Does anyone rubber-mount their midplate?

[adynes]

Does anyone rubber-mount their midplate? I plan on running a mid plate on my car, which will be used 50/50 street/strip. I would like to isolate the plate from the frame with some type of vibration dampening grommets or bushings. Has anyone seen this done before? I have searched and not found much. I am considering a few of PN: 6309K2 from McMaster Carr, maybe 3 per side.

Any Ideas?

Thanks

[BruceWilkie]

I'm planing something similar with an "L" bracket and some leftover radiator support bushings.

[BVR421]

whats the point of the plate then? The purpose of front and midplate is to tie the engine and chassis togather as a single unit. Any soft connection allows flex/torsional twist and defeats the purpose. For any street driving the factory mounting is much nicer.
FWIW, early American cars including pre 59? Pontiac used rear engine mounts such as you propose.
Mounts at the bellhousing area with rubber insulators and a single front not side mount. The remnents of that front mounting point can still be seen on the lower ears of all Pontiac front timing covers. Those two little ears were used in later years to attach the auto trans cooling lines as they passed that area.

[adynes]

The purpose for my application is to reduce stress in the block. Specifically, the stress one the sides of the block where the factory mounts bolt to.

I do not currently run forward down-bars in the engine bay. Because of this, I feel that by running hard mounted mid and front plates, I would actually be stiffening my frame with the block, which is not good for block durability.

[goquick]

Actuly, I have seen blocks/midplates with bushings and the result was a disaster. Allowed to much flex, which overloaded the blocks and bushings.

[adynes]

Well, I did a few calculations. It looks like I would need 2 of the large (6309K4) 75A-durometer grommets per side. These are 2.5" diameter and require a 1.58" hole in the plate, with a .66" through-hole for the bolt (5/8" bolt? Seems loose). I'm not sure if I have the room for grommets that big.

How bad would the vibration be with a solid mounted midplate, assuming a properly balanced and tuned engine? A little shake wouldn't bother me, but an annoying vibration would.

[Brian Baker]

How much HP are we talking here? If you're at the point where having a midplate is beneficial, why would you be concerned about any additional vibration? This reminds me of guys that are wary of installing Rhoades lifters for fear of the loud ticking...if you've got any kind of performance sounding exhaust on the car, you'll never hear the tick.

[adynes]

It depends on how much boost I run. Potentially 8-900 when I get serious at the track. Initially it will be kept lower, maybe 600 or so. Obviously things will be turned down while on the street. The reason I ask is I have never driven/ridden in a car with motor plates. I would rather dampen vibration if possible, as it tends to shake things to pieces. Being a turbo motor, the cam will be relatively mild compared to what a lot of you guys run, so that should calm things a little. I don't want the car to turn into something that I stop driving because its a pain in the a$$.

[Brian Baker]

What kind of car is this going into? 800-900 HP when the wick is turned up? That car should have a full cage in it, and as such, will already ride like a buckboard, so adding a midplate is not going to make a difference in how the car feels on the street.

Plan ahead because once you get a taste of that level of HP, you're not going to want to turn back because the chassis and cage are not up to the task.

[adynes]

Plan ahead because once you get a taste of that level of HP, you're not going to want to turn back because the chassis and cage are not up to the task.

Haha, Yeah, I fully intend on that happening. I don't plan on turning it to full kill mode until I get the system sorted out. Don't worry, there will be no 900hp no cage passes!

Actuly, I have seen blocks/midplates with bushings and the result was a disaster. Allowed to much flex, which overloaded the blocks and bushings.

Goquick, could you you elaborate on the setup you saw? Like what type of bushings, quantity/location, usage of the car, etc? I'm going to venture a guess that the seup was not properly scienced-out, but I don't want to blindly make that assumption. If I have the room, I'm going to go ahead and try it. If it works, great, if not, solid-mounted it is.

[vin63]

I would agree with the other posts questioning the need for a midplate if you bolt it to a flexible mount. While it may reduce some of the noise from engine vibration, you'll probably go through a lot of transmission cases and driveshaft-related parts. The midplate in my Super Gas car is mounted directly to the chassis and I still have to run a torque limiter (chromoly tubing with spherical rod ends on each end mounted from the block to the chassis) to keep my engine and trans from moving forward/back - this is just a 650 hp SBC. If my midplate was bolted to a flexible mount, I imagine I would have to fabricate a dual torque limiter set on each side of the block, which will defeat the purpose of isolating the vibration from the engine with the flexible midplate mounts.

[adynes]

The engine will be fore-aft limited by the factory rubber engine mounts. Trans will also use a rubber mount. The reason I will be running a midplate is to react the driveline torque reaction. This was actually discussed in a few other posts that are now burried. If only running factory mounts, I would have 800 ft-lb x 3 (first gear ratio) / 1ft? (distance between mounts) = 2400 lbs of load at each mount on the side of the block. This does not even account for impact loading of clutch dumps, TC multiplication, etc. By running a midplate, that torque is reacted to the frame, and the engine only has its internal 800ft-lbs to deal with. It seems to me like an easy and inexpensive way to eliminate a lot of stress on the block.

The grommets are listed to have a .030" horizontal deflection per 690lb of load, so if two are used per side on the midplate, and 1040lb of force at the grommets (because they are ~28" apart) that comes out to .023" deflection, or .05 degrees of engine twist, That is a lot less twist than I remember with the stock mounts.

[screamingchief]

I would'nt even attempt to soft mount a midplate,the idea defeats the whole purpose of adding the mid-plate to begin with.

I think your putting too much concern into the theory that the block motor mounts might cause block/main issues,and not thinking the whole thing thru while trying to work this design around that theory.

Now dont get me wrong,I'm not saying there is'nt some amount of truth in that theory,but using that to justify what your considering is just taking one bad idea,and replacing it with another bad idea,an idea who's actual end result may or may not be as bad than the bad idea it's replacing...

It almost sounds to me as if your willing to risk the block to try out this new idea.

Would that somehow be different than loosing a block using the stock mounts?

Like has been said,you loose a healthy amount of fore/aft control with a mid-plate unless you leave the stock motor mounts in place,and then you still have those loads being exerted on the block with the soft mounted mid-plate,so likely your improvement there will be minimal as then the mid-plate is just acting as a defacto travel limiter.

And even adding a front plate only helps the whole situation somewhat by itself,you would still be well advised to use the fore/aft travel limiter as well,when you look at all these pieces (front plate/mid plate/fore-aft travel limiters) you can see they are all well designed and function very well when used as intended,but go trying to re-invent them and you are on your own,as they say,results may vary.

In the end no solution will likely be perfect,almost surely there will be compromises made somewhere,it's just the nature of the beast.

And a stock block will still only be so strong...

But if your still intent on doing this,good luck with it.

:-ohwell:

[Brian Baker]

The engine will be fore-aft limited by the factory rubber engine mounts. Trans will also use a rubber mount. The reason I will be running a midplate is to react the driveline torque reaction. This was actually discussed in a few other posts that are now burried. If only running factory mounts, I would have 800 ft-lb x 3 (first gear ratio) / 1ft? (distance between mounts) = 2400 lbs of load at each mount on the side of the block. This does not even account for impact loading of clutch dumps, TC multiplication, etc. By running a midplate, that torque is reacted to the frame, and the engine only has its internal 800ft-lbs to deal with. It seems to me like an easy and inexpensive way to eliminate a lot of stress on the block.

The grommets are listed to have a .030" horizontal deflection per 690lb of load, so if two are used per side on the midplate, and 1040lb of force at the grommets (because they are ~28" apart) that comes out to .023" deflection, or .05 degrees of engine twist, That is a lot less twist than I remember with the stock mounts.
Now you've completely lost me. You're inquiring about a midplate to avoid stresses on the block transmitted through the rubber OEM mount location, yet you also want to mount it with rubber bushings? You can keep the OEM mounts, but make the midplate solid.

[adynes]

I would'nt even attempt to soft mount a midplate,the idea defeats the whole purpose of adding the mid-plate to begin with.

What do you consider the purpose of adding a mid plate? For my purpose, its to reduce stress in the block. I am not trying to reinforce my frame with the block.

Like has been said,you loose a healthy amount of fore/aft control with a mid-plate unless you leave the stock motor mounts in place,and then you still have those loads being exerted on the block with the soft mounted mid-plate,so likely your improvement there will be minimal as then the mid-plate is just acting as a defacto travel limiter.

Well, I was planning on using the factory mounts to limit fore/aft movement (thought I mentioned that earlier), but now after thinking about high-G launches, a limiter attached to the midplate/belhousing area would be better to eliminate the stress under that condition.

Now you've completely lost me. You're inquiring about a midplate to avoid stresses on the block transmitted through the rubber OEM mount location, yet you also want to mount it with rubber bushings? You can keep the OEM mounts, but make the midplate solid.

I think you guys are hearing "rubber mount" and thinking there will be a lot of movement. Because of the stiffness of the material, number of grommets (read: cross-section), and distance from the centerline, deflection will actually be very small, essentially it will be solid mounted, at least compared to the factory soft rubber mounts. The whole idea behind this was to dampen vibration which can be bad, depending on the magnitude. Kinda the whole idea behind harmonic dampeners. Maybe the vibrations present will be insignificant, I'm not sure. Thats why I wanted to discuss this idea.

[adynes]

After re-reading a few of the earlier post, I think I need to emphasize something. I never intended to run JUST a mid plate.

Stock motor mounts to support engine.
Midplate to react driveline torque.
Rubber trans mount to support trans.

[screamingchief]

Even if you solid mount a mid-plate the deal is gonna move around,you can count on that,nothing in a car is perfectly rigid,and a chassis were perfectly rigid it would likely have at least a few other serious issues to deal with.

The hope is that the midplate keeps the engine rotational forces in check,and it helps locate the engine in the chassis where it is wanted,same deal with the front plate.

The fore/aft limiters keep the engine in check in that respect (front to rear orientation).

And the side benefit of all this stuff is that the frame/chassis is stiffened somewhat as well,and that is in a specific area when these are all used in conjunction as intended.

That's why most who know this stuff use the front plate and fore/aft limiters as well when using a mid-plate.

A mid plate can be used with stock mounts no doubt,but the block "mounting boss" loading benefits have mostly then been lost to the rotational control issues when that is the case,as the frame in the engine mount area is still gonna flex and exert forces on those stock engine mounts,and thus the block as well.

Will that set-up help reduce those forces on the engine mounts/block some,maybe yes,maybe no,that one is kinda a crapshoot,and has been debated here a bunch in the past.

But do understand that by using the rubber isolators on the mid-plate your loosing even more of the mid-plates primary function as it will abolustely be more prone to "give" under the engines rotating (and to a lesser degree,fore/aft) forces than a solid mounted mid-plate would be prone to move under the same forces.

Ask me your never gonna get rid of all the vibrations in a given car,and rubber mounting is not a guarantee that it'll reduce the vibrations nasty affects,sometimes hard durometer rubber or poly can actually make vibration affects worse overall.

Do enough work on OE cars with serious NVH issues and you learn that in a hurry.

I've known plenty of folks who've thought poly bushings or such would isolate vibrations too,and then they learned otherwise.

For what your using the midplate for here,you could just as easily fashion a "strut" style engine torque limiter deal and get pretty much the same affect with a whole lot less hassle...

I'm not against what your doing here,I'm just saying you should'nt lull yourself into thinking your doing the block any huge favors by doing it that way,as I would say the benefits will likely be seen in other areas mostly.

And the merit of this helping with any vibration related issue will be very hit or miss IMO as well.

:2cents:

[adynes]

Agree to disagree...

We're loosing scope of the thread. I do not intend to argue the validity of my theory. I've already presented the math and engineering principals to back it up.

What I'm wanting is input from people with experience, firsthand or otherwise pertaining a vibration isolated midplate. I kind of suspected there would not be much experience in this area. I might have to be the first to prove it out, whether it works or not.

Lets hear from some other people....

[BVR421]

food for thought, I have before and after pics of my 64 at the line, a little over 600hp
before pic shows the driver side tire up about 9", pass side up about 2". This is with a 12pt cage on a factory frame. Soild side motor mounts. the front horns are quite rigid with heavy downbars, never the less the frame twist was significant. After the install of DMR rear roll control (sway bar) the launch is level, no frame twist. The car was run by former owner and builder several years (200 passes?) before the rear bars were installed and the twist may or may not have contributed to a catastrophic main web/cast crank failure later on. Point is, the addition of the Dick Miller Racing anti roll bar certainly has taken the twist out of the front of the frame and I believe that may be one of your concerns. It did nothing to stiffen the front but by equalizing the reaction at the rear it stopped the action at the front.
quote:
"I don't want the car to turn into something that I stop driving because its a pain in the a$$."

As far as a chassis capable of handling 900hp, I predict you wont be in Kansas anymore Toto LOL
600 or so I think is doable and still some street civility. Im sure yours will be well thought out one way or the other.

My only experience with a car with a solid midplate wont give much input as far as how much an annoyance it is because by the time that TA got that far it was hacked up so much any street use would be miserable.
Power adder cars have the advantage of power on demand in an otherwise street friendly car.
Im sure you are striving for a pro _street_ type car that lives up to its promises but somewhere that line gets drawn in the sand when it comes time to safely and effectively put the power on the ground.

[Brian Baker]

Agree to disagree...

We're loosing scope of the thread. I do not intend to argue the validity of my theory. I've already presented the math and engineering principals to back it up.

What I'm wanting is input from people with experience, firsthand or otherwise pertaining a vibration isolated midplate. I kind of suspected there would not be much experience in this area. I might have to be the first to prove it out, whether it works or not.

Lets hear from some other people....
Like was stated, even with a solidly mounted midplate, there is still going to be some movement, if not in the rotational plane of the crank, in the fore/aft plane of the block. I don't care how hard the durometer rating is on the rubber bushing, it's going to move more than a solid mounting. There's no getting around that. And like I said, with that much power on tap, you'll be needing a full roll cage at a minimum, and that alone will make the cars ride on the street so harsh to the point that adding a solid mounted midplate will not make a difference in ride quality. As for vibration, it's not going to induce any more into the chassis than a set of solid OEM location mounts will.

If you're looking for someone with experience running a rubber bushed midplate, I think you'll be hard pressed to find one, in any racing circle. There's a reason why they don't run them that way. If there were any benefit, more people would. Good luck.

[vin63]

I agree with Brian...if your car is not a full-cage chassis, as I mentioned before, you are going to run into transmission case and drive shaft issues because the midplate connects the engine/trans to the chassis - the engine/trans will no longer be able to move/rotate independently with the factory mounts. I've seen drive shafts in Pro Stock cars that have kissed the drive shaft tunnels from tire shake and chassis torque. With longer drive shafts, you may cause the drive shaft to jump-rope even more - really bad things happen when the trans case and drive shaft fail.

As far as protecting the block, I think there are other more important issues in chassis set-up before you start compromising the strength of an engine block. I appreciate the research and calculation you did, but I think what most of us are trying to do is save you some money from breaking other parts, and keeping you safe in case something more catastrophic fails as a result.

[taff2]

You could replicate the early Pontiac mounting system as BVR421 mentioned earlier,but use harder poly bushings at the bellhousing mounting points. This should transfer rotational torque into the chassis as you desire but absorb some engine vibration. Leave out the factory sidemounts to remove stress at the sides of the block,but use a front mount similar to the early setup ,again using poly mounts.

[455Turbo]

Agree to disagree...

We're loosing scope of the thread. I do not intend to argue the validity of my theory. I've already presented the math and engineering principals to back it up.

What I'm wanting is input from people with experience, firsthand or otherwise pertaining a vibration isolated midplate. I kind of suspected there would not be much experience in this area. I might have to be the first to prove it out, whether it works or not.

Lets hear from some other people....

I made my own polyurethane motor mounts for my twin turbo Trans Am. I leave the line at 18 psi of boost with a transbrake. The car has made 860 rwhp and 870 lb-ft on a Dynojet. That's with a factory block.

Forget the midplate.

[455Turbo]

Here are some pics:

[Brian Baker]

I made my own polyurethane motor mounts for my twin turbo Trans Am. I leave the line at 18 psi of boost with a transbrake. The car has made 860 rwhp and 870 lb-ft on a Dynojet. That's with a factory block.

Forget the midplate.
What block, what mods were done to the block, and how many passes so far? Enough passes to warrant a freshening up? If so, what did your internal inspection of the block and bearings reveal?

[455Turbo]

What block, what mods were done to the block, and how many passes so far? Enough passes to warrant a freshening up? If so, what did your internal inspection of the block and bearings reveal?

Good questions...

Block is a 1970 455 from a Bonneville. I don't know the numbers on it. It has steel 4 bolt mains on 1-4, factory rear main cap, and ARP studs. About 50 passes so far, half of them with the transbrake. And about 8,000 street miles.

I've had the engine apart three times for inspection, mostly to inspect the factory crankshaft for cracks (have never found any). The main bearings are perfect, they've been in the engine since I built it four years ago. I replaced the rod bearings once when the uppers were a little shiny, no copper was visible.

[vin63]

Here are some pics:

Very cool mount. That's very similar to what I'm fabricating for my Pontiac-powered truck project...going back to a polyurethane engine mount from the motor/midplate - the last nitrous shot cracked the trans case, which prompted this tear down and re-mount.

http://i268.photobucket.com/albums/jj33/vin63/Pontiac_PolyMount1.jpg

[Jack Gifford]

adynes- check your PM box.

[Brian Baker]

Good questions...

Block is a 1970 455 from a Bonneville. I don't know the numbers on it. It has steel 4 bolt mains on 1-4, factory rear main cap, and ARP studs. About 50 passes so far, half of them with the transbrake. And about 8,000 street miles.

I've had the engine apart three times for inspection, mostly to inspect the factory crankshaft for cracks (have never found any). The main bearings are perfect, they've been in the engine since I built it four years ago. I replaced the rod bearings once when the uppers were a little shiny, no copper was visible.
Cast? What rods?

[RAIV55]

Adynes, I understand what you are asking. The theory of the motor plates for taking stress from the block is good. Front and rear plates will remove stress that's created by side mounts trying to split the block. In most motor plate applications the block is used for a load bearing member and the plates are used in place of crossmembers. If you have a flexable chassis and a semi rigid engine mount, I wouldn't have an idea how to start figuring the multi-axis loads imposed on the block.

Also lateral mounts are needed because of motor plate flex, not just because of the rubber mounting.

[455Turbo]

Cast? What rods?

Yes, cast. Eagle rods.

[Brian Baker]

455Turbo, is the block filled to the water pump holes?

[BruceWilkie]

http://turbopontiacs.com/
Also in recent magazine. Hot Rod???

[455Turbo]

455Turbo, is the block filled to the water pump holes?

No fill at all.

[455Turbo]

http://turbopontiacs.com/
Also in recent magazine. Hot Rod???

Hot Rod, September 2007, pages 54-60

[Brian Baker]

Large journal, unfilled factory block, with a cast crank and heavy rods making in excess of 900-HP...keep us abreast of its longevity.

[adynes]

Block is a 1970 455 from a Bonneville. I don't know the numbers on it. It has steel 4 bolt mains on 1-4, factory rear main cap, and ARP studs. About 50 passes so far, half of them with the transbrake. And about 8,000 street miles.

I've had the engine apart three times for inspection, mostly to inspect the factory crankshaft for cracks (have never found any). The main bearings are perfect, they've been in the engine since I built it four years ago. I replaced the rod bearings once when the uppers were a little shiny, no copper was visible.

I have been wondering how your block has held up. It seems that rigid side mounts + trans brake launches would be very harsh on the block. That is very encouraging. I have a thick casting 400 block, and was planning a partial fill. Still debating whether or not to go splayed 4-bolt. I think would be in my best interest if I did. I thought your engine used stock two-bolt main caps at one time. Did you change that at some point, or am I just remembering this wrong?

Also, how stiff are your poly-mounts? Could you estimate how much your engine 'twists' with respect to the chassis when under load? That would give me an idea how soft or hard they are.

Thanks

[adynes]

455Turbo-

I can't seem to find that video you used to have on Putfile. Do you have any others out there? If not, You should put some up on youtube. That car is very impressive. We might have to talk HPP into doing a turbo pontiac shootout (if they haven't already) when I get mine done.

[455Turbo]

I have been wondering how your block has held up. It seems that rigid side mounts + trans brake launches would be very harsh on the block. That is very encouraging. I have a thick casting 400 block, and was planning a partial fill. Still debating whether or not to go splayed 4-bolt. I think would be in my best interest if I did. I thought your engine used stock two-bolt main caps at one time. Did you change that at some point, or am I just remembering this wrong?

Also, how stiff are your poly-mounts? Could you estimate how much your engine 'twists' with respect to the chassis when under load? That would give me an idea how soft or hard they are.

Thanks

I've used only steel 4-bolt main caps and ARP studs with the turbos.

I have no idea how much the engine 'twists' when under load. My guess is that it doesn't move much. The engine mounts are made with a piece of polyurethane, Shore A 60, about 1.5" O.D. and .75" I.D.

The launch sounds worse than it is, it's only at about 3200 rpm and barely lifts the front tires. Sixty foot times are usually in the 1.60 sec range. The transbrake allows the car to come out of the hole under boost instead of having to wait for the turbos to spool up.

I'll try to post the video on Youtube again later.

[adynes]

The launch sounds worse than it is, it's only at about 3200 rpm and barely lifts the front tires. Sixty foot times are usually in the 1.60 sec range. The transbrake allows the car to come out of the hole under boost instead of having to wait for the turbos to spool up.

Yeah, I guest a TB Launch would be worse on the trans and rearend than it would the engine. I don't know what I was thinking yesterday, tired I guess.