It's a ratio of before and after.

There is a difference between internal volume and displacement.

For a stock 400EX, the bore and stroke is 85mm x 70mm. Do the math, and you get 397cc (cubic centimeters) The compression ratio is 9.1:1

When the piston is at the very bottom of the stroke, the internal volume of the cylinder is not 397cc, it is actually about 440.6cc. When the piston is at the top, the volume is about 43.6cc (this is called the "combustion chamber")

The compression ratio is calculated by dividing your displacement by your combustion chamber volume.

397cc / 43.6cc = 9.1

A high-compression piston has a dome on top of it that will decrease the volume of the combustion chamber. If your piston has a dome on it of 7.5cc, then your combustion chamber is now 36.1cc instead of 43.6cc. For the same displacement of 397cc, your compression ratio is

397cc / 36.1cc = 11


I take no credit for writing this, I stole it from another site I cannot name.

 

Well I'll give you credit for sharing it then! thanks![img]i/expressions/face-icon-small-smile.gif[/img]


OK, so to achieve 21: compression ratio I would have to have a Compression chamber of 31cc's right?

stock DS is 653cc...So divide that by 31,and you get 21.06.


So what kind of custom piston would have to be made to get me to 31cc of compression chamber? Hmmmm.


What is the HIGHEST compression piston made for the DS today?(stock bore).







Bigger.

 

This is where I had many problems with my last Honda build.
You need to cc the head, you will need to get your deck height set up as zero, then machine a bunch of material off your head, and or weld up the combustion chamber.
It now gets tricky. Your clearances would be so minute at that compression, I would be scared of it. Not to mention the octane of fuel required.....
Good Luck

 

Would alky run with that high of compression? aslong as you didnt advance the timing.


How can diesels get by with these ratios? and still be so reliable. Iknow that they dont have Sparks plugs but they still have valves.




bigger.

 

Well when guys run top fuel thats what it is, well w/ a high percentage of Nitro. I am not sure if the nitro makes it work at higher comp. or just makes more power wherever.
You are now getting into an area that guy don't like to share info on. If I knew I would, but I am just a dumb dude from Utah, who likes to sound like I know whats going on...lol
I found some interesting info on it, mostly in relationship to car motors. Do some searches and see what you can turn over. One of the sites (the one I stole that info from, what quad was it talking about?) had a thread going on about setting up deck heights, compression ect.

 

Thanks Man.


great pics BTW.

if nothing else I will learn even more about engines[img]i/expressions/face-icon-small-wink.gif[/img]





BIgger.

 

diesels get away with it because the engine parts are made much bigger for strength. you might be able to get there if all was made out of ty and carbon fiber.

later

 

well i learned something here thats cool.

 

didnt mrhp have numbers for a 17.1 piston?

 

you cant run 21:1 on your ds buddy. try max 15:1 and thats even pushing it. 21:1 can only be turned over with special starters like they use to turn over the top fuel cars and o man detonation and overheating city. not to mention blowing up almost every run down the strip. if someone fealt in their own mind that 21:1 could be run in a ds they would have a kit for it right now so obviously not a smart thing to do dont even think about it. you want some decent gains while still not blowing up every 1in3 runs get some nitrous and line it up. spray about a 30HP shot in then let everyone eat your dust. to run the 21:1 you would also need nitromethane or extremely specially designed racing fuels that are over $40/gal. alky cant handle those kind of numbers.