305 Engine Myths

305 Chevy Small Block

I have owned a few 305 engines and I have a little knowledge when it comes to them. Debuting in 1976 the 305 was to be the small block that carried Chevy into the eighties. With more and more constraints placed on the automotive industry to meet government mandated EPA standards an engine had to be produced that would be the replacement for the 350 CID. With a smaller bore and less aggressive cam profile these engines were basically designed to just get the job done. The 305 has a 3.48" stroke and a bore of 3.75",not 3.87" that the 283 has as some will tell you. The smaller bore restricts the size of the intake valve that can be installed on that engine. The 305 also has a dished piston.
  I was looking at a third Gen Firebird to build and I came across one sitting in a yard just off the hi-way one day. I went into the yard and found the owner and started talking to him about the car and had a good look at it. The interior was completely shot and it would need a little body work. After talking with the owner and finding out that the car had a "high performance 305" I was a little taken back. Now,I'm going to be honest with you. No matter what you do with a naturally aspirated 305,you will never see more than 325 horsepower out of a 305 that is naturally aspirated and that is with a lot of work done to it. The cylinder walls do not have enough metal in them to make boring them worth the time. The block just wasn't designed with that in mind. Remember that the engine itself was designed to meet emissions standards. Anyway,the price that I offered for the car was about 2000 less than what the owner wanted for it. I offered to take the engine out and return it to the gentleman as further talk revealed that it had been driven about 150 miles after the motor was rebuilt and then parked and had not been fired it 10 years. The owner insisted that everything was fine and that the engine was "top notch". It became painfully clear that this was going to be something I would have to pass up and I did.


   Now,one of the things you can do with a 305 is install a bigger cam than you could into a stock 350. The reason for this is the dished pistons. You will however need to install valve springs that can handle the increase in valve lift,yes I'm talking from experience here. Taking into consideration that engine have the highest vacuum at idle says that the velocity of the air charge going into the chamber is very good at the low end. You get better velocity flowing 200 cfm of air through 180 cc runners then if you had 210 cfm of air going through 200 cc intake runners. With that said,you can create some good low end power with a 305 and a decent camshaft. A good set of headers and exhaust system coupled with a good after market intake manifold and carb set up also helps. Basically all the things you would already do with 350 Chevy aside from installing 2.02"  heads. The largest valve you can install on a 305 is 1.84" without hitting the cylinder walls. If you want the most you can get,hogging out the exhaust runners will also help. As I said before,the most power you can reasonably expect is around 310 - 325 horsepower.

Forced Induction

  If you plan on any type of forced induction,then a Turbo would be the way I would go. The reason for this is simple,given the limited valve size it makes sense. You would be creating more low end velocity through the intake runners. A supercharger will of course increase horsepower but,it would also increase the thrust load on the bottom end bearings more than a Turbo charger. It just wouldn't be as efficient as a turbo. Remember if you do plan on rebuilding a 305 it might be an idea to balance the internals and think about a complete roller valve train. It's just freeing up a little more horsepower. There are also a lot of guys out there that can do some amazing stuff with a set of heads so it would pay to do your research.



  The research and development that went into the 305 lead to some serious improvements to the technology that went into the small block chevy. Making a lot of the horsepower improvements implemented into the 350 in the 80's and the return of big horsepower. In 1991 the TPI 305 was for all intents and purposes just a fast as the TPI 350.