Compression Ratio vs. Cylinder Height
Thread Starter
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Tourer
Joined: Aug 2014
Posts: 302
Likes: 49
From: Gambrill, MD
Compression Ratio vs. Cylinder Height
Folks,
My '93 Ultra engine is stock with a 3.498" bore, 4.250" stroke, and a compression ratio of 8.5:1. So, if I have a machine shop true up the cylinder bases so I'll get a good oil tight seal with new gaskets, how much does my compression ratio change for every, let's say, 0.001" of material that's removed? By my calculations, removing as much as 0.005" would decrease the compression chamber volume by 1.6cc which would increase the compression ratio by maybe 2%?
Just curious.
-Dave
My '93 Ultra engine is stock with a 3.498" bore, 4.250" stroke, and a compression ratio of 8.5:1. So, if I have a machine shop true up the cylinder bases so I'll get a good oil tight seal with new gaskets, how much does my compression ratio change for every, let's say, 0.001" of material that's removed? By my calculations, removing as much as 0.005" would decrease the compression chamber volume by 1.6cc which would increase the compression ratio by maybe 2%?
Just curious.
-Dave
Club Member
Joined: Aug 2008
Posts: 22,861
Likes: 17,917
From: Lafayette, Louisiana
You need to deal with things like deck height and use the appropriate gaskets for this. After your deck height is set you can start dealing with cranking pressure. The cam shaft you use will have an effect on the cranking pressure. Its not all about ratios which is something some people get hung up on.
Cranking pressure, piston to valve clearance, etc. Different gaskets will come into play with this as well.
Thete are a number of calculators available online including some that let you get an estimate with different cams, different amounts of material milled off the heads, and other variables. Be advised that these are educated estimates and are not to be substituted for real measurements in your particular engine.
Its been a few years since I was trying to figure things out and decide what to do even though my build was very very mild. What I ended up with was very satisfying despite the components listed would indicate not very noticeable than stock. Its about achieving synergy.
Now is a great time to get rid of that shitty stock cam. And replace the cam bearing with a Torrington bearing.
Cranking pressure, piston to valve clearance, etc. Different gaskets will come into play with this as well.
Thete are a number of calculators available online including some that let you get an estimate with different cams, different amounts of material milled off the heads, and other variables. Be advised that these are educated estimates and are not to be substituted for real measurements in your particular engine.
Its been a few years since I was trying to figure things out and decide what to do even though my build was very very mild. What I ended up with was very satisfying despite the components listed would indicate not very noticeable than stock. Its about achieving synergy.
Now is a great time to get rid of that shitty stock cam. And replace the cam bearing with a Torrington bearing.
Last edited by Prot; Jan 8, 2020 at 01:38 AM.
Road Captain
Joined: Aug 2012
Posts: 625
Likes: 102
From: New Albany, Ms.
Here
http://www.bigboyzheadporting.com/EVOComp.htm
This site will help you figure out what you can do.
Ride Safe,
Harold
This site will help you figure out what you can do.
Ride Safe,
Harold
Grand HDF Member
Joined: Apr 2007
Posts: 3,727
Likes: 3,071
From: Brighton, MI
In reference to changing your cylinder length, in my opinion its not really intended for boosting compression ratio.
Look up the terms like squish or quench. And as mentioned, deck height.
Its about a more efficient and complete burn of the combustion process.
Very common in the 2 stroke racing circuits but still applicable with our engines. YD
Look up the terms like squish or quench. And as mentioned, deck height.
Its about a more efficient and complete burn of the combustion process.
Very common in the 2 stroke racing circuits but still applicable with our engines. YD
Last edited by Yankee Dog; Jan 8, 2020 at 04:39 AM.
Thread Starter
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Tourer
Joined: Aug 2014
Posts: 302
Likes: 49
From: Gambrill, MD
In reference to changing your cylinder length, in my opinion its not really intended for boosting compression ratio.
Look up the terms like squish or quench. And as mentioned, deck height.
Its about a more efficient and complete burn of the combustion process.
Very common in the 2 stroke racing circuits but still applicable with our engines. YD
Look up the terms like squish or quench. And as mentioned, deck height.
Its about a more efficient and complete burn of the combustion process.
Very common in the 2 stroke racing circuits but still applicable with our engines. YD
In any event, in that calculator at http://www.bigboyzheadporting.com/EVOComp.htm, deck height is initially set at 0 inches with a note stating that "Negative number if piston is ABOVE cylinder." On a purely stock 80 EVO engine at TDC, how far is the piston from the deck? Is it really at 0"? Is it slightly below? If so, how much? I could have measured it on my bike, but I've already removed the jugs in preparation for sending them to the machine shop.
Thanks!
Club Member
Joined: Aug 2008
Posts: 22,861
Likes: 17,917
From: Lafayette, Louisiana
Actually, my question was more academic. My plan wasn't to machine the cylinder bases to obtain any improved performance, I was just wondering if my assumptions about how it affects compression ratio were correct.
In any event, in that calculator at http://www.bigboyzheadporting.com/EVOComp.htm, deck height is initially set at 0 inches with a note stating that "Negative number if piston is ABOVE cylinder." On a purely stock 80 EVO engine at TDC, how far is the piston from the deck? Is it really at 0"? Is it slightly below? If so, how much? I could have measured it on my bike, but I've already removed the jugs in preparation for sending them to the machine shop.
Thanks!
In any event, in that calculator at http://www.bigboyzheadporting.com/EVOComp.htm, deck height is initially set at 0 inches with a note stating that "Negative number if piston is ABOVE cylinder." On a purely stock 80 EVO engine at TDC, how far is the piston from the deck? Is it really at 0"? Is it slightly below? If so, how much? I could have measured it on my bike, but I've already removed the jugs in preparation for sending them to the machine shop.
Thanks!
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Grand HDF Member
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Thread Starter
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Tourer
Joined: Aug 2014
Posts: 302
Likes: 49
From: Gambrill, MD
On a related note, I measured the cylinder bores. Here are the numbers:
Front Cylinder, Bottom: -0.0010/0.0000 (measurements made at 90 degs apart)
Front Cylinder, Middle: -0.0010/-0.0005
Front Cylinder, Top: -0.0015/-0.0005
Rear Cylinder, Bottom: +0.0010/-0.0010
Rear Cylinder, Middle: +0.0005/0.0000
Rear Cylinder, Top: 0.0000/+0.0005
These measurements were not made with the cylinder in place nor in a torque block.
The Service Manual says that the service wear limit on the bore is +0.0030, so it looks like I'm good, right? Also, the numbers are within the Taper and Out-of-Round limits. So, I'm thinking I should be able to get away with just having the bases trued up and avoid any other machining.
-Dave
Front Cylinder, Bottom: -0.0010/0.0000 (measurements made at 90 degs apart)
Front Cylinder, Middle: -0.0010/-0.0005
Front Cylinder, Top: -0.0015/-0.0005
Rear Cylinder, Bottom: +0.0010/-0.0010
Rear Cylinder, Middle: +0.0005/0.0000
Rear Cylinder, Top: 0.0000/+0.0005
These measurements were not made with the cylinder in place nor in a torque block.
The Service Manual says that the service wear limit on the bore is +0.0030, so it looks like I'm good, right? Also, the numbers are within the Taper and Out-of-Round limits. So, I'm thinking I should be able to get away with just having the bases trued up and avoid any other machining.
-Dave
HDF Community Team
Joined: Dec 2005
Posts: 5,421
Likes: 2,621
From: NC USA
Those cylinder measurements are typical of original OEM cylinders and should accept honing/new std rings without issue.
But if you're determined to have the bases machined, that's all out the window! You should be prepared for at least +.005 oversized bore, pistons etc.
But if you're determined to have the bases machined, that's all out the window! You should be prepared for at least +.005 oversized bore, pistons etc.