vetteguy75

I'm thinking about swapping my 670 Street Avenger with a 650 Holley Ultra Double Pumper. Everything between the carbs are the same EXCEPT the Avenger has vacuum secondaries whereas the Double Pumper has mechanical secondaries. What type of performance and/or economies can I expect? What is the difference between vacuum & mechanical secondaries?

72SLEEPER

Less economical w dbl pumper but who cars about gas mileage in a vette... Lol

vetteguy75

What about performance? Will I notice a difference?

oldalaskaman

pends' how deep you stick your foot in it

birdsmith

A vacuum secondary carb will generally be better for automatic-equipped cars. If yours is a fairly mild 350 the 20 cfm won't make any difference. A double pumper will use more fuel just because of the secondary squirters. If economy is a concern you may want to stick with a vacuum secondary carb; with the correct setup ( right power valve+right primary jets+right secondary spring) a vacuum secondary Holley can still really scream!

Street Rat

Put on that double pumper and there goes your mileage friend.

Just curious why you would want to switch from the Avenger?

72SLEEPER

Holley 4779 and don't look back

Brass Pass

I did the switch from a 770 Street Avenger to a 750 Ultra DBL Pumper. I never could get the 770 SA to run good. I even gave all of the tuning tips from Lars a try, but it would still fall on its face. Granted I'm a novice when it comes to carb tuning, but the SA was a disappointment.

I finally got tired of the SA and put the 750 Ultra DBL Pumper on and haven't looked back since. It is like a different car. It is on a 383 and has run great from the start right out of the box. I fell for the whole vacuum secondaries running better on an auto line of logic, but never again. The difference mechanical secondaries made for my car was dramatic.

As for economy... Never checked and never really cared.
I don't think you will be sorry you're making the switch. Good luck!

Street Rat

Good info Brass Pass. I was thinking about trying one of those Avengers out. I've always been a double pumper man myself. The mild motor that I'm building seemed to be meant for vacuum secondaries. Not having much experience with vacuum secondary carbs it's good to get your perspective.

b71vette

You should give these guys a call, they specialize in Holley Carbs.
http://www.aedperformance.com/

vetteguy75

What kind of mileage are we talking about? Right now (Avenger), I'm getting 14-15 mpg on the road and 9-10 mpg in town. Don't get me wrong, I like the Avenger and it runs great. I'm in that "what else can add, replace or modify (quick & easy) that will make the Vert MORE fun to drive..." mind set. And I thought about the carb. I've had the Avenger for about 8 years.

vetteguy75

Thanks B71Vette.....That link is very helpful...in fact, I book-marked the "Tuning Tips" section of the website.

DRIVESHAFT

The mechanical secondary carb will make your car more fun to drive.
The problem is that its enjoyable enough to get into the secondaries that you will probably do it more than you really need to, and that does use more gas.

427Hotrod

THIS is the main reason double pumpers get worse mileage. If driven normally they do fine. I've had cars get well over 20mpg with them.

A 750 DP Holley works on just about anything...and well.


JIM

FKING1

My Holley 750 dbl pmpr has never passed a gas station it doesn't like.
Having said that, since I have a moded 383 that is an autocrosser, don't care about the poor MPG.
Had to increase squirter sizes to eliminate a stmble.

billla

The two carbs are going to make exactly the same power. DP carbs have an avantage only in zero-to-WOT, otherwise they offer less responsive low- to mid-throttle and as noted (significantly) worse fuel economy.

Vacuum-secondary carbs do require more careful tuning, but are worth the effort - especially on the street and more especially with an auto trans. The DP will cover up a bad tune with more fuel

Fix what you have - timing first, then carb. At 8 years, the Holley is due for an overhaul for sure, and a dyno tune is always worth the money!

77vetteluva

The Street Avenger makes the most sense to me. Especially if you have a mild build and auto trans. I originally had an AED 650 DP on my 77 with mild build 350 w/uto trans. I took of the 650 DP and installed a 570 SA and it made all the difference. If you are not running wide open on the track above 5000 rpms you will never need the cfms from either of the carbs you mentioned. The vacuum secondaries will open when needed and give you better low end response in around town driving or the occasional tire burner. This is just my experience after doing a bunch of internet research and actually making the change. By the way, My AED DP is for sale!

FlyLS6

Had my 454 rebuilt about 4000 miles ago and tossed the 750 DP and put on a 770 SA. I suppose the DP will give more "performance" when you nail it, but I haven't regretted the SA at all.

The engine builder went up a couple of points on the squirter and jets. The choke pull off adjuster needed to be backed off a couple of turns to open the flapper more on cold start (easy to do once you remove the goop that's covering the piston adj screw)(don't bend the choke flap rod in an attempt to adjust).

So, it was a little work, but it starts easy and runs great. I guess I'd rather have the DP during a stop light grand prix, but I can live with that.

63mako

I have posted this before. If you bought a 1970 LS6 chevelle in 1970 with a 4.11 posi, rockcrusher 4 speed it came with 11.25 to 1 compression, 454 CI, solid lifter cam, and could be powershifted at over 7000 RPM. It was designed as an all out race car, had horrible street manners, no A/C available and came from the factory with a 780 vacumn secondary holley. Tune your Avenger instead of dropping $400+ on a new carb. That $400 will go a long ways toward a new set of heads. JMHO

cv67

Teh vacuum secondary gives your car the amount of air/fuel it demands.
Dumping more with a double pump wont do ANYthing for your performance.
Did that switch yrs ago and it was a waste of money. Ran the same, actually the v.s. was smoother overall.

Nowadays with a stick and a rowdy motor the DP works out.

Keep what you got and tune it.

AED makes killer carbs but unless you have an over the top build you dont need one.