Best conversion, Compression ratio 10:1 or 9.5:1 - 1200 or 1250

Posted this on an other site. Looking for all info Ican read.

Going to convert my 2002 883c to 1200 / 1250.

What is the better compression ratio for today's **** gas ? Compression ratio 10:1 or 9.5:1

 

Why not 10.5:1 ?
http://www.hammerperf.com/xlbigborekits.shtml

 

The biggest issue isn't static compression ratio, it is your dynamic compression ratio. My kit has flat-tops for a 10.5:1 ratio once I install it. You can go even higher if you pair it with some monster cams and other work. Read about compression ratios here: http://hammerperf.com/ttcompression.shtml

 

Scuba has the right answer. It depends on your cams.

Static compression ratio is the compression ratio counting the full compression stroke. However, as the piston comes up on the compression stroke, the intake valve is still hanging open for a period of time. No compression builds until that intake valve closes.

Therefore the intake valve close point has a lot to do with how much compression you actually build. So the more useful compression ratio number is not the commonly used static compression ratio, it's the compression ratio after the intake valve closes. That compression ratio is often called "corrected" compression ratio. Some people call it "dynamic" compression ratio as well, however, dynamic compression ratio has another meaning as well (compression ratio considering cylinder fill), so I avoid the term in order to avoid confusion, and I use "corrected" compression ratio instead.

To have a pump gas friendly motor, stay in the 9:1 to 9.3:1 corrected compression ratio range. I fully realize that many shops routinely advise 9.5-9.7:1 corrected; if your tune is spot on, you can push it like that, but it's safer to stay down a little.

 

If that's true then why is Harley upping the compression ration on the 2014 Sportster to 10.01? (http://www.harley-davidson.com/en_US...om.html#!specs)

 

I am willing to bet that is not the corrected compression ratio..also these days you can run higher compression with the "right" tuning, and fuel/timing controllers are really good now at keeping the tune right on the mark, taking in a lot of variables such as air pressure, air temp, engine temp. and listening for knock...

 

I'll make a call to HD and ask them.
One thing I did notice is that the torque changed but so did the RPM at which
they were measured. So not sure what's going on here.
2014: 70.8 @3500 10:1
2013: 79 @3250 9.7:1

 

No need to call. That is the static compression ratio. Their corrected compression is usually under 9, so you have nothing to worry about.

 

My Nightster 's cr is 11.3:1. It has been extensively dyno tuned and runs fine on 91 octane pumping out 98.3 ponies.

Bwana

 

An so the question remains as to what is HD is using to rate it's compression ration..... corrected or static.