So I have a new project for the winter that has a modified k frame with the motor mounts cut out of it. Previous owner used a mid plate that cracked. Car has an ar engineering front plate in it.

My question is at what point does a mid plate become necessary? Reason I ask is because I am thinking of replacing the k frame to one with mounts, running solid mounts with the front plate and eliminating the mid plate. Its easy enough to put a new k frame in at this point. Car is 9.80-10.0 ish back half race car. Any input would be helpful.

I've been running a mid plate and a front plate since I built my car.. I run 2 limiter bars all along
and nothing has broke... love it this way(full tube chassis) running 8.90s in my SB
IF you keep the front plate wide you will have better leverage on the engine(less movement)

My S/P car has both plates as well as a tranny mount, no travel bar limiter front to rear
I've raced the car that way since 2013 when I got it, runs high 8s at 148+ MPH in the 1/4 mile, 5.5 at 125 MPH in the 1/8.
it is a full tube chrome moly chassis car with 4 link rear coil overs and A arm with coil overs in the front and a rack and pinion steering
It weighs 2850 Lbs. with me in it racing it.

I would just replace the cracked plate I don't know if it helps but on my -090- steel mid plate I welded a 3/4" wide strip perpendicular to the plate in the area from approx the bottom corner of the block to the area where it bolts on the frame

No mid plate in mine. It does have fore/aft travel limiters. 750 passes so far. 1.25/9.00/150@3325lbs.
Doug

Same here. Front plate only w/ a poly trans mount and one fore/aft limiter. Been mid/high 8s at almost 3900 lbs.

No mid plate in mine either! Just front motor plate solid mounts and poly trans mount. Don't even run any limiters. 3550 lbs. /8.72-157.1.27 60ft.

A mid plate is more than an engine mount, if it is properly mounted it stiffens the firewall area of the chassis and adds as a safety net of sorts if there is a catastrophic transmission failure. Not knowing what chassis aids you have ( full cage, roll bar only?? ) a properly sized mid plate can be mounted at the bottom and top to chassis/frame members and significantly reduce twist between the engine mount and forward rear suspension mount . Again more needs to be known about the car to give more in-depth opinion. but my vote if it’s a race car with modified chassis/body components it should at least have a 1/8" mid plate as a trans failure safety mechanism.

when using a mid plate, where is the best place to put the flex plate/torque converter spacer ?
behind the flex plate or in front of it ?

Between the flexplate and crank

When you get down into the 1.20s in 60, you would be surprised how much that stuff moves with motor plates. They work great for torsional forces. Are lousy for fore and aft movement when it has to control hundreds of pounds at 2+ Gs. The car is literally trying to drive out from under the heavy engine/trans assembly that doesn't want to move. Newton's first law of motion.

I always control converter clearance with shims/washers between the converter and flexplate, and bolt the flexplate directly to the crank. Some of the Powerglide bellhousing/adapter packages are done differently.

This takes care of the fore/aft movement.
Doug

I built that exact same set up and on the first pass broke the tabs off the tranny.

I was thinking the same thing would happen.

I didn't know if it would work when I built it either. That was 9 years and 750 passes ago. 60 fts mid 1.2x to low 1.3x depending on if it's being slowed to run 9.25 index. Weight varies from 3325 to 3550. Same case and tail housing since day one.
Doug

Small block car would go low 1.20s. Limiters on either side of the engine. Had 3/8" G8 bolts in the block in single shear. Broke one of the bolts one day. Bent the motorplate. Got my attention.

You can use .090 steel(MIN) and dont need any spacers.. thats the way I'm set up

All I am implying is those limiters aren't doing anything then, your motor plate and trans mount must be doing the job. Because if it comes to those limiters its going to take the outside of the aluminum ears off.

What I was talking about was the min mid plate thickness... I have my limiters mounted from
my block over to the chassis on both sides

It has a stock replacement S-10 rubber trans mount. You'll notice that the heim joints are 90 degrees to each other. If there is any twist they wont bind at the trans ears or the crossmember tabs. The motor plate tabs are probably over built. Look at the bottom of the plate. It is also tied in at the bottom of the timing cover. Don't know if these items made a difference. I know a few guys have copied it with no failures. To be honest I would have put a mid plate in it, but the headers wouldn't fit.
Doug