The small block chevy is the iconic hotrodders engine. Its been around since 1955 and is even still made new today for the aftermarket. Without too much trouble, 400hp out of one of these engines isn’t hard to achieve. But in this modern age where we see 350hp out of NA V6’s and now turbo 4 cylinders with that level of output, 400hp doesn’t really sound or feel all that special. Is it possible for the hotrodders mainstay to compete with modern engines?
My current state of affairs
Throughout the 80’s and 90’s, having 400hp on the street was considered a pretty big deal. In those days, even exotic cars didn’t tip much more than that. With a decent set of heads and a good cam, it was easy get to this power level while still maintaining easy driving characteristics – this is what earned the small blocks place in history.
But now with many cars coming from the factory with this level of power, is it possible to get, say, 500hp out of these old engines to stay ahead in performance?
The answer is more of a mixed bag than anything. There are a few issues that need to be addressed before building an engine capable of putting this kind of power out. One of the first things is addressing the block itself. GM never designed these blocks to withstand much more than 400hp, and because of this, you will find things like 2 bolt main caps and weak castings. In the hotrod forums, people seem to be all over the place in estimating how much power a 2 bolt main engine can hold, but the general consensus seems to be 400-500hp. It is also worth noting that many in these hotrodding forums only do drag racing, which is a very short race and does not stress the engine the same as road racing. While a high HP small block might last years drag racing, the life could be quite short on a road course.
Another way to make power is to turn more RPM’s, and again, there is a debate on how many RPM’s can be turned with a basic 2 bolt block reliably. But, how well the engine is balanced, and how much stroke the crank has, has an impact on the stresses encountered at the main caps and the lifespan of an engine. Furthermore, some of the factory cast GM blocks were just better than others, and if a serious engine is going to be built with a GM block, then it is a good idea to have it sonic tested to ensure its free from any defects.
The next issue is having a set of heads that can flow 500hp worth of air. This is the major problem with the classic small block. The common heads for these engines use a 23° valve angle which is extremely inefficient. While there are plenty of heads that can support 500hp and well beyond, they require opening the valves very far to get big flow numbers. While this might not be a problem for a drag race engine, high amounts of valve lift really puts a beating on the valve springs and valve guides. Opening the valve much more than .600″ really effects how long the valve springs last, and going as wide as .700″ means you will be replacing valve springs even if you don’t drive the car. This is due to the fatigue they take simply sitting with the valves open.
There are exotic head options that have narrower valve angles, and these heads flow substantially more than a 23° head at lower lifts. The problem with these heads is that they are expensive and require special pistons and valve train components that cost a fortune; not to mention a limited selection of intake manifolds that are specialized and are also expensive. In many cases these exotic heads, their valve train, and the intake can cost more than a running 23° engine.
Finally there is the cam choice. Because 23° heads are so inefficient, a large cam is required to extract ever ounce of power. This means that the cams are going to be optimized for higher RPM, have long duration, and poor low rpm performance. These cams make driving around town a challenge due to their choppy nature at cruising rpm’s.
So while its possible to build a 500HP classic small block and even one far beyond this, the engines start to have questionable durability at this point. This is why the LS based engines were such a leap forward. LS heads, being designed with modern technology, are much more efficient than the old 23° head, not to mention, the main cap design of the LS is vastly superior. Because the LS heads flow so much better at low lift, lower lift cams can be used and the engine can sustain higher RPM’s because the valvetrain takes less of a thrashing.
The main issue with the old small block is the 23 degree head. These heads just don’t have the flow characteristics that a modern engine does, and this is the big limiter in overall power, driveability and durability.