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Post by madmax on Dec 4, 2014 11:31:25 GMT -5
I'm possibly being naiive Griz but my idea was, deck height the same as with 650 barrels therefore any boost in compression is just down to the slight extra height of the 750 pistons and also because deck height is exactly the same then camshaft (& valve) timing should be the same when using the stock cam.
Thinking about this a bit more (can you smell burning?),the pistons may not have any extra height so what I should be doing is setting the static CR using gasket thickness & CCing the head and then worrying about cams/timing later?
So OK, I set static CR at around 10:1 which should be fine with 95ron E5 petrol (97 and 98 are easily found locally) then as discussed decide what I'm going to do about camshafts, either refit the stock one but "blueprint" it to factory spec or............
How do we know the XS performance cam is a clone of the Megacylce 250-00? Has someone measured it or have they (XS perf') published data for it? I've been advised elsewhere by an "XS expert" to advance the cam 3 degrees from published spec, this I am told will "bring the power down the rev range a bit" Just need to figure out if it can be done with the crappy 2 bolt sprocket, although a mate slotted the sprockets on his Z900 (2 bolt) and "blueprinted" them to factory spec (he said they were a fair way out), reckons it was a flying machine when done.
I should really buy a good book on engine building, can you recommend?
Cheers again
Max
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Post by grizld1 on Dec 4, 2014 12:31:06 GMT -5
Max, as far as procedure goes, you have the sequence right; get static CR where you want it, then degree in the cam. Modern combustion chamber design is less prone to detonation than the old XS650 with its domed pistons and hemispherical chamber, so ignore the fact that later designs can run north of 10:1 on low octane fuel. If you drop the deck much at all, be sure to use clay to check valve-piston clearance and take solder crushes to check clearance between piston edges and head ("squish").
Re. the XS Performance camshaft, I know it's a clone of the Megacycle 250-00 because published valve timing and lift specs are identical; refer to Mike's XS and Megacycle web sites. If you're going to mount the sprocket on the slots, find your position first, then install using a high strength thread locking compound and paying close attention to torque.
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Post by madmax on Jun 14, 2015 9:44:54 GMT -5
Here's a thought.
I've been running Yamaha spec valve clearances with the XS Perf' cam (.003in, .006ex) but the Megacycle spec is .006 both therefore, my inlets are open slightly longer than they should be, would this have much (any) effect on running?
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Post by headcase on Jun 14, 2015 18:05:46 GMT -5
Any worries about oil getting past the rings and throwing the measurement off? Referring to the tdc/bdc volume measurements.
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Post by grizld1 on Jun 14, 2015 21:58:14 GMT -5
Max, .003" will have some effect; why not set your intakes at .006" and see what happens?
Headcase, you're not letting oil sit in the cylinder overnight when you cc. out a motor. The possibility of seepage can be minimized by using heavy oil and applying a layer of grease to the rings.
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Post by headcase on Jun 21, 2015 12:00:48 GMT -5
Forgot the head was off anyways, makes sense. I used to use a piece of 1/8" plexiglas with a thin layer of Lubriplate around the chamber edges to seal to the head or deck surface with a small hole drilled at what would be the highest point in the chamber/cylinder (tilted) to flow isopropyl alcohol into it with a cc burette. Then do the math for compressed gasket thickness for that volume. Of course not everyone has a glass burette stashed in the toolbox. Just an alternate way to go if you have the tools, and less messy than oil.
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Post by grizld1 on Jun 22, 2015 17:05:40 GMT -5
Dan, the problem with that method is that it gives you only the gross volume of the combustion chamber, and doesn't account for the reduction of combustion chamber volume when the piston crown enters it at TDC.
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