4-VALVE HEAD - OPTIMUM PORT VELOCITY??
4-VALVE HEAD - OPTIMUM PORT VELOCITY??
(OP)
I just recently found out about this forum and am very impressed so far with the topics and discussions. I decided to post a question.
Quick background is that I am designing a head for a V twin from a clean sheet of paper. It is a DOHC, 4 valve design. I have a mechanical engineering background and I have experience with flow simulation software and would like to utilize it to design the proper port sizes and shapes for this engine. The engine has 4.25 bore and 4.00 stroke. The rev range should be 7000 to 8000. I would like to see it make peak power well above 6500.
Most of what I find for literature exists for 2 valve designs.
What equations /rules apply for max port velocity at 28 inches H2O ( intake and exhaust)for a 4 valve design (2 intake, 2 exhaust?
Any other suggestions on good port design practices would be appreciated.
Thanks
Quick background is that I am designing a head for a V twin from a clean sheet of paper. It is a DOHC, 4 valve design. I have a mechanical engineering background and I have experience with flow simulation software and would like to utilize it to design the proper port sizes and shapes for this engine. The engine has 4.25 bore and 4.00 stroke. The rev range should be 7000 to 8000. I would like to see it make peak power well above 6500.
Most of what I find for literature exists for 2 valve designs.
What equations /rules apply for max port velocity at 28 inches H2O ( intake and exhaust)for a 4 valve design (2 intake, 2 exhaust?
Any other suggestions on good port design practices would be appreciated.
Thanks





RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Don't forget the velocity around the valve head as well, there is a calculator at
http://www.rbracing-rsr.com/machcalc.html
There is also another calculator for runner area vs torque
http://www.rbracing-rsr.com/runnertorquecalc.html
Of coarse i wouldn't take these as being 100% correct but it might help get a better understanding of whats required as starting points only!
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I think these rules of thumb are very usefull to get one started on a design. Simulations may have their place, but eventually you will have to slug it out with a good 2d or 3d cad program to figure out what will fit within the bore, how you will run the ports, how to get the spark plug in, and how to get enough cooling for the exhaust. This is what will make or break a design, and there will be lots of compromises.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Mean inlet port speed = 100m/s
Mean exhaust port speed = 120m/s
Based upon average port diameter (exc. throat area which was CNC machined)
Any higher and the powerband just went higher and torque dropped off, and vice versa.
This was also shown to be optimum based upon 1D engine simulation.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
For Andyv8:
I assume that the velocities of 100 intake and 120 exhaust were measured/calulated based on flow bench work using 28 inches H2O? Is that correct?? If so, these are well in line with what I have been using so far.
One more question for all: We have built a prototype of this head design and flow tested it. The intake port is very "noisy". Piecing whistling sound. I am working with an experienced builder and he suspects turbulence. The port flows 300 CFM at .200 lift, 360 CFM at .500 lift at 28 inches and the ports are not overly small. Problem is this engine made best power at only 5200 rpm. Could the noisy port be responsible??
Any thoughts??
Thanks, Jim
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
The 100/120 port velocities were calculated from mean piston speed.
port vel = (bore^2/port_dia^2)*mean_piston_speed
where port_dia is the equivalent dia of the combined 2 inlet ports. (all in mm)
I'll try to dig out some flow bench data from the same heads.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Couple of more questions:
1. We are using 246 duration and .460 lift for both the intake and exhaust. Does that sound reasonable for peak power near 7000 rpm?
2. "Furthermore this fast action (and the multi valves much higher low/mid lift flow) require a relatively larger volume behind the intake valves (less velocity)."
Do this mean that the intake runer length should be longer or bigger in cross section than on a comparable 2 valve??
3. Basically, this engine has very good low end torque but will not make power past 5200 rpm. We are tying to figure out why. Both cylinders now are sharing a single 48mm carb and we suspect that we may need more. The engine is 4.25 bore and 4.00 stroke.
thanks
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Found the flow bench info (25" water):
Hope it's of some use.
Bore = 87mm
Stroke = 82.6mm
Bare Cyl. Head with plasticine bell mouth
Cylinder 1 - Forward Flow
Lift (") Flow % Range CFM
0.050 13.10810811 4 38.8
0.100 27.90540541 4 82.6
0.150 40.97972973 4 121.3
0.200 52.63513514 4 155.8
0.250 61.68918919 4 182.6
0.300 68.31081081 4 202.2
0.350 71.99324324 4 213.1
0.400 74.69594595 4 221.1
0.450 75.70945946 4 224.1
Cylinder 1 - Reverse Flow
Lift (") Flow % Range CFM
0.050 4 41.9
0.100 4 81.4
0.150 4 120.5
0.200 4 147.7
0.250 4 166.8
0.300 4 177
0.350 4 183.5
0.400 4 188.9
0.450 4 191.9
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
One 48 mm carb may be starving your twin.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
John Haskell
Aire Research Engr.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
My goal at this stage is to use a port flow simulation program (which I have access to) that I can set up to simulate flow performance on a flow bench. Input to my program are pressure differential (28" in this case) and also port velocity.
Thanks again
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
No one is answering our question about flow bench velocity because thats not the way most of us approach the problem. The demand placed on the head comes from piston motion (and exhaust scavenging). You need to calculate the demand first, and then apply the demand across the cross-sectional area of the head, and this gives you the velocity in the head. Most people look for intake manifold mean port velocities in the 300-360 ft/sec range.
The flow bench can be misleading because when you put a bigger head on it, the pressure drop goes down, and common practice is to crank the motor up to restore the 28 in pressure drop. Does an engine do this? No. The piston motion generates the available potential energy, in other words you only get so much to work with.
Shrieking ports on the flow bench are not good. Because you have super high low lift flow, and not much gain as the flow increases (what size valves?, the short side turn may be too high, however this usually is not noisy.
360 cfm should be plenty to take a Harley to 8,000 rpm. Are the exhaust pipes proven with regards to length and diameter?
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
"Shrieking ports on the flow bench are not good. Because you have super high low lift flow, and not much gain as the flow increases (what size valves?, the short side turn may be too high, however this usually is not noisy."
Only the intake ports seem noisy. There is a divider where the single runner divides into separate ports leading to the valves. There may be something there that is causing the problem with noise. I just don't know whether that noise has anything to do with the limited peak power rpm we see on the dyno. Can the port be limiting rpm because of turbulence??
"The demand placed on the head comes from piston motion"
What is the correct calculation for the demand from piston action?
"Are the exhaust pipes proven with regards to length and diameter?"
Nothing on this engine is proven. It is a completely new design. We are learning what it needs. On the dyno, we tried two completely different exhausts and the peak power would still not go beyond 5200 rpm.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: Carburetor
Your description of the carburetor as "48mm" is somewhat vague. The airflow limiting feature of a carburetor is usually the venturi. Assuming you have a 48mm butterfly, the venturi ID would be smaller in diameter. You may need a larger venturi ID. You probably need a much larger carburetor as well.
I used to build V-8 road race engines in the early seventies with 48mm Weber carbs. I used four 48mm Weber IDA's (butterfly diameter). The engines were about 300 cubic inches displacement. These carburetors had removable venturi's and various venturi ID's were offered. These were two throat carburetors, i.e. one throat per cylinder. We had excellent power to around 8500 RPM.
If your carburetor has a venturi that is too small, you could be going into choke flow easily. Hence, your 5200 RPM upper limit. We used to size the venturi's to match the track. Small relative diameter for tight "torque" tracks. Larger venturi's for a track like Riverside International Raceway (long track). Venturi size will move the torque curve like a dial on a radio.
Flow test your carburetor and compare the test results with what the engine needs at say 90% to 110% VE or so.
I have a Porsche 996 GT-3 Cup head (four valve) in my shop that I would flow test if I had a bunch of extra time. Whatever the Porsche factory did to this head, they did right.
Will
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
The carb used is an HSR48mm Mikuni flat slide. No buterfly. It measures about 48mm at the bore.
"Flow test your carburetor and compare the test results with what the engine needs at say 90% to 110% VE or so."
Mikuni states that the carb will flow 270CFM at 12".
I calculated the theoretical flow rate needed for 114 cu.in. at 8000 rpm to be 264 CFM (assuming 100% VE). Given this, what is the recommended intake/carb flow for the application? Is it safe to design to only 264 CFM? I assume there is a multiplication factor needed for a design flow rate based on theoretical flow required.
If the theoretical flow is 264 CFM
The intake flows - 360 CFM at 28"
The carb is capable of 270 CFM at 12"
Could the carb and intake port be a limiter????
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I've used 106-108 degrees
"How does the area of the smallest point in the port or manifold compare to the valve curtain area?"
I don't have this info handy. What do you recommend?
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I envy your project. OK if the carb flows 270 CFM at 12" it would flow 413 CFM at 28" in a perfect world. I would want to flow the head first with a radius inlet at max valve lift plus .1 inches (for good measure), then add the intake manifold with a radius inlet, it should be within -1% or better yet show a gain. Then add the carb to the whole thing with air cleaner (if you run one), it should again be within -1% to -2 %.
Port area, if it is a CNC port contact the porting people they will know the minimum cross sectional area, and possibly the average area. Not a CNC port, get some two part pour-able rubber low durrometer (12-15) from say Walco (www.walcomaterials.com) I use GE-25. Make a plug of the port, cut it into sections ink it with a stamp pad and stamp it on a .1" grid graph paper, count the squares (average the small parts around the edges) and divide by 100, poof port area. Do this several times down the lenegth of the port, look for the smallest cross sectional area (the choke point)around the valve guide boss or other obstruction.
As for the whistle port, yep it could be a problem. Try this little test, flow the head at 5" of water, do the flow correction for 28", does it flow the same as the perfect world correction? Try flowing the port at the highest depression your bench can provide, back the numbers down (with math) to 28" are they equal? The test is looking for high speed port problems the higher the test depression the higher the test velosity.
As for max port speed, average port speed etc. I have yet to see any concrete numbers, as for one of the posts " Ideal velocity is .0000001ft/sec less than the speed of sound, and you can get that from a .05cm/2 port, but it's the combination of volume and velocity that's going to equate power." I think you would be looking at F1 technology to prove a port at this speed. The best numbers I have found are around .55 MACH, say 614 FPS (max not average), this can make as much as 2.5 hp per cubic inch at near 10,000 RPM in a pure race engine.
Good luck,
PFM
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Does 110 sound reasonable??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
On a 2 valve design, going from 106 say to 110 would imporve the top end HP and going from 106 to 102 would improve the low end torque. Is this not the case on a 4 valve?
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I'm new to this forum, and have been reading this with greate interest, and trying to use the formulars to find the best port size for my 3ltr v6 motor peak power at 6850-7000rpm, has 83mm stroke-87mm piston diamiter, 4 valve, very simular to andyv8's examples, but if best air speed is 100meters a second, thats going to need a big port, considering the piston is moving faster than that, so the port will need to be bigger than the diamitor of the piston, to slow it down to that speed
What am I missing here, is it to do with avarage port speed, so do you calculate it at 3500 rpm?, how do I calculate best port size for peak power around 6800-7000 rpm, or for any rpm,
Thankyou,
Ryan
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I'd like to toss in a suggestion.
Why don't you get one of the available engine simulator software applications (available from www.performancetrends.com among others) and do a rough model of your engine. You can then play with a lot of variables and get a feel for how they interact.
No doubt you'll find - at the end of the process - that the dyno gives a slightly different optimum than the program, but right now you're facing an awful lot of interacting unknowns - and that's a great time to use computer simulation.
I've tried several of the inexpensive engine simulators and like the Performance Trends Engine Analyzer the best. I'll confess I haven't yet validated the results, but the results vary in a way consistent with my experience modifying production engines for racing.
Good Luck!
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I went to the performance-trends site. From your experience, what engine analyzer would be a good package for getting close - I was thinking the PLUS model and not the full blown PRO model. I would be looking for general trends out of this simulation.
I like your idea - I didn't think the software was this affordable.
Good suggestion
Thanks
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
thanks Incandescent, hate that when you doing the math late at night, I looked at the piston speed, twice even, forgot to div by 60 for per second..
All looks good now on calculations.
Cheers
Ryan
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I use a ratio between exhaust & intake sizes of 1.2-1.3 after looking at everthing from Cosworth to Ducati. I use a 1.8:1 rod ratio gatering info from Jerry Branch Grumpy Jenkins and others. One last point that flow bench numbers can be deceptive as there is no piston in motion...
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Now with theese caculations for peak power/rpm port sizes, would the port size be the same for turbo charged engines?.
Thanks
Ryan
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
If you're mostly looking for trends, either the regular or the plus version should do fine. The plus version is mostly convenience enhancements.
If you're going to actually model your engine in detail, you might want to consider the pro version as mentioned by Ron Hamp. I think it's a rather different, heavier duty program that allows modeling of actual port profiles, etc.
But you'll certainly learn a LOT playing with the regular or plus version, and it should get you in the ballpark as to port sizes, valve sizes, required cfm flows, approx cam specs, etc. Actually quite fascinating to play with, and really helps develop a feel for how all this stuff interacts.
Al Seim
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
earlier you posted a rely stating "How does the area of the smallest point in the port or manifold compare to the valve curtain area?"
The valve curtain area here is approx 4.75 sq in for both intake valves together at max lift
The smallest x-section of the port is approx 2.7 sq in.
The existing intake x section is approx 3.00 sq in.
question is: what should the comparison look like?
thanks again for the help
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
I mentioned earlier in the thread about shrieking intake ports. As we test more on the bench, it seems that the noise does not interfere with the ability to flow. The ports continue to increase flow as we increase lift. We tested up to .500 lift.
In your opinion, is the noise something to be concerned with? I am ready soon to put the motor back together and get in on the dyno again. But if there is something to do about the port noise, I'd like to address it now.
Any thoughts? Are 4 valves more likely to have ports more noisy than 2 valves?
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Might try putting a radius on it with clay while on the flow bench.
If the engine's on the dyno it might be worth retarding the cam a few degrees to simulate a little more cam.
What's the Brake Specific Fuel when the power is starting to drop?
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
We have experimented with a few designs for the intake manifold configuration. The one that had relatively large cross section runners and relatively large volume flowed much less than the more streamlined, smaller cross section design. Not sure why? More importantly, not sure if one is more likely to work (make more power)than another.
The next round of dyno work will happen soon, but it would be nice to anticipate/predict Hp gains based on flow bench data.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
but I generally try and get the choke point to flow
a ceratin % of the valve capability. That is what was
mentioned above, use 61% of cross sectional area at
the divider, and yours calculated to be 56% and I believe
that is where the problem is. I use sims too, and admittedly,
the Harley V Twin not being an example motor on it, I simmed
your exact combo (within reason) and it needed a much larger
cam than a 246* cam. Were you using the Harley standard .053"
lifter rise or the more common automotive .050"?
Makes little difference, the cam was much larger than
this would allow for, I was getting a cam in the 290 range
but using program supplied head models. I will adjust from
there and see what it suggests.
But back to the point I started on, I work it from
choke point and taper it both directions from there.
If Rich's suggestion that your choke is the divider section,
then I would enlarge it some, and IMO the noise might
subside too. That part bothers me, some reading I have
done that quotes (Ken?) Chapman indicates that a "perfect"
port will be just about ready to sound off (make noise)
and this indicates the flow/velocity is well balanced.
I know that is somewhat vague and applies to 2 valve
stuff he works with, but the concept may follow thru to
the 4 valve arena.
Flow on both sides of the choke is critical to get
just so, whether it is the valve, guide or some other
point of minimum flow/max velocity.
Sounds like your 520M RPM HP peak is quite low, I'd really
inspect the choke point suggestions and the carb sizing.
The carb seems adequate if you used 2 of them, I know a
common plenum reduces overall carb needs but I do not know
the factor for a 2 cylinder. I'd exceed the the calculated
CFM needs of the engine by ~10% to start, after the choke
point is addressed. This assumes the common plenum arrangement.
Those appear to be the 2 problem areas from my viewpoint,
but I'm just a Jr. compared to these fellows around here!
I love this board! That's me, finally
a BBS where I can read more than post!
PS: ran a quick sim with your flow figures and
a cam very similar to yours shows a 9k HP peak,
Either I am using too many wrong assumptions
(you list very sparse specs) or your setup is
quite overbuilt after you work out the RPM problems.
More specs please? :D
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
FWIW
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
i spent many time modifying cosworth and bmw M power heads, both of them they are "silent". You test them on the flow bench and you just hear the motor of the flowbench.
try to add material (clay) to see if the turbulence stops.
george
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
Probably way too late (been away from the forum for a while), but the answer your looking for is probably 27mm diameter ports for a 4v head on the v6.
Andy
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??
For practical purposes Fueling{s work (Ok I dont have apostrophies) most reflects your own inquiry.
RE: 4-VALVE HEAD - OPTIMUM PORT VELOCITY??