Mechanical vs Vacuum Secondaries for this build

[RDJ]

I would like to hear some opinions on vacuum vs mechanical secondaries on this engine build. This is being built as a street car with occasional track time.

408 small block
10.3:1 compression
Stock Edelbrock heads
Hughes 3237 hydraulic cam, 232@.50 intake, 237@.50 ex dur, .548 lift, Hughes valve springs
Hughes roller rockers
Edelbrock Air Gap intake manifold
TTI headers
Accel 71301 dist, it has a vacuum canister
Accel 300+ cdi ingnition
8 ¾ rear with 3.23 gears
Stock 727 trans
3,000 stall Spec-Rite Pro Street converter (check these out if you need a converter, local to my home town, not cheap, high quality http://www.specriteconverters.com/ )

I plan on using a Speed Demon 750, annular boosters, however, I’ve had mixed advice on the mechanical vs vacuum secondaries. I’m pretty green on this, so I need to listen to those with more experience than me.

Things I’ve heard: Mech is for racing only, Vac for street

The Demon Tech guy says to get the mech secs, because with this cam and the high stall converter, the vac secs won’t work properly.

The Hughes guy (who sells Demon carbs) says lots of folks with similar builds to mine have used the vac secs just fine.

I would like to hear some more opinions, I’ve tried to build a nice engine, I don’t want to screw up this part

also, how hard is it to dial in mech secs.

Seems to me lots of other folks on this site have successfully used vac secs on similar builds.

BTW, some will chime in on the bad rep of Demon carbs, however, they have rectified their problems and probably do more QC than anybody else at this time.

 

[dgc333]

The Demon tech guy is full of s$%^. I bought a Road Demon 725 and found the Demon tech line to be of no help at all. I would recomend that you go with a Holley or one of the tuner outfits that sets up Holleys.

Unless you are good at tuning a carb you will be better off with the vacuum secondary carb. The VS carb will be easier to set up, will generally have better low mid range throttle response and will give up little or nothing to the DP in th emid to top end.

 

[ramcharger]

Mechanical secondaries is my vote and they are no harder to tune than any other carb. That's a big cam and it's going to really dig all four barrels at stall. I would suggest maybe a 3.55/3.73 gear with that cam though...

 

[moper]

I'm no fan of Demons even when they are properly prepped and set up. But it's your engine. I'll also add annular booster is not needed, and the reason for vacuum or mechanical has nothing to do with convertor, but rather the tuner. In your case I'd run a vacuum secondary. First, it's a street car and regardless of the BG tech, the VS will be more responsive, smoother, and cleaner than a DP. Second, it's a fairly mild build over all and a DP is more work to tune and with a Demon, keep in tune. I run vacuum secondary 870s which are 80cfm more and a direct wet flow to wet flow comparison to yours. Stroker cranks eat up carb cfms.

 

[RDJ]

Thanks for the replies. A holley 870 with VS should be the choice? Seems big, but this is my first small block build and looking for the best and easiest for my limited skills. Not a 770 or Edelbrock 800? What model exactly should I get then? Dominator? Street Avenger? I'm interested in your opinions.

 

[BobsDart]

I vote for mechanical. Had both on my car. I have no lag or sputter off the line, actually just spin from a dead stop. You need more gear. FYI my set up
if you care.. 360-40 over, edelbrock heads, 9.5cr 284-528 solid cam, airgap, 750dp, 2500conv. 391 posi. hooker headers, 2 1/2" x pipe, tti tailpipes.
12.75@106 thru exhaust best run to date

Bob

 

[IfItsGotWheels]

IMHO you should go with a Holley 4780 (800DP)

 

[70aarcuda]

as far as tuning difference between VS and DP...the DP are easier to tune..

almost all DP now come with 4 corner idle circuits vs the 2 corner for the VS..the DP offers alot more adjustability with larger camshafts..

getting the VS to open smoothly can be a problem...you will need to get the holley spring kit to make those trial and error adjustments...

the most you would have to do with a DP is adjust the size of the rear squirter...

I have used both carbs on the street and dragstrip....and I perferred the DP.....but I have a 750 VS on my street duster...except the times it goes to the track for some fun..then the DP goes on it...

Quick Fuel Technology makes a very nice billet vacum cannister that is adjustable with only a needle and seat screw....very nice...got one of those in use also on the VS...

My duster has a 360 that is same as your except for the crank.....

 

[Longgone]

I like the vacuum on the street and mechanical for racing. Vacuum secondaries will be much smoother on the street through variable throttle settings and will give you what you need when you need it.

 

[kielbasa]

I'm wondering about the same thing (408 stroker for street use), and I initially assumed vacuum secondaries for street use - however, my brother and buddies that also run street cars swear by mechanical secondaries.
Actuallly, I did experience a stroker big block running vacum secondaries this past summer. Doggy, but smooth....
I guess I'm still on the fence - in theory the vacuum secondaries make a LOT of sense, but every time I ride others cars, the mechanical secondaries make me a believer.
I suppose it also depends on your driving habits too - we beat our cars, always doing burnouts from every stop sign, reverse drops, making noise, powersliding, etc.
My buddy with the vacuum secondaries drives in a much tamer manner, and he's perfectly happy...

 

[forphorty]

I've had both and prefer mech sec. A good old 4779 750 DP would probably work great on that 408

 

[SpeedThrills]

I suppose it also depends on your driving habits too - we beat our cars, always doing burnouts from every stop sign, reverse drops, making noise, powersliding, etc.

Wow! Don't borrow my car! Lol!
I like that user name. There's some good kielbasa in a few shops in Phila.---
With REAL rye bread and butter, oh man!

 

[T67POWER]

buy a regular double pumper 4150 holley(cheapest)
Why spend the money where you don't need to.
Use a 850 cfm double pumper mech. secondary carb.
get a wilson 4 hole spacer even with the air gap.
Don't get a smaller carb then that.
You could use more gear.
You have a pretty good set up for torque.
That motor will make that 850 feel like a 750 on a 360.
Thats a big engine that needs lot's of air.
If you had a little bigger cam and 4.56 gears you could run a bigger carb then that.
850 mec. all the way GOOD LUCK :thumrigh:

 

[djwhog]

A large modern AVS jetted with adjustable mech is a good way to go, you have lots of torque in a stroker , go mech and setup correct is fine on the street too!

 

[1wild&crazyguy]

double pumper.

when you tromp on it, which one do you want.. that eases in? or one that does it now and gits! hit'er hard

one of the choke;ess race holleys would work nice, you'll never want a NON annular carb again!

the 750- 830cfm annular holley=$435'ish from jegs. you'll thank me.

if it were me and your setup? I'd go with the 830 holley

 

[turbodart68]

double pumper.

when you tromp on it, which one do you want.. that eases in? or one that does it now and gits! hit'er hard

one of the choke;ess race holleys would work nice, you'll never want a NON annular carb again!

the 750- 830cfm annular holley=$435'ish from jegs. you'll thank me.

if it were me and your setup? I'd go with the 830 holley

Yeah, what he said!

 

[RDJ]

Looks like I have some decisions to make. But an 800+ holley seems to be a favorite. Seems pretty split on the MS vs VS though.

 

[stroked 340]

Quickfuel 850...thats what i run on my 416 with a similar size cam,best carb i ever used great street manners,excellent results on the strip..Demons=garbage...

 

[cudaman51]

if you run mechanical secondarys you will also have to run manual valve body trans.

 

[ramcharger]

Second, it's a fairly mild build over all... Stroker cranks eat up carb cfms.

I agree with moper here. I didn't see that it was a stroker. Your gearing will be fine. :read2:

 

[ramcharger]

if you run mechanical secondarys you will also have to run manual valve body trans.

??????? I disagree.

 

[1wild&crazyguy]

if you run mechanical secondarys you will also have to run manual valve body trans.

no you don't...

 

[mopardude318]

I have a 750 Mighty Demon with mechanical secondaries on my 408, i also have a 4 spd manual tranny though...I do like Demon carbs, and haven't had a problem with them yet...Holly's are good too, but I ike demon

 

[cudaman51]

I tried it and couldnt get the shift points to work when i thought they should. are you guys saying it worked for you. I meen for daily driveing not 1/4 mile.

 

[340sFastback]

My stroker is very similar to the original posters. I went with a new 750 Holley Double Pumper because that is what Ryan at Shady Dell told me to use.

I thought a double pumper would bog when stomped from a dead stop but it doesn't. It just wails and creates a huge cloud of tire smoke instantly. Matter of fact it never bogs because the engine revs faster then my foot can put the pedal down.

I did have to crack the secondaries open some so I could close the primaries to get a good idle (over exposed transfer slots due to cam). But other then that I am running stock jetting etc and it works really well. Very happy with the carb.