Aftermarket Parts - 750 Carb Swap

Not surprisingly, both 750 HP versions delivered the best overall fuel curve, which resulted in the best average and peak power. Using the adjustable main well bleeds, we might have been able to fine-tune this carb to reduce fuel flow at the lower end of the curve.

Not surprisingly, both 750 HP versions delivered the best overall fuel curve, which result

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.

Both the Street HP and the HP have adjustable screw-in air bleeds on the top of the carburetor. The classic 750 air bleeds can also be modified; it's just more difficult if you have to make the bleeds smaller. Larger is merely a matter of drilling with a small pin vise.

Both the Street HP and the HP have adjustable screw-in air bleeds on the top of the carbur

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