air fuel velocity calculation

[vega]

http://en.wikipedia.org/wiki/Cylinder_head_porting

 

I have been doing some further reading and research on different parts of the intake systems of the 4 stroke engine. There are three things I am struggling with when it comes to this article, AND what "everyone has been telling me". MY MAIN CONCERN is the fuel/air velocity calculation.

 

"The "Porting and Polishing" myth

 

It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Larger ports flow more fuel/air at higher RPM's but sacrifice torque at lower RPM's due to lower fuel/air velocity. A mirror finish of the port does not provide the increase that intuition suggests."

 

So how does one find a medium port flow cc on for the intake port on a head for a mean of what would be best for high rpm and what would be best for low rpm and whatever the middle is for engine "x"?

 

Also reading this same article it talks about efficiency of air flowing through the intake being heavy almost a thick liquid making it difficult for the engine to pump. Is there a way to filter it down (...yes i know what an air filter is but this seems to be a bit different than that)? To filter it down so it is not longer thick for the lack of better terms?

 

It also talks about fuel coming in the same port and how this affects the air coming in the intake (to me it sounds pretty much negatively). So, why don't we have a separate port entirely JUST for fuel until it reach the exact point where the air fuel has to be mixed before it reaches the cylinder? Or is this what direct injection is?

 

http://www.engineeringtoolbox.com/ductwork-equations-d_883.html

This also shows a "way" to calculate velocity. Albeit how much is needed for particular applications eludes me. For example low velocity seems to build Peak power but loses tq at bottom end (no one person or article i have found counters this) and with high velocity seems to build more power at lower rpm. Well to get a higher velocity one needs to close (make smaller) the the intake port (on the cyl head), but by how much? If one continues to close it (perhaps too far) air won't be able to get through it efficiently. Something that adds to the dilemma in my opinion.

 

Any insight would be grand!

 

Vega-

Edited December 24, 2011 by vega

[Leon]

Search around here for posts on internal combustion engine design, porting, flow, velocity, ad nauseum. If you are truly interested in the way things work, don't listen to the internet. Some articles may be true, and others false, but both sides will be convinced that they're correct. I highly recommend reading Heywood's "Internal Combustion Engine Fundamentals". I can't go into a dissertation on engine flow, but I'll leave you with these points:

 

• Velocity IS Volumetric flow over a cross-sectional area, i.e. V = Q / A.
   
  
• Find Q (flow) by taking into account cylinder displacement and RPM and assuming a volumetric efficiency for your engine.
   
  
• The air/fuel mixture within the intake has mass, and therefore has kinetic energy when moving.
   
  
• Kinetic energy increases with the square of velocity and increasing mass.
   
  
• Frictional losses increase with the square of velocity and with decreasing diameter.
  

Draw your own conclusions!

[vega]

One of the rods and pistons with cap

http://i938.photobucket.com/albums/ad226/faytmorgan/rodandpistonwithcap.jpg

 

One of the pistons close up

http://i938.photobucket.com/albums/ad226/faytmorgan/40overpiston.jpg

 

Bottom end

http://i938.photobucket.com/albums/ad226/faytmorgan/327bottomend.jpg

 

Top of piston

http://i938.photobucket.com/albums/ad226/faytmorgan/topofpistoninmotor.jpg

 

The cylinder wall scoring I was talking about that I may have to sleeve :/

http://i938.photobucket.com/albums/ad226/faytmorgan/cylinderwallscoring.jpg

 

I was stuck (going back through my paper work of cam comparisons) between two cams.

 

12-673-4

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=232&sb=0

 

or

12-678-4

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=227&sb=0

 

The 12-673-4 cam is not as high a lift is in the same operating range if not a little better than the 12-678-4 cam so I honestly don't really see why I did not decide to go with the 673 cam.

 

Either way- the motor is already 40 over may have to 50 over or a sleeve for one the cylinder walls has a small groove. The crank is forged - I am staying with that crank (3.25 small journal crank). I don't need to stay with my flat tops I don't know the specs on them other than they were are 40 over. I am looking to have 10:1 to 10.5:1 compression so whatever I needed to get there with my bore and stroke etc. I have also considered 6 inch rods over 5.7 - although I don't know if it will make a huge difference asides what piston I go with.

 

2800 pound car

3.545 rear ratio

.7 forth gear

184mph at 6000rpm in 4th gear aerodynamics forgiving

 

4.040 bore

3.25 stroke

flat top pistons

Intake

http://www.jegs.com/i/Weiand/925/7547-1/10002/-1

Rockers

http://www.jegs.com/i/Harland+Sharp/851/1001/10002/-1

1500-6500 operating range (power range)

6800rpm red line

shifting at 6000 to 6500 rpm

 

I do not know the combustion chambers or the intake runners or the piston deck height. I am not sure how to find this.

-----------

Lastly this is the email I received from my cousin (currently going to college for this kind of stuff) I figured I would ask him for the math at least. He is far from hands on and knows more about engine theory than building one.

 

"This is an interesting topic. I wish I had more time to help you dive into the specific calculations for your engine. But here are some considerations.

 

First of all, when the article mentions air being thick and heavy they are referring to the intrinsic properties of air. Even perfectly clean air has a nonzero viscosity and a nonzero density. Viscosity (or stickiness) is a measure of how hard it is to rapidly deform or shear a pocket of air. Density is the mass of air per unit volume and is thus directly related to how hard it is to accelerate. During our everyday experience with air we seldom have to quickly accelerate much of it or force it through confined areas. So we dont experience its stubbornness and naturally think of it as being light and free moving. But within an intake port, the stickiness and heaviness of air become very important. The air is required to fill the volume displaced by a cylinder in like 10 milleseconds which means the air is required to travel at up to several hundred mph through a restriction the size of a penny. Try inhaling as fast as you can with you mouth wide open. Then try inhaling as fast as you can with a straw in your mouth. The second way is noticeably harder and prevents your from inhaling as quickly. This is basically the same phenomenon that robs the performance of an engine.

 

Your mention of wide versus narrow ports has to do with inertial ram which is an important and relatively simple phenomena this article seems to gloss over. If you have somewhat long intake pipes, the column of air within the intake pipe gains appreciable momentum as it rushes toward the cylinder. Because of its momentum, toward the end of the intake stroke when the cylinder is nearly full of fresh charge, this column of air tends to keep moving and forces itself into the already "full" cylinder. This is analogous to turbocharging where additional air is pumped into the cylinder by a turbine. If you didn't have intake pipes and your intake valves opened str8 into the throttle body chamber this inertial ram effect would not occur. In this case you would always want the biggest possible port diameter since restriction would be the only possible issue. But intake pipes are used because of the inertial ram effect even though they offer additional restriction. Narrow intake pipes tend to work better because of higher velocity and hence greater momentum of the air column. But unfortunately there is a trade off because as you say, at high enough speed, the narrow pipes offer too much restriction for the higher airflow. I like to call it the coffee straw effect. Its a matter of what rpm range you plan on running the engine. You can't get optimum performance over the entire range you quote.

 

This aspect is closely related to intake valve timing. To take advantage of inertial ram the intake timing needs to be sufficiently delayed so the intake valve stays open past when the cylinder would normally be "full" at BDC.

 

You have to be very careful which velocity you are dealing with. To achieve increased airflow through inertial ram you want the velocity along the length of the intake pipe to be high which means narrowing it. You never want to further increase the velocity across just the valve seat by limiting valve lift. I dont see what this buys you. Its just increases the overall restriction. Its not very convenient to think of torque in terms of either of these velocities. Your glossing over the big picture. Its simpler than that. Torque depends solely on how much air-fuel mixture you can get into the cylinder and burn. So you do whats necessary to get maximum air into the cylinder. And the higher the speed at which you can achieve good airflow, and hence good torque, the better the power. Remember power is rotational speed times torque. If you ultimately care about power, **** low end torque and go for the wide intake pipes so you can achieve inertial ram and minimal restriction at high speeds. In my opinion, the best way is to tune your engine for maximum power. If you want the vehicle to lurch forward from a standstill, that can always be done through a transmission. So many sluggish vehicles are on the road because people have had suffered from obsession with low-end torque.

 

The article overemphasis the importance of wave dynamics. Yes, by taking advantage of what is sometimes referred to as the organ pipe effect, you can slightly boost your performance. But it can be very cumbersome to deal with and maybe not worth it unless you have everything else perfect. This is mainly because each calculation is only valid for one particular speed so you can boost performance at only very specific speeds. For example, with a certain design you may have a noticable boost between like 2100 and 2200 rpm and then again between like 4500 and 4700 or something. If you've ever driven a vehicle where stepping on the throttle at certain speeds gives great power but stepping on the throttle at other speeds is quite sluggish, this is most likely what's happening. Often designs that take advantage of this effect have different length intake pipes (aka runners) for each cylinder. This ensures that each cylinder gets a boost at different speed. The overall effect is to smooth out the power delivery so engine gives more consistent performance across a range of speeds. GM's quad 4 for example used this principle which I'm guessing you see a lot of. And they are quite powerful engines for their size and cost.

 

Yeah direct injection is where the fuel is directly sprayed into the combustion chamber. The detrimental effect of fuel in the intake port is there (otherwise no point of DI) but its a rather unimportant second order effect. Remember that air-fuel ratio is about 15:1. So the mass of fuel involved is less than 10% of the air mass. An engine is basically an air pump. In fact, quite accurate computer simulations of intake performance can be achieved without even considering fuel.

 

As they say, the smoothness of the port wall is not very important unless its terribly rough. The air molecules next to the port wall are necessary stationary regardless of the wall texture or material. So, the only molecules that are going to see the surface protrusions are those already stopped near the wall. "

 

Anything else?

[ozconnection]

http://forums.hybridz.org/index.php/topic/72851-small-port-heads/page__view__findpost__p__692619__fromsearch__1

[vega]

info from another page

 

"It is possible to calculate the areas fairly well. You must know at what rpm your your camshaft will give peak power rpm. The formula I use looks like this:

( cc of 1 cyl. X RPM X VE ) / ( 3000 X m/sec. ) = area in mm squared.

The velocity at the csa is 140 m/sec. and at the port entrance around 106 m/sec.

The entrance area depends on where the CSA ends up, how long the intake runner is and how much of it is in the head. It should of course not be bigger than the carb or throttle body. Sportbikes have very large entrances in the head with calculated velocities around 70 m/sec.

On a 4 valve head the csa is just before the valve guide on a downdraft head and just before the bend on a more horizontal head."

 

Does this seem right?

[vega]

OZconnection

I read your stuff. We already know that smaller ports creat more tq at lower rpm. we also already know that bigger ports will create more hp at high rpm sacrificing tq at low rpm. I am looking for something in the middle of this. My operating range is all the way down from 1500rpm all the way to 68 or 69000rpm that is a WIDE RANGE. so what would the best cc port be then to achieve the flattest tq and hp curve be?

[ozconnection]

Oh gee, if only there was a simple answer to this....

 

In that other thread, I wanted to run small ports and a high quench head with a biggish cam and a dual plane manifold to get the torque that I needed. With plenty of static timing and longish runners I would have a stump puller, all done by 5000 rpm but strong from say 1.2K rpm. Tony suggested the P65 manifold and now I have one of those too. I have megasquirt but I now have another car that is going to be ET spec. So I have a whole bunch of parts waiting for another engine construction.....and an LD28 stroker crank.

 

Vega, its the combination of parts that will give you what you want, or need. My 280C is a street cruiser and the thought of bulk torque appeals so a large capacity, high comp, small ported and appropriately timed engine with a P65 EFI manifold with a biggish cam to get the whole thing breathing. I think it's easier to swap cams than rip off heads all the time to try that illusive 'optimal' port size.

[vega]

I already have the cam that is optimal and i am stuck between two weiand manifolds.

[vega]

The head options I am looking at (1 having the highest numbers on the bench);

 

1 AFR 195 Eliminator

2 Jegs 195

3 AFR 180 Eliminator

4 Trickflow twisted 190

5 Jegs 180

6 AFR street 190

7 AFR street 180

 

I have a range of either a 180, a 190, or 195

 

The highest flowing head under 180 is the Edelbrock RPM 170 head That seems too small to me.

[letitsnow]

I have a 355" sbc with trick flow 195's with a smallish cam(comp 270hr magnum) in a firebird, it's not short on torque by any sense of the word. Plenty of power from ~2000rpm-6500rpm. You don't start losing any appreciable torque down low until you start going 210+ on a 350" engine. Even if you are down on torque a little, with an engine this size in a car this light, it's going to GO no matter what. You're not going to miss 20ft/lb right off idle ever.

 

If/when I get sick of the L28 in my car I think this engine might end up in it with a t56, it went 12.3 in a 3rd gen camaro previously, should be good for 11.5's in a Z.

[vega]

Here is the formula to calculate the size the smallest cross section in the cylinder head should be.

 

.00353 X bore X bore X stroke X rpm/ 690 = cross section in sq"

 

Rpm= rpm you want max. horsepower

690= 60% of the speed of sound

 

Here is a calculation:

 

.00353 X 4.03 X 4.03 X 3.25 X 6800rpm = 1267

1267/690 = 1.836 sq"