I have a C4 race car and this weekend I noticed the shifter (zf 6speed) was jumping like 3 inches under hard accel. Its a 600hp race car.

I checked and the rear c beam bolts were lose. So I tightened them up, and it help a lot, but there is still a good inch or so of jumping movement.

How much is normal? and what else could be causing it since I tightened the C beam bolts? ALso, could there have been damage from when they were lose that is now causing the problem to continue even after re-tightening the C beam bolts?

Thanks in advance for the help.

Eddie

 

Check the motor mounts and the trans mount. Rich

 

No trans mount on a C4

Check the bolt holes in the C Beam ; common for stick cars to elongate the holes so you have movement even though bolts are torqued to spec.
Beam plates help give better clamping over a wider area

 

Check the mounting holes in the C beam. Mine were elongated. I replaced the C beam.

The mounting holes in the trans tailshaft housing and nose of the differential can also become elongated and / or crack. The mounting webs on my differential had minor cracking and were repaired by tig welding.

To strengthen the tail housing and differential further I machined steel bushings that fit between the webs cast into the tail housing and differential and through which the mounting bolts pass. I set these bushings in place with a steel reinforced epoxy used to set heavy machinery mounts.

Besides strengthing, these bushings also eliminate possible areas that the mounting bolts could move about in.

Beam plates help a lot; I made my own of angle iron which is stiffer than flat bar, The mounting holes for the C beam bolts were drilled as small as possible; additionally I drilled extra holes and bolted the plates directly to the C beams, thereby capturing the beam plates onto the C beam. Any movement between the C beam and its mounting bolts is met by the harder steel of the beam plate and the bolts do not wear against the softer aluminum of the C beam.

Don't forget to use some form of adhesive when re-mounting the C beam. I used construction grade Liquid Nails. Some claim that the factory used adhesive to prevent electrolosis; maybe; my personal opinion is that was intended to glue everything together in an attempt to mask a defective design.

One last thing; replace the rubber batwing bushings with polyurethane.

 

when i got my 94 6 speed, the shifter jumped a good 2-3 inches when i got on it. turned out to be loose c-beam bolts at the tranny. tightened them up and problem went away. i later added beam plates and it felt even better

 

Thanks for all that good info. I have a couple of questions because I really want to do what you have done.

1. where can I get a new C beam?

2. do you have any pics of the steel bushings for the tail housing?

The rest sounds pretty straight forward. I like the idea of making beam plates out of angle and also bolting them to the actual C beam itself. Sounds good.

 

Looking through my records: I got a used, but unharmed, C4 Manual Transmission Torque Arm, 1984-1996 (C4-MTTA-8496) from DINO's CORVETTE SALVAGE for $150. Any salvage yard should be able to help, the main issue being any elongation of the mounting holes (mine showed no signs of elongation).

I also see a cancelled order from ZIP CORVETTE, item SU943, new 85-96 Dana 44 Differential to Transmission Support Beam, $250; was probably on back order and I couldn't (or didn't want) to wait.

Dont have pictures of the bushings, but they are very simple. I took lengths of mild steel (black iron pipe) with the closest, but undersized, inside diameter. Chucked up in a lathe, I bored them out to the first drill size that was larger (by approximately 1/64th inch) than the bolts which were to pass through them. I then cut them to the various lengths to fit between the webs of the tailshaft housing; some required extra grinding along the outside surface to allow it to "snuggle up" and fit properly. I set them in place, ran the correct bolt through them, and filled void around them formed by the webs, with epoxy paste. While the epoxy set, I moved the bolts a bit to ensure they did not become locked in place.

The available beam plates all weld the nut to the top of the beam plate, which is one way to "capture" the nut. I did mine differently. Taking a six sided socket of the proper size for the nut (16 mm - 18 mm ? old impact sockets work best) I cut a ring from the end of it and welded that ring atop the beam plate; I set the the nut inside this ring; the nut is now captured; the nut has not been weakened or deformed from the heat of welding, and should the threads become worn or damaged, the nut is easily popped out of the ring and a new one inserted in its place.

Best of luck. sounds like you've got an exciting car.

 

Thanks for all the info. Im going to do this asap. Im sad to report that racing this past weekend, I blew up the motor.

Apparently the timing belt broke at 8000+ RPM. and with a 13:1 compression motor the valves hit the pistons. I haven't torn into it yet to see the damage, but Im already scared because the valves are Titanium and at $250 each its going to be expensive

Here is a pic of it

 

Very (apart from the engine blowing up part..)

I had no idea the ZF6 was good for 600hp!

 

Not only is it good, its freakin bulletproof. No coolers or anything. Thats what I really like about the C4, its a lot less needy(in terms of cooling and such) than newer vettes.

 

I stand corrected. That's a VERY exciting car. Sorry to hear about your losing the engine. I can only imagine the amount of time and money...especially money, that you had in that engine. What a loss. Slighty off topic, but reading Smokey Yunick's POWER SECRETS is an eye opener. He had no faith in timing chains but was a great fan of gear drives in small block Chevys.

 

Well Im running a Jesel timing belt for that reason. What happened is the throttle for the car is an old FAST system with quad barrels with quad blades. A screw fell out of the throttle linkage (that opens the 2nd set of blades) shutting them completely, and instantly at around 8400 RPMs.

The resulting backfire no doubt cause the timing belt to break. Sucks to have all this damage do to one freaking 1/4in screw coming lose..... but thats racing.

 

Omg !!!

 

Don't even have to go to that much trouble
I just used a piece of 1/4 x 1/4 bar each side to prevent nut turning
Push bolt through top plate , start nut on bolt ,( as you would without the plates ) drop bolt down and nut is captured between the bars

 

Where are you located?

I was wondering what happened to you in that race

My 92 now has the same shifter jumping issue. Sounds like its time to get it up on stands and check the c-beam.

 

Im from Knoxville TN. But it happened last weekend at Road Atlanta.

I checked my C beam before the race saturday and the 2 rear bolts were lose. We tightened the crap out of them, and that helped a lot, but its still jumping more than it ever did before.

Im worried that the holes got elongated when the bolts were lose. Im sure under a good amount of load it doesn't take long for them to get like that.

Where are you from Jaa? were you at Road Atlanta?

 

I'm in Stone Mtn/Lilburn/Tucker GA, just outside of ATL.
Yep I was in Time Trials at Road Atl.

Still working on the nut behind the wheel and I need bigger marbles

 

I hear ya....I will be racing and Time Trialing at the CMP event at the end of April....assuming I can get my engine back together by then....