Milling pistons to lower compression

[DigitalFusion]

It turns out I have to rebuild my turbo Phazer motor sooner than I expected. I could always re-ring the stock pistons & toss in a MCX head shim. But the shim is stupid expensive ($250) and gets me to 9.6 CR, which can be 'doggy' on the phazer.

I contacted JE pistons about getting some lower compression versions done as they promise a two week turn around for custom pistons, but I have yet to hear back, and from what I remember they used to charge like $250 / piston when they did offer them as an option. I can get two Wiseco pistons (12.5) with rings, pins & clips for $210.

Could I just have them milled down to get my CR lowered? They are already so small I would be worried about taking any material off of them. A really quick estimate just using the compression calculator in the sticky and playing with the head gasket height shows I would need to do something around:

• .038 for 10.5
• .051 for 10.0
• .065 for 9.5

 

[blueironranger]

I really wanted to “pump gas” turbo my Phazer and ran into the same road blocks. Custom pistons would be the only way to go. If you cut the top of the pistons down you loose quench area and run into the same issue as a head shim causes. You could dish the center of the piston if there is enough meat under crown and leave the outer edge at full height to maintain quench. Proper quench causes a lot more turbulence in the combustion chamber which allows the mixture to burn faster, hotter, and more efficiently. It allows you to get away with more timing and a leaner A/F ratio so it adds a safety factor also. I’d be willing to wager that a Phazer running 8psi with custom low compression pistons would make just as much HP/TQ as a Phazer running 10psi with a headshim, and safer doing it.

My original plan was aftermarket rods/pistons with a PCV PTI running 8psi on 91 octane. After finding NO rods/low compression piston decided to can the project after I figured I could only run 5psi safely on the stockers which would only be 105-110hp

 

[DigitalFusion]

Good info blue. Some of this stuff is coming back to me. Its been 15+ years since I edumacated myself of this type of stuff when I built a custom turbo setup on the automotive side of things, and most of that knowledge has long since been forgotten. Hopefully JE will just get back to me asap so I have an idea of what they would cost vs going through all the trouble of modifying Wiseco's.

I have a PCV with Autotune, non PTI model, but from what I read you can still use that with boost if you have a map sensor and use the aux 5V input of it. That was part of my long term goals as well. Speedwerks and their snowmobile chassis dyno is literally just down the road from me and its only $250 for 4 hours of tuning.

Do you go so far as to get a target compression ratio?

 

[blueironranger]

I was looking to drop to 10:1, should have been low enough to run 3-4psi more boost without using excessive amounts of fuel or timing retard to keep the engine alive. I had planned 8psi on 91 octane and safe at 11.5 a/f with only 2-3* timing pulled. Would have made 125-130hp. The The single/twin cylinder PTI versions allows you to adjust timing also, not sure the standard PCV does without the extra ignition control.

 

[DigitalFusion]

I heard back from JE. Its $550 for custom pistons. I just need to figure out what CR I want to go with if I go that route. Kinda seems silly not to compared to the $250 price of the head shim.

 

[DigitalFusion]

And.. I just heard back from my machinist. Its 20 bucks per piston to mill them while-I-wait, a little more depending on if I need to get fancy.

So $550 plus shipping and a two week wait for for custom JE's that I would have to fill out a crazy long order form for, or $189 shipped with a one week wait plus machining cost and time spent figuring out where/how to mill for Wiseco's

 

[mbarryracing]

If you are taking material off the crown, less material therefore less strength = weaker when you need stronger under boost and added torque. Plus thinner has less material to displace the heat and can burn a hole through from any detonation very quickly.
All in all, a bad idea...

 

[DigitalFusion]

Man, some of the stuff on the JE custom piston order form would take a LOT of time and measuring to determine, and if I screw up even one little thing I could get myself in trouble and be stuck with a set of $550 paper weights! I called Dustin @ Outlaw motorsports as they had a set made years ago for their crazy Phazer turbo and JE said if I could get the job number from it I wouldnt have to figure that stuff out.

He was going to dig and get back to me in a few days, but he said he highly doubted they still had that information.

 

[blueironranger]

Another note, Stock fuel system will max out around 6-7psi boost from what I was told. Maybe toss it back together, run 5psi and work the bugs out this season. 105hp at high altitude is dang close to where a stock 800 is. Don’t know how much I’d trust the rods either, I blew one with no boost!