compression : V-Twin Forum: Harley Davidson Forums

bigbadal · 10-14-2009, 11:14 AM

Who knows how to figue out the ratio for compression?? I have 174lbs in each cylinder on a stock 80" evo. That's with both plugs out, in nuetral and the carb wide open. What would be my compression ratio?? Alan

gsxrboy96 · 10-15-2009, 11:33 PM

Quote:

Originally Posted by bigbadal

Who knows how to figue out the ratio for compression?? I have 174lbs in each cylinder on a stock 80" evo. That's with both plugs out, in nuetral and the carb wide open. What would be my compression ratio?? Alan

Its hard to say, I know with 10 to 1 my bike was 190lbs.

93 FXR · 10-16-2009, 07:23 AM

I believe that the stock compression ratio on an Evo motor is 8.5:1.

route66paul · 10-16-2009, 10:28 AM

what cam is in the engine?

bigbadal · 10-16-2009, 11:50 AM

Quote:

Originally Posted by 93 FXR

I believe that the stock compression ratio on an Evo motor is 8.5:1.

Yes, I believe that is correct. What I'm looking for is the formula to compute compression ratio given particular parameters...stock bore, stock stroke and compression. I'm not a math whizz so I don't know if that's possible, but logically, I would think it is. Alan

bigbadal · 10-16-2009, 11:56 AM

Quote:

Originally Posted by route66paul

what cam is in the engine?

The cam is an ev27. Where are you going with this information?? I think I know, but I'm not totally sure. So maybe the durations of the valves being open and the overlap will change the compression ratio due to the air not being compressed till both valves are closed and the piston being "x" amount of distance from the bottom of its stroke?? Alan

bigbadal · 10-16-2009, 11:59 AM

Quote:

Originally Posted by gsxrboy96

Its hard to say, I know with 10 to 1 my bike was 190lbs.

Yes, but I seem to remember that your engine was bored twenty over, so it's not stock. So it would be slightly different than mine. I know I'm splitting hairs, but I'm just looking for a formula, if there is such a thing. Alan

springer- · 10-16-2009, 12:34 PM

This can help figure it out, it may take a couple of educated guesses since you are "reverse engineering" it.... http://www.bigboyzheadporting.com/EVOComp.htm

route66paul · 10-16-2009, 01:16 PM

Quote:

Originally Posted by bigbadal

The cam is an ev27. Where are you going with this information?? I think I know, but I'm not totally sure. So maybe the durations of the valves being open and the overlap will change the compression ratio due to the air not being compressed till both valves are closed and the piston being "x" amount of distance from the bottom of its stroke?? Alan

You need to know the intake close degree for the formula - it does go a little further, but this will give you a good guess. It also helps to know the baratric pressure when you took the comp test, but for best guess purposes, you elevation would be ok.

bigbadal · 10-16-2009, 02:13 PM

Quote:

Originally Posted by springer-

This can help figure it out, it may take a couple of educated guesses since you are "reverse engineering" it.... http://www.bigboyzheadporting.com/EVOComp.htm

Thanks Springer. I'll explore that avenue.

bigbadal · 10-16-2009, 02:17 PM

Quote:

Originally Posted by route66paul

You need to know the intake close degree for the formula - it does go a little further, but this will give you a good guess. It also helps to know the baratric pressure when you took the comp test, but for best guess purposes, you elevation would be ok.

Wow, I had no idea this was going to be such a complicated situation. Maybe, I'll try to find some chart that somebody else already figured out with all the right parameters and it will give me the ratio. Thanks, Alan

springer- · 10-16-2009, 02:55 PM

If you use the link I supplied that is exactly what I have done. You pick your cam and that calculates the intake close and adjusts your cranking compression accordingly. Then enter in any known values like the bore, stroke, head gasket thickness etc. Then adjust your combustion chamber volume up and down until your cranking compression matches what you have. At that point it should give you a rough idea of the static compression ratio that you are looking for. I don't think you will find a better "chart" out there since there are way too many possible combinations. If you have any issue feel free to send me a PM and I will help with it.

TXCHOP · 10-16-2009, 03:45 PM

9.16 to 1 compression in a perfect world at sea level. Too many variables to be exact, but that's "close".

blackjackcustms · 10-17-2009, 01:59 PM

To measure an assembled motor, you'll need to orient it so the spark plug is it's highest point, set it at BDC and fill it with fluid from a graduated buret, note the volume, and then move it to tdc compression, and recheck the volume. Any number you get from the cranking compression and the cam closing will be a WILD guess.

The only way I would imagine this would matter is if you want to increase output, and if that's the case, then just take it apart and CC it, or change your cam until you get the performance you want from it. I have run on the street with as much as 230 psi cranking, and with a good ignition and careful tuning, it can be done. It will be Gasoline sensitive, and you'll have to watch what you fill it with, but it does work.