New Slant Heads for 80cc's: Compression Boost!?

Man, my new motor came with the slant head... once the motor heats up you cannot start the damn engine. At first I though it was a seized crank, but now I think it may just be to high compression.
 
bseelbach said:
Great for you. 2x compression reading! That is amazing. I would expect an increase but not to that magnitude.

Even though its still raining, I ran a compression check. About 95+ for the slant. Its a cheap test so the readout isn't very precise. This is in the neighborhood of the test I ran on the regular head.

I pulled the slant head and noticed that the rear ward side of the gasket didn't seem to have a good seal. I didn't see blow by but it wasn't as defined as the front side even though all of the studs were equally tight. I reset the head and we'll see if there is better weather tomorrow.

Did you use a torque wrench or just go reasonably tight?
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I just went reasonably tight. :)
 
I wonder if they may have given me a 50cc head instead of what I should have recieved (66cc head) Do you guys know if a 50cc head will even fit a 66cc? Maybe that is why I am getting such a high compression reading. When I compared the inside of the heads, the slant had a much smaller area. I figured this is just how it was designed. I have not had any piston slap issues. Hmmmmmmm
 
People please just cc your head and you will see the difference is tiny!!!!! Seriously if your getting a 100psi varience ........ You guys are takin somethin different to me.
 
i cant wait to get my slant head motor :D

thanks to this thread i suppose i will go ahead and tear it down right off the hop and replace the studs and seal the gaskets proper
 
Alright Bare Bones, I'm with you on the compression readings. 180 psi is reasonable for the slant head.

I tightened the studs again. It was a bit tighter than I was comfortable with but since this motor has M8 studs I wrenched down a bit more. Ran 3 more compression tests and got 180psi.

I'm not certain where the variance came from seeing I got 95psi last time. It could be 1)torque on head bolts 2) fit of compression tester in plug hole 3) duration of compression test.

The compression tester I have is a cheap harbor freight model and the end is quite funny. It has a screw on extension that looks like the threads are too long for the HT engine and would lead to possible contact. If you take the extension off, the thread diameter is fine and length is acceptable but it is tough to torque down. There is no formed nut to use a wrench. The outer shoulder is simply a smooth circle. Dumb design. This could have caused the variance from the first test.

I also noticed that to I had to turn over the engine for about 15 feet to get max reading. Previously I had only turned it over for about 5 feet and thought that it was adequate. On the second round of tests, I set the dial so that I had a view while riding and noticed that while 95psi was the initial reading, the number kept increasing with subsequent revolutions. The readings for the slant plateaued at 180psi.

Still raining here in MI so I haven't been able to see if there is a noticeable increase in torque.
 
bseelbach said:
Alright Bare Bones, I'm with you on the compression readings. 180 psi is reasonable for the slant head.

I tightened the studs again. It was a bit tighter than I was comfortable with but since this motor has M8 studs I wrenched down a bit more. Ran 3 more compression tests and got 180psi.

I'm not certain where the variance came from seeing I got 95psi last time. It could be 1)torque on head bolts 2) fit of compression tester in plug hole 3) duration of compression test.

The compression tester I have is a cheap harbor freight model and the end is quite funny. It has a screw on extension that looks like the threads are too long for the HT engine and would lead to possible contact. If you take the extension off, the thread diameter is fine and length is acceptable but it is tough to torque down. There is no formed nut to use a wrench. The outer shoulder is simply a smooth circle. Dumb design. This could have caused the variance from the first test.

I also noticed that to I had to turn over the engine for about 15 feet to get max reading. Previously I had only turned it over for about 5 feet and thought that it was adequate. On the second round of tests, I set the dial so that I had a view while riding and noticed that while 95psi was the initial reading, the number kept increasing with subsequent revolutions. The readings for the slant plateaued at 180psi.

Still raining here in MI so I haven't been able to see if there is a noticeable increase in torque.
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Cool! Thanks for letting us know what your readings were/are! Once your weather calms down, let us know how your bike performs. Mine has way better low end. I have a 36 tooth and I can climb hills starting off with a slow peddle much better now. The mid and high are better as well :)
 
wow 180 psi is definitly high compression! right where a two stroke should be.

id be pretty interested in the compression reading of a stock head....

180 is a CORRECT psi for a 2 stroke engine, anything above 200 and you will need to use high octane racing fuel. this is definitly a head that should have high test gas used with it.


wen you get below 90psi you usually rebuild
 
180 is actually pretty damn high for an aircooled motor, especially with the minimal squish these things have, which is like only 25% area of chamber. Change out a actual expansion chamber that works and i would not be surprised to see some detonation and preignition.
 
stv1jzgte said:
cylinder compression has nothing to do with fuel or an engines compression ratio.
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wait... cylinder compression has nothing to do with an engine compression? Explain that one aussie...


Compression leads to heat, and the way the comb. chamber is designed designates how the cylinder charge is effected by the heat. The point is, these have very small squish bands. 20-25%... 25 being rather generous. So more or less you might even call it just a standard dome head, which are generally no good above 6.5:1 compression ratios, because they will then start to detonate and that leads to preignition (and no those are not the same thing). The larger the squish band, the more the charge is centralized over the piston crown, leaving a larger amount of area left in a small layer to be cooled by the nearby head and cylinder. The downside is the larger the squish the less power you make.
 
foximus, your all over the place with your explanations man. let me elaborate, and this goes for you too there aussiethunder with the big brains....

cylinder compression has EVERYTHING to do with an ICE's compression ratio. As mentioned, more compression more heat different compression ratio. Take a diesel engine for example. They ignite the mixture with HIGH COMPRESSION. (therefore always having a high ratio)

So foximus wins there.

But aside from that, stv1jzgte you are totally wrong.
Build engines for a few years and let me know how your making out running 220psi on 81 octane....
 
I believe stv1zgte has somewhat of a point. By taking a single compression reading, you are only measuring how well the cylinder is sealed. Essentially you are checking the rings and gaskets ability to hold pressure.

This does not (necessarily on it own) reflect engine compression (such as 6.5:1) or change in power. By noticing an increase between compression checks of slant vs reg head, I would think you could summize that engine compression ratio has increased if you are confident that there are/were no losses at the head gasket.

Essentially we are hoping that the slant has a slightly smaller combustion bowl. For the simplest formula. Pressure = Force/Volume. Force stays the same within the engine and pressure on the slant heads seems higher. This would mean that volume is smaller on the slant which is increased compression ratio.
 
you can mathematics me all you wish but you cant tell me you only take one compression reading and expect that to tell you if your top end leaks. thats just. wrong.


*forum logic has told me to edit my post*

anyway, ive got my ways, others have theirs
 
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so, uh, do those slant heads work good or not?

(i wonder what would happen if you called the factory in china and said "do you guys have slant heads?")

jeez. i'm going to h*** for that one...
 
I may convey my thoughts haphazard but none the less they are correct in their reasoning and logic, and all relevant to the point at hand.
 
Humsuckler said:
But aside from that, stv1jzgte you are totally wrong.
Build engines for a few years and let me know how your making out running 220psi on 81 octane....
Click to expand...


If you build motors you'd know there are plenty of cars and especially motorbikes with comp ratios from 12-14:1 that run on regular unleaded, from what you say they'd be way more than 220psi.

Also what do you think the comp ratio of a happy time with a slant head is taking into account that a centre fire motor is 6.5:1 and holds 95psi cylinder pressure.
 
That is an incorrect assumption of making a directly correlation between compression RATIOS, and compression readings. The compression ratio of a centerfire head is 6.5 to 1.... or something like that give or take a bit... However thats the plain compression ratio as you would measure it on a 4 stroke, not on a 2 stroke. The correct way to measure compression ratio on a 2 stroke is (Bore x Stroke) - stroke below exhaust port closing + Combustion Chamber / Combustion chamber.

So your actual compression ratio is more like 3.23 : 1
 
bare bones said:
I wonder if they may have given me a 50cc head instead of what I should have recieved (66cc head) Do you guys know if a 50cc head will even fit a 66cc? Maybe that is why I am getting such a high compression reading. When I compared the inside of the heads, the slant had a much smaller area. I figured this is just how it was designed. I have not had any piston slap issues. Hmmmmmmm
Click to expand...

A 50cc slant head will not fit on a 80(66) cc engine no way no how
 
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