Three-foot-high flames shooting out of your carburetor are not a good thing when your engine is screaming on the dyno at 6,000 rpm, but it happened to us, and our stock-block Ford 5.0L lived to tell about it. And what a story it is. It’s the tale of a heroic little engine saved from the crusher and reborn into a fire-breathing, 650-horse hell-raiser.
We could lie and say we started with a “used” block or even a cheapo rebuild to stave off critics who say we’re cheapskates, or we could tell you that we “recycled” it out of the local “auto dismantling center” to make it sound more PC, but we won’t mince words. We’ll be straight up: We got the engine out of a wrecked ’86 5.0L Mustang at a junkyard, cleaned it up, and bolted on the right speed parts. That’s it. If it blew up on the dyno, at least we’d know the limits of the stock bottom-end so we could go yank another one and try again.
From the outset of this project, we had no intention of building a typical torquey street/strip motor that compromises top-end horsepower in lieu of more street-friendly low-end grunt. This time, we wanted a high-rpm screamer capable of putting a lightweight car solidly into the 10s with the right chassis setup. It won’t be a daily driver, it won’t get 20 mpg, and it won’t purr like a kitten, but it will at least be streetable enough that it could be driven to and from the track. Our goal was to take a factory 5.0L short-block and dress it out with an awesome set of aluminum heads, a big cam, and the right induction and valvetrain gear to support them. Then we’d turn it loose on the bottle. A big bottle.
We’ve seen 5.0L Mustang racers do this for years, getting away with 200hp nitrous hits and more on bone-stock, high-mileage short-blocks for round after round of sub-10-second passes. How? Mainly because from 1985 to 1992 all 5.0L HO engines—the version that came in Mustangs and some Lincoln LSC coupes—were built with forged-aluminum pistons (1993 5.0s used hypereutectic pistons) and hydraulic-roller camshafts. These are stout engines, easily capable of revving freely past 6 grand until the stock lifters reach their limit around 6,200 rpm and pump up, floating the valves. Ford blocks of this era were also cast with thick, partially siamesed bore walls out of high- nickel-content iron, so bore wear is minimal and ring seal is excellent, plus the 4-inch-bore/3-inch-stroke combination is inherently an excellent power producer with the right size heads and valves. We’ve seen plenty of well-maintained, high-mile 5.0s thrive under harsh dragstrip abuse.
Check out the photo captions and sidebars for more on the parts we used, why we picked them, and what we had to do to put the combo together. All in all, we had little fear that we were about to pull the pin on a nitrous grenade, but then again, you never know until you try.
Round One
We brought the assembled engine to Westech Performance group for a dyno session, and started by running it with the Edelbrock nitrous system hooked up but not activated to see how much power we could make on motor only. All testing was done with a set of 1¾-inch-primary full-length dyno headers hooked to a set of Flowmaster race mufflers and an MSD Billet distributor running through an MSD Digital-7 ignition computer. Westech’s previous testing with the AFR heads on several other stock-type 302s has shown that the heads work best with 34 degrees of total ignition lead, so we started and stayed there. We also put the stock 70/80 jets back in a used Holley 4779 750 double-pumper we had in the garage (but we priced a new one anyway).
The first pull resulted in 385 hp at 5,800 rpm, but the power curve hadn’t even peaked before the test was stopped at that rpm limit. Figuring the springs were good to at least 6,200 rpm, we made another pull to 6,300 to see where the stock hydraulic lifters would give out, and were rewarded with 398 hp at 6,100, which we backed up with a 399 at 6,100.
Round Two
The dyno’s fuel flow meter showed that we were running a little lean at the top end, so we added three jet sizes to the secondary side (70/83) and made another pull: 398 hp at 6,100, with the valvetrain just starting to float. We tried again, but the second pull netted only 340 hp, and the sickening feeling that our day was about to end early began to rise. Suspecting a lost cylinder, we pulled the valvecovers and found that the rocker stud on the No. 5 intake valve had come loose (mental note: always double-check rocker stud torque). We pulled all the rocker arms, retorqued all the studs to 60 lb-ft, and reset the valves.
The third pull was a charm, providing us with a baseline of 406 hp at 6,100 and 360 lb-ft at 5,600, a surprisingly high peak torque level. Westech’s John Baechtel suspected that the wide 114-degree LSA and aggressive exhaust valve duration of the Nitrous HP cam was costing us some naturally aspirated power, especially given the AFR head’s already exceptional exhaust port flow. But we weren’t here to test naturally aspirated power.
Round Three
We decided to start off the nitrous testing conservatively with the Edelbrock Performer RPM kit’s smallest 100-horse jets. We set up the system to come on at wide-open throttle with a carb-mounted microswitch, and kept ignition timing at 34 degrees total. Edelbrock suggests retarding timing 2 degrees for the 100-horse stage, but we felt that the AFR head’s superior chamber design would keep us out of detonation problems at this level. Baechtel eased into the throttle to build engine speed, then hit WOT at about 4,000 rpm. The engine practically lept off the dyno stand as it howled past 6,000 rpm, and we practically fell off our chairs at the result of the pull: 531 hp at 6,100 rpm and 4,900 lb-ft of torque at 4,900.
Most nitrous kits’ advertised horsepower levels are optimistic to say the least, and we’d expected to net no more than 75 hp at this level. Why the big difference? Edelbrock’s kits are designed to run rich with an 11:1 air/fuel ratio. Westech’s Lambda meter revealed consistently leaner 13.4 to 13.6 A/F ratios throughout our pulls with our carb jetting and bottle and fuel pressure. This probably explains the higher horsepower gains we experienced with the suggested jetting levels, but you won’t see us complaining about getting 125 hp from a 100-horse hit on 92-octane pump gas. At the beginning of the day, we’d hoped to wind up with 550 hp; now it looked like our junkyard dog could go higher. But how much higher?
Round Four
If a little of something is good, more must be better. We checked the plugs and everything looked fine, so we swapped in the 150-horse jets, turned back the timing 2 degrees to 32 degrees total, and pulled the pin. The 5.0L again sang past 6,000 rpm, and the results were more impressive than the last: an even 600 hp at 6,000 rpm and 570 lb-ft of torque at 4,800. Once again, the Edelbrock system delivered more than advertised, with a 70hp increase on a 50-horse jetting increment. The plugs still looked good, with no “peppering” of the plugs with molten piston, a surefire indication of detonation, or other signs of damage to the electrodes.
Round Five
If more is better, why not try a lot? A 200-horse nitrous hit is serious business on any motor, let alone a stocker, and we knew we were embarking into the danger zone. At this point we were betting what rpm the bottom-end would let go at, but we had to at least go out with a bang. To give the motor a fighting chance, we drained the fuel tank and the carb bowls and filled up with 100-octane unleaded, threw in a can of octane boost for good measure, and dialed back the timing 2 more degrees to 30 degrees total. The first pull resulted in an impressive but somewhat disappointing 641 hp at 5,800, with the Lambda meter showing the engine was running very rich.
Most nitrous companies, including Edelbrock, recommend keeping bottle pressure between 900 and 950 psi, and pressure drops fast as the bottle empties; a quick weight check showed that the bottle was more than half empty, so we switched to a full one and took three jets out of the secondary, putting it back at the original 70/80 jetting. Full bottle pressure rewarded us with a mind-blowing 655 hp at 5,900 rpm and 647 lb-ft at 4,700. We were more than 100 hp past our initial goal, and we still had another set of nitrous jets to try.
Round Six
Punch-drunk and greedy from our staggering success so far, we decided to test the Windsor’s limits by pulling out the 250-horse jets—the biggest in the Edelbrock kit—in a bid to crest the 700hp mark. Keeping timing, carb jetting, and fuel the same, we pulled the handle for what we expected might be our last run of the day—and nearly was. Midway through the pull, with the motor screaming like a Top Fueler on nitro, the dyno room echoed with a cacophonous blast as flames shot three feet high out of the carb. Expecting the worst, we dashed into the dyno cell and were mildly surprised to not find a rod hanging out of the block. Suspecting that we had at least popped a piston, we yanked the plugs to check compression, and immediately found that the electrodes had melted off the plugs in cylinders No. 1 and 2, indicating that we’d run the engine way lean and into heavy detonation. The rest of the plugs looked good, and we crossed our fingers during a compression test that miraculously revealed eight good holes.
We scratched our heads for a while before deciding that we’d screwed up by not loading the tank with 110-octane race gas as Edelbrock recommends for the 250-horse hit, and for not retarding timing at least another 2 degrees. After looking at the dyno’s fuel curve record, it appeared that the fuel solenoid had lagged when the system was activated at WOT, which allowed the engine to load up with nitrous. Fortunately, the ensuing backfire occurred with the carb’s throttle blades open, or otherwise the carb might have been shot right off the manifold. As it was, the only damage was some slightly singed venturi boosters.
Round Seven
Feeling lucky that we’d dodged a load of shrapnel, we made another run on motor alone to see how badly we’d hurt our scrappy little engine. Despite our near-disastrous tuning mistake, the 5.0L kicked out 392 horses, nearly backing up the previous baseline. Most of that loss is probably attributable to the higher-than-needed 100-plus octane fuel in the tank, which often drops power on a naturally aspirated motor by slowing the rate of burn when it’s not needed to fend off detonation.
Conclusion
From the outset, we had high expectations for the stock, forged-piston 5.0L short-block, but nothing in our experience prepared us for the spectacular results of our dyno session. We knew 500 hp was within our reach, but 650 hp from a $100 junkyard short-block? That works out to 2.16 hp/ci, which is full-on race-engine power production. For Pete’s sake, that’s Winston Cup territory, and it’s just abso-friggin-lutely incredible for a 10.0:1 motor that doesn’t need to run on race gas. We’d taken the 5.0 to the limit, passed it, and lived to see another day. It ran great on the dyno, but we really can’t wait to see what it does in a car. Stay tuned while we work on getting it into an ’86 Mustang LX coupe and down the track.
