for better mpg, something has to give...I can't afford it with the way gas is going with this being my daily driver. O.o





75 Block 350
Bored 60 over
Forged Flat Top Pistons 8.5-9 to 1 compression
Vortec heads with new springs for higher lift
Edelbrock Performer EPS Intake
Seegers Cam 292/288 advertised duration at .005
224/224 @ .050
.300 @ .450 intake
.307 @ .460 exhaust
Hooker Competition Painted Headers
High Flow Cats
Dynomax Super Turbo Mufflers
2.5" true dual exhaust
Holley 750 Vacuum Secondary
4.11 Richmond Posi
2600-2800 PTC Stall w/ Lockup
700r4 Transmission Swap

Last edited by luda : 02-19-2013 at 11:19:34 PM.
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Old 02-19-2013, 10:54:39 PM   #2
markw
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4.11 gears + gas mileage = no
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Old 02-19-2013, 11:06:41 PM   #3
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Quote:
Originally Posted by markw
4.11 gears + gas mileage = no

I was gonna say, 12 mpg is pretty dang good for 4.11's (if you have a th350 that is). I don't get much better with the stock 2.41's in mine, i get somewhere in the 15 mpg range. OD trans i think is going to be your next best step!

I DD mine as well
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Old 02-19-2013, 11:18:55 PM   #4
luda
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OH...will edit, forgot to change that. We swapped it for a 700r4...so I do have overdrive and lockup.
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Old 02-20-2013, 12:03:21 AM   #5
dale68z
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Learn how to drive for fuel economy. Look ahead,lift early, take off slowly.
Optimize your ignition timing and keep it in overdrive.

My engine is larger, carb bigger, and cam more aggressive, but has 3.73 gears and OD. It gets better fuel economy than you are getting.
When I want, I use my power. But because I daily drive my 71, now that I'm repainting the 68. It's not often.
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Old 02-20-2013, 12:14:42 AM   #6
Todd80Z28
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Are you using vac advance on the distributor?

That cam is wrong, IMO, for best mileage also.
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Old 02-20-2013, 04:35:22 AM   #7
ZS10
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Gotta look at what you're doing and how you drive.

Low rpm is key, so 14* less @.05 cam, set up the converter lock so it comes on as soon as the engine can handle it, 3.42 gears would help.

After that, it's all how heavy your foot is.
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Old 02-20-2013, 06:43:27 AM   #8
luda
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Quote:
Originally Posted by Todd80Z28
Are you using vac advance on the distributor?

That cam is wrong, IMO, for best mileage also.


Well that's fair enough. I didn't pick the cam though, shop did. Gotta make the best of it for a while, probably won't change that until I tear it down to rebuild which hopefully won't be for a while.

As far as vacuum advance, yes I am.

Also, I'm keeping my foot out of it as much as possible. Right now I need to work on the shift points and lockup. My lockup is hitting AS SOON as it hits OD so I either have to keep it in 3rd longer OR let it hit OD but barely tap hold the break so lockup doesn't occur until I want it to. You guys are thinking my secondaries could be opening up even at normal acceleration? If so...I might take the time to spring it as stiff as possible until I get my shift points worked out. Currently I have 68 jets in it.

A side note, we went 4.11 because I mainly do city driving and it allows me to lock up as low as 50mph. I really don't know that I feel comfortable doing it much less than that...what do you guys think? Would it build up too much heat or lug the engine too badly at speeds lower than that?

Thanks for all the help, I really appreciate y'alls responses...trying to not go broke while I finish my last year of my masters.
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Old 02-20-2013, 07:47:37 AM   #9
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You should be getting a lot better than that! I get 11mpg with two carbs on a 6-71, 3500 stall converter, 4.11 gears and a TH350.
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Old 02-20-2013, 09:42:13 AM   #10
hhott71
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Quote:
Originally Posted by luda
Hey guys, we recently made some changes to my car hoping to get better gas mileage and went from 10 to 12mpg. I realize my carb is too big for my setup. I have a few questions with regards to this.

1) Can this Holley be tuned to give me better gas mileage?
2) If not, how hard is an Edelbrock to tune?
3) Could I expect a significantly better mpg with an Edelbrock 650?
4) How much performance can I expect to lose if I drop to a 650? I'm guessing none but figured it was worth asking.

If I can't tune this one for better mpg, something has to give...I can't afford it with the way gas is going with this being my daily driver. O.o





75 Block 350
Bored 60 over
Forged Flat Top Pistons 8.5-9 to 1 compression
Vortec heads with new springs for higher lift
Edelbrock Performer EPS Intake
Seegers Cam 292/288 advertised duration at .005
224/224 @ .050
.300 @ .450 intake
.307 @ .460 exhaust
Hooker Competition Painted Headers
High Flow Cats
Dynomax Super Turbo Mufflers
2.5" true dual exhaust
Holley 750 Vacuum Secondary
4.11 Richmond Posi
2600-2800 PTC Stall w/ Lockup
700r4 Transmission Swap


The 3310 Holley is NOT too big, It is the same size as a 2 bbl if you keep your foot out of hearing those secondaries roar!
Flat top pistons and 64cc chambers in 10.25:1 compression.
You should use 91 or 93 octane and full advance.$0.10 more per gallon is only $1.50 more for a fill up.
4.10 x .7 OD = 2.87 rear gears.
So that is not hurting you much.
3.42 x .7 = 2.39 so the 2.87 ain't bad with the 4.10's.
Lock up stall is excellent choice.

The 224* camshaft is not an econo cam

10-12 mpg sucks with OD and a lock up.

A similar engine but with 221* cam. 041 heads 10:1 comp and 2400 stall 350TH 4.10 gears got 12 mpg everywhere.

SO carb tuning, stock 72 jets are fine.
A 8.5 or 6.5 PV should do it, stock 10.5 is too high and may open at idle.

Ignition, 12* initial, 36* at 2500 rpm then add in the vacuum advance get an adjustable one so you can set it up to where it is just under the detonation threshold.
You should see 15-17 with some tuning of what you have.
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Last edited by hhott71 : 02-20-2013 at 05:30:36 PM.
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Old 02-20-2013, 09:59:55 AM   #11
luda
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Quote:
Originally Posted by hhott71
The 3310 Holley is NOT too big, It is the same size as a 2 bbl if you keep your foot out of hearing those secondaries roar!
Flat top pistons and 64cc chambers in 10.25:1 compression.
You should use 91 or 93 octane and full advance.$0.10 more per gallon is only $1.50 more for a fill up.
4.10 x .7 OD = 2.87 rear gears.
So that is not hurting you much.
3.42 x .7 = 2.39 so the 2.87 ain't bad with the 4.10's.
Lock up stall is excellent choice.

The 224* camshaft is not a econo cam

10-12 mpg sucks with OD and a lock up.

A similar engine but with 221* cam. 041 heads 10:1 comp and 4.10 gears 2400 stall 350TH 4.10 gears got 12 mpg everywhere.

SO carb tuning, stock 72 jets are fine.
A 8.5 or 6.5 PV should do it, stock 10.5 is too high and may open at idle.

Ignition, 12* initial, 36* at 2500 rpm then add in the vacuum advance get an adjustable one so you can set it up to where it is just under the detonation threshold.
You should see 15-17 with some tuning of what you have.

I appreciate the heads up. Timing is set right around there with 12* initial and I believe in the neighborhood of 36* total. My current vacuum advance isn't adjustable though to push it just a hair further so I may look into that.

I should correct myself on jetting. I think I only pulled it back to a 70 and was thinking of pulling it back even a hair more to a 68. Going to pull a plug today when I get home to see what it says. (Rode the V-Star in 33 degrees to save gas O.o)

Going to grab a vacuum gauge tomorrow from my Dad to see what my cruising vacuum is...you may be right on the power valve...but I didn't think the stock one was 10.5. If so, I could definitely see that being an issue even though I have a dual plane intake. As for carb model, the one I'm running is a 4160 vacuum secondary with electric choke...brand new out of the box.
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Old 02-20-2013, 10:52:14 AM   #12
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Regardless of the carb used getting a wide band to tune it for ideal power and be sure it is not too fat. You can also then lean cruise AFR some.
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Old 02-20-2013, 12:42:28 PM   #13
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Quote:
Originally Posted by hhott71
if you keep your foot out of hearing those secondaries roar!
maybe a heavier secondary spring to slow their opening?

Edit: I mean in his current carb
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Old 02-20-2013, 05:42:50 PM   #14
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The Holley 3310 was a 4150 (about 1968) they became 4160's when they became 3310-1 (in the 70's) the dash 2, dash 3, dash 10 by now. The 3310- whatever 4160 style is the basis for the electric choke model that you have. Same carb essentially.
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71 Camaro 355 NA
11.1650 @ 119.30
1.5028 60'
Quote:
Originally Posted by Lowend
For the record I agree with Aces and hhott71
Quote:
Originally Posted by Lowend
If the rear end grenades it blows your nuts off...
Quote:
Originally Posted by Strange81Z
That's because we have more to give....ask any women who's been with a conservative and a liberal! LOL!

Last edited by hhott71 : 02-22-2013 at 01:13:54 PM.
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Old 02-20-2013, 06:42:57 PM   #15
Todd80Z28
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Back to the vac advance-

If your specs are right- flat tops and Vortec heads, you are about 10:1, as hhott mentioned. If you are using the stock can without limits, that is going to be way too much total advance (init+mech+vac at cruise). Put a limit tab/can on the one you have to limit the total to around 12° or so.
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