Dart's new 9 Degree Chevy...
Dart's new 9 Degree Chevy small block heads are rapidly gaining popularity among dirt track racers and their engine builders because they are capable of producing both good power and a broad powerband. Here, raw heads are stacked up like cordwood in preparation for final machining at MBE Cylinder Heads and Manifolds.
"When you think you've reached the limits of power for an engine combination," Matt Bieneman says, "then it's time to back up and take a closer look at what you think are the limiting factors keeping you from making more power. As you take a closer look, you often will find that the real reason isn't what you originally thought."
Matt Bieneman is the owner of MBE Cylinder Heads and Manifolds. A longtime cylinder head specialist, Bieneman provides heads and intakes to many of the top teams in all forms of motorsports, and one area that recently has seen a great change is the Dirt Late Model class.
"The new 9 Degree cylinder head from Dart is really making some waves," Bieneman says. "We've had a program done for it and had people racing all last season, and it's obvious that when an engine builder gets a feel for how to make it work, he can really get some power out of it."
Port length is a critical...
Port length is a critical factor when it comes to cylinder head design because it plays a role in where the completed engine will produce peak power. The simplest way to measure port length is to run a length of wire or welding rod along both the roof and floor of the port. Measure both and take the average to get the port length.
Despite the fact that it's a brand-new casting, Dart's new 9 Degree head is an evolutionary step in dirt track engine performance, and not a revolutionary one. Over time, the trend has been to reduce the valve angle from Chevrolet's standard 23 degrees. Reducing the valve angle improves cylinder head flow by less "wall" area around of the combustion chamber around the intake valve, which in turn reduces shrouding.
Over time, smart engine builders and cylinder head specialists have cut the valve angle in steps. For years, an 18-degree Chevy small-block head from Dart was the hot ticket, and it still sees a lot of use. But then came the 15-degree heads with even better performance. And that was followed by 13-degree and 10-degree heads.
"But the problem was the 10-degree heads didn't make any more power than the 13-degree pieces," explains Bieneman. "And the flow was actually worse; I saw a drop-off in flow of around 3 to 5 percent. And that's a lot when you are talking about performance race heads."
Here, MBE owner Matt Bieneman...
Here, MBE owner Matt Bieneman compares the port length on a few cylinder heads. The SB2 head has a significantly shorter length than the 9 Degree Dart, which is a major factor when it comes to producing an engine with a useable powerband for dirt racing.
"At first I thought, 'OK, so that means 13 degrees is about as far as we can go when it comes to changing the valve angle in these heads,'" he adds. "But it turns out the valve angle wasn't really the limiting factor. With that casting there was only so much you could do with the port location, and as you made the valve angle shallower, it made the turn the air and fuel had to make to get from the port entrance to the valve too sharp. That's what was killing the flow."
But, Bieneman says, the original Dart casting, which had been a part of the evolution from 18 degrees all the way down to 13, had reached the limits of how much it could be modified. Thirteen degrees was simply as far as it could go.
The result, as you probably already have guessed, is a new casting designed to work with shallower valve angles. Bieneman says the new 9 Degree casting from Dart is an intelligent design. By raising the intake port entrance it reduces the turn the incoming air and fuel charge has to make to get past the valve.
"As a result, the new head helps make power. The 13-degree head, which had been the standard for a while now, flows a peak of 400 cfm. But with the new 9 Degree head with our CNC porting package, we are able to get it up to 430 cfm.
Cfm numbers may not mean much to everybody, but when you consider that at this level of race engine every cfm you gain equals approximately one horsepower, that's really eye opening. And it's even greater if you are looking at torque, which is very important in dirt track racing. There, every count of cfm you gain is about 1.2 lb-ft of torque."
Here, you can see one reason...
Here, you can see one reason why the ports are shorter on an SB2. The SB2 head on the left features offset intake and exhaust valves. The intake is closer to intake side of the head, making that port shorter than the 9 Degree on the right which has an in-line valve arrangement.
In the foreground is an intake...
In the foreground is an intake manifold designed and ported to work with the Dart 9 Degree head. In the background is a Cup manifold. Notice how much smaller the plenum area on the manifold in the foreground is. The plenum is the open area inside the manifold before the runners start. A large plenum is necessary for a high-rpm motor but can kill low-end torque. This is why the 9 Degree manifold has a much smaller plenum area with visibly longer runners.
This is the reason a new casting...
This is the reason a new casting was required to create the Dart 9 Degree head. On the left is a ported 9 Degree while on the right is the Dart 18 Degree casting (which can be ported to create a 13 Degree head). You can see that the intake port has been raised 0.330-inch versus the 13 Degree head. This reduces the radius of the turn the air/fuel charge must make between the port entrance and the valve seat which allows it to flow significantly better than the 13 Degree head.