76projectstingray

Hi, I'm looking for some confirmation on my carburator pick.

I have a stock L-82 , automatic.
I do have headers and dual exaust.
I have an RPM air gap intake that I want to put on the car this summer and want to change the carb to a holley 670 street avenger.

My question is, next winter i'm looking at putting in a new roller cam and brodix heads and want to make sure the 670 cfm will still be fine with the new topend? I pretty sure it will be enough but just wanted some opinions.

Thanks Jim

yel76low

A 350 running at 7000 rpm with a volumetric efficiency of 95% (which is very high) is 673cfm. If VE=.9, 638cfm. You won't be running above7k rpm, I hope.....

Seems like 670 should be enough. But why are you changing? What do you have now?

ED69ray

I bought a 650 Holley for my recent 350 build - it looked like a nice size carb for a moderate 350. I figured it would give up a few ponies near redline but it would make better midrange and torque vs a 750. With that said,,,,,,,,,,,,,,I was persuaded to send the 650 back and have a 750 tailor built. I used this guy:

http://www.chucknuytten.com/pro_street.htm

my motor guy hooked me up and he built me a real nice 4150 model 750 DP. She did pretty well on the dyno with the 750

http://forums.corvetteforum.com/c3-t...440-440-a.html

You might do best to pick up a used 600-650 for now and then spend the cash on a new carb when you do the heads.

Good Luck, Ed

DRIVESHAFT

The 670 will be fine, but the car would likely be slightly quicker with a 750.

yel76low

Why do you say that?

Is the 750 inherently a better carb? Does it have less frictional resistance than the avenger would have (not being sarcastic, being serious here)? Do carbs just in general perform better if you size them 10-20% larger than you'd actually need? Because he won't use more than 670 cfm, realistically, he won't even use 600! (350ci, 6000rpm, 95%VE = 577cfm) So a 670 is already 14% larger than he needs, and a 750 is 23% larger.

Or, are you just telling him to get a mechanical secondary instead of a vacuum secondary (avenger is vac, right?) That I understand, but the sizing in cfm is significantly more than he'll need.

MotorHead

Don't want to sound rude or anything like that but those mathematical calculations are not as sound as what has been tried and worked. A 750DP with mechanical secondaries ( 4779 ) works fine on any mild to wild 350ci - 383ci, I used it on my 355ci and my 406ci

cargo247

maybe this will help????
http://www.lbfun.com/warehouse/tech_...Carburetor.pdf.

Todd

dmaaero

I just went through this with a professional carb builder, what i was told is a 750 will be fine as long as you have some gear. If you have 3.08 or similar gears go with the 650.

DRIVESHAFT

Lets just say that mathematical calculations and the drag strip dont always agree.
Also, it is a lot easier to hit 100%+ volumetric efficiency than the magazine would have you believe.
Jims car will have Brodix heads, a roller cam, a Performer RPM intake, and headers.
I would be very surprised if his engine did not exceed 100% VE.

ED69ray

ironic you say that, on my recent dyno runs my 360ci peaked at 100.5 VE% - that was at 4600rpm and happens to be when I posted my best torque #


I am making 412ftlbs at 3200rpm - the 750 DP does not appear to be hurtin it one bit in the midrange

Good Luck, Ed

76projectstingray

Thanks for all the replies,

Now i'm second guessing. I have used a 750 dp before on a chevelle i had, but was not sure if it was too much.

My L-82 is a TH400 auto and I do have 3:36 rear.
I was going to go with the vacuum secondaries this time due to the auto, but should i just go with the 750 DP? I will be putting in a 2500 stall converter when i do the heads and cam.

Thanks again.

76 VETTE

I am not sure about all the cfms but another concern might be hood clearence .Iwas thinking about a holly for my 76 with my Edelbrock performer intake and didn't have enough hood clearence.

76projectstingray

I have a drop base air cleaner that I hope works, If not I might add on a L88 or cowl hood scoop.

yel76low

Not rude at all... that's why I'm asking the questions. Mathematically he doesn't need even close to a 750DP, but that's what everyone with experience recommends. I'm wondering what the disconnect between the two is. I can think of a few reasons (efficiency close to the max flow, better because of the mech. secondaries, or simply just a better carb design than the avenger) but no one ever says anything except something along the lines of "get a 750DP."

1972warship

The speed shop that I use(which built my current engine) will see how many cfm an engine uses on the dyno, and then recommends a carburetor about 100 cfm more. Not sure if they know what they are talking about, but a lot of racers use their engines and win.

baxsom

maybe they are just good drivers.

on VE. i am not an expert but i just dont think its possible to go over 100% unless you are using forced induction

rham

Nascar does it and even a good street engine can do it if spec'd right. You can only hit it at a small rpm range (around peak torque). Exhaust scavenging and cam overlap has more effect on intake velocity than the intake stroke itself, the cylinder is still drawing in intake while the compression stroke has started (look at intake closing points of cams) this is how it is possible to draw in more than the cubic inch of the cylinder, ie more than 100%.

ratflinger

It's all about air flow, that's why a 750 has been recommended. Your engine might be be pulling on the 650 at the upper end, but the 750 will still be flowing fine. The 750 will not deliver more air than your engine can handle. A properly jetted & tuned 1100 would work too. One can tune & jet a larger carb to work just fine, not much can be done with one that is too small.


BTW - the original qjet was probably a 750.

yel76low

This is what I'm talking about. Why do they recommend 100 cfm more?

It is possible... it has to do with the inertia of the air flowing in. If you have high velocity air flowing into the cylinder, you can (somewhat, not a lot) compress the air that is in the cylinder to higher than atmospheric pressure because it will require pressure higher than atmospheric to slow down the oncoming air. This is similar to a 'water hammer' event in a pipe where you open a closed valve that has a high pressure source of water. You can significantly exceed the pressure source's original pressure downstream, as you compress the air that was originally in that region. (EDIT: Note: You can significantly increase the pressure in water hammer because water has approximately 1000 times more inertia that air (per unit volume) due to it's high density. You can not significantly compress in the cylinder using the incoming air, but you can a little.)

However, this is only going to happen in a small range of rpm, and not at the top of the curve. ED69ray said he had 100.5% ve at 4600 rpm, but I can virtually guarantee you he did not have that at 6000 rpm. (Now, with a different cam it is possible that he could, but we don't have race engines, so it isn't going to happen on the street.) ED, if you are still following this thread, I'd like to see the air flow rate (or VE) graph vs. engine speed.

I understand that you can use a larger carb, and that the qjet is a 750. But you saying it's all about air flow is exactly my point. If his engine can't handle more than 600 cfm, then there is no reason to use a carb that is larger than that (again, I keep stressing, all other things being equal.)

Even Holley's recommendation for a "very efficient drag race or circle track engine with quality porting, high lift/duration ... " for a 350 running 6000 to 6500 rpm is a 600DP. http://www.holley.com/types/4150%20HP%20Series.asp

MotorHead

I'm not saying anything about air flow or using charts or reading magazines. And the last thing I would ever do is phone Holley tech line. The guys who designed and know how to tune these carbs are long gone.

When this forum was only around for a year or two I bolted a Holley 750DP with mechanical secondaries on Victor Jr single plane intake, and a solid flat tappet cam on my mild 355ci. Everyone thought that was crazy.

A forum member by the name of MountainMotor told me that carb would be the best for my combination and he was right. This guy forgot more than most know about motors.

So you can argue all the theoretical points about this that you want, proof is in the pudding as they say
Chassis dyno



I am talking from personal experience and have use a 750 Holley DP on a mild 355ci to a wild 406ci without any problems. A little tuning is needed as would be with any carb you bolt on.

You should never take a carb, no matter what is recommended and bolt it on your motor and expect it to run perfect. These are aftermarket carbs and have to be tuned to your specific motor. A wideband O2 sensor like the LM-1 is required to do this properly