4-VALVE HEAD - OPTIMUM PORT VELOCITY?? 4

[v114]

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

 

[richdubbya]

If the carb is too small, you will see an increase in manifold vacuum at 5200. If the problem is in the manifold or ports you won't. What is the crankshaft degree at the intake lobe center? How does the area of the smallest point in the port or manifold compare to the valve curtain area?

 

[v114]

"What is the crankshaft degree at the intake lobe center?"

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?

 

[PFM]

V114,

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 (

http://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

 

[flowmaster]

First off your cam is to small the Rotax engines that i had worked on were only in the 670 cc range and required a cam with 270 degrees of duration @.050 judging from your flow figures your going to need a cam with 260 to 270 degress i also stated in here that average port velocitys should be in the 300 to 360 range higher of course for a wider curve .the 48 mm mikunis are probally a bit small for a cylinder this large i had built some special mikunis for a 80" dual carb project that had 48 mm venturis and tapered out to 52 mm. its going to be hard to over carb an engine with individual runners that is this large .my experience comes from building and porting single cylinder dirt track engines to working with the late Jim McClure on his fuel bike 4 valve heads .these numbers are just based on past experiences theres knowledege on this forum well beyond mine .

 

[v114]

Any experience out there on what to use for crankshaft degree at the intake lobe center for an engine like this??

Does 110 sound reasonable??

 

[richdubbya]

I'm sure opinions vary quite a bit on this. My experience shows that, combined with very fast valve opening and relatively short duration, figures in the 102 to 106 range give much broader torque and hp figures. I know it sounds low tech, but the most important aspect in a 4 valve is to make sure the intake valves open as quickly as possible (not earlier but faster), meet the piston, and follow it down the bore. If the ports are sized correctly to supply the very strong draw that this achieves, then the valves need to be closed much earlier because the cylinder fills fast and well and any long delay will cause back flow out of the cylinder into the port. The other way to try to accomplish all this is to have smaller intake ports, longer duration to try to fill the cylinder, and later closing of the valves to take advantage of the velocity in the smaller ports. This will work to some degree, but IMO you are giving up the basic advantage of the 4 valve head, it's ability to keep up with the demand in the cylinder as it occurs.

 

[v114]

Richdubbya,

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?

 

[richdubbya]

Yes, the numbers work the same way. Advancing the intake cam will favor the low and mid range. The difference is that the 4 valve IMO, with correct porting, will fill the cylinder much quicker and the intake valves can be closed sooner without hurting the top end. When the valves follow the piston down as mentioned above, and the duration is shorter this moves the intake centerline forward resulting in a lower number. I didn't mean to imply that these numbers would work with your ports and cam, I was just trying to explain the difference between 4 and 2 valve porting/velocity/cam timing. I have seen 4 valve engines with intake timing at 101 make good power to 12,000. IMO the sooner you achieve your max volumetric efficency the better and closing the valves at that point makes the best power.

 

[NzRyan]

Hi Guyz,

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

 

[incandescent]

Ryan, I think you have your piston speeds miscaculated. They can't be more than 100m/s even at peak - more likely around 40m/s.

 

[ads22]

V114:

I'd like to toss in a suggestion.

Why don't you get one of the available engine simulator software applications (available from

http://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!

 

[v114]

ads22,

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

 

[NzRyan]

Hi,

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

 

[thundair]

The head design is one of those "The more you know the more complicated it gets." I thought I knew everything about head design untill I spent time at Branch Flowmetrics and Feuling R&D. My point is the velocities , delamination reversion waves, converging port walls,valve size and shape,rod ratio,lobe centers etc. The best chance of hiting your mark is use as many imperical formulas as possible with heads that are working in your rpm/bore requirements.
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...

 

[NzRyan]

Hi All,

Now with theese caculations for peak power/rpm port sizes, would the port size be the same for turbo charged engines?.


Thanks
Ryan

 

[rhcycle]

I use the Performance trends soft ware and have had some good results on HD engines .theres a couple of examples in the pro version that are ether engines i built or worked on and they have produced very close results .i have also ran Hondas CRF 450 in the program and have shown very similar results in stock and modified engines these are a 4 valve engine. Ron Hamp, RHC

 

[ads22]

V114:

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

 

[v114]

richdubbya,

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

 

[schmidtj86]

Does that valve curtain area at max lift take into account valve to cylinder wall clearance? May need to approximate that part if you're trying to simulate it.

 

[richdubbya]

You seem to be fairly close to what I use, port x section about 61% of curtain area. Your figures are a little lower. Making the transition from the single port to the 2 short ports is important. There has to be an increase in x section to accommodate the 2 valves and curtain area but try to make this gradual rather than abrupt to avoid a sudden loss in velocity. There is a trade off here, between velocity and the ability to keep up with the excellent mid lift flow of the 2 valves. Four valve engines need very fast opening intake lobes with less duration than you might think. It sounds simplistic, but IMO the intake valves must closely follow the piston down the cylinder. If the typical 2 valve cam lobe is used, then you need high velocity and long duration, and you end up with very little benefit (just combustion efficency) over a 2 valve engine.