Trans/engine mounts for unibody car

March 11, 2019, 02:42 PM

Bruce Williams

After breaking two PG cases in a row with my wifes 74 Nova I'm getting ready to change out the factory motor mounts with solid as well as tie the subframe together,
Any thoughts on the trans mount?
On my Corvette, trany is solid mounted as well as front and mid plates.
Don't know if a sold mount on the Nova is a good idea or not?
This is a low 12 second street/strip car, mostly strip.

March 11, 2019, 03:11 PM

TomR

I would recommend solid engine mounts and good urethane mount for the transmission. Check the driveshaft and check the front sub-frame body mounts since they usually rust out and allow it to flex more than it should.

March 11, 2019, 03:52 PM

Bruce Williams

Thanks Tom.
Subframe mounts are all new as well as the driveshaft.

March 11, 2019, 06:39 PM

banjo

Where is it breaking the case? Anything change or significant work done since this starting happening?

Bill Simpkins
74 Nova
SBC 406
3240 pounds
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60 1.27 (10/16)
1/8 6.03@111 (10/16)
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March 11, 2019, 10:53 PM

Bruce Williams

quote:
Where is it breaking the case? Anything change or significant work done since this starting happening?

Bill it is breaking the PG case at the reverse piston.
I did not know it at the time but the 350TH cracked the case in the bell housing area close to the front pump.

March 11, 2019, 11:23 PM

banjo

If you bottom out your driveshaft yoke in the transmission, how far does it come out when you hook it to the rear end. If the driveshaft is too short and it pulling out to far and putting things in a bind. I chased a problem like this until I got the correct length driveshaft. I kept cracking cases.

March 12, 2019, 03:39 PM

Bruce Williams

Bill I will check it out.
Thank you for the suggestions.

March 13, 2019, 09:11 AM

Eman

When you check the driveshaft length place the car on blocks or boxes. With the suspension loaded you should have 1" from bottoming the yoke.

March 13, 2019, 09:17 AM

Lenny5160

quote:
Originally posted by Tom Reyer:
I would recommend solid engine mounts and good urethane mount for the transmission.

This is what I use. I still break an occasional PG case (one every few years), but I figure that is the cost of doing business at 650 hp, 3600 lbs with no motor plate or midplate.

March 13, 2019, 10:10 AM

Calvin7613

Bruce, with the factory mounts and the subframe not tied together, the torque of the engine is trying to lift up on the drivers side. When this happens, the transmission tries to twist with the engine and the tail housing connected to the rear mount is trying to keep it from twisting. The transmission case is now the only thing trying to keep the engine from twisting at the tail housing mount. The case is going to break at its weakest point. What I found to work the best was to leave the bolts out of the rear tail housing mount and just let the tail housing rest on it. This will keep from breaking cases until you are able to make the modifications to keep the engine/chassis from twisting.

March 13, 2019, 11:45 AM

SlyFox

My 69 Chevelle ran mid-11's with a mild 496 and a TH400. It would pull the wheels slightly off the ground. I put a solid Moroso engine mount on the driver's side, because it would break a stock mount. The passenger side mount was stock, as was the transmission mount. Never any problems, raced it that way for 9 years and hundreds of passes.

On a faster car, yes, maybe you need different mounts, but on a car running low-12's like your Nova, I very seriously doubt the mounts are your problem.

JMHO

March 13, 2019, 11:48 AM

SlyFox

Also, My Chevelle did not have a cage or roll bar, which is why I never tuned it to go quicker than 11.50. The factory frame is really pretty much a wet noodle, so I'm sure it flexed about as much as your unibody/subframe car. Again, I'm having a really hard time believing you have a mount and/or flexing problem.

March 13, 2019, 02:12 PM

Bruce Williams

quote:
Originally posted by Calvin7613:
Bruce, with the factory mounts and the subframe not tied together, the torque of the engine is trying to lift up on the drivers side. When this happens, the transmission tries to twist with the engine and the tail housing connected to the rear mount is trying to keep it from twisting. The transmission case is now the only thing trying to keep the engine from twisting at the tail housing mount. The case is going to break at its weakest point. What I found to work the best was to leave the bolts out of the rear tail housing mount and just let the tail housing rest on it. This will keep from breaking cases until you are able to make the modifications to keep the engine/chassis from twisting.

Kevin, thanks for the tip.
Just received the solid mounts and sub frame connectors. Pulling the trans out after lunch.
I'm getting way to old to be flat backing this kind of work.

March 13, 2019, 02:13 PM

Bruce Williams

quote:
Originally posted by SlyFox:
My 69 Chevelle ran mid-11's with a mild 496 and a TH400. It would pull the wheels slightly off the ground. I put a solid Moroso engine mount on the driver's side, because it would break a stock mount. The passenger side mount was stock, as was the transmission mount. Never any problems, raced it that way for 9 years and hundreds of passes.

On a faster car, yes, maybe you need different mounts, but on a car running low-12's like your Nova, I very seriously doubt the mounts are your problem.

JMHO

Slyfox, thank you for your input.
Do you think the Chevelles full chassis helped you out over the unibody Nova?

March 13, 2019, 02:24 PM

banjo

Btw, my car is a 74 nova, unibody car. I have frame connectors. I have a solid motor mount on one side, rubber on the other and a rubber on the trans. Make around 650+ hp. I dont have any issues.

I am assuming the car has at least a roll bar. With rubber mounts all the way round, I dont think it's your issue. My money is on driveshaft length.

March 13, 2019, 02:50 PM

SlyFox

quote:
Originally posted by Bruce Williams:

Slyfox, thank you for your input.
Do you think the Chevelles full chassis helped you out over the unibody Nova?

If you saw the frame flex when lowering onto jack stands, you would likely agree with me that it's not much more rigid than a unibody

March 13, 2019, 02:52 PM

SlyFox

quote:
Originally posted by banjo:
I have a solid motor mount on one side, rubber on the other and a rubber on the trans. Make around 650+ hp. I dont have any issues.

I am assuming the car has at least a roll bar. With rubber mounts all the way round, I dont think it's your issue. My money is on driveshaft length.

That's how my Chevelle was mounted, as I described (one solid, two rubber) and again, I agree with banjo....just not seeing mounts as your problem on a low 12-sec car - unless the mounts are broke.

March 13, 2019, 02:54 PM

SlyFox

Also, a Chevelle is a factory 4-link rear suspension and works really nice even in stock form. A Nova is on leaf springs - possible the rear axle is moving fore & aft and/or wheel hopping? Wheel-hop will definitely break stuff, and would amplify a driveshaft length issue.

March 13, 2019, 10:59 PM

AlkyIROC

I'm probably living on borrowed time but I don't use a trans mount for my powerglide.

Front and rear motor plate and a triangulated link at the back of the transmission to prevent the motor plates from flexing forward and backward. Car also has a full cage so there is no chassis flexing.

March 14, 2019, 10:09 AM

TomR

There is A LOT of chassis flex in a Nova.

I could tell an immediate difference when we installed frame connectors and a roll bar in my Nova. Then I could feel a bigger difference when we added "a" pillar bars to my car. The front subframe is held to the main body of the car by only 4 bolts. I had a problem with the front end moving on those 4 bolts and added front strut bars to solve the problem.

The more bars you add, the stiffer the chassis and better it will work.