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Discussion Starter · #1 ·
Is it common for a 396 to get about 11.5 mpg? The block has been bored .30 over to a 402, Oval port heads that now have .188 / .202 valves, 10.25 CR, Edelbrock performer rpm air gap, Holley 670SA vac. secondaries with 65 main jets, 35 squirter nozzle, purple secondary spring and a 8.5 PV. The cam is ISKY 270 grind hyd. .293 cam lift, .510 valve lift, 216 duration @ .050, 114 lobe center. Cam p/n 396270. Turbo 400 with 3.55 gears. Tachs 3200 @70mph with 26 inch tires. I'd like to squeeze another mile or two per gallon out of her if it's possible.
 

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You might get it to 13 or 14 but it’s going to take some judicious tuning and staying off the loud pedal. I think slowing down the accelerator pump will make the biggest change. Otherwise you need an OD trans and economy still won’t be great.
 
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You should be dang happy you are getting that kind of mileage - I can only dream. My small block 377 averages about 7 MPG, with a Turbo 400, 2,500 RPM stall converter, 3.42 gears, and an Edelbrock Thunder AVS 2 800.
 
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Discussion Starter · #4 ·
Thanks for the response Alan,
When I checked the mpg the other day, 95% of the miles put on were interstate and the rest of it was light footing it around town. Not sure what you mean by slowing down the accelerator pump? Are you referring to squirter size or pump cam profile?
Steve
 

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How’s your timing curve set-up? You running vacuum advance? I would think that combo should do 15-16 mpg easy unless you are running down the highway at 80 mph.

I would ditch the air-gap manifold and put some heat into the manifold. Good atomization of the fuel is important for efficiency, and the air-gap isn’t helping that cause. Even a non air-gap manifold with a blocked crossover would be a better fit.
 

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Is it common for a 396 to get about 11.5 mpg? The block has been bored .30 over to a 402, Oval port heads that now have .188 / .202 valves, 10.25 CR, Edelbrock performer rpm air gap, Holley 670SA vac. secondaries with 65 main jets, 35 squirter nozzle, purple secondary spring and a 8.5 PV. The cam is ISKY 270 grind hyd. .293 cam lift, .510 valve lift, 216 duration @ .050, 114 lobe center. Cam p/n 396270. Turbo 400 with 3.55 gears. Tachs 3200 @70mph with 26 inch tires. I'd like to squeeze another mile or two per gallon out of her if it's possible.
28" tires will help some. A 4L80e would really be nice but pricey

How’s your timing curve set-up? You running vacuum advance? I would think that combo should do 15-16 mpg easy unless you are running down the highway at 80 mph.

I would ditch the air-gap manifold and put some heat into the manifold. Good atomization of the fuel is important for efficiency, and the air-gap isn’t helping that cause. Even a non air-gap manifold with a blocked crossover would be a better fit.
Vacuum advance will help a lot if you don't have it hooked to full manifold vacuum.

I'm switching to a factory q-jet intake (cast iron) that has delivered some amazing dyno numbers by CPMotorworks (a member here). You can get a q-jet with 800 cfm capacity. The spreadbore speed demon 750 is only $350.
 
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I dont think anything cruising at 3200 rpms will get any mileage.
 

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Discussion Starter · #8 ·
Nova,
Thanks for the response. Yes, MSD ignition hooked up to full manifold vacuum. As far as the timing curve goes, initial at 18* total 38* all in around 2500. I have been considering swapping out the manifold back to the original cast iron and a QJet. Or a Weiand Stealth or regular performer. I would have to restore the original intake as it has light rust from being left outside when I was much younger and not as wise. 😕
Steve
 

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Discussion Starter · #9 ·
Wakko, thanks for the response. I don't recall checking the mpg years ago in its stock configuration before I built the engine. I don't have any idea what type of mpg I can get out of it.
Steve
 

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Discussion Starter · #10 ·
Nashville, thanks for the response. I won't be changing tire size as I'm very happy with the setup. As for the trans, I just built it and the expense would be great as you stated.
Steve
 

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I'd swap rear gears down to 3.08s or get a 200R4. And that's about it.

I wouldn't do anything else b/c you'll hurt performance.....and that motor was built with modest performance in mind.

Unless you are driving this every day (which you shouldn't be) trying to gain one or two MPG is purely an academic exercise. Your wallet will not notice it. But your right foot will.

Put another way, think about how much money you'd have to spend to do what was mentioned above. Contrast that to your actual fuel costs. Where is your break even point? This is an exercise unlikely to gain material results. If you want good MPG, you're in the wrong car.

So why bother?
 

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Put a wideband on it, and get the cruise tuned to a crisp 14.5:1 range. That is the only way to improve the steady throttle economy. You may be able to dial in a couple more degrees timing also.

For economy when you are opening it up more at times, a stiffer black secondary spring will slow the secondary opening and may also help.
 

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Id think the best mpg is where that sweet spot is not so much rpm


Full size 5.3 truck gets the same mpg in D around 60-65 as it does in OD and actually feels better. Over that mpg drops without that extra gear.
On the street I dont think OD is worth it
 

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I had a 66 SS396-325 hp years ago with 4 speed and 3:55 gears that got a consistent 18 mpg @ 3000 rpm.

That 325 hp cam was pretty small. :)
Absolutely. My 3.36 gear equipped Nova cruises at 3000-3200 rpm on the highway and pulls down a legit 18+ mpg with a 217/225 cam on a 108 LSA. I run a Q-jet though, which is probably the best MPG carb. You can have gas mileage without OD, but it's a bit of a challenge.

To the OP... sounds like you are doing everything "right", but IMO, that timing curve is way too fast. I think the "all in by 2500" deal is more for race cars, and not street drivers, but I'm not sure that's what is causing your MPG issues. I'm a little puzzled why that mild cam, 396 combo isn't doing better.
 

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Put a wideband on it, and get the cruise tuned to a crisp 14.5:1 range. That is the only way to improve the steady throttle economy. You may be able to dial in a couple more degrees timing also.

For economy when you are opening it up more at times, a stiffer black secondary spring will slow the secondary opening and may also help.
I run my Q-jet Nova at 15.3:1 A/F. Cruises fine with a lean ratio, but I also have Vortec heads, with fast burn chambers, tight quench, and small ports. A BBC with old school heads might not like being that lean.
 

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Thats pretty good...not all mpg claims are true you see on the web...even my 02 1500 thats supposed to get 16-20..avgs 12.

My 67 with 383 cant figure it out. Literally 20 bux for 20 minutes of fun..tahts with 6 gears....worth every penny too.
You're doing something wrong then. If that's freeway. My 02 with a 5.3 and 3.73s got 18 but city it got 12.
You mention od doesn't make a difference, it definitely does in mine. It have me about 4 mpg compares to D.
 

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Should knock down 14-15, had a 68 GTO with a 400 & 3.55 gears it would get 15 hiway.
421 in my Monte will do 19+ hiway.
 
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