how do i keep my car from twisting when i put a new motor in it. should be around 400hp if i can do what i want (if the wife lets me lol)

 

weld in subframe connectors will help.

 

do some chassis reinforcing but realistically a stock thirdgen will handle that fine without help they are very strong uni-bodies as long as there is no trans brake or you dont dump the clutch and hook up.
My 84 T/A has about 420ish hp no frame reinforcement and its a t-top so you should be just fine

 

Unibody cars will flex. Many stock coupes, for exampe, will have stress cracks behind the drip rails (mine does). I'd strongly suggest SFC's.

JamesC

 

As said before in so many other threads, weld in SFC's should be one of your first mods if you're going to build a higher HP motor then stock.

 

Hmm, so that's what that is? Mine has 1" cracks on both sides.

 

mine has no cracks...

 

1991 RS--Does your car have T-Tops? If its a hardtop you should be fine without the SFC's...

 

Hard top stress crack, no sub frames, old combo was about 450hp. Its hard to tell from the picture but it has about a 3" by 3" buckle in the quarter next to the crack (both sides). Sub frames are on the to-do list.

 

chassis flex is the thirdgens worst fault IMO it's not
just the passenger area that flexes,the whole front
structure of these things also flexes and "matchboxes"
mightily-noticed this with mine the first time i drove it
with the hood off and was shocked to notice the radiator
support area jiggling independent of the cowl area
Shortly after noticing this(in 1994,100,000 mi)added
stout SFCS,extra x-member under fan area,cowl struts,
a few gussets, and added extra welds to certain areas.
this did add some weight alright,but cured the flex
problem 95% and it has stayed tight for over 17 yrs
since i did this (over 245,000 mi. now)
BTW when i bought my z28 in 1990 with 86,000 mi it was actually pretty tight yet,but after driving it through
a couple winters and thrashing on it the flex started to
creep in....
A rigid thirdgen drives so much nicer than a stock,flexi
one does

 

Get some subframe connectors, and a strut tower brace. Maybe a stronger torque arm as well. These will also improve handling, which is a nice addition.

 

hard top here
mine also has about 1" behind the drip edge passenger side.

 

Just curious,do the fourth gens flex too?
The second generations honestly don't seem any more rigid than the third gens,especially the t-top cars.

 

I've never driven a 4th gen,but GM did put some effort
into designing the 4th gen to be more rigid as the 3rd
gen flex problem was well known by the time GM started
to design the 4th gen.'92 model 3rd gens had "strucural
adhesive"applied to welded seams to help them be more
rigid than earlier thirds though i don't know if this made
much improvement over the long run or not.
I did notice some chassis flex with my '69 camaro,but it
was not as obtrusive as my '82 was before i fixed the
flex problem.Also my '69 was much less stiffly sprung
and has far smaller sway bars as well as lighter weight
than my '82
I have a porsche 928 and '68 roadrunner that also have
some chassis flex problems that need corrected
The mopar has front end shake and "matchboxing"
at least as bad as a stock thirdgen When i bought the
porsche i thought i had at least gotten a car that would
have a rigid chassis...Wrong!

 

I'm kind of surprised a Porsche would have noticeable body flex too,although I believe the 911 Targas were know for that.

I know the new Mustang convertibles flex like crazy too and have leak issues and second gen Trans ams especially those with t-tops flex as much as third gens.

 

my 83 z28 (t-tops) had 400+ hp with nitrous and the chassis was all kinds of twisted and it was way to expensive to fix and now I am a huge advocate for subframe connectors.