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Re: compression
« Reply #15 on: January 08, 2009, 07:25:11 AM »
isnt compression ratio, How much it compresses? Masterfabber some tech here...
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Offline fabr

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Re: compression
« Reply #16 on: January 08, 2009, 08:15:59 AM »
I haven't read all of this but
Compression ratio is simply the comparison of the swept volume of a cylinder divided by the compressed volume above the TOP RING of the piston. 

Earlier post asked about raising the exhaust  port and does it effect calculated compression ratio.NO. It may/will effect achieved compression ratio(cylinder pressure) tho and in turn affect the octane necessary.
 This is a BIG subject for something so simple.ASk all the questions you wish.I'll give the  best answer I can.
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Re: compression
« Reply #17 on: January 08, 2009, 08:35:20 AM »
Quote from: Masterfabr on January 08, 2009, 08:15:59 AM
I haven't read all of this but
Compression ratio is simply the comparison of the swept volume of a cylinder divided by the compressed volume above the TOP RING of the piston. 

Earlier post asked about raising the exhaust  port and does it effect calculated compression ratio.NO. It may/will effect achieved compression ratio(cylinder pressure) tho and in turn affect the octane necessary.
 This is a BIG subject for something so simple.ASk all the questions you wish.I'll give the  best answer I can.

Is there a set psi that will run premium gas, and were does that cutoff to need the use of race fuel? so in essence, you could have a large comp ratio with a low number still, I think i understand what your saying there...
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LiveWire

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Re: compression
« Reply #18 on: January 08, 2009, 09:02:07 AM »
Two strokes are sometimes calculated from the top of the exhaust port to the top of the stroke which gives a much lower compression ratio. Raising the exhaust port height may give a lower static measured compression ratio. Yet that does not mean your octane requirements are lower. You could actually end up with higher cylinder pressures. Where an FL400 will start having detonation issues with pump gas above 160, I run FL350s at 170 on pump gas with no problem. Your best bet is to see what others running the same engine have run.
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Offline Nutz4sand

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Re: compression
« Reply #19 on: January 08, 2009, 12:31:43 PM »
I agree with Livewire. 155 to 160 PSI is all you should do for a play engine. Going higher gets touchy with a two stroke.

A lot of the compression depends on the squish band. You get a bad squish band and the motor will have issues.

The squish band is the area on the head near the outside of the cylinder thats a different angle than the rest of the dome. This amount of area and its thickness can make or break a two stroke and cause it to be low on power (best thing) or detonate badely (worst thing) if its not right.

The squish band creates turbulence in the mixture as it closes that last little gap and in turn it helps promote burn in the cylinder's center and keep the burn aways from the edges which help the motor remain cool. I will admit I barely understand this but I have studied in quite a bit due to having a foolish "pro" engine builder screw up some good engine parts on me. Doing radical head milling can alter the squish and kill your motor. So study up if you mess with the head or pistons that raise the compression with a taller crown/dome.

 
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SPEC

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Re: compression
« Reply #20 on: January 08, 2009, 12:42:51 PM »
Nutz has agood point...I was just looking for those pics of a destoyed set of heads off a 500...Water skip guy, did the heads on a blanchard grinder...That motor pounded the pistons to bits..And totaled the enire motor...Water skipping you can just let off the gas...Or your going swimming...I'll keep looking
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Offline Nutz4sand

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Re: compression
« Reply #21 on: January 08, 2009, 12:51:14 PM »
When we used to mess with Banshees the factory compression was in the 135 range (longevity= long life) You could mill the head to get the compression around 155 for reallly good power on premium pump gas. Apparently Yamaha (Bless thier souls) made it so the the squish band got better to that point as you milled material off the head.

Going much farther meant having the squish bands recut or else.

Do your homework on squish especially how it affects the motor your playing with.

   
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LiveWire

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Re: compression
« Reply #22 on: January 08, 2009, 01:59:28 PM »
Yamaha was probably just being safe with squish clearance. An FL350 with that max shaved off the head of .008" before you hit the squish band and no head gasket will still have a large piston to head clearance. You actually have to shave the top of the jug to tighten it up. The stock squish band is 13.5 degrees and leaves a bit of trapped volume at the outer edge. I run 14 degrees to correct that.

Speaking of water skipping, ever see the Top Gear episode with a race between a rock crawler and a snowmobile across water.
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SPEC

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Re: compression
« Reply #23 on: January 08, 2009, 02:07:04 PM »
No I didn't
Is it out on youtube somewhere?
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Offline Nutz4sand

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Re: compression
« Reply #24 on: January 08, 2009, 02:21:46 PM »


Dunno if I would call that a rock crawler though! lol

It seems to be able to generate a bit more wheel speed than most crawlers.

I think the sled draggin it is in that video too. But I have dialup and down loading it all will take over an hour at home.

Sorry for the Hijack...
« Last Edit: January 08, 2009, 02:23:46 PM by Nutz4sand »
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Offline fabr

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Re: compression
« Reply #25 on: January 08, 2009, 02:43:28 PM »
Quote from: Nutz4sand on January 08, 2009, 12:31:43 PM
I agree with Livewire. 155 to 160 PSI is all you should do for a play engine. Going higher gets touchy with a two stroke.

A lot of the compression depends on the squish band. You get a bad squish band and the motor will have issues.

The squish band is the area on the head near the outside of the cylinder thats a different angle than the rest of the dome. This amount of area and its thickness can make or break a two stroke and cause it to be low on power (best thing) or detonate badely (worst thing) if its not right.

The squish band creates turbulence in the mixture as it closes that last little gap and in turn it helps promote burn in the cylinder's center and keep the burn aways from the edges which help the motor remain cool. I will admit I barely understand this but I have studied in quite a bit due to having a foolish "pro" engine builder screw up some good engine parts on me. Doing radical head milling can alter the squish and kill your motor. So study up if you mess with the head or pistons that raise the compression with a taller crown/dome.

 
You touched the surface on a very good point.That is the combustion chamber shape and the flame front are related closely.Too slow a flame front and you can have detonation occur because of the rapid increase in cyl pressure but the flame front cannot propogate evenly and BOOM detonation and a dead piston.I run over 14:1 on all my race stuff and up to 15:1 on 110 octane. A good combustion chamber that complements the piston dome can handle very high compression but a bigass dome with a pisspoor chamber will die at 11:1. Compression is only a rough guideline at best when determining octane needs.
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"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
 " You have all the right in the world to believe any damn thing you'd like, but you don't have the right to demand that I agree with your fantasy"

Offline fabr

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Re: compression
« Reply #26 on: January 08, 2009, 02:44:48 PM »
Quote from: Nutz4sand on January 08, 2009, 12:51:14 PM
When we used to mess with Banshees the factory compression was in the 135 range (longevity= long life) You could mill the head to get the compression around 155 for reallly good power on premium pump gas. Apparently Yamaha (Bless thier souls) made it so the the squish band got better to that point as you milled material off the head.

Going much farther meant having the squish bands recut or else.

Do your homework on squish especially how it affects the motor your playing with.

   
Absolutely Agree.WAYYYYY too much to rewrite about here in detail.
« Last Edit: January 08, 2009, 04:13:07 PM by Masterfabr »
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"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
 " You have all the right in the world to believe any damn thing you'd like, but you don't have the right to demand that I agree with your fantasy"

SPEC

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Re: compression
« Reply #27 on: January 08, 2009, 02:51:15 PM »
Thanks Nutz that was fookin great  :s
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