Individual runner induction systems are not new to horsepower junkies. You probably vacillate between memories of vintage gassers with 8-stack horns protruding from hoods at theatrically wonky angles and lengths, and images of boutique exotic race engines with equally exotic price tags.
The name Hilborn has been tied to such voyeuristic automotive fantasies since 1948, when Stuart Hilborn pioneered performance automotive mechanical fuel injection, chasing the horizon at El Mirage. Fast forward to today and individual throttle body technology is playing a new role in engine performance. No longer does ITB induction systems mean mechanical fuel injection purely for the track, EFI management allows hot-rodders to “have it all” when it comes to performance and drivability.
Shafiroff Racing 615 Big-Block Chevy Twisted HR Low Deck Specs
Dart Big “M” Low Deck Block w/Splayed 4 Bolt Main Caps & .904-inch Lifter Bores
Moroso Race Prepped Oil Pump w/Tig Welded HD Pickup
GM Performance Chrome Front Cover w/Billet Timing Pointer
Hi-Performance Harmonic Balancer
Complete Assembly Internally Balanced
We caught wind of some exciting research and development untaken by famed engine builder Scott Shafiroff and Hilborn EFI specialist Andy Starr. Shafiroff Racing is well-known for high performance Ford and Chevrolet engine programs to suit the needs of the drag racer, and pump-gas street machine alike. Among his pump-gas, big-block Chevy offerings is the 615 Twisted Low Deck – this package generally comes equipped with a 1,250 CFM Dominator carburetor, but would become the recipient of a set of Hilborn stacks for testing. In carbureted trim this engine is advertised at 925 horsepower and 815 lb-ft, however the particular engine used for this testing included an upgraded ring and cam package, and a bumped compression ratio from 10.8 to 11.0:1, and produced 969 hp and 836 lb-ft.
“Here’s what wound up happening, we had done some previous testing with a 509 W engine and Scott [Shafiroff] was so impressed with the stuff he said ‘I think we can do something with these, I’d like to get a consignment injector from you.’ We don’t typically do that but it was a great opportunity,” began Starr.
The existing off-the-shelf offerings for big-block Chevy applications from Hilborn include 2-7/16 inch and 3-inch bores, but custom sizing is a common thing. For this application Hilborn provided one of their EFI-R kits featuring electronics from Holley, and fuel system from Weldon Pump. Starr sized a middle-of-the-road combo and took to the dyno.
“We put together one that had a decent butterfly size because we know he’s typically building horsepower, yet wouldn’t be too aggressive as far as throttle tip-in. I picked a 2-5/8 inch butterfly system and shipped it to him,” he explained.
“We do customize all of our butterflys, and quite frankly with that particular combination I would have went bigger — because of all the camshaft in it and all those things that would allow us to still have good throttle tip-in. In actuality that butterfly size would be considered on the small side for that particular type of engine.”
When an engine builder or starry-eyed customer gets to the point of speccing out a custom induction system with Hilborn, a lot of considerations are at play. Sizing butterflys, ramp tube lengths, injector sizes and more are decisions based off not only the goals of the builder, but also some hard science and theory behind the designs.
“Looking at the displacement and sizing it with what the camshaft is doing is our first primary goal with street-driven engines,” Starr assured us.
“Most of the streetcar engines Shafiroff has been doing have to be drivable, we can’t have the light switch at throttle tip-in. The bigger cubic-inches are more difficult because they create so much more pressure or vacuum on the backside of the butterfly. As we start to tip it in, that air rush is greater than the same engine with 150 less cubic-inches. It’s just physics.”
“This whole entire kit was based on a combination somewhere around 850 horsepower, injector size was sized as such. We put it on and I said, ‘what do you mean this thing only has 65 pound/hour injectors?’ We were able to use fuel pressure in order to stay right around 82 percent duty cycle. We do generally size the injector for the application but this was a big unknown because of the consignment deal.”
One of the iconic and sometimes perplexing things we associate with ITB systems are ramp tubes of different lengths. If you’re into reading about engine theory, it’s not a mystery to you that engine performance can be altered drastically by tapping into harmonics just like a musical instrument. Changing the lengths of ramp tubes can shift the powerband, but sometimes the results can be counter-intuitive.
“We did play with the runner lengths, we have dyno testing on that engine with different ramp tubes. We started out with a 12-inch ramp tube, our standard base tune-up and got it sorted out with some fuel correction. We then went and played with what is called injector offset, Holley has this neat gadget where using camshaft opening and closing events and injector spacing, it will develop its own injector offset table. We imported that into the tune and we picked up power all across the board, at that point we were at 983 horsepower.”
“We expected to start eclipsing those numbers by putting a shorter ramp tube on it. We went from a 12-inch to basically 6-inch ramp tube and it lost some ponies on top, but the whole mid-range picked up 5-7 horsepower,” Starr unexpectedly explained.
While a shorter tuning fork rings at a higher frequency, lots of conditions in the dynamic environment of an engine can lead to unexpected results. We were as surprised as Starr and Shafiroff. Starr hypothesized possible differences that lead to this scenario;
“What’s interesting is we’re dealing with combinations today that are so much better than the combinations of yesteryear. These heads work so good, the ring seal is so good and now the harmonic where that frequency in the ramp tube is different than we would see in the old days.”
In the end the final numbers were 999 horsepower and 879 lb-ft, a healthy 30 hp and 43 lb-ft gain over baseline. Not only are peak power and toque numbers increased, but overall area under the curve was amplified as well.
“Torque at 4,500 rpm was an impressive 850 lb-ft carrying that number to 6,000 rpm. The average numbers showed a considerable increase as well. Average horsepower for the carb was 878 hp vs 918 hp for the Hilborn with the torque average of 815 lb-ft for the carb compared to the 854 lb-ft for the Hilborn,” Starr concluded.