I have a ZZ4 with an Edelbrock 600 on it currently. Want to go with the Barry Grant Speed Demon, but I'm trying to decide on the size. If you do the formula it says I should be running a 650, but I'm concerned about later add-ons. I want the 750 but I think it may actually be too much??? Its just a toy car and doesn't get driven that much, strictly street. Also are there advantages to a mech secondary on the street or for a street car are you better off with the Vac secondary? The 600 I currently have on there is restricting the motor. I know it has more potential.

 

from what ive been reading and from what RB said in an earlier post the equation gives you the bare minimum cfm needed. so when it says 650 you can safely run a 700. do a search on mechanical vs. vacuum secondaries and you will have a whole new world opened up to you right before your eyes. its a topic that has been beaten to death.


this thread is what RB said about the equation on cfm ratings.this

 

I'd go with a 750 mechanical secondary. Hands down.

 

I would not...

It is better to under carb than to overcarb the engine. The edelbrock 600 carb is not the best for performance. If you go strap on a mechanical secondary carb, chances are you are goign to have ****ty throttle responce, and be dumping in too much fuel off the line. I wouldn't use a mechanical secondary carb on any automatic car with less than a 3,000 stall converter. That said, I would suggest a more potent carb, but not oo much as to drown the engine in the fuel. I think a Speed Demon 650 (or 750 if you prefer) vacuum secondary carb will be the best choice. It will provide excellent streetability, without holding back the motor.

 

How would you figure that a mechanical secondary carb would "dump fuel"? Maybe if it wasn't tuned properly, but thats the same with any carb.

A properly tuned mechanical carb will function just like any vacuum secondary carb only better. Its just that a vacuum secondary is more forgiving (tuning wise).

 

I figure that both the primaries and secondaries opening with a stall converter lower than 3000 will be too much for it use at that RPM. It wouldn't be a problem if it was higher in the RPM range and able to use the extra fuel. Thats why I always suggest vacuum secondaries with auto for anything less than 3,000 RPM. he says he wants to a bigger CFM carb, and if he is using a mechanical secondary on an auto, he is going to need a smaller carb, the big double pumper carb will be too much for it. If you insist on mechanical secondary, a 650 will work better than a 750, it will have much better accelleration, though the top end might be lacking. I'd say 750 with vac sec would the the perfect combination, especially since he can swap secondary springs and have them open earlier if he wants.

 

There's nothing magic about 3000 RPMs. The chart on this link http://www.holley.com/HiOctn/TechSer...fo/TI-225.html tells you how much DP CFM you can live with.

Having finally "seen the light" this past summer, I'll never bolt on another vacuum secondary carb - unless I want to slow down.

In another thread, a member building a similar engine was told by BG to go with a 650 DP. He was told little details like air velocity will make a difference, enough that a 750 would not be the right choice.

And, yes, you can "dump fuel" into the engine when the carb is too big. In order to compensate for the reduced booster signal, you'll have to go up in jet size, and un-vaporized fuel (which will be required to get the throttle response desired) will cause the final mixture to be too rich.

 

FWIW, my switch from 1901 Edelbrock Performer Q-jet to 750 VS Holley netted me no change in ET or MPH (although the Holley was more consistent - the q-jet held the all-time high-MPH mark over the Holley). That was with 3.08 gears, by the way. Going to 3.73's took .4 off the ET, MPH stayed about the same.

The switch from 750 VS Holley to 650 DP Holley kept the same MPH but took .2 off ET. The switch from 650 DP to 750 DP Proform mainbody took off about .05 and MPH stayed the same. Up to this point stall was 2200 RPM.

The switch to 3500 RPM stall and cool air intake took off another .3 from ET, MPH stayed about the same. The DP didn't "bog" with the 2200 stall, but the engine wasn't in it's powerband until 3000 RPMs.

All this is with the '57, in case you were wondering.

 

I know Barry Grant has a specific carb for the ZZ4 weather a VS or MS. But I'm thinking later on this year when I put in the hot cam and do some head work that carb is just too small. My thinking is why spend the 450 on a carb that is too specific to a particular motor? I understand the differences between the VS and MS. I'm thinking the 750 maybe dejetted a tad would be a good building block for the future, allowing me to keep the same carb. My car doesn't really get driven that much, and has only seen the track 1 time. The Edelbrock that is currently on the motor is a great carb hands down. I have never had any problems with it at all. I just think the motor has sooooo much more potential, therefore I'm making the step to a different one. I like the Barry Grants, I just think they are better built carbs. Thanks for all the responses,

Clint

 

People seem to think all you need to do to fix an "over-carb" situation is jet down. Not true.

You jet for proper mixture. When over-carbed, you will never get the jetting/transition/power/accel correct for all circumstances. Driveability will suffer.

If the Proform mainbody came in a 650 CFM version, I would have bought it. I was told my engine would want 750 CFM, but the 750 Proform really did not make a significant difference for me over the 650 Holley. I'll keep the Proform, but I did lose the ported vacuum source & choke (not issues for you with the BG's, but facts, none the less).

Even with the Hot Cam, your ZZ4 isn't going to out-flow my 401. Although a 750 might produce more peak power on the dyno, for a street toy, a 650 will do you better.

As for VS vs. DP: With sufficient stall, a DP will respond better to WOT, even on the street.

 

Stall on mine is 2800, I guess if I went with the 650 I could could always upgrade the main body?? I ran a 700 DP Holley for a while but didn't have enough ports to run everything from the carb. Kind of like haveing that clean look. Like I posted earlier, its more drivability for me, not all out dyno power. Just a fun toy, not an all out race car. If I wanted the race car I would have built a different motor all together. Thanks

 

I might be the guy on the "other" thread... My best advice to you is to call the BG tech line. They'll ask you specific questions about your setup such as intake type, cam duration, gears, weight of car amoungst other things. Of course, give them the setup you PLAN to run. FWIW.