I was reading of a guy who modified his factory subframe by removing it and building a a custom jig. After seeing some pictures on here and from what I understand, It seems like I could leave the subframe in, (engine removed of course) cut out whats needed and box the factory subframe while it never leaves the car. Just a thought anyway to save some time/fabrication.. thoughts?

 

That might work. It might not. Depends on how much cutting and welding, I suppose. The problem is that the suspension pivot points are fixed by the subframe. So if you start cutting the subframe, the suspension mounting can move around, making for unpredictable handling later on. Part of the idea of making a fixture is to maintain the relative alignment of all those important points like steering and suspension pivots. Cutting while its bolted to the car only fixes the mounting points at the subframe/chassis interface. The upper suspension mounting probably doesn't move much, but the lowers certainly can.

Keep in mind that you are cutting a weldment, which relieves all those built in stresses, causing movement, then you are welding in more metal, which introduces a whole nother set of stresses. You really want to restrain all that as much as possible.

Mike

 

I understand that for sure. I just can't seem to imagine it moving that much when id be only cutting the inside out. I would definately do one side at a time, that would help on keeping things straight . Do you think it would/could move more than an alignment world fix?

 

My opinion is that it "could". It's a crapshoot. Keep in mind that the adjustments are only in one plane. The frame can "potato chip" in 3D. If you're determined to do it that way, get some conduit or pipe or angle or something and tack it to the subframe across the bottom in a couple of places to keep it tied together, then cut and weld away. The tacks can be cleaned up when you are all done. Report back.

Mike

 

We are in the process of doing 2 LS1 conversions modifying the subframes our self. I bought an extra front subframe for $50. Since I wanted to run the exhaust inside the tunnel and not through the fender wells we decided that the best way to keep the strength in the subframe was to first add to the outside of the frame. Using 2" x 2" angle we notched the angle to clear the suspension points and then welded the pieces in place, including welding to the bases of the suspension points. We then cut out the center and boxed it back in. The above suggestion to cross brace is a good idea. Even welding a brace across, we still had some movement, but we were still able to force it back open and bolt it in place.

The advantage of using a spare frame was that we could do all of the fabrication on the engine mounted to an engine stand. The only guess that we had to make was to estimate where the steering shaft would go. We fabricated the motor mounts and headers on the stand, taking careful measurements to gauge the distance from the firewall to the motor mounts to make sure the engine would fit. Test fit the engine and transmission and the fit was perfect the first time.

 

Any chance of some pics? Did you fabricate the headers from scratch? Oil pan? Pictures would really be great.

To the OP - notice that even with the precautions taken, there was a need for some "adjustment". So don't be surprised if you need to massage with the big hammer or other some other form of persuasion.

Just a thought, the temptation to just slot the holes is not necessarily the best approach. Better to move the frame around so the holes line up.

Mike

 

I don't have the photos hosted anywhere, I don't know how to post them here.

We are fabricating the headers by modifying the sheet metal Corvette manifolds. They tuck very tightly to the block and we were able to adjust them so that they miss the steering shaft. We had to eliminate the inner layer of metal so they may not last long, but for now they look good. I will go to block huggers which should work, if these don't hold up.

I didn't modify the pan, but I did notch the top of the cross member. I strengthened it by adding some additional angle under the crossmember. I have test fitted the hood and while it will be close I think that I will be able to get it to close.