Optimal CFM for a 405?
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Optimal CFM for a 405?
I'm looking at two things, first, the best CFM for the engine I am building from a carb, and later, a throttle body when I finally shift into fuel injection.
Here is the engine.
SBC 405
Roller Retrofit
64cc Aluminum Vortec Heads (200cc Intake Runners.)
10.15:1 Static CR
Crane 119661 Roller Cam - I292 - E300 - Dur .50 - 230 / 238 @ 112 - LI.539 - LE.558
It's going in front of a TH2004R, and I have 3.08 rear gears.
What would be the optimal carb CFM rating for this engine, considering I like cruising and low RPM torque. Usually won't play above 4000 except on special occasions.
I have an Edelbrock Performer 750 CFM carb that I will start out with but want to switch to a good flowing throttle body down the road for fuel injection. I know their CFM rating is a bit different with regards to how the engine reacts so I just need a baseline.
A member might be working with me to bore out a 454 TB for use on my engine and we just need some info on where to start.
Thanks.
Here is the engine.
SBC 405
Roller Retrofit
64cc Aluminum Vortec Heads (200cc Intake Runners.)
10.15:1 Static CR
Crane 119661 Roller Cam - I292 - E300 - Dur .50 - 230 / 238 @ 112 - LI.539 - LE.558
It's going in front of a TH2004R, and I have 3.08 rear gears.
What would be the optimal carb CFM rating for this engine, considering I like cruising and low RPM torque. Usually won't play above 4000 except on special occasions.
I have an Edelbrock Performer 750 CFM carb that I will start out with but want to switch to a good flowing throttle body down the road for fuel injection. I know their CFM rating is a bit different with regards to how the engine reacts so I just need a baseline.
A member might be working with me to bore out a 454 TB for use on my engine and we just need some info on where to start.
Thanks.
Team Owner
Joined: Apr 2002
Posts: 62,198
Likes: 1
From: All humans are vermin in the eyes of Guru VA
Cruise-In IV Veteran
Cruise-In V Veteran
If you look at the holley TBI kits: I think your going to want their 900cfm kit.
In either case the other option if you are looking to guh-hetto rig it is the Offy dual quad intake for 2 qjets, and then use 2 of the spreadbore to OE gm TBI adapter plates from summit and use 2 cheap throttle bodies off of the ebay.
No idea how it would work but I think the OE throttle bodies were meant for motors with a lot less output than you are looking at.
In either case the other option if you are looking to guh-hetto rig it is the Offy dual quad intake for 2 qjets, and then use 2 of the spreadbore to OE gm TBI adapter plates from summit and use 2 cheap throttle bodies off of the ebay.
No idea how it would work but I think the OE throttle bodies were meant for motors with a lot less output than you are looking at.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
If you look at the holley TBI kits: I think your going to want their 900cfm kit.
In either case the other option if you are looking to guh-hetto rig it is the Offy dual quad intake for 2 qjets, and then use 2 of the spreadbore to OE gm TBI adapter plates from summit and use 2 cheap throttle bodies off of the ebay.
In either case the other option if you are looking to guh-hetto rig it is the Offy dual quad intake for 2 qjets, and then use 2 of the spreadbore to OE gm TBI adapter plates from summit and use 2 cheap throttle bodies off of the ebay.
I want to stick with my intake as I have Vortec heads, a polished Vortec Air Gap manifold, and I only want one unit on top of the engine.
I have been 'told' by a local carb expert that my optimal would be around 700 CFM from a carb. Problem is he didn't know how my particular engine build would play into this.
The member that might be doing the throttle body work has a 454 TB ready to modify and test on to get it bored out to work for me. seeing as how it's already rated for economy on a 454, I would imagine it wouldn't take much to feed a 405.
I see lots of guys on the forum with 400s, and none have carbs as big as 900.
Instructor
Joined: Nov 2005
Posts: 129
Likes: 1
From: Plano Texas
TBs were never meant for performance, but with a big enough chain saw, they can be opened up to a very respectable size:
Funny you should mention the Offy dual quad. Did you mean the crossram or an inline? Crossram might looks something like this with a pair of 2.375" TBs . . 1,100 CFM:
Alas . . the Durango Boy has Vortec
In-line with a pair of 2.125" would put you over the 900CFM mark. Is there a dual quad manifold for Vortec?
Anybody care to throw an opinion in the pot on what a stock 454TB flows?
Anybody know how much bore a 454 TB can take?
Funny you should mention the Offy dual quad. Did you mean the crossram or an inline? Crossram might looks something like this with a pair of 2.375" TBs . . 1,100 CFM:
Alas . . the Durango Boy has Vortec
In-line with a pair of 2.125" would put you over the 900CFM mark. Is there a dual quad manifold for Vortec?
Anybody care to throw an opinion in the pot on what a stock 454TB flows?
Anybody know how much bore a 454 TB can take?
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
lugging around at low rpm really sucks gas with most carbs.
the qjet can go down to 1500. But, one guy got 9-10 mpg with a qjet at 1200.
A funny side note:
C&D tested a Viper and said it lugged in 6th 80-140mph. after 140 it was fine
.
if your car is noisey and rattles a lot at higher revs, try earplugs
anyway, either get a "towing" cam
or a 3.70-4.11 rear
the qjet can go down to 1500. But, one guy got 9-10 mpg with a qjet at 1200.
A funny side note:
C&D tested a Viper and said it lugged in 6th 80-140mph. after 140 it was fine
.
if your car is noisey and rattles a lot at higher revs, try earplugs
anyway, either get a "towing" cam
or a 3.70-4.11 rear
Last edited by Matt Gruber; Aug 4, 2008 at 03:13 PM.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
lugging around at low rpm really sucks gas with most carbs.
the qjet can go down to 1500. But, one guy got 9-10 mpg with a qjet at 1200.
A funny side note:
C&D tested a Viper and said it lugged in 6th 80-140mph. after 140 it was fine
.
if your car is noisey and rattles a lot at higher revs, try earplugs
anyway, either get a "towing" cam
or a 3.70-4.11 rear
the qjet can go down to 1500. But, one guy got 9-10 mpg with a qjet at 1200.
A funny side note:
C&D tested a Viper and said it lugged in 6th 80-140mph. after 140 it was fine
.
if your car is noisey and rattles a lot at higher revs, try earplugs
anyway, either get a "towing" cam
or a 3.70-4.11 rear
Are you confusing MPG and CFM? I don't really care about the mileage I will get...I just might need to know how large to have a 454 TB bored so it'll work well with my torquey 405. The carb I have slated for use is a 750, and rumor has it that it might be too big for my lower RPM tastes.
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
Are you confusing MPG and CFM? I don't really care about the mileage I will get...I just might need to know how large to have a 454 TB bored so it'll work well with my torquey 405. The carb I have slated for use is a 750, and rumor has it that it might be too big for my lower RPM tastes.
i once built a TQ 355, it could pull away fine in high w/3.08 at 20 mph. cam was a 206/212 was fun for a month, then i got bored. good luck. get a towing cam if u want low rpm TQ. Wont need high cfm down there. since u have a carb, try it and see if u can live with it until u talk to some rv'ers about a low rpm TB.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Are you confusing MPG and CFM? I don't really care about the mileage I will get...I just might need to know how large to have a 454 TB bored so it'll work well with my torquey 405. The carb I have slated for use is a 750, and rumor has it that it might be too big for my lower RPM tastes.
with FI, you can put on damn nearly anything 800 cfm is fine....
flow enough to keep it happy....sounds like 30 lbs injectors to me....
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Pro
Joined: Apr 2007
Posts: 736
Likes: 1
From: Chester VA
Hey Durango, a good place to check for more info on factory throttle body info is the crossfire forum since that is a popular upgrade over crossfire setups. I spent alot of time there when I had my 84 and you'll get good info there.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Awesome, I'll spend some time over their searching for info on the 454 TB. Thanks.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
The optimal carburetor with your engine parts is the CFM you already have, a 750. Vacuum secondaries and not that double pumper that so many thinks is better. That is if you want it to run good. Otherwise, its your car and it strikes me you probably have already made up your mind as to which carburetor you want. One of those big pretty and fancy designer carburetors that looks good and dont run worth a damn. ...
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
The optimal carburetor with your engine parts is the CFM you already have, a 750. Vacuum secondaries and not that double pumper that so many thinks is better. That is if you want it to run good. Otherwise, its your car and it strikes me you probably have already made up your mind as to which carburetor you want. One of those big pretty and fancy designer carburetors that looks good and dont run worth a damn. ...
The carb is an Edelbrock 1411, 750 CFM. I'm using it because I have it, and because buying a different brand would cost extra money that I don't have.
The question about the proper CFM is because I want to go fuel injection, and I like the looks and simplicity of a single TB that can bolt in place of the carb on the existing manifold.
Instructor
Joined: Nov 2005
Posts: 129
Likes: 1
From: Plano Texas
Couple of comments . .
The Crossfire forum is all about single barrel Crossfire TBs. Two barrel TBs are not popular over there. This forum or Third Gen are better resources.
I go by the plate size in estimating flow on TBs. Carbs are not equivalent to TBs because they are not an unobstructed straight hole the same size as the plate like a TB.
Several manufacturers like this one publish numbers for their different size TBs. I checked several and their numbers vary, but the curves are similar.
http://www.fbthrottlebodies.com/products.htm
There are all sorts of opinions on factory TB flow: The above referenced manufacturer's numbers would put the 350 TB a bit over 600, and the 454 at almost 800. And stock Crossfires at about 525.
The point I am making here is that the CURVES from several maufacturers as well as from my engineering ref texts match closely, so I am comfortable utilizing them to make COMPARISONS. The ABSOLUTE numbers are open to intepretation and depend on the amount of pressure used in the measurments, and manuf are free to use whatever they want and not disclose. Obviously the higher the pressure, the more flow you get. Different manufacturers have some pretty different numbers for the same size hole. I quoted only ONE manufacturer and applied their curve to the subject TBs. That should not be taken as gospel across the board. My opinion is that they (like many others) are . . . hmmmm . optimistic
Also as someone already mentioned, TBs can be a lot bigger than carbs because they have none of the problems of carbs. TBs do not rely on a ventouri to draw fuel up out of a bowl.
The Crossfire forum is all about single barrel Crossfire TBs. Two barrel TBs are not popular over there. This forum or Third Gen are better resources.
I go by the plate size in estimating flow on TBs. Carbs are not equivalent to TBs because they are not an unobstructed straight hole the same size as the plate like a TB.
Several manufacturers like this one publish numbers for their different size TBs. I checked several and their numbers vary, but the curves are similar.
http://www.fbthrottlebodies.com/products.htm
There are all sorts of opinions on factory TB flow: The above referenced manufacturer's numbers would put the 350 TB a bit over 600, and the 454 at almost 800. And stock Crossfires at about 525.
The point I am making here is that the CURVES from several maufacturers as well as from my engineering ref texts match closely, so I am comfortable utilizing them to make COMPARISONS. The ABSOLUTE numbers are open to intepretation and depend on the amount of pressure used in the measurments, and manuf are free to use whatever they want and not disclose. Obviously the higher the pressure, the more flow you get. Different manufacturers have some pretty different numbers for the same size hole. I quoted only ONE manufacturer and applied their curve to the subject TBs. That should not be taken as gospel across the board. My opinion is that they (like many others) are . . . hmmmm . optimistic
Also as someone already mentioned, TBs can be a lot bigger than carbs because they have none of the problems of carbs. TBs do not rely on a ventouri to draw fuel up out of a bowl.
Last edited by simpson36; Aug 8, 2008 at 08:22 AM. Reason: Added paragraph empasizing need to interpreting flow numbers
Burning Brakes
Joined: Sep 2006
Posts: 1,071
Likes: 86
From: Rotonda FL
While you are researching, also consider going to a dry-flow port injection system, which will give you both better diriveability and mileage. All you'd need is to weld your current manifold to accept injectors, fuel rails, and a throttle body such as an Edelbrock (1000 CFM from their Pro-Flo), You then have your choice of controllers, from the rudimentary GM open loop used on their RAM JET through various stock based or aftermarket units including the DIY Mega Squirt.
Installing the custom port injection system above will probably cost a bit more (depending on how you source materials), but it has great development potential and will give you both better diriveability and mileage.
Installing the custom port injection system above will probably cost a bit more (depending on how you source materials), but it has great development potential and will give you both better diriveability and mileage.
Pro
Joined: Jun 2007
Posts: 634
Likes: 0
From: Hastings MN
750 cfm is pleanty!
Even with 100% VE (which you won't have), 750 cfm is good to 6400 rpm with 405ci. With 90% VE, it's good to 7100rpm. I assume that your heads aren't going to support >7000rpm, so 750 is pleanty big!
Even with 100% VE (which you won't have), 750 cfm is good to 6400 rpm with 405ci. With 90% VE, it's good to 7100rpm. I assume that your heads aren't going to support >7000rpm, so 750 is pleanty big!
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
I go by the rule of thumb:
CFM = ((CI x RPM)/3456) x VE
Figure VE @ 90% or so, the 750 is plenty
For a square-bore carb on the street, you're looking for sizing that a) meets the max airflow at target RPM + 500 and b) ensure you're not getting into the secondaries in cruise.
Too big of a carb can reduce intake velocity...and this is where it gets interesting and a little more dependent on the type of carb. The Holly Street HP carbs with the annular boosters would have no problem here, but the traditional "performance" Holly carbs will probably be too big.
I'll make a flat statement - and as a generalization it's suspect, but a good place to start - the best performance and driveability on the street will be delivered with the smallest carb consistent with the airflow needs of the engine. Drag, road racing, etc. - different story completely. There may be a couple of ponies on the dyno with an 850 DP...but on the street, you'd hate it.
CFM = ((CI x RPM)/3456) x VE
Figure VE @ 90% or so, the 750 is plenty
For a square-bore carb on the street, you're looking for sizing that a) meets the max airflow at target RPM + 500 and b) ensure you're not getting into the secondaries in cruise.
Too big of a carb can reduce intake velocity...and this is where it gets interesting and a little more dependent on the type of carb. The Holly Street HP carbs with the annular boosters would have no problem here, but the traditional "performance" Holly carbs will probably be too big.
I'll make a flat statement - and as a generalization it's suspect, but a good place to start - the best performance and driveability on the street will be delivered with the smallest carb consistent with the airflow needs of the engine. Drag, road racing, etc. - different story completely. There may be a couple of ponies on the dyno with an 850 DP...but on the street, you'd hate it.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
I like cruising so I will very rarely see much over 5500.
Racer
Joined: Oct 2006
Posts: 435
Likes: 18
From: Florida
I would give C&S a call, and see if their billet aerosol line of carbs would work for you. They claim their booster design gives significant advantages over conventional booster in the part throttle area of a carb.... Its centralized over the butterly and has a more uniform distribution of fuel and better atomization of the fuel. So they claim.....
