boost psi vs head flow for 427
03-12-2009, 01:39 PM
#41
03-12-2009, 02:07 PM
#42
Well it has to all be going into the motor. Where else would it go? That is what my point it. Now I'm not saying it won't hit a HP asymptote at some point due to excess heat and other similar factors, but my point is, if the blower is cranking out X amount of cfm, the motor is consuming that same X amount of cfm.
03-12-2009, 04:42 PM
#43
Well it has to all be going into the motor. Where else would it go? That is what my point it. Now I'm not saying it won't hit a HP asymptote at some point due to excess heat and other similar factors, but my point is, if the blower is cranking out X amount of cfm, the motor is consuming that same X amount of cfm.
03-12-2009, 07:04 PM
#44
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what ever you try to force into the cylinder , you first have to get the 'fired charge" out first...
the engine is an "air pump"
the better the head flow ( ie; sbc 15* head is better flowing than 23 * head)
Now,
since the exhaust flow is @ temperature, and less dense , and higher pressure, it flows out quicker...
if the engine is badly designed , or has inheriate back pressure problems , your not going to get ALL the previous fired charge out...
example:
you have a open top stainless beer keg ( that you drank the beer last nite with you mates...)
put a 3/4 garden hose in the top....
the 1/2 tap at the bottom , turn on full...
what happens ??
the beer keg over flows..
change the 3/4 hose filling it , to 3/8 hose,...
you will never over fill the keg...
the keg now has equal or better flow OUT , than in...
this is a simiplied example only, and complex fluid dynamics is more intense..
design the engine to flow in AND out ,
you get more "clean" air ( read boost) into cylinder,
and therefore you can put more fuel in , to match up required air / fuel ratio
equals
more power...
As mentioned before, more air flow , with LESS heat gain from compression ( boost) , the air is more dense, etc,etc..
From my R & D
355 cubes , OEM 882 iron heads , ( 1.9 IN & 1.5 EX) @ 35 PSI with twin 61mm turbonetics my car went 4.7 @ 156 1/8mile
the newer 23 deg heads ( dart pro 1 , with 2.1 In & 1.6 EX) , @ 18 psi , with twin 66mm turbonetics , car went 5.0 @ 155 mph 1/8 mile...
same cam , same turbines, same turbine housing , same converter , same car ....etc,etc
better head flow , requires LESS boost to put out simular power
the engine is FLOWING more air , with less pressure behind it ( less boost)
Basicly,
get the air in and out , with least restrictions..
cheers
ash
the engine is an "air pump"
the better the head flow ( ie; sbc 15* head is better flowing than 23 * head)
Now,
since the exhaust flow is @ temperature, and less dense , and higher pressure, it flows out quicker...
if the engine is badly designed , or has inheriate back pressure problems , your not going to get ALL the previous fired charge out...
example:
you have a open top stainless beer keg ( that you drank the beer last nite with you mates...)
put a 3/4 garden hose in the top....
the 1/2 tap at the bottom , turn on full...
what happens ??
the beer keg over flows..
change the 3/4 hose filling it , to 3/8 hose,...
you will never over fill the keg...
the keg now has equal or better flow OUT , than in...
this is a simiplied example only, and complex fluid dynamics is more intense..
design the engine to flow in AND out ,
you get more "clean" air ( read boost) into cylinder,
and therefore you can put more fuel in , to match up required air / fuel ratio
equals
more power...
As mentioned before, more air flow , with LESS heat gain from compression ( boost) , the air is more dense, etc,etc..
From my R & D
355 cubes , OEM 882 iron heads , ( 1.9 IN & 1.5 EX) @ 35 PSI with twin 61mm turbonetics my car went 4.7 @ 156 1/8mile
the newer 23 deg heads ( dart pro 1 , with 2.1 In & 1.6 EX) , @ 18 psi , with twin 66mm turbonetics , car went 5.0 @ 155 mph 1/8 mile...
same cam , same turbines, same turbine housing , same converter , same car ....etc,etc
better head flow , requires LESS boost to put out simular power
the engine is FLOWING more air , with less pressure behind it ( less boost)
Basicly,
get the air in and out , with least restrictions..
cheers
ash
Last edited by crashly; 03-12-2009 at 07:11 PM.
03-12-2009, 07:17 PM
#45
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From my R & D
355 cubes , OEM 882 iron heads , ( 1.9 IN & 1.5 EX) @ 35 PSI with twin 61mm turbonetics my car went 4.7 @ 156 1/8mile
the newer 23 deg heads ( dart pro 1 , with 2.1 In & 1.6 EX) , @ 18 psi , with twin 66mm turbonetics , car went 5.0 @ 155 mph 1/8 mile...
same cam , same turbines, same turbine housing , same converter , same car ....etc,etc
better head flow , requires LESS boost to put out simular power
the engine is FLOWING more air , with less pressure behind it ( less boost)
Basicly,
get the air in and out , with least restrictions..
cheers
ash
03-12-2009, 08:00 PM
#47
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Well it has to all be going into the motor. Where else would it go? That is what my point it. Now I'm not saying it won't hit a HP asymptote at some point due to excess heat and other similar factors, but my point is, if the blower is cranking out X amount of cfm, the motor is consuming that same X amount of cfm.
I guess the op question would need to be qualified with a better description of the parts he intended to use. I'm curious as well.
03-13-2009, 03:59 AM
#48
@crashly :
At what rpms did these different combo's have their max power ?
and do you perhaps have 2 dyno sheets showing the different combo's ?
And did you use a shorter gearratio for the lower boosted (=more rpm ?) setup ?
just to see how much your combo would apply to my previous post.
At what rpms did these different combo's have their max power ?
and do you perhaps have 2 dyno sheets showing the different combo's ?
And did you use a shorter gearratio for the lower boosted (=more rpm ?) setup ?
just to see how much your combo would apply to my previous post.
Last edited by jeejee; 03-13-2009 at 04:32 AM. Reason: typo
03-13-2009, 04:45 AM
#49
TECH Apprentice
@crashly :
At what rpms did these different combo's have their max power ?
and do you perhaps have 2 dyno sheets showing the different combo's ?
And did you use a shorter gearratio for the lower boosted (=more rpm ?) setup ?
just to see how much your combo would apply to my previous post.
At what rpms did these different combo's have their max power ?
and do you perhaps have 2 dyno sheets showing the different combo's ?
And did you use a shorter gearratio for the lower boosted (=more rpm ?) setup ?
just to see how much your combo would apply to my previous post.
jeejee:
max power @ 6100 / max torq @ 5300 ish rpm
race rpms where, 4400 laurnch , 6200 shift, 6700 ish finnish line
dyno sheet for first engine only, then used the '1320' dyno....
second combo , only the '1320' dyno
no, same gears, same car, same p/g, same tyres...etc
heads:
882 chev irons , 166 cc intake , 1.9 intake valve VS dart pro 1 238cc intake ,2.1 intake valve
03-13-2009, 08:54 AM
#51
Interesting thread. I'd like to expound upon a few things...
Head flow is in cfm, or cubic ft/min. That is a volumetric flow rate, not a mass flow rate. Mass flow is what makes power, whereas volumetric flow rate is what relates to frictional losses. When you use forced induction, you increase the density, hence more mass per unit volume. The gas velocity in the head may not actually change much. It's like putting twice as many people in the hallway, but they all walk the same speed as before. So, a head that flows 350 cfm at ~1 psi dP (flowbench) will still flow 350 cfm at ~1 psi dP and 15 psi boost, but since there will be double the lb/cubic ft, then the mass flow rate will double.
That said, I think it's worth noting that head flow in cfm is a pretty terrible way to measure frictional flow losses in the port for the very reason that some are getting confused in this thread. First off, it's measured at ~1 psi dP. Who says we want 1 psi dP? I want as close to 0 as possible. Next, there will be some point that more cfm at 1 psi dP simply isn't needed. For instance, most might think that a 700 cfm head flows twice as a 350 cfm head. While that is true at 1 psi dP, your engine will not hold the dP constant when the head flow increases. Your goal is to reduce the dP, which would result in the highest pressure in the cylinder.
To further explain my point, if you have a 305 TBI head (160 cfm) on a 427 and run it at 7000 rpm, then you might find 5 psi of dP across the intake port. Install a set of ported LS7 heads (400 cfm), and the dP across the intake port drops might drop to 0.5 psi. Install a set of hypothetical 500 cfm heads, and the dP across the port may only drop to 0.4 psi. There are definitely diminishing returns as head flow rises, not to mention the loss in inertial effects caused by large cross-sections.
In any other industry, flow losses are expressed as a pressure drop coefficient. Not quite as good, but certainly more intuitive, would be to express head flow as a dP at a constant cfm.
Mike
Head flow is in cfm, or cubic ft/min. That is a volumetric flow rate, not a mass flow rate. Mass flow is what makes power, whereas volumetric flow rate is what relates to frictional losses. When you use forced induction, you increase the density, hence more mass per unit volume. The gas velocity in the head may not actually change much. It's like putting twice as many people in the hallway, but they all walk the same speed as before. So, a head that flows 350 cfm at ~1 psi dP (flowbench) will still flow 350 cfm at ~1 psi dP and 15 psi boost, but since there will be double the lb/cubic ft, then the mass flow rate will double.
That said, I think it's worth noting that head flow in cfm is a pretty terrible way to measure frictional flow losses in the port for the very reason that some are getting confused in this thread. First off, it's measured at ~1 psi dP. Who says we want 1 psi dP? I want as close to 0 as possible. Next, there will be some point that more cfm at 1 psi dP simply isn't needed. For instance, most might think that a 700 cfm head flows twice as a 350 cfm head. While that is true at 1 psi dP, your engine will not hold the dP constant when the head flow increases. Your goal is to reduce the dP, which would result in the highest pressure in the cylinder.
To further explain my point, if you have a 305 TBI head (160 cfm) on a 427 and run it at 7000 rpm, then you might find 5 psi of dP across the intake port. Install a set of ported LS7 heads (400 cfm), and the dP across the intake port drops might drop to 0.5 psi. Install a set of hypothetical 500 cfm heads, and the dP across the port may only drop to 0.4 psi. There are definitely diminishing returns as head flow rises, not to mention the loss in inertial effects caused by large cross-sections.
In any other industry, flow losses are expressed as a pressure drop coefficient. Not quite as good, but certainly more intuitive, would be to express head flow as a dP at a constant cfm.
Mike
Last edited by engineermike; 03-13-2009 at 09:04 AM.
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03-13-2009, 01:18 PM
#54
Thanks, guys.
dP = delta pressure
In other words, dP is pressure in the plenum minus pressure in the cylinder. This is the pressure that causes flow through the runner and port itself. You want this to be as low as possible, even though flowbench testing is done at 1 psi dP.
dP = delta pressure
In other words, dP is pressure in the plenum minus pressure in the cylinder. This is the pressure that causes flow through the runner and port itself. You want this to be as low as possible, even though flowbench testing is done at 1 psi dP.
03-17-2009, 12:19 PM
#55
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Well I was thinking it was a difference number of some sort, but couldnt be sure. Thanks for clearing that up, you explanation in post 51 makes perfect sense now.
