Love my new engine/trans;hate the MPG!

[Lancialonnie]

My tired old 307/TH350 was struggling and needed to be replaced as it would run hot and only got 6 MPG! I justified the expense of the upgrade by thinking I would see a sizeable increase in mileage but it hasn't turned out that way. Talking with some friends and wanting to keep a period look, I was convinced that a 383 stroker (Blueprint 430 hp) would be the best bet with plenty of torque and enough power that I wouldn't second guess my choice.

I went with an early 700R4 from a local shop. I talked with several of their satisifed customers that were happy with their transmissions and no electric lock up needed sounded good to me. FITech fuel injection, new headers and dual exhaust and now it has 1100 miles on it. I will say it is a blast to drive now! Before at 65 mph it was screaming loud and it was not comfortable driving it on the freeway. Now at 85 mpg it is very quiet and I have to watch the speed as it is easy to go 85 and not realize it! Plenty of power off the line and it will light the tires like it never could before, but, it is only getting 8.5 mpg with mostly city driving. I commute 6 miles each way to work and about half that trip is at 55-65 in the country and of course drive it around town.

I'm sure pure freeway miles would get into the teens but boy I was sure hoping to get 12-14 mpg! Smiles per gallon though can't be measured and the amount of thumbs up and waves I get proves that other people love this truck but then again, they are not burning up their cash to fill it up!

[toolboxchev]

Originally Posted by Lancialonnie

My tired old 307/TH350 was struggling and needed to be replaced as it would run hot and only got 6 MPG! I justified the expense of the upgrade by thinking I would see a sizeable increase in mileage but it hasn't turned out that way. Talking with some friends and wanting to keep a period look, I was convinced that a 383 stroker (Blueprint 430 hp) would be the best bet with plenty of torque and enough power that I wouldn't second guess my choice.

I went with an early 700R4 from a local shop. I talked with several of their satisifed customers that were happy with their transmissions and no electric lock up needed sounded good to me. FITech fuel injection, new headers and dual exhaust and now it has 1100 miles on it. I will say it is a blast to drive now! Before at 65 mph it was screaming loud and it was not comfortable driving it on the freeway. Now at 85 mpg it is very quiet and I have to watch the speed as it is easy to go 85 and not realize it! Plenty of power off the line and it will light the tires like it never could before, but, it is only getting 8.5 mpg with mostly city driving. I commute 6 miles each way to work and about half that trip is at 55-65 in the country and of course drive it around town.

I'm sure pure freeway miles would get into the teens but boy I was sure hoping to get 12-14 mpg! Smiles per gallon though can't be measured and the amount of thumbs up and waves I get proves that other people love this truck but then again, they are not burning up their cash to fill it up!

Gear Vendors Overdrive, and/or change the rear gear ratio...depending.

[HO455]

A couple of things. You said no electric lockup, is the lock up wiring for the torque convertor not installed?.
How are you verifying your fuel mileage? Are you recording the milage and fill up amount at every fill up and averaging them over 10 fill ups? Have you verified the accuracy of your speedometer? (With something other than a cell phone)
Do you have the fuel tank still behind the seat?
Did you use a wide band fuel ratio gauge to set up the Fitech?
You made no mention of your distributor and how you had it curved. What is you total timing with full vacuum advance.

My Burban had a 305 with 3.07 gears and now a 350 with 3.73 gears both combinations 12 city 14 highway (flat highway, steady 60 mph, no wind, otherwise it's 12 mpg)
I would wager that you have several things contributing to your poor mileage.

[68Gold/white]

8.5 MPG isn't too bad for a 4000+ pound vehicle, in town...needs to be a little better.

What is your rear gear ratio??? Me not knowing the ratio, it sounds to be spot on, going from your highway driving comments...

I'm not sure why folks don't want/or like the lock up converters in these transmissions??? Maybe having the lockup feature weakens the converter???

Unrelated...sorta...I have a TH350 with the lockup converter. Had it in my 67 Bel Air, plan on using it in my 67 C-10. It knocks about 350 RPM's off the engine speed going down the highway...

[tdangle]

With my stock 350 auto and 4.10 gears I was getting 9-10 around town and maybe 11 on the hwy. I swapped the TH350 for a TH2004R overdrive trans and the best I have gotten on the hwy is 16 at 75 MPH. I do love the lower rpm the overdrive lets you cruise at. Around town I still was at about 10 MPG.

Just installed a new GM Crate motor the SP350/357 which is suppose to be 357 HP 406 ft/lb torque. I'm having the setup dyno tuned this week to get everything dialed in a little better. Hoping for a increase in mileage. 1st tank with the new motor all in town driving was 6 MPG. I will admit I was driving it pretty hard though

[crazyjeenyus]

Originally Posted by Lancialonnie

.....Now at 85 mpg it is very quiet....

At 85 mpg my truck is quiet too, I only get that kind of mileage when the engine is off.

[TwoFiftyShifter]

A 430HP engine is a performance engine. It's almost double the horse of a 1970 350. Power sucks fuel. Add into the mix a heavy vehicle with Nixon era aerodynamics - poor mpg.

[BCOWANWHEELS]

Imo your combination is off

[70STOVEBOLT]

I average about 10-11 with my mild 402TH400/3.73 combo but if I put my right foot on the floor it drops considerably. By comparison, I averaged 13.5-14 with the stock 250, same teams and rear gear.

[Steeveedee]

I get 10 MPG in combined driving. I have yet to go to the gas station to fill the one saddle tank I used when I towed the trailer camping a few weeks ago. It used to be 7. I'm hoping for 8. That would be huge.

[randy500]

For only a few bucks more you could have installed an LS engine and got at least double the mpg....

[jocko]

I could get better mpg in a new K2500. Or I could drive an old 72 and get half the mpg, but with about $60k extra in my pocket - which can buy a lot o gas. I think the in grand scheme of things, mpg becomes irrelevant if we're choosing to drive an old truck because we like old trucks. But I put a T5 in my 66 anyway

[Lancialonnie]

Originally Posted by HO455

A couple of things. You said no electric lockup, is the lock up wiring for the torque convertor not installed?.

It locks up but the early models do so hydrauliclly and don't require wiring, there is no power needed to lock it.

How are you verifying your fuel mileage?
Are you recording the milage and fill up amount at every fill up and averaging them over 10 fill ups? I've tracked the mileage of every tank and fill it up till fuel comes up the neck.

Have you verified the accuracy of your speedometer? (With something other than a cell phone) I also installed a Dakota Digital dash and calibrated it as per their directions.

Do you have the fuel tank still behind the seat? No, aluminum tank at rear under bed, in tank pump.

Did you use a wide band fuel ratio gauge to set up the Fitech? No just using the installed O2 sensor and letting it do its thing.

You made no mention of your distributor and how you had it curved. What is you total timing with full vacuum advance. It is a cheapy distro, probably need to upgrade and it is timed as per Blueprints recommendation.

My Burban had a 305 with 3.07 gears and now a 350 with 3.73 gears both combinations 12 city 14 highway (flat highway, steady 60 mph, no wind, otherwise it's 12 mpg) I have the original gearing which I believe in a 72 350/TH350 would have been 3.73? Would different gearing increase mileage without sacrificing all of the performance? I can sacrifice some if the trade off is worth it.

I would wager that you have several things contributing to your poor mileage.

It locks up but the early models do so hydrauliclly and don't require wiring, there is no power needed to lock it.

I've tracked the mileage of every tank and fill it up till fuel comes up the neck.

I also installed a Dakota Digital dash and calibrated it as per their directions.

No, aluminum tank at rear under bed, in tank pump.

No just using the installed O2 sensor and letting it do its thing.

It is a cheapy distro, probably need to upgrade and it is timed as per Blueprints recommendation.

I have the original gearing which I believe in a 72 350/TH350 would have been 3.73? Would different gearing increase mileage without sacrificing all of the performance? I can sacrifice some if the trade off is worth it.

I think a new distro couldn't hurt and maybe get it to a dyno to see if any adjustments are needed. I'm certainly willing to listen to any suggestions on what I can do to improve the mileage.

[randy500]

I was teasing about the great mileage with an LS power plant, good to hear your getting that mileage with a 700r4 and fuel injection, neither of those provide huge mileage gains like people like to talk about.

You should be getting at least 11 or so though.
Possible causes of low mpg.
At part throttle what rpm are your shift points, with the 700r4 its likely shifting real early and your lugging the engine all the time.

You didnt say 2 or 4wd and tire size?

If its a C10 the stock gear ratio is 3.07 which is far from ideal with a 700r4.

[Jrainman]

Originally Posted by randy500

I was teasing about the great mileage with an LS power plant, good to hear your getting that mileage with a 700r4 and fuel injection, neither of those provide huge mileage gains like people like to talk about.

You should be getting at least 11 or so though.
Possible causes of low mpg.
At part throttle what rpm are your shift points, with the 700r4 its likely shifting real early and your lugging the engine all the time.

You didnt say 2 or 4wd and tire size?

If its a C10 the stock gear ratio is 3.07 which is far from ideal with a 700r4.

Agree ,I think you are spot on with your reply ,I think you can obtain 10-11 around town and maybe 13- 14 highway if you keep your foot out of it ,if you look in to what randy has prescribed.

[Myself]

Sounds right for a performance build, but we need all the specs to know for sure. Probably has a high rise dual plane intake BUT if it's single plane then there goes mileage. Also, what about the cam? I'm assuming a roller type retrofit but what are the specs? If it's a pretty lopey cam that makes good power from 3000-6500 rpm then it's all wrong for street driving. there are several variables that could be contributing to the poor mpg. Another thing I have to ask is why no lock up on the trans?? That's just mpg wasted. I daily drove a mild built sbc withh 700r4 trans for 16 years. Had lopey cam, low rise dual plane intake, 58cc heads, 9.5:1 flat top pistons, Holley Street Avenger carb, headers, dual exhaust, 3.42 gears. I'd get 15mpg every day mixed driving with about 19mpg on the highway!!

[Lancialonnie]

Thank you everyone for helping me come up with potential solutions! It is a 2WD long bed fleetside and yes, it does shift early at low rpm and then the engine seems to be lugging when it goes into OD so early. It is only going like 9 mph when it shifts into 2nd. Does anyone know more about the earlier model 700r4s that lockup hydraulically? The way it was explained to me is that these models go into full lockup but do not require any power to do so as it is locking up based on internal hydraulic pressure, does that sound right? Apparently they were not as robust but the shop I bought it from has a great reputation for upgrading them and making them work much better than they did originally. I believe they installed a 2500 rpm stall converter and it isn't lopey at idle.

Tires: 235/55/18 front and 255/55/18 rear

Here are the engine particulars, it is a Blueprint 383 #BP38313CT1
HP & Torque: 430 HP / 450 FT LBS
Compression Ratio: 10.0:1 (I use premium)
Hydraulic Roller Cam
.528 Intake .536 Exhaust
221 Intake / 226 Exhaust duration
@.050 - 110 degree lobe separation
Cast Steel Crank
Hypereutectic Pistons
BluePrint Aluminum Cylinder Heads - H8002K
64cc chamber
2.02" intake/1.60" exhaust valves
195cc intake/75cc exhaust runners
Weiand Warrior dual plane intake

I know a new distro can't hurt and then I should take it to be dyno'd and tuned. If the orignal gearing was 3.07 maybe I should go to 3.42 and keep the revs up a little so it isn't lugging as bad? With the OD it should still be a fairly low rpm at freeway speeds, shouldn't it?

Thanks again!

[HO455]

To start I would suggest taking a careful look under the truck for any fuel leaks.
Having your distributor curved by a competent person will give you both better mileage plus horsepower. Not mention peace of mind from knowing you won't be pinging when the weather gets warm. A competent distributor technician will want to know your rear end ratio and your rear tire size as well as compression ratio and camshaft specifications. Check to see if your vacuum advance is working by using an adjustable timing light (Or a properly installed timing tape on the balancer) Record your timing at idle with vaccum advance disconnected and plugged at the source. Then use vacuum pump to pull a vacuum on the advance unit while watching the timing mark with the timing light. You sho8 see at least 10 degrees of advance from the vacuum.
If you do have 3.07 gears you most assuredly would benefit by going to 3.42's or 3.73's. (As randy500 said) I know that GM used 3.42's on 2 wheel drive S10's and 3.73's on 4x4 ones with the 4.3. I believe that 3.42 is as low as you could go on a 3.07 carrier. So that would be the way I would go. Plus the 40 mph 4th to 2nd gear down shift in my S10 (3.42's) kicks A$$!
I would also talk with the transmission shop about how the transmission is shifting. There are different springs that can be installed to change the torque convertor lock speed, as well as adjustments to the govenor for upshift rpm.
This thread is large but full of 700r4 information it may help you. Plus it would be a good place to pose a question.

https://67-72chevytrucks.com/vboard/...d.php?t=634121

Believe it or not but much of the MPG gains seen in new cars in the 2000's came from transmission advances. The ability to always be at the optimum RPM makes great mileage possible.
And finally find a way to verify your speedometer is accurate.
Good luck and keep us posted.

[Daves70]

With that cam and with the lockup working, you'll want 3.73 gears. It'll be grumpy at 50 -60 mph with 3.42's. I have a 408 small block in mine a little wilder than you're combo and on a long trip at a steady 70 mph I get 16 mpg easily. I have 3.42's but I know 3.73's would be better. My truck hates running below 60 mph. I unlock the converter in town.

[garyd1961]

I had pretty much that same motor and it's made for high rpm power. It's made to go fast not get good gas mileage. Either enjoy your high performance gas guzzler for what it is or do as I did and sell it and get a stock L31. My L31 doesn't have the sound of my old 383 but it still has a lot of low rpm torque and twice the gas mileage.
As I said your 383 was designed to drink gas and go fast, nothing wrong with that but trying to get good gas mileage out of it is a waste of time.
Another thing you need a big gear in the rear to match that cam.

[Rick Bollinger]

The 383 is suppose to be a lower rpm high tork motor. I have one being built now around the same hp 420 and 450 torque estimated. Designed for under 5k rpm with peak torque around 2500 rpm. Going with the Fi Tech with there Go EFI 4 600 hp and there Force fuel pump. I too am running a 700 r4with all upgrades and a stock rpm torque converter matched with the low rpm cam. Lock up with the B&M system locks it up at a certain mph. I have it wired through the brake pedal so it unlocks when I hit the brakes. It works really well the only thing I don't like about it is it wont unlock when passing but will down shift. After I get the 383 in I will run it through a vacuum switch as well. 3:73 gears 29" tall tires. Msd Street Fire HEI distributor. I really hope I get much better MPG than this. I am hoping for at least 16 as I am planning on seeing the country in this truck.

[Lancialonnie]

Hey Rick, keep us posted on how things go with your 383, I also thought they were supposed to be decent at lower rpms. I have the GO EFI 400 hp system and their tech said it was rated conservative and "should" be aliright of course he would have been happy to sell me a 600 hp system. But I have spent enough on the engine and trans for now. I am going to get a new distro and look at going to 3.42 or 3.73 rearend gearing and drive it that way however it turnes out. It is a blast to drive, but getting a little better mileage would sure make it more enjoyable!

[68Gold/white]

Originally Posted by Rick Bollinger

The 383 is suppose to be a lower rpm high tork motor. I have one being built now around the same hp 420 and 450 torque estimated. Designed for under 5k rpm with peak torque around 2500 rpm. Going with the Fi Tech with there Go EFI 4 600 hp and there Force fuel pump. I too am running a 700 r4with all upgrades and a stock rpm torque converter matched with the low rpm cam. Lock up with the B&M system locks it up at a certain mph. I have it wired through the brake pedal so it unlocks when I hit the brakes. It works really well the only thing I don't like about it is it wont unlock when passing but will down shift. After I get the 383 in I will run it through a vacuum switch as well. 3:73 gears 29" tall tires. Msd Street Fire HEI distributor. I really hope I get much better MPG than this. I am hoping for at least 16 as I am planning on seeing the country in this truck.

Do you have a vacuum switch???
There's a thread here, I just brought it to the top (revised lockup TC wiring) it may help you...

[Rick Bollinger]

Mine should be done at the machine shop in a week or so. He is going to call me for the dyno test so I can be there. Hopefully I can get it in in a couple months and driving. I will post the numbers when I get them.

[garyd1961]

The 383 is a high torque motor with the right set up. With the right cam and intake it will make a lot of torque. My 383 had a little larger cam than the OP and a rpm airgap intake and it was good for one thing , going wide open for the 1/4 mile.
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.