I put a new polyurethane tranny mount bushing on, but my engine mounts are still stick along with all my other bushings at 140k miles. I'm getting a dyno tune this Friday and I drive the car rather hard I think. Should I be worried? Ive been hearing bad things about replacing some polyurethane bushings and not others

 

If you are on stock rubber motor mounts you don't want to be on a poly trans mount. You'll have too much flex in the engine and none in the tranny which can lead to problems. Especially if you drive as hard as you say. Pick up a new stock rubber tranny mount and save the poly for if/when you do a motor mount swap.

 

yes deffinately, i knew a guy who swaped the tranny mount to poly with stock rubber bushings for the engine and took it to the track and cracked the bell housing. also if your tuner ratchets the car to the dyno by the body, then you could crack the tail housing with would lead to another whole mess of problems, they can tie the car to the dyno from the axle

 

Thanks you guys. Can't afford that to happen haha

 

I cracked a bellhousing doing this. Take it from me, before you put a poly trans mount on it you need to buy either solid motor mounts or poly motor mounts. I went out and bought some spohn solid motor mounts and a new bell housing to fix this problem. Money money money.

 

I've been running a poly trans mount for the past 30,000 miles and haven't broken anything with stock motor mounts. Poly mounts still flex some.

 

Have you dynoed your car/drive it hard since the poly trans mount?

 

Haven't dyno'd it, but it was a daily for 3 years with poly trans mounts, and I didn't drive her easy.

 

Alrighty. I'll keep that in mind and try not to freak out while it's getting dynoed tomorrow haha

 

Having a urethane trans mount is not going to break anything. Even if your motor mounts are still the oem rubber. It's been done many times. Now, the only draw back is more vibration inside the cabin.

 

Im running the opposite, solid motor mounts and stock tranny mount. Im sure its the same dynamics, relatively, and I havent had any issues. Ive been dynoed a couple times also.

 

Taking the 140k miles on my car into consideration, I just want to be as careful as possible and address any common risks people seem to have I guess haha. I definitely don't feel as concerned though so thank you both

 

My stock rubber motor mounts had 120k of fun driving when I swapped them for poly's and they looked pretty bad. Chewed up, sagging, and was only really there because the clamshell was holding them together.

So for my piece of mind, I wouldn't put a poly trans matched with stock rubber motor mounts in that regards.

It is generally acceptable to do poly motor, stock trans though.

 

I wouldn't take the risk. Think of the motions and forces in play here. Your motor wants to rotate, which the motor mounts allow, but the poly trans mount is triangulated and not allowing for movement, or at least not a lot. You are putting a lot of rotational force on the trans mount, which it's not designed to take. It's purpose is to keep the transmission from moving in a vertical plane.

If it were me, I'd pull the poly trans mount until you at least replace the motor mounts with poly. It's easy enough to do. I just read a thread yesterday about a guy who brought the tailshaft off his t-56, snapped the driveshaft, and did other damage because he had a poly trans mount with factory rubber motor mounts. Not worth the risk in my opinion.

 

Gotcha. How much to poly Motor mounts go for? And is it hard to swap?

 

Poly motor mounts are ~$30 each if I remember correctly. To swap them, is another story. Not hard, just time consuming and can be stubborn.
The best time to do a motor mount swap is when doing headers because they are readily accesible. I did mine on a lift, so it wasn't bad, but it's definitley not a 30 minute job. Figure a few hours at least, and get a couple friends to help work through some sticking points because it can get stressful when things look like they won't fit/work.

 

Here's the stupid question - how's he know it was the mount that was responsible for the failure?

The transmission should only be taking very little torque in the first place, so regardless of what the trans mount is made of, if the engine mounts are allowing enough flex to crack a tailshaft, poly or rubber mounts aren't going to make a difference. Poly still deflects, and if you need more deflection than that to keep your transmission in one piece, you should be more worried about the condition of your engine mounts, rather than thinking about a transmission mount.

 

Your right, the engine mounts should be controlling engine torque, but the fact is, gm engineered the three mounts to flex quite a bit. Just look at the factory ypipe on the driver's side. They left a good 3-4" of clearance and flattened it because they knew the motor would torque over that much.

Now add a bunch more torque/power, and now replace the soft rubber trans mount with a much stiffer poly mount and guess what's going to happen? All of that rotational force is going to get transmitted to the trans tailshaft.

Beleive what you want. I still say putting a poly mount on the trans mount with worm out, old factory rubber engine mounts is a recipe for problems.

 

That's my point. In a situation where the tailshaft is seeing that much rotational play because of shot engine mounts, even replacing a blown trans mount with an OEM rubber replacement could cause the tailshaft to crack.

 

Now I'm scared haha