Third Generation F-Body Message Boards - Rebuild my 305 or swap 4 ls1?

It depends on what you want to do. For '85 the LB9 was new. It had the highest output 305 that was ever paired with an automatic for our cars. Keep in mind though that if you have the original ’85 700r4 in the car it’s not as strong as the ’87 and up 700r4’s. They got some upgraded components over the pre ’87 700r4’s.

That being said it’s your car so you have the freedom to do with it what you want.

The first question that I would ask is what your end goal (s) are for the car.

Do you want something a little faster than stock?

Are you going to be road racing it or drag racing it?

Do you still have to pass emissions testing?

How much money do you have to spend?

How much down time do you have with the car? If this is your daily driver instead of a 2nd or 3rd car, you will have to include that in your plans for the car.

Those are some of the factors that you will have to decide. Since you are planning on working over the motor, don’t forget about the transmission or the rear end either. Our cars are old enough now that attention needs to be given to the entire drive train not just one component.

The LS-swap has been done to many thirdgens. You can look around on this website and other sites for help with that. Just understand that you will have to change certain components (such as the engine mounts, fuel system, computer, wiring, etc.) Even if you stay with the Gen 1 small block you may have to get a custom chip for your specific combination of components depending on the extent of modifications that you do.

If you stay with the Gen 1 small block (like you already have) then you won’t have to change the mounts. You will then have to decide if you want to rebuild the 305 or step up to a 350. You can make good power with the 305. The TPI system was designed for the 305 to create more torque. At the time GM was figuring out if it was going to stop making the 350 engines. Part of the reason they developed the TPI system was to give the 305 more torque down low in the power band. This was an effort to give the 305 some of the attributes of the earlier carbureted 350’s. With the TPI set up the torque would be comparable to carbed 350’s (of the same era) even though the 305 was 45 cubic inches smaller.

GM did decide to continue producing the 350’s but didn’t redesign the TPI system for the larger air requirements of the 350. This is why many members on here that keep the long tube TPI set up port or install a new base, install and port the intake runners and plenum. This is all in an effort to get more air flowing in to the engine. With a stock TPI system you will make good power to 4500-5000 rpm. After that the power drops off sharply. TPI will never be a high winding, high horsepower system. Its long tube runner design leans more on the side of slow engine speed torque production than ultimate top end performance.

You can make good power with the 305. I am sure though that many people will tell you to get a 350. With more cubic inches there is more power to be had with the 350 than the 305. The cost of the 350 may be a little lower as well.

It’s your car so you are free to do what you want. The most important thing is to set your goals out first and do modifications that will help you meet them. For example there would be no need for you to install a Ford 9-inch rear end in your car if all you do is mildly modify the engine that you already have.