Holly 770 Street avenger final tunning results

[genolan]

I wanted to close out my Carb tuning, I started with a 408 engine and a 770 Holly street avenger with Vacuum secondary.

Carb was stock with the following
pri jet 72
sec jet 75
pump nozzle .25 Green Cam
pwr valve 6.5
secondary diap spring silver (vacuum secondary)

Final Tuning resulted in
pri jet 75
sec jet 83
pump nozzle .35 Green Cam
pwr valve 6.5
secondary diap spring White(vacuum secondary)

[jhv48]

Just for comparison, put a 750 double pumper on and discover another barn full of horses.

[genolan]

Anyone have a 750 Double Pumper for sale?

[genolan]

Does a Double pumper really make that much more power than a well tuned vacuum secondary carburetor, Is it worth investing $400 to try a rebuilt one.

[GLS-BDR226]

I changed out a 670 Street Avenger with a QF 650 double pumper on a 302. Really woke up the engine. A nice upgrade.

[genolan]

I found this one on ebay do you think an 830 DP is to big or can I just change the jets to dial it in.

Holley 0-80509-1 Race Carburetor, 4150HP 830 CFM 4 Barrel Dual Inlet Square Bore

[jhv48]

Originally Posted by genolan

Does a Double pumper really make that much more power than a well tuned vacuum secondary carburetor, Is it worth investing $400 to try a rebuilt one.

Yes, most definitely!

I've done it twice, with both a 302 and a 427. Both made a huge difference in performance.

[jhv48]

Originally Posted by genolan

I found this one on ebay do you think an 830 DP is to big or can I just change the jets to dial it in.

Holley 0-80509-1 Race Carburetor, 4150HP 830 CFM 4 Barrel Dual Inlet Square Bore

A little on the large side for a 408, but can be made to work. New ones are almost $800. I would look for a nice 750 if possible.

[genolan]

Since I need a new project I'm going to try a Double Pumper. I was leaning toward Holley since I have tuning kits and some experience with them. I'll be removing a 770 street avenger with Vac secondary's and was thinking a 750 Dp is this the best size to use. It's a 408 stroker with supposedly 520hp but I don't have the engine internal details.

[Maricopa]

Is it worth investing $400 to try a rebuilt one.

In my experience, NO. Go new and save yourself some headaches.

[jwd]

Originally Posted by genolan

Does a Double pumper really make that much more power than a well tuned vacuum secondary carburetor, Is it worth investing $400 to try a rebuilt one.

Absolutely NOT. A correctly set-up vacuum sec. carb. is a much better street carb. I've replaced several DP Holleys & Barry Grunt Demons with Street Avengers and the throttle response, gas mileage and general drivability is always much improved. If you're gonna do nothing but drag race, put on a DP, that's what Holley made them for.

[Maricopa]

Yet that's exactly the opposite of what Holley themselves recommend and what I've read over and over by Cobra owners that have had both. DPs for lightweight cars with manual trans and vac secondaries for heavier cars with automatics.

I think the big mistake people make is simply over carbing.

[jwd]

Originally Posted by Maricopa

Yet that's exactly the opposite of what Holley themselves recommend and what I've read over and over by Cobra owners that have had both. DPs for lightweight cars with manual trans and vac secondaries for heavier cars with automatics.

I think the big mistake people make is simply over carbing.

I suppose you're right. My almost 40 years of experience building/tuning street and drag race engines has obviously been wrong.
Oh, by the way, when you have a few spare moments, contact Roush and ask them why they only use vacuum secondary carbs on all their engines.
While you're at it, call Keith Craft and ask the same question, however he does use DP carbs. on his "Drag Race" series of engines.
I've tried almost every combination and I know what works, both on the track and the street. I would NEVER run a DP on the street and I'm confident I can improve the performance/driveability on the cars that do.

[Maricopa]

Please re-read my post as I wasn't quoting my own experience, I was quoting Holley (you know, the guys that make 'em) and others. Obviously opinions differ but if you want I will publicly state that your ___ is bigger than mine.