[Hillard]

Seeing the other "motor mounts??" thread got me thinking and I didn't want to hijack his thread. I've got a 69 GP that I was going to make a pair of solid billet mounts for but wondering if I should use plates. Car will be street/strip. Engine is a 455 +60 w/ a 4.5 stroke, aluminum rods, ported Wenzlers, solid roller (256/264@.050 w/.644/.624 lift), about 12.75 compression. I know the solid mounts would hold it just fine. But we also know that the 4.5 stroke/3.25 main in a stock block can result in a split block. Just wondering if plates would help keeping the block more stable since they hold front and back instead of the two sides. Maybe I'm just over-thinking this but.....opinions??

[Chupa]

I would run a mity mount with a front plate - just worried about stok block weaknesses.

I'm about 600 hp have the mitt mounts in with fort and rear plates now.

I would get rid of the mount but need to accout for the fwd aft engine movement.. Just wasn't ready to address this yet. When I cut the front xsection I will

[70 bird]

Proper answer is motor plate. But... I have been using solid Indian adventures motor mounts for 7 years now without issue at 700+hp, stock block 462.

[janderson]

Would utilize front and mid plate(s) if you have the option.

Lessen the stress on the side of the block and create some additional header clearance.

As Chupa mentioned - will need to control longitudinal / fore-aft movement.

Will also support the engine and / or trans if you have to pull one and not the other.

What we did on the '78 T/A: front and mid plate(s) with poly trans mount.

[Hillard]

I've heard solid mounts on sides of block can actually distort the mains. Any truth to this? Looking at a block comparing mount holes to main caps, I can see where this is a possibility.

[BLANK1]

Have you thought about the polyurethane mounts that Butler sells?

[60man]

Originally Posted by 1050goat

I've heard solid mounts on sides of block can actually distort the mains. Any truth to this? Looking at a block comparing mount holes to main caps, I can see where this is a possibility.

I had a nice long reply for you but the internet devils ate it . you heard right . l lost #3 cylinder with solid mounts. I went with stock mounts and IAII front plate on GP. The 69 G body makes it a challenge but doable. Front cross member location . I !eave on brake at 5K and all is good so far. 867 Hp& 780 tq.. & 4.5" crank. No rear plate. You could pull the bolts to frame from front plate for street. If you want.
If yod a search I made a post about it.

[Hillard]

Originally Posted by 60man

I had a nice long reply for you but the internet devils ate it . you heard right . l lost #3 cylinder with solid mounts. I went with stock mounts and IAII front plate on GP. The 69 G body makes it a challenge but doable. Front cross member location . I !eave on brake at 5K and all is good so far. 867 Hp& 780 tq.. & 4.5" crank. No rear plate. You could pull the bolts to frame from front plate for street. If you want.
If yod a search I made a post about it.

Any particular reason for no rear plate? The more I think about this I was considering front and rear plates plus stock rubber mounts.

[Elarson]

The advantage to a rear motor plate comes from reacting transmission torque. For every reaction, there is an equal and opposite reaction.

Lets say you're making 800 ft-lbs of torque in the engine and your transmission/converter/whatever has a 2:1 multiplication ratio. That means your transmission output shaft is running at 1600 ft-lbs, so your transmission case has a reaction torque of 1600 ft-lbs in the opposite direction of rotation. Impact loads from a hard launch will make that number several times higher.

That 1600 ft-lbs of reaction torque has to be "grounded" to the chassis somewhere. If you have a rear motor plate, the 1600 ft-lbs reacts from there into the chassis and does not twist the block. If you have side motor mounts, you are making that transmission case reaction torque travel up though the block and push/pull the sides of the block. If on the other extreme, you have soft stock motor mounts and a solid rear transmission mount, the 1600 ft-lbs will try to be resisted by the tailshaft housing of the transmission.

Overall, loads will react at the stiffest location in your "system" and your choice of mount configuration determines what gets twisted and what doesn't.

If using front and rear motor plates, one of them needs to be soft fore/aft to allow for axial thermal expansion of the block.

If you have a stock flexible frame, the motor should be solidly grounded at one end only and soft anywhere else so that frame flex will not try to flex the block. The factory system of (3) soft mounts prevents chassis flexing from inducing loads in the transmission case or block. Our funny car frame is very flexible, so we have a really beefy aft motor plate and not much at the front (saddles and hose clamps ) with nothing at the aft end of the transmission.. The Grocery Getter has a stiffer tube chassis so it has full motor plates fore/aft (front one is soft axially) and no real support at the end of the transmission.

Hope this sheds some light.....

Eric

[Hillard]

Originally Posted by Elarson

The advantage to a rear motor plate comes from reacting transmission torque. For every reaction, there is an equal and opposite reaction.

Lets say you're making 800 ft-lbs of torque in the engine and your transmission/converter/whatever has a 2:1 multiplication ratio. That means your transmission output shaft is running at 1600 ft-lbs, so your transmission case has a reaction torque of 1600 ft-lbs in the opposite direction of rotation. Impact loads from a hard launch will make that number several times higher.

That 1600 ft-lbs of reaction torque has to be "grounded" to the chassis somewhere. If you have a rear motor plate, the 1600 ft-lbs reacts from there into the chassis and does not twist the block. If you have side motor mounts, you are making that transmission case reaction torque travel up though the block and push/pull the sides of the block. If on the other extreme, you have soft stock motor mounts and a solid rear transmission mount, the 1600 ft-lbs will try to be resisted by the tailshaft housing of the transmission.

Overall, loads will react at the stiffest location in your "system" and your choice of mount configuration determines what gets twisted and what doesn't.

If using front and rear motor plates, one of them needs to be soft fore/aft to allow for axial thermal expansion of the block.

If you have a stock flexible frame, the motor should be solidly grounded at one end only and soft anywhere else so that frame flex will not try to flex the block. The factory system of (3) soft mounts prevents chassis flexing from inducing loads in the transmission case or block. Our funny car frame is very flexible, so we have a really beefy aft motor plate and not much at the front (saddles and hose clamps ) with nothing at the aft end of the transmission.. The Grocery Getter has a stiffer tube chassis so it has full motor plates fore/aft (front one is soft axially) and no real support at the end of the transmission.

Hope this sheds some light.....

Eric

Lot of good info there Eric. Thanks.

Correct me if I'm thinking about this all wrong, but from what you're saying I'm now thinking soft mounts (stock location) and a mid-plate w/ no front plate or front plate also but mounted soft axially. This is a stock frame/chassis. Keeping in mind it is a GP chassis which is the boxed frame so it is stronger than say a GTO hdtp frame. The whole idea behind me using a front and/or mid-plate is to lessen the strain on the block as much as I can.

[bigchief468]

I would go with a front and mid plates with what you are building. I cracked two blocks in the past at the 550 to 650 HP levels. I have a current old motor at the 700 HP level at 7000 RPM level and has been running for years (approx. 10) with front and mid plates with no issues on a stock block.

[60man]

Originally Posted by 1050goat

Any particular reason for no rear plate? The more I think about this I was considering front and rear plates plus stock rubber mounts.

Nope...just a logistical thing for me. Nothing against rear plates..I understand it would take a lot of stress off tranny and back of block. Since I do have a IAII block I wasn't that interested. Just can't get my head around the install and side effects on other items such as headers,trans removal, and so forth..It tight down there!!!! If it were a chassis car.....no brainer.

Not sure how offensive a rear plate would be with street manners either..
The front plate transfers a lot of noise as it is..

Got any pics of GP ??

[Chupa]

Originally Posted by 60man

Nope...just a logistical thing for me. Nothing against rear plates..I understand it would take a lot of stress off tranny and back of block. Since I do have a IAII block I wasn't that interested. Just can't get my head around the install and side effects on other items such as headers,trans removal, and so forth..It tight down there!!!! If it were a chassis car.....no brainer.

Not sure how offensive a rear plate would be with street manners either..
The front plate transfers a lot of noise as it is..

Got any pics of GP ??

I got a allpontiac rear plate in my 1st gen bird - I understand your concerns but plenty of header clearance. Actually relatively easy to install.

[Hillard]

Originally Posted by 60man

Got any pics of GP ??

I have pics on my old PC which has been retired to the basement. When I get a chance I'll retrieve them.

[guidos]

Anyone install a rear plate with a 4 speed?

[60man]

Originally Posted by Chupa

I got a allpontiac rear plate in my 1st gen bird - I understand your concerns but plenty of header clearance. Actually relatively easy to install.

Any pictures? IIR a first gen Bird is not a full frame car? I'm paranoid about difficulty R&R tranny.
My main concern. With just a front plate it not a piece of cake.

[Chupa]

I'll try an snap a pic today...

I did have to trim out a area on the front plate to provide room around the lower radiator hose and front cross member.

[455firebird1969]

Originally Posted by Chupa

I got a allpontiac rear plate in my 1st gen bird - I understand your concerns but plenty of header clearance. Actually relatively easy to install.

My last engine (600hp) went 7 years and over 1000 passes before spinning a rod bearing. The mains were starting to split upon tear-down. It used solid mouts and a polyurethane trans mount, and seemed to work well in my application. My new engine (stock block) should be close to 700 hp, so I am interested in this thread, and your 1st gen application. The way I am reading your info is you use mity mounts and a mid plate. Please elaborate on the entire eng/trans mount system, and the length of time or number of passes. I'd like to see how the mid plate mounts and any other info it takes to make it work. Thanks.

[60man]

Originally Posted by Chupa

I'll try an snap a pic today...

I did have to trim out a area on the front plate to provide room around the lower radiator hose and front cross member.

Ditto.......and for steering shaft on drivers side.

[Rob B]

I just finished re-doing my entire car, and I mean everything. Previously I used solid stock mounts with a rubber trans mount with a small turnbuckle on drivers side for piece of mind. Never had any issues for 15 years running mid-high 10's. Did the same setup for Carole's car except I also installed an AP front plate like troy did instead of a turnbuckle to take the stress off the sides of the block. For her car its overkill since its only a 475hp motor and runs in the mid-high 11's.
On my car, I decided to eliminate the stock mounts and all the brackets to go to front and mid plates (also from AP) and I still have a rubber mount on the trans. I know alot of people have no mount at all when they go to plates on the trans but I just couldn't do it. Motor is still in the stock location so if I wanted to switch back, it would be no big deal. Basically I set the motor and trans in the subframe with the old mounts (subframe was out of the car) and then fabbed up and mounted the plates. I also fabbed up fore-aft bars to keep the motor from moving back and forth. I really didn't have any issues with clearances with the mid plate when it came to headers and bellhousing. I converted my car to a rack so I can't comment on the stock steering shaft and rag joint but I don't see any reason why they would be an issue. The whole process was pretty simple and in the end, everything worked out perfect. Drivability wise, I can't tell a difference from the old setup. Definitely doesn't ride like a cadillac.