[ho428]

I recently install an app 525hp 11.5:1 407 in my Firebird. Sims showed tq over 500 as well. After talking to a few road racers and round track guys I decided to try a set of solid engine and trans mounts.
I broke in the engine at fast idle at 1500 for about 15-20 minutes to break it in before I even got in the car.
Once I got in the amount of vibration is up quite a bit over the rubber mounts, and so far I've only zipped it to around 4000 a couple of times.
The amount of torque going through the frame seems very high as well. Never had the fenders quiver and the entire car lurch over like it does now. Kinda cool but...

My wonder is if these harmonics will increase exponentially at 100+ mph and 6000 rpms.

I'm taking the old rubber mounts to the track, I only get one practice that I'll use to finish the break in, just a few laps at 4000 or so and come in to check everything. I was planning on changing out the break in oil after P1.
Not sure if I'll have time between practice and qualifying to also swap the mounts if needed so it becomes an either or. Change the mounts after P1 and run the break in oil through Q? Or go ahead and change out the oil to VR1 before I go. The engine has only been run about 20 minutes so far.
Or will the harmonics not be an issue?

The car does not have a single rubber mount anywhere. Solid bushings and body mounts everywhere.

I figure you guys running plates and/or tube frames deal with this a lot. What can I expect?

[Half-Inch Stud]

Seems a Urethane Rear Trans mount will help. OR;
a solid Trans Mount with Stock Clamped-Rubber on the Crossmember ends.

Updated after Post #3; yea, toss the Solid Trans mount and go with Rubber there.

[screamingchief]

Never EVER use a solid trans mount on ANYTHING...

Just replace the solid trans mount.

I dont even care for poly trans mounts all that much,as many of them are acutally slightly taller than the OE rubber trans mounts are,and as a result of that they can throw off the driveshaft angles if you are'nt watching/checking for that sorta thing.

I mostly just use the plain old OE style rubber trans mount with a HD hose clamp around it to prevent it from seperating.

2¢/FWIW

Bret P.

[ho428]

Makes sense.
Since the center of the car is caged and won't twist having a rubber mount at the trans lets the engine-trans and front frame twist as needed.

Never really thought much about the tranny mount being a pivot, or stress relief point, which is what it'd be in this case.

[BruceWilkie]

With a road racer, I think I'd go all rubber and just add some cable or chain to restrain excess movement/prevent mount separation. Agree, as a minimum lose the solid trans mount.

[ho428]

Thanks guys,
Since the tranny mount is the hardest I'll take it back out tonight and put the rubber back in.
I can swap the engine mounts fairly quick at the track if needed while 10 quarts of break in oil drain. $$$

[Matt Meaney]

i remember a discussion on this site about not using solid engine mounts. something about stressing the block at a weak point and/or possibly contributing to cracking the block someplace. a plate type mount put the stress else where and was perfered if the engine needed to be solidly mounted. it may not be applicable to your intended use. i can't find the thread searching, but wanted to mention it.

anybody with experience/information regarding this?

[blueghoast]

Rubber trans mount should do it.

[Kevspontiacs@aol.com]

Originally Posted by Matt Meaney

i remember a discussion on this site about not using solid engine mounts. something about stressing the block at a weak point and/or possibly contributing to cracking the block someplace.

i remember this also

[screamingchief]

Well,the problem with all that ^^^^ is there really is'nt much 100% conclusive proof one way or the other to put that debate to rest once & for all.

The problem is the fact that there are indeed a few guys that have had some isolated block problems (for whatever reason) while running solid motor mounts,and then there are guys out there that have ran solid motor mounts for years & years w/o any issues what-so-ever.

Both have had 100% valid experiences none-the-less.

Neither experience can be 100% wrong nor 100% right to the exclusion of the other.

So a kind of "grey area" begins to exist on that whole topic.

Now we have to ask ourselves were those few isolated block failures a direct result of the solid motor mounts?

Or could those blocks have possibly had some sorta underlying/contributing issue(s) that went completely unnoticed (like freeze damage or such) till said block failure occured???

Internal corrosion of a blocks coolant jacket is a pretty common thing too,it's not all that uncommon to find a block that has had cylinder wall that has collapsed due to the inside of the coolant jacket corroding & pitting enough to weaken the block castings integrity.

That ^^^^ is especially true in race motors that tend to use only water in them,typically with minimal rust inhibitors or such.

And what may have worked fine in one chassis,might not work for $#!t in another chassis.

So throw in a car-2-car variable to that whole situation too.

So what pretty much happens with this deal is any given fella picks a failure mode theory that suits his mindset,and then he typically sticks to that POV.

And of course various POV exist simultaneously,hence the debate continues to rage on & on.

Though often it amounts to some degree of what's "good,better,best".

Me,the solid motor mounts dont bother me much here @ all.

But the solid trans mount,IMO there is no arguing/debating that deal.

The solid trans mount has gotta go.

Bret P.

[ho428]

I don't think I'd run a solid motor mount drag racing with a high hp and tq Pontiac engine. Just to big a hit on the block, especially off the line, and just watch those chassis flex when the front end lifts.

Road racing I'd prefer a solid mount since the engine is getting torqued back and forth over and over with so much upshifting and downshifting.
You do see a few broke rubber mounts here and there at the track, fatigue would be my guess.

I pulled the solid trans mount last night. The more I thought about it there needs to be a pivot-stress point somewhere.

[Skip Fix]

MY TA has a solid engine on the left rubber on the right and tranny and no increased vibration when autocrossing or drag racing. I did have the driveshaft balanced after the new U jounts I put in also.

[Sirrotica]

Gordon, in my circle track cars (Which is going to be as close to road racing as your on an off the gas all the time) was breaking the rubber mounts constantly so I decided to leave the rubber mounts and tie both sides of the engine (from the heads to the frame) with adjustable turn buckles for acceleration and deceleration. I used that setup for 3 years and had no ill effects such as the harmonics nor any broken motor mounts. I consider this to be the best of still having the rubber to absorb vibration but not stressing the sides of the block to control the torque. I never have liked the idea of using solid mounts and have avoided them in everything I have ever built, still up to the individual to make their own choice though.

The car I had the most trouble breaking mounts was my 69 GP which has the same mounts that your bird has, they are not a good design mount for high performance use and are prone to rip right through the rubber so I have a very good handle on what your problems are.

[JSchmitz]

Originally Posted by ho428

I was planning on changing out the break in oil after P1.

You should change the oil and filter right after the cam break-in. I wouldn't leave that stuff in there.

[Half-Inch Stud]

Since frames wobble like gumby, it seems a proverbial pothole could crack a block that is solidly mounted.

[lust4speed]

I feel somewhat safe running my solid engine mounts since I have the stock cross member in the GTO, and the middle of the cross member is pretty beefy. Hopefully I won't jinx myself, but have went years leaving off the trans brake at 5,500 RPM with no problems. I do like Brad's suggestion of normal engine mounts and the turnbuckles - seems much safer than other alternatives.

As far as the height of the trans mount. I was delivered two new mounts to see which one fit best on a '72 TA, a standard mount and a poly mount. The poly mount was a quarter inch lower in height than the stock replacement mount. It looked like the new stock piece was way too tall rather than the Poly being shorter than normal. Just something to keep in mind when searching out trans mounts.

[gtoroadracer]

Hi Gordon. I would suggest run the solid motor mounts and the rubber transmission mount but what I did was also supported the back of the engine as well. The idea of using solid mounts is to keep the engine from moving around while cornering. If you hang a stone on a string from your rear view mirror you can see what happens while the g forces are being applied. Imagine what is happening to a 600 pound motor. The ideal situation is the motor should be supported by front and rear plates and then the transmission will just be hanging there and then you can mount it with a rubber support. I know a couple dirt track racers that do not even run a transmission mount.