Ok i have a 357 small block with Vortec heads. (Other info in sig) A cam designed to run to 6K rpm's. I run a Holley 600cfm carb with vacuum secondaries. And lately i have been reding that a 750cfm is the smallest i should see on a 350. And i have been tossing around the idea of mechanical secondaries b/c the car is a 6 Speed now. What do you guys think i should do? This winter i'm converting to a Roller cam and probably 3.73's in the rear. The car seems to run pretty good but doesn't pick up in seat of the pants feel until over 4k... it will go go go then 4k and bam puts you in the seat until over 6k.... any ideas? Also timing should be right around 31 degrees total like the Vortecs like.(from what i have heard) Any ideas?

 

Ok, assuming 100% volumetric efficiency..... (yeah, uh huh, that's gonna happen...) at 6500 RPM, the engine is moving approximately 660cfm. (about 610 @ 6000) So, I am thinkin' that a larger carb is indeed going to give you better performance, but, it is going to be most noticeable in the higher RPM range. It might actually give you worse off the line performance, due to the larger primaries...... But, I am not precisely a carb expert, and will defer to others on that particular point.

 

Going from the high ratio you have now to a 3.73 gear is going to take
care of most of your problem.

750 vac secondary would be a good choice for your combo. Also helping
low end lag.
750 Vac also gives you some wiggle room for future.
Better gas milage also.



Later

 

Its normal for an engine to just gain power in a "powerband"? It feels like my friends Turbo Eclipse... Just feels like it hits boost when i get on it. it'll go go go then bam it puts you in the seat and starts pulling a lot harder... I was thinking maybe that's when the secondaries are opening... It stumbles at low rpm when you get on it too sometimes..

 

So you think vacuum secondaries are better than a double pumper?

 

I hear about a "Kick" that the mechanical secondaries are common for when opened up and i thought that would be cool. Fuel mileage is not a problem for me either.

 

As to the vac versus mech secondaries it all depends on what your
intentions are for the car.

For the street the vac secondaries would be the best choice.
Not going to hurt strip use either until you start getting to what I call
getting carried away.

Also probably save a quarter of a tank of gas per fillup if you get into it alot.
Been there done that over many years, cars and alot of Holleys.
As to kick if thats your thing, probably get more when the diaphram opens
the secondaries exactly when the motor needs it.

HTH

Later

 

Vacuum secondaries is a bad choice for your car (could be argued it's the worst choice).

700 or 750 CFM DP would do you right.

 

Afraid going to have to disagree with you.
Moderator or not with over 30,000 posts and that statement, kind of sets
me off, as, I was offering been there done that advice.
As I'm sure you are also.

He's not building a race car but good running street car from what
I gather.

Not going to get into a p-----g match with you and just offered my opinion
on my experience over the years.
Worked with carbs, mainly holleys for over thirty years and ex engine
builder, retired now, but still in it.

So (could be argued worst choice) kind of terminal.
Ends here on my end.

Have a good one

Later

 

if your car is making a lot of power, like 400+, it would probably like the mech sec carb better. i have a 4779 holley on my 84. when i punch the throttle it just lites up the tires. i have a 3310 holley on my s10 (daily driver) and i love it. if you're looking to buy a new carb you can't go wrong with the vacuum sec. if its a street driven car.

 

What's your reasoning? i'm just wondering here.... I hear about ppl punching it down low and it lites the tires up like skirkland1980 said.. and i want that kind of punch....

 

If you need to limit low-RPM power, then a detuned vacuum secondary carb is one way to do that. I haven't seen anything that indicates you will need to do that.

Your current kick at 4k may be where your combo comes into its power band, or your secondaries could be opening up too early or too late. You'd have to have a pretty stiff spring in there to open them too late, but I suppose it's possible.

On the street, vacuum secondaries have a nasty habit of not closing when you let off the throttle. If you're feeling frisky and run it up at 3/4 throttle then let off, it'll take a second or so for the secondaries to close - doesn't sound like much, but when you're needing to slow down, shifting, whatever, a second can be an awfully long time.

If you're keeping your foot out of it, there is no reason a double pumper can't be tuned to sip gas as well as a vacuum secondary.

 

Are DP's really that much worse on gas? Gas really isn't much of a problem for me anyways.... Also i get pretty bad mileage now so it doesn't really bother me. lol

 

unless you're on a prepped drag strip with a tuned suspension you'll never put all your power down on launch. so the vacuum secondary which opens slowly will work best on the street.

 

I'm not talking on launch, i'm talking when i'm already rolling..

 

yes, the mechanical secondary carburetor is the best since your car is already rolling and you never stop

 

Based on my sig what do you think i would run at the track now? I know i've beaten 13 sec cars and what do you think after my mods? Mods being a nicer roller cam and rear gears? (a roller to yield about 40hp gain over flat tappet i hear)