butchering a miata subframe

before i do something stupid tomorrow....

2000 miata

2004 gm 3400 FWD engine mounted in RWD configuration

the oil pan on my engine swap is sitting on the crossmember right now. im pretty sure that everything will clear by chopping the center 8 inches out of the crossmember, right behind the steering rack. im almost positive the rack will then nestle into the notch in the oil pan, thereby allowing me to drop the engine another 5 inches or so, and get it 3 more back, and make everything play nice. id then put some sort of tubing in place to tie the two halves back together with the available space. this may be lower than the height of the factory crossmember. 

im worried about strength, safety, and chassis rigidity here. challenge car, blah blah blah....

heres what im working with at this point in time:

oil pan to crossmember interference

20171209_153543 by Michael Crawford, on Flickr

amount able to come back (its actually about 2 inches all told)

20171209_161812 by Michael Crawford, on Flickr

amount of drop to make everything actually fit

20171209_153622 by Michael Crawford, on Flickr

what issues do yall see with my plan of chop apart and weld back together? am i missing a better solution? (other than different engine) oil pan hackery is NOT an option. firewall may be, but must be able to retain factory HVAC stuff and dash without mods. 

If you own a welder I say go for it. 

im a redneck with a welder, sawzall, a rabbits foot, and no fear of failure. 

but a fear of injury or safety issues. my 9 year old needs me to kick the E36 M3 out of her future boyfriends.....

RWD pan and sump from S-10/Camaro?

Deflection. Will stuff go to heck once you cut the frame?

I'd put a piece of bar, tubing, etc, on the ground at ride height, weld down to it with tubing, then do the cut. Once you have the engine in place, you can rebuild the crossmember however makes sense. 

Note, I may be overthinking this, and the body is totally strong enough to hold stuff in place, even with the center of the subframe out.

cant use RWD sump due to crpssbolted mains and the oil pan being a stressed member in the FWD block. 

good call on welding something across to hold the halves together just in case. 

If you need any reinforcement as to how much you can  whack on the cross member check this...... http://grassrootsmotorsports.com/forum/build-projects-and-project-cars/resto-modding-an-autobianchi-bianchina-micro-car/123915/page1/

Take a look at the FM V8 conversion crossmember.  Seems like they removed quite a bit of material.  I'd weld the tube where it needs to go and cut away what you need to from the stock crossmember.

The crossmember we use is still the same basic shape as stock, though. And it's not as stiff as the stock part, I'll admit. I've torn apart a few under cornering and braking loads, including ripping the steering rack off an early one.

That cross bar behind the steering rack is really important. All of your cornering loads get fed in there, and the steering rack is pushing against it as well. Make it as strong as you can and as close to a straight line between the front control arm mounting points as possible. If you have to give up ground clearance, well, it's a Challenge car. And a low bar may protect the pan.

Keith:

Thanks for the experience.  Ill make sure that whatever i do increases strength to the steering rack mounts.

Would moving the rack an inch or two forward to create space for uninterrupted box tubing squarely behind it be beneficial or detrimental?

Take a page out of the subaru subframe book and build the front crossmember around the rack?

I can't tell from your pics-Have you trimmed the rear "ears" of the rails right at the firewall in the engine bay?

Moving the rack forward or back will affect both bump steer and ackermann. Given the use of the car, I'd probably sacrifice them in order to get as much strength in the cross bar as possible. Bump steer isn't likely to be an issue on a challenge car, but the high levels of steering lock will make ackermann a factor in handling. You may be able to improve on it, I don't know.

In reply to MrJoshua :

Building the subframe around the rack may not be a bad idea.

Shouldn't have to notch the ears in the engine bay. I have inches at the tightest spots right now, and it will only get better lower down in the bay.

60 degree v6 is a very compact package.  Except for the freaking oil pan. 

Custom oil pan?

You've got a welder. 

In reply to SVreX :

Nope. Thats the only thing that i am not willing to cut.

What would it look like to remove the cross member, get the engine in place, and then offer the crossmember back up to the car?  Would you need less material removed?

I'm surprised that it doesn't clear the stock crossmember.  I figured rx7s and miata had similar dimensions from firewall to crossmember.  

Maybe I can take some measurements of my car for comparison sake.

Im going to try something similar today. Take the trans off and see if i can get it slid where i want it. But i dont believe that it will go.

Dropping the crossmember is a possibility, however.

Drop the cross member. Do whatever it takes to get the engine attached to the cross member and clearing the rack while it is on the bench.

Start jacking the engine and cross member assembly up into the chassis.

Remove any Miata tin that gets in the way.

There is a fair bit of dead space around the tunnel so HVAC should not be an issue. I would much rather rebuild the tunnel entrance than the cross member.

Totally spitballin' here, but if the oil pan is a stressed member of the engine block, and this is a Challenge build, why not make the engine part of the crossmember? Noise / vibration will increase to be sure, could mitigate in other places.

In reply to NOHOME :

 Since you have cut apart a lot of Miatas and made things fit, how much Dead Space would you say is around? Especially concerned about the heater core module to firewall Gap

In reply to oldrotarydriver :

I don't like that udea, but don't quite know why. No offense, just doesn't sit right at first blush. 

Dusterbd13 said:

In reply to oldrotarydriver :

I don't like that udea, but don't quite know why. No offense, just doesn't sit right at first blush. 

Understood, it's a weird concept.  Powerplant as a frame stress member is common in motorcycles, but rarely in automobiles.

I threw it out there as an 'outside the box' concept.  I'm mulling over how to make it work, but I don't have a 3400 and Miata to stare at.
 

Let me just trot out to the shop with a picturometer taker....

Back. 

Get an arm in from the DS

Maybe a bit tighter from the PS.

Unobstructed view

When you come south because you are sick of the snow, i owe you lunch. You just made my life easy. Thank you so much for that.

When we replaced crossmembers in the round track frames we made the new crosspiece of several 1" sguare tube pieces welded togeather to make like a 1x5 x whatever it takes to span the distance It's Stronger than stock and can be shaped to give lots of clearence