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Aftermarket Parts - Are Premium Parts Worth The Price?

We Test A Pile Of Speed Parts To Discover

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750-CFM Carb Swap
Carburetor cfm ratings seem to be those magic buttons that everyone wants to push to make more horsepower. The truth is that large carburetors will make more power on most, but not all, combinations. But there's really a more important story that needs to be told than just the old sneeze that bigger is better. The truth is that not all fuel mixers are created equal.

The trick is matching the fuel-delivery characteristics of a carburetor to the engine it sits atop. Holley has taken a cue from the custom carb tuners and now offers several levels of performance carburetors, each with special features that can create a little more power. For this test, we flogged three different Holley carburetors, all rated at 750 cfm but with different features. A simple test would have been to see which carburetor made the most power while also comparing the prices of all three carbs, but we decided to add a little more data to this comparison by also evaluating the amount of fuel delivered at each rpm point by each carb (see the chart).

As a starting point, all three carbs tested here are mechanical-secondary versions. Our baseline carburetor is the classic Holley 750 mechanical-secondary 0-4779-C. This is based on the original double-pumper design complete with the integral choke housing and fixed air bleeds in both the main body and the metering blocks. The second carb is what Holley calls its Street HP carburetor, an in-between 750 offered in both mechanical- and vacuum-secondary configurations. The Street HP offers most of the features that have made the HP popular but at a more affordable price. The main body has contoured venturi entries, screw-in air bleeds, and a tumble finish, but is only $80 more than the 0-4779-C. As you can see from the dyno results, it does deliver a slight power increase over the standard 750 version with no major tuning required.

The final test utilized the 750 HP carb, which offers all the same advantages of the Street HP along with Dominator-style fuel bowls, notched floats with jet extensions in the secondary side, stainless steel throttle plates for increased durability, and high-flow metering blocks with screw-in main-well bleeds. Each of these carburetors is individually wet-flow tested to ensure the calibration is correct. The HP version is $230 more expensive than the entry-level 750 but with many features that can otherwise only be found on custom carbs costing much more. If you look closely at the overall curve of the three tests, note how the 750 HP tended to deliver more fuel throughout the entire curve yet made more power only from 4,900 rpm and up. It appears that an engine designed to make serious power above 5,000 rpm would really benefit from an HP carb more than a mild, 400hp engine. According to Holley, the metering characteristics are supposed to be very similar between the Street HP and the HP carburetors, which is reinforced by our testing where the fuel curves overlap. From this test the Street HP seems to be a better deal since it made similar power for $150 less than the HP.

Both the HP and Street HP offer the opportunity to really get in depth to dial in the fuel curve on a particular application, and with sufficient time on a chassis dyno using a wide-band air-fuel ratio meter, it's possible to dial in the carb to fit your engine's specific fuel-curve requirement. What we've learned is that there is benefit to more expensive yet infinitely more tunable HP-style carburetors. Even if you never mess with air bleeds (we didn't for this test), there are still advantages to selecting these fuel mixers. It comes down to whether the additional power and/or adjustability is worth the additional cost. Now you have the numbers to make a more educated decision.

750 carb test
Note in this comparison that we evaluated overall power with average torque and horsepower, but also fuel use. Note how both HP versions delivered a fatter fuel curve at lower engine speeds that cost power, yet above 5,000 rpm the curve leaned out and the engine made more power. As an example, at 3,900 rpm, the HP delivered 7 more pounds of fuel per hour (lb/hr) than the 750 classic carb. Had we been able to customize the fuel curve in the HP to reduce the fuel flow around that engine speed, it's possible that power would increase beyond what the classic carb version produced.

  TEST 1 TEST 2 TEST 3 DIFFERENCE
  750 Classic 750 Street HP 750 HP    
RPM TQ HP Fuel TQ HP Fuel TQ HP Fuel TQ Fuel
2,500 392 187 84 395 188 80 392 187 81 0 -3
2,700 384 197 86 384 197 85 383 197 85 -1 0
2,900 381 210 89 384 212 97 381 210 91 0 +2
3,100 393 232 97 400 236 98 396 233 97 +3 0
3,300 411 258 103 419 263 96 417 262 105 -1 +2
3,500 426 284 111 428 285 104 425 283 114 -1 +3
3,700 436 307 121 435 307 118 433 305 124 -3 +3
3,900 444 330 126 439 326 126 441 328 133 -3 +7
4,100 453 354 134 448 350 133 448 350 141 -5 +7
4,300 457 374 146 455 373 144 452 370 149 -5 +3
4,500 458 393 157 456 391 155 457 391 158 -1 +1
4,700 460 412 166 456 408 164 460 411 167 0 +1
4,900 461 430 172 457 426 172 464 433 176 +3 +4
5,100 458 444 182 460 447 180 461 448 182 +3 0
5,300 456 460 194 456 460 184 457 462 189 +6 -5
5,500 449 470 204 448 470 189 452 473 193 +3 -11
5,700 436 473 208 438 476 196 441 479 199 +5 -9
5,900 423 476 205 427 479 201 428 481 206 +5 -1
6,100 413 480 208 416 483 205 418 485 215 +5 +7
6,300 401 481 216 404 485 210 405 486 219 +4 +3
6,500 385 477 227 387 479 222 389 481 220 +4 -7
Peak 461 481 227 460 485 222 464 486 220 +6 ---
Avg. 428.5 369.2 154.2 429.4 367.9 150.5 429.8 370.4 154.6 +1.4 +2.4
Note: Averages were taken from the entire dyno run at every 100-rpm data point. The last column is the power difference between Test 1 and Test 3.

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