TBI or 700R?

[herb101]

Which single mod (TBI conversion or 700R4 swap) would give the best mileage improvement?

For the requisite background, the truck is a 71 K5 with 350, Edlebrook carb, TH350 and 3.73 gears, running 35's. I get about 8-9 mpg now and the 30% gas price hike has me thinking...

I have the ability to perform either swap, so install costs won't be a factor. I'd like to do both eventually, but which should be first?

Thanks,
Herb

[metalmassfab]

I say hands down, no contest....700R4. I was getting that same 8-9 mpg with the same engine in my C-20 with a TH 400, 4.10 gears and 31" tires. She screamed at around 3200+ on the highway. The 700R4 dropped the rpm down to under 2400, and the mpg jumped to 14. A while later I installed a 510 hp 383 (pic is for proof...) and I got just over 16 mpg with that combo. Sounds like BS, I know! But a well worn 320 hp 350 still wastes more fuel than a well tuned torque monster. Of course when the pedal hit the metal on the 383, all that went right out the window...go for the tranny, its the best thing you can do for your rig!

[RACINJASON22]

Atleast on the more power part. I've seen it first hand on a friends truck he was selling. He wanted to keep his well built higher HP engine so he took it out and we put together a new well rebuilt stoker and his gas milage went ta s%$. The harder ya have to run them the more gas your gonna use.....hmmmm....well I guess that answers your question too working the engine less with the overdrive would kinda be the same but different right?? Well thats my point. RJ22

[gringoloco]

Overdrive is more beneficial to fuel economy than EFI. EFI just multiplies the effect by more precise metering of fuel at lower rpms...

[Chris27]

Overdrive is more beneficial to fuel economy than EFI. EFI just multiplies the effect by more precise metering of fuel at lower rpms...

Do the overdrive first. If you had a Q-jet on that thing you wouldn't notice much difference between that and TBI for gas mileage.

[chiggly626]

With your current gear ratio and tire size I would go for the TBI. You will need at least 4:11's to use OD with 35's.

[metalmassfab]

Another note: TBI is not much better, if any, than a carb of the right size and decent tune up. TBI squirts fuel in the same manner as a carb. For the serious mpg gains, tuning, and power, a multiport fuel injection system unit like the Holley MPI Commander 950, Edelbrock Pro-Flo, Accel DFI, Fast unit...blah blah...These have the injectors placed as close to the cylinder as possible. TBI swaps are pretty popular, and I am not just talking smack about them; I just dont think its a worthwhile swap, especially if you will be adding performance upgrades later. TBI units just dont handle performance applications well. Like Holleys $900 2BBL digital TBI unit...it'll handle up to 275 hp...!! Three times the price of a carb, and no better performace, marginal economy. I do understand that a TBI is probly better on a stock type engine on a 4X4 that sees a lot of climbing at angles that can flood a non-prepared carb, so there is a plus side.

The 700R4 will increase your mileage, and with its deeper initial gear set, your acceleration will be vastly improved over your current tranny, tire size, and gear ratio. This is the best thing for 4X4's and heavier vehicles...and your highway cruising rpm will be down around 2000 at 70 mph or so. you wont be disappointed!

[herb101]

Thanks for the replies so far. I'd really like to go with a TPI setup, for the performance and looks, but they just don't exist in yards around here & the prices get pretty insane on eBay - maybe someday.

On the 700r4 topic - I have seen writeup for the swap, but only on the 2wd trucks. What is a good doner for a 4x4 swap? I have the TH350 & NP205 setup, so the tailstock pattern and spline counts are in good shape. What years would be the best for durability? I have heard of folks burning up multiple 700s, but is this due to improper setup or weak internals? I would go with the proper TV linkage setup. with teh swap.

Thanks,
Herb

[ffrider]

I know this is basic but can someone explain gearing to me what does 3.73 4.11 etc. gearing mean?

[ABLAZN]

Originally Posted by herb101

Thanks for the replies so far. I'd really like to go with a TPI setup, for the performance and looks, but they just don't exist in yards around here & the prices get pretty insane on eBay - maybe someday.

On the 700r4 topic - I have seen writeup for the swap, but only on the 2wd trucks. What is a good doner for a 4x4 swap? I have the TH350 & NP205 setup, so the tailstock pattern and spline counts are in good shape. What years would be the best for durability? I have heard of folks burning up multiple 700s, but is this due to improper setup or weak internals? I would go with the proper TV linkage setup. with teh swap.

Thanks,
Herb

I have talked the tranny guy that use to build my drag race autos, , about this swap. He also happens to run a 700R4 in his 1975 Blazer with 35's (3:73's) and a 454 in front of it. This is the information he gave me. You want a 4X4 88'-92' 700R4 because these have the thicker case on them. The 700R4 case is longer then the Turbo 350, so you will need to move the T-case back about 2 inches, but on a lifted rig, that is not a bad thing. The output shaft in the needs to be replaced with a longer one that can be bought for about $150, but there is no no need for any other adapters . When he rebuilds them, he puts extra clutch packs in and uses the "high energy" factory stuff. His tranny had been in his Blazer for 7 years without a problem, so I think he may know what he is doing. I think that most of my information is right, but other please correct me if I am wrong.

[herb101]

Awesome stuff! Thanks!

ffrider - on the gear thing...the gear ratio is commonly expressed as a single number like 3.73 or 4.11, but it actually means a ratio of 3.73 pinion rotations to 1 ring gear rotation. What this means is that for the wheel to complete 1 full turn, the driveshaft turns 3.73 times. Assuming no overdrive, the final gear ratio in the transmission is 1:1, so 3000 engine RPM = 3000 driveshaft RPM = 802.3 axle RPM with a 3.73 gearset (729.9 axle RPM with a 4.11 gearset).

To take it one step further and expalin why tire size affects your 'effective gearing' and speedometer calibration, consider the tire circumference. A 35" tall tire would travel approximately 110 inches in one rotation, while a 30" tall tire would only go about 92-1/4". That means that the taller tire rotates 576 times per mile, while the short one has to turn about 687 times. Since your speedo is triggered off dirveshaft RPM, you can see where the difference occurs.

When you hear people talk about going to a "lower gear" or a "deeper gear" for more low end grunt, it means a lower ratio or a higher "big" number because 1 / 3.73 is a larger number than 1 / 4.11. A "taller gear" means a smaller "big" number for lower highway engine RPMs.

Hope that helps,
Herb

[pound41]

Check out Bowtie Overdrives web site they have everything you need. Even built trannies for a 4x4 I am thinking about getting one from them. Only bad thing for me they are on the west coast. Not sure about shipping Their stuff is highly recommended by some of the other posts I had read

[Jason915]

I did the TBI on my K5 its a mild build, 355 w/ L98 Iron heads, CPU friendly CompCam, headers, stock GM TBI intake & TB, TBI spacer and it's chipped. With a TH350 3.73's and 33's I am in the 10MPG Club so really the TBI is not the best MPG set up as far as EFI's go becasue my truck with a more powerfull engine and 4BBL gets the same or slightly better. I do love the TBI for 4wheeling it's good for that but not for MPG. I would imagine that a 700R4 would help.

[Scrubby]

I recommend pricing either swap and then doing a cost analysis on it to see if it is worth it for you.

If a 700R4 swap runs you $2000 and gets you 18 mpg (gaining 10 mpg), at $3/gal gas, you have to drive almost 12,000 miles to pay off the swap. If your blazer is a daily driver, that might be 6 months of driving. For my blazer, that might be 5 years of driving. Not worth it to me.

Just something to think about. Now, if you just want to do the swap because it is cool, then go for it!

[onetonk5]

I have never done/worried about a cost analysis for anything i have ever done to anything i have ever owned. Figure if its worth my time to do it's worh it to me. A 700r4 not only will make your rig more fuel efficient but it will be more enjoyable to drive. Deeper first gear and overdrive is always a plus.

[herb101]

What would be a good price to expect to pay for a 700R4?

I have a line on one from an 88 4x4 burb, rebuilt 42K ago, with convertor.

Thanks,
Herb

[BUDDY]

Originally Posted by herb101

What would be a good price to expect to pay for a 700R4?

I have a line on one from an 88 4x4 burb, rebuilt 42K ago, with convertor.

Thanks,
Herb

If you can get that one on EBAY for 400 or less I say go for it.

However, if you double that price you 'll get one that's been completely rebuilt with a warranty of some kind. You could go to autozone or some other local place & get one outright that way as well. Or you could go to the JY, pick one out, & have it rebuilt by the most highly recommended shop around you. If you do it that way it doesn't matter if it's 2WD, 4WD, just have it done the way you want it. Oh the possibilties!!

I think you can pay as little as 800 for a chain store rebuild or as much as you want for a full performance setup. Just remember all the other ancillary things, TV-cable bracket, TC, cooler, etc., etc. Check this site for some GREAT info. I got that link from here.

Good luck,
Buddy

[martinkh]

I am thinking about this for the future also. It seems the weak link on the 700r4 is the lockup. I was thinking of 2 solutions:

Go with non-lockup converter. More heat, but little chance of breaking the tranny this way.

Go with electric controlled lockup, with pushbutton on, only works when a sensor says it is in overdrive/4th gear, and have it like cruise control so that tapping the brake disengages it. But is there an electrical or mechanical output that would tell you when it is in 4th gear?

[Chris27]

Go with electric controlled lockup, with pushbutton on, only works when a sensor says it is in overdrive/4th gear, and have it like cruise control so that tapping the brake disengages it. But is there an electrical or mechanical output that would tell you when it is in 4th gear?

That's kinda how the 700R4 lockup converter in my car works. There are four wires in the lockup harness. You only need two. One is ground, the other power to the lock up in 4th gear. The power wire goes through a brake light switch, to a vacuum switch, and down to the converter. The vacuum switch unlocks the converter during low vacuum conditions (full throttle). Brake switch unlocks converter when brakes are applied. After I get my car out of storage I can check what wires are for what. By the way that "weak link" lockup converter is surviving just fine behind 479HP & 472lbs.ft. of torque. Biggest issue is TV cable adjustment, get that right and it will last a long time.

The 700R4 lockup in my K5 is controlled by the ECM. It locks up in 3rd & 4th gear. Has brake switch, but uses VSS (vehicle speed sensor) instead of vacuum. This one uses all 4 wires on the tranny.

[herb101]

Well - I lost the one I had an eBay line on in the last 14 seconds. I am really beginning to HATE eBay. Now I can't even see the name of the person who sniped to be able to put the proper voodoo mojo on them - friken eBay

Herb

At least I made him pay and extra $60 over what he would have stolen it from me for....

[martinkh]

If you go all electric, can't you bypass the tv cable?

[Chris27]

You still need a TV cable, unless you use a 4L60E. Those are the electronically controlled version of the 700R4. I think the 4L60E's are '93-up. The TV cable has nothing to do with converter lock-up.

[martinkh]

OK, what does the cable do? Is it needed with a non-lockup converter?

[Chris27]

TV cable controls shift points, line pressure, shift firmness, and kickdown. TV cable is still needed with non-lockup converter.