Milling off top of stock pistons to drop compression??

O

oldsmokey97

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Since i am in the process of slowly rebuilding my engine. I was wondering how many of yall daily drivers have dropped compression??..
How do yall like it and what are the advantages besides less internal pressures especially with higher boost numbers???

ALso.......... is there a somewhat rule of thumb as to how many thousandths taken off equals a certain compression drop?? I know there are a LOT more factors than that like gasket, block deck, and head surface.. but there has got to be somewhat of a rule of thumb for us with stock parameters.

one more question..
who has marine pistons with the 155* bowl???

Lance
 
MAXTORQ took .030 off top of my 12v pistons and Swain Tech did the coatings. Use the truck on trips to events. Compete and then drive back with no problems. Got haze all the time but that is due to injectors mostly. Has increased mpg little. Don't use a lot of boost due to head work. IMO its worth it. When I get it broke in good will crank things up. Nice thing is I dont have any drive pressure issues. Dont know about the milling to compression formula. Maybe MAXTORQ, Smokem and Jim will chime in.
 
Can't remember what mine have off, either .050 or .060 and that dropped it down 2 points to 15.5, it's not too bad but it doesn't get that cold here.

Marine pistons are by either Cummins or Mahle

Jim
 
I'm running .020 off my pistons, stock deck height and a .020 over headgasket.

Smokes nasty white gray when cold and sure hates cold weather (I'm talking 30*). Search Youtube for "Cold Start DOdge" (Yah, with the funny O in Dodge)

The motor is much softer... Turns over when cranking faster, turns off more gentle, coasts longer/farther, revs up quicker in neutral, runs much lower EGT.

I daily drive mine, in fact, I haven't been in any type of competition for a few years now.

I'm running a HX-35/14 over a 3B/22 twin setup with a PDR stage 1 cam and head, back when it was still Piers Research...

I'm also running some Mach 4.8 injectors..

I can get 30Lbs boost by 1,500RPM (pretty smokey though, and the trans hates it), EGT's at 345rwhp never go over 1,300 or so. However, at my 500HP setting the EGT's peak at 1,600 and boost peaks at 50 - 58PSI, depending on ambient air temp.

Since '03, the only problem I've had was that I changed to some used "250 horse" injectors and dropped a valve seat...

I'm running about 15MPG driving normal (mixed, and occasionally spinnin' the tires), but can easily sneak out 20+MPG without A/C on, windows up and under 65 on the highway.

When towing,, about 3 times a week,, I gross anywhere from 26,000Lbs to 34,000Lbs.

I wouldn't be afraid of doing it.

the cons are hard starts in cold weather, smokes a little more on throttle "tip in", and looses some of it's "retarding power".

Pros are, more air = cooler EGT or more power, less engine wear, and quieter.

Dropping the compression simulates a very slightly larger turbo...

Merrick
 
how are you gettn away with unbalancing your motors by cuttin the pistons? or are ya all spendin the time to get the motor balanced?

Although the cummins may not be the most balanced motor out there from what i hear, makin it more unbalanced cant be good for it?

or did I miss something?
 
I have haisley modified stock pistons in my pulling truck. They are fly cut and tapered cut to allow for the heat expansion. The compression is a little below 15:1. All i know is that low compression, 5x.018 injectors, and silver 71 makes a very spooly combination.
 
seeker1056 said:
how are you gettn away with unbalancing your motors by cuttin the pistons? or are ya all spendin the time to get the motor balanced?

Although the cummins may not be the most balanced motor out there from what i hear, makin it more unbalanced cant be good for it?

or did I miss something?
Click to expand...


20 or 30 thousands out of the tops does'nt even come close to a gram of BOB weight.
 
Jim Fulmer said:
Can't remember what mine have off, either .050 or .060 and that dropped it down 2 points to 15.5, it's not too bad but it doesn't get that cold here.

Marine pistons are by either Cummins or Mahle

Jim
Click to expand...


What is the compression ratio on the marine pistons from mahle or cummins???

Lance
 
My CR pistons are cut a full .100
White smokes when a little cold out but not terrible. Totally clean when warm.
Comp. Ratio is somewhere round 14.5 : 1
Daily drives fine. Power right off idle is a little soft but picks up nicely when boost comes up.
 
XTrEmE9156 said:
I have haisley modified stock pistons in my pulling truck. They are fly cut and tapered cut to allow for the heat expansion. The compression is a little below 15:1. All i know is that low compression, 5x.018 injectors, and silver 71 makes a very spooly combination.
Click to expand...

Mine are done similar. They say compression is 12.5:1. But saw a set of 12.5:1 Scheid pistons and they looked way different. They Haisley set takes a couple cans of either to get fired up!:doh:
 
2Tone12V said:
Mine are done similar. They say compression is 12.5:1. But saw a set of 12.5:1 Scheid pistons and they looked way different. They Haisley set takes a couple cans of either to get fired up!:doh:
Click to expand...

Don't worry I'll buy you a whole case of ether so you can spray all you want..... Weather or not it goes into your engine isn't my concern LOL.......
 
oldsmokey97 said:
What is the compression ratio on the marine pistons from mahle or cummins???

Lance
Click to expand...

There are two different versions of the Mahle pistons, the ones I have for my new motor are 17.5 but do not have the teflon skirts.

Jim
 
Big Swole said:
My CR pistons are cut a full .100
White smokes when a little cold out but not terrible. Totally clean when warm.
Comp. Ratio is somewhere round 14.5 : 1
Daily drives fine. Power right off idle is a little soft but picks up nicely when boost comes up.
Click to expand...

Why would you take .100 off the top, especially on a CR? I would think (after looking at the spray pattern on a CR piston) that you would be getting close to spraying out of the bowl and on top of the piston. Then you add programing with timing. Looks like trouble to me, but thats just my thought.

What would the benefit be to cutting the whole top as opposed to just making the bowl a little bigger? I don't know, thats why I am asking.
:thankyou2:
 
BigYellowIron said:
Why would you take .100 off the top, especially on a CR? I would think (after looking at the spray pattern on a CR piston) that you would be getting close to spraying out of the bowl and on top of the piston. Then you add programing with timing. Looks like trouble to me, but thats just my thought.

What would the benefit be to cutting the whole top as opposed to just making the bowl a little bigger? I don't know, thats why I am asking.
:thankyou2:
Click to expand...

I agree. I would think making the bowl bigger would be the best but if you did the tops you should have a different spray angle on the injectors to compensate for height and timing. JMO
 
Ive got mine cut .080 and the spray patter is still very deep in the piston. It would take more than .100 to get outa the bowl on a Cr.
 
First the balance issues
A inline 6 cylinder is neutral balanced meaning you do not use bob weights on the crank to balance , just the crank itself in the balance machine
The pistons , rods, rings , bearings , and wrist pin need to weight the same with in a a small percent..

As to taking .100 off a CR piston, you asking for big trouble in both the top of the piston , and the ring lands being close to the top .

Now as to compression in a diesel.

Compression gives a mechanical advantage , cutting it dose just as many on this thread have said soften up acceleration , or just plan killed HP under the curve
If you are building any thing under 1000 hp, then I recommend no drop in compression at all , and over that 16.4 to one is the optimal point for a well planed program, as in cylinder head , cam program, and fuel delivery.
Now if your building a 750 hp or less street engine , then I would even look at increasing the compression slightly , a little gets you a lot. The way to do this is mill the deck slightly , and the piston the same,
With say 17.75 to 18.25 to 1 the engine will be extremely responsive, and faster on the street in real life.

There is a lot more to the compression ignition then this , and I’ll get back on later and explain, to fully realize you must take in to account cylinder pressure rise , maximum cylinder pressure , and when the actual ignition takes place

Also a thoughtful cam program will only need .060 piston cut to fit , this is my 6500 rpm profile like I used in the Predator truck
 
This is my new piston. I took older 24v pistons that have a lower ring land and no cooling passages for more strenght and turned the bowl out to the diameter of a 600 series piston all the while trying to take as little material away as possible. Wound up with a 60cc bowl. Should make my cr around a 14.5 or so, lower then I wanted but it will work.

DSC00233.jpg
 
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