Was looking around at crate engines and was wondering to myself what size engine could you put into a 1991 Rs before having to mod the frame to support the HP .

I would mod the frame with a stock V6.

The issue is body twist as much as horsepower. Personally, I consider subframe connectors as a first "must" mod, regardless of what you have planned after that.

Unless you're running in some racing class that doesn't allow that modification.

By stiffening the frame I assume you mean only subframe connectors. To five7kid's point the primary purpose for subframe connectors is to increase chassis stiffness with the end result of improved vehicle handling.

If your engine is very powerful it can cause handling problems during hard acceleration from a standing start but I don't think you're going that radical. You would have to develop a lot of power and have enough traction to plant the power to the ground before this is really an issue.

well i was looking at Gmperformance website and thats when i started thinking about it . i was thinkin about putting in the 350 H.O/330 Hp or zz4 350/355 Hp crate engine.

You can run without subframe connector but it will benefit handling. With those engines I would be more concerned about the strength of the rearend and health of the transmission. Both are going to need to be in tip-top shape to last a long while.

More power will certainly produce more body twist. But, the unibody/subframe construction of 3rd gens really needs to be beefed up in order to help these poor cars out regardless of power. Cracks above the door window openings of hardtops, doors that don't close tight, leaking T-tops are all symptoms of this. Get subframe connectors.

Next weakest chassis link is the rubber suspension bushings. Poly bushings all around make a huge difference.

Next is the open press-formed suspension members - lower control arms, track arms, torque arm. Boxed or aftermarket do wonders for handling and traction, and controlling improved power over factory.

BTW, what engine do you have now, V6 or TBI?