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High RPM
https://slantsix.org/forum/viewtopic.php?t=22363		 Page 1 of 1
Author:  Bren67Cuda904 [ Thu Mar 22, 2007 6:17 am ]
Post subject:  High RPM
It seems that the engine is turning pretty fast at 70 MPH with 2.76 rear gears.
I verified speed with GPS. Tach reads 3050 RPM. I felt 3 shifts. Is the tach wrong. Tach is an Autometer and the dip switches have been set for a 6 cylinder. The reading is coming from the MSD and it has the correct wire cut for 6 cylinder. The rear gear was verified some time ago by rotating and counting drive shaft. This can't be a normal RPM. I plan to install 3.23 or 3.55 gears and already turning 3000 rpm, I won't be good. The tach must be wrong.
Oh 26" tire
Author:  argentina-slantsixer [ Thu Mar 22, 2007 6:57 am ]
Post subject: 
wire cutting on a MSD only has to do with rev limiting feature. Tach output send out a 20% duty cycle square wave signal. You have to verify if your tach is indeed capable of reading this signal. Otherwise you might be needing a MSD 8020 tach adapter. Double checking your settings on the tach wouldn't hurt... other thing you might want to do is get a alternator field tach (diesel tach) so you can install it side by side with your autometer and see how bad it is. 3050 rpm with what size tires? I don't think that's correct. Plus the 2.76 rear... with 205/70 14 you should be seeing less than 2.5 k rpm... unless... :roll: your trans is slipping BAD... :cry:
Author:  Dart270 [ Thu Mar 22, 2007 7:09 am ]
Post subject: 
Tach or ign signal must be wrong. You should be turning more like 2700-2800 with an auto. I have seen some tachs overestimate RPM, probably to make you think you are revving higher for bragging rights or something stoopid like that.

3.23s at 3000 should run you around 70 mph with an auto. Drop 2-300 rpm for a stick. I have cruised my 64 Dart many 1000s of miles at 3500+ rpm with 3.23 gears and had no troubles.

Lou
Author:  Bren67Cuda904 [ Thu Mar 22, 2007 7:36 am ]
Post subject: 
As I remember the instructions that came with the tach showed a MSD configuration. Green wire in the side of the MSD 6AL box. I would think that this means that the tach is compatiable. I will check the dip switches again but I am 99.9% sure it right. The calucaltions I coming up with is 2500 RPM. Is it possible for the stock 2000 stall torque converter to slip over 500 rpm?
Author:  Bren67Cuda904 [ Thu Mar 22, 2007 7:42 am ]
Post subject: 
What can I use to check RPM? I have seen a tool that you apply a reflective strip to the damper and point an electronic device at it and it displays the RPM. What other tools are easily available and cheap to test this. Cheap being under $25
Author:  Bren67Cuda904 [ Tue Mar 27, 2007 5:38 pm ]
Post subject: 
Tach is MSD capatiable. Haven't found a cheap way to verify tach. Can't check the dip settings because I lost the instructions. Looked online for the Autometer Z series 2698 tach and found instruction, but mine has a square box with 2 dip switches and the instructions online don't. If I were off on the settings I think it would be off by more than a 1000 RPM and I am not.
I just had a though. Do you think I could just install the 3000 rpm MSD "chip" and put it to the floor and see what the tach says?
PS the tach output is 2.5volts at idle.
Author:  SlantSixDan [ Tue Mar 27, 2007 9:31 pm ]
Post subject: 
26" tire circumference is 81.68"
There are 63,360 inches in a mile
Therefore, 775.71 tire revolutions per mile
times 2.76 rear axle ratio = 2141 driveshaft rev/mile in third (direct) gear, therefore approximately 2141 engine RPM at 60 mph (one mile per minute). Therefore 2498 engine RPM at 70 mph.

If you "felt three shifts", you weren't counting correctly. A Torqueflite will give you TWO shifts (1-2, and 2-3).

If your trans were stuck in 2nd gear, however, you'd see approx 3104 engine RPM at 60 mph.
Author:  Bren67Cuda904 [ Tue Mar 27, 2007 11:31 pm ]
Post subject: 
I am not stuck in 3rd.
Author:  70valiant [ Wed Mar 28, 2007 3:48 am ]
Post subject: 
I turn 2900rpm at 70mph with 24" tires and 3.23 gears.
Author:  SlantSixDan [ Wed Mar 28, 2007 9:12 am ]
Post subject: 
Quote:
I am not stuck in 3rd.
Clean your eyeglasses and try again.
Author:  Bren67Cuda904 [ Wed Mar 28, 2007 9:19 am ]
Post subject: 
Ohhh-my irritating friend. Correction not stuck in 2nd.
Installed the 3000 rpm chip and the engine climbs to 3000 rpms on the tach and climbs no more. Humm I am going to retest with the GPS.
Author:  Dart270 [ Wed Mar 28, 2007 7:35 pm ]
Post subject: 
OK, my guess is that with 26" tires you are actually going 75-80 MPH when the speed reads 70. The stock speedo/gears would have been calibrated for ~24.5" tires. How accurate is the GPS? Seems like it would be better than 5 MPH error?

Lou
Author:  Bren67Cuda904 [ Thu Mar 29, 2007 3:42 am ]
Post subject: 
The GPS is dead nut. Tested in 4 differrent cars. I haven't had time to recheck but it's got to be me smoking something, because it not making sence.
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