Tri Z Engine Build

[onformula1]

I always thought it was yours.

Nice present!

[El Camexican]

I always thought it was yours.

Nice present!

Nope. I did put it on my bikepics page under "my bikes" as they don't have another option to post photos, but I've never claimed ownership. I do like that if you Google 1992 WR250 two of the four images are of that bike.

[El Camexican]

Picked the cylinder up this evening and got to work. Made a new port map so I have some data for picking a pipe. Here are the short & sweet results are compared to what I started with.

- Exhaust port was raised another 1mm (now 34.5mm from top of cylinder)
- Exhaust was opened 1 mm (now 48 mm wide)
- Transfers and boost port were raised 2mm (now 52.6mm from top of cylinder)
- Main transfers were widened 2mm (now 29.5mm wide)
- Secondary transfers were widened 1.25mm (now 24.5mm wide)
- Boost port was widened 1mm (now 19mm wide)

With the port map done the piston was reshaped at the base to match the cylinder. Unfortunately a perfect match would have required removing pin boss material so I stopped there.

The cylinder was then installed and I took some timing measurements:
- Exhaust opens 83 deg. ATDC
- Exhaust is open for 194 deg
- Transfers open 117 deg. ATDC
- Transfers are open for 127 deg.
- Exhaust closes 97 deg. ABDC
- Transfers close 63 deg. ABDC
Then the head was bolted on and the squish band was measured. It is now .025” at the outer edge of the piston and .042” just before it transitions into the doom. There is a .005” difference from one side to the other which seems decent considering I lapped the cases and cylinder base by hand.
Some other numbers I’ll need to make changes in the future:
- Piston dome volume = 7cc
- Head volume = 22cc
- Bore = 68.50cc
- Stroke = 68mm
- Rod length = 130mm
- Trapped compression ratio = 8.14

All you 2 stroke gurus please feel free to play with these #’s and tell me what I did wrong.


BTW special thanks to my anonymous 2 stroke coach that likes car that go in circles and turn BOTH directions

[onformula1]

I like the Motul pre-mix, excellent choice.

What paint did you use on the engine, I dig it!

Very clean, as always.

Lay out fluid too? Nice I see more Sharpies these days.

[barnett468]

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Some other numbers I’ll need to make changes in the future:
- Piston dome volume = 7cc
- Head volume = 22cc
- Bore = 68.50cc
- Stroke = 68mm
- Rod length = 130mm
- Trapped compression ratio = 8.14

All you 2 stroke gurus please feel free to play with these #’s and tell me what I did wrong.

Hi Nico, I wasn’t going to bother checking any of your numbers because I figured you already checked them six ways to Sunday, but after actually looking at them, I saw what appeared to be an error so I decided to check them and the following is what I got . . Unless I have alzheimers, or my calculator is wrong [either is possible, lol], assuming your head gasket thickness is 0 [if you are using 0 rings], and your deck height is 0, your static/uncorrected, or "trapped" compression is 17.707 . . If I convert the piston dome number to a dished number of 7 cc, I get use 9.64.

If I'm wrong, please feel free to tell me where my error is so I and everyone else can tell myself how stupid I am, lol.
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[El Camexican]

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Hi Nico, I wasn’t going to bother checking any of your numbers because I figured you already checked them six ways to Sunday, but after actually looking at them, I saw what appeared to be an error so I decided to check them and the following is what I got . . Unless I have alzheimers, or my calculator is wrong [either is possible, lol], assuming your head gasket thickness is 0 [if you are using 0 rings], and your deck height is 0, your static/uncorrected, or "trapped" compression is 17.707 . . If I convert the piston dome number to a dished number of 7 cc, I get use 9.64.

If I'm wrong, please feel free to tell me where my error is so I and everyone else can tell myself how stupid I am, lol.
.

I wouldn't say stupid, maybe I just have the nomenclature wrong. I agree with 17:1 uncorrected, but I take trapped to mean corrected.

I used this site http://www.torqsoft.net/compression-ratio.html and instead of a 22cc combustion chamber I used 15cc to account for the 7cc piston dome and got 9.48.

In the earlier post I said 8.14:1, but that was because I decided to add the squish volume space to the equation. In hindsight I think it is accounted for in the chamber cc number, so 9.48:1 corrected compression is likely the correct #.

But enough boring math! I bolted it in and hooked up the gauge and got a reading of 177psi. At our altitude on a hot day that works for me. It might even run on pump gas.

I also bored out the from mount to 10mm and shimmed the head mount. The front mount was using an 8mm bolt with over 1mm of slop in it! The current hole is less than .010": larger than the bolt and it fits like a glove. I could not do the lower mount as there isn't enough meat on it to go to 10mm. also made a mount for a new rear brake reservoir, but had to run a nose in a hose to get it to work in the old large I.D. hose. I put a sample of the new hose (I think it is modern high pressure fuel line, its grey) in a plastic bag of brake fluid to see how it reacts.

Oh, and to use that website formula you need to know the top of the exhaust port is 34.5mm from the top of the cylinder and that the piston is pretty much flush with it at TDC

[El Camexican]

[QUOTE=onformula1;1376512]Also, check how close the reed cage is to the Boyesen boost ports, if it's blocked off a lot run a reed spacer.[/QUOTE

Forgot to mention the boost ports are over 1cm from the cage, so no worries

[barnett468]

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ok, xlnt . . 177 is great but you already knew that and its only 7 higher than your calculated projection of 170 which is actually incredibly close . . you probably already know that you will gain almost 5 psi per every 1000 ft drop in elevation and vise versa which is why im guessing you were asking what the elevation is at the racetrack.

just for reference, the more common terms i see and use for the type of compression you are referring to is dynamic compression or corrected compression . . it's probably a bit more universally understood . . the term "trapped" could be understood either way depending whom you ask but it probably would more commonly mean dynamic so the error would be more on my part.

you ,may already know this too but just in case you don't, E85 gas, which I think you said you don't have, is equivalent to around 105 octane, so at least in the states, it is a cheap alternative to race gas if you just need a bit more octane than 91 or 93 etc . . In fact, I know a few people that run it in their V8 engines with around 12 to 1 compression . . or a more comparable number might be around 195 - 200 psi cranking pressure.

Anyway, it looks way too nice to run now anyway.

Also, in case you didn't know, the squish band area should be included in the head volume simply because it has volume . . One easy way to accurately measure head volume is to install the spark plug you plan t use . . turn the head upside down . . put a thin layer of grease around the gasket sealing area . . then place a CD on top and give it around 1/8th of a turn to seal it well . . then just use a graduated syringe or cylinder and fill the head up.

0 deck would be when the edge of the piston is flush/even with the top of the cylinder.
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[YTZ drew]

. . . then place a CD on top and give it around 1/8th of a turn to seal it well . . then just use a graduated syringe or cylinder and fill the head up.

Nico, this is an awesome thread, and an amazing build. Your work is outstanding! And Barnett, you sir are a genius! Why didn't I ever think to use a CD? Great idea.

[El Camexican]

Barnett, I did use a CD because some guy named Aramid that used to be on here suggested it. It worked very well. (Whatever happened to that guy) but instead of filling with the syringe I found it easier to fill from the tap and when all the air bubbles were out I extracted the water with the syringe. Much easier and if I'd have had a helper to hold the head it would have been super easy.

The term "trapped" came from one of the compression calculation sites and made perfect sense to me even though I'd never heard it before. As the piston passes the top of the exhaust port the gases are "trapped". Makes so must more sense to my simple brain that the term "corrected" which makes it sound like you need to fix something you didn't know was broke. It might have originated from England, or Australia. They have their own way of saying things there eh.

Ok, so back to brass tacks. Please explain to me why I need to consider the squish volume if the outer edge of the piston crown is flush with the top of the cylinder and the combustion chamber volume has been measured? In other words all the squish volume is machined into the combustion chamber of the head.

If the squish band ring of the head was flush with the mounting surface of the head (like my shaved stocker is) and the space below it was taken up by head gasket thickness, or the piston crown was below the top of the cylinder at TDC then I could see this being necessary to account for, but with the elimination of the gasket and the decking of the cylinder I no longer have that volume.

I am 99% sure there is no need to consider additional volume on my setup and I think the proof is in some rough pre-assembly math that indicated that if that space did exist my measured psi would have been significantly lower than what it ended up being. Good thing too, I was very worried about that. The only way remaining to get more psi in this setup would have been to buy a smaller dome, or do a bunch of machining to the one I have.

Thanks for posting everyone, you too Aramid, wherever you are...

as far as measuring the head volume . . if you don;t currently have a plate you can just use your least favorote cd . . the size is perfect pluss it already has a hole in it . . if its something good like ozzie, you can just clean off the greae afterwards and it will be just as good as it ever was . . i think.

[barnett468]

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Barnett, I did use a CD because some guy named Aramid that used to be on here suggested it. It worked very well. (Whatever happened to that guy)

Xlnt…I didn't see that post because I didn’t read all of them . . He was a friend of mine whom got back into 3 and 4 wheelers recently so I told him he should sign up because he can find parts and info if he needed . . He actually used to work on them too and knows quite a lot, but I was the one whom told him about the CD trick so I’m glad he passed the info along to you and that it was useful . . He got very busy again with his work and general life issues.

Ok, so back to brass tacks. Please explain to me why I need to consider the squish volume if the outer edge of the piston crown is flush with the top of the cylinder and the combustion chamber volume has been measured? In other words all the squish volume is machined into the combustion chamber of the head.

I apologize in advance if I don’t understand your post correctly, or if like some of my answers, this one is confusing.

Even though the squish band closely follows the crown/dome of the piston, there is still a space between them when the piston is at tdc . . In fact, you already measured this space/clearance with your solder.

In your case, if you spread the grease so you had the same diameter as the cylinder bore, you already included the squish band when you measured the volume, in which case, there is nothing else to calculate on the combustion chamber.

[El Camexican]

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Xlnt…I didn't see that post because I didn’t read all of them . . He was a friend of mine whom got back into 3 and 4 wheelers recently so I told him he should sign up because he can find parts and info if he needed . . He actually used to work on them too and knows quite a lot, but I was the one whom told him about the CD trick so I’m glad he passed the info along to you and that it was useful . . He got very busy again with his work and general life issues.





I apologize in advance if I don’t understand your post correctly, or if like some of my answers, this one is confusing.

Even though the squish band closely follows the crown/dome of the piston, there is still a space between them when the piston is at tdc . . In fact, you already measured this space/clearance with your solder.

In your case, if you spread the grease so you had the same diameter as the cylinder bore, you already included the squish band when you measured the volume, in which case, there is nothing else to calculate on the combustion chamber.

I think we are saying the same thing (my previous post and your last sentence) so let me try to say it another way. The space within the combustion chamber that the solder was squished in is the same space that water occupied when the doom cc was measured, BECAUSE there is no head gasket and the piston dome stops flush with the top of the cylinder. Therefore, adding the squish volume to the formula as a negative would duplicated that amount of void as it was already accounted for in the 22cc chamber volume.

I just noticed that aramid is on the banned list. Must have been that crack about the Ozzy CD.

[barnett468]

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I think we are saying the same thing (my previous post and your last sentence) so let me try to say it another way. The space within the combustion chamber that the solder was squished in is the same space that water occupied when the doom cc was measured

That's all you need then . . Whatever is measured, is exactly what the volume is, irregardless of what any calculations say.

The calculators then have a place to enter compressed gasket thickness and gasket bore id . . Obviously, if there is no gasket, you would enter 0, however, some calculators are finicky and require 0.0, or if your bore is 4" for example, they require 4.0 instead of just 4 . . I can also tell you that at least when using calculators to determine dynamic compression on a 4 stroke, some calculators vary by around 10% for some unknown reason, so I suggest ones I know are fairly accurate to people like the three below.

This one asks for piston ring height,piston clearance, and the amount of camshaft advance etc.

http://www.projectpontiac.com/ppsite...tio-calculator

http://www.wallaceracing.com/dynamic-cr.php

https://www.uempistons.com/index.php...tors&type=comp


This being said, calculating accurate dynamic compression is a bit more complicated because the compression varies with the amount of intake/fuel charge in the cylinder, so to calculate it more accurately, you need a special computer program that is around $150.00, but it also calculates other things like hp and tq based on different intakes and heads from different mfg’s . . It asks for cam duration and valve lift and several other things . . One decent one is called desktop dyno 2000, but you need to buy the upgrades and various intake and cylinder head downloads to get the most accurate results.

Also, cylinder filling is also affected by the exhaust due to the scavenging affect it has . . The amount an exhaust scavenges is mainly determined by the header tube size and length and collector type and size of the tail pipes and the muffler type etc . . This being said, you can install just about any exhaust system on a 4 stroke and it will still decent, but that is far from the case with a two stroke . . I did a lot of 2 stroke pipe testing at Kawi as well as some with Harry Klemm and Dave Miller, and of the pipes we dynoed, the ones that had the most peak power weren't often the ones that worked the best because the power was not in the right area or the right type for the app.

If you just want to play around on a super budget version for fun, you can download the one below from Comp Cams . . It takes less than 10 minutes to do it all . . I entered maximum everything on it one day just for fun so it was like 600 ci with 22 to 1 compression and a supercharger with 20 lbs of boost and nitrous etc, and it said something like 3,000 hp at 1800 rpm.

http://www.compcams.com/v002/Softwar...itle=CamQuest6

[Bren_downe]

"Ozzie" should read Ozzy. We don't want the prince of darkness after you guys…
Good thread. Carry on.

[barnett468]

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"Ozzie" should read Ozzy. We don't want the prince of darkness after you guys…

He was referring to the Greatest Hits album by Ricky Nelsons dad from the Ozzie and Harriet show.


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OMG...I think my spleen just burst.

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