Hello, Can someone tell me if maybe they have run into a similar situation? Im running a 67 pontiac t-400, that was modified into a switch pitch at tri-shield by Jim. Low and high stall seem to work fine. But my cruising rpms are very high. Im running a 3.50 rear gear. Took a buddy out for a ride and he swore that the car is running to fast for it to be a 3.50 gear, so we jacked it up, put it in neutral and spun the rear wheels 1 full rev while counting the rotations of the drive shaft. 3 1/2 turns of drive shaft for each full rotation of the wheel. Took the car out and recorded rpms at various speeds and got on the computer program for calculating ratios (using tire dia, etc) and everything points to a 4.86 gear. All speeds are right in this zone, and I dont feel as if tranny is slipping or anything out of the norm, Fluid looks and smells good. Ive driven approx 5k miles like this. Would a torque converter do this? I thought all t-400s had a final drive of 1 to 1?. Im kind of stumped as to whats going on here.o No:
All turbo 400's are 1:1. You can feel the difference between low and high stall? Foot brake each and see what rpm's you get on the tach? You shouldn;t be over 2500 rpm @60 mph.
4.86 gear? Give us the RPM's at various speeds. If the car wasn't upshifting into 3rd gear, your overall gear would be 1.48 X 3.50 = 5.18. Are you sure it is shifting from 2nd to 3rd?
Re: switch pitch Just got back from following my wife and son down the road and had them go exactly 60 while we stayed on the cell phones and I follwed right behind them, because I never really checked to see if my speedometer was accurate or not. @3740 was the where it was holding rpm wise, and my speedo was @58mph. I have 3 gears in manual shift and I feel all 3 engage when in auto. 1st to 2nd shift at @10mph at light throttle 2nd -3rd at @20-25mph. Its funny you should mention that 2nd normally would put me pretty darn close to where Im at. Im using msd 6al- with one of those shift lights that have a small digital read out of rpms, I feel it is accurate as I have been using it to tune with (right around 800 rpms at idle) - (all in at 2800RPM). tires are 225/60/15 all around Im missing something here, dont know what yet
Re: switch pitch That makes no sense to me at all. Unless the trans is slipping bado No: Let me know when you find out what you are missing.
Larry, the way we checked the rear end ratio, is that the correct way to get in the ballpark? Could I check this with the rear jacked up start the car and put it in drive, and let off the brake enough to let the tire spin 1 turn and see how many times the driveshaft turns, should still be 1 to 1 right. If not the trans has some funky final drive gear or slipping disks or converter malfunction
That method is only somewhat accurate if you have positraction and it is functioning. best method is to remove the cover and check the ring gear stamping
regretfully I put a 9" back there, so I cant just pop the cover off any more, but what Im thinking now is this winter Ill take it all out for cleanup and detailing and see whats going on in there and make changes then. In the meantime Ill give this some more thought, and do some research on the tranny. thanks for your input on this, at least Ive got a place to ask this kind of stuff and not feel ashamed, thanks again
Are you certain you are getting both high and low stalls? Do the foot brake test and switch the converter manually. You can tell when it switches, no question. It sounds like it may be stuck in high stall which will slip quite a bit compared to low stall. 3450 is wacked for 60 mph. I have a TH400 SP with 3.73 rear and turn just over 3000 at 70 mph. Good luck.
Did you actually do the brake torque test? As norbs suggested, pull the electric wire and do a reading and then one with the wire connected. That will tell you if the SP is actually working or not. Assuming you have a stock converter, I'm thinking maybe 2000/2500 stall speed? o No:
Took the car out this evening to try see if the switch isnt actually switching and to attempt a proper foot brake test. I have the foot switch for the s/p going through my "Fasten Seat Belts" dash light to come on when high stall is activated only. It comes on and off with each press of the pedal but I still didnt rule it out. I then found an empty parking lot held my foot firmly on the brake and slowly ran it up in low stall till both rear slightly let loose @1900 2x, high stall @2800 also 2x, put the car back in low stall and drove home noting @2500 at 40 mph, 3000 at 50, and it did seem to be pretty much a 500 rpm spread for every 10 mph from here on down. Ill try and bum another tach from someone at work and see what it has to say. thanks guys
Carry I had the exact same thing happen with my 65 GS..my tach was showing rpm's that would have meant i had deeper gears than what i had.. turns out my auto gauge tach made by auto meter was off by 500 rpm
Those autogauge tachs are pretty crappy, get a half decent autometer one for $100 or so and it will be right on +/-. www.autometer.com
