"CFM" Discussion
"CFM" Discussion After doing a little research on the web, I came up with the following:
Engine builders have seen heads which flow great on the bench, but get beat at the track and on the dyno, by heads that flow less…There is definitely more to cylinder heads than just great flow numbers…One important aspect to look at here, is the motion of the incoming fuel/air charge. Generally heads with good motion or swirl will perform better because they will burn the fuel/air mixture faster. When heads burn faster you gain in 2 ways:
1. The engine will require less spark advance. By firing the mixture closer to TDC there will be less cylinder pressure to fight the piston rising to reaching TDC.
2. The burn will be completed sooner after TDC. This creates more cylinder pressure after TDC to push the piston down and make more power.
Is there a point when to much CFM will hurt you?..
Kevin
Engine builders have seen heads which flow great on the bench, but get beat at the track and on the dyno, by heads that flow less…There is definitely more to cylinder heads than just great flow numbers…One important aspect to look at here, is the motion of the incoming fuel/air charge. Generally heads with good motion or swirl will perform better because they will burn the fuel/air mixture faster. When heads burn faster you gain in 2 ways:
1. The engine will require less spark advance. By firing the mixture closer to TDC there will be less cylinder pressure to fight the piston rising to reaching TDC.
2. The burn will be completed sooner after TDC. This creates more cylinder pressure after TDC to push the piston down and make more power.
Is there a point when to much CFM will hurt you?..
Kevin
Last edited by Camaro01SS4life; Jul 1, 2004 at 08:29 PM.
TECH Senior Member
Joined: Mar 2004
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From: BFE
Well this is where Velocity comes in play. A theory that AFR just put to practice and so far has yielded good results. The more mods to help them breathe the better they perform.
Although there is still not enough data and indepandant testing to surely confirm it, they have been putting ourstanding numbers on the dyno.
Now is the real test, THE TRACK.
Although there is still not enough data and indepandant testing to surely confirm it, they have been putting ourstanding numbers on the dyno.
Now is the real test, THE TRACK.
TECH Fanatic
Joined: Sep 2003
Posts: 1,570
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From: San Diego, CA
i think CFM is the last thing you need to compare two cars at the track....
pay more attention to a cam that is best fit for the heads you have...
a good intake manafold, TB, filter, lid etc....
good headers, plugs, compression, etc...
and the heads them selfs id be more concerned about the quality of the work done,
good valves, good springs, good seals, properly assembled!!!! and most of all
VACUM CHECKED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
i dont think CFM means much unless you to low! which means STOCK!
if someone is making 10-15 more CFM than you.......you can easily make up the differance somewhere else! focuse on more important things!
jmo
pay more attention to a cam that is best fit for the heads you have...
a good intake manafold, TB, filter, lid etc....
good headers, plugs, compression, etc...
and the heads them selfs id be more concerned about the quality of the work done,
good valves, good springs, good seals, properly assembled!!!! and most of all
VACUM CHECKED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
i dont think CFM means much unless you to low! which means STOCK!
if someone is making 10-15 more CFM than you.......you can easily make up the differance somewhere else! focuse on more important things!
jmo
Nice topic.
I've seen it and done it 
There is much more going on than meets the eye.
Richard
Originally Posted by Camaro01SS4life
Engine builders have seen heads which flow great on the bench, but get beat at the track and on the dyno, by heads that flow less…
Kevin
Kevin
There is much more going on than meets the eye.Richard
6600 rpm clutch dump of death Administrator
Joined: Dec 2001
Posts: 4,983
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From: Texas
Originally Posted by Richard@WCCH
Nice topic.
I've seen it and done it There is much more going on than meets the eye.
Richard
I've seen it and done it
There is much more going on than meets the eye.Richard
The heads that have made the most power for us, and ran the fastest at the track had big ports (230+cc) good flow numbers, and NO mixture motion damn.
Food for thought, the new AFR head has no mixture motion damn either!
Futher food for thought, GM has prototype heads that have so much mixture motion that it blows the spark out!
Drag race heads don't need mixture motion as much as someone trying to pass emissions, at high RPM's the air charge has enough mixture motion to keep the air and fuel homogenized.
The total package is what makes a car fast at the track, weight of everything, especially anything that rotates, wheels, axles, spools, driveshafts, etc. Proper stall, gear, suspension, etc. I have a customer that runs the Fun Ford Street Warrior series, which requires a N/A 306 with flat top pistons and .500" lift hyd roller cam. He has runs 9's on the motor, with 480 RHWP, so how efficient do you think his drivetrain is??
BTW, the biggest head we offer for a small block anything is a Brodix Neal for a small block Ford that has a intake port of volume of 318cc!! and flows almost 440 cfm. We have this head on my cousins Mustang with a 438" engine it makes over 900 HP on motor and runs 155 MPH in the quarter ON MOTOR, with the spray it has gone 7.70 @ 178, and just won the last NMRA Outlaw race, how much velocity does this port have?? Mixture motion?? Total package !!
My .02
Food for thought, the new AFR head has no mixture motion damn either!
Futher food for thought, GM has prototype heads that have so much mixture motion that it blows the spark out!
Drag race heads don't need mixture motion as much as someone trying to pass emissions, at high RPM's the air charge has enough mixture motion to keep the air and fuel homogenized.
The total package is what makes a car fast at the track, weight of everything, especially anything that rotates, wheels, axles, spools, driveshafts, etc. Proper stall, gear, suspension, etc. I have a customer that runs the Fun Ford Street Warrior series, which requires a N/A 306 with flat top pistons and .500" lift hyd roller cam. He has runs 9's on the motor, with 480 RHWP, so how efficient do you think his drivetrain is??
BTW, the biggest head we offer for a small block anything is a Brodix Neal for a small block Ford that has a intake port of volume of 318cc!! and flows almost 440 cfm. We have this head on my cousins Mustang with a 438" engine it makes over 900 HP on motor and runs 155 MPH in the quarter ON MOTOR, with the spray it has gone 7.70 @ 178, and just won the last NMRA Outlaw race, how much velocity does this port have?? Mixture motion?? Total package !!
My .02
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From: Spring Hill, TN
What a flow bench doesn't show is the velocity of the mixture. If 2 heads flow the same CFM and one does it with smaller runners then it's moving the air faster.
This is preferrable because there will be a bit of a ram effect that in a certain RPM range makes a difference.
Then there's the rest of the package as others have said.
This is preferrable because there will be a bit of a ram effect that in a certain RPM range makes a difference.
Then there's the rest of the package as others have said.
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Originally Posted by M6HuggerSS
and the heads them selfs id be more concerned about the quality of the work done,
good valves, good springs, good seals, properly assembled!!!!
good valves, good springs, good seals, properly assembled!!!!
For example, way back in 89, Ford and Yamaha colaborated on a project called the SHO. I owned a 91 for several years, and always marveled at it's advanced technology and implementation, especially for it's time. The centerpiece was the Yamaha 3.0L DOHC HO engine, of course...the induction system on this engine was the first I know of to utilize a variable intake system. The intricate snaking pipes of the intake manifold utilized 2 sets of runners: one long set with high velocity for robust low-end tq, and a much shorter and fatter set for good midrange and high-RPM hp (i.e., max cfm) without running out of air up top. There was a servo-operated butterfly valve for each runner that switched from the long to the short runner at ~3800 rpm (depending on PCM tuning...yes there is a ls1 comparable software package for tuning this pocket rocket engine...).
When the secondary runners opened up, it sounds as close to a F1 car or ferrari for that matter that you can get for ~$3k! The low end tq on these engines was good enough to propell the 3500 lb 4-door sedan with suprising grunt below 4 grand, and yet the beast redlined at 7k and spins that reliably for well over 200k miles. The combination of velocity and tq made for a dyno graph as smooth as some of the variable timing systems out there today, and in fact the BMW 7-series utilizes a similar (although infinately variable) systems as well. Pop in a 3.6L stroker kit and stage I cam with tuning and look out N/A cobras!
Last edited by Carvinta; Jul 3, 2004 at 03:19 PM.
I agree with the velocity issue...Forcing air into a semi-smaller opening as opposed to a larger one will create a higher velocity(Bernoullis Principle)...Faster moving air, along with everything else=more power...
Kevin
Kevin
FormerVendor
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From: Houston, Tx.
The newest LSx heads coming out are secret but will be the highest flowing heads ever put on an automibile. So I guess we know what Chevy thinks.
People that usually poo poo flow and flow benches are also usually people that can not get results either. The REAL flow numbers are very important but the heads need to be designed and ported for what you want out of them. No 265 CFM head will ever beat a 300 cfm head on a 400 inch engine if you are trying to go fast on each and a 350 cfm will beat those both handily.
NOW if you have ten 350 cfm heads you will have some differences in power and track performance and this is where good and bad porters come into play. There are more than a few ways to get flow out of any given head casting and the heads could be way diferent in size and shape of the runners so of course flow is not all that matters as long as you have enough! That's where this stuff gets a little more complicated.
People that usually poo poo flow and flow benches are also usually people that can not get results either. The REAL flow numbers are very important but the heads need to be designed and ported for what you want out of them. No 265 CFM head will ever beat a 300 cfm head on a 400 inch engine if you are trying to go fast on each and a 350 cfm will beat those both handily.
NOW if you have ten 350 cfm heads you will have some differences in power and track performance and this is where good and bad porters come into play. There are more than a few ways to get flow out of any given head casting and the heads could be way diferent in size and shape of the runners so of course flow is not all that matters as long as you have enough! That's where this stuff gets a little more complicated.
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From: San Diego, CA
its true that CFM does play a part in making power!
but i feal that a GOOD valve seal, GOOD valve seat surface, and proper vacum testing
(to insure a GOOD seat surface has been made!!!) is MORE important!
if you dont start with those 3 FIRST, i feal that a 20-25 CFM differance will not help any!
therefore make sure you vacum test your heads! and try to inspect your valve seals!!!
if they are improperly installed the installer can put small cuts in the top of the seal!!!
depending on there style of installing them!
and then its just a matter of time before your $2000 dollar 325CFM heads start burning
a **** load of oil! or worse!
jmo
but i feal that a GOOD valve seal, GOOD valve seat surface, and proper vacum testing
(to insure a GOOD seat surface has been made!!!) is MORE important!
if you dont start with those 3 FIRST, i feal that a 20-25 CFM differance will not help any!
therefore make sure you vacum test your heads! and try to inspect your valve seals!!!
if they are improperly installed the installer can put small cuts in the top of the seal!!!
depending on there style of installing them!
and then its just a matter of time before your $2000 dollar 325CFM heads start burning
a **** load of oil! or worse!
jmo
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Originally Posted by M6HuggerSS
its true that CFM does play a part in making power!
but i feal that a GOOD valve seal, GOOD valve seat surface, and proper vacum testing
(to insure a GOOD seat surface has been made!!!) is MORE important!
if you dont start with those 3 FIRST, i feal that a 20-25 CFM differance will not help any!
therefore make sure you vacum test your heads! and try to inspect your valve seals!!!
if they are improperly installed the installer can put small cuts in the top of the seal!!!
depending on there style of installing them!
and then its just a matter of time before your $2000 dollar 325CFM heads start burning
a **** load of oil! or worse!
jmo
but i feal that a GOOD valve seal, GOOD valve seat surface, and proper vacum testing
(to insure a GOOD seat surface has been made!!!) is MORE important!
if you dont start with those 3 FIRST, i feal that a 20-25 CFM differance will not help any!
therefore make sure you vacum test your heads! and try to inspect your valve seals!!!
if they are improperly installed the installer can put small cuts in the top of the seal!!!
depending on there style of installing them!
and then its just a matter of time before your $2000 dollar 325CFM heads start burning
a **** load of oil! or worse!
jmo
When I was doing the Holley SysteMax development in 93'-94' we would run the engines at 4000 RPM, wide open throttle, full load for 24 hours at a time to do the durability testing of the heads. At the end we would do a leak down and power pull to check the heads, and then put a fresh set on and recheck power. With every cylinder leaking 50%-90%, we would install new heads and the leakdown would drop to less then 10%, rerun the power pulls and the HP difference was 10 HP or about 2.5%. I have freshened heads that had bad leakage, put them back on the car, and not see hardly any faster times at the track. The reason that this occurs is because once the mixture lights off in the cylinder, it pretty closes the valves, and tiny amounts of leakage are almost undectable in terms of real power. But most people would never know this.
I agree that a head has to be of good quality, we leak check every chamber of every head.
I agree that a head has to be of good quality, we leak check every chamber of every head.
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From: Fort Collins, CO
Originally Posted by Carvinta
I couldn't have said it better. In addition, velocity is an underrated variable imo.
For example, way back in 89, Ford and Yamaha colaborated on a project called the SHO. I owned a 91 for several years, and always marveled at it's advanced technology and implementation, especially for it's time. The centerpiece was the Yamaha 3.0L DOHC HO engine, of course...the induction system on this engine was the first I know of to utilize a variable intake system. The intricate snaking pipes of the intake manifold utilized 2 sets of runners: one long set with high velocity for robust low-end tq, and a much shorter and fatter set for good midrange and high-RPM hp (i.e., max cfm) without running out of air up top. There was a servo-operated butterfly valve for each runner that switched from the long to the short runner at ~3800 rpm (depending on PCM tuning...yes there is a ls1 comparable software package for tuning this pocket rocket engine...).
When the secondary runners opened up, it sounds as close to a F1 car or ferrari for that matter that you can get for ~$3k! The low end tq on these engines was good enough to propell the 3500 lb 4-door sedan with suprising grunt below 4 grand, and yet the beast redlined at 7k and spins that reliably for well over 200k miles. The combination of velocity and tq made for a dyno graph as smooth as some of the variable timing systems out there today, and in fact the BMW 7-series utilizes a similar (although infinately variable) systems as well. Pop in a 3.6L stroker kit and stage I cam with tuning and look out N/A cobras!
For example, way back in 89, Ford and Yamaha colaborated on a project called the SHO. I owned a 91 for several years, and always marveled at it's advanced technology and implementation, especially for it's time. The centerpiece was the Yamaha 3.0L DOHC HO engine, of course...the induction system on this engine was the first I know of to utilize a variable intake system. The intricate snaking pipes of the intake manifold utilized 2 sets of runners: one long set with high velocity for robust low-end tq, and a much shorter and fatter set for good midrange and high-RPM hp (i.e., max cfm) without running out of air up top. There was a servo-operated butterfly valve for each runner that switched from the long to the short runner at ~3800 rpm (depending on PCM tuning...yes there is a ls1 comparable software package for tuning this pocket rocket engine...).
When the secondary runners opened up, it sounds as close to a F1 car or ferrari for that matter that you can get for ~$3k! The low end tq on these engines was good enough to propell the 3500 lb 4-door sedan with suprising grunt below 4 grand, and yet the beast redlined at 7k and spins that reliably for well over 200k miles. The combination of velocity and tq made for a dyno graph as smooth as some of the variable timing systems out there today, and in fact the BMW 7-series utilizes a similar (although infinately variable) systems as well. Pop in a 3.6L stroker kit and stage I cam with tuning and look out N/A cobras!
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Brian, we've had intake seats move and leak and lost 50+ hp on 800-1000 hp engines and fixed it and the power's back. On a very low power engine with good exhaust like low power engine's have you probably wouldn't lose as much. Basically though you almost can't even tell if the exhaust is leaking as it leaking has almost no impact whatsoever on power. Probably just like you said it closes mostly under the power stroke anyway.
