Started with '84 GMC conversion van with a slug 305 and 700r4. Swapped in fresh '70 spec 455 and old '73 TH400. Use van to travel and tow 20' boat around Florida. Gas mileage was great, at around 16 to 18 MPG highway (not towing). Then TH400 went bad a year later, so looking to get a little better MPG, and already owning the original 700r4, rebuild 700r4 with help of a friend, using $1200 worth of stronger internal parts. When a choice was available, went with part better suited for towing, not racing (RV converter, etc) Now I am only getting around 14 MPG. Couple thoughts and questions. 700r4 is running at lower RPM, so carb is more critical? I also need to set up HEI distributor. It is stock curve. Carb is close, but not right. In 4th gear at say around 70MPH, RPM's are lower of course than they were with TH400, and I can feel motor running a little jerky, and not so smooth on light acceleration at highway speeds. When I coast in 4th, it feels like engine is slowing down the vehicle and not coasting as much as I think it should. I think speed drops too fast when coasting. Any 700r4 experts that can comment on 4th gear coast with a 455? Am I thinking correct about carb and ignition more crucial at lower speeds? Any advice is greatly appreciated! Rob
I'm no expert, but it sounds like the converter is locked up and when you let off the gas, the engine slows down, and is like an engine brake. You need the converter to unlock so the car can coast. Does that make sense? Maybe I'm way off. The OD stuff is all new to me. Brian
What gears are in the rear? If you are using a highway gear you might be overtaxing the motor at cruise.
What is your rearend ratio? If it is lower than 3.23, you could be lugging the motor. Also, you need lots of spark energy when you run lower RPMs. Get some great wires in there and a strong coil. You might also want to get some slightly stronger springs in your mechanical advance. What spark plugs are you running? What gap? Is your convertor locking up? Is your TV cable properly adjusted? -Bob Cunningham
Thanks for the replies. Don't know what rear end ratio is. Stock for the '84 with 305 and OD, so probably no higher than 3.23. When I stated it seemed to be jerky at highways speed, lugging motor would also be how it feels. How am I supposed to get better gas mileage if I can't run lower RPM? I am guessing 70MPH is around 1500 to 1800RPM. Running spark plugs for stock HEI, with gap for HEI, again sorry but forget exact plug number. On second thought, I think we might have put in some Bosch Platinum's at one time, and that might be what is in there now. Yes, convertor properly locks up and disengages with brake pedal or slow speed (factory wiring harness to factory tranny). I think this early truck version valve body always stays locked when in 4th, unless brake pedal switch is activated. Yes, TV cable is properly adjusted. That was another question. It was a royal pain to do with quadrajet and stock '70 intake manifold. Had to modify carb linkage, cut and weld, extend out away from carb so it would clear intake, etc. Would like to get another carb, but fear throttle plate linkage would not fit. It is properly adjusted using a line pressure gage at various speeds. This post was a quick one. I can recheck all the numbers for exact if you think it would help. Hitting around 18MPG with TH400, thought 20 to 22 MPG was within reach using OD. EDIT. Looking at some past posts, and gears are 3.06, and gas mileage was around 16mpg not 18mpg. Cam I went with was CC 268H. From there site: The 268H cam is an ideal cam for big blocks towing heavy loads equipped with lower optional rear end gears.
The factory set-up would look for minimum Throttle Postion Sensor voltage on a coast down and disengage the torque convertor clutch anticipating a stop. If you aren't using the TPS, this could be why it feels like the TCC is staying engaged on a 4th gear coast.
Thanks for the input. You can not disengage this versions lockup when it is in 4th gear. Even hitting the brake does not disengage lockup until it slows enough that it downshifts to 3rd. Lockup is controlled in 4th by line pressure, not voltage. You can remove voltage and it will still lockup in 4th. Looking at transmission rebuild manuals, and out of the 10 or so 700r4 valve bodies, this is the only one like that. Adding the electrical connection does allow the converter to lockup/disengage in 2nd and 3rd gears. I think anyway... It's been 6 months since I was into the manuals and research and road tests with meters and gages. Either way, if it is only lockup, then if some amount of throttle was used, as would be the case running at 70MPH, converter would stay normally locked up, and therefore the engine braking effect at coast would not have much effect on gas mileage? (Flat Florida roads). Under normal driving, I would generally not be disengaging lockup under normal highway cruising. Seeing the coast results, I was concerned something was dragging down or braking all the time, causing additional throttle pressure to counter, which adds to worse gas mileage. If it is just converter lockup, I guess I can skip that coast issue as it relates to gas mileage.
OK, couple of things come to mind. first off the van likely has 3.42s. A carbed 305 in a huge van needed help. second, the 700r4 needs no computer input. its TV cable only. The TPS comes into play the 4L60E, the late model 700r4. I thought the only input from the computer is lock-up. I've played with several 700s, and every one had electrical lockup. Fancy 700 swaps can be had with a auto lockup type deal, that is run off of line pressure. its by no means a computer though. perhaps this is what your tranny had installed, since you say it was built properly. Third, if the carb isn't right, the timing is off, good mileage isn't going to likely happen. lastly, your a sick man. vans a pain to work on, and you swapped in a BBB. you may consider electro-shock therapy... :moonu:
it is a bigger engine but i think you need more tuning the stock computer will try to bring the timing way up at cruise to get mpg if you really try i bet you could get the computer to work on the 455. use a v6 computer dist and add the v8 pickup parts and an o2 bung 305 carb with some tuning. i also think you might have a problem with the roller clutch in the trans as you should get more freewheel on coast then your describing
I think there is some confusion here. It's an '84 GMC Vandura G2500 hi top custom conversion van. Stock '84 had no computer in the 305 truck or used in 700r4 Trans. Old way under powered 305 with 700r4 got about 12 MPG at 60 MPH struggling and kicking in and out of OD at slightest hills in the road. 305 was removed 5 years ago and replaced with 455/TH400. (had a few left over 455's). Van running 455 with TH400 got 16 MPG at 70MPH, never struggling. The 455 was the perfect replacement motor for the van. The only change made relating to this thread was replacing the TH400 with the 700r4, and seeing gas mileage go down. Motor stayed that same. But, I think this releasing lockup in coast as one of the causes of poor gas mileage can be eliminated, because it would generally stay locked up in any vehicle (any year 700r4, 4l80), traveling down the highway with some throttle pressure. Which leaves carb and ignition as more probable causes.
Personally I'm thinking that cruising RPM is too low; need to get up further toward the engine's torque peak to get better mileage. Dropping RPM at cruise is most often where you see improvement, but there's such a thing as overkill. Isn't third gear 1:1? Maybe a trial running awhile in third would give you an idea of what's up, mimicking the final drive of the TH400. Devon
Yes, 3rd in the 700r4 is the same ratio as 3rd in the TH400, so that would be a good test for me to try. I guess I am thinking wrong when seeing OD transmission improving gas mileage, because I guess for most here with muscle cars, they would have had a different rear end ratio, and for them it would help. My goal of 20MPG is fading fast. The sweet spot for crusing RPM running the stock van rear end gears might have been where the TH400 was putting me. Perhaps rebuilding the TH400 and different gears in the rear would have been the best choice. Since I've invested so much in the 700r4 already, gear change might be cure, plus it would make it even more fun around town.
It might help to check the vacuum on the engine in overdrive. If the engine is carburated, it may be that you will have to play with the metering rods as the vacuum may be low enough to engage the power circuit:TU:
Thanks! That is a great tip that might lead to the solution!!!! Guess it's time to buy the Cliff Ruggles book, unless some quick pointers can be suggested here. BTW, old photo of the two motors side by side show the 455 is actually smaller overall than the smog 305. http://www.robstv.com/Buick/c-305vs455.jpg The Pontiac in the background, as well as a not in view '68 Cutlass, also have BBB 455's in them as well!
Just did a road test, and learned a few things. First, I was wrong about lockup in 4th. Sorry. If I tap brake, lockup disengages, and RPM's rise about 300 RPM. Speed at RPM in 4th with converter locked up: 46mph 1100rpm 50mph 1250rpm 55 = 1350 60 = 1500 65 = 1650 70 = 1750 76 = 1900 80mph = 2000rpm all +/- 25 rpm. Also test showed that it has bad carb issues. In 4th, at any speed, slight acceleration cause stutter, bucking and other nasty things until pressed enough to kick into 3rd gear, then it takes off. I'll go to carb section and get it right first before continuing this thread. Problem is, throttle plate for 700r4 that clears stock '70 manifold. My throttle plate was butchered pretty bad getting it to fit. Thanks for the help anyway. It got me motivated to fix issue.
I know something was mentioned earlier- but- do you have a 4 pin wiring harness running down to your 700?
WOW! Those are some LOW RPMs... See what your gas mileage is in 3rd gear with the T/C locked. I bet it is better. -Bob C.
I have a 3.31 axle in my GS with 275/60X15 tires and the tach reads 3000 at 70 MPH in conventional third gear. I have a 200-4R which has a little stiffer (.67 vs the 700R4s .72) overdrive gear, and the tach reads 2200 at 70 MPH. Before I installed the 200-4R I got 14 MPG on the highway. With the overdrive I get consistent 17+ I have a switch for the lockup, and don't even use it below 60MPH as the cam makes it lug. I also have a 69 Sport Wagon with a 350 4bbl and a 3.42 axle. When I got the wagon ,it was a 14 MPG car and with tuning and a200-4R I have it running 20MPG at 75 MPH There is a good possibility that you are pulling too much gear and the carb is way down on vacuum.
I have a 66 skylark with 3.90 rear posi and 2004r tranny and at 4th lock up im at 2050 rpm at 55 mph 255/60/15 2100 stall conv.
