shaving pistons/ and what CP ratio

[hawaiian808]

Hey!
i have been lurking as usual
few questions
can we shave our pistons safely?
and by how much? what is a good compression ratio for 15/20 psi? that i will still be able to start in the dead of winter? and allow me to in theroy run 25 psi? now im am by no means looking to have a great start up just be able to start XD

also the hx35 i tried to follow that thread while at work on my phone and that didint work out as planned, i cant "search" with it and i would like to know how to achive 20 psi with it max and still build boost down low? ( not expecting 10 psi off idle by no means )someone suggested changing housings , like i said tried to follow it did not pan out to well. and one final question, whats a good compund set up? just out of curiousity with max boost being 20lbs? and how many cfms' is it for the hx at 20 lbs ? ( i seriously try to goodle these equations and i never can find them, and im a math retard)


as an up date thank u towcat for the offer onthe cab, i ended up telling them to "splice" in a new roof, now i have a brake like on the back of my cab haha, turned out good they will be done tuesday and ill take pictures for you guys to see the amout of work i should have NOT put into my truck


in the next month its off to have the motor rebuilt!! and thank you to that special someone for those injectors!

 

[racer30]

Well cutting the piston tops is one way to lower CR it seems a little scary after seeing most of the pistons in both my turbo engines cracked in the middle. the CR is reduced 1.2 points for every .010 cut off the piston. "If the piston top shape dosen't change" If you were to cut .010 off the piston top, The shape would be differant. The compression would change but not as much because the area cut is not the total surface. The shape is important to flame travel so changing it might not be a good idea. I have read here that there are replacement pistons for a engine that has been decked, They are made aprox .010 lower for that reason. those are the pistons I hope to use in my build. I also found that for every CC of space added to the combustion chamber reduces CR .47 points. I have measured the valve pockets on my heads and found the intake pocket is aprox 6 CC and exhaust is 3 CC the pre cup area with a injector and glow plug installed is aprox 20 CC. The depression's on the surface of the piston totals about 8 CC With this info and the aprox gasket thickness of .062 is what I used to figure CR on the computer engine analyzer to get 21.5 to 1 the piston comes up about .014 above deck. I plan to gently round the edges of the valve pockets in the head on the center of the combustion area "Not on the out side of the valve pocket where the head meets the block" I hope this will be enuff to get me a few more CC of area to drop my CR to about 18 to 1 . I plan to run 20 to 25 psi after my intercooler "Head studs a must"and I have lots of room for turbo upgrades as this engine is going in my rear engine Motor Home. I hope this will give you some ideas.

 

[icanfixall]

The oem factory builder of our pisons is Mahle. I talked to their head engineer about 4 years ago about this milling. He told me that 14 or 15 was tops. Any more will allow too much heat to reach the top ring.... So with this info some here have milled 25 off the tops and they say they are running fine.... How much you feel you can get away with is on your shoulders. Pushing the limits is a test of what works and what doesn't work....

 

[hawaiian808]

ok awesome thank you,
i was under the impression that the 6.9 pistons are basically flat? i would imagine if you shaved 10-11 off it would help a little bit as long as the integrity of the piston was not effected by a large margin, so porting the chambers would also net me a little less compression ... hmm i will email the builder to get a new quote, im not trying to reach out and set records by no means or spend 5 million dollars , but since its apart just do a few things here and there, hopefully it will add up,
any ideas on a good compound turbo set up?

 

[icanfixall]

Mahle sells alow compression piston. What they do is machine the pin 10 thousands higher upin the piston so it ends up lower in the bore when its at top dead center.

 

[hawaiian808]

So they do sell low com pistons awesome!