"CFM vs Horsepower"
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From: Rockville, MD
"CFM vs Horsepower" For most street/strip and mild race motors generate up to 1.7hp per cubic inch, improvements in airflow translate directly into horsepower. However, at anything beyond that specific outut, the flow bench is no longer the holy rail of predicting power. "Once you hit 2hpp per cubic inch, things start gettin crazy", says Judson. "Take a Prostock engine, for instance, that puts out rifht around 2.7hpper cube. On a motor like that its a given that you have a serious cylinder headthat can fill the cylinder quite well. So when the exhaust valve opens the residual pressurereaching hundreds of psi escapes out of the cylinder. The velocity of the exhaust coming out the headis a bazillion time higher than wha can be replicated on a flow bench." To put it simply, experts can find no correlation between airflow and power on the exhaust side beyond 2hp per cubic inch. "There are thins going on at that power level in a motor we just dont understand. Ypu can have one head that's down 50CFM to another head on the exhaust side that makes just as much power. Dont get me wrong, the flow bench has contributed more to horsepower than any other pat or tool in the last 30-40 years, but after a certain pont flow numbers just dont mean squat."
ripped from this months CHP where they talked with Judson Massingill
so what ARE those other thins going on? any ideas, explinations, theories?
ripped from this months CHP where they talked with Judson Massingill
so what ARE those other thins going on? any ideas, explinations, theories?
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Well I think some people understand as they (prostock) find repeatable incremental power increases every year. Maybe not the interviewed "professor" however.
The heads are just one important part of the equation. ProStock power includes everything from the airflow over the car into the scoop to the pressure underneath the car where the exhaust exits. Hell its not just ProStock. There are many areas we all can improve on if we focus on a particular area of our combination. Some people are fortunate to be able to make a living focusing on these areas.
I personally feel that people that are on the outside of a class like prostock think there is more spooky stuff going on there than what really is. There's more than one competitive shop in the country that you can go into and see absolutlely everything that goes into the engine.
Patience and finesse are important traits of a prostock engine builder. Finance is also pretty high up. : )
More to your question though to make the power like ProStock does you try to make the most use of wave tuning as possible. Hence the very narrow powerbands.
I think the program PipeMax? helps a person focus on those areas to maximize gains in a certain rpm band.
As one famous ProStock crew chief likes to say "Its all math"
The heads are just one important part of the equation. ProStock power includes everything from the airflow over the car into the scoop to the pressure underneath the car where the exhaust exits. Hell its not just ProStock. There are many areas we all can improve on if we focus on a particular area of our combination. Some people are fortunate to be able to make a living focusing on these areas.
I personally feel that people that are on the outside of a class like prostock think there is more spooky stuff going on there than what really is. There's more than one competitive shop in the country that you can go into and see absolutlely everything that goes into the engine.
Patience and finesse are important traits of a prostock engine builder. Finance is also pretty high up. : )
More to your question though to make the power like ProStock does you try to make the most use of wave tuning as possible. Hence the very narrow powerbands.
I think the program PipeMax? helps a person focus on those areas to maximize gains in a certain rpm band.
As one famous ProStock crew chief likes to say "Its all math"
That was a great interview with Massingill. I appreciated what he had to say about the balance between flow and velocity...minimizing the differences between localized velocities. "If air moves too fast, it won't want to make the turn at the short-side radius..." His common sense approach and admission that you can not figure everything out based on the flow bench, is refreshing. Lots of good advice to amateur porters like me.
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From: Rockville, MD
Originally Posted by ChucksZ06
That was a great interview with Massingill. I appreciated what he had to say about the balance between flow and velocity...minimizing the differences between localized velocities. "If air moves too fast, it won't want to make the turn at the short-side radius..." His common sense approach and admission that you can not figure everything out based on the flow bench, is refreshing. Lots of good advice to amateur porters like me.
although i found that his view on the intake/exhaust port ratio a little odd. "no matter how bad the ratio gets, just keep going" 
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What was said "experts can find no correlation to power and flow on the EXHAUST side". On the intake side, thinks are pretty predictable.
I've always said, especially on a NA intake flow is where the power is made.
On the exhaust side, as long as you have a properly sized and shaped port all is well.
I've always said, especially on a NA intake flow is where the power is made.
On the exhaust side, as long as you have a properly sized and shaped port all is well.
Originally Posted by Wnts2Go10O
although i found that his view on the intake/exhaust port ratio a little odd. "no matter how bad the ratio gets, just keep going"Larry Meaux has a pretty good system of determining how well a port makes power...
Peak_HP = Flow_CFM * .257 * Number_of_Cylinders
is estimated potential Peak HP to expect
you multiply .87 percent times cam's theoretical max lift , round off to nearest .050" in Flow Test, then see what CFM is at 28 inches
example=> .700" Lift cam
.700 Lift times .87 percent = .609" Lift
Flow head at .600" Lift , then take CFM at 28 inches and calculate HP potential with above formula
.257 Factor = for beginning engine builders and engines near 10.0:1 Comp Ratio
.285 Factor = would be for Professional engine builders with wet sump pans, lightweight rotating assemblies, low tension great sealing rings, deep oil pans, etc.
excellent use of inertia/wave tuning with 9.5 to 11.5:1 Comp Ratios or
11.5 to 13.0:1 CR ranges without fully utilizing inertia/wave tuning effects
.300 to .310 Factor = Current ProStock Technology with dry sump, unlimited carburetion, Hi Comp Ratio, ultra lightweight rotating assembly, etc, max use of inertia/wave tuning, etc, 14:1 to 17:1 Comp Ratios
(usually no better than .3200 efficiency or no worse than .2980 eff %)
For a "State -of- Art" type engine, if you know how to build the rest of the Engine you should get around .300 Factor if you have 15-16:1 CR,etc
Backwards calculation =>
Current ProStock 500 cid = 1345 to 1350 HP "Quote Bill Jenkins"
1350 / 8_Cylinders = 168.75 HP per Cylinder
562.5 Potential CFM @ 28" = 168.75 / .300
544.4 Potential CFM @ 28" = 168.75 / .310
pretty close to what they claim as Head Flow numbers
Darin Morgan claims as high as 116.5 CFM/Sq Inch Valve Area
a 2.525" od valve = 583.4 cfm
583.4 cfm = 168.75 / .289271 Factor (Sounds too high)
562.5 cfm would be 112.33 cfm/sq.inch sounds more reasonable ??
and that would be a .300 Flow Factor on that sort of engine
Some of the Math descrepencies are due to the fact that often Dynos and FlowBenches don't all read the same ...some read High , some read Low, so the Flow Factors might seem off a little
Bret
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Right about the exhaust. A 1 psia test at 70°F is not an accurate representation of 100 psia at 2500°F. People report differences on exhaust changes between 28 and 50 inches of water (approximately 1 to 2 psia). When people report that an LS7 exhaust port looks weak at 28 psia, it probably reflects GM tests closer to the real environment.
One noted engine developer claims using very small exhaust flows (so the flow is supersonic) works best. Not something you can test at 1 psia and 70°F.
Another point is that exhaust flow is pretty much done at BDC. Maybe .3-.4" lift. So even what you look at needs to be different.
Attached are two sample outputs of a simulation program for a stock '97 LS1 at 4000rpm and 6000rpm
One noted engine developer claims using very small exhaust flows (so the flow is supersonic) works best. Not something you can test at 1 psia and 70°F.
Another point is that exhaust flow is pretty much done at BDC. Maybe .3-.4" lift. So even what you look at needs to be different.
Attached are two sample outputs of a simulation program for a stock '97 LS1 at 4000rpm and 6000rpm
Last edited by DavidNJ; May 5, 2006 at 11:43 PM.
2HP per cubic inch. Hmmm 346 x 2 = 692 HP. I don't have to worry about the unknown area then.
In all seriousness I just think they don't want to share what they know. Those guys have serious R&D behind them and data log every possible point on the engine and constantly go over the data again and again. They know what's going but just aren't saying because they want to be faster than the competition.
In all seriousness I just think they don't want to share what they know. Those guys have serious R&D behind them and data log every possible point on the engine and constantly go over the data again and again. They know what's going but just aren't saying because they want to be faster than the competition.
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you think that part of it has to do with the fact that the pulses moveat supersonic speeds? what im saying is, its a combination of the massive pressure and the speed it travels. like it creates a pressure wave simular to the intake pressure wave and the speed combined with the pressure creates a slip stream of sorts. someone got any other ideas of whats going on?
Originally Posted by MadBill
But doesn't this formula in effect say that displacement is irrelevant? Put a pair of "X" flow heads on a 283, 350, 434, whatever. It doesn't matter, they all make "Y" HP? Presumably at least at different RPM? 
Well it is relative.....on a 283 you would have to spin the snot out of it to make the same power as a 434 with the same heads..I would be more interested as to what happens to the torque under the same conditions. On the other hand, most of the people who utilize these formula's would/will put the correct CC head for the inches they are working with.
David
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quote from FASTFATBOY: I would be more interested as to what happens to the torque under the same conditions.
Torque is more directly related to displacement. This is because it's a more a factor of cylinder fill rather than cylinder fill * speed. The same cam heads intake (assuming that this is the limiting factor, not strength of parts or knock or float) will usually produce the same (similar) max power within a wide (but limited, i.e. don't use a 1 c.i. engine compared to 400 c.i.) range of displacements. The torque (at lower RPM than peak power) will always be greater with more displacement. The intake system has more of a %efficiency factor for obtaining torque, but a max flow limit for obtaining power at high speeds.
Harmonics and component mass and friction will fight this theory a little, but it's more of a generality.
In general with the same heads, cam, intake:
Big engine will have greater torque, obtain peak hp sooner, and keep a flatter peak hp curve. Also will have more area under the curve.
Small engine will have lower torque peak, and peak hp late and peak in a sharp manner, but can achieve about the same peak power.
Just plug this into a desktop dyno or whatever software, and you'll see this.
Torque is more directly related to displacement. This is because it's a more a factor of cylinder fill rather than cylinder fill * speed. The same cam heads intake (assuming that this is the limiting factor, not strength of parts or knock or float) will usually produce the same (similar) max power within a wide (but limited, i.e. don't use a 1 c.i. engine compared to 400 c.i.) range of displacements. The torque (at lower RPM than peak power) will always be greater with more displacement. The intake system has more of a %efficiency factor for obtaining torque, but a max flow limit for obtaining power at high speeds.
Harmonics and component mass and friction will fight this theory a little, but it's more of a generality.
In general with the same heads, cam, intake:
Big engine will have greater torque, obtain peak hp sooner, and keep a flatter peak hp curve. Also will have more area under the curve.
Small engine will have lower torque peak, and peak hp late and peak in a sharp manner, but can achieve about the same peak power.
Just plug this into a desktop dyno or whatever software, and you'll see this.
Think about airflow and displacement this way. Your head and cam package is a restrictor. It only allows so much air to flow over a given period of time. So, by understanding this, you can see why both a large displacement engine and a small engine should make the same horsepower.
The reason they don't is because the smaller engine has to spin faster which creates more loss due to mechanical efficency. The faster something moves, the more power it takes to move it.
The reason they don't is because the smaller engine has to spin faster which creates more loss due to mechanical efficency. The faster something moves, the more power it takes to move it.
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From: Houston, Tx.
Originally Posted by MadBill
But doesn't this formula in effect say that displacement is irrelevant? Put a pair of "X" flow heads on a 283, 350, 434, whatever. It doesn't matter, they all make "Y" HP? Presumably at least at different RPM?
For instance:
If I put 450 cfm heads on a 250 inch LS1 with two dominators I won't see any 950+ HP.
I "might" see 635 hp at 9500-10000 rpm. It's hard for pushrod engines to peak in this range but it happens with huge massive dollars.
If I put 450 cfm heads on a 450 inch LS1 with two dominators I could easily see 950+ HP.
I might easily see 990 HP at 8600 rpm. We already do stuff like this in racing quite often. 8500 rpm in friction and valvetrain control is much easier. Also this engine would last ten times longer than the 250 incher even making 50 percent more power.
OTOH if you really could twist the 250 incher to say 15500 rpm then you would at least be close to the 450 inchers throughput at 8500 rpm but with huge frictional losses and a MUCH lamer cam that the 250 incher sees the heads through as compared to the 450 incher. The 450 incher will destroy the 250 and that's why we never see unlimited displacement in racing or racing engines in general built anywhere but right at the limit of displacement for their minimum weights.
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Originally Posted by MadBill
But doesn't this formula in effect say that displacement is irrelevant? Put a pair of "X" flow heads on a 283, 350, 434, whatever. It doesn't matter, they all make "Y" HP? Presumably at least at different RPM?
no the formula tells u what power n at what rpm its can be made at so if u increase displacement then the rpm range goes down at least thats what ive learned correct me if im wrong