I am thinking there is a better carb for my ride. I currently have the holley street avenger 870 Vac secondaries carb. It has 82 and 87 jets with the standard pink secondary spring. The car runs good but wanted to know if there is possibly a better all around carb for performance and if possible to have both, gas economy as well. I currently get about 12 mpg highway if I take it easy. Normal driving gets more like 9-10 mpg. The engine is a 428 30 over bore. 600 lift 242/246 dur cam, headers, rpm manifold and alum heads.

Any and all opinions appreciated. I care more about performance but a few mpg's more would be helpful. Thanks in advance

1957 Fairlane Club Sedan
433 Cobra Jet-5 spd TKO600
3.70 Detroit true track

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Mickey Thompson's 63 1/2 #997 S/S Hi-Rise 427 Lgt/Wgt Galaxie,
1957 C-600 Cab-over carhauler w/390-4V, 2-speed rear-end
FGCofA member #4908
MCGC member #75

"There will ALWAYS be an FE in my LiFE"

look at the numbers...IF you could pick up 15% milage (12 to 14mpg), and drive 12000 miles per year..(lots for a toy !!)...at $2.50 a gallon ...you gain about $355.00...is that worth a carb change? / performance drop? / re-tune hassle? ..JMHO

mileage was a secondary consideration. How about a better carb for performance? I would go through some hassle for performance gains.

1957 Fairlane Club Sedan
433 Cobra Jet-5 spd TKO600
3.70 Detroit true track

Holley lists the venturi / throttle sizes as
1-3/8 1-3/8 1-11/16 1-11/16
This is the same size as the 750 double pumper.
So, I think it is a misprint.

Holley lists the stock jets as 78+pvalve primary and 82+plug secondary.

Do you by chance have correct venturi/throttle bores for the 870 SA?

Well in any case it is about the right size, but if you have a chance to borrow or try an 800 double pumper see if it runs different/better. But the 870cfm is about the right size for your engine (496hp per the Gonk) so I'd experiment with jets and the secondary spring.

As far as mileage, the new "Gasolator" I wrote to go with the "Gonkulator" says you should get about 13mpg on 60mph road trips, 12.5mpg at 70mph, and yes, 9.5mpg in the city. So you might be a bit rich but not too bad.

On this front, if you have a good Autolite 4100 or even better Autolite 4300 (top dog for mileage in my experience) try them - they will NOT run as good but you might get as high as 15mpg highway. I used to run a 4300 Autolite on long trips, then swap on a 780 or 800 Holley for fun.

See the following link

http://www.schwartzperformance.com/ygen.htm>



Some nice pictures but mainly, it is a 474hp small block Chevy, which is kinda like a Ford now that GM is in the Ren Center......LOL. Anyway, they report best dyno results with the 870sa. If it is really a "780cfm" downleg carb that maybe true since the downleg Holleys are hard to beat on the dyno. The other sa carbs, 570sa, 670sa, 770sa look to be straight leg boosters.



Other observations (I am learning a lot or at least a lot of questions here.....)

1] you can see the downleg boosters in the link above

2] on the Holley site, the venturi and throttle are (edit - almost) same as the old 780 Holley

3] I think that Holley rated their 570sa, 670sa, and 870sa at 2.06" Hg (28" water), not 1.5" Hg, and even then did some approximating so they could use the convenient sequence 570,670,770,870. Will try to confirm, maybe somebody knows?

4] It looks like the 770sa is a "750 straight leg" style carb rated at the good old 1.5" Hg.

5] I don't know why Holley is fooling around wtih In Hg like this, but I believe Demon does it too, in the opposite direction.

6] If all this is true, even the stock jets (78+pvalve) in the primary of the 870sa look too rich. I'd try some Gtech or timeslip runs, lock out the secondary, and start going leaner on the primary til it runs best. My guess is about a 73 jet in front. Then open the secondary and repeat procedure. The secondary jetting from Holley maybe about right if the 1-3/8 venturi they show is correct.



Has anybody else observed the 870sa, is it really just a shined-up reinforced "780 downleg" with a slightly smaller secondary venturi, 1-3/8 for the "870sa" vs 1-7/16 for the old "780" we all know and love?

However, the new HP and SA metering sure isnt.

The IFR design changed, the main jet responsiveness changed due to what I think is a redesign of the emulsion tubes.

Ironically, I am not a Street Avenger fan for the street. WOT they work well, but I dont think the main circuit comes in clean off the transition circuit, forcing a richer mixture to make up for it.

When I put early metering blocks on my HP series, matched in IFR, jet size, PV rating and PV channel restrictions, the difference in low end and tip in was dramatic.




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- 70 Fastback Mustang, 489 cid FE, TKO-600 5 speed, 3.70 9 inch
- 71 F100 shortbed 4x4, 397 cid FE, headers, Street Dominator, 280H, 5 lug Dana 60, 4 speed