Morgan6t9

I have heard many conflicting stories about carb sizes. first, I'll give you the details. I have a 69 coupe that I am doing a body off restoration (lose translation). I have the original SB on a stand. I bought a ZZ4 bottom end and put on GM Fast Burn heads with the LT4 Hot Cam Kit (Cam and 1.6 Roller Rocker Arms). This is very close to the limited edition ZZ430. GM ran the dyno on this engine with a 750 Holley. I think there is little arguement that a 750 will give you the most power when doing a dyno pull where the objective is to get big numbers at high RPMs. On the other hand, Demon states that a 650 will actually create more power in the "useable" RPM range. Demon and Inglese actually make a special carb for the ZZ4 (both 650 CFM) but since my engine is a little more that just a ZZ4, I have my doubts. I talked to an engineer at Inglese and he thought that their ZZ4 carb would need to be re-jetted for my particular combination. I will not be racing this car, but hate the thought of cutting myself short when I really want to get on it. I did the math and I think that my engine would need to run about 8000 RPM before the the CFM requirements were above 650. I did find an article that had my exact combination dynoed and they used a 750 Demon with 84 Primary and 91 Secondary jets and got nice numbers, but once again this is on a Dyno and not the road. http://www.findarticles.com/cf_/0/m0...41/print.jhtml

I am just really confused! Any thoughts would be greatly appreciated.

mrvette

650, oversize on the pot is nothing but a PIA on a street car.....

GENE

Morgan6t9

Gene,

What does this mean???

SanDiegoPaul

Dunno what he's trying to say. But I can tell you that I'm running a 795 CFM Edlebrock Q-jet (performer series) on mine. It is *very* streetable and the transitions from mild to wild performance are seamless.

The Money Pit

Budget minded as I am,I opt for the larger of anything I buy. This will give you something to grow into. If you stick with vacuum operated secondaries,the carb will size itself to the engine,even though it says 750cfm on the carb,the engine may only pull 600 cfm through. Till you stroke it,solid cam it or otherwise improve it. This is when you'll be saying"Shoulda gone bigger!" By the way the 795CFM carb I now run was on the 355 with no problems what so ever.


[Modified by The Money Pit, 5:08 PM 12/28/2002] :steering:


[Modified by The Money Pit, 5:09 PM 12/28/2002]

Robert N

:iagree:

I have a 650 DP on mine now (previous owner) and talked to the local speed shops about going to the 750 DP since the car had a 780 cfm vac secondary for a stock carb. The 650 is enough. Holley offers a calculation. My car, with a 6500 redline would only need a 655 cfm carb if it operated at 100% efficiency at that rpm range. Given that the car is rarely at that rpm range and the car probably operates at 85% efficiency AT BEST, the 650 is large enough.

I was also told that the 650 would offer greater benefits at the lower RPM ranges for "off the line" performance, but would suffer in high RPM runs. The 750 was the choice for the person who "wants to rule the street and the strip".

While the 750 is still a very usable carb on the street, I still didn't see the point.

For what it is worth, I jumped the primary jets to 71 from 67 and the back end now wants to race the front around corners from a dead stop. Pretty good compromise to me.

My .02.

redwingvette

Use big carbs for high RPM's and smaller carbs for better throttle response. Vacuum secondaries per above reply. There are web sites that calculate what size you need and it is based on: Cubic In, RPM and exhuast. A big carb won't do you any good if the engine can't push enough air through it.

Fevre

I have some inout on this question now but still have no 'that one' answer. I recently built up my pretty similar to what you have and started with a 600 vac sec holley which worked well and I felt like a I had great power witht he new build up. I decided to rebuild my 750 dp and give it a try. I switched the carbs out and left the sec unhooked on the dp. It felt like I changed engines agin the first time I hit it hard. I could not believe the diff. After hooking the sec up and tuning it, this engine screams pretty nice. Now it does not idle bad but still I think it is rich but cannot seem to get it to run as nice as the 600. I was seriously thinking of getting a 650 dp just to see how it would behave but the tpi bug has bit me. Do you know any motor heads around you with spare carbs laying around? I would suggest a 650 dp.

MotorHead

I had this discussion a while back and could not decide what carb to go with 650 or 750 CFM or vacuum or mechanical secondaries. I did the calculations for air flow on a 350ci motor etc. Many said that the 750 would bog the motor, it was too much carb etc.

A fellow forum member, MountainMotor told me to buy the Holley 750DP with mechanical secondaries and I could not be happier with the results. The motor idles fine and I can cruise around at 40MPH in 4th gear at 1500 RPM and hit the gas and there is no hesitation or bog it just goes. And if I hit it at higher RPM's you get immediate gratification.

Now I feel I can pass on to you some advice, if you motor is going to making the kind of power you say will need the Holley 750 and I would go with the mechanical secondaries. If everything is working correctly you won't regret it.

One more thing, a quadrajet (spreadbore) carb is a little different than squarebore in that it seems yocan run a higher CFM for any given motor.


[Modified by MotorHead, 4:39 PM 12/28/2002]

Schmucker

I put my vote on a 650. Why? Because my dad said so and he is never wrong. ;)

Morgan6t9

Keep the opinions coming! Lots of good advice. Right now it is about a dead heat between the two.

ED DINAPOLI

I did not see what intake you were using, but if you are using a single plane the 650 will run better. Dual plane you can run a 750 vacuum secondaries or dp depending on you setup and use. Chevrolet only used vacuum secondaries on there performance motors.
Ed

Titanium Rat

now how are cfms calculated,,,,,cubic feet pet min i know,,,, but i have a holley 750 on my vette now,,,but am going to a carter afb competition with my new motor. now the holley is a 750 cfm and the carter is an 850 cfm but if you look at the primary and secondary holes on both, the holley's total space is more where as the carter looks much smaller, but the cfm is higher on the carb with smaller total hole openings.. why is this?
-------------------------
something else,,,, on a 300 hp 350 cid motor thats 6000 max rpm holley calculation shows that the best carb for the engine is a 600 cfm,,,,,but a quadrajet that is suposed to flow 750 cfm comes on a stock 190 hp L48????the factory q-jet is not to much for that motor, but by holley calculations the L48 would need something like a 450 cfm carb??
anyone else confused?






[Modified by Titanium Rat, 5:33 PM 12/28/2002]

Robert N

I am as confused as you are TitaniumRat. The performance SB engines as mentioned by Ed cam with vacuum secondaries, but DPs are better.

ED DINAPOLI

Titanium Rat,
Thats a mouth full.:rolleyes I will IMO offer what I have experienced and some of what I know. All the carbs are measured in CFM as you indicated. How a manufacturer flow tested it may not be consistent with the test of another manufacturer. I have read some test where two carbs of a similar size did not produce the same hp on the same engine. We know that Holley carbs of the same size dont alawys run the same.
As far as the Q-Jet this was a one size fits all [most of the time]. It was not application sensitive, as the primaries were small and offered good response on small motors with out over carbing, due to the vacuum secondaries. It also offered good performance on larger motors with its good flow characterics. :)
I really cant tell you the reason a high performamce engine runs better on the top end with a carb that exceeds the theoritical CFM of that particular motor, but it does. How big is too much is the on going question.
Someone else offer some imput?
Ed


[Modified by ED DINAPOLI, 8:19 PM 12/28/2002]

Morgan6t9

I am Still confused.

Here is the calculation for Maximum airflow potetntially demanded by a given engine:

CFM = Engine CID X Max RPM / 3456

Example 350 CID engine with a 8,000 Max RPM would need a 810 CFM Carb if the engine was 100% volumetrically effiecient.

Most performance street engines are not more that 95% V.E. So in the example above if the engine had a VE of 95%, the 810 CFM would need to be multipled by .95 to equal 769.5

Still loads of underlying issues!!!

Shock

So, when are you revving your engine to 8000? I am running a Holley 750 Pro performance carb with vacumn secondaries and it seems great. Better overall performance and fuel mileage than the 650 Holley spreadbore it replaced.

Mike

Crash Dummy

Does anyone know what the DP 650CFM stock # is comparable to a Holley 4150 series vacuum secondary carb? I have a 355 going in shortly with Dart heads & Weiand 7530 single plane intake that calls out for a 4150 series carb and I'm not convinced to go to 750CFM.

Would this be comparable.... E-bay auction....
http://cgi.ebay.com/ebaymotors/ws/eB...tem=1875504684


[Modified by bgrice, 6:49 AM 12/29/2002]

BSeery

650 CFM should do just fine.

By the way, that 795 CFM Q-Jet is has smaller primary ports and larger secondaries. That way you get the great street driveability and response of a smaller carb, and the awesome top end of a bigger carb without killing the throttle response.

Since Holleys are all equal size for all 4 ports, going too big kills driveability for those very few times you have it open all the way.

650 if its a street driven car, 750 if it's a race only.

JMO.

Titanium Rat

[quote]
Titanium Rat,
Thats a mouth full.:rolleyes


is that meant in a good way :blueangel: or a bad way :mad ,,,i hope not in a bad way :nono: