vacuum vs. mechanical demon 750
Pro
Joined: Feb 2004
Posts: 564
Likes: 0
From: Pittsburgh PA
Originally Posted by GrandSportC3
mech. secondaries = more power
vacuum secondaries = better fuel economy and less power
vacuum secondaries = better fuel economy and less power
You can dial in vacuum-secondary carbs much more than mechanical secondaries.
With the vacuum-secondaries, you can adjust the carb to kick in the secondary side exactly when the engine needs it. Mechanicals will dump the fuel into the engine whether your engine can handle the excess fuel or not.
If the ratio of fuel to air is too rich, you lose power. If it's too low, you'll not only lose power, but the lean condition can cause catastrophic damage to the engine. Mechanical carbs tend to run overly rich, because the the fuel dumps in before the engine needs it. You stand on the gas-pedal, and the carb dumps in the fuel, even though the engine doesn't yet have enough air to make the proper ratio, and so not only does economy suffer (wasted, unburnt fuel), but your power will go down...
A vacuum carb, when properly dialed-in, will perform as well (and I would submit, better) then a double-pumper. Because with the right spring controlling the vacuum secondaries, you can stand on the pedal, and you'll stay on the primaries until the vacuum and air-flow are at a point where the secondaries are necessary. And then they open. You therefore will make more power, because the fuel/air ratio will not go overly rich, you won't blow unburnt fuel out the exhaust (or simply foul your plugs), and you'll have trouble-free performance that will keep you happy for a long time...
Only in max-effort engines, where the engine is set on "kill" all the time, will a double-pumper outshine a properly tuned vacuum secondary carb. And then, only by a very small percentage!
Setting up the carb, is the real key. IF you know what you're doing, the "double-pumper is HP, vac-secondary is economy" idea is clearly what many already know it to be: an old wive's tale.
Be well,
Eric
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
If the vacuum secondaries could make the same power as a mech. Secondaries carb, why doesn't anybody at the track use vacuum secondaries... Vacuum secondaries carbs are a lot cheaper than mech. Secondaries carbs...so it would be a great deal...same power and less $$ 
I've made many dyno pulls with my car and the mech. Secondaries 750 HP made 20 HP+ more than the vacuum secondaries 750 Holley.. Numbers don't lie.. I'd take anybody on a dyno challenge to show me that you can make the same power with a vacuum secondaries carb...
I've made many dyno pulls with my car and the mech. Secondaries 750 HP made 20 HP+ more than the vacuum secondaries 750 Holley.. Numbers don't lie.. I'd take anybody on a dyno challenge to show me that you can make the same power with a vacuum secondaries carb...
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
Originally Posted by GrandSportC3
On the dyno and on the race track - you'll always get better numbers with mech. secondaries carbs... (if both carbs are equally well tuned)
The reason why people say that vacuum seconaried are better with AT cars is because the rpm drop between gears is higher on automatics because there is usually 1 less gear and when often the torque curve is a little better with vacuum secondaries.. but the HP is just about always lower with vacuum secondaries.. I gained 20 RWHP when I switched from vacuum secondaries Holley 750 to mech. secondaries Holley 750 HP series.. I have the dyno sheets with both carbs somewhere..
The reason why people say that vacuum seconaried are better with AT cars is because the rpm drop between gears is higher on automatics because there is usually 1 less gear and when often the torque curve is a little better with vacuum secondaries.. but the HP is just about always lower with vacuum secondaries.. I gained 20 RWHP when I switched from vacuum secondaries Holley 750 to mech. secondaries Holley 750 HP series.. I have the dyno sheets with both carbs somewhere..
