I would like your opinons on this subject. Interested in the pros & cons of each type.

 

I suppose that means that if you don't have a double pumper/mechanical secondaries you're pretty ignorant as far as fuel delivery goes, huh? I had no idea people with mechanical secondaries/double pumpers were the only ones that knew anything about fuel delivery and how to use their right foot. Now I do. Thanks.

 

Well, as we've seen before, there are varying opinions in this area.
Nastee, you clearly like your Q-jet & for good reason- it obviously works very well for you. Either type of carb can be made to work quite well.

I found it interesting that conventional wisdom usually said that if you're running an auto trans that you should run vacuum secondaries. I had a Holley 780 cfm w/ vacuum secondaries on my Vette(w/ TH400 trans) & it was optimized for my car in terms of jetting & secondary activation. There was no bog or hesitation at all. When Barry Grant was still modifying Holleys(before he was forced by Holley to give birth to the Demon line) I looked into his line, as I was ready to spend the money on a modified carb to gain some ponies. His staff assured me that "Your motor will love this carb." when discussing buying their modified 750 double pumper rated up to 1040 cfm. I went w/ their advice & w/ no other changes dropped an immediate .35 in the 1/4 mile along w/ gaining about 4 MPH.

I realize that this was largely due to the increased flow & the mods that matched fuel delivery to that flow. I must add that drivability was improved also but that it does come on noticably quicker under part & full throttle. This does make it a bit more challenging to drive since traction is more of a factor. Maybe that's what gkull was trying to get at w/ his comment, although I certainly don't intend to put words in his mouth.

Bottom line is, there is no right & wrong here- just varying opinions based on various experiences. Let's try not to get into a flame war here when a member is looking for info- especially at a time when our country is dealing w/ such a traumatic situation.

 

Everything's cool. I knew my Dodge emblems would catch on!

 

Gkull, thanks for withdrawing that post...

Well, let's see.
Vacuum secondaries:
Advantages include good street manners for daily driven cars. This is because they are more forgiving. The engine only draws the amount of fuel that it needs. Because of this reason, you can also oversize a vacuum secondary carb. However, you'll still notice more of a midrange bog than a properly-sized mechanical secondary carb, if you have a big cfm vacuum carb on a small block, for example. Vaccum carbs also do very well with auto transmissions. However, they are not a requirement. I might suggest running a vacuum secondary carb if you have one available, on a new rebuild just so you don't have the phenomenon of washing the cylinder walls with excessive gasoline while trying to get the mechanical carb adjusted right.
Vacuum carbs are also very easy to set up. Basically, the variables are the primary jets and the secondary spring. You can tune the opening of the secondaries simply by changing a spring. Heavier opens later and lighter opens earlier.

Mechanical secondary carbs:
They are much harder to dial in. But they are also much quicker responding and offer better throttle response when they are dialed in. The midrange seemed better when I used a small 650 DP Holley on my 355 compared with the 750 VS. But the 750 gave me a better top end.
If you have a stick shift car, mechanical secondaries with their improved throttle response really help when you're charging through the gears, downshifting, heel and toeing around apexes, etc.

Even though I got a 775 cfm Gold Claw DP for my new 427, I am definitely going to break it in on the 750 VS Holley first, before I even worry about dialing in that Gold Claw. Might not be a bad idea to have both types of carbs on hand, because if you ever need to do maintenance on your double pumper, you don't have to ground your "fighter-Vette".

Let's extend this discussion a little further. How about comparing Holleys to Rochester, AFB, and Edelbrock's in the family of spread bore carbs.
The thing I like about spread bore carbs are the tiny primaries that dramatically improve torque, and then the monstrous secondaries for top end. I think if I were going to run a small block, I'd use a dual plane intake with a spread bore type carb. But for an engine that has a high top end and runs a single plane, I prefer the symmetric design of the square bore carbs such as Holley, Demon, and Barry Grant.

Open to discussion...



[Modified by MoMo, 8:31 PM 9/26/2001]

 

I've had a few Rochesters in my time, and I was never able to get one to work well. For some reason, I could almost never get the secondaries to open properly with the Rochester. When I switched to a Holley 650 double pumper, my problems were gone. However, I can't completely knock the Rochester. I know people who've had great success with them. A good friend of mine had a Carter AFB on a similar car that ran better than any of my Rochesters, but not as well as the Rochester dual jet that he removed!

Enough of my ramblings...

 

Since this is a carb discussion I have a few questions. I have hear a few mention latley how satisfied they have been wit their Demon carbs. I have just a 650dp box stock. I am satisfied with how it is tuned and runs. I wonder how much better a demon is??? Is it worth profiling the top area of my carb to look like one. You know mill off the choke tower and smooth out the sharp edges. Would this make any improvements however small?? Or would it create a junk carb?

 

Had a 650 dp on my '64. 331 ci, 10:1, 350 hp cam (i.e., I/E @ 222 degrees at 0.447"), Performer RPM with 3/4" open spacer, Roller Tip rockers, 2:02/1.6 valves, 4" K&N, Thorley headers, 2.5" mandral bent exhaust w/crossover, Dynomax max-flow mufflers, Mallory High Fire IV ignition, solid copper wires, CR 4-speed, 3:70 rear.

650 was jetted too fat (71 primary, 79 secondary). Replaced it with this carb jetted at 67/73. Difference is like night and day in driveability and torque. Don't know about top end, but seat of the pants feel was well worth the investment. And wow factor is like... WOW!
http://cgi.ebay.com/ebaymotors/aw-cg...367592&r=0&t=0

 

I have a 1980 vette (automatic) with a ZZ4 engine that replaced the original L82. The car runs great with the original Rochester (jetted right and secondary spring is set correctly). Runs very well when setup right.

Now I also run a 650 Holley double pumper spreadbore at times(in warm weather) . While the Rochester runs well, this carb seems to have a real advantage in the midrange.

I think either carb can be setup to work well, but for pure power the Holley seems to have an advantage in the midrange as mentioned in the other posts. I think the fact that one I use is a spreadbore and double pumper all in one might help with having the auto trans.

I recently picked up another version of this holley carb that was for oldsmobile and ponitacs as a driect replacement for the rochester in the 70's. This version has an intergral choke which will allow me to run it all year around. The chevy version has an external choke which doesn't fit my ZZ4 intake.

 

Well it seems that each of us perfers both types, myself included. I have had 5 carbs on my car over the years. Three vacuums & 2 mech. I think matching the carb to the engine modifications, trans type, and most important is the gears is more inportant than the secondary type. My latest is a 650 speed demon w/ mech. secondaries. I think it performs the best for me. I am running an XE 274 cam, Gm fast burn heads, performer intake, and 3:90 gears. There is no bog at wot off the line, mid range and top end is steady. The engine with this combination takes all the fuel it can get. Down side is 9 mpg, maybe 10. It is really hard to accelerate slowly from a red light, this engine sets you in the seat so well.
Thanks to all who responded.

 

That is exactly the carb I was thinking of, Bud. I have 3.70's and an M21 with a modified L46. My Holley 750 DP ran great when the foot was on the floor, but fouled plugs every 500 miles or so. My Holley 4160 runs great every day, but I am not seeing the power that the 750 gave. The Rochester I have is not in good shape :lol: . Maybe I will go with the 650 Demon like you. Is yours the Speed Demon, or Race Demon?

AC


[Modified by A C, 9:37 AM 9/30/2001]

 

GOOD info guys. I have a holley HP 750 vac sec. It has a metering block in the secondarys for top end jet adjustment it runs very well at WOT and part throttle. being i run an auto trans. i went with the vac sec. But latley have been thinking of going with a DP and a single plane intake to try it out. I guess i'm never satifyed.

 

Ac, Hi!
I have a 650 Speed Demon on the engine now. The Road Demon is rated at 625cfms and according to their literature it is on adequate up to 350 hp. I don't know where your at hp wise or may be planning to be in the future as to which is right for you. They both come in vac. or mech. secondaries. I love this carb! the four point adjustments make the flow smooth. I had a 650 dp Holley w/ mech. on it last. I was not impressed with it's performance, but in all honesty I am not a Holley man to begin with. Before the Holley I had a 750 Edelbrock on, it performed well except it was too big for the engine.