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Post by 90GTVert on Jan 25, 2011 14:58:36 GMT -5
It's a little warmer today, so I spent some time checking squish and compression.
With no head gasket I got about 1.26mm or clearance and 177-178psi on a compression test.
With one head gasket (roughly .5mm uncompressed) I got 1.71mm clearance and 161psi.
I know that with 185psi the Venus had some pretty rough detonation. It on;y took a couple of miles to get so hot that I had to be gentle with the throttle and it would run-on when I shut it off. The current setup has 148psi (or had at the time of initial setup), and I've had no issues. I always use 91-93 octane pump gas, depending what is available (usually 92-93). I'm thinking 150-160 would be my absolute max, especially since the porting is more aggressive on this engine than the Venus. The theory there being that even though the cranking cylinder pressure is lower, the larger and longer duration ports should allow more mix to be drawn or forced into the cylinder and increase dynamic cylinder pressures beyond that of the milder cylinder. Also, I believe that raising the exhaust port is supposed to lead to increased piston crown temperatures so detonation would be more likely with the same compression. I may be wrong, on those points. If you know, please do correct me. Thinking farther, maybe I should stick to something closer to what the Venus does, maybe 140-150psi. Perhaps all of my logic is wrong and I should be cc'ing everything to figure out CCR and UCCR?
I could just go with the 1.71mm squish to hit 161psi and see how that goes I suppose. Normally I end up looking for around 1.2-1.4mm. If 160 works out I could then shave the head enough to get 1.4mm of clearance or so, then see if I could mod the combustion chamber enough to make up for the volume I remove with tight squish. I know in the past I went to a lower squish setting (1mm-ish IIRC) by having a head ball milled to add a couple of ccs. I really didn't see a difference in performance from loose squish and tight squish with the same compression figures. I'm wondering if that would make worrying about a .3mm drop in squish worth all of the trouble. I mean, there has to be a cutoff point somewhere where squish clearance becomes too great for optimal performance. For example, you wouldn't wanna run a nearly flat head with 4-5mm of squish clearance.
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Post by stepthrutuner on Jan 25, 2011 15:58:03 GMT -5
I am curious as to the theory behind longer exhaust duration meaning higher piston crown temperatures? My way of thinking is that an earlier exhaust port opening would allow less heating of the piston crown by combustion gases due to less contact time. The escaping combustion gases would be hotter since less of their energy has been converted to work driving the piston down and perhaps create a hotter spot on the exhaust side of the piston which could contribute to detonation. Since longer exhaust durations allow higher rpm (I suppose mainly because of less trapped mixture to be compressed and thus less slowing of the ascending piston) there is less time between combustion events and less time for piston cooling. This is an interesting discussion.
Concerning squish. I don't understand why everyone is so concerned with squish clearance for a street scooter(although I've been guilty of this as a 2t newbie). Tight squish clearance is only important in delaying the onset of detonation in race applications which limit the time in which the throttle is held wide open. Tight squish really just helps delay onset of detonation in "edge" tuned engines (lean for ultimate power). We all know that street 2ts can't be tuned this way and live for more than a few minutes of cruising. A. Graham Bell says to not worry about squish clearances for street bikes. :2cents:
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Post by lshigham on Jan 25, 2011 16:50:10 GMT -5
I don't think there is a direct link between crown temps and exhaust port duration. CR is another matter altogether, definitely something to watch on a street scoot.
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Post by 90GTVert on Jan 25, 2011 19:02:22 GMT -5
I read about the crown temps some time ago, but it was very brief. I can't even remember if it were a truly reliable source, but for whatever reason I must have believed it. lol
If it is true, perhaps it could also have something to do with heat transfer from the piston to the cylinder walls? The exhaust area of the piston rings will be out of contact with the cylinder wall longer than they are in contact with the cylinder wall (196 degrees of exhaust port duration). Couldn't that potentially lead to increased heating (or more appropriately, reduced cooling) of the piston in the exhaust locale? Maybe not significantly, but perhaps a bit?
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Post by 90GTVert on Jan 25, 2011 19:16:52 GMT -5
Here is a quote found from a MacDizzy member... "Lots of exhaust duration creates overheated piston crowns which leads to detonation.The expanding combustion charge still has energy left to push down on the piston, when the port opens early that energy(heat)rushes past the piston crown scrubbing off the protective boundary layer that insulates the piston , with the boundary layer gone and the friction of the escaping charge directly on the vulnerable piston crown the temperature rises until detonation occurs followed by catastophic failure." |
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Post by 190mech on Jan 25, 2011 21:30:54 GMT -5
Usually when the Ex port is raised,the squish is tightened to bring back the lost low end torque..We all know that tuning a 2 stroke is a juggleing match and must be approched with great caution!My readings on the Big Motor Mina's is they tend to detonate easily and the squish should be bigger than say a 70cc motor.I'd err on a big squish,then tighten it if needed,likely wont need to though.. :2cents:
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Post by 90GTVert on Jan 25, 2011 22:22:58 GMT -5
Alright, well I think I'll try the 1.71mm squish and see how it acts. Might as well see if I have a problem before I try to solve it I suppose. Luckily I can work on T2's engine without removing it, so dealing with an issue after some testing is a lot easier than I've become accustomed to with the Venus.
BTW, while we're talking about this stuff. With all my screwing around with stuff I found a way to make detonation appear if it ever will. Heavy rollers. It;s a lot like how I'd check out 5 speed cars for friends when they wanted to be sure their timing was safe after being advanced. Stick it in 5th gear and hit the gas. Putting all that load on the engine will bring out the spark knock if it's anywhere near the edge. So you stick heavy rollers in a scoot to do the same thing.
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Post by stepthrutuner on Jan 25, 2011 23:08:17 GMT -5
Good idea with the rollers. :idea:
And I'm going to do some more research on exh. duration. :banghead:
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Post by lshigham on Jan 26, 2011 18:21:31 GMT -5
Alright, well I think I'll try the 1.71mm squish and see how it acts. Might as well see if I have a problem before I try to solve it I suppose. Luckily I can work on T2's engine without removing it, so dealing with an issue after some testing is a lot easier than I've become accustomed to with the Venus. BTW, while we're talking about this stuff. With all my screwing around with stuff I found a way to make detonation appear if it ever will. Heavy rollers. It;s a lot like how I'd check out 5 speed cars for friends when they wanted to be sure their timing was safe after being advanced. Stick it in 5th gear and hit the gas. Putting all that load on the engine will bring out the spark knock if it's anywhere near the edge. So you stick heavy rollers in a scoot to do the same thing. [/b] I opt for the less labuor intensive ''thrash it up a steep hill''. The end result is the same, lots of load on the engine. |
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Post by 90GTVert on Jan 26, 2011 18:29:46 GMT -5
What if you live where there are no steep hills? lol I can think of 1 steep hill and it's maybe 1/10 of a mile. That's about an hour or so ride from my house. There's one maybe 10 miles away, but it's in the middle of a town and a very short distance up.
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Post by 2strokd on Jan 27, 2011 3:27:48 GMT -5
What if you live where there are no steep hills? lol I can think of 1 steep hill and it's maybe 1/10 of a mile. That's about an hour or so ride from my house. There's one maybe 10 miles away, but it's in the middle of a town and a very short distance up. By the time you get there, if it has a overheating problem you should know :doh: :bfg: .?. "Take it REAL easy until you get to the hill A$$hole", "i dont wanna mess my motor up!!!" "Shhhh, was that a ping" LMAO :lol: :lol: :lol: ! I like both idea,s! Not sure if hill idea would work as well with a lighter cvt set-up? Seems to me it would just stay in the "power" and rev going up a hill? Mine does anyway..... Heavy rollers would probably get it hot pretty quick if its too tight.?. I just like to have my head temp gauge to look at, other than that my ear does the rest. If it rattles and knocks, "or sounds like a ball bearing is in the crankcase at quarter throttle it aint right  |
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Post by lshigham on Jan 27, 2011 3:45:22 GMT -5
What if you live where there are no steep hills? lol I can think of 1 steep hill and it's maybe 1/10 of a mile. That's about an hour or so ride from my house. There's one maybe 10 miles away, but it's in the middle of a town and a very short distance up. I live in a valley, so can't go anywhere without going up a steep hill  |
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Post by 2strokd on Jan 27, 2011 4:19:22 GMT -5
What if you live where there are no steep hills? lol I can think of 1 steep hill and it's maybe 1/10 of a mile. That's about an hour or so ride from my house. There's one maybe 10 miles away, but it's in the middle of a town and a very short distance up. I live in a valley, so can't go anywhere without going up a steep hill Good point, i bet your scoot rips up it too . We have some pretty good hills around here but nothing to complain or even tune for |
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Post by 90GTVert on Jan 27, 2011 13:39:42 GMT -5
I did find the variator I was missing yesterday, so I installed that and some 8g sliders. The belt rides way too high in the vari right now, but it'll work to start out with.  I got T2 back in the garage too before the rain changed to snow.  I had planned to try and get some work in this afternoon, but that's not gonna happen now. I didn't feel like freezing, so I fired up the Kerosene heater with some old K1 from last year. I lit it, got it going, then went inside. 10 minutes later I checked it to see if it was still good. I figured it would take about 2 hours to get to maybe 40-45 degrees from the current temp. 2hrs later, I look out the door connecting the garage to the house and all I can see is black. "Oh shit I burned the garage down", I thought. I opened the door and it wasn't hot, just dark, couldn't really see any flames. Opened all the bay doors and smoke poured out, ran over to the heater and put it out. I guess the wick is shot. I've seen em smoke, but wow. Needless to say, it's nasty out there and I'm letting it air out, so I guess I'll get to T2 another day. |
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Post by lshigham on Jan 27, 2011 13:46:11 GMT -5
That's a nice garage you've got there, Brent.
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