Intake Manifold Secrets - When Bigger Really Is Better

At Circle Track we have the luxury of spending lots of time with the very best chassis fabricators and engine builders in the racing business. And it doesn't take long to realize that the common bond between all of them is they are never satisfied. Win or lose, they are always trying to find ways to make their stuff just a little bit faster, more dependable, or simply easier to work with.

Of course, if you aren't one of the big guys, the R&D department is really just a fancy name for staying late to try out something new after hours. And that applies to most of the best people we know in this industry. There's always a project hidden away in a corner that someone is messing with whenever they can, simply trying to build a better mousetrap.

Such is the case with a new intake manifold design that engine builder Keith Dorton of Automotive Specialists has developed for big-inch small-block race engines. Designed for high-rpm performance, Dorton says this intake has surprised even him by producing an additional 60 horsepower more than his previous best on a 358ci SB2 engine.

Normally, Dorton would keep an advantage such as this a secret for absolutely as long as possible. In fact, as this went to press there were only four of these intakes in existence, and he's gone to great lengths to make them look as ordinary as possible. But after much begging he's agreed to allow us to photograph two of the intakes in different stages of completion and share what works and how. It will probably cost us a couple lunches down the road, but it's worth it.

Land Speed Testbed

"We originally developed this intake--we don't even have a name for it yet--for our Land Speed Racing engine programs," explains Automotive Specialists' owner Keith Dorton. "In Land Speed Racing you're always looking to really maximize top-end horsepower. This intake did that to the tune of 60 horsepower, but it didn't hurt the bottom end too badly, so we think it can really be an advantage on an unlimited Dirt Late Model engine on the bigger tracks."

Although his bread-and-butter is stock car racing engines, Dorton recently began developing engines for several Land Speed Racing efforts, mostly using his NASCAR Nationwide and Truck Series engines as a foundation. Dorton has already had a lot of success at the Bonneville Salt Flats with his engines, but because the rulebook is wide open in many of these classes he had been looking at ways to improve the performance even further.

"We started by looking at how we could get more air into these motors," he explains. "Originally, we thought maybe the carburetor was the restriction. We were running a cast intake made for the Chevy SB2 engine and an 850 Holley carburetor. We tried putting a Dominator-style carb and adaptor on the engine, but the adaptor was too much of a restriction to high-rpm airflow and there wasn't much of a power gain.

"So our next step was to rework the intake itself. We cut the top off of the manifold and made a plate for it to fit the 4500 Dominator pattern. But while we were at it we also started experimenting with the plenum size and runner locations. We were using a 2866 Edelbrock intake, which already has a pretty large plenum, but we found that increasing the size even more helped make power up top."

After several runs, Dorton settled on a design that not only expanded the plenum's volume but also moved the runner dividers outward to shorten the overall runner length. This was done to also help straighten the runners out.

"We wanted to take the curve out of the runners," Dorton says. "We did that so that we could get all eight as equal as possible. With a more conventional intake, the outside four runners are longer than the inside four. What we wanted was a straight shot to the backside of each of the intake valves. We also ground the runners so that there is almost no taper."

On the dyno the baseline SB2 produced approximately 820 hp at more than 8,000 rpm (Dorton doesn't want to get too specific here), but after dialing in the new intake with a few different versions he was able to increase peak power with the same Dominator carburetor by 60 to push that number all the way up to 880. Peak torque also moved up the rpm range closer to peak power.

"The interesting thing is we started out testing on one of our biggest Land Speed motors," he continues, "and after we were able to get another 60 hp out of it with the new intake, we decided to try it on a few other motors.

"We tried it on a 303ci motor, a 358, and a 370, and the results were all about the same. The three motors didn't have the same peak power numbers but they all improved by about the same percentage.

"I think some of that power comes from the fact that the intake design helps equalize each cylinder so the engine can be tuned better. We put lambda sensors on it, and we were able to see that we had very even air/fuel mixtures in all eight runners. And on top of that, the EGTs (exhaust gas temperatures) were all within 40 degrees.

"By straightening out the runners and making them all as equal as possible, it really took away the variations in the air/fuel charge from cylinder to cylinder."

Dorton says that tuning on the dyno revealed that the same head with the new intake required less timing, which also points to a better, more consistent fuel atomization in the combustion chambers. The engine also appeared to be less sensitive to air/fuel ratios. This is good because the ability to run richer without a horsepower penalty helps the racer avoid detonation that can come from going too lean.

And the results speak for themselves. After the great holiday for all Land Speed Racers known as Bonneville Speed Week (held every August) Dorton's engines came away with five new Land Speed record times. It's hard to argue with that kind of success.

Going in Circles

"This intake design will hurt the low-end torque a little bit, but not as much as I had thought," Dorton says. "But with the big Dirt motors you make so much torque that this shouldn't be as much of a problem, and then you can also work with the gearing to help it."

Of course, even if you can get just half of the 60 hp gain Dorton saw with his LSR engine packages, many Dirt teams would be more than happy to give up a little throttle response.

Unfortunately, it will be a little bit of time before we'll get to see dyno and track results. So far, Dorton has only produced four of his new intakes, and all four are on customers' engines. He is in the process of creating a new one, but he says all the cutting, welding, and grinding required eats up approximately 80 man hours. "The first one took me about 100," he says. "So we've cut down the time significantly, but it is still a pretty serious project. We're working with Edelbrock to try to create a casting but that's still in the early stages.

"For now, it's still a lot of hand work and patience, but if we see them benefit our customers, we'll make 'em."