Back to your 305 heads. You didn't say which heads they were. If you're trying to use 305 TBI-style heads, they have a very poor intake port, which would kill the power potential. If you're taking about LG4 or L69 carbureted 305 heads, they are the best 305s out there. Their casting number is 14014416 and they feature 1.84/1.50-inch intake/exhaust valves with 58cc combustion chambers. Why I'm getting down to the numbers is because this is the exact head we have to use on our 305 Stock Eliminator engine for our wagon. I had a set of virgin 416 heads that had never had a valve job except from the factory and had never been surfaced. After we had done our performance valve job, we cc'd the heads to estimate how much we needed to cut off to get down to the 55cc rule limit. The heads cc'd at 64 cc! Boy, was that a surprise. We've now cut more than 0.060 inch off the heads and we still are not down to the limit.
Let's say that you drop your combustion chambers down 12 cc-you should boost your compression around 1.2 points. If your mid-'70s small-block is an 8:1 engine (should be), you will have raised the compression to 9.2:1. If you apply the 4 percent rule to the 250 hp you're probably making now, you have a whopping 260 hp! After all that work, you would want more power than just 10 hp. Also, the exhaust ports on the 305 heads leave a lot to be desired.
We would look to a set of aluminum performance street heads. For nearly the same money to freshen up a set of iron heads properly, you can score a set of aluminum heads. The swap to aluminum heads from the iron 350 heads would give you an easy 40hp bump. If you play your component right, you will see even more. Don't go backward, use new technology and move forward.
When They Go Boom, They Sink
Q I purchased a used Airboat with a 350 small-block. I fixed it up and put in aluminum heads, a stainless steel exhaust system, and an aluminum manifold. After all these upgrades, the bottom half of the engine blew a hole in the block, resulting in catastrophic engine failure! I was thinking about replacing the block with an aluminum one. Is the weight difference worth the price and effort, and which 350 aluminum small-block would be best for the job? Airboats need low-end torque. Also, keep in mind that the mounts on the cage are set up for a '75 Chevy small-block 350. Thank you.Jim Delaney
Deerfield Beach, FL
A Wow, I bet that that engine failure was exciting. Nothing went through the hull of the boat? You're still around to write about it!
It never fails. You take a perfectly good, running engine and boost its speed and power output, and the engine expires. Think about it for a second. You buy a little old lady's Camaro or Nova with an engine that never saw the top side of 3,000 rpm with her driving it. It only has 80,000 miles on the clock. And because the engine never really sees operating temp, the cylinder wall wear is terrible. The top ring has worn a nice ridge into the cylinder. Now, you get your hands on this really clean musclecar. You give it a tune that it really deserves and take it out for a drive. Maybe it lived through this, but when you swap in a small camshaft, aluminum heads, and a set of headers, all of a sudden the engine loves to run to 6,000 rpm. When this happens, everything stretches to the point that the top ring is running into that ridge in the cylinder and either breaks the top ring or-worse yet-breaks the top ring land in the piston. Power drops and you're wondering why. It ran great for the first trophy dash. Once you've cracked a cast piston, it's just a matter of time before it scatters.