Recently, we posted an article in which Richard Holdener dyno tested the five-liter powerhouses put out by the Blue Oval and the General in the 1980s. That made for an incredibly close series of tests, which proved that regardless of how you get your displacement, similar displacement engines will make similar power and respond similarly to modifications. However, it also led to a lot of upset Bowtie fans for some reason.
“Try the DZ 302 and see what happens!” many cried in the various comments sections, to which the Ford faithful replied, “Maybe against a BOSS 302!” Well, Holdener was reading those comments, just like we were. And, he decided to make that happen. So now, we have a pair of legends, being compared on the dyno. Both engines feature a four-inch bore and three-inch stroke, so there are no excuses available for either side regarding minor discrepancies in displacement. At the brochure, the two engines are identical power-wise, according to Holdener, but we prefer to let the dyno tell us who makes what.
Although we thought that the previous 302 Windsor vs 305 TPI was a pretty fair test of two era-and market-specific competitors, this has to be the ultimate level-playing-field. “The DZ 302 in the Z/28 was a natural marketplace competitor to the Boss 302 from Ford. Both these motors were factory-rated at the same 290 horsepower, but that number is suspect,” says Holdener.
“Back in the day, there was all kinds of speculation that they made more than 400 horsepower. And the reality is, they did, in modified trim — the way they were raced in the Trans-Am series. The real question is, what do these factory motors make, and how do they compare to each other?”
Where They Come From
Before jumping right into the dyno numbers, we need to look at the engines’ backgrounds to truly understand why they were built, and why they are so equal. “When the SCCA started the Trans-Am series. they mandated a 5.0-liter displacement limit. That means 305 maximum cubic inches. Because they didn’t allow destroking [of an existing engine design] initially, both companies had to build actual displacements of five liters,” explains Holdener.
Both companies decided to develop a 302 combination via the previously mentioned four-inch bore and three-inch stroke. However, Ford already had that architecture in its lineup, while Chevrolet did not. “They had to combine a four-inch bore from a 327 block with the three-inch stroke from the 283,” Holdener elaborates.
“Basically, the DZ 302 shared all the specs with the previous 365-horsepower 327, with the fuelie heads, the Duntov cam, an aluminum intake, and Holley carb. All they had to do was make the piston dome bigger on the 302 to keep the same 11:1 compression ratio.”
Since the Windsor block was already a thing, the Blue Oval team decided to fit a set of Cleveland style heads to the short-block and create the Boss 302. “[The Boss] cylinder head came about after Ford’s experience with the tunnel port cylinder head where they learned ‘lots of head flow equals lots of power,’” Holdener relates.
“While the tunnel port wasn’t a complete success, that’s how the Boss heads came to be. They topped a 302 Windsor with the Cleveland -style heads, making the first Clevor. So, a big head, lots of camshaft, an aluminum intake, and a Holley four-barrel. The Ford had slightly less compression, at 10.5:1, so out of the gate, Chevy had a little bit of an advantage.”
The Heart of the Matter
When it comes to the valve events, Ford and Chevrolet took two different approaches. One side opted for mild lift and more duration, and the other went for more lift and milder duration. The DZ 302 has a reputation for being a high-RPM screamer, and the camshaft is definitely part of that. The Duntov cam that GM used in the DZ 302 had over 250 degrees of duration at .050 [inch]!” Holdener explains.
“While it only had .480-inch of lift, that duration would really allow the RPM to climb. Combined with the big bore and short stroke, everyone thought ‘these things wanna rev.’ The reality is that the head flow and dual-plane intake really limited the RPM.”
Moving to the Blue Oval, we see that they went with a “milder” profile. “The Ford is spec’d out at 228 degrees at .050-inch lift, but lift was a different matter. The Chevrolet used a 1.50:1 rocker, while on the Cleveland head, they used a 1.73 ratio. That puts lift over .500 inch — .514 according to our calculations, minus the lash,” Holdener reveals.
“Both engines are solid flat tappet cams, and the Chevy especially responded to changes in the lash. Both had dual-plane high-rise intakes and big Holley carbs. So the induction system is kind of a toss-up. The biggest difference between the two engines is the cylinder heads.”
Comparing the cylinder heads on paper, using data collected by Holdener for a previous article, the Boss heads clearly come out ahead. “A stock Boss head will support way over 500 horsepower, where the Chevy will struggle at 400 horsepower. And really, it’s a difference in the philosophy behind these two motors. Chevy used higher compression and lots of camshaft, combined with mild cylinder heads. The Boss used a mild camshaft, mild compression, and lots of head flow,” Holdener says. So how’d they stack up?
Putting Them On The Dyno
For the testing, Holdener put together a non-numbers matching “replica” of the DZ 302. It features a forged dome piston to bring compression up to the original 11.0:1 compression ratio with the 64cc “186” casting fuelie heads. He used a correct Duntov camshaft and set the lash a little tighter than the original spec, at .020-inch. A factory high-rise dual-plane intake manifold was used, with one part not period-correct.
“The factory 780cfm carburetors are really, really expensive, and we didn’t want to spend a few thousand dollars on a carburetor,” explains Holdener. So, instead, they ran a simple 750cfm Holley carburetor, along with 1-3/4-inch dyno headers.
After it’s time on the pump, the DZ 302 posted 357 horsepower at 6,700 rpm and 333 lb-ft of torque at 4,300 rpm. The Ford, however, made a peak horsepower number of 374 horsepower at the same 6,700 rpm as the Chevrolet, and again, had the same peak-torque RPM as the Chevy, but made slightly less peak torque, at 324 lb-ft.
Looking at the graph, it looks like the DZ 302 actually outperforms the Boss 302 from 3,700 rpm until 5,800rpm. However, in the upper RPM range, the Boss 302 shows a definite, significant advantage over the DZ 302.
“The results of this test didn’t really surprise me, except for one thing,” reveals Holdener. “I figured the Boss would make peak power. A big head and lots of airflow means lots of power. I also figured that the DZ 302’s smaller heads, with better port velocity and more compression, would make more peak torque. However, way down low, where the big-port Boss 302 motor isn’t supposed to work, it made decent low-speed torque. That was a surprise.”