high frequency vibration above 70mph but only on throttle

[gc3]

took my car (88 fc with 5.3, t56)  out for the first drive this season last night.
over the winter i swapped

n/a diff out to a t2 diff (and halfshafts)
solid mounted rear subframe, diff, reinforced front diff mount
tires
stripped all interior in the "trunk" area including tar sound deadening

it never had this vibration before, so i assume it's from something i added or touched...

what is strange is that the vibration significantly lessens off throttle, but on throttle it's bad enough i immediately got off on the next exit thinking maybe my driveshaft was going to fall off.
it is too high a frequency to be a tire/wheel/halfshaft as far as i can tell.

it also does not seem to happen at 35~40mph which id think would show some symptoms with how aggressively it comes on at 70-80

i'll be checking u joints and driveline angles today, but does anyone have any other ideas?
(ujoints and dshaft are only about 5000 or so miles old, from grannys. i got a brand new pinion yoke to swap to the t2 diff)

[largeorangefont]

Yep I think you are on the right track. You may try rotating the drive shaft on the yoke 90 degrees to see if that helps.

[Bowtie7]

If it gets worse the faster you go I will bet that it is the solid diff mounting. The subframe mount will not cause it but the diff sure will. I chased this one for awhile in my old car. At 100mph+ it sounded like the car was coming apart. Mazdaspeed mounts made it go completely away.

[gc3]

driveline angles are 3deg off b/w the trans output to the diff flange so thats probably not it.

ill try rotating the dshaft 90 on the flange like ash said next.

it doesnt get much worse between 70  to 80. havent had a chance to try anything beyond that yet.
i think this is a bit beyond just nvh issues, but it seems okay for city driving.

so i went to cars and coffee.
i make Ferraris look good

[JapCrap]

I hope whatever it is vibrates the mounting screws loose from that ugly fucking wing.... That thing is uglier than my hood!!

[digitalsolo]

You realize that wing is actually functional on that car, right?

[JapCrap]

If you say so...

[gc3]

its okay, the wing isnt as ugly as my hood.
havent tested it to see if it's actually functional, but i sure hope so.

but whatever the vibration, i guarantee the wing will be one of the last things to vibrate off sorry

[largeorangefont]

3 degrees difference in driveline angles should not be too bad, but is there anything you can do to get it them closer to see of that helps?

I wish corksport still made those front fenders 

[MPbdy]

I hope whatever it is vibrates the mounting screws loose from that ugly fucking wing.... That thing is uglier than my hood!!

[gc3]

3 degrees difference in driveline angles should not be too bad, but is there anything you can do to get it them closer to see of that helps?

I wish corksport still made those front fenders 

clocked the pinion 90 today, no test drive though.

i'll make some subframe mount spacers to try to get that better.

yea, they seem to fit the 275s pretty well

[gc3]

quick maths tells me i need to space down the subframe by nearly an inch to get the diff to nose down 3 degrees.
i need to do a little measuring to see how much room on the stud there is or a spacer, but i doubt i can get all that much in there.
i think roughly .5" might be doable according to the old measurements i have from when i made the bushings, but that only gets me 1.5 degrees of pinion angle change.

is this a typical situation for other people too? having the diff front pointed too high?


-edit-  aha, i see people are extending the slots on their subframe to lower their diff nose down.
anyone know how many degrees they got from that?


-edit2-

Slot subframe mount deeper to get the diff down a bit more

That works, but I found mine slipped back up.  I suggest you shave the tops of the delrin diff mounts to move the rear of the diff up, (same effect)

http://www.norotors.com/index.php?topic=17018.msg225537#msg225537

hmm..

[largeorangefont]

The problem is probably the diff mounts are to thick at the top. That's not the easiest fix though.

[markfluko]

I thought I posted this a long time ago... but turns out I didn't press submit.

I had this exact problem, but in an FD.

Turns out one of my outer CV joints was bad... Discovered this after I replaced nearly everything else possible in the drive-line.

[gc3]

last night i managed to cut the rim of the tophat in the diff bushing in half, and used that donut spacer over the subframe bushing, and managed to get the diff to point down exactly the right amount (lucky me, trig doesnt work with all the various linkages apparently haha) anyways, it snowed 3" so test drive isn't going to happen soon, but at least the driveline angles are better.
i did have to reroute a fuel line so it wouldnt get pinched between the diff and the floor. (not much more room to shave that bushing anymore, and up front i see a single thread left on the stud beyond the nut, so no more room to space that anymore)


one issue solved, hopefully it's all fixed now, but if not i'll look into the CVs.
Nothing felt bad about them when i bought them (used) from a member here, aside from checking for play, is there a way to test if they are bad?
would swapping sides help diagnose? as in would something show up in engine braking as opposed to accel in that case?