Big block 460
Member
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cfm vs velocity
- Thread starter hustler
- Start date
Bob Evans
Administrator
At what RPM?
I'm just a big ol dumb small town boy, but a quick calculation shows that to approach 100% VE at 7000 rpm with a 200 cc motor, the carb and intake would need to flow in the neighborhood of 49 cfm at near 0 intake vacuum.
Overlap and reversion would help a little, but I doubt it would get you over the hump. DOHC with variable cam timing and 4 valves would help, but you're still dealing with poppet valves that can only be so big in an OHV configuration before they start hitting the sides of the cylinder.
At 7000 rpm, during the intake stroke, the motor has just .016 seconds (16 milliseconds) to fill the cylinder.
I just have a hard time getting it.
Please show me where I went wrong
Bob 49CFM of flow is incorrect. For 100% VE using the pipemax formula which gets you a very close number you would need 70CFM @ 28" for a power peak of 7000 and a VE of 100% (for a 13ci 212 predator) that is for the whole intake path not just the head. Larry Meaux author of pipemax uses this formula. (CID*Peak RPM*.000978474*VE) / (# of cyl*127.5) and I do believe those 20+ HP motors like barry young builds are a bit above 100%VE.
Please show me where I went wrong[/QUOTE]
Pauls theory/ideas are 100% correct.. when formula 1 was naturally aspirated and flow benches were few, rare and little understood it was soon discovered and proven by a few that... the suction created by the downward stroke of the piston accounted for less than 15% of the forces at work drawing in the fuel charge... which resulted in short stroke very high rpm engines being developed... 85% of the forces to draw in the fuel air charge were created via the exhaust
p
Pauls theory/ideas are 100% correct.. when formula 1 was naturally aspirated and flow benches were few, rare and little understood it was soon discovered and proven by a few that... the suction created by the downward stroke of the piston accounted for less than 15% of the forces at work drawing in the fuel charge... which resulted in short stroke very high rpm engines being developed... 85% of the forces to draw in the fuel air charge were created via the exhaust
p
Bob Evans
Administrator
I know, I hesitated on putting that number down, it's sort of been a rule of thumb for sizing carbs for a long time, and takes drivability into factor to some extent. Never heard of the pipemax formula.
In my simple scope of thinking, 100% volumetric efficiency means having the cylinder full of a fresh air/fuel charge at 1 atmosphere, ready for compression. And to exceed 100% VE would require positive pressure in the intake.
But, there ya go.
Big block 460
Member
You would need positive pressure in the intake .. but that’s only made from forced induction .. instead on a naturally aspirated engine you need to create a vacuum in the cylinder ..which is the opposite thinking but same result... the exhaust is creating a high pressure wave and behind it is a low pressure zone .. thru correct cam selection and exhaust length/width the low pressure area created behind the high pressure exhaust exiting the system can be harnessed ... creating a vacuum in the cylinder that allows for more that 100% of the static volume number of air to be brought into the cylinder .. this is how you get an engine to preform at its peak capability...
Big block 460
Member
If the volumetric efficiency of a 13ci engine like a 212 is like 30 cfm “ just ruff numbers “ there are actual equations based off rpm etc ... but as a ruff estimate ....and the conversion of Cfm to hp is .275 hp to 1 Cfm ... a predator 212 should make about 6.5 hp .... so if it’s making more . . . . Than there is your answer ... it’s performing at over 100% volumetric efficiency
Big block 460
Member
In addition ! All the record hp numbers quoted made by a 212 are fake .. it’s impossible to make even 25hp with that bore and stroke ... and that’s the truth
flattop1
Dawg 89
Almost afraid to jump in here, but here goes.
I think as soon as the intake valve opens there is no more vacuum.
When I was vacuum testing weld seams . you could pull 20 inches of vacuum then as soon as you opened the ball valve it dropped to zero immediately. I can't see any reason an intake valve opening would be any different .
Well one thing the throttle blade would be a restriction.
I think as soon as the intake valve opens there is no more vacuum.
When I was vacuum testing weld seams . you could pull 20 inches of vacuum then as soon as you opened the ball valve it dropped to zero immediately. I can't see any reason an intake valve opening would be any different .
Well one thing the throttle blade would be a restriction.
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It depends upon what you consider a vacuum, the true definition is a space completely devoid of matter. It is virtually impossible to cause a vacuum in a cylinder of an internal combustion engine. So we will call it it an area of low pressure, the goal is to make the area in the cylinder and the area on the other side of the intake valve to be at the greatest pressure difference as possible. This is much easier to do with a multiple cylinder engine than what we are dealing with here. If you run one of these on the dyno without a filter you will quickly learn that you cannot maintain that positive pressure flowing toward the cylinder at all RPM. If we are talking about most kart sanctioning ryules the exhaust pipe is too short, As you adjust camshaft lobe overlap and or event timing, you will be able to attain improvements in narrow bands of your RPM range. Without rely on text books and calculations it would be foolhardy to think this engine design will ever reach 100% VE I would be suprised if you reach a clean honest 90%. That is of course if we are solely dealing with normal aspiration at typical atmospheric range where a human can exist.
silverback
Member
You are trying to bring science to the shade tree!
Bob you want the greatest pressure difference you can achieve. The cylinder should have the lowest residual pressure you can get. You have the volume beyond sealing, the next task is making certain all pressure is relieved and it must relieve itself through the exhaust tract. If the oressure in the cylinder is higher than atmospheric or it is turbulent it will attempt to relieve pressure through the intake as soon as it begins to open. Simply put you want the pressure to be exiting leaving a void behind it that the intake charge can fall into. If you were to design an engine for flow the first design you would discourage would be an L head flat head. You would go right to the T head and then the OHV.
jsstump#70
Member
So is it possible to make more than one HP per cubic inch in a small block chevy? How about 2 HP per cubic inch? Naturally aspirated of course. If so, why not on "these" engines? If not, does that mean the whole performance world is lying?
paulkish
old fart
Not on the subject of question but my understanding is top 410 sprint car engines are now over 1000hp.
... and usta be without weight rules some cars were under 1150 pounds with driver
PD Engines
Member
Chev engine has had millions of hours of development. Not relevant to a kart engine.
Bob Evans
Administrator
I meant as in, flowing a carb and intake through the block with the exhaust closed and the intake at maximum lift.
But if you want the greatest pressure difference across the intake valve, you could just close the throttle. That would do it.
But don't do it with a manometer connected between the intake port and the combustion chamber. Been there, done that
But that's ok.