what cfm to run?

[OldSchoolin86]

Three motors:

460ci Ford completely stock
4500rpm red line

350ci GM totaly built(cam, porting, higher compression, free flowing exhaust)
7500rpm red line

360ci mopar mildly built(performer intake, free flowing exhaust)
6000rpm red line

Can you match the proper size carb to the motors? What would you put on them? What is the max cfm you think these motors will benifit from?

Good luck!

[MTS]

460 ford:479 cfm
350 chevy: 721-759 cfm
360:562 cfm

[OldSchoolin86]

I said "max".

[MTS]

oh wait then i did do i right,.,i think,,,am i in the relitive ball park :D?

[Studytime]

Oh, shoot! I'm up to my neck in a thermo project and will be all night and Sunday. What are the chances that you guys will wait until I can post my answers!?

Mr. OldSchoolin'86, but what if VE goes over 1!!!!? I remember reading about a BBF that went from 140% VE down to like 110% and gained all sorts of power! Think about that one!!! Sounds weird, but there's a great explaination why.

Let me get my answers up before you guys post your *opinion* as to what size carb should be run.

Studytime

[Studytime]

I'll just guess before I whip out the calculator... but these are not my final answers...

On the ford, I'd put a model 1850 Holley.

On the chevy, I'd put a 4779 Holley with the proform mainbody ( :p This is the one I run on my truck). I'd thin the throttle shafts and smooth/flatten the screws holding the throttle blades on. Here, we're taking a gain of ~70 cfm above a regular 4779. For a real world application, this would be a SUPERB choice on what carb to run.

On the mopar, I'd machine a block of aluminum to plug up the whole intake. haha, only kidding. I'd stick a 4776 or a 4776 on the Dodge.

It would be nice to know what kind of vehicles we're talking about too.

Studytime

[Studytime]

460 ford:479 cfm
350 chevy: 721-759 cfm
360:562 cfm


yeah, those numbers look right. I'm guessing those are at 100%VE.

Okay, post what the "answers" are. This was a cool exercise.

BTW,

1850 - vacuum secondary 600cfm
4779 - 750 cfm double pumper
4776 600 cfm double pumper
4777 650 cfm double pumper

Studytime

[OldSchoolin86]

Mr. OldSchoolin'86, but what if VE goes over 1!!!!? I remember reading about a BBF that went from 140% VE down to like 110% and gained all sorts of power! Think about that one!!! Sounds weird, but there's a great explaination why.

Not gonna happen on the motors I listed. I could make it, but I didn't list anything that would make it happen.


It would be nice to know what kind of vehicles we're talking about too.

Doesn't matter what it's in, I listed everything you need for cfm.


Let me get my answers up before you guys post your *opinion* as to what size carb should be run.

CFM is no "opinion".

[Studytime]

Not gonna happen on the motors I listed. I could make it, but I didn't list anything that would make it happen.


Of course not on those motors. I was talking about a totally different engine. I doubt it could ever happen on a N/A engine. I'm sure you're familar with the situation I was talking about- excessive overlap.

Studytime

[Studytime]

Not to get into a debate, but it depends A LOT as to what carb you're going to run if your vehicle is heavy or a light weight.

This goes back to what carb to run being an OPINION and a compromise.

Studytime

[OldSchoolin86]

Not to get into a debate, but it depends A LOT as to what carb you're going to run if your vehicle is heavy or a light weight.

This goes back to what carb to run being an OPINION and a compromise.

Studytime

You can debate all you want, I just don't want the ones trying to learn here get confused by you. CFM has nothing to do with what vehicle or the weight of it or anything else other then what I listed. Tuning those cfm's def need that info but I'm not looking for tuning answers.

[Studytime]

This should make you happy :cool: ,

Edit: Please leave this part out for a couple of days please. Osin86 ;)


Yes, an engine's air demand is a relative hard number. Using that number to decide what carburetor to go with IS an opinion.

After re-reading what you've typed I think we are both right here.

Please, see the question directed at you in the trike forum.

EDIT: Just read you with to move our friendly debate to over here. I await your respose as to why you thought my statements were off. :)

Studytime

[OldSchoolin86]

Here you go buddy:
Additionally oldschoolin' validate why you siad this, "other 2 are pretty off and they are all missing one major part of the formula".If this motor your talking about is running pretty much stock rpms your really high. If your turning the extra R's(the missing part) then it may need the bigger carbs.

[Studytime]

Here you go buddy:If this motor your talking about is running pretty much stock rpms your really high. If your turning the extra R's(the missing part) then it may need the bigger carbs.

Yeah, I'm talking close to stock engine speed. <6500 RPM.

What size do you suggest for a 1 hp/ci 350 chevy? Everyone knows now that with today's designed cylinder heads and more aggresive lobes that larger carbs certainly show better gains. This is why I mentioned a 800 CFM carb.

I would certainly not say "pretty off". My experience backs up what I said and I stand by it.

Let me just say again, that a larger carb even on a stock motor will show a power increase, say 1500 RPM, below stock max rpm. You'll trade some torque down low for it. When I say "stock", I mean a mild performance motor with factory heads/cam but with a 4-bbl and long tubes.

Studytime

[OldSchoolin86]

Yeah, I'm talking close to stock engine speed. <6500 RPM.

What size do you suggest for a 1 hp/ci 350 chevy? Everyone knows now that with today's designed cylinder heads and more aggresive lobes that larger carbs certainly show better gains. This is why I mentioned a 800 CFM carb.

I would certainly not say "pretty off". My experience backs up what I said and I stand by it.

Let me just say again, that a larger carb even on a stock motor will show a power increase, say 1500 RPM, below stock max rpm. You'll trade some torque down low for it. When I say "stock", I mean a mild performance motor with factory heads/cam but with a 4-bbl and long tubes.

Studytime

CFM is probably the biggest mis-understanding in the building/tuning world. The motor you described would never use more than 660cfm. That's at 100%, in reality it would be more like 600.

[Studytime]

CFM is probably the biggest mis-understanding in the building/tuning world. The motor you described would never use more than 660cfm. That's at 100%, in reality it would be more like 600.


Yes, if you do the calculations you are correct, but in real world applications on an engine dyno or at the track bigger carbs do help.

I have a 1hp/ci 350 with a ~820 CFM carb I run on it. It runs WAY, WAY better with the big double pumper than with my vacuum secondary 600 carb. In this case, it has better bottom end and certainly makes more PEAK power with the bigger carb. This carb is built around a 750 Holley so it still has great velocity. An out of the box 800 cfm holley would make my truck go to crap. So, like eluded to tuning is everything.

The formula aside, if you're making a street machine from warm to HOT a 750 Holley is a safe bet even if you're only spinning it to 5500 RPM.

Studytime

[OldSchoolin86]

Yes, if you do the calculations you are correct, but in real world applications on an engine dyno or at the track bigger carbs do help.

I have a 1hp/ci 350 with a ~820 CFM carb I run on it. It runs WAY, WAY better with the big double pumper than with my vacuum secondary 600 carb. In this case, it has better bottom end and certainly makes more PEAK power with the bigger carb. This carb is built around a 750 Holley so it still has great velocity. An out of the box 800 cfm holley would make my truck go to crap. So, like eluded to tuning is everything.

The formula aside, if you're making a street machine from warm to HOT a 750 Holley is a safe bet even if you're only spinning it to 5500 RPM.

Studytime

Like I said, cfm is one of the most mis-understood builder/tuner items there is, lol.

[MTS]

yeah, those numbers look right. I'm guessing those are at 100%VE.

Okay, post what the "answers" are. This was a cool exercise.

BTW,

1850 - vacuum secondary 600cfm
4779 - 750 cfm double pumper
4776 600 cfm double pumper
4777 650 cfm double pumper

Studytime
actually no they where not at 100%VE becuse for 1 a "stock" motor dose not run at 100%VE, thats just a fact, also not even the warmed over 350 would be pusing 100 maby closer to 95 which i belive if i remember correctly is what i based it on, its all about guessin basicly with out a actual computer and what not to measure it, as i said "rough" ;)

[Studytime]

Just wanted to try out this picture posting feature that we have;

plus these are the carbs that I run on small blocks. They flow around 810 CFM.

Studytime

[OldSchoolin86]

lol, I forgot about this. Anybody else want to take a stab at this?

[200x Basket]

Three motors:

460ci Ford completely stock
4500rpm red line

350ci GM totaly built(cam, porting, higher compression, free flowing exhaust)
7500rpm red line

360ci mopar mildly built(performer intake, free flowing exhaust)
6000rpm red line

Can you match the proper size carb to the motors? What would you put on them? What is the max cfm you think these motors will benifit from?

Good luck!


well differnt cams pull differnt vacuums. the Holley CFM rating is based off of a certain vacuum. so the CFR rating on a carb is really just a generic number and not very accurate. that is why i always call rick at www.pro-system.com

here is some of is advice

http://www.pro-system.com/pjames011900.html

Choosing A Carb Size

One of the most asked questions we receive at PRO-SYSTEMS is, how big will my carburetor be, how much will it flow? Most customers are expecting to hear big numbers. They want to hear that their 4150 will flow 1150 cfm or that their 2 inch throttle blade Dominator will flow 1400. Unfortunately, that's not going to happen. In all the designs I've flowed, designed, calibrated and tested, it's just plain physically impossible. Builders will give out dry numbers (meaning cfm ratings without the disturbance of the fuel cone) but these numbers are not what the engine actually sees and are for reference only. Some builders still use the inflated cfm creating 28 inch rating used by an aftermarket manufacturer. Enough has been said on that subject.

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

Remember, if we have a heavy vehicle and a two speed we will require a slightly smaller carburetor, than a light vehicle and a stick. Also, if we have a booster/emulsion/air bleed configuration designed to operate and shear fuel at lower rpms we can increase the cfm. An increase in cfm is usually a guaranteed increase in power, but it takes a design that'll still pull and shear fuel at launch to pull that off. That's when the builder starts altering the entry and exit angles of the booster, the emulsion layout, air bleed configuration/well diameter, etc. All in an effort to fan the fuel cone to increase impact, supply the proper air to fuel ratio throughout the rpm band and emulsify the mixture prior to decrease plating for the air speed being encountered. All those mods cost money and they're not easy to do.

But return on investment is the deal when purchasing a carburetor. Oftentimes a customer is thinking of purchasing a this or a that, when the same money spent customizing his current model will yield more performance.

Remember, as we talked about earlier, the loss of torque we record at launch and the subsequent rate of acceleration you lose at the start of the race will be carried throughout the rest of the shifts. So a good leave (excellent fuel shear and proper air to fuel ratio at launch) is getting the reciprocating mass to carry this rate of acceleration to reduce E.T.'s. But if you have too much restriction at the finish-line, the mass will be slowed as a result and E.T.s will increase and none of us want that.

[OldSchoolin86]

Well I think you just finnished it up for me, lol. Studytime wanted to do the same thing but I asked him to wait untill some other got to try first. I was going to start out with the standard generic for the street numbers (Holley math) and then get into what studytime was working on. The motor specs I gave was enough to get some close numbers. Cams will change the VE but RPM play a way bigger role in cfm's. RPM's probably is the biggest misunderstood part of the equation. I can count numerus times that I've heard of bad combo's like a 750 being on a mild street 6000rpm 350sb. That's just a waste and you could have much better launches with a smaller carb. Just because a buddy makes it work doesn't make it right.

Good work guys, once again I am impressed with the intelligent answers given. Anybody with a question about a carb size on thier project should be able to get into the ball park with the above info. Feel free to add anyother questions around this topic if you want to.

[cliff2302]

Any ideas on mechanical fuel injection?

[OldSchoolin86]

VE is pretty universal on Natural motors but what do you have in mind?

[200x Basket]

Any ideas on mechanical fuel injection?


i have run an enderle buzzard catcher on a 565 cubic big block chevy. i have also tunes several ron's toilets. what do you have in mind?

[Studytime]

... I can count numerus times that I've heard of bad combo's like a 750 being on a mild street 6000rpm 350sb. That's just a waste and you could have much better launches with a smaller carb. Just because a buddy makes it work doesn't make it right.

Wait... for a second I thought you were talking about me, but then I realized you weren't because I only spin my 350hp/350ci to ~5800 and have a carb that flows 810 cfm.

:)

OH, and it makes more power throughout the entire curve; however, I do agree that too much carb can reach a point where it hurts. Such as this guy I know with a <9/1 318ci with a huge single plane and a 750 double pumper and a big roller cam (even less dynamic compression ratio).

Studytime

[cliff2302]

Well, right now i have a 327 small journal motor. I'm in the market for pistons right now. I'm looking at somewhere around a 10:1 comp. ratio, but it might be anywhere from 9.5-10.5:1. I have a set of ported 461 fuelie heads. The cam is going to be around .488 solid lifter. What i'm looking at are the old hilborn/enderle/crower fuel injection. The kind with the stacks, not a scoop. Do you guys know anything about these? I know they come with different diameter runners, but i'm not too sure what they will affect. I'd assums i'd need a set with a small diameter because the motor isn't super high performance, just a healthy SBC. Any ideas?

[Studytime]

Yeah, I have a little info to share- more if you have questions.

Small runners with the same volume going through them provide high velocity for better cylinder filling. This give you more torque. As engine speed increases the small runners cannot flow enough air and then become a restriction. ***think TPI***

Why not take advantage of the vastly superior cam grinds we now have available? I am "in the know" about cams and might could help you out if you're in the market for suggestions.

Studytime

[200x Basket]

for cams go see www.flowtechinduction.com tell ed curtis that chris vaught sent you

[OldSchoolin86]

Wait... for a second I thought you were talking about me, but then I realized you weren't because I only spin my 350hp/350ci to ~5800 and have a carb that flows 810 cfm.

:)

OH, and it makes more power throughout the entire curve; however, I do agree that too much carb can reach a point where it hurts. Such as this guy I know with a <9/1 318ci with a huge single plane and a 750 double pumper and a big roller cam (even less dynamic compression ratio).

Studytime

You are running too much cfm.

[Studytime]

You are running too much cfm.


and this would me marked by what?

STudytime

[200x Basket]

http://impact-designs.net/2tuff/images/145.jpg so i have a 418 cubic inch motor that spins to 6400 rpm. it is a drag race motor with the following cam spec

Gross Valve Lift 1.60 Ratio: .6528” .6608”
Specs at 106 Degree Intake Centerline:
Valve Timing at Open Close
.050” Tappet Lift: Intake: 25* BTDC 57* ABDC
Exhaust: 70* BBDC 22* ATDC


the carb is a custom carb by pro systems. the plugs are perfect and i run 7psi FP at the end of the track. what size carb do i run?

[cliff2302]

what cam/cams would you guys suggest? I would LOVE to get a roller cam, but unfortunitly thats not going to happen for a couple of years.

[Studytime]

what cam/cams would you guys suggest? I would LOVE to get a roller cam, but unfortunitly thats not going to happen for a couple of years.


Well, if any body was to suggest a cam this far along then I wouldn't listen to what they had to say.

You'll have to feed us 10 minutes of typing first. What kind of set up is this 327 going into? Be as descriptive as possible.

What are you looking to get out of it? What kind of idle quality? Drive it how often? What's your intake, exhaust, etc?

Studytime

[Studytime]

http://impact-designs.net/2tuff/images/145.jpg so i have a 418 cubic inch motor that spins to 6400 rpm. it is a drag race motor with the following cam spec

Gross Valve Lift 1.60 Ratio: .6528” .6608”
Specs at 106 Degree Intake Centerline:
Valve Timing at Open Close
.050” Tappet Lift: Intake: 25* BTDC 57* ABDC
Exhaust: 70* BBDC 22* ATDC


the carb is a custom carb by pro systems. the plugs are perfect and i run 7psi FP at the end of the track. what size carb do i run?

I'd rather not make any suggestions at all for a drag motor. Sounds like what you have now is working okay. Good luck.

Studytime

[OldSchoolin86]

so i have a 418 cubic inch motor that spins to 6400 rpm. it is a drag race motor with the following cam spec

Gross Valve Lift 1.60 Ratio: .6528” .6608”
Specs at 106 Degree Intake Centerline:
Valve Timing at Open Close
.050” Tappet Lift: Intake: 25* BTDC 57* ABDC
Exhaust: 70* BBDC 22* ATDC


the carb is a custom carb by pro systems. the plugs are perfect and i run 7psi FP at the end of the track. what size carb do i run?
Don't know what your running but anything over 950cfm is unnecessary.

[cliff2302]

sorry, i wan't thinking. Its going in my 64 Nova, i thought it was 2000lbs, but most people don't agree with that, so we'll say its 2200-2300 lbs. $ speed close ratio tranny, 373-411 gears. 11 inch slicks. I will drive this on the street occasionaly, but it is going to be primarily my race car for friday nights. if i don't get an injection this winter, i have an edelbrock 600cfm electric choke carb that i will keep on the motor and a wiend iual plane or an edelbrock single plane intake i could also use. Here are the motor specs again. 327 small journal motor. I'm in the market for pistons right now. I'm looking at somewhere around a 10:1 comp. ratio, but it might be anywhere from 9.5-10.5:1. I have a set of ported 461 fuelie heads. For ignition i have a vertex magneto, or a reworked HEI distributer from a very reputable local builder. i'm probab;y going to use the mag though cause i'm going for a nostalgic look.

[OldSchoolin86]

sorry, i wan't thinking. Its going in my 64 Nova, i thought it was 2000lbs, but most people don't agree with that, so we'll say its 2200-2300 lbs. $ speed close ratio tranny, 373-411 gears. 11 inch slicks. I will drive this on the street occasionaly, but it is going to be primarily my race car for friday nights. if i don't get an injection this winter, i have an edelbrock 600cfm electric choke carb that i will keep on the motor and a wiend iual plane or an edelbrock single plane intake i could also use. Here are the motor specs again. 327 small journal motor. I'm in the market for pistons right now. I'm looking at somewhere around a 10:1 comp. ratio, but it might be anywhere from 9.5-10.5:1. I have a set of ported 461 fuelie heads. For ignition i have a vertex magneto, or a reworked HEI distributer from a very reputable local builder. i'm probab;y going to use the mag though cause i'm going for a nostalgic look.

Cliff, I'm gonna chime in on the compression ratio here. If you have the 461 fuelie heads and not the 461x then you have a 62cc combustion chamber. To achive your target compression you want to use a flat top piston(0 dome). Now you have a little play here with your compression. If you use a 1044 Fel-pro head gasket(.051 compressed thickness) you'll get a 10.23:1 compression ratio. That's the one I recommend. You could also use a 1010 fel-pro(.039 compressed thickness) and you'll have a 10.55:1 compression ratio. For you application I would stay with the 10.23:1. These numbers are for a stock bore. If your .030 over then they will be 10.34:1 and 10.7:1.

[Studytime]

What converter are you running on your car? Depending on what camshaft you go with you could certainly get away with 10.5/1 compression on the street provided you intake valve closed late enough. This way you can go with a large cam and still have GREAT low-end torque because you maintain a high dynamic compression ratio.

What converter did you say you were running? Did the shop that flowed your heads provide any head flow data?

EDIT:: What RPM do you leave the line at? Are you comfortable revving to 3000+ and dumping/slightly feathering the clutch?

Studytime

[OldSchoolin86]

What converter are you running on your car? Depending on what camshaft you go with you could certainly get away with 10.5/1 compression on the street provided you intake valve closed late enough. This way you can go with a large cam and still have GREAT low-end torque because you maintain a high dynamic compression ratio.

What converter did you say you were running? Did the shop that flowed your heads provide any head flow data?

EDIT:: What RPM do you leave the line at? Are you comfortable revving to 3000+ and dumping/slightly feathering the clutch?

Studytime
You know study, with a 600cfm carb he's not gonna have the RPM's to make the window your going for here. I agree where your going but not for his set-up.

[Studytime]

You know study, with a 600cfm carb he's not gonna have the RPM's to make the window your going for here. I agree where your going but not for his set-up.


Well, actually I'm going to tell him to get a carb like I run. Next, tell him where i order from and that I have an extra one all ready assembled and ready to go. It was on a big block for four weeks.

Yeah, his carb will certainly hold him back... it's small PLUS it's an edelbrock.

Oh, what valve springs are you running? The spring pressure at the seat and the max amount of lift are two pertinent pieces of information we'll need to know. Also, depending on how they are installed, you can affect the safe amount of lift you can run.

Studytime

[Studytime]

Oh, let me just point out about my favorite carb. It's essentially a 750 cfm carb. It has the same diameter venturi, but with no choke housing flows more air than stock.

Plus with the proform mainbody it has all of the features that a $700 carb has, but for less.

Studytime

[OldSchoolin86]

Oh, let me just point out about my favorite carb. It's essentially a 750 cfm carb. It has the same diameter venturi, but with no choke housing flows more air than stock.

Plus with the proform mainbody it has all of the features that a $700 carb has, but for less.

Studytime
I don't think your paying attention here, he doesn't want another carb. He's only using that untill he can get his injection system. Besides a 750 on a 327 is a horrible idea.

[Studytime]

I don't think your paying attention here, he doesn't want another carb. He's only using that untill he can get his injection system. Besides a 750 on a 327 is a horrible idea.


No, it's not a "horrible" idea. You also said it was too large for my motor, but you never said what this would cause. Infact, I'd even let the above poster try it and return it if he wasn't happy- provided he's paying shipping, of course. You're forgetting this is a VERY light car and it has a standard tranny.

As for the increase in CFM, this comes from better molding of the area around the venturi. An increase in venturi size will simply mean a lower pressure drop through the venturi of the carburetor. If you can still manage good fuel atomization, then this can happen great- i.e. my case with my truck. After all, his car is a RACE car.

I'd love to debate this with you. You're overlooking an important two important aspects. His VE will be higher (well, not that big of a factory) and most importantly CFM required is a function of RPM. Yes, my carb is too big to go WOT at 1300 RPM, but I never do this. At 3000 RPM plus a 750 will work way better than you're thinking for him. It's also all about they system. If he has 1.625" headers on this thing with 2" pipes and only spins it to 5000 rpm then, you would be right- but he says it's a race car.

Studytime

[200x Basket]

Don't know what your running but anything over 950cfm is unnecessary.


actually you are right and wrong, sort of. make sense? my carb started live as a 950 HP main body. everything else is constom. it actually flows 880 cfm on my motor.

[OldSchoolin86]

actually you are right and wrong, sort of. make sense? my carb started live as a 950 HP main body. everything else is constom. it actually flows 880 cfm on my motor.

880cfm is a great match for your motor. By the way, I love the looks of your car.

[OldSchoolin86]

No, it's not a "horrible" idea. You also said it was too large for my motor, but you never said what this would cause. Infact, I'd even let the above poster try it and return it if he wasn't happy- provided he's paying shipping, of course. You're forgetting this is a VERY light car and it has a standard tranny.

As for the increase in CFM, this comes from better molding of the area around the venturi. An increase in venturi size will simply mean a lower pressure drop through the venturi of the carburetor. If you can still manage good fuel atomization, then this can happen great- i.e. my case with my truck. After all, his car is a RACE car.

I'd love to debate this with you. You're overlooking an important two important aspects. His VE will be higher (well, not that big of a factory) and most importantly CFM required is a function of RPM. Yes, my carb is too big to go WOT at 1300 RPM, but I never do this. At 3000 RPM plus a 750 will work way better than you're thinking for him. It's also all about they system. If he has 1.625" headers on this thing with 2" pipes and only spins it to 5000 rpm then, you would be right- but he says it's a race car.

Studytime

lol, no it is a "horrible" idea. This is why I started this thread. Most people do over do it like you are. Look at 200x Basket's motor and what carb he runs with it. That's a good match. When you go big like you like to, it's harder to tune and you get less responce. Your forcing your motor do deal with more flow then it's ment to. With the right size carb you'll get better launches and faster acceleration.

Out of the 2 intakes that cliff mentioned, neither one is any good over 6500rpm. You'd have to spin that little 327 almost 8000 rpm to start thinking about a 750. Please stop giving bad advice to people looking for good answers. Your not even listening to what he's asking.

[Studytime]

Not listening? Well, actually this was a carb thread and we made it back to discussing carbs- so no harm done.

I'll post more when I get back- running to pick up some black vinyl to cover my seat.

More later,
Studytime

P.S. Prepare to be proven wrong! :) :) Only kidding.

[Studytime]

Will a 750 cfm carb work on a 383 at 2500 RPM?

Studytime

[OldSchoolin86]

Will a 750 cfm carb work on a 383 at 2500 RPM?

Studytime

Sure but it won't be optimum.

[Studytime]

Sure but it won't be optimum.


What would be in your opinioin?

Studytime

[cliff2302]

studytime, wouldn't i choose my valve springs to match my cam profile? I was walking back from class today when this hit me like a brick. I can't beleive i forgot to ask about this. We have been having problems with this for years. My dad has a 94 S-t0, 700 miles on it when the owner took out the engine. He bought a 400 SBC, turbo 350, 2000 RPM stall converter. The motor has stock heads and a full edelbrock performer package-cam, valve springs, intake and 600cfm carb. My dad made the headers, im not sure of the primaries but they aren't overly big so i'd assume they are the most common size (1 5/8? i can't remember) The headers are jet hot coated and the problem we have is that some of the coating burnt off. We're speculating the carb is too small. We had it adjusted to give as much gas as it can. Also, the engine runs at around 200 degrees. It has a 4 core radiator, 180 degree thermostat, and two of the best electric fans we could get.any ideas besides putting in a 160 degree thermostat?

[OldSchoolin86]

What would be in your opinioin?

Studytime

I've already laid it out for you. Quit playing this silly game, your just wrong.

[Studytime]

studytime, wouldn't i choose my valve springs to match my cam profile? I was walking back from class today when this hit me like a brick. I can't beleive i forgot to ask about this. We have been having problems with this for years. My dad has a 94 S-t0, 700 miles on it when the owner took out the engine. He bought a 400 SBC, turbo 350, 2000 RPM stall converter. The motor has stock heads and a full edelbrock performer package-cam, valve springs, intake and 600cfm carb. My dad made the headers, im not sure of the primaries but they aren't overly big so i'd assume they are the most common size (1 5/8? i can't remember) The headers are jet hot coated and the problem we have is that some of the coating burnt off. We're speculating the carb is too small. We had it adjusted to give as much gas as it can. Also, the engine runs at around 200 degrees. It has a 4 core radiator, 180 degree thermostat, and two of the best electric fans we could get.any ideas besides putting in a 160 degree thermostat?

Well, I would suggest removing the thermostat and seeing how it runs. If it's close to 160 you may try that. Of course, everyone has heard the horror stories about 400ci engines running hot. Most of this is because of head swapping without the steam holes being drilled? Do these heads have the correct holes in them? You can add them with a .125" drill bit and a head gasket and good directions.

I'd start there and make sure it runs nice and cool, then add a 160 if it runs too cool for your liking or never really gets up to operation temp. I'd be willing to say with only electric fans that it'll probably run about 160 without the thermostat.

Yes, the headers will likely be 1.625" as you were saying. Big tube for S10 V8 conversions are very rare. Even the really fast running S10s still have the small-ish sized tubes with the exception being custom headers.

OH, JUST READ you have stock heads, disregard the steam hole info. Also, if your dad built the headers then they could be any size, but probably either 1.75 or 1.625" primary tubes.

As for the carb being too small, this will not be a factor in it getting insufficient milage. In fact that much engine air flowing through a 600 will probably run a little rich. yes, rich.

You say you've adjusted it all the way to add more fuel? Elaborate on this.

______________
yes, your springs need to provide enough pressure to keep your vavles closed or you'll get into valve float. Also, if you compress your springs too much you run the risk of the retainers smacking down on your valve guides, but if this is the only thing stopping you, you can get around that.

Spring pressure is very important though. So, you are very right in saying you need springs that coincide with your cam. Edelbrock did this for their "power package" which I really don't like, but they did it nonetheless.

Oh, and why such a tiny carb? Did you see the pictures of the carbs I posted? You need something more like that. You'd be SIMPLY AMAZED at the power you'd gain from a bigger carb on that motor.

Best of luck,
Studytime

[J.D.]

Ok I have a question for you guys. What size carb would be good for my car?

1981 El Camino. It has a SBC 350 with flattop hypereutectic pistons.(I don't know what my compression ratio is). It has .188 heads, I'm putting headers on it, here's my cam specs: Advertised Duration: 290/300, @ .050 224/234 and Gross Lift is .465/.488. This is just a street car....I'm going to hit it sometimes, because I expect it to be fairly fast. I doubt it will ever see past 4500/5,000 RPM tops. What do you guys recommend?

[cliff2302]

I'm not too big on carbs, thats why i didn't reply to this thread, but what they did was put in bigger metering rods i believe, the biggest ones you can get. I think they put in bigger jets? not even sure if edelbeocks have jets? But, i guess what im asking is is the 600 too small? Will a 750 or something like that work better? also, if we go bigger, its probably going to be a holley, so do we want vac. secondaries or a double pumper? I think vac secondaries because its not a wild motor and a WOT dump off the starting line may bob it down. am i way off or on the right track?