I have a HPR 12:1 FSR piston in my stock bore that is gasketed for 11.5:1 and i have been using a 50/50 mix of 110 and pump premium with it and getting good results.Today i tried straight 110 and purely by seat of the pants feel it seems to pull harder especially in the low end.I assume that port work can effect compression to some degree but just how much can it alter compression?I am very pleased with the port work that i had done and i think a big part of my stock bores very good performance is because of it but if my compression is raised even slightly then that could explain why the 110 worked better?Any ideas or input?

 

Not alot of answers but good reading!!!!Text Porting

 

That all depends on how much the combustion chamber was opened up. The more port work you do to it the less cc the chamber becomes. Most of the port work on the ds head is just a clean up of the casting flash for the combustion chamber. It is the only part of the port work that would effect compression.

 

Porting doesn't really change the compression ratio. It greatly effects cylinder pressure though. Static compression ratio is simply a mathmatical ratio of starting volume at BDC compared to ending volume at TDC.

You raise an excellent question though............

Lets say you have a compression ratio of 11.5:1 with stock porting and cams and the bike runs the best on straight 92 octane premium. Now lets say you improve your cams and porting and that results in an increased amount of air/fuel entering the cylinder on the intake stroke creating more pressure earlier in the compression stroke. This will create more heat earlier in the compression stroke which will raise the optimal octane level (burn rate, specifically). This all assumes equal spark advance curves.

This is why if you are going for that last 10th of a horsepower that even fuel should be dyno tuned for the days conditions. This is also why you cannot make a blanket statement that 92 octane is always best for 11.5:1 static compression motors or 50/50 for 12:1. Those are good places to start but try a few different combinations with your build and find the fuel that makes you bike run the hardest without detonation or pre-ignition and use that regardless of what someone else says you should have.[img]i/expressions/face-icon-small-smile.gif[/img]

I hope that helps[img]i/expressions/face-icon-small-smile.gif[/img]

 

I was trying to think of an analogy to compare the difference between static compression and pressure. The simplest comparisons work the best I guess..........

Take a 32oz jar and put 24oz of liquid in it. It is still a 32oz jar even though you chose not to fill it completely. It doesn't matter how much liquid you try to pour in a 32oz jar..........it is still always 32oz capacity.

 

And that would be called Volumetric efficiency. Ds motors do not have a 100% VE. But when piston manufacturers make the piston, they have to assume a 100% VE. So really all porting can do, is raise the VE. Thus bringing the VE up closer to the 100% the manufacturer spec. It is possible to go over 100% VE, but it is very hard to do.


So it your case, lets say with your 11.5-1 comp, and an assumed 85% VE, your static compression ratio is more like 10.625-1. Now with everyones research, and since most these motors are similar, you could have ran straight pump. But, you have porting and cams that brought up the VE to say 93%, and that raised your static to day 11.25-1, and you chose to run a mix fuel. Now you decide to try something, add more octane, and it gives you better performance. Well we know it cant be because of the fuel. But the higher octane could have stopped some deto that you didnt know you had. The numbers I used are just an illustration, and are not exact numbers. I just used them to make the argument.

One thing to note, alot of the things we do, add, remove, alter, do nothing more than make us thing we are going faster, while at the same time, they are doing nothing but taking our time and money.[img]i/expressions/face-icon-small-wink.gif[/img]

 

This is the same conversation Cape and I had last week. Static compression, cams , and porting all have a hand in creating cylinder pressure. This is why Hpr 3 cams and 12-1 will create better torque then 14-1 and 109's all else being the same. I'm not trying to say one is better then the other just your whole package needs to work together for the intended purpose of the engine. Trying to make up for poor cam selection by jacking up compression, you still won't get the full potential of your engine.

 

Great responses to my question guys.I am not sure if i was actually going up the hill faster but i am sure that the throttle snap for lack of a better word was improved.I am trying to figure out the best ratio of 110 and premium that will work well when i am just on motor and what will give me some margin of safety when i start using Nitrous.It seemed that i may have been just about the same except for the low end snap but the power of suggestion may have been at work there.So a 50/50 mix of premium and 110 should be sufficient for 11.5:1 and Nitrous use?I had heard that port work could effect compression but it sounds like that is not the case although the effects of porting can alter fuel needs?I need to buy a radar gun for my testing so that i can get some hard numbers to go by.

 

No, porting can and will effect ratio. Its filling the cylinder better, so its raising the VE, and in turn raising the static comp. It all works hand in hand like was mentioned.

At sea level I wouldnt recommend using nitrous on mix. I ran it, but at 5000', and depending on timing, 20-25 shot. I run straight 110 with my 30 shot. I can run pump 91 with my 12-1 at 5000'. The 110 kills my bike on all motor, but I guess you got to give a little to get a little.

While I like HPR 3's, I diagree they make more pressure than a 14-1 with 109's. But thats just my opinion.

 

Just an FYI.
Static compression ratio is set and can be calculated by the CCs about the piston at TDC and the bore and stroke. And like Viper said, if you had a VE of 100% this would match you dynamic ratio.
Dynamic compression ratio is what you get in real life, and is affected by all the variables in an engine.
Scott