TRX250R.ORG
Workshop => Engine and Bottom End => Topic started by: SBarclay on July 07, 2015, 12:51:07 PM
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Hey I'm working on freshening my top end going with an 86 68.5mm piston in my 87/9 motor. Going to add spacer plate so piston clears head and port timing stays the same. Any ideas or concerns would be great guys. Just wanting to make sure I'm not missing something.
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Anybody have any idea or tips on what info or measurements he would need to have to give to his builder ? Pretty sure he's looking To get his compression around 200-210. I know he runs vp 110 so that part isn't an issue.. I know there are some gurus lurking out there so any advice would be appreciated
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Adding the spacer is to keep the 85-86 piston skirt from hitting the crank. If you have an 87-89 cylinder why not just stick with the 87-89 piston?
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Adding the spacer is to keep the 85-86 piston skirt from hitting the crank. If you have an 87-89 cylinder why not just stick with the 87-89 piston?
I guess I didn't mention my cylinder is already bored to 68mm which is the largest piston I could find for 87/9. I have damage on inside of cylinder from melting down the piston so now I need it bored. I have heard of others doing this as a cheaper option.
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So I put everything together and my squish came out to .040". Calculated compression next and got 18:1 which is crazy. I have a 68.5 bore and 72mm. 15cc in combustion chamber plus .5cc would equal 15.5cc.
265 + 15.5 = 280.5 divide that by 15.5 = 18.09
does anyone have advice on what to do?
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that dome is way too small. I think a 20cc dome with the stock .063 head gasket puts you right at 210psi so I can only imagine what a 15.5cc dome would do!!
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that dome is way too small. I think a 20cc dome with the stock .063 head gasket puts you right at 210psi so I can only imagine what a 15.5cc dome would do!!
so your saying I shoud up the size of the dome to lower compression down? 20cc would put me at 14:1
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Is that 15cc measurement the flat plate volume of the combustion chamber in the head or the trapped volume above the piston measured at TDC on the assembled engine?
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Is that 15cc measurement the flat plate volume of the combustion chamber in the head or the trapped volume above the piston measured at TDC on the assembled engine?
Not sure if I'm following your question but the motor was assembled head was bolted down and we held the flywheel to keep the piston at TDC then added the oil..... If it was really pushing 18:1 compression would you even be able to kick it over?
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Not sure if I'm following your question but the motor was assembled head was bolted down and we held the flywheel to keep the piston at TDC then added the oil..... If it was really pushing 18:1 compression would you even be able to kick it over?
Good that is your trapped volume, not the actual volume of the dome in the head (probably ~18cc dome). Yes you would be able to kick it over, remember a two stroke doesn't start compressing until the rings close off the exhaust port. Depending on the exhaust port height I would estimate your Corrected Compression Ratio (CCR) to be somewhere between 10:1 and 11:1 and statically crank around 220-240 psi, obviously use race gas only.
Here is a good read if you haven't seen it already: http://homes.ottcommunications.com/~red/uccr.html
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Good that is your trapped volume, not the actual volume of the dome in the head (probably ~18cc dome). Yes you would be able to kick it over, remember a two stroke doesn't start compressing until the rings close off the exhaust port. Depending on the exhaust port height I would estimate your Corrected Compression Ratio (CCR) to be somewhere between 10:1 and 11:1 and statically crank around 220-240 psi, obviously use race gas only.
Here is a good read if you haven't seen it already: http://homes.ottcommunications.com/~red/uccr.html
So how would you go about figuring out what cc the dome it is?? It is an oem head but we're not 100% sure that it hasn't been milled at some point... We have already started the bike and done 2 heat cycles the way it is and it started right up and purred like a kitten. Didn't sound funny no leaks nothing .... We just want to be sure that it's safe to run and it's going to be reliable as can be
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You would take the head off, put a spark plug in, place a flat piece of lexan with a small hole in it across the gasket face of the head and fill the head using a burette to measure the volume. Sounds like you did a good job putting it together and it is running good. Did you perform a static compression test on it yet for reference?
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A builder runs the piston to top dead center. Fill the dome with tranny oil and let the piston down and catch the oil. Measure the cc to see what you have. Much easier said than done.
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why would someone do it that way when you could measure it as you put it in :tongue-new:. then suck it back out with the same instrument you put it in with. never heard of someone trying to catch the oil but whatever works i guess
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A builder runs the piston to top dead center. Fill the dome with tranny oil and let the piston down and catch the oil. Measure the cc to see what you have. Much easier said than done.
we performed a uccr test with the motor assembled had the piston TDC then added oil and measured. It came out to 15cc plus 1/2cc for the spark plug so that's why we were thinking its a 15cc dome... That seems small but I don't know what a oem head has machined into it. If I've read everything correct you don't want any more then 14:1 comp ratio, in order to get there he would have to buy a new head with 20cc dome
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You would take the head off, put a spark plug in, place a flat piece of lexan with a small hole in it across the gasket face of the head and fill the head using a burette to measure the volume. Sounds like you did a good job putting it together and it is running good. Did you perform a static compression test on it yet for reference?
We are working on getting a quality gauge from a friend so we can do compression test before he does any riding other then heat cycle. also going to build a setup so we can do a leak down test for piece of mind
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you don't want any more then 14:1 comp ratio, in order to get there he would have to buy a new head with 20cc dome
The builder then puts the dome back in the lathe and takes a lil more off going by experience. Put the head back on and remeasure
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we performed a uccr test with the motor assembled had the piston TDC then added oil and measured. It came out to 15cc plus 1/2cc for the spark plug so that's why we were thinking its a 15cc dome... That seems small but I don't know what a oem head has machined into it. If I've read everything correct you don't want any more then 14:1 comp ratio, in order to get there he would have to buy a new head with 20cc dome
That 15cc measurement you came up with is not your dome volume it is your trapped volume, which is a function of dome volume, head gasket volume, deck height volume (if not set at zero) and piston crown volume.
Where did you hear that you don't want anymore than 14:1? What octane fuel are you planning on running?
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That 15cc measurement you came up with is not your dome volume it is your trapped volume, which is a function of dome volume, head gasket volume, deck height volume (if not set at zero) and piston crown volume.
Where did you hear that you don't want anymore than 14:1? What octane fuel are you planning on running?
I just thought 14:1 is where it should be. Im simply basing this off what I've read in other post and builds that guys do. Usually the guys that have 200-220 psi is at 14:1 I've also read that 230 is kinda a thin line that you shouldn't cross unless you want a time bomb??? I'm not a motor builder by any means that's why I'm posting so many questions. I jut want my buddy to have a good strong running motor to have fun on without to much worry of it blowing up from pushing to much compression ... we will take his head back off and measure the dome itself... I guess we misunderstood how the uccr worked. Also he will be running VP 110 straight with maximum pre mix I believe. 4 oz per gallon of fuel
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When I am ccing the trapped cylinder volume @ TDC, before I torque the head down, I'll smear a bit of grease on the edge of the piston to seal off the rings so none of the oil will go down beside the piston. Then once the head is on, I use a plug hole dial indicator & find correct TDC & then lock the crank down in that spot so it cant rotate & get off TDC.
Your only getting the trapped cylinder volume when your ccing like that & not a true head dome volume.
Head dome volume cc, is as detailed earlier with using a flat plate over the gasket surface of the head, sealed good with the spark plug in place & then inject the oil in & note the total cc amount.
Just by reading what your saying so far, that you've already ran that engine a few times & it was not a monster to kick over to start up, I would say the compression shouldnt be to high, nothing over normal that would kill the engine if left as is.
Now if it was a bugger to kick over to start up, hurt your foot with boots on, then I would say you better be ready to hang on when riding & be on your toes jetting that carb before you put a foot on that kicker, (every time) & use good quality race gas so you wont melt another piston down.
This is to any & every 250R owner.
It makes no difference who built a 2 stroke engine, being the highest quality it could be built, be it Allen @ CT Racing, Arlan @ LED or even BDT building it, that engine will run no better or last no longer, than how much that person knows that is tuning on that carb feeding it gas mix.
That carb is feeding it what keeps it alive, so when you skimp on that, your making its life shorter.
A 2 stroke is simple in design & simple to make it have a long life, so we shouldnt try to over comp things when dealing with them.
Compression ratios are like dyno readings, thats all they are to either list or brag about & when its all said & done, either your foot or your butt will give you the true reading of what that engine is doing before & after it is running.
Neil
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He stuck a gauge on tonight with a cold motor and got just over 240. Think she's safe to run?
(http://i1222.photobucket.com/albums/dd493/jakes2c/A42D15BB-6FAA-4AAF-A4B1-18AFA07CD657_zpsi4ybdq2f.jpg) (http://s1222.photobucket.com/user/jakes2c/media/A42D15BB-6FAA-4AAF-A4B1-18AFA07CD657_zpsi4ybdq2f.jpg.html)
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He stuck a gauge on tonight with a cold motor and got just over 240. Think she's safe to run?
Has the exhaust port been raised much or at all?
If it hasn't I would run it. My XC engine after a rebuild cranks ~245 psi. You should have a nice grunt engine there with good low end power but don't expect it to rev to the moon. Re-read Neil's post, he's right on the money; use high quality fuel and jet conservatively.
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Man, 240 lbs, grunt & go baby.
Quality race gas & dont guess on the jetting. Its always better to be on the rich side to keep the piston cool, keep from melting it, because you have the compression to burn all you can put in the cylinder.
I've had some guys tell me, they were pumping over 300 lbs on an OEM cylinder, but honestly I didnt know if I believed them or not. I know I've never had one pump that high, not an OEM jug.
Neil
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Thanks to all the guys that took the time to read through this thread and help out. I think hes gonna do some break in this weekend I will post an update on how it goes!