ESS S85 Vt1 SC Kit - Page 3

rocket5979 · 1st September 2011, 16:43

Quote:

Originally Posted by built240

Didnt know that thanks. I guess i'd presume the m6's v10 was built with the fact it would be raced so the strongest parts would be used. But in laymens terms when you talk about compression are you kinda saying that the stock motor is already maxed out with power so adding more may be an issue.

Compression ratio is just the ratio between the biggest and smallest volume of the combustion chamber. The biggest volume being when the piston is at BDC (bottom dead center) and the smallest volume being when the piston is at TDC (top dead center). So if your total BDC measured cylinder volume is 744 cubic centimeters and your TDC measured volume is 62 cubic centimeters then 744/64.7=11.5 or 11.5:1 compression ratio. The simplified formula is as follows.

VBDC / VTDC = CR

The higher the compression ratio the more that the air fuel mixture is being compressed in the cylinder which means more heat build up in addition to whatever heat was already present to begin with from either forced induction or high ambient temperatures. The higher the compression the more the mixture is going to be heated. If this heat is too much it can cause the mixture inside the combustion chamber to light off prematurely causing detonation. This is where the mixture ignites before the spark plugs could fire and usually in an explosive manner. This results in stressing rotating parts much more than they may be rated for because they were not in the proper physical position for ignition to occur just yet. Think of this like pushing a child on a swing. To get the best and most swing you start pushing on them right when they are coming to full apex. If you try to push them forward while they were still on their back-swing then you will hurt your arms. The same thing applies with the manner in which the engine tries to fire off the air fuel mixture.

So with forced induction the aircharge will become heated above ambient from being compressed; on top of when it gets into the combustion chamber and gets compressed again. This "double heating" for lack of a better term is what makes forced induction more prone to detonation. If you have your engine built to a lower compression ratio then it will have less tendency to detonate due to the air fuel mixture being heated less.

My preference is about 10.0-10.5:1cr for turbocharged and centrifugal supercharged engines because I want to ensure that I have decent off-boost torque. With positive displacement superchargers (roots, TVS, twin screw) that make full boost almost immediately and their torque down low you don't need to worry about keeping low end off-boost torque so I prefer to drop compression down into the 8.5-9.5:1cr range which will allow me to run more total boost safely.

It is not that the stock S85 engine is "maxxed out with power", because it is not. But detonation needs to be kept at bay so that parts aren't overstressed. This is where either running a higher octane fuel such as E85 or injecting water/methanol to raise octane and cool intake temps helps allot. Due to differences in design, some engines are more prone to detonating than others. I have not worked on the S85 and thus do not know its specific characteristics when it comes to its tendency to detonate while at the stock 12:1cr and boosted. In most forced induction cases 12:1cr is pretty darn high. It sounds like ESS knows their stuff so I am curious to see what they did to mitigate any potential for issues to occur due to that.

Sorry for the long and probably over-detailed explanation. I get carried away sometimes.

Quote:

Originally Posted by irishbimmer

Built 240-I think he means that the stock 12:1 compression of the V-10 is typically too high for an add on blower. A motor that is designed for such an add-on my have a normal compression in the 8s or 9s for longevity.

Yep, pretty much.