Landracing Forum
Introductions => Formulas => Topic started by: Milwaukee Midget on February 18, 2011, 10:56:31 PM
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Quizzing the brain trust again.
I'm looking for the formula to calculate piston position with respect to valve events. Specifically, to determine the potential for valve interference when the piston is heading toward and away from TDC.
I'm thinking the variables have to be:
Stroke
Rod/Stroke ratio
Lift
Cam profile/rate
Overlap
Depth of valve face into the combustion chamber
Angle of valve approach with respect to the piston (radius dependent)
Rocker Ratio
Gasket thickness
I'm having a tough time putting it all down on paper into a formula, and am hoping somebody just knows it off of the top of their head.
Thanks!
Chris
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I just use modeling clay.
Will6er
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A couple more items: Crankshaft centerline offset, wristpin offset, pushrod/lifter angles, pushrod/rocker angles, pushrod/valve angles, rocker arm geometry,.......
Yeah, modeling clay is MUCH easier!
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Thanks, guys. I agree - but here's my point -
Using clay is like measuring a 2x4 after you've cut it.
Modeling clay will tell you what you have, but a formula can predict what's possible.
I intend to check it with clay on the dry build, but if my pistons need relieving, I'd like to know that before hand.
Part of my concern is that I'm using a cam with two different lobe centerlines on it. The overlap between 1/4, and 2/3 is different, so timing in the cam is really critical.
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solidworks similation
John
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Thanks, guys. I agree - but here's my point -
Using clay is like measuring a 2x4 after you've cut it.
Modeling clay will tell you what you have, but a formula can predict what's possible.
I intend to check it with clay on the dry build, but if my pistons need relieving, I'd like to know that before hand.
Part of my concern is that I'm using a cam with two different lobe centerlines on it. The overlap between 1/4, and 2/3 is different, so timing in the cam is really critical.
Call your cam grinder and tell them what you're running. They should be able to tell you if you need to cut pistons.
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Well it's not really a formula but a set of simultaneous equations based on the geometry of your system that you would solve at a number of angles around TDC. CAD software can help if you build a 3D model that allows movement but another method would be to assemble a block, head, and valve-train with light springs. A couple of dial indicators and you can chart a graph for lifter-lift vs valve-lift. Solving crank angle vs piston position is more straight forward but with a degree wheel, crank, rod, and piston you can build a second graph. Build a spread sheet, select angles +/- TDC and using your can lift data (if you have it, otherwise you get to build a third table) you can enter the valve lift and piston position. The last bit to the puzzle is with a piston at TDC and the light springs installed, how much lift does it take to touch the piston, assembled this can be measured directly.
Maybe a bit of work but since once you have your graphs you can change the cam data and again check clearance. I prefer CAD models myself, but it's a bit easier to get yourself in trouble. Do talk to the camshaft people about clearances on your specific engine, including lash (also if they have cold and hot numbers). If they don't have hot numbers you'll have to add measuring those to your work list. Cold numbers are just that, stone cold not a little warm (ask any air-cooled VW bug owner who adjusted valves using cold numbers with just a few hours cooling, instead of overnight, what happened with their #3 exhaust valve).
Anyway, I hope that helps.
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This is pretty mickey mouse, but it is what I have done. First, I put the engine together with the piston at top dead center and the intake valve, only, in the head. It was an overhead cam single cylinder engine and I left the cam out. I held the valve closed with a clothes pin on the stem. I put a dial indicator on the valve stem tip. Then I measured how far the valve dropped between being fully seated and hitting the top of the piston at TDC. Then I rotated the crank 5 degrees ahead and past TDC and measured the drop, again. Then I repeated the procedure at 10 degrees before and after TDC. I might have done it at 15 and 20 degrees, too. I do not remember. I made a graph of valve drop vs crank angle for the intake valve. Then I did it again for the exhaust valve. The plotted lines are maximum valve travel.
Next I plotted on the graph the valve lift at various crank angles based info from the cam data card. The distance between the two lines on the graph is the clearance between the valve head and the piston top. The cam lift line crossing the maximum valve travel line means interference. It is easy to use this method to see if changes in cam timing will cause piston/valve contact. It does not tell you if the valves will bang into each other, though.
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it is just not worth the effort to attempt it with some calculation
we often make a dummy blank piston and go from there.
the biggest variable in doing the calculations with a formula is that when the cam install number is advanced or retarded as much as one degree...EVERYTHING ELSE will change dramatically.
we run some of them with as little as .040" (cold setup )piston to valve clearance.
you just cannot get that close with some "calculation", especially when just a thousandth or so difference in piston to wall clearance will change the rock over at least .012"..or more....
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I'm sensible, :-D
using a flat head... valves can't hit each other, same between valves and piston...
Patrick
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For one of the Honda race teams, I digitized the cam and components on a CMM (Coordinate Measuring Machine). Then using CAD, I could track valve events.
But unless you have deep pockets, that's not realistic. IIRC, it was ~$2000 to do it that way. Very labor intensive. But when you are done, you can do a lot of "what if's" and the data is forever.
What if I:
Want a bigger intake valve?
Float the exhaust valve?
I tighten the LSA?
Retard the cam?
Use a different piston?
Alter rod length ratio?
Regrind the base circle?
Change the buckets/lifters/rockers, etc.
It not only tells you about piston issues, but geometry issues as well.
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BTW - For my personal stuff, I use machinist clay (it's stiffer than kiddy clay). That's too much time to spend on the cam.
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I've not found a reliable way to predict piston to vale without a mock up.
I recently came across this old blog by David Reher that I found very useful
http://www.rehermorrison.com/blog/?p=186
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The crank vs. piston is easy - it's straight trigonometry, but (as said) the actual valve motion is a whole different thing (unless you have cam doctor etc.).
I wrote a simple .xls for piston position down from TDC vs. crank angle in degrees, e-mail me for a copy (and there are many more complex ones out there that give stroke percentage, distance vs. deck height, thrust in degrees, percentage of cylinder fill, etc.)
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The Reher article covers this nicely, but sort of brushes past what IMHO may be important but is very cylinder head-specific.
The clearance notch is described as a cylinder, or cylindrical section, with .050" radial clearance to the valve head circumference (OD +.100").
As a general rule this is pretty safe, but the safety is limited to engines with valve stem axes fairly close to vertical (Heron, BMC Mini, GMC L6, Buick L8 etc.).
Some of these can get away with less than .050" if the stiffness of the valve at overlap lift is high (low lift, tight stem-to-guide, long guide, short stem length to the guide, generous stem-to-head radius) since this minimizes "ringing" which is one reason for more clearance than a simple extension of the head shape into the dome.
The other use for the .050" is not actually discussed (unless I went right past it?): it's slack to allow for a non-vertical valve to shuffle slightly while opening/closing.
This is the part that I thought needed more attention.
An engine with high included valve angle (Jaguar L6, Chrysler hemis, Harley Sportster, panhead, shovelhead) has considerable sideways motion even at small lifts, and if clay is used the valve head will not depress the clay (as a slow impact) but instead plow it out of the way in 2 planes.
Luckily, the math is easy to do and very accurate if you know your valve angle. Using an obvious easy choice: 45°, the sideways motion is the cosine of that angle × the calculated or measured depth for lift only.
E.g., a 45° valve lifts .200" intrusion below the upper dome surface during overlap. The notch depth is .200" (duh!) measured on the stem axis (not the bore axis), plus whatever clearance to the bottom of the notch you need.
The notch width is the valve head OD + whatever ringing space you need (.050" radial, etc.).
The length of the notch is the width (as above) + (.200 × the cosine of 45°, or .707) = .141". Note that this is more than the radial clearance, and a very small error will lead to breakage. The valves generally open toward the bore CL but there are exceptions.
To sum up: heads with shallow stem axes may need less than the .050" radial clearance suggested (and have small chamber volume), since the valve head will follow the notch location closely.
Heads with high angles need either a relief in a more complex geometric shape (is there a term for 2 identical cylinders partially overlapping?), or a cylindrical notch defined as valve head OD + radial clearance + lift × cosine because of the scrub effect across the piston.
Those familiar with Excel can easily write a sheet that allows slider or check box selection of stem axes (they're not always identical - 426 is one), valve head OD, lift at overlap, and radial clearance, and produces the notch length and width. If the notch is a regular shape, it could even calculate the volume...
</breathe>
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Another thing to consider is "piston rock". Most racing engines use pistons with more piston to wall clearance thus when cold the piston moves or "rocks" oven in the bore requiring more radial valve head clearance. YMMV.
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There are commercial programs out there that accomplish this, some expensive, some "affordable." Almost all the useful ones however, require the utilization of an electronic cam profile file, such as "Cam Doctor". You can accomplish what you need to "the old fashioned way" & accurately with the following:
1) Degree wheel & pointer, both solidly attached to the mocked up long assembly
2) .50"/1.00" travel dial indicator and stand solidly attached to cyl. head
3) VERY LIGHT substitute valve springs for cylinder being checked
4) A LIGHT TOUCH when rotating engine during overlap period if you suspect valve to piston clash (As opposed to the "Clash" which never had a light touch,
musically speaking.)
5) Opening the valve lash to reduce lift @ overlap can give you somewhere to start calculating flycutting dimensions if the dreaded clash occurs straight off.
6) Light springs on the valves & the indicator on the valve stem can be used to plot existing valve to piston clearance (from the seated valve to the piston top) from
about 20/15 degrees BTDC to 15/20 degrees ATDC, WITHOUT ANY CAM LIFT. DO NOT "ASSUME" BTDC & ATDC WILL BE THE SAME, you might be surprised.
Then graph this out and you have an idea of what that particular setup will tolerate for NET cam lift. You probably want to measure every 2 degrees or so.
7) When building up an "orphan setup" the "engine builder" (as opposed to the "engine assembler") NEEDS TO VERIFY THIS INFO FOR HIMSELF, regardless of what
any others may advise. Unless the "builder/assembler has a "bottomless wallet".
8] I like to see .080" inlet to piston clr. & .100" exhaust for pushrod engines running 9k redlines. IF VALVE TRAIN MOTION IS VERY WELL CONTROLLED, (or " IF
you're feelin' lucky, punk".) clearance can be as tight as .050" inlet/.070" exhaust.
9) I reserve the right to change my opinion, at any time, without notice.
Best regards to all,
MB
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MM
To know your piston to valve, put in a intake & exhaust valve with no springs and strech a rubber band around the valve stems to keep them in place
Install head and used gasket with a few bolts & zero indicator on the intake stem, crank a TDC put rubber band over guide and slide valve down to piston pocket and write down distance [ .400 ] lets say.
Rotate crank to 5 - 10 - 15 - 20 degrees , then the same on the exhaust valve.
Take off head install running valve springs and valve train, install head and adjust each valve with proper lash.
Put indicator on intake retainer and go to your crank settings at TDC etc on split overlap and write down the lift number under the free valve number of .400 [ say you have .300 lift then you have .100 piston to valve. this way you know what you have at each 5 degrees of crank rotation with no guess work & no CLAY.
If you are a little short on the exhaust side advance the cam a tick to pick up more clearence.
This is the proper way to know what you have, Don