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Post by Zed on Dec 19, 2018 11:00:14 GMT
The equation found all over the net for calculating the inlet valve closing crank angle from the advertised duration (not the @0.05 figure) is this: (Duration/2 + Lobe Separation Angle) - 180 So for an advertised duration of 272 degs and 108 LSA - like a scat c25 (272/2) -72 = 64 degs ABDC. On my engine and any other with stock 2l crank (71mm stroke) and stock length rods 131mm) at 64 degs ABDC the piston has travelled 15.99mm up the cylinder when the valve closes, leaving an effective stroke of 55.01mm. If I use 1.2mm deck and my 104 pistons, 55.01mm stroke and CR 7.7:1 it tells me I need 60mm chambers. If I then set the stroke back to 71mm it should tell me the static CR? 9.6:1. For the scat cam, there seems like a big difference (38 degs) between advertised duration (272) and @0.05" (234) compared to the figures webcam quote for their cams. I looked at a lot of calculators that use @0.05 degs + 15 to 25 degs. Still with me? If we use c25's @0.05" degs + 25 degs = 259, the crank angle when the inlet closes is 58 degs ABDC and effective stroke 58mm (freaky coincidence). Same numbers into calc with 58mm stroke, 7.7CR yields 63cc chambers and 9.2:1 static CR. So I already have a range pf 9.2 to 9.6 and I'm feeling sympathetic to John and respect for recommenfing anything! When I've had a cup of tea I'll do the whole thing for mr westy's actual engine stroke and bore as I appreciate his input into this disccussion. The hard bit to calculate is the trig to get to the effective stroke, but it's a piece of cake using CAD so I'll post list of advertised duration/Intake closing angle/effective stroke for his enjoyment. Then all he has to do is decide just how accurate the advertised and @0.05" numbers are and deduce what number duration he wishes to believe. Somehow Im feeling the zero lash @0.05" may be both more accurate and closer to real life actual clattery valve train duration and thusly @0.05"+10-15 may be the way to go.
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Post by Zed on Dec 19, 2018 11:29:05 GMT
Mr Westy, 80x96, JE stroker pistons and stock rods works out nicely for deck height, good choice. 👍
The first two colums are purely cam related tand apply to all engines. The third column is derived from the 80mm stroke, 131mm rod length and valve closing angle for Mr Westy to experiment with.
Duration/inlet shut degs ABDC/Effective stroke mm
204-30-76.18 208-32-75.65 212-34-75.09 216-36-74.49 220-38-73.86 224-40-73.19 228-42-72.49 232-44-71.75 236-46-70.99 240-48-70.18 244-50-69.35 248-52-68.48 252-54-67.57 256-56-66.63 260-58-65.66 264-60-64.66 268-62-63.63 272-64-62.56
Have fun, I can extend the table if you like.
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Post by popegregoryxi on Dec 19, 2018 12:13:15 GMT
I have nothing of value to add to this thread, but it is making interesting reading! Respect to you for doing all the maths, and putting up here for others benefit. 👍
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Post by Zed on Dec 19, 2018 12:39:18 GMT
80x96, 131 rods, C25 cam, 1.2mm deck, aiming for a safe (?) 7.7:1.
For 272 duration - 59cc 9.6:1 For 234+25 duration - 62cc 9.2:1 For 234+15 duration - 65cc 8.9:1 For 234 duration - 67cc 8.7:1
So it comes down to which duration number you think gives a realistic valve closing angle. Even at my most conservative number, 234, air-cooled.net's recommendations of 7.5-8.1 do look conservative. Powerband idle to 4000rpm maybe due to his low compression!
If I back work 7.5 static you'd have 80mm chambers and using 272 (worst case) your dynamic cr would be 6.1:1
8.1 static = 73cc and best case (highest cr) 234 deg - dynamic cr would be 7.3:1
Now I see why the nerds on samba get their guesstimated perfect cam and thoroughly measure it, particularly the all important inlet closing point, before they do anything else.
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Post by Zed on Dec 19, 2018 12:50:59 GMT
I have nothing of value to add to this thread, but it is making interesting reading! Respect to you for doing all the maths, and putting up here for others benefit. 👍 I have sporadic flashes of mental agility but I still don't really understand what it all means. However this time last week I didn't even have a rough idea how to calculate dynamic cr or any clue to where the valve shuts in relation to piston travel or that this is how to derive static cr for a particular combo of stroke, rod and cam. Hopefully I haven't cocked up but having cut almost all the calculation out and being Prince of the CAD operators (apart from Barney), I don't think so. It's all down to "these figures are for comparison only" duration, so we're still pissing in the wind without accirately degreeing our actual in my hand cam. I wonder if Mr Westy is at a point where he could do that.
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Post by 77westy on Dec 19, 2018 18:49:21 GMT
Mr Westy, 80x96, JE stroker pistons and stock rods works out nicely for deck height, good choice. 👍 But I’m not using standard rods or JE stroker pistons. I’ve used 135.25mm (5.325”) H-beam rods and KB pistons with the pin as high as possible (it breaks into the oil control ring groove). With the 80mm stroke crank, the rod ratio is 1.69:1 compared to 1.84:1 for a 71mm stroke and standard-length rods. Most engines are in the 1.6:1 to 1.8:1 range so I’m happy with 1.69:1. Incidentally, for a standard 1600 type 1 the ratio is 1.99:1 – VW liked very high rod ratios. Now, I’d like a dynamic CR of 8.0:1 for no other reason than I agree with John Maher. And I’d like a deck height of 1.0mm to 1.2mm because I like a good squish. The pistons are flat top so what static CR do I need and what should the combustion chamber volume be? I’m using 1800 heads but I can’t remember the volume off the top of my head – I think it was about 56cc. I’ve just checked, Jake Raby says the 1800 heads are 55cc to 56cc. I can’t check mine ‘cos I’m going to New Zealand tomorrow and I need to pack.
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Post by Zed on Dec 20, 2018 8:22:12 GMT
Mr Westy, 80x96, JE stroker pistons and stock rods works out nicely for deck height, good choice. 👍 But I’m not using standard rods or JE stroker pistons. I’ve used 135.25mm (5.325”) H-beam rods and KB pistons with the pin as high as possible (it breaks into the oil control ring groove). With the 80mm stroke crank, the rod ratio is 1.69:1 compared to 1.84:1 for a 71mm stroke and standard-length rods. Most engines are in the 1.6:1 to 1.8:1 range so I’m happy with 1.69:1. Incidentally, for a standard 1600 type 1 the ratio is 1.99:1 – VW liked very high rod ratios. Now, I’d like a dynamic CR of 8.0:1 for no other reason than I agree with John Maher. And I’d like a deck height of 1.0mm to 1.2mm because I like a good squish. The pistons are flat top so what static CR do I need and what should the combustion chamber volume be? I’m using 1800 heads but I can’t remember the volume off the top of my head – I think it was about 56cc. I’ve just checked, Jake Raby says the 1800 heads are 55cc to 56cc. I can’t check mine ‘cos I’m going to New Zealand tomorrow and I need to pack. Damn, guessed wrong! I'll do the trig for your rods later. When it comes down to what cc for 8:1 that's going to be up to you to decide which duration number to use but I'll be persuing that also on samba where I started just that thread yesterday. Have good trip.
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Post by 77westy on Dec 20, 2018 13:05:38 GMT
Your Samba thread is interesting. Brian mentioned BMEP and that Gary Lewis article is excellent. Alstrup (Torben? he knows his stuff from what I’ve read) says ‘80 x 96 mm was widely used 10-20 years ago. Today we do not use it much because the RR simply gets too low for a type 4 and results in premature blowby and very high ring and wall wear. Stay above 1,70, especially on a slow rever.’ Well, I’m living in the past and have a RR of 1.69 so I’m pretty close. And I don’t know nearly enough to add to or question what others have said. Incidentally, I’m a rotating equipment engineer (retired), involved mainly in high-speed centrifugal gas compressors and power turbines so not really relevant but I have been messing with tuning and modifying engines for more years than I care to remember. The case is packed, leaving for the airport soon, overnight flight to Dubai then 16+ hours on a A380 to Auckland, arriving Saturday – I’ll be keeping an eye on the threads whenever I can. Keep up the good work, every day is a school day.
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Post by Zed on Dec 20, 2018 13:53:19 GMT
Writing numbers was getting annoying. I'll do it later. Meanwhile here's a graphic with piston top lines every 2 degrees through the compression stroke. Dimensions should show you what's what. It shows a big difference Mr Westies stroker at the top (hopefully got the pin height right?) and my stock crank one at the bottom. The one in the middle is what I guessed Mr Westy had built and to my innocent eyes doesn't look much different. I'm guess his cylinders are 2mm over stock length or the pistons would be sticking out. I might do one with spark timing as it's pretty weird to think that in some situations the spark will be sparking before the valve shuts. I may have to stop thinking about this and just guess.
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Post by Zed on Dec 20, 2018 14:55:08 GMT
I've added pin posion at the bottom of stroke and 90°. At 90° where the greatest rod angle appears to be I drew in the rod and a rod angle dimension for comparison.
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Post by Zed on Dec 20, 2018 15:33:54 GMT
Same thing again but with the piston tops lined up at TDC. Microscopic difference between the piston speeds/position ref TDC between the rods but Mr Westy has reduced the max rod angle by 0.6° (0.58° actually). That must be good? 👍
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Post by sANDYbAY on Dec 20, 2018 16:47:32 GMT
Mmmmmm pretty lights :-)
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Post by 77westy on Dec 21, 2018 5:13:01 GMT
The cylinders are standard length, the pistons do stick out the top and I’ll be fitting base spacers to get the deck height around 1mm. The piston compression height is 1.11"/28.21mm but that doesn’t affect rod angle. And I’ve used the longest rods I could without the engine width becoming a problem. I’ve built the bottom end but I haven’t mocked up the cylinders yet to calculate the base spacer thickness.
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Post by 77westy on Feb 5, 2019 15:22:44 GMT
I needed 0.090” cylinder base spacers to get a 0.062” deck height and with a 57cc chamber volume I have a 9.4:1 static CR. What do you think Zed, is 9.4:1 too high for a C25 cam, what does it give me as a dynamic CR?
I don’t want to increase the deck but I could open up the chamber a bit, if I have to.
By the way I’m just about to purchase a set of new valves (41mm and 34mm), when Robert built your engine did he use standard valves, or stainless steel, or sodium filled? Or none of the above.
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Post by Zed on Feb 6, 2019 9:10:58 GMT
Stainless valves. I'll work out the DCR later but all I read on Samba would say 9.5 is too much for a bus.IIRC Jake Raby says 8.3 is max bus CR, but of course we don't know what cam he would be using because he's secretive. I sometimes wonder if the reason he is so cloak and dagger is that he's not doing anything very clever
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