Carb Sizing

onefastmustang · #1 (**permalink**) 06-25-2007, 05:18 PM

So i have this 351W that has a stroker kit to take it out to 392 + the overbore which I think takes it out to 395ish.. Anyhow I swing by the dyno shop today to see about getting a tune done. He looks at the car and tells me my carb is undersized. I have a Holley 600 and he says I need to up to an Avenger 770. Anyone agree with that??

jwd · #2 (**permalink**) 06-25-2007, 05:24 PM

Definately agree. That's what I have and it's a great carb.

Tongue Pirate · #3 (**permalink**) 06-25-2007, 06:50 PM

If you are pushing on the rpms then yes. I ran a 454 at 40 over with a 600 double pumper & it ran better than a 750 because of the low end response it needed due to the large gearing in that car. If you like to putt around town & go out on leisurely cruises then keep the 600 on there.

DAVID GAGNARD · #4 (**permalink**) 06-25-2007, 07:32 PM

I think a lot depends on your engine build and the rpms you want it to run the best at.......

I have tried no less than 6 carbs (from 600 cfm to 770 cfm)(both vacum and double pumpers) on my 350 hp, 351-W over the years and a 20 year old Holley 600 double pumper I rebuilt gave the best results, both on the drag strip and on the street mpg wise....

My motor rarely sees anything above 5500rpms........most street driving/cruising done at or below 3000rpms and she still gets 23 mpg cruising on the interstate, the motor is in a 65 Mustang Fastback.......

David

Tongue Pirate · #5 (**permalink**) 06-25-2007, 07:52 PM

I refuse to sell my 600 dbl pumper cause i like it so much. well, not quite as much as the webers

olddog · #6 (**permalink**) 06-25-2007, 08:51 PM

395 cid / 12^3 * 6000 rpm / 2 rev/4 strokes = 686 cfm

So at 100% volumetric effeciency and 6000 rpm you will suck in roughly 700 cfm. Adjust your estimated maximum rpm and effeciency from there.

Pete Munroe · #7 (**permalink**) 06-25-2007, 08:58 PM

You can ALWAYS spend more money later...like some of the fellas posted, what are your plans for the car? Street mainly?...keep the small carb for the torque.

I just had the opportunity to swap out my 750 dbl pmpr for a 650 dbl pmpr for the weekend.

(Hyde....care to swap out the Webers for a weekend?)

After an open track weekend at ButtonWillow raceway with the Pantera Club I would report:

the engine starts about the same, better?
seems to idle with less fumes
otherwise runs about the same/better? as with the 750.

The engine is 289/347, cam a mild roller, 224/228, TF heads, max rpm with the chip is 6000.

The 650 has 67's in the primaries/73 secondarys, the 750 has 73's/80's

There are not many points on a tight road track where you are at max RPM for more than a few seconds, and once I am at 6000, thats it...so more carb isn't going to help me rev higher. (could change the chip to 6500 but wear goes up with RPM so I would rather get lots of events on the engine rather than see the 500 RPM on my top speed)

For the street and a tight track, torque is better...for the drags HP is better.

You have the 600 carb in hand, RUN it...when you really need more to go drag racing or serious track, or get spanked BAD on the street...get the bigger carb and a dyno tune.

Above all, HAVE FUN, if you want braggin' rights...go ahead and get the big carb and dyno tune...see what the engine can do and frame the dyno sheet!

I would like to have dyno sheets for a range of carbs just out of curiousity...this stuff is Sooo much fun...

Pete

Tongue Pirate · #8 (**permalink**) 06-25-2007, 09:45 PM

Trade? You crazy?!!!

if you have the option have him dyno it with the 770 & he 600

YOu may find something in the results to sway you one way or the other.

onefastmustang · #9 (**permalink**) 06-25-2007, 09:47 PM

I do have that option.. the guy that owns the dyno owns the speed shop next store.. It won't be for a couple of weeks though as he is getting married and will be out of town. He wanted to be around when my car was done. Something about his dyno operator and carbs don't mix..

CHANMADD · #10 (**permalink**) 06-25-2007, 10:16 PM

Try this,
There is a formula that will get you very close to the proper cfm for your engine. The formula goes like this:

cubic displacement divided by 2 (ex: 351=175.5)

maximum rpm divided by 1728 (ex: 6000 rpm=3.47)

multiply those two figuires (ex: 175.5 x 3.47=608.98 cfm)

Hold on we are not done. You now have to factor in volumetric efficiency. A Nascar race engine runs at 85%-90% volumetric efficiency. A normal street car wil be 75-80%, depending on what you have done with your heads and exhaust.

Ex: 608.98 x 75%=456.74cfm

onefastmustang · #11 (**permalink**) 06-25-2007, 10:20 PM

Oh.. Now I am really confused..

CHANMADD · #12 (**permalink**) 06-25-2007, 10:23 PM

Ok ,What will be your Redline,and I will work it out for you.

onefastmustang · #13 (**permalink**) 06-25-2007, 10:26 PM

Well my current MSD ignition has the 6000 chip in it..

Pete Munroe · #14 (**permalink**) 06-25-2007, 10:28 PM

In my case, it was a "borrow" for the weekend...not a permanent swap per'se

The fun/economy is to find someone you can trade carbs with to see what happens as you go larger/smaller on carb size.

If it works/(or not) you have learned something without THROWING MONEY at the problem.

If I could push a button, my 750 would be a 650...but that is for my engine, for street and open track, not all out racing.

I would still REALLY like to try a set of Webers...but that is not a quick on/off...!

Pete

Pete Munroe · #15 (**permalink**) 06-25-2007, 10:34 PM

In my case, it was a "borrow" for the weekend...not a permanent swap per'se

The fun/economy is to find someone you can trade carbs with to see what happens as you go larger/smaller on carb size.

If it works/(or not) you have learned something without THROWING MONEY at the problem.

If I could push a button, my 750 would be a 650...but that is for my engine, for street and open track, not all out racing.

I would still REALLY like to try a set of Webers...but that is not a quick on/off...!

Pete

CHANMADD · #16 (**permalink**) 06-25-2007, 10:35 PM

Here it is,
395cu/2 =197.5
6000rpm/1728=3.47
197.5 x 3.47 =685 cfm assuming 100% efficiency,but take it you did a good job on cams head exhaust etc we will use 80%

685 x 0.8 =548 cfm
So if your carb is jetted and set right a 600 cfm will be perfect.Her's a link for you to find your carb and it will give all the correct jet and power valve sizes.
Good luck.It does not help to overcarb your motor!
http://www.mortec.com/carbs.htm

rdorman · #17 (**permalink**) 06-26-2007, 07:23 AM

I would say it is probably not worth spending the money. I recently read a test of a engine of similar cubes putting out about 460 ft lbs where they tested Holley carbs from 390 to 1000 cfm on the same motor. After getting the AF correct, there was very little difference between 600 and 1000 cfm in terms of power. The difference between the 600 and the 750 was only an average of 1 ft lb and hp with the peaks being the same. So you may see gains, but I would not expect much.... at least not enough for me to change the carb. The stronger signal of the smaller carb may be better at the engine speeds and loads that you run most of the time. Now if I didn't already have the 600 on the motor and tuned... it would be a different story!

CobraEd · #18 (**permalink**) 06-26-2007, 07:38 AM

But, . . . . . You want to get the jetting right. Jetting can make as big of a difference as air flow!!!!!!

.

onefastmustang · #19 (**permalink**) 06-26-2007, 05:05 PM

Well what I think I may do is have him dyno pull the car with the 600 and then swap the 770 in there and have him pull it with that one. Then we can see if there are any significant changes. He did mention that the 770 was easier to work on than the 600 but that may just be a sales tactic.

jwd · #20 (**permalink**) 06-26-2007, 05:14 PM

Deleted as to not get into a pizzing match.