383/700r4/3.08........and 10.5MPG!!!!!!

[americanmusc1e]

OK, the title says it all.

Vortec-headed 383 (actually a 388), Comp Cams XE268 cam, Holley 600 VS, and a Weiand Stealth intake......34 degrees total ignition timing

Built 700r4

stock 3.08 gears with an open diff


I'm getting 10.5mpg.

is this normal????? or do I still need some tuning????

1900rpm at 70mph--- is this too low???

with the old stock350/th350/3.08 I got 12mpg!!!!!!

[knsmodels]

seems low to me....I get better than that with my stock K/10.....

[1971 GMC SHORTY]

Wow Thats Horable....

[smoky02]

I have the same setup, well minus the 383, and I'm getting around 18-23. Highway of course, in town is around 14 or so. I would say that's about right

[67ChevyRedneck]

With all of that stuff put together, how sure are you that your odometer is accurate?

I have a stockish 350/700R/3.73. About 12 around town and 16-17 highway. My odometer is off by 9%, so I take that into account in my fuel mileage calculations.

[Hart_Rod]

Seems like you would be lugging the motor with the OD and 3.08 gears. I bet 3.73s would move you up a couple of MPGs....

[BarryB]

Yeah with the cam you have at lower RPMs your motor is struggling you need to get some RPMs in the motor to benefit from your cam, either swap out the cam to something more stcok, or put a deeper gear ratio in. 3:42 probably do you pretty good. BUT a 383 where it has a longer stroke, it does tend to suck in more fuel.

I basically had the same set up in a Nova, once I change out the rear gears. I put in a 3:23 ratio. It did better.

[69GMCLonghorn]

As stated, you should be getting higher than that.

As for the "lugging" thing... YOU ARE NOT LUGGING THAT ENGINE! If it's working too hard, you'll feel it in your right foot. If you want to prove me wrong/right, drive it in 3rd for an entire tank of gas. If it gets better than what you're getting now, I'll eat my shoe...

[Jim_PA]

I bet it's running way rich, and are you using vacuum advance? At 1900 RPM's, I doubt you have much mechanical advance. I think you would see some great increases in MPG if you were to get the carb cruising on the idle circuit, and throw some vacuum advance ontop of that. Is your power valve working, and is it properly sized?

As it is now, you're probably on the mains right off idle, and it's just wasting fuel.

[bamabaxter]

i have a 383/700r4/3.73 gears. I also want to know if your speedo is correct. That could be the main problem. At that rpm you definitely need some vacuum advance. what do your plugs look like?

[Longhorn Man]

your RPMs are too low, you are under the torque curve. While you aren't ligginh it, you are under the curve and it takes too much throttle to try and maintain the speed.
9 times out of 10, a 3.07 and overdrive are not a good combination.

[special-K]

3.42s are my limit for o/d.I consider 3.08s to equal o/d running 1:1.I also think you actually need a bigger carb for that motor.Sounds funny,but the circuits in a carb increase flow progressively and a bigger carb will be pulling less fuel at 70 mph because it`s at a lower point of the carb`s range.

[bamabaxter]

highly recommend some 3.73's. At 65 mph I am at about 22-2300 rpm. I don't have my converter lockup wired yet but I am told that it will drop me another 200 rpm at cruise speed. I remember someone on this forum talking about running 3.08 and 700r4 with a 454 motor. I think his was a low rpm motor with an rv cam or something similar. He got really good gas mileage, like 18 or something but like Longhorn said, you are probably opening the vacuum secondaries. I found the best thing that helps you "see" what your carb is doing is a vacuum gauge. I just got one that actually mounts on the dash. Definitely taught me a lot in the past few days.

[69GMCLonghorn]

I disagree with the folks saying he's overdriven. I DO agree that there is probably a timing advance problem. Trying to get the carb to be on the idle jets when cruising will not happen as the carb is OFF the idle jets by the time you get just over 1000 - 1200 RPM. Maybe he could put a Gear Vendors behind the 700R and get those R's down a little more???

Seriously, just to prove me wrong or everybody else wrong, drive it in 3rd for an entire tank of gas and see what your mileage does. If it's worse, then gearing isn't your problem and tuning IS.

[americanmusc1e]

OK, so everyone pretty much agrees with the tuning ideas I had:

swap the holley 600 for a modified (and properly calibrated) Q-jet (750cfm)

tune the ignition timing

3.73 gears-- that is why I got the OD in the first place, cuz 3.73's are on my list of modifications

check the odometer for accuracy (pretty sure the speedo is correct, we checked it against my brother's truck)

I WILL try to drive it in 3rd with the converter locked up to see what my mileage does

[DANTIP]

I had a ZZ3 350 with a 700R4 and 3.07 gears and I was able to get 20 mpg on the highway. The torque curve is excellent on the ZZ3 even down to 1800 rpm. Your 383 set-up may not be as friendly at low engine speed.

[americanmusc1e]

OK somebody help with my ignition timing!!!!

stock HEI

without (ported) vacuum advance hooked up: 12 initial, and 20 total mechanical

with (ported) vacuum advace hooked up: 34 degrees total

all the advance is in before 3000 rpm

somewhere I read that you are supposed to hook the vacuum advance to full manifold vacuum????

[bamabaxter]

It is a never ending battle about what vacuum port to use. Depends on what you want. First thing you need to find out is how much advance your vacuum canister adds and at how many inches of vacuum it comes in. You may need an adjustable canister. You said you are running stock hei, but with and aftermarket motor setup up it changes the vacuum levels in the motor. This is when a vacuum gauge and pump come in really handy.

[Ackattack]

what size of tires are you running?

My gas mileage improved when I went from 3.08 to 3.73s (14mpg to 18mpg) with a TPI 350 and 700R4.

[americanmusc1e]

Originally Posted by ackattack

what size of tires are you running?

My gas mileage improved when i went from 3.08 to 3.73s (14mpg to 18mpg) with a tpi 350 and 700r4.

31"

[bamabaxter]

What did the hei come out of. Give me the year and specs and I can look up the vacuum specs on the canister.

[69GMCLonghorn]

You need to hook the vacuum advance up to a ported outlet on the carb so you don't get advance when you don't want it.

Also, I don't know anything about cams and timing, etc, but i have a built 350 and the guy that tuned mine has a total of about 44° of advance dialed into it. Once he did that, my bottom end came alive (Did I really just type that?!?!). In fact the low end is so much better now that my tranny whines if I pour it on around 1200 RPM (it's a stick).

For sh**s and giggles, try giving yourself more base timing and keep the vacuum advance disconnected. That is how mine is set up and it works great. Just no idea how it will work on yours. FYI, my base timing is something like 25 - 30°!!! I dont' know how it works like that without pinging, but it does!

[americanmusc1e]

Originally Posted by bamabaxter

What did the hei come out of. Give me the year and specs and I can look up the vacuum specs on the canister.

sorry, I have no idea, I bought it used and I don't know what it came out of

[nightmare]

I kinda have the same quandry. But, with a GM crate motor (built sometime around 2001), edelbrock 1901 (750cfm), 700r4 (non-lockup), and posi 3.08's, I get around 10-10.5 mpgs in the city.

I just went on a 250 mile trip to Houston, and averaged 12.2 mpg. My cruising speed was 70, and the rpms were around 1900. I know that I need to change out the rear, as most have said that the 3.08s are too low, but I talked to one of the places recommended by this board, and the guy said that 3.42s or 3.73s would decrease my fuel mileage, not increase it. I just don't understand how some people are getting 18-20 mpgs in these trucks. I'm not expecting phenomiminal mileage, but my stock dodge dakota (extended cab with a 360 and a 3.92 gear) averaged 16.7 mpg. I plan on changing to a 3.42, since that should put my rpms around 2100.

[smoky02]

Originally Posted by nightmare

I kinda have the same quandry. But, with a GM crate motor (built sometime around 2001), edelbrock 1901 (750cfm), 700r4 (non-lockup), and posi 3.08's, I get around 10-10.5 mpgs in the city.

I just went on a 250 mile trip to Houston, and averaged 12.2 mpg. My cruising speed was 70, and the rpms were around 1900. I know that I need to change out the rear, as most have said that the 3.08s are too low, but I talked to one of the places recommended by this board, and the guy said that 3.42s or 3.73s would decrease my fuel mileage, not increase it. I just don't understand how some people are getting 18-20 mpgs in these trucks. I'm not expecting phenomiminal mileage, but my stock dodge dakota (extended cab with a 360 and a 3.92 gear) averaged 16.7 mpg. I plan on changing to a 3.42, since that should put my rpms around 2100.

you also said that your tranny doesn't lock up, that takes alot away from you. And the higher gear ratio, would more than likely increase your mileage in town, but not out on the highway at all.