Ok I havent been able to get a straight answer on the question of if I should go with a 600cfm carb or a 750cfm one.

I have a fresh 350 and I want very good power below 3500 rpm but also fairly decent power above 3500. I currently have a comp cams extreme energy 262 h cam(advanced 4*) and has specs of 218/224 duration and .462/.469 lift with 1.5 rockers. I also have an edelbrock eps intake manifold.

Before when I was asking everybody on this board if the edelbrock 750 cfm performer carb would be good enough for what I need, everyone said that it was too big and would kill low to mid range power. Since then I purchased a 600cfm carb as everyone told me this would suit well with the 3500rpm peak target.
However now everyone is telling me that the 600cfm is too small and will choke the 350 at my desired trq levels.
I have 3 or so days to return the 600 and buy the 750
Please all you experts and people that have tried these 2 carbs on a 350. WHICH ONE WILL GIVE ME A LOT OF POWER BELOW 3500rpm BUT ALSO GOOD POWER ABOVE……BUT MOST OF ALL, BETWEEN THE 2 CARBS WHICH ONE WILL GIVE ME HIGHER TORQUE AROUND 3500RPM. BUT STILL STRONG SLIGTHLY BELOW AND ABOVE?

 

Go with the 750. It will not be too big, particularily if you get a Vacuum Secondary or Air operated secondary. I have a well built 350 that I Autocross and Hillclimb. I have been using a 780CFM Vac Sec Holley for many years. My Cam specs are close to yours.

Tons of bottom end grunt and will pull past 7,000 rpm. 600's and 650's run out of steam at around 6,000 rpm. Been there done that.

What most people fprget, or do not even realise, is that the formula for calculating carburator size is for Single Plane manifolds only!!. Holley doesn't make this very clear, but if you have some of their Tech books this is mentioned in the fine print.

A Dual plane manifold requires about a carb 20% to 25% larger than the formula. With a Single plane manifold any cylinder can draw from all four venturies. With a Dual plane manifold any cylinder can only draw from only TWO venturies. Thus the need for a larger carburator than a Single Plane manifold.

David Vizard's excellant book " How to build Horsepower Volume two " go into great detail on modifying Intake systems and deals specifically with the nuaces of Dual Plane Manifolds vs Single plane Manifold and dispells all sorts of myths. If you don't know who David Vizard is...do youself a favor. Go out and buy some of his books. Most HP shops stock them.

 

thanks for the info, ill keep it in mind

 

i run a 600 edelbrock with electric choke works very good on my 350 with the xe 262 cam,i using edelbrock rpm intake,and have other work done internaly. 600 edelbrock HIGHLY recomended :rockon:

 

how bout 600----->650 <-----750? if a 750 is too big and a 600 is too small why not try a 650?

 

Because Performers jump from 600 to 750.

Honestly, I don't understand why you would even consider that carb. I had a 750 Holley VS on the 396 for the last couple of years, tried a 650 DP in July. What a difference! Throttle response improved, WOT performance improved, ET's took a significant dip, power was maintained. I've gone to the Proform 750 main body since, but it didn't make nearly the difference that going from VS to DP did.

I don't do enough street driving (basically to and from the track) to measure things such as gas mileage, but that's a matter of primary tuning.

Frankly, unless you're using the vehicle for towing, stay away from Performer and vacuum secondary type carbs (the Performer, AFB, q-jet, and vacuum-secondary carbs are all demand-based secondary types). A 650 DP Holley or Demon will fulfill all your dreams (as stated in the original post).

 

Ditto, Who says a 650 is not a performance carb? It's a great performer between a 600(a little wimpy) and 750 (race race). A 650 would be a good street application to 6000 rpm. with mild cam. and weekend bracket fun.

 

I called edelbrock about my application and they told me a 600 was too small and a 750 was too big. They talked me into buying one of their new "Thunder Series" 650 carbs. I don't have it on yet but hope to soon. I suggest calling their tech line and asking. The people on there get paid to know what carb to use.

 

Have you considered the Holley 670cfm Avenger with vacumn secondaries? That's what I use on my 355ci with a Edelbrock Air Gap intake, it's been a great combo thus far. Has an electric choke that works great as well, and the cam I use is a Crane unit dual pattern with .454 intake and .480 exhaust. My engine pulls strong past 6500 rpm and the carb doesn't starve the engine at all, yet from idle kicks in right now..
cc 82Z-28

 

I ran a 670 on mine and after the swap to the 750 speed demon biiig difference. I'm putting the 670 up on ebay over the weekend to get rid of it. Haven't ran it in months if that tells you anything. I tried and tried to get that 670 to perform good for me but it just wasn't happening. Still ran pretty good but not as good as the car runs now.

 

That makes sense considering all you modifications!
Whats you rpm redline?

 

I also ran a large VS Holley on my car when it had the 305 in it. 780VS Holley. Car ran hugely better than with the non-emissions Quadrabog.

VS when set up properly can be great. The trick is to use a larger VS than what the " Formula " says" . A larger VS carb will have better low end Torque than a smaller DP, make more power on the top end, and yet still give good gas milage ( often better than the smaller DP ) and great driveability. Vizard and Emanuel have tested 650DP against 750VS and the VS comes out equal or on top as far as Power production, low end Torque, driveability and milage.

Slap on a 750DP and the top end power is equaled of course...but at the expense of low end Torque, milage and driveabilty.

Can you imagine a mild 305 with a 750CFM DP on the street? Would be vastly over carbed and swill fuel. Yet my 780VS Holley gave great performance ....would buzz to 6,000 rpm at the drop of a hat,,,but also gave good economy and drivability...... on a 305. Qjet made slightly better milage...but throttle response and power with the Holley 780VS was waaayyyy better.

Downside to VS carbs. They aren't "usually "as quick at a dragstrip. Launching with the RPM's up negates the advantage that the VS has on the street. VS can also lose you time shifting. Can be a bit tricky to set up in that respect.

Mind you...five minutes from me is Buck Kinney Automotive.... something like 27 NHRA Super Stock Championships with a QJet....proves that Air Valve and VS carbs can be made to work at the strip. :hail:

 

That's why I advocate using VS carb's with auto trans at the drags or street. It don't make any difference cause your foot is on the floor the whole time with an auto and the VS makes up the difference on gear changes when the rpm drops. I've never used a VS on a man. trans. car because the gear change is a sudden, sharp drop in rpm and the DP say's "do it now" and the VS says "When your ready" Sorta like sex! Most guy's want to "do it now" (Who's on top?) instead of "when your ready!" Have you ever had any problems with reversion using the 780?

 

I'm sorry, but you're dead wrong.

As I said in my earlier reply, going from VS to DP made a huge difference. And this is with an automatic transmission. Jester tried to tell me that before I bought the VS, but it took me 2 years to realize he was completely right.

The Holley 750VS that I put on the '57 in 2001 never ran a better time than the q-jet I took off until I went from 3.08 gears to 3.73. However, the Holley was more consistent, so I was happy with that trade-off at the time. I'm so much happier now.

I cannot advocate the use of a VS carb for drag racing. Period.* The Holley website has a chart to help you size a DP to avoid bog problems. Follow that, and you will not run quicker with a VS than you will with a DP, even with an automatic transmission.

Oh, I'm only saying all this on this thread because Edelbrock Performer, q-jet, and Holley/Demon VS all have the same "issue".

Matt, I hope you've returned the Edelbrock and gotten a DP by now.



*Okay, there is one reason to use a VS carb - to slow your car down so you can stay above a minimum ET for a particular class.

 

Mucho Snippage....Or if you don't Drag race at all..then a VS or Air Valve is fine as mentioned.

One thing that has not been mentioned...and I don't really like to tell a lot of people about this ( We Hunter Motorsport Team Members are kinda tight lipped ) ..... but a VS carb puts down power smoother than a DP'r . For high horsepower, RWD traction limited cars ( such as Autocross cars ) this can really help when trying to put the power down exiting corners.

 

This is my daily driver, I guess what you'd call a mild 305 and a 750 DP LOL got 20-22 mpg on the highway, haven't checked around town. It's just runs sooo nice and I can't believe it with the big carb on. The 305 is going bye bye soon though Been a damn good engine, never really gave me fits. Anything that did go wrong was either something I did or something other than the engine failing.

 

Ohhh that's a Speed Demon 750 DP? I thought it was a VS

Just goes to prove what some Modern Booster design can do. My 780VS now has Annular boosters...made a huge Difference. Demon Boosters very efficient as well.

BTW, didn't want to mention it at the time, as I thought you had a VS Demon...but I've not been impressed with the Demon's VS models. They took some shortcuts in the air actuation circuitry that controls the Secondary Diaphram. Very crude compared to Holley's design.

Buddy of mine had a Demon 750 VS on his Poncho 463CI Ram IV. Could never get a smooth opening rate. Would always " snap " open.

Figured it was a combination of a " Big " engine and the crude VS design on the Demon. May not be apparent on a smaller engine.

Anyhoo...he's now got a new 830cfm Speed Demon DP and is going to try it out next season.

To each his own I guess. One thing that does make a difference when running a large carb on a small engine is Booster design. The Demon's do have very good boosters as do the Holley HP series. Annular boosters are best for efficiency and Booster signal...which is what it's all about.

 

I'm getting tired of this thread, seems like I'm repeating myself! Like I said, application is everything! There is 3 levels, drag, auto-cross, and street. What do you want? I'm sure that if Chickenman wanted to drag race, he would change his auto-cross app. visa-versa! What do you want? There is so many variations nothing is cut and dried. Even you torque converter makes a difference!

 

Hehe, guess I should have made it more clear earlier or in my sig. I hardly get into the secondaries at all with this thing, don't need to in regular driving. Half throttle from a snails pace and below causes noise to come from the rear tires. Only problem I'm having is when the car gets up around 200 degrees, it stumbles a bit :/ usually runs about 175 to 185 with no stumble problems. Same thing happend with the Holley and stock q-jet. Hotter the engine got, the worse the stumble.

 

You nailed it. Agree 100%