Moderator: Team
What are you using for your throat diameter %? I assume that when you say you are using deep valve seats, that you mean a fairly large radius under the seat cut, maybe like 13mm?Using a divergent Laval nozzle shape in the exhaust port works most every time,I've had good results using a small and deep valve seat shaped to make a venturi/nozzle just below the valve seat,then taper the port out to the gasket flange.
Haven't got the notes here now...but from what I remember I think it was in the 85%-range or a tad below. I used 1mm oversize exhaust valves and made the minimum throat dia 2 mm or so smaller than stock on the engine I was working on. Flow numbers hardly changed,hp did. With deep seats I mean that the seat insert was thick,if you get what I mean...so the seat could be cut at the same height,but the insert did a two-fold job as a seat and a nozzle at the same time.shawn wrote:What are you using for your throat diameter %? I assume that when you say you are using deep valve seats, that you mean a fairly large radius under the seat cut, maybe like 13mm?
shawn
After looking through multiple 4 valve heads on different import motors I have been thinking about exhaust port reconfiguration to a subaru motor. To effectively create a true funnel/bell shape you definitely would need weld multiple areas of the port. How well will this work with the center divider like these designs have? If you really look at this port and take into account the two ex valves and shared port it seems as a true bell shape may not be possible in this design layout.SWR wrote:It's at the minimum CSA,i.e. the venturi. Still learning on the topic though,so I have no "rules set in stone" about it yet. But from what I gather it goes sonic at the blowdown phase,it can because the port (if shaped correctly of course) tapers outward like a rocket nozzle bell shape,so the shock wave doesn't converge on itself and choke...that's the whole reason for the nozzles on the space shuttle,as soon as you reach sonic velocity the increase in area after the minimum CSA increases the velocity, in other words does just the opposite of what a sub-sonic flow does,more area = more velocity. That's how you get multi-mach velocities out of the shuttle's nozzle. Then you're talking blow-down velocity....which gives you a mass of air that's moving out of the port at a huge speed,not leaving much to be pumped out by the piston. Exhaust pipes are still important though...because the gas does slow down after traveling a bit,and with a very low pressure situation in the cylinder you don't want the gasses to "come back" during overlap,just like ordinary exhaust tuning tries to avoid.MadBill wrote:So, in the case of exhaust, would that target speed be at the venturi under the valve seat, at the port/header junction, at the smallest or largest step or diameter of a true tapered pipe, or??
And how is the gas volume calculated? Does the 'incompressible' reference mean it is taken as the cylinder volume and converted to a velocity by same divided by time for the exhaust stroke (with or without blowdown and ATDC flow intervals) and the applicable port area?
The more I learn the more I have to be confused about..
From what I gather,it doesn't have to have a "100% rocket nozzle shape",it's all about getting the correct area change from the "nozzle" to the port exit... still testing.. Looking at www.theoldone.com , it seems like Larry Widmer made a supersonic port work...and some rules to work by.Plasma wrote: After looking through multiple 4 valve heads on different import motors I have been thinking about exhaust port reconfiguration to a subaru motor. To effectively create a true funnel/bell shape you definitely would need weld multiple areas of the port. How well will this work with the center divider like these designs have? If you really look at this port and take into account the two ex valves and shared port it seems as a true bell shape may not be possible in this design layout.
You can always explain complex system and maths with simple analogies in almost all cases.kdrolt wrote: That's another item I can't easily discuss (here) because of the math needed.
Darin and others,Darin Morgan wrote:My personal opinion and from what I have learned about exhaust ports I have to say that small super fast exhaust ports make more power over larger high flowing exhaust ports except in the case of full exhaust systems such as Nextel cup engines. For some reason they like a slightly larger exhaust port but no where even close to what I would call large. Large and Small are ambiguous. In my book anything over about 110% of the valve area is large and anything under 105% of the valve area is very small but the exit velocity seems to play a role here as well. I try to adhere to the 105-108% in our pro Stock engines and it seems that I am not alone in my theory because many of the top notch heads I have seen are about the same or within about 2%. Another very big thing to consider in the tuning of exhaust ports is there sound or should I say the lack of sound. How smooth an exhaust port sounds and how quietly it can move the air are both very serious factors to consider. As the valve opens the sound of the ports should smooth up and get increasingly silent. The loudest portion of the exhaust flow on the bench is from .200 to .400 after that they should go increasingly silent with every lift increment. I have had exhaust ports that actually cracked and popped like fire crackers! With a little seat blending and chamber work I managed to smooth up the flow, gained a measly 2 cfm average and gained 26 horsepower and it still was not correct because the port was to big. The hardest thing I do is try and fix exhaust ports that are screwed up. Its much easier to fix intake ports!
Like an intake port, an exhaust port can be made to flow a great deal of air, Just make it big.
Some rules I live by.
(1) Exit area = 105-110 % of the valve.
(2) Exit air speed at a minimum of 300 and a max of 330 ft/sec mean.
(3) Smooth silent flow by at least .400 lift and absolutely by .500 lift.
On another note, I am not sold on the theory that the flow in the exhaust port goes Sonic. Anyone here care to prove this theory?
Did these engines have to big exhaust valves and port for the application? I have also used a smaller header with a venturi on small motorbike engines with to big ports with good results. I am thinking of trying with different sized inserts. Normally you need a diffuser with a length of 10 diameters, what is your thought on this? Erland.exhausted wrote:Just got into this topic and have not read all the posts, but I would like to toss out a couple of things.
I have been building exhaust headers for professionals for over 15 years. Cup, PS, Stock, SuperStock, Comp, IMSA, SCCA, ARCA etc. I have been busy making venturis in the exhaust systems at the head to header interface.
In more than 60-70% of applications, I can make more power and torque using a tube size with an area as much as 10-12% smaller than the given port area. I would say without hesitation that this is because head porters and engine designers have always been developing their engines and heads based on data from enignes that did not have a header or system that was helping them or working effectively. Therefore, without a exhaust header that would help protect the low piston speed power and also reduce system pressure at maximum rpm's; cam duations, intake flow to exhaust flow ratios, maximum lift flow numbers, (when low lift flow is critical on exhaust, which I think is what you folks are discussing in this thread),etc, have all gone astray.
I am often able to allow engine builders to revisit previous conclusions about many aspects of thier engines after they have seen what the exhaust system can do for them.
TOP38 wrote:Darin and others,Darin Morgan wrote:My personal opinion and from what I have learned about exhaust ports I have to say that small super fast exhaust ports make more power over larger high flowing exhaust ports except in the case of full exhaust systems such as Nextel cup engines. For some reason they like a slightly larger exhaust port but no where even close to what I would call large. Large and Small are ambiguous. In my book anything over about 110% of the valve area is large and anything under 105% of the valve area is very small but the exit velocity seems to play a role here as well. I try to adhere to the 105-108% in our pro Stock engines and it seems that I am not alone in my theory because many of the top notch heads I have seen are about the same or within about 2%. Another very big thing to consider in the tuning of exhaust ports is there sound or should I say the lack of sound. How smooth an exhaust port sounds and how quietly it can move the air are both very serious factors to consider. As the valve opens the sound of the ports should smooth up and get increasingly silent. The loudest portion of the exhaust flow on the bench is from .200 to .400 after that they should go increasingly silent with every lift increment. I have had exhaust ports that actually cracked and popped like fire crackers! With a little seat blending and chamber work I managed to smooth up the flow, gained a measly 2 cfm average and gained 26 horsepower and it still was not correct because the port was to big. The hardest thing I do is try and fix exhaust ports that are screwed up. Its much easier to fix intake ports!
Like an intake port, an exhaust port can be made to flow a great deal of air, Just make it big.
Some rules I live by.
(1) Exit area = 105-110 % of the valve.
(2) Exit air speed at a minimum of 300 and a max of 330 ft/sec mean.
(3) Smooth silent flow by at least .400 lift and absolutely by .500 lift.
On another note, I am not sold on the theory that the flow in the exhaust port goes Sonic. Anyone here care to prove this theory?
Question regarding the smooth silent flow of exhaust ports. I've noticed this on a number of BBC heads (Dart 355's and AFR's 357's) at the higher valve lifts, .500 and above, the flow manometer is nice and smooth however the port has a loud high pich whistle which seams to be related to the ex valve shape (back anlge of 25 degrees and a 3/8 radius). What are you thoughts on the whistle causes and is this a performance issue? These heads have been on good running bracket type engines, 548/555/565 making 930 to 970 HP.
Very interesting stuff. Are you willing to put up a picture of your header venturi?exhausted wrote:Erland,
Yes, the port area at least at the header flange was larger than needed. I am not saying the whole port was too large. Bowl volumes are important! the flow bench is what makes exhaust ports bigger in the castings. as the port is increased in area you can flow more air. Usually you can make more horsepower because the headers are too big too and the engine is just pushing everything out, so bigger is always better untill the same pressure that is pushing the exaust out is also keeping the inlet air from coming in. If you neglect the power of the finite pressure waves in your system and how they are effecting things, the wall comes very soon. Smaller systems usually will carry power further past power peack.
I do not put a venturi or anything in header or at port. Whatever Erland did for his morotsycle port was helping to increase velocity in the exhaust. It is very common for me to use a tube diameter off the cylinder head that is smaller in area than the port's area. (How much I can reduce the area is my info.)This in effect creates a venturi out of the whole first step of the header. The reason I believe this makes more power is because the port is or was too large and the loss in velocity during the exhaust event of each cylinder is given up. I can regain some of that velocity, and often many times more power across the entire power band. The flow bench only tells us how much air that can be made to go through a opening at a given pressure drop. It does not tell you how much you need at what velocity! While it is not incorrect to sell head porting by comparing flow numbers, just as in larger carbs and larger headers, larger heads do not mean it will be tuned properly. It is time for people to really understand bigger is not always better! You have heard it before and it is true!Bos's5.0 wrote:
Very interesting stuff. Are you willing to put up a picture of your header venturi?
Does any of this correspond to a turbocharged header design?
