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th-400 to 5spd (2wd) conversion Q's

Discussion in 'Chevy Trucks' started by silvershad0w, Aug 25, 2003.

  1. silvershad0w

    silvershad0w Junior Member
    Messages: 5

    Hello All,
    My first question here is regarding that '88 in my sig. It is my "everyday" truck and I would like to get the best milage out of it that I can. Right now I only get about 15-16mpg with that 4.3 and the 3 spd auto, and I drive it easy.

    My intial thoughts are to install a 5spd manual into it, but I need to know if anyone has any experience with this. I figure on needing the flywheel/clutch/PP assy, tranny, pedalbox+hydraulic clutch cylinder... What about anything extra like if the driveshafts are different, plus I have never swapped out a hydraulic clutch before, just the older cable style. Any heads up would be really helpful before I attempt this.

    Also, I had thought about a 700r4, but I figure I would get slightly less mileage than the 5 spd. would give. Plus with that I need to change some of the linkage and components on the steering column, I thought they might be a little more year-year dependent, like 93 stuff might not fit the 88 I have. Any other viewpoints on this would be helpful thou.

    Just FYI, a yard checked the glovebox option codes, and says I have a 3.08 rear end, which I guess was popular in the worktruck models. This seems like it should give me good milage when coupled with an overdrive tranny, just sacrificing a bit of pulling power. I do want to make sure the 4.3 has enough torque to pull that ratio without bogging out thou, like I say, most of the time this truck is unloaded except for the tool box.
     
  2. wyldman

    wyldman Member
    Messages: 3,265

    I don't think you would see a big improvement in economy with a 5 spd swap.It can be done,but it is a lot of work.You'll need a trans,flywheel,clutch setup,hydraulic master and slave cylinders,shifter,pedal assy,crossmember and driveshaft.It's easier to get a complete donor truck,and do the swap.

    You might see only 1-2 mpg with a 5 spd,or even the 700R4.It would really only be a big benefit if you do a lot of high speed highway driving.

    I'd stick with the TH400,as they are virtually a bulletproof tranny.The 5 spd,and the 700R4 aren't as good.
     
  3. silvershad0w

    silvershad0w Junior Member
    Messages: 5

    That is a bummer, I guess it might help as I do a lot of highway miles, about 80-120 a day, I would suspect the highway milage would go up by a bit, I know some of those 700r4 websites I read claim up to a 40% improvement in mpg. Well I guess first I'll look into the cost of such a thing, and then if that looks good I might go ahead.

    Another odd bit, I had a '89 2wd with a 305 and a 700r4, it got 28mpg consistently.. Is this common for an 8 to get better milage because it doesnt have to pull as hard? Since the truck I have gets so many miles on it, I really could use a savings in gas, especially with the games they like to play with the prices these days.

    Just found a donor truck here, 200 takes all the necessary parts you list, but its got 160K on the odo. Seem worth it for that price or is this more trouble than its worth? If this is too much of a pain, I may just wait until I get back from my deployment, and then buy a new truck. Should have a good surplus of extra cash floating around then. :)
     
  4. wyldman

    wyldman Member
    Messages: 3,265

    If you can get the donor truck cheap,and do the work yourself,it wouldn't be too bad.I wouldn't use the clutch\pp\release bearing unless it looks like new,so you may want to figure on a new clutch kit.The slave cylinders tend to actup too,so you may want to look at a new one.

    28 Mpg is pretty high.Maybe long highway drives may get close to that,but it is still pretty high.You may not see that with with the V6.The V8's will generally pull better mileage.
     
  5. ratlover

    ratlover PlowSite.com Addict
    from IL
    Messages: 1,325

    You sure its a 400? Not a 350? Is the pan square with a corner taken off? dont much matter......

    A manual conversion is kinda a PITA IMO and not worth much MPG compared to a OD tranny.

    88's were available with 700r4's in em. It would be a fairly easy swap but a 700 is longer so you will need a shorter shaft(easy to have done) Depending on what tranny you have now depends on what you will need to do about the yoke. but thats an easy deal.

    With a 700 you will probably notice a bit more get up since 1rst gear is lower but it will be a dog in OD. And dont even think about towing anything in OD. With a 4.3 you are going to have your foot into it fairly heavy to maintain higher speed. 65 should be ok but that 4.3 will be getting a workout at highr speeds.

    28 mpg in a truck?! I got friends with new 4 and 6 cylinder s10s that dont come any were near that. I wouldnt expect it outa this ride. I'de be happy to hit 18.

    is the only reason you want a stick is for the milage? auto would be easier
     
  6. wyldman

    wyldman Member
    Messages: 3,265

    It should be a TH400.That was the only 3 spd available in the late 80's,and early 90's trucks.Should be RPO code M40 in the glovebox.
     
  7. silvershad0w

    silvershad0w Junior Member
    Messages: 5

    Surely is a 400. I know that the 700 is a popular one to put in, I am curious about the linkage swap to get the 4spd linkage in it if I go that route. I have never liked autotranny's much, but the 700 is one option I could live with. If I go that route, all I should have to do is get the driveshaft with the tranny out of the same WB truck, right? Then do whatever is required with the linkage, it would be an easy swap. I can get yard tranny's here for 75$, with low miles, there are at least 10 of em sitting out there now.

    As for the 28mpg, probably the fluke of all time, or just the way my grandad drove it, but he could make a 34 gallon tank last a month and a half driving back and forth to work every day. I calculated it several times, I couldn't beleive he could get that much milage out of that truck. I would be happy with 18, but I would hope for 20-22 highway. I never run it over 65, 'cause I'm trying to save gas here :) The ford eats enough $$ for all my trucks. Now deisel is cheaper than gas, been long enough waiting for that to happen.

    For the life of me I can't understand why manual tranny pickups are so hard to find, these auto's sure are popular. Ever since my first 78 chevy, I have wanted a stick, whether it be a car or a truck. Gas mileage was just an added bonus to the manual. 78 didn't have the fancy 5 spd w/OD either. Do many guys plow with a manual, because all plow trucks I've seen/run around here are auto's too. I must be a minority wanting a stick shift for my truck!
     
  8. wyldman

    wyldman Member
    Messages: 3,265

    The 700R4 linkage is the same.Only difference is the detent quardrant in the column.You can use your existing column,you just won't be able to select low gear.If you get a donor truck,just swap the column.

    You could use the driveshaft out of a donor truck as long as the wheelbase and the yoke is the same.

    You will need to get and hook up a TV cable or the trans will not shift correctly,and even burn up.
     
  9. EZSnow

    EZSnow Senior Member
    Messages: 205

    Throw an automatic in it and get a floor shifter- then you get the best of both worlds!!

    Seriously- I'd go with a 700- the reliability has greatly improved since they got their bad rap in the late 80's. But you are going to have trouble gettin' her up a hill in O/D with those gears, but flat cruising should make some hay for mpg.
     
  10. Alan

    Alan PlowSite.com Addict
    Messages: 1,393

    I think that you just might find there would be no payback in doing this swap. OD works best when you have a high torque engine with a lot of displacement. The main efficiency of OD comes by reducing pumping losses which are internal to the engine and are a major factor in efficiency. Pumping losses multiply with the square of RPM so there is a real advantage in decreasing operating RPM.

    With your 3.08 rear, and assuming an OD of roughly .8 to 1 you would have a final drive ratio of somewhere in the 2.46 range. I think some of the current ODs are more in line with a .7 ratio which will give you an even taller final drive ratio, on the order of 2.16.

    With that kind of tall gearing your 4.3 is going to be way below the RPM range where it can live comfortably.

    My own experience with OD vs. Direct top gears has been that you see a highway mileage improvement of under 5%. Maybe you would see more going from auto to manual but you're going to ahve to throw a boatload of $ at it to get there.

    If you're currently getting 16 mpg and paying $1.60 a gallon that's $.10 per mile fuel costs. Let's assume you will improve mileage by 15% to be on the really optomistic side, that's 18.4 mpg for a fuel cost of 8.7 cents per gallon. Your saving will be a whopping $.013 per mile. In 100,000 miles that's a saving of only $1300.

    Unless you're really packing the miles on there is no way you will see payback on this conversion. The tall gears will reduce the driveability of the truck and increase the maintenance costs in the form of clutch problems.

    It's your nickel, but I think you would be better off keeping what you have and possibly looking into cam/manifold changes to increase the efficiency of your current drivetrain.
     
  11. wyldman

    wyldman Member
    Messages: 3,265

    Excellent post Alan.I was going to try and do a cost breakdown but just didn't have the time.I think once he see's that,it may change his mind.
     
  12. silvershad0w

    silvershad0w Junior Member
    Messages: 5

    Looking more into this 700r4, and taking into considerations of my cost for the swap, and savings, etc. Here are the numbers I have come up with.

    The 700 swap will cost me $165, if I get the tranny, crossmember, TQ, driveshaft, steering column. If I get a rear end, something recommended, like the 3.42-3.70, It would run me $240 total.

    Manual Swap runs me slightly more, 185 total w/o rear end swap.

    So, for a basic 700 swap without the rear end, it would take me 12,700 miles to recoupe my money, using your calulation, if i see a 15% increase. That would take me 3.5 months of my everyday driving. After that, for the first 100,000 miles, that leaves me 1100 dollars to spend that I wouldn't normally have, then I could drop in a 305 or beef up the 4.3 for low end. With 1100 dollars, I could do a pretty nice engine swap, and repaint the truck, and pay for all the scheduled maintence for a few years. For as many highway miles as I do, it does seem like a good investment, easpecially if gas doesn't fall back down to 1.60 any time soon :) 3.5 months for turnaround isn't bad. Now for a manual conversion, I would have to wait about 4 months for a return on the investment.

    I also ran out the RPM's at different speeds, to get an idea of what I was looking at. right now, truck would turn 2475 RPM's with 5% TQ loss added in. Doesn't seem too bad for highway driving. Since I don't have a tach on board, I can't really verify if that's what I am seeing. It seems like it is higher, someone may have switched out rear ends on me. I will measure it later today.

    With the 700r4, which is 0.7:1 OD best I can find on the 'net, it would be down around 1630 RPMS @65. Way below the RPM's a 4.3 TBI would need to see to generate enough torque I would think. Maybe I can squeze by on straight and level at those RPMs. Even if I can't, a 305 might like those speeds the way I drive it. I can pick up a low miles v8 for 170.00, I might just do that shortly after if OD doesn't seem to be working out w/ the 4.3.

    Driveability, I would have to say the 700 would be nothing but a increase over the 400. Manual is another story thou. Lower 1st in the 700, coupled with a manual overide for the TQ lockup, I think driveability would be a lot better, even if I can't use OD much with the 4.3. I will probably go ahead with a 700 swap first, and if it doesnt work out, you might be seeing a 700 for sale here cheap in the near future :)
     
  13. wyldman

    wyldman Member
    Messages: 3,265

    If you can do it that cheap,then maybe it is worth it.I could never find parts that cheap in good working order.Don't forget to add in the little stuff,like trans fluid,filter kit,trans lines,etc which should be done during the swap.

    If you can get stuff that cheap,I'd just go all the way and get a complete 305 TBI,700R4 combo,and drop it in.It will pull the 3.08 just fine,and is one of the best combo's for mileage.
     
  14. silvershad0w

    silvershad0w Junior Member
    Messages: 5

    Should be able to find parts like that all around the country, standard U-Pull-It type pricing. Check out www.sechristautoparts.com if you are in/near Western PA, East Ohio region. When I was stationed in Ft. Knox, pricing seemed about the same, and they have similar prices in U-pull-it's around Wash/Oregon state last I was there too. I don't think these are any kind of "special deals" or anything.

    Conversely, if you need used parts but can't get em cheap near you, I will pick up stuff and have it shipped out to you. I've been doing that for a lot of years with another weird pet hobby of mine, 80's Subarus. I make a trip to the yard a couple times a month on average.