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timh

Is a 750 edelbrock too much for my setup?

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I was wondering if switching to an Edelbrock 750 (1407) will be too much for my setup.

 

I have:

'75 280z w/355 sbc, 10:1 flat-top pistons

333881 cast heads, (76cc, 2.02/1.60)

cam: 272 adv. dur.

performer intake.

Holley "red" fuelpump with stock fuel line.

Stock HEI ignition.

 

Am I going to have low end power problems.

 

Thanks in advance for any thoughts!

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Guest EPS

Timh,

 

I can't imagine a problem with a 750 carb on your set up, infact you need it. Here is a question for you gear heads reading this reply. My engine builder, who has a national reputation and all the trophies and pictures in his office to profess to his racing history, looked me straight in the eye and said, "the size of the carb is not that important. The motor will take what it needs." What do you guys think about that???

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Guest JAMIE T

EPS, if I were you(I am not)I would let your engine builder continue to build engines and let someone else tune your carb. TimH, a 650cfm carb would give you much better throttle response. A 750 is enough carb for most STREET big blocks, including 454's,460's, etc...

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Jamie, I know a few racers who run their big carbs on any motor and just jet appropriately. If the venturis are appropriate to get decent metering is the main issue AFAIK.

 

TimH: I can't say why but I did definitely find a pattern among drag racers that their edlebrock 750's just weren't performing (pardon that awful joke) up to snuff and going to the edelbrock 600's had them full of smiles. A reputed builder back in AB only puts the performer 600's on his drag motors (commonly v. tweaked 400 sbc's). Some common thread came up that the 600 design just worked a helluva lot better than the 750. I love my 600 and it's independence of tuning circuits and ease of tuning. From experience of a few I now running 383's and 400 sbc's on the 600's with your power and more that'd be my pick. YMMV

 

 

------------------

Of all the things I've lost, I miss my mind the most.

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The 750 gives you great bragging rights!!

 

A local engine builder here told me if I used anything larger than a 650 on anything under 383 cubic inches, I would actually hurt the performance of the motor I was using. He stated 650 is plenty of carb for up to 383 cubic inches. However, if I had a 12.5:1 drag racing motor... smile.gif

 

David

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An engine may take only what it needs, but the low end mixture velocity is going to really suffer on the big carb. I'd bet the 600 would be closer to a bolt on than a 1000cfm dominator! You might get the dominator to run, but after how many modifications? I know you weren't talking Holleys, but for the sake of argument, Holley engineers ran tests on the 350 HO and found the 750 to be the best 1/4 mile performer. (600s were also tested) Vacuum AND mechanical secondary models both performed well. Redrilled shooters primary and secondary to .040 were used to increase throttle tip in on the double pumper, and .035 on the vacuum model were the only major changes. Jetting and springs were optimized. Center hung floats are mandatory for hard acceleration. Katech's 610 hp small blocks uses the 750 as well. Sounds like a pretty flexible carb!

JS

 

[This message has been edited by John Scott (edited February 03, 2001).]

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Guest Anonymous

I agree. The problem that pops up on street driven cars by using a large carb is intake velocity and low speed drivability. A large carb can be jetted to work just fine, but its low end response is going to be a PITA to live with on a regular basis on the street.

The two largest mistakes on the street are over-carbed and over-cammed (sometimes both). If its a strip driven car, fine and dandy, low end isn't a issue. Watch your tach sometime when your cruising on the street, unless you really like hearing the engine whine (with a M/trans) you normally are turning probably 1500-3000 rpm.

Unless the engine is a really high output engine that has huge valves and lots of duration and lift, it won't need anything over 650 cfm.

Far as bragging rights, they look the same, tell your buddies its a 750, they won't know the difference. The above is of course my opinion, but its based on a lot of reading by a good many 'experts' (which as Mark Twain said is just someone from out of town!), and my own personal experience with over-carburated soggy performing street engines.

 

regards,

Lone

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I have to agree w/those who espouse the 600 & 650's. Most engines are over cam'd & over carb'd.

 

The 750 is for an engine that will run in the high rpm's all day long. For a street engine that rarely see's the other side of 3500rpm's; you dont need the extra cfm's as the larger carb's signal will be diminished by the lower displacement engines.

 

CFM is nothing more than a by product of displacement & volume/velocity.

 

The formula for getting you in the neighborhood for carb's size is:

 

DISPLACEMENT multiplied by RPM'S; once you've got that answer it should be divided by 3456 (i think that's correct) & that is your cfm you should go with. If you'ld like to take that one more step you can multiply your theoretical cfm times .85 and that answer will be your actual cfm. The .85 represents your Volumetric Efficiency for a slightly modidied/to heavily modified street/strip engine.

 

As you can see after running the formula; very few of us actually need a 750cfm carb.

 

As for the advice from an engine builder; if he doesn'nt have a flow bench to prove his statement, then his advice is suspect. I'm not saying he's not a great engine builder; I'm only saying be careful who you listen to & attempt to get advice from more than one source....dont limit yourself. If you want advise on a carb...go to a carb guy, if you want advise on a trans...go to a trans guy.

 

One man's opinion is sometimes just, "Their Opinion" & not founded in actual expertise. AKA: Dont by your clothes from a naked taylor!

 

Remember, building an engine is a Total Pkg & not just a Bragging Rights kind of thing.

 

Hope this helps.

 

 

 

[This message has been edited by Kevin Shasteen (edited February 03, 2001).]

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Guest RON JONES

I went over to Prestage.com and used a calculator they have to figure out carb requirements.I didn't know what your peek RPM is,so I took a guess and put in 5800.it told me your motor needs 506cfm.I think a 600 is plenty for your combo.BTW they have a lot of cool car math stuf over there,Thanks John Scott for turning me on to that site.

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Guest Anonymous

Hey everyone, I recalled an article in Car craft about a year ago that covered carb size. The dyno runs showed a Holley 750 out performed the smaller carb, a 650, through out the whole rpm range including low end torque. It also made more vacuum at idle than the smaller carb. The article stated that proper carb size depends on the type of intake manifold your using. Duel plane intakes perform better with a bigger carb than the math would indicate while the math works good with a single plane. The 750 carb was the best performer of all the carbs used. Do keep in mind that they were running only Holley carbs. I don't know how the Edelbrock carb would have performed.

Glenn

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Originally posted by Glenn Richelsoph:

The dyno runs showed a Holley 750 outperform the smaller carb, a 650:

 

(Good Discussion by the way)

 

Not to confuse anyone; but I too can show 10 articles where a 650 outperformed a 750. The thing to remember is "TOTAL PACKAGE" not just one component.

 

Where will your rpm range be the majority of the time; above 4000rpm's or below 4000rpm's.

 

The rpm, weight of the car & gearing will determine what cam,intake, cyl.heads, & carb one should choose.

 

Work the formula to obtain your theoretical cfm & you will see that a 350 @ 5500rpms w/a Volumetric Efficiency of .85 (.85 representes most Street/Strip engines) only needs 473cfm to function.

 

Take that same 350 & make it an out-n-out race engine: which should yield a Volumetric Efficiency of 1.1 (110%) @ 7500rpm's brings our CFM needs to 835cfm.

 

Wow, what a difference V.E. makes; keep in mind very few of us have the $$bucks to obtain 110% V.E. from out engines.

 

Again the bottom line is "Total Pkg" & not to get stuck on one component in an engine.

 

 

 

[This message has been edited by Kevin Shasteen (edited February 04, 2001).]

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i have the holley book.it says it depends on top rpms and most street /strip motors dont turn much over 6000.my brother bought a used jet boat with a pumped 460 ford and it had a 650 dp on it.after getting rid of old points distributor and puuting on some electronic ford stuff from pick&pull i set the total ign timong at 3500 rpm and started on carb.put in rebuild kit.check balls for power valves and jetted at 78 front and 80 rear.boat ran good and pulled 5400 rpms-had a tight pump.if a 650 can run a 460 ford than it should run a sbc.the local 1/4 mile nascar racers have to use a 350 cfm carb and make 400 hp out of 358 ci.

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Thought I might jump in here. I have seen only one reference to the other specs of the "package". I know that the weight and the frontal area have a great deal to do with what carb works best.. Example:

Lingenfelter built 498" 604 hp BBC in a 92

chevy fleetside.. At the track with

a 850CFM holley hp4150 carb.. truck runs 7.80's very consistantly... Just for grins

we swapped a spare 750CFM hp4150 in place,

almost the same carb with minor differences in jetting... truck ran 7.62,7.58,7.54

in a row.. getting a little faster as the driver got a handle on the wheelspin...

 

This was in a full interiored everday truck!.

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Originally posted by MYRON:

Lingenfelter built 498" 604 hp BBC in a 92

chevy fleetside.. At the track with

a 850CFM holley hp4150 carb.. truck runs 7.80's very consistantly... Just for grins

we swapped a spare 750CFM hp4150 in place,

almost the same carb with minor differences in jetting... truck ran 7.62,7.58,7.54

in a row.

 

This is exactly my point; at what rpm's did you fininsh the run, how heavy was the truck & what gears/tires was the truck running.

 

"TO 750 OR TO NOT 750? For that is the (?).

I believe you can run a 750 in place of a 600/650 & w/the proper jetting/tuning it may even perform respectfully.

 

This is how I understand Engine/Carb flow. The 750, while flowing 750cfm, will still only flow what the engine demands/needs. Think of a larger Carb as installing a larger camshaft. Are you merely going to put a alarge cam in an engine & not think about better flowing cyl.heads & intake; or pistons & compression ratio. I hope not/that's a recipe for disaster.

 

Another way to think about it is to think of the carb as a turbo/supercharger (I said think of it that way-work w/me here). Would you put a turbo/supercharger on your engine that wasnt efficient but looked good; just because it looked good you now have bragging rights. It looks good yet lacks in performance; meanwhile your Duster/Slant 6 buddies are passing you left/right. Why, because you've forgottent the basics of engine performance....VELOCITY!!

 

We've all heard this a thousand times; an engine is nothing more than an air pump..an air pump not a FUEL PUMP. It doesnt matter how much fuel you dump in an engine or how you introduce the fuel to the engine. If you are'nt "Ramming" the air into the cylinders in a density that is acceptable then your engine is not running efficiently; hey, maybe that's where that V.E.(volumetric efficiency) term comes from.

 

When should one up their cfm; when the carb they're running becomes a restriction-then one might think about upping to a larger carb.

 

This is how I understand engine performance The air is rammed into the cyl's because the piston slides down the cyl on its intake stroke as the intake valve opens; along with the help of atmospheric pressure. This creates a low pressure drop-thus pulling air into the cyl's. In retrospec: The piston slides down the cyl as the int.vlv. opens pulling air into it thru the cyl.heads, thru the intake runners thru the intake plenum & finally thru the carb's body/venturri.

 

Take a lower cfm carb such as a 650; its intake plenum signal is 100cfm lower than a 750 & should allow for a crisp idle & good low/mid torque to boot; then it stands to reason that a higher cfm carb on that same engine will open the throat of the carb more-thus allowing for more volume of air to enter. Right?

 

This sounds good except not having done anything else to increase the engine's ability to take advantage of that increase in volume will only aid in an equalizing of pressure under the carb base & above its airhorn as the previous 650 restriction is no longer there; now you have a tendency for an equalization of pressure and this is not good for a carb.

 

Knowing the above & knowing that a larger cfm will diminish that intake plenum signal (that is: not changing anything else other than a carb) why would you want to ruin that signal.

 

Here's the run down on a 350 @ 500 rpm increminets with a .85 V.E.(volumetric eff.)

 

1)5000...430cfm

2)5500...473cfm

3)6000...516cfm

4)6500...559cfm

5)7000...602cfm..(hey, we made it to a 600)

6)7500...645cfm

7)8000...688cfm

8)8500...731cfm

9)8712...750cfm..(Well, we made it to a 750)

10)9000..774cfm

 

I dont drive a rail dragster & wont ever see the short side of 8000rpm's. As I stated earlier in the thread: its the total pkg not a specific component that makes an engine go. If you want a 750; it needs to be prorperly jetted/tuned. I still think you'ld like the crispiness of idle & accel with a smaller carb.

 

Myron; have your buddy put a 700cfm on that big block & see what happens; or even a properly jetted/tuned 650. Then we'll have some real world experience on cfm to talk about.

 

Anyway....still a fun issue to bat around.

 

[This message has been edited by Kevin Shasteen (edited February 05, 2001).]

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Guest Anonymous

Tim, I think the 600 will be better match on your set up (heads, cam, & intake).

 

 

Kevin, The motor in the test was a typicle hot street motor & was run on a dyno.The engine when equiped with the 750 performed better at all rpm's including idle. The motor had a smoother idle with higher vacuum at idle & had more low end torque. Yes, I'm talking about the bigger carb (actually the mid sized carb in the test). You need to keep in mind that most generic 750 carbs come set up for s.b.c.'s. Also keep in mind that many, even most hipo motors will run higher volumetric efficiency than 85%. Many even exceed 100%. Design of the carb & intake also plays into the equation. I will look for the article & post what magizine it was in. It makes for some good reading.

Glenn

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Quote from Glenn-even most hipo motors will run higher V.E. than 85%. Many exceed 100%:

 

This is my take on V.E. The holley book I have goes thru the stages of preping a carb for Nascar; First the carb is tinkered w/straight out of the box till its efficient. Then their tech's tear it down & put each component on a flow bench where each dry passage is optimized; then the metering passasages for fuel are equally scrutinized: 20 hours later they have a carb running 100% V.E. I'm not aware of any of us doing that-how many of us have those resources....our eng's are not running 100% V.E. Granted the components now days are pretty efficient: a good set up may push the 90% area (IMO).

 

Anyway: I hate unresolved issues & this thread has forced me to do some late night reading: I guess this is good when anyone sets out to find the truth it has to be a good thing, right(?). This is what I learned from Two books-one from Rochester & the other from Holley.

 

Holley ran a comparison on 4carbs on a stock 69Chevelle w/325HP 396 (L-35) w/10.25:1 comp ratio, automatic & 3.31 rear gears. The four carbs were: Stock 750 Q-jet, 500cfm Holley 2bbl, 650 Holley 2bbl,800 Holley Dbl Pumper 4bbl & a 600 Holley 4bbl. The only modification was replacing the intak manfld w/Edelbrocks oval port single plane manifold specifically designed for chevy big blocks.

 

Shifting @ 5000 rpms these are the results:

 

1)Stock Iron Manifold/Stock 750cfm Q-jet

14.58 E.T @ 98.18mph

 

2)Torquer Mnfld/ Stock 750cfm Q-jet

14.47 E.T. @ 98.38mph

 

3)Torquer Mnfld/ 500cfm Holley 2bbl

14.59 @ 96.69mph

 

4)Torquer Mnfld/ 650cfm Holley 2bbl

14.39 E.T. @ 97.71 (quicker ET-Slower mph)

 

5)Torquer Mnfld/ 800cfm Holley Dbl.Pmpr.4bbl

14.24 E.T. @ 98.88

 

6)Torquer Mnfld/600cfm Holley 4bbl

14.21 E.T. @ 99.35 (Best ET & Fastest MPH)

 

For a bone stock engine: smaller is better. Obviously this engine couldnt take advantage of the 800cfm's ability to flow.

 

This is what else I found when attempting to understand why a 750 works when its theoretical CFM indicates it shouldnt.

 

My "Rochester" book touches on "More Air-More Power" & claims that atmospheric pressure has more to do on filling the cyl's than does the pressure drop created by a piston's downward motion on the intake stroke. Granted the two go hand-n-hand. But, Rochester goes on to say, "On a small carb its venturi and Throttle-Blade opening actually restrict atmospheric pressure being pushed into the cylinder & peak performance will actually take a nose dive after 5000 rpm's: this is great on a street car not going to see extreme high rpms.

 

If your looking for max power: Think big-Big Intake Mnflds, Big flowing Cyl.Heads, Big Cam as all of these components aid in the removal of restrictions on atmospheric pressure. For max.output it is essential to have the carb as large as possible (consistent w/the required & intended driving range). Using a lot of flow capacity can reduce inlet restrictions & increas V.E. @ WOT (Wide Open Throttle) at high rpm's.

 

What I understood the two above paragraphs to mean is this: A normally aspirated engine will never flow 700cfms, 750cfm's nor 800cfm's; the reason one would use such a large carb is to remove restrictions on atmospheric pressure entering the engine-this allows a denser charge of air to enter. Thus a properly jetted/tuned carb of larger cfm's can be used & be used efficiently for WOT @ Higher Rpm's.

 

Another test quite interesting in this book was a test on Air Filter Houstings & their restrictions on cfm @ WOT. Test done were on a 396.

 

1)No Air Cleaner....................713cfm

2)Closed Housing & single snorkel...480cfm

3)Closed Housing & snorkel cut off..515cfm

4)Same @ above but w/duel inlets....690cfm

5)Chevy Hi/Po open element..........713cfm

6)14 inch aftermarket open element..675cfm

7)Chevy Truck (Tall Base & Lid).....713cfm

8)Foam Type Lid (Domed flat type)...675cfm

*Test Done in 1971 (well before K&N filters)

 

**Holley: Still thinks most racers over carb their engines.

 

***I've learned that bigger components dont make more HP, but, bigger components remove the restrictions on atmospheric prssr which allows a denser charge into the cyl's & that is what makes horse power. Semantices-Maybe.

 

PS: I'm smarter now (I think thats a good thing)

 

 

[This message has been edited by Kevin Shasteen (edited February 06, 2001).]

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Guest Rick Johnson

2)Torquer Mnfld/ Stock 750cfm Q-jet

14.47 E.T. @ 98.38mph

 

5)Torquer Mnfld/ 800cfm Holley Dbl.Pmpr.4bbl

14.24 E.T. @ 98.88

 

6)Torquer Mnfld/600cfm Holley 4bbl

14.21 E.T. @ 99.35 (Best ET & Fastest MPH)

 

 

***I've learned that bigger components dont make more HP, but, bigger components remove the restrictions on atmospheric prssr which allows a denser charge into the cyl's & that is what makes horse power. Semantices-Maybe.

 

The books explanation about removing restrictions and decreasing pressure drops with larger cabs is exactly how I understand the CFM size concept. I must admit that I am surprised the 600 CFM beat both the Holley 800 and Q-Jet 750 for top MPH which is usually a great judge of top end horsepower.!!

 

I encourage Z owners to share their actual experiences with changing carb sizes, and everything else forthat matter.

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I must admit that I am surprised the 600 CFM beat both the Holley 800 and Q-Jet 750 for top MPH which is usually a great judge of top end horsepower.!!

[/b]

 

Well, keep in mind this was a, healthy, but moderate bone stock 396/325hp w/3.31 gears. They replaced its manifold but didnt replace its camshaft. If it had a camshaft to match the higher cfm carb & intake: its 1/4 mile times probably would've been better w/750 or 800cfm carb.

 

That 396 @ 5500rpm's is only capable of a theoretical 536cfm; the purpose of a larger carb is to remove restrictions of atmospheric pressure; which the carb & intake will do; however, the only restriction left was the camshaft.

 

Again; remember its a total pkg & we have to be honest w/ourselves so as not to over carb or over cam our engines-if we do, then we are only hurting our performance: on a street driven car that is.

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