We're done assembling our budget-conscious 383 stroker mill, and it's time to put it to the ultimate test-the dyno-so we can learn what to expect once it's bolted in a car (yes, we'll do that too). If you've been following along, you'll know that we perked up an ailing 350 with a Powerhouse Engine Components 383 stroker kit in the Sept. '07 issue. Then we showed how to refurbish stock cast-iron cylinder heads in October's Part Two for those of you unwilling or unable to cash in on the bonanza of bolt-on aftermarket aluminum heads available today. Next, in Part Three (Nov.), we torqued it all together and, while we were at it, took a closer look at the new Super Street 215cc aluminum heads from Patriot Performance.
The 383 stroker kit already proved that it can sometimes be more expensive to revive tired factory iron parts than to simply replace them with new components from the aftermarket. Sure enough, the fully assembled Patriot aluminum heads looked to be capable of the same stunt. But the only way to validate these upgrades is to compare them side by side on the dyno. In this month's installment, we'll not only stick the motor on the dyno, but we'll also mix and match heads and intake manifolds to see what works best.
Here's a rundown of the plan: Using our Isky-cammed (0.544/0.547 lift, 284/292 duration) 383 short-block as the basis, we'll start out with the refurbished big-chamber, 76cc iron heads (9.0:1 compression) and a dual-plane intake manifold. Let's call this Test 1. Next, we'll explore the effect of a single-plane intake manifold on the combination in Test 2. Then we'll yank the iron heads and replace them with the small-chamber, 64cc Patriot heads (9.8:1 compression), fed by the same dual-plane intake for Test 3. Finally, we'll top the aluminum heads with the single-plane intake and see what we get in Test 4. OK, the crew at JMS Racing Engines has the 383 perched atop the shop's DTS 4000-G 1,500hp dyno, so let's get started.
Before doing anything, be sure to properly fit any replacement dipstick kit so it is accurate for your particular engine's block, pan, and filter combination. This is no place for assumptions, as oil starvation can quickly destroy any engine. We started by filling the oil pan to its rated capacity-6 quarts in the case of our Moroso Chevy II pan-then added another half quart to account for the oil that will be stored inside the filter. (Tall filters can hold as much as 1 quart, so add accordingly.) Once the pan was filled to the correct capacity, we gave the oil a half hour to settle, then installed the dipstick tube into the block, adding sealant at the base to prevent leaks. Next, we inserted the dipstick in the tube to see if the actual oil level was aligned with the "full" indication line marked on the stick. Our dipstick tube was a little too long, causing a false underfilled reading, so Jeff Johnson used a special compact tubing cutter to remove 1 inch. Then our dipstick reading was spot on.
Priming the oil system prior to initial start-up eliminates the threat of oil starvation. Johnson used a priming tool and drill motor to spin the oil pump clockwise until the oil pressure gauge showed 65 psi. During break-in, be sure to use mineral-based oil rated for off-road use, meaning it has zinc, an EPA-regulated ingredient absent from most highway-rated passenger-car oil formulations. Despite costing 40 percent more than most other premium motor oils, JMS uses Valvoline VR1 racing oil, and while thin viscosities yield more impressive dyno numbers, we went with 20W50 for maximum protection on this fresh build. A pint of GM Engine Oil Supplement is also good antifriction/antiwear insurance at every oil change, especially with solid flat-tappet cams like ours. Avoid synthetic oil until after break-in; it's too slippery and the rings may not seat properly.
The 15-minute cam break-in phase went without incident, but during the first test attempts in Test 1, the 383 had a nasty-sounding misfire that came on and wouldn't quit between 4,200 and 5,500 rpm. In vain, Jeff (foreground) and Don Johnson checked everything from ignition timing to the dyno's fuel pressure setting for the cause before focusing attention on the carburetor, an 800-cfm Edelbrock 1412 EPS 800. We thought maybe there was some isolated flaw with our specific unit, so another box-stock EPS 800 was inspected and installed. Even with the new carb, the 383 stumbled as bad as before! Next a box-stock 750-cfm Edelbrock Performer 1407 was installed, and viola, the misfire cleared up, and the best of several pulls delivered 379 hp at 5,400 rpm and 405 lb-ft at 3,700 rpm. A good result, but the earlier stumbling episode haunted us. Was it caused by a case of overcarburetion with the big 800? Not so fast...
Though the traditional carb-sizing formula-cubic-inch displacement (383) times maximum rpm (6,500), divided by 3,456-would dictate the use of a 720-cfm carburetor, everybody at JMS agreed the "too big" 800-cfm rating was not the culprit. Rather, the attention focused on the design of the primary discharge nozzle used on the 1412 EPS800 carb. The pen points out how the nozzle is capped at the end and opens downward facing the primary throttle blade, making it susceptible to the effects of fuel reversion. Fuel reversion is a natural occurrence in which the column of fast-moving intake air reverses direction and stagnates or actually exits the intake tract with positive force moving outward past the carburetor inlet opening. Every engine experiences reversion at some point in the rpm curve, and most run right through it with no perceived ill effects. But once reversion takes hold, the normal vacuum signal becomes a pressure front that can enter the discharge nozzles and interrupt the flow of liquid gasoline from the bowls, through the emulsion tubes, and into the booster venturi. Why did the misfire symptoms vanish once the 750-cfm 1407 carburetor was installed? Because, it seems, the Edelbrock 750 is fitted with discharge nozzles (as are its 500 and 600-cfm siblings) that are not as susceptible to the effect of reversion. These nozzles terminate horizontally (left), so pressurized reversion is not as likely to enter the primary circuit and interrupt the crucial vacuum signal. We pondered the situation at length and this is the only logical conclusion we drew. So with the EPS800 on the shelf, all tests were made with a 750-cfm Edelbrock Performer 1407 in place.
There's a lot of discussion about single-plane intake manifolds versus dual-planes, so we tried one of each, an Edelbrock RPM Air Gap dual-plane (bottom) and an Edelbrock Victor Jr. single-plane.
Dual-plane manifolds (left) essentially divide the engine in two, feeding each bank from one side of the carburetor. Aside from a small 2.65x0.5-inch machined passage in the plenum divider (pen points), each bank of cylinders only has access to one half of the carburetor's flow capacity. The idea is to keep the velocity of the air/fuel mixture that passes through the carburetor and intake runners as high as possible for optimized atomization and cylinder filling at all rpm levels, especially down low. This yields more efficient combustion, which maximizes cylinder pressure, which equates to good torque output. The downside is reached at high engine speeds where the restricted carburetor access can decrease top-end power output by starving the engine. By contrast, notice how the single-plane configuration (right) uses an open plenum that gives all eight cylinders straight-shot access to all four barrels during the intake stroke. Flow velocity may not be ideal at lower rpm so torque may suffer, but once the crank gets spinning, power potential is much greater than with the dual-plane.
We were surprised to see that the dual-plane RPM Air Gap has larger runner entry openings than the Victor Jr. While the widths are very close (Air Gap: 1.162 inches, Vic Jr.: 1.125 inches), the RPM Air Gap's runners are 0.092 inch wider.
With our iron head testing out of the way, we can now get serious and switch to aluminum heads for Tests 3 and 4. As we demonstrated in previous installments, refurbishing the stock iron heads (top) cost us $909 in parts and labor (see Part Two). Why settle for puny 1.94/1.50 valves, big 76cc chambers, and unnecessary mass when for $1,295-a mere $386 more-you can step up to 2.02/1.74 valves, 64cc chambers and superior ports and shed 38 pounds with a set of Super Street 215 cast-aluminum heads from Patriot Performance? It made sense to us. For a comparison, check out Part Three for a detailed overview.
The Air Gap name describes the fact that the runners and plenum are separated from the effects of hot oil splashing against the bottom of the manifold by an integrally-cast "hallway." Greater intake charge density is the objective, while the increased carburetor height allows more efficient runner designs. Nonetheless, these manifolds fit beneath virtually any production Chevy hoodline. With the single-plane Victor Jr. in place atop the iron heads, in Test 2, the 383 churned out 384 hp at 5400 rpm and 395 lb-ft at 4,000 rpm, a gain of 5 hp and a loss of 10 lb-ft. Though not an extreme example, this is a typical result of what happens when you trade a dual-plane manifold for a single-plane design.
After installing fresh head gaskets, engine owner Dale Snoke installed the aluminum heads on the 383. Thanks to the smaller 64cc combustion chambers, compression jumped from 9.0- to 9.8:1. Though we're well below any concerns of detonation even with 91-octane pump gas, know that a 1-point increase in compression ratio typically calls for 1-3 degrees less ignition timing advance. However, this requirement isn't as important when swapping from iron to aluminum heads, due to the reduced surface temperatures caused by aluminum's rapid heat transfer characteristics. In our case, it's a washout, and we left the total advance at 37 degrees BTDC with no detonation problem.
While it's true that head bolt threads must be lubricated for 100 percent assurance that they're properly torqued, the fact that they thread into the coolant jacket beneath the deck mandates sealant to prevent water leaks. Teflon pipe sealant safely pulls double duty, though a light coat of oil between the bolt heads, washers, and cylinder head eliminates potential frictional drag that could yield undertorqued fasteners. Is there any difference in the torque value for an aluminum head versus an iron head? Don Johnson answers, "Nope, you torque the fastener, not the head. The bolts go to 70 ft-lb, in three steps, regardless of head material."
With the aluminum heads in place, the previous, 8.050-length pushrods (arrow) were too long, so they were swapped for a set of Elgin 7.860-length pushrods (PN PR-777). Remember the pushrod guides used with all aluminum heads require hardened pushrods. Iron heads can get by with nonhardened pushrods as long as guideplates are not employed.
Aluminum heads expand more than iron heads once normal operating temperature is reached. The Isky cam card recommends 0.016/0.018 valve lash clearance. If set cold, this will grow by around 2 to 3 thousandths in the running engine. Some guys will remove a few thousandths from the cold setting (0.013/0.015 in our case) and leave it at that. But for complete accuracy, you really should set them to the recommended spec with the engine cold then recheck them after it warms up like we did on our Scorpion billet aluminum roller rockers.
All buttoned up, in Test 3 (aluminum heads, dual-plane), the Super Street 215 heads and RPM Air Gap made 431 hp at 5,700 rpm and 441 lb-ft at 4,800 rpm, an impressive 52hp and 36-lb-ft increase over the results of Test 1 (iron heads, dual-plane). Clearly the added 0.8 point of compression, free-breathing ports, and bigger valves boosted cylinder pressure and exerted more leverage on the crankshaft. What's really good to see is how the aluminum heads were already outperforming the iron heads by 26 hp and 45 lb-ft way down at 3,000 rpm, where the dyno started measuring output.
Test 4 (aluminum heads, single-plane), netted 442 hp at 5,800 rpm and 436 lb-ft at 5,000 rpm. The resulting 11hp gain and 4.5-lb-ft torque loss reaffirm the high-rpm bias of the single-plane design. Look at the dyno printout; the single-plane lags behind the dual-plane in every area until 5,000 rpm, where the single plane finally delivers higher horsepower and torque numbers. So unless you're planning on spending more driving time above 6,000 rpm than below 5,000-rpm, stick with a good dual-plane manifold, you'll be a car length ahead. Next time we'll stick it in a car and see what it does on the chassis dyno and dragstrip. See you then!