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Thread: Bigger exhaust valve?

  1. #21
    Quote Originally Posted by Rabbit8dude View Post
    Thanks for the insights Christian and Paul.
    I'm looking at building a short stroke 500 so my queries are at the opposite end of the spectrum compared to Dudas.

    I'm thinking of going the Trestle billet cylinder with Nicasil coating. It's a pricey item but from the looks of it a bullet proof cylinder with the benifit of a low friction lining.

    The friction comes from the ring package. Napier cut second rings, ring thickness, using gas ports so you can run thinner rings, etc. Good ring seal and bore rigidity is more important here than Nicasil. The drag racers and land speed guys know iron blocks will make more power over aluminum due to the bore being more concentric and rigid.

    I'm assuming a short stroke engine based on the 400 crank will in general be turning at higher rpm for a given road speed ie. shorter geared.

    It will turn the same rpm, if piston cfm demand is equal. The engine doesn't know if the volume displaced is from stroke or bore

    I'm still trying to get my head around the whole short stroke configuration in relation to valve to bore ratio and how that is related to the shorter stroke, higher rpm potential.

    See my response below

    By this I mean when I look at the motorcycle manufacturers gradually over-squaring trend over the years, this has been matched by bigger and bigger valves capitalizing on available piston area.

    The heads have been designed accordingly.

    So what are the benifits of using a 400 crank if the basic head design is limited in flow potential and what would the rpm be limited by, piston speed or keeping the valves under control?

    A bigger intake valve from a larger bore, but there are diminishing returns.
    The biggest issue here is the head and ultimately what the casting can do. It's more than just CFM as well.

    Getting to valve size:

    There is no sliding scale to larger and larger valves will equal more flow, and more importantly, more power. The port and the valve locations are a fixed geometry (more on this in a bit). A larger intake valve will need a larger corresponding throat. This will cause the short turn radius to be tighter. To get the air to make it around the short side radius, you need to gain area, especially on the bottom of the port. You do this by pushing the walls out. Ultimately, what you end up doing is running out of casting material. There is more to it than just cfm - you need to look at velocity as well as port stability.

    I have a 1.900" intake valve in the head I had done, and my cylinder head guy worked the short side radius walls until he wasn't comfortable going any further. The head flows 235cfm by .600" valve lift, , has good velocity (300fps on average) The head MCSA is set up for a 38mm (1.5") carburetor which is what is hold the port back from flowing a bunch more. Just as importantly, the flow depression moves less than a tenth of an inch and the flow never backs up. A stable port is a port that makes power. Remember, a running engine will see much higher than the 28" H2O it will see on a flowbench. If you're port isn't stable and flow backs up at 28", what do you think it's doing at triple that?

    Furthermore, you'll increase low lift flow, which has the detrimental effect of exacerbating overscavenge conditions - this is a problem on just about every hemispherical type head with long duration camshafts (whether the builder recognizes it or not - most don't).

    From a camshaft perspective, the larger the intake valve, the more you limit yourself on the camshaft events (IVO, IVC, EVO, EVC). With a hemispherical head with a 55* included valve angle, you're up against the valves clashing. Put larger and larger valves in, and you have to spread the lobe centers to keep this from happening. This will require a custom cam and it will likely require putting the cam events where they may not be best just so you can fit everything. There are only so many offerings for these engines, and with it being a OHC / rocker type head, the lobes will have to be proprietary to the geometry of these specific engines. Even at that, I'm suspicious of how many of the off the shelf offerings actually are.

    From a dynamics standpoint, larger valves are heavier. Heavier valves will require stronger springs. There is only so much available off the shelf for these heads, and so much that will fit within the available space. Remember, this is a OHC rocker engine, with the pivot points at a fixed place. You can't just keep adding valve stem length to fit a longer and stronger spring in there - it will screw up the valvetrain geometry.

    This is a just a quick overview. Into the weekend, I can get into the math of all of this (Paul will too as he has a good grasp on this).

    I did the math one time of a SR400 crank / rod with a 97mm bore vs a SR500 crank/rod and a 87mm bore. The difference in peak piston speed at 7200rpm was like 20fps (or about .04% difference). Total piston CFM demand is the same, just the curves are a little different due to the rod/stroke ratio, but the port sizing difference is the same.

    Here are some pictures:

    Intake port. This is about as wide as you can go on the short turn radius without risking the casting getting too thin. No way to get a welder in there to add material.

    Exhaust port:

    Looking down the carburetor throat towards the valve (note, everything wasn't bolted super tight here, so some of the steps you see go away when all bolted down). This photo makes obvious the kink in the port:

    Combustion chamber. 1.900 intake valve (stock is 1.85) and a 1.48" exhaust valve (stock is 1.53"):

    How it fills up a 90mm bore:

    Using the ratios Paul gave..

    For hemi and semi hemi heads, I valves are suggested to be 52-55% of the bore ratio and E valves are 37-42%.
    My head with 90mm bore:
    Intake: 54% Exhaust: 42%

    Stock head with a 90mm bore:
    Intake: 52% Exhaust: 43%

    98mm bore with stock valves:
    Intake: 48% Exhaust: 40%

    If you go larger, focus on the intake side. Just work on the exhaust side shape, hit the velocity targets, and make sure it's a good and stable port, but I wouldn't go bigger.
    Last edited by Hoffman900; 09-28-2017 at 08:56 PM.

  2. #22

    Thank you - appreciated!

    Conclusion as of now for my 98mm bore 630cc engine:

    1. Valve sizes: I'll stay with 49mm inlet and 41mm exhaust.
    2. Adding a few pics of the head ported w. 49mm inlet valve but without 41mm exhaust valve. Happy for feedback on porting and combustion chamber e.g. Click image for larger version. 

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    3. 10 years back with stock valves and no porting peak rpm was around 6000 rpms. I haven't benched her after the increase in Intake valve size, piston change and porting, but she revved 7500 now no probs with an significant increase in power and rev willingnes. Will dyno her again with Nikasil cyl & 41mm Exhaust valve after running in,

    Click image for larger version. 

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    Last edited by Dudas; 09-29-2017 at 12:43 PM.

  3. #23
    Adding some pictures of the EGU cylinder after machining to 98mm and Nikasil coating. There is indeed more porosity in this casting material than original. Yet to find out if it has any consequences in terms of durability. Awaiting new Carillo piston pin before reassemby and startup.

    Click image for larger version. 

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  4. #24
    I think you'll be alright from the porosity standpoint.

  5. #25
    Is that a crack on the metal around the guide?

    Interested in the valve to valve clearance on the overlap...especially if you go to a larger exhaust valve?

    ...'Any unnecessary items on the bike, that it doesn't really need, shouldn't be there!'. (Café racers creed). SR..

  6. #26
    Quote Originally Posted by stewross View Post
    Is that a crack on the metal around the guide?

    Interested in the valve to valve clearance on the overlap...especially if you go to a larger exhaust valve?

    Stew, mine has a crack like that as well, just not as clear. My cylinder head guy and others say it's typical and not to worry about it.

  7. #27
    Ah, something to watch out for...perhaps epoxy repair...

    ...'Any unnecessary items on the bike, that it doesn't really need, shouldn't be there!'. (Café racers creed). SR..

  8. #28
    Hey guys,

    1. Crack around valve guide: As Christian, I got the same info from 2 head guys - quite common, rarely becomes an issue. This crack didn't expand over the season I had it. Will just keep an eye on it. Worst thing that could happen is that it start sucking in oil.
    2. Valve clearance at closest position: around 2mm, see pic.Click image for larger version. 

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    Regards, Mikael

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