well, ive been researching some options for a near future boost projeect on my e36. Now i see alot of people running thicker head gaskets or low compression pistons and such and i couldnt help but think that there is a much more affordable way. Well, i wanted to freshin the motor up any way and looking at the piston design I noticed that the piston top could be shaved/milled a couple mm's without really affectin the construcion of the pistons or strength. Now this would make the piston top flatter which will not only promote better flame travel to avoid detonation, and lower compression but probably make more power as well. Last time i checked it didnt cost much at all 100-150bucs tops, but the only downside would be that a bottom end balance would be a good idea after this, which os probably a good idea anyway. I think this would lead to a more reliable high boost motor build on a budget.

Has anyone done this yet??? What are your thoughts?? Now that i think about it, polishing the piston tops would be a good idea afterwards to help deflect heat/knock and carbon build up.

How many mm´s are we talking about? Let say 3 litr R6 engine, thats 500ccm per cylinder, asumming CR 10,8:1 and would like to have 8,5:1 it seems to me about 3mm drop, is it correct? and 10:1 its just some 0,5 (pretty much nothing... I quess I have mistake somewhere :-) )...

I think you will be going the opposite way. The dish is in the piston for a reason. The edges are a squish area that keeps the combustion centered in the chamber, where you want it. Detonation would be worse, not better.

When shaving the piston top you are just simply removing material filling the combustion chamber which will lower c/r. The center will not be any closer to the cylinder head than before. The general shape of the piston will still be there but really becomes less important since c/r will be lower. Last time i checked flat top pistons made power, not dished.;) So you think that the pistons dish helps to direct the combustion outta the cylinder head. Id believe that if it was a domed piston but not a dished piston, but i could be wrong. im here to learn like everyone else which is why i brought the topic up.:)

I wouldn't modify a cast piston in any matter. Just do a headgasket and rest assured that you aren't going to blow a piston to pieces... at least immediately.

When shaving the piston top you are just simply removing material filling the combustion chamber which will lower c/r. The center will not be any closer to the cylinder head than before. The general shape of the piston will still be there but really becomes less important since c/r will be lower. Last time i checked flat top pistons made power, not dished.;) So you think that the pistons dish helps to direct the combustion outta the cylinder head. Id believe that if it was a domed piston but not a dished piston, but i could be wrong. im here to learn like everyone else which is why i brought the topic up.:)



I understand what you are saying, and it will lower the c/r, and probably work just fine.

There is a reason BMW manufatured that piston like they did. There is about .03 "squish" between the non dished part of the piston, and the cylinder head at TDC. There is about .015 of laminar (dead) air covering every surface of the combustion chamber. When those two surfaces come together at TDC, not only does it "squish" most of the combustion onto the center of the combustion chamber (right where the spark plug is), but the dead air in the "squish" area doesnt burn, meaning you are not getting false ignition, that would cause detonation.

If you want to understand more, read two stroke high performance stuff. It is where it is dead critical to get that just right.

Run on sentences rule. :)

First of all, how much HP do you want. This is the most important question of all.

This will affect the way you will build your motor entirely. Then we can help you with the least expensive way to reach this goal.

There are many angles to takle your endeavor of afforadable motor building but this isn't a great one IMO. Think about the labor to get the pistons out, worked on and back in. Might as well drop a thicker headgasket. Or might as well save up a little more to buy some forged pistions or even a Swain Tech coat over the cast. Modding cast pistons is like improving a weak material in a "patch work" way. Might as well use a stronger material to begin with. When heat is the issue with cast pistons, you may want to look into coating like Swain Tech before shaving.

I wouldn't take this path or even look at it. There are much more affordable ways to safe/reliable ratio (subjective).

So how much power do you want?

Plenty of brilliant answer here, but I mostly have to agree with Makr, and the various "cast piston = bad" posts. The dish is there for a good reason, as Makr pointed out quite clearly. And these pistons are cast, and they have failed in various ways. Check around the various threads here. You will see failed ring lands and chunks disintegrated off these pistons. Plus, the headgasket swap is easier and cheaper, when you factor in everything.

Quite a few Swedish M20s use shaved pistons with good results - 9 and 10s 1/4 mile. Not sure if the m52 pistons are constructed differently though.

I dont understand why would do this if you can get the same CR from just a head gasket swap... you gota pull the head to do the pistions so why not JUST pull the head and do the gasket?.... seems more costly ( plus the risk of a pistion breaking) to mill it.... shoot when the heads off just get a die grinder and take some out of the head/valves.... ;)

ok, I have been thinkin of alot of different approaches here. I dont really have any power goals since im not sure yet wha i am working with. I am just simply throwing some ideas out there.

1. I know cast is weak but cardcounter is doing just fine right now with his set up and i find it hard to believe that piston ringlands are cracking do to piston quality. That is all in the tune and that I am no stranger of since i tuned my last race motor with over 12.5:1 compression.

2. Milling .02-.03 off the top of the piston isnt going to change the structural integrity of the piston or effect combustion flow to the point of noticing, especially since compression will be lower. Squish and quench areas will still be in tact.

3. the true bandaid is running a thicker head gasket which is probably why we have the problems of headgasket failure so much. Plus, who is to say you cant run a thicker head gasket along with shaved pistons to obtain an extra safe c/r for future tunes.

4. Obviously this isnt a budget build if i have to pay someone to do all my work but i dont. I do it myself and i have a very good machine shop i have connections with so machining will be minimal in price.

Im not exactly sure what i will be doing myself but i would just like to throw out the ideas and talk it over with people. Pistons are very expensive for our cars along with rods, and i dont really feel they are necessary considering the strength of I6's and a good tune. Lets be realistic, i just want a safe, reliable car with about 400-500rwhp. I dont think internalls are necessary to obtain this. Just some smart parts matching.

oh, the main reason that I would consider doing this piston mod was if i was to freshin up the motor. I dont know if i would want to go throught this extra work if i didnt have to, however if i had an extra short block laying around that needed a rebuild then why not??

i just want a safe, reliable car with about 400-500rwhp.
thats what im going to be working towards. goodluck man!!

thats what im going to be working towards. goodluck man!!

meh, its not gunna happen over night, but im willing to take my time and do things right. But also not dump my life savings into it in the process. Ive made the mistake of dumping mountains of money into drag cars, and i still love the sport but can nolonger justify spending the money especially when there are no sponcers to help out when things break.

thats what im going to be working towards. goodluck man!!

I have the same philosophy too man. If it don't work reliably all the time day in day out, it don't work.

1. Key point "just doing fine right now". Not implying or believing that his set up isn't reliable. Nobody knows. Only time will tell. I'm sure he'll agree with me here. Maybe not. That car is bad ass though. Mad props.

2. Anytime I would take material from it's original design/cast etc. especially when it's a piece that sees stress like a piston or rod or crank etc., I would have it stressed relieved/shot peened. The chances of you fcuking it up is greater than improving it or matching it's original structural integrity.

3. Totally agree here. A thicker head gasket is a micky mouse job to me. It's a weak link and on top of that you're boosting more because of it. But people have their reasons and motives. It's like using lowering blocks to drop a chevy. It works but isn't the best way.

4. Nice.

How bout for a budget build Swain Tech the cast pistons cuz heat is the one of the main issue with them. Buy some used M3 con rods from me because they are plenty strong for 400+ RWH. I'll sell them to you for dirt cheap. Just get them stressed relieved/shotpeened. :dunno

Shot peening pistons shouldnt be necessary considering polishing to remove any possible weekness in the cast. swain tech is a great idea but now your talking about spending more money which isnt part of the whole "budget concept". Honestly i wouldnt consider swain tech or any other coatings unless I was looking for around 800-1000whp. 400whp can be atained without creating too much heat. Too much heat means that your timing is retarded too much or your detonating from too much timing. I really dont think heat will be a issue if tuned decent, and using a intercooler thats big enough obviously.

I would like to thank eveyone for there input and not flaming me for being a noob.:) I am new to tuning turbo applications but not new to tuning cars in general. I have tuned a few turbo honda's but not to the point where i would call myself a pro. However i have built ALOT of motors and proudly my civic drag motor. I managed to put down 226whp and 150wtq on my 2.0liter. I didnt even get the chance to tune the igntion timing or cam timing. The motor woulda seen 245whp easily with more tuning time, buuuuut i sold the car and now my g/f will be wearing an 11sec civic on her ring finger;) lol

Soo since my drag car is gone and i just picked up a vw for a daily i can start messing with the 328.

Buy some used M3 con rods from me because they are plenty strong for 400+ RWH. I'll sell them to you for dirt cheap. Just get them stressed relieved/shotpeened. :dunno

How much is your idea of dirt cheap???lol they are the same length,journal size of my 328?? correct?

Don't know if they are the same specs, anyway give me a reasonably cheap offer.