What do CFM ratings REALLY mean???
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
I tend to agree with that conclusion as I've gone to a gutted MAF as well as to a 52mm TB while building slightly under 300chp max.
Basically the intake tuning is done in the intake runners so reducing air flow resistance in the upper track could be beneficial to a point. Of course going too large at the TB can cause tip in problems.
This also applies to the exhaust system, where tuning is done in the header tubes, so reducing back pressure after the headers to a point should also be beneficial.
None the less, I've saved the above air flow tables for future reference.
Basically the intake tuning is done in the intake runners so reducing air flow resistance in the upper track could be beneficial to a point. Of course going too large at the TB can cause tip in problems.
This also applies to the exhaust system, where tuning is done in the header tubes, so reducing back pressure after the headers to a point should also be beneficial.
None the less, I've saved the above air flow tables for future reference.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by rocco16
One variable that always seems to be ignored in stating CFM figures is "at what pressure/vacuum"?
But I have no idea what we're talking about here since the first post is blank.
When someone says "this throttle body flows 500 cfm", they don't know what they're talking about. It won't flow 500 at idle, and it'll probably flow more than that at redline. If the other components connected in series with it are less restrictive, it'll flow more, if theyre more restrictive, the throttle body will flow less. The entire system must be taken into account.
Now, don't get started on exhaust flow, then we include the confusion of condensing gases (decreasing volume).
Burning Brakes
Joined: Jun 2003
Posts: 937
Likes: 0
From: Bedford Tx
Static flow testing of a cylinder head, is BS.
What should be done, for testing a cylinder head, is...
put the cyl head on a one cylinder test engine, with
some typical intake, and some typical valve timing,
run the 'engine' with an electric motor, and
then tell us how much air is moved.
.
{yes, I know that there are other variables,
and that something similiar could be done
in different ways, perhaps with the cyl actually firing}
{and , I acknowledge that a static measurement,
measures 'something'}
.
Then we would could compare cyl heads.
In addition, I think the importance of 'velocity'
is grossly exagerated.
.
What we want is, to maximixe the amount of air
retained in the cylinder when the intake valve
closes on the compression stroke,
nobody seems to get that.
.
I would be thrilled with 90 percent VE at 6000RPM,
but, what we typically get is, some crap about
some system with a VE of 60 percent at 5000RPM
and some discussion of 'velocity' that moves the
VE up by 5 percent. Velocity is not
a goal in itself.
What should be done, for testing a cylinder head, is...
put the cyl head on a one cylinder test engine, with
some typical intake, and some typical valve timing,
run the 'engine' with an electric motor, and
then tell us how much air is moved.
.
{yes, I know that there are other variables,
and that something similiar could be done
in different ways, perhaps with the cyl actually firing}
{and , I acknowledge that a static measurement,
measures 'something'}
.
Then we would could compare cyl heads.
In addition, I think the importance of 'velocity'
is grossly exagerated.
.
What we want is, to maximixe the amount of air
retained in the cylinder when the intake valve
closes on the compression stroke,
nobody seems to get that.
.
I would be thrilled with 90 percent VE at 6000RPM,
but, what we typically get is, some crap about
some system with a VE of 60 percent at 5000RPM
and some discussion of 'velocity' that moves the
VE up by 5 percent. Velocity is not
a goal in itself.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Originally Posted by silver84
I think the importance of 'velocity'
is grossly exagerated.
is grossly exagerated.
Static flow numbers play a huge role in performance, but they don't tell the whole story. They are not tested on an engine because any given cylinder head could be used in a million possible combinations. Thus all heads are tested based on a standard and it's up to you to decide how well it will work with everything else. You could spend a lifetime testing a single cylinder head with every possible engine combination.
Burning Brakes
Joined: Jun 2003
Posts: 937
Likes: 0
From: Bedford Tx
NP,
In your opinion,
how much 'dynamic supercharging' effect is
accomplished by 'velocity'?
.
I'd be surprised if it was ten percent.
.
IMO, a better idea would be to use the LS1
strategy of fat runners.
..
The aftermarket head vendors, talk about
flow, flow, and flow {and flow}.
If these vendors , somewhere, suggest a
compatible intake manifold - cross section area,
they must be using invisible ink.
In your opinion,
how much 'dynamic supercharging' effect is
accomplished by 'velocity'?
.
I'd be surprised if it was ten percent.
.
IMO, a better idea would be to use the LS1
strategy of fat runners.
..
The aftermarket head vendors, talk about
flow, flow, and flow {and flow}.
If these vendors , somewhere, suggest a
compatible intake manifold - cross section area,
they must be using invisible ink.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
I don't have any specific numbers, and it is very difficult to quantify. I can just tell you that you always want the highest CFM possible through the smallest opening possible, this equates to highest velocity. It helps atomize the fuel better, etc.
If ever given the choice between two heads that have the same flow numbers, take the one with the smaller runner size. It will perform better, period. I don't know ALL of the physics behind it so I can't explain everything.
When I worked at TEA every now and then we would get a competitor's head on our flow bench and check it out. Sure it would flow really high numbers but when you CC'd the runner it was huge. The result was a poor performing cylinder head.
If ever given the choice between two heads that have the same flow numbers, take the one with the smaller runner size. It will perform better, period. I don't know ALL of the physics behind it so I can't explain everything.
When I worked at TEA every now and then we would get a competitor's head on our flow bench and check it out. Sure it would flow really high numbers but when you CC'd the runner it was huge. The result was a poor performing cylinder head.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by Nathan Plemons
I don't have any specific numbers, and it is very difficult to quantify. I can just tell you that you always want the highest CFM possible through the smallest opening possible, this equates to highest velocity. It helps atomize the fuel better, etc.
Or at least the velocity helps keep the fuel in suspension with the air. One key advantage of velocity is the energy of the air or air fuel mixture. F= M*A. Where F is the force or energy of the air. M is the mass or weight, and V is the velocity. When the velocity and therefore the energy of the air is higher, it can help pack the cylinder full as the intake valve is closing against a rising piston. A very well coordinated, naturally aspirated engine package with the heads, cam, induction, and exhaust all tuned for a specific rpm range, can exceed 100% volumetric efficiency over a fairly narrow rpm range.
RACE ON!!!
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
I've seen published numbers where the ram tuning effect of a damn good N/A intake system got them up 110% VE at a given RPM.
Now, first off, most N/A engines max out at 85-90% VE over certain RPM ranges, so this is your starting point when you add boost. So that tells me a perfected intake could add more than 10%.
Hell, lets compare the TPI to the Crossfire motor with the same heads and cam. Obviously we'd expect a little more power from better fuel atomization, but most of the power increase was purely due to the intake design. 205 hp to 230 is a big difference. I'd venture to guess we got 20 peak hp from the intake design alone.
Now, first off, most N/A engines max out at 85-90% VE over certain RPM ranges, so this is your starting point when you add boost. So that tells me a perfected intake could add more than 10%.
Hell, lets compare the TPI to the Crossfire motor with the same heads and cam. Obviously we'd expect a little more power from better fuel atomization, but most of the power increase was purely due to the intake design. 205 hp to 230 is a big difference. I'd venture to guess we got 20 peak hp from the intake design alone.
Last edited by CentralCoaster; Jan 7, 2005 at 09:45 PM.
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Burning Brakes
Joined: Jun 2003
Posts: 937
Likes: 0
From: Bedford Tx
Does anyone remember old time cars, that had
a 'trumpet thing' {aka velocity stack}{one per cyl}
right over a carb?
Something like that, is what we want.
The velocity stack, is a continuation of the intake
manifold's intake tract.
Overall lenght of the velocity stack, maybe
limited by packaging issues.
question...
Under any circumstances {if yes, define}
would two cars, that had the same cyl-head intake
opening, want velocity stacks that had different
cross-sectional-areas?
a 'trumpet thing' {aka velocity stack}{one per cyl}
right over a carb?
Something like that, is what we want.
The velocity stack, is a continuation of the intake
manifold's intake tract.
Overall lenght of the velocity stack, maybe
limited by packaging issues.
question...
Under any circumstances {if yes, define}
would two cars, that had the same cyl-head intake
opening, want velocity stacks that had different
cross-sectional-areas?
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
I do not have a car example but I have a bike example of diffeerent intake tracts for different cylinders. The hot ticket in todays supersport bikes is to use two of the GSXR 750 (longer same diameter velocity stack), and retain two of the original GSXR 1000 (shorter same diameter velocity stacks).
As for the CFM question, I always understood that you double the max head flow rating at the lift value that corresponds to your max cam lift and that would equate to the max NA (very important to remember this is normally aspirated) FWHP that the heads would support. As far as the inlet plumbing, I have always went with the rule of thumb of "the bigger the better" up to the point of the TB. What I have followed on TB sizing is (regardless of engine cubes) that up to 300 RWHP, the 48MM is fine, up to 400 RWHP the 52MM is fine and the 58MM is for HP levels above 400 RWHP.
I am now challenged with when to go to the 1300CFM monoblade TB. I was told by TPIS that a 600RWHP is going to pick up 25RWHP above a 58MM. ASM told me that a 650RWHP car will pick up 40RWHP, but possibly at the expense of some "tip in" response.
BTW, what happened to the original "CFM vs ci" table? I would be interested in seeing this.
Aaron
As for the CFM question, I always understood that you double the max head flow rating at the lift value that corresponds to your max cam lift and that would equate to the max NA (very important to remember this is normally aspirated) FWHP that the heads would support. As far as the inlet plumbing, I have always went with the rule of thumb of "the bigger the better" up to the point of the TB. What I have followed on TB sizing is (regardless of engine cubes) that up to 300 RWHP, the 48MM is fine, up to 400 RWHP the 52MM is fine and the 58MM is for HP levels above 400 RWHP.
I am now challenged with when to go to the 1300CFM monoblade TB. I was told by TPIS that a 600RWHP is going to pick up 25RWHP above a 58MM. ASM told me that a 650RWHP car will pick up 40RWHP, but possibly at the expense of some "tip in" response.
BTW, what happened to the original "CFM vs ci" table? I would be interested in seeing this.
Aaron
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by CentralCoaster
Hell, lets compare the TPI to the Crossfire motor with the same heads and cam. Obviously we'd expect a little more power from better fuel atomization, but most of the power increase was purely due to the intake design. 205 hp to 230 is a big difference. I'd venture to guess we got 20 peak hp from the intake design alone.
Actually, the 1984 and '85 are the same long block, with the exception of the camshaft, which I'd bet isn't much different. You make a good point. I do think, however, the the long TPI runners were intended more to bolster low and mid range torque than to boost higher end horsepower. The Crossfire (L83) was rated at 290 ft.lbs. of torque at 2800 rpms. That is quite a low engine speed, for the max torque. The horsepower was 205 @ 4300 rpms. Does anyone have the figures for the 1985, L98? Also, for anyone that hasn't seen the inside of the Crossfire manifold, the runners are surprisingly long. Probably not half as long as the L98, but longer than *I* expected, until I saw it opened up.RACE ON!!!
