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Discussion Starter #1
How much compression gain is realized from using the .030 head gaskets in a HO 103? The stock compression is supposed to be 9.7 to 1. The old compression ratio was 9.5 to 1 and most results show a .2 to .3 compression ratio increase. I haven't found any specifics for the HO 103's. I'm in Denver so all of the compression numbers are a little skewed to start with.
 

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Your figures are probably pretty close, .25 is the norm. The real value of a .030" HG is the tighter squish = better burn.
 

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When you pull your heads, check how for down in the hole each piston is. Put piston at top dead center and take a straight edge across the top of each cylinder and take a feeler gauge and measure the distance between the piston and the straight edge. Each side could be a little different. Usually not much though.This is called measuring the deck height. The volume of how far down in the hole the pistons are and the volume of the combustion chamber of the heads will give you how many cc's your total combustion chamber is. This now can be figured into what size head gasket can be used to get the compression ratio you desire. I will look for the conversion of cc's to compression ratios and post up again. Most shoot for around 83 t0 85 cc's.


Here you go: http://www.bigboyzheadporting.com/TwinCamComp.htm
 

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You can see if you play with the numbers on Big Boyz site things change quickly. Such as if you heads cc at 83 instead of 85 it increases compression ratio very quickly. Also when you measure deck height and enter these numbers the effects are pretty dramatic.
 

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The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO
 

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Discussion Starter #6 (Edited)
Thanks for the responses. I'm not ripping into the motor. I did it last year. My buddy and I were discussing the best build for him. He'll probably do the same thing I did. Mild port/polish, valve springs, cam and pushrods. The question is really will the thinner head gasket help his performance? I say it can't hurt.
 

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The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO
The cam has absolutely zero effect on compression!!!
 

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The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO
Ironmark, I want to hear the explanation as to why a cam would change the compression of of a build. I understand the term corrected compression but as for our compression which is the determined by the cc's of the combustion chamber which is the size of bowl in the heads and the deck height.
 

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For an additional .2 bump in compression, use .030" thick head gaskets
This is what S&S says
 

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It is the corrected compression that changes not static compression.
The cam changes the timing so the amount of compression in the cylinder when the spark plug fires is the corrected compression.
If the spark plug fires before or after the piston is at TDC than the compression changed because the volume of air and fuel is not at the max compression.
The different cams change the opening and closing of the valves some open before TDC some open after TDC and the ignition timing is different on most cams this all leads to corrected compression.
 

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It is the corrected compression that changes not static compression.
The cam changes the timing so the amount of compression in the cylinder when the spark plug fires is the corrected compression.
If the spark plug fires before or after the piston is at TDC than the compression changed because the volume of air and fuel is not at the max compression.
The different cams change the opening and closing of the valves some open before TDC some open after TDC and the ignition timing is different on most cams this all leads to corrected compression.
Makes no difference. You either build the engine with the pistons and compression you want and choose a cam that works with that, or you choose a cam and build the engine for the cam. It's a catch 22. If you want more horsepower, bigger pistons, bigger valves, more compression, then the cam comes into play. It's all about how radical you want to get and how reliable you want it for the long haul and distance trips. Rule of thumb, lots of highway miles and long trips, stay in the 10 to 10.2:1 compression on your build and a cam that makes good power down low and it usually gives out at around 5000 to 5200 rpm. If you want a bar hopper, one up rider, 10.5:1 all the way to 11.5:1 compression, a cam over .625 lift and will keep on trucking past 6500 rpm but that build will beat a motor up over a short period of time.
 

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I use 79cc heads and a 585 cam my compression is 11:1 I use .030" thick head gaskets I had to use an S&S easy start cam because I have no compression releases in the heads
 
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Makes no difference. You either build the engine with the pistons and compression you want and choose a cam that works with that, or you choose a cam and build the engine for the cam. It's a catch 22. If you want more horsepower, bigger pistons, bigger valves, more compression, then the cam comes into play. It's all about how radical you want to get and how reliable you want it for the long haul and distance trips. Rule of thumb, lots of highway miles and long trips, stay in the 10 to 10.2:1 compression on your build and a cam that makes good power down low and it usually gives out at around 5000 to 5200 rpm. If you want a bar hopper, one up rider, 10.5:1 all the way to 11.5:1 compression, a cam over .625 lift and will keep on trucking past 6500 rpm but that build will beat a motor up over a short period of time.
I disagree! the bikes engine always runs on the corrected compression. The cam makes all the difference in corrected compression.
For example a stock 103 has a static compression of 9.87:1 but with
HD OEM cam- corrected compression - 9.43:1 with cold cranking compression @196.7 @ sea level
SE 255 cam - corrected compression 9.56:1 with cold cranking compression 202.2 @ sea level
Andrews TW55 cam - corrected compression 8.84:1 with cold cranking compression @ 181.1 @sea level
Woods TW777 - corrected compression 9.01:1 with cold cranking compression 185.6 @ sea level
This is just a few examples. Performance builds that have higher compression will change these #s but they will still run on corrected compression.
 

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I disagree! the bikes engine always runs on the corrected compression. The cam makes all the difference in corrected compression.
For example a stock 103 has a static compression of 9.87:1 but with
HD OEM cam- corrected compression - 9.43:1 with cold cranking compression @196.7 @ sea level
SE 255 cam - corrected compression 9.56:1 with cold cranking compression 202.2 @ sea level
Andrews TW55 cam - corrected compression 8.84:1 with cold cranking compression @ 181.1 @sea level
Woods TW777 - corrected compression 9.01:1 with cold cranking compression 185.6 @ sea level
This is just a few examples. Performance builds that have higher compression will change these #s but they will still run on corrected compression.
I agree with you Ironmark, we are saying the same thing but actually the compression is based on the amount of squish area (cc of the combustion chamber) Without that number calculated correctly, you will not get the most out of the cam you choose. Pretty simple actually.
 
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