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calfroper_06 09-07-2009 07:33 PM

Pinion Angle
Working on a 68 mustang coupe, running a 351w based motor,backed by a tko600, thats been milled on topfor the 68, from American Driveline. The rearend is a factory 8inch. The car developes a buzz (pulsing) vibration at around 90mph with 2.78 rear gears, and at 70 with 4.11 rear gears.

Weve taken degree measurements from the valve cover, starter, trans yoke, driveshaft and rear yoke. What were running into is the valve cover,starter, and trans measure out at -2.5*, the DS measures out -2.5*, and the rear is 0*.

Trans_______ -2.5
Drivesahft ___-2.5

We would like to get the trans up to 0* giving the rear and trans te same degree with a negatice degree onthe DS, but the trans is all the way to the trans tunnel.(well as close to hitting itas it could be). Need some help r infoon what to do. Should we try shimming the rear up towards the trans 2.5*, giving the trans and rear a -2.5 and the DS somewere around a 1*or so? any info will help greatly.

Also bee thinking going to a 9inch, is ittrue that the 9"pinion sits lower. Is so would it helpto set the pinion angle.

Gun Jam 09-07-2009 07:37 PM

In theory setting the rear end to +2.5 would be the answer. Since that is a very cheep and easy possible fix I would try that first. Simply buy a set of 2.5 deg wedges and slide em in. They call this parallel... Setting the engine line parallel with the pinion line.

It might just work.


2+2GT 09-07-2009 07:40 PM

The driveshaft angle is irrelevant. If you have transmission -2.5, and pinion -2.5, you are golden. Or 0/0 or whatever.

There are wedges available for just this purpose.

Gun Jam 09-07-2009 07:42 PM

And of course no two answers you get will be the same.....I found that out a while ago.... and worst of all they are probably all wrong


calfroper_06 09-07-2009 07:58 PM

thanks guys ill try and get back asap, had to ask cause ive seen so many problems with this

Gun Jam 09-07-2009 08:32 PM

yeah if you find the answer let me know I have been actively pursuing that same issue for over a year now. I think in my case its a DS balance issue that doesn't show up until the DS exceeds 5000 RPM which is easy to do with 3:80 rearend.

DO get the shims and DO try both of the above suggestions if the first one you try doesnt help or makes things worse.

Good Luck


67mustang302 09-07-2009 08:56 PM

2+2 beat me to it. Get the wedge spacers and shim the rear axle housing, then shim the trans mount if you need to. Proper pinion angle actually varies from vehicle to vehicle depending on application and suspension types etc. Generally -2 is considered a good starting point, leaf springs sometimes need more like -3, but again, it will depend on how stiff the suspension components are and how much power you have.

racer_dave 09-07-2009 09:35 PM

You need to have the pinoin angle at -2 to -5 degrees. When you step on the gas, the rear end will try to rotate back. If you start out at zero, and it rotates back then you end up with what is commonly called a broken-back alignment. It will wear out U-joints and cause a vibration, and will make the tailshaft bushing wear prematurey as well.

On a race car we set it at between -5 and -7, but we have a lot more suspension movement, and probably more HP too. Going a little too far to the negative will not hurt in a street car at all. I'd get as close as I can to -5 with the shims.

kalli 09-08-2009 05:05 AM

ya. on hard accelleration the (rear axle) pinion will rotate downwards, since my pinion is already too low (according to the proper geometry above), the vibration in my car is way worse when I accellerate in 3rd at 80 than when I cruise at 80.
Gonna shim it as well this week or next week. Have a 2.5 and a 4.5 shim. I'll probably even need the 4.5 degree one from what I measured.
If you change the axle to a 9" you'll have to re-measure again

And yes, as all the others say: driveshaft angle doesn't matter. You have to have the same angle at tailshaft against the pinion. just the other way around.

Another thing to look out for is that the tailshaft is centered in the car. not pointing to the right or left unless the yoke is offset

Aussie66Fastback 09-08-2009 07:20 AM

what are you running for a drive shaft and what revs is the vibe? does it go away if you increase the revs?

i cant do the math but there's a relationship between driveshaft length and diameter and the rotation speed. at some point the shaft will start to bow in the middle (harmonics) and that causes a vibe. but the kicker is that it can happen at exactly half the revs too. so you tend to build a drive shaft suited to more revs than the engine can do but then you can sometimes have an issue within the range. example...your engine is good for say 6000. You use a drive shaft that's good for 8000. between 3900 and 4100 you may experience a vibe.

assume you've checked wheel balance and axle runout? 8 inch axles are prone to twisting under decent torque with modern stickier tires.

good luck

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