Drivetrain Thunking - SPF

[MidOHasp]

Any SPF owners experience this?

Pulling in and out of driveway, real low speed stuff, if things start to "buck" a little bit, there's a thunking noise in the tuneel toward the rear of the car.

Last night, was letting a buddy test drive and I assumed that he respected the whole 500HP, 2500 pound relationship. He was immediately sideways with an oncoming car, and I think he got a little nervous... Basically shut it down and turned into a development off the road he was on. Pretty sure in a fit of panic he grabbed 5th gear and was going about 10 miles per hour. The bucking was violent and so was the noise.

I think he was so scared he broke it he couldn't find first gear. I kept telling him to keep the revs up and find first gear and he'd be ok, but I really thought something was wrong.

We got back the 1/4 mile to his driveway and I took it out for a spin. Everything was fine.

I'd noticed the thunking occasionally before, perhaps the new park brake setup they're using, but had never heard it this violently before. Now it raises the question...is this normal?

Setup:

SPF 1670 (new diff mounted hand brake)
351W
Tremec 3550

Thanks guys...
JP

[Mike Simard]

Aside from the usual differential mounting points that have come up with SPF's, some other things to look at are the motor mounts. If you have a broken motor mount you will notice it mostly in low gear when the torque on the drivetrain mounting points is at its greatest. My car had a similar clunk and the main culprit turned out to be the driveshaft to diff bolts were loose.

[MidOHasp]

Definitely will have it up on a lift within the next week or so, and checking d/s to diff bolts is a top priority (along w/ all motor/trans mounts)

Glad to hear you had similar problem and found culprit. That's a positive for me.

Thanks, Mike.

JP

[Goose]

I've had a similar noise with my car, both at low speeds and while in relatively no load situations (e.g. constant speed, flat road).

I have been told that Superformance has increased clearances in the differential and this is contributing to this noise. I am not sure what acceptable play is at the wheel, but would like to know so that I can determine if I have a "normal" amount of play or too much.

I have also been told that poor ignition can induce the situation that I sometimes experience in the low-load circumstances through a mild "bucking" of the drive train.

[Excaliber]

5th gear at 10 mph would make ANY car "clunck".

Two of you in the car? Thats what the glove box is for you know, holds two changes of underwear.

Ernie

[MidOHasp]

No kidding, Ernie. Joe (the guy who kinda well...messed up) is one of my best friend's dad. Lot of years ago, he wouldn't let his son ride with me b/c he heard me do a burnout in my 240SX near his house. He kinda looked at me after the whole Cobra incident as if to say "dude, this was way worse than when you did that burnout in the Nissan!"

It was funny b/c we were both 'keeping our cool' even though I'm sure he was forgetting his name at that point and all I wanted to do was jump out of the car and get underneath it to see if something bad was going on. When we got back to the house, he was out of the car immediately and when I told him I was gonna take it for a drive to see how it was, he said he'd probably have his pants clean by the time I got back.

Thing was, I couldn't tell he was in 5th from the passenger seat. I'm not over there very often.

If anyone hears how much "play" is acceptable in the diff, let me know. I'd like to check that out further.

Thanks
JP

[Mr.Fixit]

Check motor mounts, tranny mount, and diff mounts.
Check to make sure driveshaft bolts are not loose.
Rotate driveshaft back and forth to see of you can locate a clunky Park-brake pad.
Check axles for indicators that they might be rubbing on the frame rails. See if the CV boots have been destroyed, which will let the CV joint get dirty and go bad.
Rotate tire in one direction, then the other, checking for excessive backlash in the diff, axles, and U-joints.

[MidOHasp]

Mr. Fixit,

What would be excessive backlash?

Definitely planning to check torques on all of those bolts in the *very* near future.

Thanks!

[G.R.]

Always had a "whine" in my diff and an ocassional "clunck"--after almost 1500 miles it got worse, turned out is was a bad diff from the get-go, a dye test showed numerous "stress" fractures on the ring gear plus the pinion looked as if it had been put on a grinder--evidently it was a "bad" diff that made it through the process and the tolerances were not set right. Only problem I ever had with my SPF.

I would definitely check it out--I was told if I had gotten on the throttle real hard I would have destroyed the rearend in the condition it was in.

[MrMagoo]

I have a performance cars venom 427 (similar to backdraft). I have the thunking in the same place, when I am starting in 1st from a standstill. Really have to go easy on the gas when letting in the clutch to get a smooth takeoff, almost letting the engine stall before I give it easy gas, or she bucks like a bronco.

From what I have been told, this sort of thing is usually related to the improper alignment of the driveshaft relative to the differential. In the cobras, our driveshaft is so short that any small angle can make a big difference.

Running 26psi instead of 34 in the rear fatties helped absorb the shock a tad. I may also try some red line anti-shock differential oil. Other than that, I think I need to fiddle around with the tranny/engine mounts to get the angles perfect.

I have a 2001 Mustang GT, and it has wheel hop absorber units mounted behind the differential at an angle to the rear frame. Very similar to the setup that ERA is currently testing for the Jaguar IRS in it's cobras. That would probably help this problem, but thats a bit of a design effort for the BMW rear that I have.

[blykins]

You need about .008" to .012" backlash in the rearend...you could check that then paint the gears to check the pattern out....

But that low of a speed in high gear would cause a lot of bucking....Ernie's right....

[Lowell W]

J. P.

My car has what I consider a fair amount of on/off the gas clunk in the rear end. According to Mike at Dynamic and Eric at PE, some of the rear ends have more clearance than others. When I first got my car I really thought the noise was excessive, so I called and talked to Mike at Dynamic, who said to be sure all the rear end mounting bolts were tight. From my position on the creeper, I thought I got 'em all, but when I had the car in for inspection, alignment, etc. at about 3,000 miles, Mike found the rearmost bolt (way up in the back, behind the pumpkin) was loose. Tightening that bolt helped considerably. I still have some clunk, but not nearly as bad as it was. I'd take the car over to Dynamic and have them take a look at it.

I've been amused to watch a couple of friends start out in third gear. One drives a VW bug, so he didn't think it was too unusual to give it a little gas until he just kept going and going and going... The other guy started out in town and the car pulled it fine, but he had this bewildered look on his face, like "Now what?" When he asked why I was laughing, I said, "You're in third gear" and he couldn't believe it.

[Mr.Fixit]

It can also just be the stacking of backlashes
2 U-joints
ring and pinion
2 CV joints per axle

all those are very near one another, and if you are "floating" (not in or out of the throttle) you may very well hear them clunk as load is transferred directions and the backlash is taken up.
Coupled with the fact that these are all right behiand you with only a thin layer of fiberglass between you so you get to hear them.
"How much is too much backlash?"
I don't have a number, but if you check each one by one, you will get a feel for how much rotation you can get back and forth and feel what is normal vs abnormal.

[MidOHasp]

Thanks guys, good advice from all. From the sounds of things, what I'm experiencing isn't too bad, although checking all the mounting bolts will be important next time I have it on the lift. But yeah, the only time it ever alarmed me was in 5th gear at creeper speeds, which.... well, the whole thing was kind of scary!

Next time I think I need to explain to people exactly what the power/weight ratio means and that as wide as those tires look, they're nothing for keeping it planted.

I think Joe from my story was most worried about getting it sideways. That was the nicest thing he did to it.

[Tony Ripepi]

Hi JP,

I owned SPF 343 and have seen a similar probelm on other SPF cars. The bolts in the rear diff housing are undersized and with a high torque 460, 427 etc. engine they break. The fix is a larger bolt/washer combination in a grade eight. If the holes are elongated from wear a drill out/plate weld may be required.

Be careful until you do an inspection and really find out what is going on.

Another problem was the original rear shocks broke on the lower end where they attach to the wheel carrier ----- new shocks required and then you may as well go to the Bilsteins.

Tony R.

[Mr.Fixit]

I never saw the diff bolts break, but I have repaired several older SPF's when the chassis' passenger side forward diff monting ear tore apart. They changed the design several years ago.