Love my new engine/trans;hate the MPG!

[68Gold/white]

I think there's tuning to be done here, before buying ANYTHING! Not trying to sound like a Beavis here...
From what I can tell the rear gear is still unknown...the common 350/350 gear was a 3.07.
Every engine is different. some won't due well lugging at lower RPM's, many have voiced this, I agree.
Do you know what your total timing is???

What type of distributor is it??? Does it have vacuum advance? Is the vacuum advance hooked up???

You must have dished pistons, with 64 cc heads on a 383. Your timing will need to be carefully dialed in with this combo, not even considering the MPG situation...

[Boog]

Try running it in drive instead of OD to see if it makes any difference in mpg.

[Myself]

Originally Posted by garyd1961

I don't know but a 528/536 lift cam seems a big for low end torque. Everything needs to work together. The high compression needs a big cam, big cam needs a big stall to keep rpms up in power range, big stall needs a big gear in the rear. All of this is contradictory to gas mileage.

Bingo!

I'm guessing from that combo it's the Speed Warrior intake.
All lockup style converters work hydraulically. After switching into overdrive,AND other parameters are met, a 12v signal is applied to a solenoid which opens a fluid passage travelling into the converter to cause the lockup. Without that 12v signal going to the solenoid there will be NO lockup. Unless there have been serious valve body mods to allow lockup always in the 4th gear range which is also not great for mileage OR daily driving for that matter. I've rebuilt several 700R4 and 4L60 trans and have always used a Vacuum switch and brake switch as they did from the factory on all the early 2004r, 700r4, th350c and th200c trans. As somebody else pointed out, the weights and springs of the governor can also be played with to dial in the shift points. Keep in mind, the shift points WILL change with different gears in the rear. AND....with your engine combo, I would start there. You need a 3:42 gear at the highest, the 3:73 would be just right. I also think if you will be driving this truck frequently, I would swap out that converter to a 2000 stall instead AND add a temp gauge to your trans line. The 700r4 DOES NOT like to be run over 200 degrees PERIOD. It WILL fail prematurely.

[Rick Bollinger]

68 Gold/White Thanks for that info. I do not have a switch yet. I was assuming I would need to wait to see what my actual vacuum was at highway speed to know what to get.

[TwoFiftyShifter]

The 383 can be a low rpm torque motor. Look up that GM HT383. It makes 345 or so HP and 440ish TQ. Peaks at lower RPM. That's what you buy for efficiency. The motor you have is better for acceleration.

[HO455]

Trying clear things up here. There are nonelectricaly operated lockup convertor setups for some 700r4's. Please see the link below. From what the OP has said he has one of these. No vacuum switch required.

https://transmissioncenter.net/shop/...speed-choices/

[Lancialonnie]

Thank you HO455! This is very helpful as this is the early style 700r4 that I have which does fully lockup but without any electrical switching required.

[68Gold/white]

Originally Posted by Rick Bollinger

68 Gold/White Thanks for that info. I do not have a switch yet. I was assuming I would need to wait to see what my actual vacuum was at highway speed to know what to get.

The vacuum at highway speed should not matter. When you go full throttle, vacuum goes to zero, which allows the switch to unlock the converter, same as the brake light switch. When you push on the brake pedal, the lockup is turned off...
The lockup TH350 I had in my 67 Bel Air worked flawless, did help MPG, and provided better gearing driving around town as opposed to the original powerglide...

[Scott Liggett]

I have a 383 that I built with BluePrint's 195cc heads. it has a far bigger camshaft than the one BluePrint uses in their 430 HP 383. It's in a 67 GMC with a A833OD trans, with near identical gear ratios to the 700R4, 3.73 geared 12 bolt. I get 16-18 mpg on the freeway doing about 2500 rpm, 75 mph.

I work at BluePrint and talk to customers all day, and I can tell you that our customers with the 815 HP 632 with a 3 circuit 1250 Dominator are getting better than 8 mpg. So, you have something wrong. Start by looking at the AFR's on your FAST EFI's system. You should be able to see that number on the handheld. Cruising AFR's should be in the 14's, WOT should be in mid 12's. Anything below that and your EFI system needs tuning and that is killing your mpg. Also, if you have any kind of exhaust leak between the engine and the O2, it will give a false lean signal to the ECM and the EFI system will over richen the fuel mixture.

A simple test is to pull the spark plugs, if they are black and sooty, it is running rich. Also, if you oil smells like gas, then it is running WAY rich and you need to fix the EFI settings ASAP before you fuel wash the cylinders..

[Lee H]

Agree with Scott, needs tuned. The 383 in my 72 is 352HP @ 4,929 and 393TRQ @ 3,644 at the rear wheels, coupled with a 350THM and I'm getting 12 as my DD, and I'm not gentle with the skinny pedal. Running a quick fuel 750 four barrel.

[LuckyScott]

I have always wondered why nobody makes a crate "Milage" motor. Every crate motor is more horsepower than stock. I would love somebody to come out with a small block chevy motor designed for good gas milage.

My 72 suburban gets 13 MPG with the stock 350 motor. Dual exhaust, HEI distributer with a custom curve. edelbrock intake manifold. Quadrajet carb. Carb has very small primaries so if you keep your foot out of it it uses just the primaries. It has been rebuilt by a custom carb shop.

You put a high horsepower motor in, it will use more gas.....