Help Please – Vibration

Original configuration: 76 Gran Torino, 400ci connected to a original FMX transmission, 391 posi gearing.

New configuration; 2800 stall, C6, Gear Vendors Overdrive unit.

Over the winter I had my transmission replaced and the Gear Vendor overdrive installed. Vibration appears when I am on the highway with the s Overdrive off, around 3500rpm (70mph). Vibration starts, then disappears, starts again, then disappears – cycle is every second or so – rhythmic. Slow down the vibration stops, or turn on the overdrive and the vibration is almost completely gone, even when I speed back up to 3500rpm in OD (90mph).

Could this be a resonance vibration? Any ideas?

Thanks!

Resonance <U>sounds</U> resonable, as you've described it. But the only thing is that when you engage the OD, the tranny internals slow down... the driveshaft, U-joints and pinion speed remain the same ... which makes me wonder if the vibration is specific to the tranny/GV mechanism. ????

Honestly don't know for sure, but I'm all ears because I'm seriously considering the Gear Vendor route, myself! Although in my case I'm also closely looking into swapping my C6/2000 stall to a C4/2800 stall converter and 4.33's out back in place of the 3.50's that are in there now. What are your general driving impressions of the GV unit? Had much chance to pay with gear splitting? I think that would be killer at the track as you'd be able to go 1st to 2nd, then 2nd to 2nd/over. Keep from dropping as many revs on the big end. Just curious.

Best of luck, and can't wait to hear what you find!

Kevin
[addsig]

The Gear Vendor's unit works great. I love my car so much it my daily driver in the summer (except when it rains).

The Overdrive unit can come on automatically at either 20mph, or 50mph, will automatically disengage when going below 20mph. There is also a foot switch to turn it on/off manually. Day to day driving I just let it come on at 20mph and the car drives like my 3.91 rear end changed to a 3.05 – don’t even notice it engage unless I am stepping on it – just feels like I have 4 gears.

I have played with it a bit to try gear splitting, really nice. Easy way to lengthen 2nd gear!

I tried splitting 1st gear, but it takes some coordination to time it right, on just before the up-shift to second, this drop the rpm 22%, then off right as it shifts to second. I gave up on this after a few tries. I have heard there are options available to help with gear splitting but I did not follow-up so I’m not too sure on this.

The OD engages/disengages very firmly (nice!) easy to chirp tires if it engages while stepping it a bit in 1st gear.

The vibration is not a real problem, I always drive on the highway with the OD engaged anyway – but I would like to get it fixed just the same.

I will drop the car off mid in June while I am on vacation, I have a few more pressing “odds and ends” to have addressed first, but hopefully they can look for vibration at the same time.

I hear that the Overdrive engagement takes a little bit - ie, wouldn't be good for shifting on the strip.

Someone had one a while back and I think he said that it takes a second or two for the OD to engage.

Hmmm… when activating manually the delay is short enough I did not notice – but then I wasn’t looking for it either. I would think less than a second, but not sure, I will pay more attention tomorrow.

Check your pinon angle. Comparing the rear end and the trans (gear unit ). they should have the same angle and should not be straight into one another. Also maybe the person that shortend and balanced your driveshaft made a BOO BOO.

I would start searching the web concerning this vibration issue and see if its related to the resonance. In fact there might be a tech article here on Fordmuscle.com
good luck

[addsig]

I've got one in my '68 Mustang. 289, C6, GV and 9-inch.
Once installed, it's been bullet proof over the last 5 or
so years. Installation is tight on a '68, to put it mildly.
I have similar vibration issues that have been greatly
reduced but not eliminated. Vibrations are worse with
the GV unit dis-engaged and almost non-existant when
in O/D. I chased one thing after another to try to
optimize the drive train angle, which is the source of
the problem. Bear in mind that, to begin with, the car
is short and angle problems in the drivetrain are made
more noticable when distances lessen. The number two
conclusion I came away with was related to the above,
probably works without any vibration or resonance in
an RV/Motorhome because the distances are greater-
and that is the original audience that GV marketed to.
On a practical level, I verified the "angle of the dangle"
of pretty much everything in the drivetrain on the Mustang.
I went to a carbon fiber driveshaft (about $1000), used
a better isolating truck transmission mount.
I've yet to perfectly locate the engine, because at the
time I did the installation, an easy solution to drop the
engine down in the chassis did not exist.
I'm about 90-percent vibration free at this point and
only a pair of modified Total Control engine mounts and
about an afternoon with a protractor and welder away
from "solving" the puzzle.
As far as rapidly splitting the gears on the GV unit, they
have a sequencer out now that will do just that. It's
computerized and probably pricey- I'll just use mine as
an O/D......

It takes approximately 1/2 to 3/4 of a second for the GV overdrive to engage. Disengaging take much less time, almost instantaneous, perhaps 1/10 of a second.