Chevy Small Block Testing - Legendary Small-Block Shootout
We Pit The DZ302 Vs L76 327 Vs LT-1 350 In A No-Holds-Barred, Dyno Dogfight.
Where else but in the pages of Super Chevy are you going to see a test of three legendary small-block Chevys? Under the Comp Cams valve covers sit a DZ302, an L76 327 and an LT-1 350.
If you have even an ounce of Bow Tie blood, you will immediately recognize the three names presented above as nothing less than legendary small-blocks. During the muscle car era of the '60s and early '70s, this trio carried the torch into battle for their respective displacements. Sure, the fuel-injected version of the 327 was rated 10 hp higher than the carbureted L76, but both shared the same internal components.
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The DZ302 came about because of the need to produce a 5.0L displacement powerplant for the popular SCCA Trans-Am sedan racing series. The LT-1 350 represented the last hurrah for the high-squeeze, solid-lifter small-blocks, as the '71 version made due with just 9.0:1 compression compared to 11.0:1 for the 1970 model.
We all know now that the power ratings in the old days were somewhat suspect. The manufacturers played games with the official power ratings in order to appease the buying public, insurance companies and racing officials alike. Offering their engines with elevated ratings was not uncommon, as horsepower sold cars and with the lack of available dynos to verify the claims, it was difficult to tell the difference between a 350hp engine and one rated at 370hp.
Before we could officially test these legends, we had to source three blocks. Our trio came from L&R Automotive in Sante Fe Springs, California.
Speaking of ratings, it is also important to note that these differed wildly from those used today by the OEMs. Where the new LS3 small-blocks are tested to the latest SAE net (flywheel) standards and are rated with full accessories, exhaust and the factory induction system (basically the way it comes in the car), pre-1972 engines were rated in optimized condition sans accessories (open dyno headers, no accessories, no air inlet restrictions, etc.). What this means is that a 370hp LT-1 from 1970 is actually much closer in actual power output to the 300hp LT1 circa 1995.
Regardless of the differences in the power ratings, what we really wanted to know was just how each of these legendary small-blocks compared to each other. Was the output of the high-winding 302 really closer to the often-touted 400hp than the rated 290 hp? Was the 365hp 327 more than a match for the later 370hp LT-1 350? After all, both the 302 and 327 shared slightly wilder cam timing than the LT-1, but would the milder cam timing offset the additional displacement? Would the shorter stroke small-blocks make more peak power than the long-stroke 350? These were all questions that deserved answers and these answers had to come from more than the recollection of some racer who ran one back in the day.
This is one of those times when a simple session of bench racing snowballs into a monster of a dyno project. Obviously, the only definitive answer had to come from the dyno. Real back-to-back testing was the only surefire way to provide not just peak power numbers for each, but overall power curves. After all, man does not live by peak power alone. It is the average power production that provides the key to a successful street powerplant, and since all three of these production small-blocks were indeed street motors, we needed to test them.
Testing three legendary engines meant building three legendary small-blocks. Luckily for us it was unnecessary to reproduce the motors down to the last casting number. Basically all we needed to do was pay close attention to the engine specs of each and duplicate these where necessary. There were a few mandatory items in each build up, namely things like 11.0:1 compression, the proper cam specs (duplicates of the original) and original big-valve "fuelie" cylinder heads. While each of the motors was adorned with different casting numbers, the reality is that the big-valve performance heads were much more alike than different.
Though two-bolt blocks would be more than sufficient for these production motors, we were fortunate enough to locate three four-bolt blocks for our testing.
L&R gave the cranks a once over including polishing. Without its support this major project would be just another good idea.
Flow numbers and port volumes indicate that a set of 492 heads is fundamentally identical to a set of (big-valve) 461, 186, 041 or 040 castings. Having run a number of these heads on the flow bench and through various small-block build ups, they produce near-identical power numbers. In fact, you're likely to see as much or more power differences between two sets of the same casting numbers (given core shift and production tolerances) than two different castings.
Rather than opt for the restoration, our original game plan was to build three 11.0:1 short-blocks using the various stroke combinations and top them with the same 492 head castings. You might be wondering how the 302, 327 and 350 all share the same 11.0:1 compression ratio, given the use of the same 64cc combustion chambers and decidedly different stroke lengths. All of the small-blocks originally featured 5.7-inch connecting rods, but the difference in stroke was made up by a change in compression height of the pistons. The 3-inch stroke 302 originally featured a 1.80 compression height, while the 327 and 350 featured compression heights of 1.675 and 1.56, respectively. Since the static compression ratio is a function of the displacement (bore and stroke), the smaller-displacement 302 featured a taller piston dome than the 327 and 350 to achieve the same 11.0:1 compression.
Installation the small-journal cranks in the large journal blocks was made possible with the use of Clevite spacer bearings (PN MS1110H). Clevite also supplied the standard bearings for the large-journal 350 crank from Pro Comp.
Our short-blocks were likewise equipped, but because of the pricing and availability of the original forged pistons, we opted to equip all three short blocks with 6.0-inch connecting rods. This allowed us to reach our goal of 11.0:1 compression on each combination with the use of (nearly off-the-shelf) Probe Racing forged pistons. By altering compression heights, Probe was able to produce three different dome pistons for each of the combinations. The 302 required 12.5cc domes, the 327 just 7.5cc and the 350 produced the requisite 11.0:1 compression with just 2.5cc domes.
The three short-blocks were assembled using crankshafts from different sources. The 302 featured a steel 283 crank courtesy of L&R Automotive, ditto for the 3.25-stroke, cast crank used in the 327. The 327 crank was a large journal so it dropped right into the large-journal 350 block, but the early 283 crank was of the small-journal variety. The cure was to utilize bearing spacers from Clevite, which allowed the installation of the small-journal 283 crank into the large-journal block.
The blocks sourced by L&R Automotive included a trio of 4-bolt blocks. After machining (thanks again to L&R), the blocks were stuffed with balanced assemblies including the proper cranks, 6.0-inch, H-beam rods from Pro Comp (small journal rods from Crower for the 283) and Probe Racing forged pistons. Also included in each were high-volume oil pumps, HD pump shafts and main studs, all from Pro Comp. The L76 327 and DZ 302 were equipped with Elgin factory reproduction, solid flat-tappet (single-pattern) cams that speced out at 0.484 lift, 254 degrees of duration (at .050) and a 114 degree lobe separation angle (the Duntov 30-30). The LT-1 received a dual-pattern cam offering a 0.459/0.485 lift split, a 242/254 duration split and a slightly wider 116 degree lobe separation angle. The cams were run with Comp Cams double-roller timing chains, 6.75-inch (neutral) dampers and factory front covers and oil pans.
The pistons and rods for our legendary trio came from a variety of sources. All three sets of pistons came from Probe Racing, while Pro Comp supplied the forged rods for the 327 and 350.
The heads all received new 2.02/1.60 stainless steel valves, valve springs (with 125-130 pounds of seat pressure) and factory retainers. Each head was given a factory-style valve job but no porting of any kind was employed to the otherwise stock castings.
As indicated previously, our original intention was to top each of these legendary short blocks with the same set of 64cc chamber iron heads. As luck would have it, we found period-correct head castings for all three of our test motors. The 492 castings would be run on the LT-1, while the 327 received a set of big-valve, 461 heads and the DZ 302 was toped off with a set of 186 castings. Our pair of 492 heads actually came from two sources, one from Westech Performance and the other from L&R Machine. Both 492 heads were in excellent condition (the one from Westech was untouched) and prior to assembly on the awaiting LT-1 short block was treated to a fresh valve job using new stainless steel 2.02/1.60 valves from Pro Comp, a set of Comp Cams valve springs that offered 130 pounds of seat pressure at 1.825 installed height and a light surface to ensure straightness.
We measured the chamber volume of each head and they checked in at 62cc in both cases. The 461 heads for the 327 and the 186 heads for the 302 were given the same treatment, basically bringing them up to factory fresh condition (but no porting of any kind or any special valve job).
Given the wild cam timing and more-than-adequate static compression ratio of each of our small blocks, it was actually the cylinder heads and intake manifold that held back each of these factory motors from offering even more power. It might be interesting to see what one of these short blocks motor might do with a set of AFR heads and a single-plane intake.
The flow rates for all of the heads are listed in the head flow data chart, but know that most big-valve performance (production) heads offer very similar flow numbers. There seems to be as much difference between two sets of heads of the same casting number as two set of different casting numbers.
Speaking of intakes, our mills all received period-correct intake manifolds. The early 327 featured an original L76 dual-plane high rise (461 casting) courtesy of a seller on Ebay, while the same source was used for the LT-1 intake. Since the intakes used on the 1969 DZ 302 (610 casting) and 1970 LT-1 (110 casting) were nearly identical in terms of design and performance, we elected to run both of these with our over-the-counter factory replacement (024 casting).
All three were originally equipped with Holley 780 (3310) 4-barrel carbs, but since that carb was no longer offered by Holley and a restored unit was well out of our price range, we decided to top all three with a modern Holley 750 Street HP. Pony Carbs offered up an original 780 Holley for testing but we just couldn't bring ourselves to dismantle the carb for jetting and secondary spring changes necessary to work with each configuration.
In terms of the ignition system, these engines were originally equipped with points distributors, but we elected to run an electronic ignition on the dyno. We wanted to ensure adequate spark energy to maximize the power potential of each legendary combination. We also ran the engines with optimal timing rather than factory-spec.
It should also be mentioned that the Elgin reproduction flat-tappet cams were combined with the factory long-slot, stamped-steel rockers (from Comp Cams) and a set of hardened pushrods. If not factory equipped, the heads received an upgrade in the form of screw-in rocker studs (from Comp Cams), but the factory valvetrain components were run to ensure accurate dyno data.
Basically, we were running optimized combinations of each of these motors in the same way enthusiasts might have tuned their motors back in the day. Rather than choke each combination with the restrictive factory exhaust manifolds, we opted to run the same 1.75-inch dyno headers on all three combinations.
No question, the Street HP carburetor added some extra horsepower, too. A Meziere electric water pump replaced all of the factory accessories.
All heads received rocker studs and guide plates from Comp Cams along with hardened pushrods (7.8-inches in length).
After flow testing, the heads were installed on the awaiting short block using the Pro Comp head gaskets and ARP head bolts.
Given we had not one but three impressive small blocks to test (actually four counting the low-compression LT-1), we fully expected there to be a few issues and true to form, this dyno session was not without its incidents. The first issue occurred when we brought the finished 461 heads to the airflow bench. What we thought was a set of untouched, factory castings with a fresh valve job was actually a ported set. With no replacement set of 461s at our disposal, we opted to run the 327 with the same set of 186s used on the 302. Given the similarities in airflow between a set of 461s and 186s, this was an accurate representation of the true power of the 327.
Next up was the intake manifold, The author purchased the required intake via E-bay but the intake was actually for a 1963 L76 and not the later (and higher HP) 1965 L76. The difference was that the 1963 version was equipped with a Carter carburetor and not the more desirable (and available for our dyno session) Holley carb.
With time running out, we opted to run all three small blocks with the 1970 LT-1 Z/28 intake. Correct for the LT-1 and as-near spot on (in terms of performance) for the DZ302, the intake was actually slightly better than the original manifold for the L76. We'll run a back-to-back test on the two intakes to clear up this issue and provide the results for the interested parties, but know that the L76 might be up slightly from where it would be with the stock manifold.
Naturally the stock exhaust manifolds would kill some of the power compared to the long-tube headers, but the factory tested them with open headers in the late-'60s and early-'70s (SAE gross) so we were comfortable with this (especially since all of the test engines were run in the same configuration.
When we test the stock L76 intake, maybe we'll toss on a set of stock manifolds just to see what the headers were worth. Let's hit the dyno and find out which combo is king.
If you are looking for an original carb for your legendary small-block, look no further than Pony carburetors. It supplied this 780 Holley (3310) for the DZ302, but rather than ruin this numbers-matching carb by disassembling it for the necessary jetting and secondary spring (required due to the use of headers and lack of accessories), we opted to run a modern Holley 750 HP carb.
*461 heads eliminated for test due to obvious porting; 327 was run with 186 heads.