Pete is right but there is another issue. Its all about rigidity. There is so much pressure when a cylinder fires under load things move around. A crank shaft can flex up to an 1/8 of an inch. I know you think of the spinning mass in this precision rotation but the fact is the crank is trying to shot out the front,back or straight out the bottom. It is the blocks job to keep the crank put. In a 305 the wall thickness was not designed to hand powers much above 250 HP dependable. Remember the 305 was designed as a light weight fuel mileage emission engine.

Now when we talk old school 350's there where two basic types the 2 bolt mains and the 4 bolt mains. The 2 or 4 refers to the number of bolts that held each main bearing cap (aka crank controller). The thing is just adding 2 bolts would not do much. The 4 bolt main is a different casting. The cast walls that contain the bearing caps are much stronger. The extra bolts where added so the cap did not become a weak point. Don't get me wrong you will not break you 305 block at 300 hp its just not rigid all that moving around and vibration will show up in a failed rod or afailed main bearing. Even the 350 2 bolt can handle more HP then the 305.

The 302had a 4 bolt main and a steel crank. That is a nice rigid setup. GM advertisedthe302 at 275 HP. One reason they did this was they did not want the 302 to over shadow the new 350 SS that was listed at 290HP and the other was the insurance company's. I have seen a couple test where a 302 was build to factory specs and the where pushing 490 HP. The race team wouldtweak the heads and match up the ports and balance the engine.With themods they where doing no reason to think those old Trans-amcars where pushing 525+ HP.

 

I'm glad I decided to read this one. I've always wanted to know more about the 305, and knew almost nothing about the 302. I always thought it was a Ford motor. Our old Crown Vic has a FI 302, and it makes decent power, and moves the giant boat. I even raced my friends 79' Camaro in it and won. But I'm a better driver. And the Camaro had badly running 305, so nobody really cared, it was just for fun

 

i have played around with a few 305's. the 305 can make decent power for pretty cheap. the 305 responds to the same old tricks as all SBC's. the camshaft in the 305 was about as sorry as they come. the heads were not so great either, stopped flowing at 4600 rpm and had serious valve shroud issues. a healthy 305 short block with a decent camshaft , some L31 Vortec heads (shaved to bring compression back up) and the supporting mods (intake , carb , ignition) will make good power........ the 302 Chevy engine was not a destroked 327. the 302 was 4 bolt main , the 327 NEVER had 4 bolt mains , they were all 2 bolt. the 302 also had a steel crank , the only other 3.00" stroke crank out at the time was cast. you can turn a 350 or 327 block into a 302 cu in engine , but it won't be a DZ302 unless you use a 4 bolt main block and find a steel 3.00" stroke crank (and good luck with that)