D
Deleted member 12676
Guest
.02" is only half a millimeter which isn't enough to boost from 90 to 150psi
Buying one of these this payday
- Thread starter Randall
- Start date
gary55
Well-Known Member
Thank you Steve. All I'm doing is calling them like I see them with my own two eyes. I wouldn't post it otherwise. Much respect to both of you guys. I haven't ventured much into head modification, but that 10,000 rpm slant plug mod. is something I have to try. To Zippinaround I haven't cc'ed the heads. I think the comp. test gives some answer, but I am going to get the tools to try a accurate volume measurement. Since I have received jugs from different suppliers varying stock from 2.738 to 2.779 the material removed varies greatly. I'm going to see what the psi is on a stock jug with these heads. I have a feeling the huge difference between jags comp and mine could also have to due with varying jug overall heights and maybe different piston configurations.
panmines
Active Member
I've had the same experience too. Originally, I had a stock slant plug head that gave me 175 psi, then I bought a 7cc "Fred head" and the compression was lowered to about 117 psi (although I was using an extra head gasket when I installed the new head). I broke my compression tester by the time I took out the extra gasket after seeing the psi was low enough to not worry about. I know this is doesn't seem possible, but maybe the stock head was modified by the person who built my engine to give more compression. I bought my kit from Dax
springfieldscooter
Member
Jennings!
The more I read, the less I understand.
The more I read, the less I understand.
Steve Best
Well-Known Member
"Listen Grasshopper, when you can take the stone from my hand, you are ready" (Kung-fu)
Jennings is the master. He pulled it all together by listening to what was going on in the heady years of the early 70s and trying out what worked and didn't work. His book was like an expose of all the little secrets the racing teams didn't want you to know. 40 years later it is still valid and still a primer for anyone who wants to know Schnurl scavenged 2 stroke engines. Grasp Jennings, go on to AG Bell, do a little GP Blair for numerical theory and you are pretty much up to this millennium in theory. Now build some engines to test it out. That is what I am doing with these China Girls, cheap experiment kits to verify 2 stroke theory. Just like an electronic Breadboard Kit.
As for compression ratio and specific parts, there ain't no specific parts with these motors. Huge variety in deck heights, piston domes, and port designs. Like Jag says, they are restricted to meet the laws of most countries on purpose, and I'd add to keep price low. No need for work class uniformity. I have found the stock head is a good place to start for a high rpm motor but it is soft and I have had recurrent head gasket leaks and warpage along the plug axis under compression and any detonation. The shape is surprisingly not bad for what it is asked to do, broad rpm range power. The earlier center plug head was even weaker and cracked under detonation.
10,000rpm China Girl takes more than just the head.
Intake and exhaust port windows into the cylinder need to be opened up and raising the cylinder helped too.
Just used a stock Grubee non-catalyst exhaust too. Imagine if you used a tuned pipe?
My point was that putting the aftermarket head on reduced the rpm about 10% but increased torque a similar amount.
Steve
JunkyardDog
Active Member
I also have Gordon Jennings book. I've had 2 stroke motorcycles since the early '70s. I have rebuilt several 50cc moped (I mean factory built pedal mopeds like Puch, Tomos, and Minarelli) engines with 80cc kits, porting and polishing, higher compression, larger carburetors, no air filters, expansion chamber exhausts, etc. to the point where they would double their 30 mph top speed. They all blew up after a few hundred miles. Why? Simple. They were seriously overstressed. If you want to build a racing engine and don't care that it will not be long lived, and are ok with what it costs you, fine. One big difference in this engine and the engines I built is that this engine is Chinese. Those moped engines were MUCH higher quality to begin with. They were all European. This thing looks like it would make a good boat anchor if it were a bit heavier.
gary55
Well-Known Member
Guess which one is actually a 6 cc head. Here goes. left to right. Stock straight plug head= 10.1 cc, stock slant plug head= 6 cc, pancake head= 7.9 cc, large so called 6 cc head= 8 cc. These after market heads in most cases (unless replacing a stock straight plug head) do not increase compression. I am guessing that as Steve pointed out earlier their performance improvements are due to chamber design. Unless I am missing something as to a adverse effect the large squish bands may create I don't think they will cause early damage to wrist pin bearing or chrome linings of cylinders. Having said that, their are more then likely considerable differences in these after market heads as well depending on who made them. I would be interested in others test results.
Randall
Active Member
Hey Jag, can't I just add another head gasket to lower the compression?
gary55
Well-Known Member
You should check what the compression is before adding another gasket cause it probably won't be needed.
Randall
Active Member
Check with a compression gauge ..... got it.
Similar threads
- Deleted member 35519
- 2-Stroke Engines
