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408 Stroker Build & Dyno Test - Double Trouble
Building and dyno-testing a 408 stroker to double the power of a tired 351W
Who among us doesn't want more power? Where we differ is in the chosen route. When you think of improving performance, power adders immediately come to mind-after all, there is big power to be had from nitrous, a blower, or turbo system. Bolt-ons constitute yet another route, as heads, cam, and intake can dramatically improve the power output of even a stock short-block.
The final route, and the one chosen for this adventure, is to add displacement.
Reaching a specific power level is both easier and ultimately more driveable with additional cubic inches. In truth, our displacement was combined with bolt-ons, as we did more than toss a stroker under the stock top-end components.
The first item on our to-do list was to set a power goal. Our starting point was a stock '69 351 Windsor, originally rated at 290 hp (gross flywheel). The tired Windsor put out about 200 rwhp, or slightly more than a bone-stock 5.0-liter, but less than a PI 4.6L Two-Valve. In its defense, the 351 Windsor offered more low-speed torque than the others, but by no means could be considered a powerhouse.
We decided to double the power output of the stock 351 Windsor, so our goal was to exceed 400 rwhp, the equivalent of over 500 flywheel horsepower. In addition, we decided that the 500hp Windsor must also combine driveability and a broad, usable torque curve.
Acceleration is a function not of peak power, but of average power production over a specific rpm range. We knew right away that combining our peak power goal with improvements in average power production meant one thing-a stroker motor!
In truth, it would be possible to reach our goal of eclipsing 500 hp with a stock-displacement 351 Windsor, but that will be made much easier with a stroker. In addition, the increased displacement will improve power through the entire rev range. The extra torque offered lower in the rev range can be enjoyed every time you get behind the wheel.
With this in mind, we decided to build a 408ci Windsor. This was achieved by combining a 4.00-inch, forged-steel stroker crank from ProComp Motorsports with forged (4.030) pistons from Probe Racing. The two were connected via a set of 6.20-inch, forged-steel connecting rods (also from ProComp Motorsports). After adding the 28-ounce damper from ProComp Motorsports, a set of Total Seal rings, and C77 bearings, the stroker combination was balanced and assembled by L&R Automotive.
With the additional displacement taken care of, we took a hard look at the power producers-namely the heads, cam, and intake. Wanting 500 hp and plenty of torque for this daily driver, we selected our power producers accordingly.
For cylinder heads, we went to Brodix, which supplied a set of KC LH F 17 heads. Available in two different intake port volumes, we chose the smaller 195cc intake ports. The 210cc heads might work equally well, but the 195 heads offered nearly 300 cfm, more than enough to feed our power needs.
We suspected the smaller intake ports might improve part-throttle response and torque production, but the Brodix KC LH F 17 heads certainly had a lot going for them. They feature 17-degree valve angles, a 2.02-/1.60-inch stainless steel valve package, and full CNC porting. The 60cc combustion chambers combine with the dished pistons and Fel-Pro 1011-2 head gaskets to produce a streetable static compression ratio of 10.0:1. The Brodix aluminum heads were secured using a set of 1?2-inch ARP head studs. ARP also supplied a number of other fasteners, including a balancer bolt, oil pan studs, and HD oil pump drive.
Once the heads were secured, we made sure we had enough cam timing to take advantage of the head flow. What better cam to install in our stroker motor than one designed specifically for that purpose?
The Xtreme Fuel Injection (XFI) series of cams from Comp Cams were designed specifically for stroker applications. For our street stroker, we chose the smaller XFI236HR-14. Though smaller, the XFI236HR-14 offers impressive specs that include 0.579 lift, a 236/248-degree duration split at 0.050-inch lift, and an idle-friendly 114-degree lobe separation angle. This would be a healthy cam in a smaller 302, but combines an abundance of power with improved idle and driveability in the larger 408.
According to Comp Cams, the effective operating range for the XFI236HR-14 is 2,600 rpm to 6,300 rpm, making it a perfect match for the dual-plane Performer RPM Air Gap intake from Edelbrock. Sure, there was more peak power to be had from a single-plane design, but we didn't fancy the inherent trade off in low and mid-range torque. We'd gladly give away 10-15 hp at the peak for an additional 30-35 lb-ft in the low and mid range. Since the motor will spend most of its time in the lower rev ranges, we like the idea of optimizing the area of primary use.
The 408 was starting to come together now that we had the heads, cam, and intake installed on the short-block, but we were still missing a few key items. Complementing the cam is a set of link-bar, hydraulic roller lifters that eliminate the use of the factory spider hold-down. In addition to the cam and lifters, Comp Cams also supplied the hardened pushrods, double-roller timing chain, and aluminum 1.6 roller rockers.
Needing both fuel and fire, we installed a Holley 950 HP carburetor and MSD ignition system that includes a billet distributor, cap, and wires run with a Digital 7 ignition amplifier. Down below, lubrication comes from a new Melling standard-volume oil pump.
Due to time constraints, we were forced to run the motor with a conventional pan, but testing has shown a race-type pan, pick-up, and windage tray to be worth extra power at the top of the rev range. We fully expect more power from the combination once we install the Milidon pan that we have for this engine.
The finishing touches include Hooker Super Comp headers, a Meziere electric water pump, and Lucas 30W break-in oil. After a computer-controlled break-in cycle, we installed a fresh K&N filter and changed the oil to Lucas 5W-30 synthetic.
After installation on the dyno and a complete break-in procedure, the 408 was finally run in anger. Minor jetting and timing revealed peak power numbers of 530 hp and 534 lb-ft of torque. Not only had we eclipsed our goal of 500 hp, but the stroker offered an exceptionally broad torque curve, bettering 500 lb-ft from 3,400 rpm to 5,400 rpm. Even below 3,000 rpm, the 408 thumped out serious torque, and we fully expect traction to be in limited supply.
The test was not without incident, as we discovered the spring package on the Brodix heads isn't quite sufficient for the aggressive ramp rates offered by the XFI cam profile. We suspect valve control is limiting power at the top of the rev range, so a spring upgrade may unearth additional power.
When combined with the Milodon pan and windage tray, we suspect the 408 may post even greater power numbers. After addressing these two issues, the motor will be installed in the awaiting '69 Mach 1. Knowing the combination will exceed 400 rwhp, we have officially doubled the power output of the original 351W. With so much more power, only time will tell if this means double trouble.