Danger Mouse Part 8

We Tested 12 Different Holley Carbs for Power

Mike Petralia Feb 13, 2003 0 Comment(s)

This month, we took Danger Mouse (DM) in a completely different direction. In the past seven months, we've used DM to tackle various ideas on the dyno and see just how much power we really could make with a genuine street small-block. For the most part, the results have been fantastic, and DM has usually surpassed all power expectations. For our eighth installment on DM's testing schedule, we wanted to see if there really was power to be gained or lost by running the right or wrong carburetor.

Now, this type of testing can open up a whole, big can of worms, so to keep testing simple and fair, we chose to only run carbs from the same manufacturer, Holley. And we also chose to test only 4160- and 4150-style carbs, and not the giant 4500-series Dominator lineup. We gathered up 12 different size/style carbs, ranging from Holley's smallest four-barrel, the 390-cfm vacuum secondary unit, up to the big HP950 double pumper, and bolted each one of them onto DM to see if one stood out from the rest.

We expected the smaller carbs to make good bottom-end torque but sacrifice peak power, which they did. But the thing that really threw us for a loop was that the biggest carb we tested--the HP950--made the most power of any carb! Does this mean that big carbs are best? Probably not, but it does mean that Danger Mouse liked the bigger carb. However, DM also liked the HP 750, and we think these tests really show that Holley's engineers have done their homework and dialed in the HP carbs to work best on just about any engine. It also means that all of those carburetor modifiers out there probably do have something good going, because most of them can take an old, abused Holley and make it work as good as a new HP, but the cost is about the same as if you just bought a new one, so which choice you make is up to you. From what we learned in this test vs. what we'd already known is that not all carbs are created equal, and if you're building a daily-driven street car, you should still go with something smaller than 800 cfm. But, you certainly can't judge a carb by its size alone. Check out the dyno charts to see what we found.

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We established a baseline using Holley's old-faithful 0-4779 750-cfm double-pumper. We tried to sway the fuel curve into making more power by changing jets, but Holley's stock settings worked best.

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After the baseline, we bolted on Holley's smallest of the line, the 390-cfm vacuum secondary (List number 0-8007). It made good low-end torque, but not the most of all carbs tested. Top-end power really fell off with the little carb.

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When compared side-by-side, you can really see the difference in throttle blade size between the 390-cfm carb (left) and the 750 double-pumper. It's the small throttle and venturi size that kills top-end power with the little carb.

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The 570-cfm carb was part of Holley's new Street Avenger line and has quite a different appearance from the other carbs.

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One of the nice extra features of the Street Avenger line are these fuel level sight plugs that make setting the float level a little easier.

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On carbs with chokes we had to make sure the choke didn't close when we made a power pull. A small vacuum cap wedged in tight did the trick, but don't try this on your daily driver as it might vibrate loose and get sucked into the carb.

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Shop owner and dyno operator George Vrbancic personally checked each carb to make sure it was getting full throttle before a test was made.

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Vrbancic Brothers' DTS dyno has been very good to the editors at SUPER CHEVY. Its reliability and repeatability have made testing with it very consistent.

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Both of Holley's HP-series carbs (the 750- and 950-cfm models) made more power than all the other carbs tested. The HP950 even made more peak and average power than any other carb. That's really saying something for the crew at Holley who designed the HP lineup.

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One other interesting comparison is that the venturi boosters--the things in the middle of each venturi--can change drastically from one carb design to another. On the left is a standard 750 double-pumper with regular down-leg-style boosters. On the right is the only carb we tested (the 830-cfm 0-9381) with annular-style boosters.

This month was the first time we bolted DM onto the Vrbancic Brothers Racing DTS dyno in Ontario, California. Shop owners George and Bob Vrbancic both supervised the test sessions with George at the dyno controls. Yours truly performed all the jet changes and kept a running tab of the results.
To establish a baseline we first ran Holley's old standard, the 750-cfm double-pumper (List 0-4779-8). After the baseline was done, we tested from smallest to largest, trying every different size Holley carb we could get our hands on. And in order to ensure accurate results we tried tuning fuel curves for the best peak power. It turned out, however, that the stock fuel curves worked best, and any time we strayed from Holley's settings for each carb, we usually sacrificed some power.
(For the complete flow rating chart, check page 62 of the April 2003 issue of Super Chevy magazine).

SOME INTERESTING DYNO OBSERVATIONS

* Only three (850 dp, HP750, and HP950) of the carbs averaged over 400 lb-ft of torque, and only the two HP carbs (750 & 950) peaked over 430 lb-ft of torque.
* The HP950 made more torque at 2,500 rpm (353 lb-ft) than any other carb, but the 650 dp was right behind it with only 1-less lb-ft (352).
* The 650 and 750 dps tied for most tq and hp at 3,500 rpm.
* The 750 vacuum secondary carb (List 0-3310) had the lowest mid-range figures of all carbs.
* Peak torque occurred between 4,100 and 4,300 rpm, and peak hp occurred between 5,400 and 5,700 rpm with every carb.
* Average power tells the story and the two HP carbs (750 and 950) made more average power than any other carb.
* Note: Faithful DM followers may note that peak power numbers for this test were down by a couple percent from previous tests. That discrepancy is most likely just the difference between how a DTS dyno and a Superflow dyno collect and interprets the data. It's kind of like stepping on the scale at your house vs. your friend's house; either scale may be correct, but you'd still weigh the same regardless of the difference in readings.

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