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Some silly head porting related musings.

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  • Some silly head porting related musings.

    Just this morning I flowed a stock head from .050 to .420 valve lift on a SF600. Stock numbers are always depressing to me, but at least the ratio is right around the magic .7 (.68 average is what I got both pre and post porting, though I think I would rather see around .75+ for a turbocharged engine).

    For this (my own personal development and interest) work, all I worked on was the blending around the seat insert, long turn and bowl area of one intake and one exhaust port. Minimal material was removed, spent MAYBE 5-7 minutes on each port.

    From memory all final flow numbers up to .250 lift lost between 3 and 7CFM, but after that they continued to rise to an overall gain of over 10CFM for both intake and exhaust. Keep in mind I did not touch the chambers, port inlet/outlet size any bigger or alter port volume substantially. Looking the data and thinking of stock vs aftermarket cam specs, there is really NO reason at all to lightly port the heads unless you plan on using aftermarket cams with longer duration and additional lift. The additional duration and lift will more than make up for the typical low lift losses you're going to see from basic porting (some of which can be regained through more extensive work which is what I plan to do over time). As I got to thinking about it, this is compounded even more by the timing of the peak pressure wave that is pushed through the port by the piston in mid stroke; you are at the peak airflow volume close to your peak lift and hence airflow numbers. Thus, the pressure drop through the port will be far less during the timing of the greater pressure "front" if you may. This partially negates the theory I had been taught time and time again which is the valve spends more time at lower lifts so more power can be had in the end because there is more airflow to be had with additional time. Actually, you may just end up encouraging backflow/reversion by having better flow numbers towards the lower lifts (and either end of your I & E durations, exhaust especially
    in turbocharged applications), because it's effectively doing the same thing as changing the cam lobe profile to be much steeper.

    Feel free to comment, just some food for thought.

  • #2
    Scratches head...

    I may not be following you completely here. This is what I am reading here initially, Please correct me if I am wrong.

    Mild porting, assuming no valve or valve seat alterations, may actually have a negative effect on performance depending on cam lobe duration because the porting could actually counter the valve lift duration? (effectively make it shorter which is bad for turbo applications?)

    My thoughts:

    wouldn't this depend on the amount of lobe overlap as to how much back-flow/reversion would be present? Also the angle of the cam lobe would have some significant impact on this if I am thinking correctly. Short/long duration???

    adjustable cam gears ( Russ' or Morgan style) simply thinking here..., I wonder how much slightly changing the cam/valve opening closing points in relation to the piston would help this effect? Probably hard to quantify.

    What happens with an "aggressive" port/polish and valve cut? Theoretical at this point I suppose but, it would be interesting to see some differential numbers on: stock, mild port, and full port and valve seats.

    I could just be thinking of this from an NA perspective though. If so...sorry.
    Just stand back and throw money.
    Performance costs money.
    Reliable performance costs more.

    Comment


    • #3
      as is always the case you have to think of the air like a liquid, this applies to the exhaust too

      so what can happen is you improve the low lift flow numbers to the point where the cylinder is filled prematurely and a long duration pressure wave is created by the "impact" .... basically you approach the static quantity of air as or more quickly than the increase in space the piston going down creates causing a logjam for the rest of the intake air behind the valve

      not a big issue once boost hits, but pre-boost or unboosted you can have a loss of efficiency.... see why vvt is so popular now?

      Comment


      • #4
        OR-Zman wrote: I may not be following you completely here. This is what I am reading here initially, Please correct me if I am wrong.

        Mild porting, assuming no valve or valve seat alterations, may actually have a negative effect on performance depending on cam lobe duration because the porting could actually counter the valve lift duration? (effectively make it shorter which is bad for turbo applications?)
        Perhaps a negative effect, but I am thinking more along the lines of little to no benefit overall. IE wasted time. The bowl port work is just the beginning of my toying with different ideas, but it's the primary area most "head porters" focus on along with the short side radius for making peak lift flow gains.
        It would not entirely "counter" the effects of your larger duration camshaft, but rather contribute a minuscule amount on either end of the lobe. The longer duration with increased lift means the valve is open more distance and for more degrees of rotation, so it has more time to make up for the low lift losses.

        OR-Zman wrote: wouldn't this depend on the amount of lobe overlap as to how much back-flow/reversion would be present? Also the angle of the cam lobe would have some significant impact on this if I am thinking correctly. Short/long duration???
        Um... yes. But also the manifold and pressure ratios in the system are playing a large role. If by the cam lobe angle you mean the profile, then yes also. The thing is we can only run so steep of a profile with a flat tappet camshaft. With a roller cam you can be much more aggressive with opening and closing rates, which means peak flow is much more meaningful on those engines with something like a 300 degree .800" lift cam like a BBC 400. They may go from 0 to .250 lift in half of the distance or less than a VG cam. So low lift numbers are a little more important to us than on that engine as an example.

        OR-Zman wrote:
        adjustable cam gears ( Russ' or Morgan style) simply thinking here..., I wonder how much slightly changing the cam/valve opening closing points in relation to the piston would help this effect? Probably hard to quantify.

        What happens with an "aggressive" port/polish and valve cut? Theoretical at this point I suppose but, it would be interesting to see some differential numbers on: stock, mild port, and full port and valve seats.

        I could just be thinking of this from an NA perspective though. If so...sorry.
        I really don't mean to rag on them, but adjustable gears like morgans are useless for this application because you don't know where you will truly want your degreeing until you either measure, degree using idle vac as a reference, or dyno the engine. Then you don't have the kind of adjustment you are looking for anyway, just a guesstimated 3 degrees in either direction.

        Valve angles will make a bigger change in the low lift numbers, but I have yet to play around with different back cuts, seat widths and top/throat angles. The rest of the port work will come with time, but I at least have my method down for when I get to it.

        My understanding is everything in airflow is taken from an NA perspective to begin with, only small changes are made for forced induction.

        Comment


        • #5
          Jason, one of us is confused. I am not sure if you are talking about intake or exhaust, turbo or NA.

          Are you saying a more efficient port has no effect on the engine, or, are you saying the port you made has no effect on the engine.

          So it seems that you understand the need for a valve job that complements the 'new' shape you made in the bowl. (Sometimes a backcut on the tulip side of 35-32 degees will help). I would agree that you won't get the performance you should unless you finish the job and work with the valves.

          I am not clear on the pressure 'wave'. On a blown car, isn't the pressure the same at the throttle body as it is at the valve seat. You said 'pushed' so I am assuming here, that you are refering to a blown application. In an NA engine the piston causes a vacumn.

          My understanding is there is very little backflow/reversion on a blown car.

          Did you do any grinding on the metal seat when shaping the bowl?

          Comment


          • #6
            JK wrote: Jason, one of us is confused. I am not sure if you are talking about intake or exhaust, turbo or NA.

            Are you saying a more efficient port has no effect on the engine, or, are you saying the port you made has no effect on the engine.

            So it seems that you understand the need for a valve job that complements the 'new' shape you made in the bowl. (Sometimes a backcut on the tulip side of 35-32 degees will help). I would agree that you won't get the performance you should unless you finish the job and work with the valves.

            I am not clear on the pressure 'wave'. On a blown car, isn't the pressure the same at the throttle body as it is at the valve seat. You said 'pushed' so I am assuming here, that you are refering to a blown application. In an NA engine the piston causes a vacumn.

            My understanding is there is very little backflow/reversion on a blown car.

            Did you do any grinding on the metal seat when shaping the bowl?
            Just the guy I was hoping would comment, thanks. I'm just saying light porting alone, with stock cams, valve grind and seat/valve angles is borderline on a waste of time. The seat grind is going to be my next focus, and I agree it needs to be done to compliment port work. I will try your suggestions, it's really about what I was planning for the next time I get to it.

            This "wave" I was thinking of, for lack of a better understanding/description, is when the piston is mid-stroke and the air mass is starting to build velocity. It may be I am thinking much too deeply with too little information already crammed into the brain. This is thinking along the lines of (airflow vs piston velocity vs valve lift) the effect that yields increased cylinder filling with just an increase in intake duration. Am I way off base here?

            Reversion on the exhaust side is what I was referring to. I have always heard there does tend to be substantially more on turbocharged cars simply because the exhaust manifold pressures are so much higher.

            I did a small amount of grinding on the seat insert when shaping the bowl, but just enough to blend it into the port. Not more than a few thou was taken off.

            Comment


            • #7
              Jason84NA2T wrote:
              This "wave" I was thinking of, for lack of a better understanding/description, is when the piston is mid-stroke and the air mass is starting to build velocity. It may be I am thinking much too deeply with too little information already crammed into the brain. This is thinking along the lines of (airflow vs piston velocity vs valve lift) the effect that yields increased cylinder filling with just an increase in intake duration. Am I way off base here?
              hmm, i wonder if this is why Gregg at Hekimain Racing runs SHORTER rods and his own cam grinds? in language i barely understood, he pointed out how the lower overall piston speeds (yes faster at tdc/bdc) that come from a shorter rod complimented the comparitively low port volume in a vg. WAY the hell over my head! in fact i may have it exactly backwards as far as the piston speeds. http://www.hekimainracing.com if you're curious.

              Comment


              • #8
                [quote]benz-tech wrote:
                Originally posted by Jason84NA2T
                This "wave" I was thinking of, for lack of a better understanding/description, is when the piston is mid-stroke and the air mass is starting to build velocity. It may be I am thinking much too deeply with too little information already crammed into the brain. This is thinking along the lines of (airflow vs piston velocity vs valve lift) the effect that yields increased cylinder filling with just an increase in intake duration. Am I way off base here?
                hmm, i wonder if this is why Gregg at Hekimain Racing runs SHORTER rods and his own cam grinds? in language i barely understood, he pointed out how the lower overall piston speeds (yes faster at tdc/bdc) that come from a shorter rod complimented the comparitively low port volume in a vg. WAY the hell over my head! in fact i may have it exactly backwards as far as the piston speeds. http://www.hekimainracing.com if you're curious.
                This makes perfect sence. Generally gases should not exceed roughly 450ft/sec in a pipe. After this point drag increases signifigantly. Having a shorter rod to compliment a smaller port volume, makes perfect sence. I havent figured out the perfect set up yet. But then it would depend on what size you make your ports, only then could you begin to decide how much shorter to make your rod to compliment your port size.

                It all comes down to head design. :wink:

                I am not sure what Jason is meaning when he says "the bowl" but I can only assume he means right around where the valve stem comes into the port. This makes sence to me because this is the, how do I say it, flattest part? of the whole port. It needs to be more deep to make a smoother transition.

                Now as far as a light porting goes, after the numbers that Jason posted up, I am of a firm belief that if you do just a polishing on stock ports, that would be the best thing to do when running stock cams. I cant see polishing hurting anything as you are not changing port shape at all. However I can see it helping for obvious reasons of the air having a smoother surface to glide over. All is well, until you upgrade your cams.
                85 Z31 6.0 LSX turbo 766whp/792wtq
                04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.

                Comment


                • #9
                  Allthough, this would make your wave or pulse more "dence" if that makes any sence, dut to the faster piston speed at TDC. It would compress that wave faster than a longer rod would. I dont know if that would make any difference. Over all the piston has to move the same amount of air in one stroke to have the same displacement, wether it be a long or short rod. So I dont know how changing the rod length would really alter one pulse of the exhaust stroke.

                  The pulse happens from roughly BDC (varies greatly depending on valve timing) to TDC. You can change the rod speed at different places in that stroke, But ultimatly you are still stuck with the "frequency" of whatever RPM you are at.

                  I guess basicly what I am saying is that if you run a short rod it makes piston speed slower at 90* but faster at TDC. If you run a long rod it makes piston speed faster at 90* but slower at TDC. It has to balance out either way. No matter which one you choose, I would think the exhaust pulse would be dictated by the rpm's of the engine, not rod length. So maybe the sence isn't as perfect as I thought it was last night.
                  :? :?: :?: :?:
                  85 Z31 6.0 LSX turbo 766whp/792wtq
                  04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.

                  Comment


                  • #10
                    I think what primarily that engine builder was working with is the limited overall volume of the ports. vg ports can only get so big. in his thinking, since the port size is relatively small, a quick suck from the piston mid stroke could theoretically not pull as much air as a longer more even pull. since the valves are nearly closed at tdc/bdc that extra piston speed there would not affect port flow, but would as you mentioned affect the inner cylinder dynamics. he is dealing with n/a so i'm sure they are more sensitive to flow since there is no force-feeding.
                    I hate to say it but the VG has many limitations that require $$$ to work out. small ports, small valves, small turbos or low compression and possibly the worst: "biggest cam avail has .450" lift?!"
                    sorry to jack your thread Jason, this was about ports.

                    Comment


                    • #11
                      ....perhaps there's an inefficiency overlap in relation to rpm they've discovered/utilized, there has to be a point where the pressure front is either given multiples of time (multiple wave reflections?) and is inefficient for short bursts kinda like resonance frequencies, or the inverse given too little and becomes almost a barrier of constant resistance.... which is all dependant on port velocity

                      if you can match the cross-over point in a force fed car so that you just begin to hit the wall at the same time as boost, you can probably transition really nicely .... however with the side effect that you limit ultimate top-end flow which agressive cams can offset somewhat

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