Opinions on a rubber vs. solid mounted engine

[adynes]

Hello, newbie here.

I am building a dual purpose (street/strip) twin-turbo 77 T/A. I plan to run a midplate between the bellhousing and the block which ties into the frame. The midplate will react the driveline torque to the frame instead of through the stock mounts, which bolt to the side of the block. I feel this will take a lot of load off of the block sides, as 800ft-lb engine torque x 3:1 first gear ratio = 2400 ft-lbs. If this torque is reacted at the stock mounts (~1ft apart), that would be an unnecessary 2400lbs-force on the sides of the block. Midplates are common on high horsepower drag cars, mainly because its an easy way to mount an engine to a tube frame chassis (and I don't think most racers realize the benefit I mentioned above, based on feedback from other forums). A common failure mode in high hp Pontiac engines is structural failure of the block near the stock mount location.

Virtually all cars built with midplates solid-mount the plate to the frame. Since my car will be a street-car 50% of the time, comfort and longevity will be important. I am a little uneasy about solid mounting the engine to the frame through the midplate. I am considering using vibration dampening grommets to reduce vibration in the rest of the car. I am also worried that If vibration is large enough, it could shake the car to pieces over time.

Now for my question:
Are the grommets necessary? What could be some pros/cons of each configuration? I know a lot depends on the specific vibrations present, materials used, and geometry of parts. Could dampening the vibration energy through grommets increase the durability of engine block/components by any appreciable amount?

I missed out on the machine-vibrations class in college, but understand some of the basic concepts, so feel free to keep the discussion technical.

All input is welcome

 

[Danger]

Welcome to PF, Adynes.
I'd stick with the solid mounts, myself. As long as your driveline is properly balanced, vibrations shouldn't get out of hand. If they happen to hit a frequency that irritates you, try isolating the body from the frame with rubber spacers. (I'm not sure how to accomplish that, but it should just be a matter of washers on the panel mounting bolts.) For additional stiffness, you could add 'elephant ears' to the front of the engine and maybe chain it down to the the shock tower to confine reaction torque.
There are a couple of other guys around here who know more about it than I do, so hang around for them to chirp up.

 

[RonL]

Might be a real "chick-magnet", check a few biker bars and find out why the girls like the Harley's so much.

On the serious side, solid mounting will produce failure in areas you least expect, unless you like working on your car on the side of the road, i think even if only a small amount, the engine should be padded with something.


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[adynes]

For additional stiffness, you could add 'elephant ears' to the front of the engine and maybe chain it down to the the shock tower to confine reaction torque.

If I chained to the front of the block (or head), the torque would still be twisting and stressing the block. That is why I want to react the torque to the frame with a midplate between the block and bellhousing. This way, only the belhousing will be stressed by driveline torque. The engine will still have to deal with its internal torque, though.

On the serious side, solid mounting will produce failure in areas you least expect, unless you like working on your car on the side of the road, i think even if only a small amount, the engine should be padded with something.

That is exactly what I was thinking. The plan right now is to use 4 grommets (two per side) of PN: 6309K4 in a 75A durometer from McMaster Carr. These are two-piece neoprene vibration-dampening grommets. Max load capacity: 1330/690 lbs Vertical/Horizontal, with a deflection @ max load of: 0.08"/0.03" respectively. Distance between grommets would be ~28". I will be using them in the horizontal config. Assuming a first gear ratio of 3:1 and 800 ft-lb enigne torque that would be a loading of 1030 lbs per side, due to static torque loading. I assume the grommet's load ratings have a factor of safety built into them, so this should take care of any shock loading (clutch dumps, wheel-hop, etc.) Based on the deflection ratings, engine movement (twist) should be very low. I am pretty sure I'm going to give this a try, despite all the flack I've been catching on the 'other' forums. I really don't see a down-side.

 

[Danger]

If I chained to the front of the block (or head), the torque would still be twisting and stressing the block.

Actually, my chain is attached to one of the left bank header bolts. It transfers the torque to the sub-frame through the shock tower. Elephant ears would do the same thing from the front.
I agree that there's no downside to using rubber mounts; I just happen to prefer solid ones myself.

 

[trambolin]

On the serious side, solid mounting will produce failure in areas you least expect, unless you like working on your car on the side of the road, i think even if only a small amount, the engine should be padded with something.

Indeed ! Be aware that constraining the vibration without damping cause it to be transmitted to the nearest unconstrained part. And then the vibration will effect a part that is not meant to vibrate at all. You definitely will have significant wear and/or failure if you don't dampen with anything. It might be your butt or plastic paddings but you need damping at some point. Thus, it is not a downside issue, it is necessary.

 

[Danger]

it is not a downside issue, it is necessary.

Let's not get hyperbolic about this. It isn't necessary; it's an option. There are a lot of machines running with solid mounts. The only disadvantages that I've ever seen were more noise and a bit of increased vibration in the cockpit. I admit that my Roadrunner has only a puny 650 hp, but I would expect a potential problem to have showed up at some point in the 45,000 miles that I put on her.

 

[trambolin]

Yes, but we are discussing in an engineering sense. If nothing happened, then something is definitely more than needed in your design.

 

[RonL]

Let's not get hyperbolic about this. It isn't necessary; I admit that my Roadrunner has only a puny 650 hp,

Not all horses are the same

 

[Danger]

Man, that was funny! I live in horse-country, and my wife's an Indian, so I can't wait to show her.

 

[DoctorDex]

The plan right now is to use 4 grommets (two per side) of PN: 6309K4 in a 75A durometer from McMaster Carr.

When do you think you'll have the grommets installed? I'm very interested to see how this turns out.