70 MPH vibration and rear end woes....

[Killer_Mopar]

My dad's challenger has a couple of problems that I was hoping I could get resolved. First off, his car has always had a vibration when above 70 mph...I always thought it might be a trans or engine mount so when I put the cam in a couple of weeks ago I changed all the mounts. Well to make a long story short the car still has the vibration at anything over 70 MPH, so I am trying to figure out what the problem is and have narrowed it to two things. The first one is when I was working on his 340 I noticed that the engine has a cast crank but a thin balancer in the front, so could the engine be out of balance and could that cause a vibration at 70 mph? I would imagine that the vibration would be apperant at all high RPMs, but it isnt......it only happens when the car is in 4th gear. The only other thing I can think of is that the driveshaft is out of balance and causing the vibration. Also, since I am asking what gear ratio would be in a car if the engine is doing 4200 RPM @ 60MPH??? I turned the driveshaft around till the wheel spun twice (non-suregrip) and came up with 3.23, but that has to be wrong. Any help with any of these problems would be greatly appreciated and I am hoping to get them resolved so I have a better chance of going to carlisle.

[MEK-Dangerfield]

For a young guy, I'm really impressed with your wisdom.  An unbalanced drive shaft is where I would look here. I think I have that right now.  If it only vibrates at high speeds(above 50), that sounds reasonable. As for your rear end gear... boy you are built for the drag strip.  I have an auto with 3:23's. So in my 3 speed tranny, I'm only pulling about 2,600 at 60 MPH. I would say you have 4:10's or more, but the size of the rear tires matters too. The taller the tire, the lower the RPM.


  Mike

[fishn4cuda]

I think you may want to change the balancer to the correct one. I had an exact situation with a 340 and with the wrong balacer took out the main bearings. Not cool! I also agree to check your u-joints. I had a customer with a bad joint and he shattered his matching # trans.

[tactransman]

If the balancer is wrong I wonder if the flywheel is wrong too? Yes it should vibrate at all high RPM's no matter what gear your in but I would check it out . The weight difference between a cast 340 and a steel 340 is minor compared to all the others but if it is wrong it needs to be fixed for engine longivity.
Your problem sounds like driveshaft or tires.

[Chryco Psycho]

if the engine was balanced with the thin damper it may be internally balanced so replacing the damper May cause a problem not solve it , I agree that the vibration should be present in all gears if it is a engine balance problem
 I feel it is a drive shaft or tire / rim problem

[Mt.St.BigBlock]

Hi, I would try putting the rear up on jackstands, have a helper crank it up in 4th gear and
you watch the driveshaft. You will see right away if the shaft is bent or wobbling. I'm assuming
you've already crawled under there and made sure the u-joints are not loose in the yokes, if
not check that first. I had a 72 Satellite Sebring that had the same vibration you describe and the
shaft had a bow in it that wasn't obvious until it was spinning  faster than by hand! New driveshaft
and joints and it was sweet thereafter!  Goodluck!   Jeff

[Killer_Mopar]

if the engine was balanced with the thin damper it may be internally balanced so replacing the damper May cause a problem not solve it

I think you may be right because when I was checking the crank I noticed that the connecting rods looked like they had been ground down and balanced.

Im pretty sure the driveshafts are the same for 340 and 383s, right? If so, I will just pull the driveshaft out of my Callenger and put it on his challenger and see if that remedies the problem. Would work out because I could have him give me a little money for my shaft and buy myself a stronger one to put behind my 451 Now all I have to do is figure out why the car is running at 4200 RPM at 60 MPH....

[Chryco Psycho]

the driveshaft could be the same If the diffs are both 8 3/4s , & both have 727 trannys & if the the U loint size is the same in both cars

[Killer_Mopar]

Both cars are 4-speeds with 8 3/4s with 7290s, so the driveshaft should bolt right in. Ill try switching the driveshafts by Friday and see if that fixes the problem. Also, I did the 'spin the driveshaft to get the rearend ratio thing' and found it to be 3.23 again...so what would cause the engine to rev so high at 60 MPH??? I thought the tac may be off a little, but the engine really does sound like its at 4200 RPM. Is it possible to have a transmission that doesnt have a 1:1 final gear ratio???

[Chryco Psycho]

are both tires turning equally , if not the reading will be off

[Killer_Mopar]

are both tires turning equally , if not the reading will be off

I see no reason why they shouldnt be turning equally. I think the tach is off even though the engine sounds like it is around 4200 RPM, I will hook an engine tester to the engine and see if the tac is off.

[ntstlgl1970]

I agree wtih Mt St Bigblock, run it on jackstands at the speed it vibrates and see what is going on. Had a problem similar to this on my car once, when I ran it on stands, one of the tires would start to deform when the vibration happened. Turned out that there was water in the tire!!! I had a leak and kept filling it at one gas station a couple of times a week. Guess they never drained their compressor. When I dismounted the tire there was about a cup of water in there! Vibration solved.

[Killer_Mopar]

Well I just got finished working on my dad's challenger and I am happy to say that I fixed the vibration at 70 MPH. Turns out the driveshaft must be out of balance or bent because the driveshaft from my car instantly fixed the problem. Also, I hooked up an engine tester and found the tach to be off about 500 rpm per 1000 rpm, so Im guessing the engine is somewhere around 3000 RPM at 60 MPH, but I dont know for sure because I am pretty sure the MPH is off. Anyhow, I remedied the two problems but ran into a new one....while on the highway I started to see smoke and when I came home I saw that the rear of the intake is leaking AGAIN. Guess I will have to pull the intake tommorow and hopefully I can stop the leak....doesnt really matter because of all these problems I know I will definatly not be taking the car to Carlisle. Right now I'm 50/50 about going.....

[MyMopar]

are both tires turning equally , if not the reading will be off

I think he is refering to when you spin the tires to check ratio.  If both tires aren't turning, the ring and pinion may not be turning to give you the correct reading.  I thoguht with a one legger that one tire could stay on the ground while you turn the other tire in the air and get a more accurate reading.  I could be wrong, I have been in the past.
I ran 4.30's in my Charger and it would turn around 3800 rpm on the highway w/automatic.  I ran a tall tire, about 30 inches. 
I would imagine like stated before it is a driveline issue or an out of balance wheel.  Try swapping wheels fron tto back and see if that changes anything (only if your wheels are same size all around).

[Grancoupe]

Is there a chance the tach was for a 6 cylinder?