We've all heard the talk. It seems to run the gamut from, "Dude-headers are worth 100 hp," to the naysayers who snivel "Headers? They aren't worth the trouble." The truth is actually somewhere in the middle. Because results can vary so wildly, we decided to take a look at some past testing just to see what works and what doesn't.
If you're new to performance engines, and all headers tend to look alike to you, you might want to start with Jim McFarland's treatise on header design on page 62. This will give you the basics on why headers are designed the way they are, and why they don't all look the same. For this story, we're going to perform several header comparisons to shed some light on that age-old question of how much power headers are really worth.
All the dyno tests in this section were performed on different small-block Chevys, but when it comes to headers and exhaust tuning, this same information is completely relevant to any engine-only the header sizes change to match the displacements. This is a relatively busy subject with plenty of variables, but once you get to know all the important characters in this horsepower and torque stage play, the ending is always fairly predictable. Most of all, have fun with this. This is one session involving lots of hot air that's actually helpful.
Headers vs. Cast-Iron ExhaustWe'll start with a very mild Goodwrench 350ci small-block test with stock cam timing. The idea here is to compare the power output of a stock small-block 350 with cast-iron exhaust manifolds and headers. Because this is a mild engine, the peak power difference occurred relatively early in the power curve, with a maximum 34 hp and 53 lb-ft of torque gain at 3,400 rpm. That's an excellent 19 percent improvement in torque. Had the engine been fitted with a decent camshaft to allow the engine to make more power around 5,000 rpm, the improvement would have been even greater. However, this test does point out that even a stock engine will respond to a decent set of headers. As power potential increases with a good set of heads and a camshaft, headers and a free-flowing exhaust system become absolutely essential components.
Test 1: Stock GM Goodwrench 350 with Q-jet and aluminum intake, stock iron exhaust manifolds, a stock camshaft, and a 211/42-inch exhaust system with turbo-style mufflers.
Test 2: The only addition is a set of 151/48-inch Hooker street headers with the same exhaust system.
Test 1 Test 2 Difference RPM TQ HP TQ HP TQ HP TQ% 2,600 272 135 283 140 11 5 4% 2,800 288 154 299 159 11 5 4% 3,000 283 162 303 173 20 11 7% 3,200 266 162 292 178 26 16 10% 3,400 271 176 324 210 53 34 19% 3,600 320 219 349 239 29 20 9% 3,800 321 232 338 245 27 13 8% 4,000 311 237 330 251 19 14 6% 4,200 298 239 317 253 19 14 6% 4,400 285 238 304 255 19 17 7% 4,600 270 237 286 250 16 13 6% 4,800 253 231 267 244 14 13 5% 5,000 238 227 248 236 10 9 4% 5,200 219 217 229 227 10 10 4% Average Gain 20.3 13.8 7.5%
Looking for a Header? Tri-YsThe book on Tri-Y headers is that they pump up the midrange torque over a set of four-into-one headers, which is exactly what we saw with this test. In the midrange between 3,600 and 5,200 rpm, the torque jumped as much as 27 lb-ft, but this comes at a price. Note that the average torque gain throughout the entire curve is only a scant 2.6 lb-ft. This is because at the lower engine speeds like 3,000 rpm, the Tri-Ys lost 32 lb-ft compared to the four-into-one header. Would these Tri-Ys be quicker in a mild street car? When we plugged the two power curves into the Racing Systems Analysis Quarter Pro simulation, the two curves produced very similar results. The four-tube-header version ran a scant 0.08-second and 1.1-mph quicker through the quarter than the Tri-Ys. Much of the difference was due to the slower 60-foot time exhibited by the Tri-Ys because of the torque loss below 3,400 rpm. The simulation used a 2,600-stall converter. It's possible that with a little carburetor work, that torque loss could be overcome. The Tri-Ys might have over-scavenged the engine at low rpm, requiring more fuel, and that could make these two headers more even in the car. But there appears to be no significant gain in performance with the Tri-Ys. It's mostly about where you want the torque gain to occur within the engine's power curve.
Test 3: Four-into-one 151/48-inch headers on a mild 355ci small-block with an Edelbrock RPM dual-plane intake.
Test 4: Same as above, except we added a set of Doug Thorley Tri-Y headers.
Test 3 Test 4 Difference RPM TQ HP TQ HP HP TQ 2,600 330 163 311 154 -19 -9 2,800 325 173 308 164 -17 -9 3,000 335 191 303 173 -32 -18 3,200 330 201 308 188 -22 -13 3,400 332 215 330 213 -2 -2 3,600 328 225 333 228 +5 +3 3,800 330 239 351 254 +21 +15 4,000 341 260 362 275 +21 +15 4,200 353 282 371 297 +18 +15 4,400 355 297 383 321 +28 +24 4,600 362 317 389 341 +27 +24 4,800 374 342 391 357 +17 +15 5,000 391 372 393 374 +2 +2 5,200 393 389 395 391 +2 +2 5,400 389 400 386 397 -3 -3 5,600 378 403 378 403 0 0 5,800 375 414 377 416 +2 +2 6,000 360 411 362 413 +2 +2 Average Difference +2.6 +3.6
The Long And The Short Of ItThis is an interesting test of header length. Test 5 compares standard-length 151/48-inch headers to Test 6 with the same diameter but shorter-length intermediate headers. By looking at the Difference column, you can see that the shorter headers sacrificed torque below 3,600 rpm but made up for that by making as much as 23 more horsepower at the top. The average torque made by both engines is within 1 lb-ft, which is negligible. Basically, the shorter headers moved the torque higher up the rpm curve. If you plugged the intermediate-length header curve into a light car like an early Mustang or early Chevy II, for example, where you could not use a large tire, then the shorter headers might make traction a little easier to achieve. But you'd want to use the long-tube headers in a big, heavy vehicle or a truck where the stronger torque curve would help acceleration.
Test 5: A mild 355ci small-block with Hedman 151/48-inch, 34-inch long-tube headers with a 211/42-inch exhaust system and a pair of Flowmaster mufflers.
Test 6: Same as above except with a set of Hedman 151/48-inch intermediate-length headers.
Test 5 Test 6 Difference RPM TQ HP TQ HP HP TQ 2,600 430 213 422 209 -8 -4 2,800 434 231 420 224 -14 -7 3,000 439 251 427 244 -12 -7 3,200 438 267 431 262 -7 -5 3,400 434 281 433 280 -1 -1 3,600 431 295 432 296 +1 +1 3,800 424 307 432 312 +8 +5 4,000 415 316 422 321 +7 +5 4,200 402 321 411 328 +9 +7 4,400 389 326 398 333 +9 +7 4,600 374 327 386 338 +12 +11 4,800 355 324 369 337 +14 +13 5,000 334 318 351 334 +17 +16 5,200 314 311 337 333 +23 +22 5,400 294 302 316 325 +22 +23
The Big Tube TestThis is a great example of how larger headers affect the power curve. The larger 171/48-inch headers lost a tremendous 47 lb-ft of torque at the bottom of the curve. By 3,400 rpm, the big headers were actually making a little more than the smaller 151/48-inch headers. This may be a hiccup with the smaller header power curve that could be addressed with jetting or timing had we spent more time on tuning. By 4,400 rpm, the smaller headers were making as much as 33 lb-ft more torque than the larger headers. Then by 5,600 up through 6,600, the larger headers took over and made as much as 26 more horsepower at 6,600 rpm.
If you look at the average power curve numbers, you can see that the larger headers actually made slightly less average power. That's because the horsepower gain at the top did not completely make up for the torque loss at the bottom of the curve. So what's the bottom line here? If this engine is going into a light car with a manual trans and a deep rear gear, or at least an automatic with a stall speed of no less than 3,600 rpm, the larger headers might be a slight advantage. The important point here is that a 131/44-inch header is probably the answer, since it would improve the low-speed power while not sacrificing nearly as much horsepower at the top end.
Another point worth mentioning is that what we're seeing here with these two primary-pipe-diameter power curves is not really as much about power loss or gains as it is about moving the curve around where the engine makes its best power. Had we tested a set of 131/44-inch headers, we're confident we would have witnessed an average power increase over the other two primary pipe diameters. This merely reinforces the idea that engines operate best when outfitted with the right set of parts that complement the rest of the engine package.
Test 7: A 406ci small-block Chevy outfitted with a set of Dart Pro 1 heads, a healthy mechanical-roller camshaft, a set of Hedman 151/48-inch-diameter primary-pipe four-into-one headers, and a 211/42-inch exhaust using a pair of Borla XR-1 mufflers.
Test 8: The same engine outfitted with a set of Hedman 171/48-inch-diameter primary-pipe four-into-one headers with the same Borla exhaust system.
Test 7 Test 8 Difference RPM TQ HP TQ HP HP TQ 2,600 432 214 385 191 -47 -23 2,800 433 231 390 208 -43 -23 3,000 429 245 411 235 -18 -12 3,{{{200}}} 433 264 440 268 + 7 +4 3,400 443 287 455 294 +12 +7 3,600 454 311 460 315 + 6 +4 3,800 464 335 456 330 -8 -5 4,000 464 353 444 338 -20 -15 4,200 466 372 436 348 -30 -24 4,400 469 393 436 365 -33 -28 4,600 470 412 451 395 -19 -17 4,800 474 433 462 422 -12 -11 5,000 472 450 461 439 -11 -11 5,200 467 463 461 457 -6 -6 5,400 457 470 457 470 0 0 5,600 447 476 449 478 +2 +2 5,800 438 484 445 492 +7 +8 6,000 429 490 437 499 +8 +9 6,200 417 493 434 513 +17 +20 6,400 402 490 418 515 +16 +25 6,600 385 484 {{{405}}} 510 +20 +26 Average Difference -7 hp -3 lb-ft