7.3 cylinder heads.......????

[freebird01]

Consequently, when you mess with the height of the piston you're doing 2 detrimental things. First off you're increasing the ration of the prechamber volume to the cylinder volume. This decreases the turbulence in the prechamber to some extent (less gas compression into the chamber and lower velocity of gas entering) and also means more of the combustion is going to occur in the cylinder proper. This is more important since all of the fuel is injected into the prechamber. Essentially what happens is you wind up with is incomplete combustion within the prechamber and unburnt and partially burnt fuel flowing out into the main chamber. The more of the combustion volume is in the cylinder, the more of this flow there is and the less efficient combustion is due to the less effective mixing in the cylinder as well as the fact that combustion is going to rapidly cool once it gets into the much cooler chamber.

why do so many people decompress these engines when looking for more power and bigger turbo's? are you saying to properly do this you would mill less off the pistons and re-machine the recardo cups in the piston crowns

In this statement your saying its detrimental to efficiency and possibly performance to change piston height. I understand WHY you lower the piston height and lower compression ratio...but your statement contradicts that.

Maybe I am off base here. Please don't take this as calling anyone out...I'm just trying to paint a complete picture of the discussion...

What I am getting out of the information above is your saying leave the compression ratio alone and don't mess with the original pistons due to the detrimental effects it will have on combustion gases and create incomplete combustion and improper swirling. But to add more air and fuel and bigger turbo(s) you need to lower the compression ratio.

So your answer would be (this is my assumption based on what im reading above please correct me if im wrong)....is to either re-machine the recardo chamber into the milled pistons or complete custom pistons...

However this does nothing to address the ratio change of cylinder volume to pre-chamber volume....



but read your statement above then read my last one...

 

[racer30]

The static compression ratio is a calculation of space above the piston compressing the air in the cylinder. Dynamic compression is the calculation based on when the intake valve closes on the compression stroke. Boost compression ratio is a combination of dynamic calculations with a given boost pressure added to give the new ratio under boost....Now I think The IDI was designed as a NA engine and they determined they needed 21.5 to 1 to get the engine to run well over a given range of temps and elevation. Many hours of diesel engine performance has gone into designing better Turbo's and injectors for engines and most of those Guys lower the compression when they raise boost. this is because when they add more fuel and air they need more room for the larger combustion charge to burn completely in the cylinder. Now how much to lower the CR is what everybody is trying to figure out...My thoughts our that I wanted a cam that would move more air and more duration to raise the power up a few hundred RPM, I cut .030 off the pistons to lower the CR to about 19.5 and I will try to push 15 to 20 PSI Boost. This brings my Boost compression ratio to 39 to1 at 15 PSI so I think that's plenty of compression when under full power. Now Just like any performance engine the more radical the full power performance the worse the engine will run when not at full power in most situations. This means you need a CR that will make the engine start and run off boost. So modest modification is best for Daily driver Rigs and full out performance engines stay on the track or off road. This is my opinion not a slam on anyone that wants a more radical build. you just have to look at the complete engine not just one aspect of it.

 

[icanfixall]

Lets add one more thing to turbos and boost. If we have 12 lbs of boost. That means we have 12 lbs pressing on the intake valve and its stored up in the intake manifold. So as the valve opens there is a rush of compressed air chargeing the mt cylinder. As the valve becomes fully open that 12 lbs does not deminish. It continues to fill the ever opening cylinder as the piston is traveling downward in the cylinder. Think about all those extra atmospheres force filling that cylinder. Then the valve closes and we begin compression on said extra forced volume of air in that cylinder. Now do that 3000 times per minute.. Wow.. Thats a beating going on in there. A few years ago when I was at ARP talking to the engineers about why my studs they sold me gaulded and why my DPS studs broke they told me about a customer wanting intake studs for his psd engine. He kept breaking his. They had no idea why his were breaking till they asked what boost are you running. Simple straight faced answer... Oh, 250 lbs.... They sent him on his way telling him they had nothing to help him. Thats all I know bout that engine. I think its an impossibility to run that high of boost on a psd but what do I know...

 

[freebird01]

both are good points... i am just trying to make sense of conflicting information.

racer30... that's exactly why in my particular 6.9 build I am planning on not milling the pistons as radically as it came to me. seems the original owner had the pistons milled .060 or .065 not sure of the exact number. figures were that the engine was at 17:1. I had plans of milling these new stock pistons i got .040" figuring it would start better in cold weather as well as having them ceramic coated.