Conventional carburator wisdom is to drive it at an rpm that the engine will be pulling the most vacuum. I*think*the whole lowering the rpms with an overdrive is more suited for a fuel infected car where you can get precise spark and fueling at any given rpm and load (kpa).
Thanks for posting your results for comparisons. I'll road test later today, checking RPM and feel in 4th with lockup off, and also check RPM and feel in 3rd with lockup on and off.
Have not had a chance to road test again, but did find a site with a great calculator: http://www.angelfire.com/fl/procrastination/rear.html Using the above calculator, and the results indicate I was correct when stating it has a 3.08 gear. The calculator agrees with me on speed at each point measured, so it is 100% accurate. Here are numbers again when using 235/75R-15 tires (28.9") (Tire size calculator http://www.miata.net/garage/tirecalc.html) Road test results (matches calculator): 700r4 (final drive = 0.70) w/3.08 (converter locked up) 46 = 1100 50 = 1250 55 = 1350 60 = 1500 65 = 1650 70 = 1750 76 = 1900 80 = 2000 Calculator numbers based on original trans would have been: TH400 (final drive = 1.00) w/3.08 46 = 1650 50 = 1800 55 = 1950 60 = 2150 65 = 2350 70 = 2500 76 = 2700 80 = 2850 Basically, Old TH400 @ 2500 RPM = 70 MPH New 700r4 @ 1750 RPM = 70 MPH So I guess I should be thinking Target 2100 RPM @ 70 MPH = 3.70 gear needed? That should improve MPG from TH400 by lowering 70 MPH needed engine speed by 400 RPM Or, if I turn off lockup converter, RPM's will increase about 300 RPM. That would be the easiest thing to do, but heard it should never be done, since it causes trans to run too hot, which leads to quick breakdown. I am running synthetic fluid to help with temps. What do you think?
I sent you a few pms- I don't use my lockup in my truck very often, and I have a trans temp gage and an external cooler- trans temps run in the 180s- no difference with the lock up on- my .02- driveability in your case will improve with the lockup off during all but freeway driving.
Thanks for the replies. Has your 700r4 been built, and have heavy duty parts in it making it suitable for 4th gear towing? I tow a 20' center console and occasionally a loaded car trailer to the track. Just did another road test for 3rd gear as another poster suggested. Lockup does not engage in any gear but 4th with current stock wiring. What that means is that in 3rd gear, the converter is not locked up, so it slips, and therefore I am turning higher 700R4 3rd gear RPMs 1:1 than I was with TH400 in 3rd. 700R4 in 3rd (no lockup) 46 = 1850 50 = 2000 55 = 2150 60 = 2350 65 = 2550 70 = 2700 76 = 2900 80 = 3050 So using 3rd with lockup will at best only be as good as TH400 3rd gear was. I've learned that comparing lockup to non-lockup type converters, and it is opposite of what I had first thought. Lockup converter basically is a non-lockup when engaged, and a slipping non-lockup when disengaged (this is proven by 3rd gear tests above). It might not improve performance when locked (compared to non-lockup types), and improves performance when you disengage lockup (slips, raises RPM's, which is needed when stopping and desired when accelerating). It's key is the ability to slip when disengaged, not help when engaged (comparing to non-lockup). If I switched to a non-lockup type converter, it would run as though it was always locked up, run at lower RPM's, and not slip.
the 700 was built to handle 400/400- its strong- I just don't see the need for 4th gear lockup in my application. I never get on a straightaway over a mile or 2. Traffic on this island sucks- so basically all i do is town driving.
Yep, very different application. Here in Florida, I rarely see a corner or hill every 20 miles. Across alligator alley it is pretty much 100 miles of straight flat road with one exit near the middle.. A typical monthly trip to Ft Lauderdale or the Keys would be 300 to 500 miles each way. I think 400/400 is also weak considering the 455's had 510 lbs of torque stock, and the '70's compression had close to 400 HP with very slightly modded stock motor. The torque was my biggest concern. The 700r4 was never made to handle a big block, but can be modified/upgraded to run it well. EDIT: Friend just found an old set of 3.73 gears buried in his garage, so I get a free upgrade to 3.73's. He gave away the 10 bolt 8.5" posi chunk with a car, so would be nice to find one of those to swap in at same time. (hint hint anyone, cheap $50 - $100 range). Will post back with performance and economy results in a couple weeks. Funny thing is, I've been preparing to upgrade rear in my Mark VIII, and the going rate on ebay for posi unit "with" 3.55 to 3.73 gears is $40 to $80 total, and just now won a good condition 3.55 set of gears with posi carrier @70k miles for $40!!! Too bad the GM stuff cost 4x the price.
i dont know if its too late, and others can assess this, as i dont know if its true or not. My first thought when reading it was, at that low of an rpm your timing is backed out, and therefore leaving some MPG to be had by recurving the distributor to be all in earlier.
Thanks, it is not too late for suggestions. Looking for a posi carrier to install at same time as gears. Distributor is stock curve HEI, so that makes a lot of sense. I planned on installing a curve kit anyway, so probably a good idea to do that first.
Has anybody calculated the cruize rpms of a 70 225? they might be close. with enormous tires and 2.56 gearing the are probably close. I know when I had the 2.56s in my skylark the tach was rarely over 2k even at highway speeds.
The 3.73's are free, so make a good choice :grin: This morningg I bought the Crane vacuum advance kit, plus the MG mechanical advance kit, and while in there found I had no mechanical advance. It was tough to budge, so was pretty much static at what ever it was stuck at. WD40 loosened it up pretty good, but still not enough for springs to fully return weights from extended position. Will keep messing with it until it is correct, then test drive to see how properly setup distributor helps before continuing with gears.
also I would power time it... then look at your initial. those hei units have way too much advance in them for a lot of guys. I have a lot of cam so it's even worse. You may need to limit the advance somewhat, depending on what you find. so be aware. I bet it runs way better just freeing the advance.
Timing light just took a dump in test, so initial settings rough guess. Weights move freely now. Since I want advance all in very early RPM's, started with both yellow springs. It does run much better out of the hole, and G-Tech showed 10.13 in the 1/8th mile, upper 15's in the 1/4, spun bad and it auto short shifted in drive.. Cruising in 4th at 60MPH feels a tad more responsive, but still bogs or lags under light acceleration. I'll try a new timing light and finish the job, but looks like it will still need the gears.
i think you should try to get the timing all in before your standard cruising rpm, that way its advanced when cruising. I would do the gears also :Brow: It would be nice to smoke some cars in an endura van :grin:
Using the two yellow springs is as early RPM that the kit provides. Unless I try the two black springs from the other kit? Thought the yellow was lightest springs. That was also why timing light was not as much of an issue right away, since I wanted to go all in as soon as I could, with lightest spring anyway. Will use timing light this weekend. Edit. Updated with speeds after changing to 3.72 gears and 100% correct speedo gears (gray/black w/29" tires) GPS confirmed MPH. 700r4 w/3.72 46 = 1400 50 = 1500 55 = 1650 60 = 1750 65 = 1900 70 = 2050 76 = 2300 80 = 2425 Before was rough speeds due to close but not exact speedo gears: 700r4 w/3.08 46 = 1100 50 = 1250 55 = 1350 60 = 1500 65 = 1650 70 = 1750 76 = 1900 80 = 2000 This puts motor RPM's right between old TH400 speeds and 700R4 speeds, and it feels better. HUGE improvement out of the hole, although that was not what I was after. Power timed helped a lot. But, still spits and sputters in 4th under moderate acceleration. Next up, proper carb setup.
