Minimum CFM for 385 hp?
Thread Starter
Teching In
Joined: Feb 2005
Posts: 11
Likes: 0
From: near Sears Point Raceway CA
Minimum CFM for 385 hp? This car must be emissions-certified. The only EGR-smog-certified Eddelbrock carb (personal preference closed to debate) is #EDL-1400, 600 cfm. Motor is GM Fast Burn crate 350 cid/385 hp/385 lb-ft.
What is the minimum required cfm rating to produce full power in the above motor? Will the above carb limit power in the above motor?
There are conflicting answers to the above questions. An equation at Holley's website indicates my motor requires less than 550 cfm. Conversely, Holley phone tech implied 600 cfm will limit power. In a Summit catalog, the text in a Holley Street Avenger ad states "670 cfm, 300-400 hp", implying my 385 hp motor requires about 650 cfm. Is the minimum cfm 550 or 650? I'm lost. Please share the basis for your answers.
Thanks a ton.
What is the minimum required cfm rating to produce full power in the above motor? Will the above carb limit power in the above motor?
There are conflicting answers to the above questions. An equation at Holley's website indicates my motor requires less than 550 cfm. Conversely, Holley phone tech implied 600 cfm will limit power. In a Summit catalog, the text in a Holley Street Avenger ad states "670 cfm, 300-400 hp", implying my 385 hp motor requires about 650 cfm. Is the minimum cfm 550 or 650? I'm lost. Please share the basis for your answers.
Thanks a ton.
Last edited by ro9397; May 19, 2005 at 04:17 PM.
Every orifice reduces airflow@pressure. I think flow
is like the square root of pressure. 4BBL Carbs are
rated at a low pressure drop of 1.5 inches of mercury.
-Every- carb "limits" power. Not as in a hard limiter,
it's just a "tax" on manifold pressure.
The (ambient air - pressure drop) is your manifold air
pressure, your density and your mass air flow. A
750CFM carb will produce a lower drop than a 600
(duh) but presuming there's nothing nonlinear going
on, the difference at, say, 700CFM is going to be
something like:
drop750=(700/750)^2*1.5inHg=1.31inHg
drop600=(700/600)^2*1.5inHg=2.04inHg
Ooohhhh, big difference, right? Not really, because
atmospheric is 29.9inHg and the manifold pressure is
(atmospheric-drop):
MAP(ideal)=29.9
MAP(750)=29.9-1.31=28.6
MAP(600)=29.9-2.04=27.9
Now if you look at those pressure drops as percent of
ideal, you have a rough idea of how much HP is left
on the table. Say you'd have a 400HP motor if only you
could get a CARB-approved 20,000CFM UberDominator
9BBL carb in pink anodized finish...
1.31/29.9*400HP=17.5HP lost to 750CFM carb
2.04/29.9*400HP=27.3HP lost to 600CFM carb
The difference in loss is 9.8HP on a motor drawing
700CFM.
At 500CFM you would be at about 5HP less, running a
600CFM carb, than a 750. Presuming they were equally
well tuned and stuff like that.
Which is not to be ignored, because where you live
on the street is midband torque and transitional
metering, a smaller carb running at more of its
max vacuum signal will have better mixture fidelity
at the lower airflows.
is like the square root of pressure. 4BBL Carbs are
rated at a low pressure drop of 1.5 inches of mercury.
-Every- carb "limits" power. Not as in a hard limiter,
it's just a "tax" on manifold pressure.
The (ambient air - pressure drop) is your manifold air
pressure, your density and your mass air flow. A
750CFM carb will produce a lower drop than a 600
(duh) but presuming there's nothing nonlinear going
on, the difference at, say, 700CFM is going to be
something like:
drop750=(700/750)^2*1.5inHg=1.31inHg
drop600=(700/600)^2*1.5inHg=2.04inHg
Ooohhhh, big difference, right? Not really, because
atmospheric is 29.9inHg and the manifold pressure is
(atmospheric-drop):
MAP(ideal)=29.9
MAP(750)=29.9-1.31=28.6
MAP(600)=29.9-2.04=27.9
Now if you look at those pressure drops as percent of
ideal, you have a rough idea of how much HP is left
on the table. Say you'd have a 400HP motor if only you
could get a CARB-approved 20,000CFM UberDominator
9BBL carb in pink anodized finish...
1.31/29.9*400HP=17.5HP lost to 750CFM carb
2.04/29.9*400HP=27.3HP lost to 600CFM carb
The difference in loss is 9.8HP on a motor drawing
700CFM.
At 500CFM you would be at about 5HP less, running a
600CFM carb, than a 750. Presuming they were equally
well tuned and stuff like that.
Which is not to be ignored, because where you live
on the street is midband torque and transitional
metering, a smaller carb running at more of its
max vacuum signal will have better mixture fidelity
at the lower airflows.
a 600 will be fine. as long as u jet it right all will be well. i knw magezines arent to be totally believed but look back at a few danger mouse articles, theyve done wonders wit 600-650 cfm carbs. u could also have the carb bored out a lil...yes it has been done.. good luck.
Thread Starter
Teching In
Joined: Feb 2005
Posts: 11
Likes: 0
From: near Sears Point Raceway CA
Jimmy, thanks for your reply. I only wish my math was better!
Here are my questions, better distilled:
1. On this particular 385 hp motor, what happens to the 5 hp bump between 5500-5800 rpm when the OEM 750 cfm Holley 4160 is replaced with a 600 cfm Edelbrock 1400?
2. Does the bump become a plateau or a downward cliff?
The plateau is tolerable, especially if performance improves below 5500 rpm.
Here are my questions, better distilled:
1. On this particular 385 hp motor, what happens to the 5 hp bump between 5500-5800 rpm when the OEM 750 cfm Holley 4160 is replaced with a 600 cfm Edelbrock 1400?
2. Does the bump become a plateau or a downward cliff?
The plateau is tolerable, especially if performance improves below 5500 rpm.
Thread Starter
Teching In
Joined: Feb 2005
Posts: 11
Likes: 0
From: near Sears Point Raceway CA
Hey, it's a jimmy convention!
Yes, jim #3 you are very astute! The source of the consternation is that my builder & a couple other apparently reliable sources prefer the Edelbrock for long term reliability, a big priority for me.
Also, Edelbrock was convincing that the 600 would work significantly better below 5500 rpm.
Your opinion comparing the two carbs?
Thanks a ton.
Yes, jim #3 you are very astute! The source of the consternation is that my builder & a couple other apparently reliable sources prefer the Edelbrock for long term reliability, a big priority for me.
Also, Edelbrock was convincing that the 600 would work significantly better below 5500 rpm.
Your opinion comparing the two carbs?
Thanks a ton.
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Staging Lane
Joined: May 2005
Posts: 68
Likes: 0
Here is a quick formula that I usually go by. The RPM's you are going to run definetely matter. You don't need a big carb if your not going to run high RPM's, although there are other factors that come into play, this is just a quick formula. Anyways, here it is.
( Engine Size (CID) x Maximum RPM ) /3456 = CFM
For example, I had a 396 ci. Big Block, and my max RPM was 6500, so
(396 x 6500) / 3456 = 744.79
The motor made 720 hp on the dyno, and I ran a Holley 750 double pumper. I had numerous people tell me I needed more carb, but the 750 was plenty after it was tweaked.
By the way, Jared, nice Camaro. Doug Herbert racing sells the best and the cheapest cams there are. You can get any size hydraulic non-roller cam you want, with springs and lifters for under $300.00. Just thought I would let you know.
( Engine Size (CID) x Maximum RPM ) /3456 = CFM
For example, I had a 396 ci. Big Block, and my max RPM was 6500, so
(396 x 6500) / 3456 = 744.79
The motor made 720 hp on the dyno, and I ran a Holley 750 double pumper. I had numerous people tell me I needed more carb, but the 750 was plenty after it was tweaked.
By the way, Jared, nice Camaro. Doug Herbert racing sells the best and the cheapest cams there are. You can get any size hydraulic non-roller cam you want, with springs and lifters for under $300.00. Just thought I would let you know.
Last edited by white_ta; May 23, 2005 at 11:42 AM.
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Staging Lane
Joined: Oct 2004
Posts: 90
Likes: 0
Originally Posted by white_ta
Here is a quick formula that I usually go by. The RPM's you are going to run definetely matter. You don't need a big carb if your not going to run high RPM's, although there are other factors that come into play, this is just a quick formula. Anyways, here it is.
( Engine Size (CID) x Maximum RPM ) /3456 = CFM
For example, I had a 396 ci. Big Block, and my max RPM was 6500, so
(396 x 6500) / 3456 = 744.79
The motor made 720 hp on the dyno, and I ran a Holley 750 double pumper. I had numerous people tell me I needed more carb, but the 750 was plenty after it was tweaked.
By the way, Jared, nice Camaro. Doug Herbert racing sells the best and the cheapest cams there are. You can get any size hydraulic non-roller cam you want, with springs and lifters for under $300.00. Just thought I would let you know.
( Engine Size (CID) x Maximum RPM ) /3456 = CFM
For example, I had a 396 ci. Big Block, and my max RPM was 6500, so
(396 x 6500) / 3456 = 744.79
The motor made 720 hp on the dyno, and I ran a Holley 750 double pumper. I had numerous people tell me I needed more carb, but the 750 was plenty after it was tweaked.
By the way, Jared, nice Camaro. Doug Herbert racing sells the best and the cheapest cams there are. You can get any size hydraulic non-roller cam you want, with springs and lifters for under $300.00. Just thought I would let you know.
Thanks alot white ta. That's the most help I've recieved on this forum yet and I didn't even have to ask a question. Good thing I made that stupid comment.
By the way, I better make sure my dad doesn't see you making 720 HP with a 750. I am stealing his 750 as soon as he puts an 850 on his 396. I dont have money for a carbright now! I am running a 600 cfm edelbrock. He's making a little less than you, he suppose to be about 550 rwhp in a 396.
Take care.
Staging Lane
Joined: May 2005
Posts: 68
Likes: 0
His car is a street car right? If so, 550's not to shabby! Mine camaro was a street car when I first put that motor in it, but now it's just going to be my drag car. It wasn't in the best of shape, and it would have took too much work and money to fix it up right, so I just bought a nice 72' chevelle, and decided to drag the Camaro. Good luck with your car, and glad to be of any help.
