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Old 11-23-2012, 05:51 PM
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mirrored combustion chamber

I have heard of people doing this on their pistons. What is the bennefit of this modification? How much do they mill out of the piston. (Depth)

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Old 11-23-2012, 07:09 PM
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Originally Posted by birkey View Post
I have heard of people doing this on their pistons. What is the bennefit of this modification? How much do they mill out of the piston. (Depth)
By placing the dish directly over the combustion chamber of a wedge-type head, the quench area is maximized and the flame front may have less distance to travel than if a round dish piston were used. But this is generally only used if the compression ratio would be too high if flat top pistons were used. Also, instead of machining flat tops into an inverted dome piston, a D-shaped dished piston is usually used. Not quite as good as a true mirrored dish, but close.

The amount of the dish depends on the target compression ratio, so there's no magical number for depth.
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Old 11-24-2012, 08:11 AM
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I have heard of people doing this on their pistons. What is the bennefit of this modification? How much do they mill out of the piston. (Depth)
You cant modify pistons in this manor you need to buy them forged or cast in the proper shape. But usally its just a dish that fits under the combustion chamber. For the most part the pistons have to be matched for the head type. This is only used to increase the volume of the combustion chamber for lower compression ratios.

Now when you say mirrored there is also a polishing proceedure to bring a mirror finish to the top of the piston and combustion chamber. This reflects heat out of the piston and forces it to do more work and reflect the heat down the exhuast pipe in stead of into the engines. This is almost always done on racing engines. It does really help with heat soak. The same holds true fpr the top of the exhuast valves and the exhuast port. Intake port is not done as it flows a little better on the fresh air fuel side not to have a polished surface.
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Old 11-24-2012, 11:56 AM
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You cant modify pistons in this manor you need to buy them forged or cast in the proper shape.
Pistons have been custom machined like this since the beginning. The main caveat is the thickness of the piston deck has to be sufficient and the balance needs to be corrected.
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Old 11-24-2012, 10:27 PM
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Pistons have been custom machined like this since the beginning. The main caveat is the thickness of the piston deck has to be sufficient and the balance needs to be corrected.
I should add this is best done only to forged pistons.
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Old 11-24-2012, 11:28 PM
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Maybe I missed something but exactly how do you get the "mirorred" finish?
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Old 11-25-2012, 12:00 AM
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Maybe I missed something but exactly how do you get the "mirorred" finish?
Polishing the chambers and piston tops is a waste of time on a street engine, the chambers and piston gets a carbon coating the first five minutes the engine runs.

The MAIN thing you want to do is remove any/all sharp edges or casting irregularities from the combustion chamber (including the exposed spark plug threads) that could cause preignition. This can be done w/a die grinder and sanding rolls. A Dremel will even work but takes longer.

Smoothing/polishing intake ports might help a little. But this is after you've done other far more important things- like smoothing the lip below the valve seat and port matching the intake and head (NOT gasket matching, port matching is a different thing).

Giving the valve reliefs of the piston a small radius (just enough to remove the sharp edges, nothing more!) can help a tiny bit, too. Do not mess w/the rest of the piston unless you know exactly what you're doing.
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Old 11-25-2012, 09:42 AM
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Polishing the chambers and piston tops is a waste of time on a street engine, the chambers and piston gets a carbon coating the first five minutes the engine runs.

The MAIN thing you want to do is remove any/all sharp edges or casting irregularities from the combustion chamber (including the exposed spark plug threads) that could cause preignition. This can be done w/a die grinder and sanding rolls. A Dremel will even work but takes longer.

Smoothing/polishing intake ports might help a little. But this is after you've done other far more important things- like smoothing the lip below the valve seat and port matching the intake and head (NOT gasket matching, port matching is a different thing).

Giving the valve reliefs of the piston a small radius (just enough to remove the sharp edges, nothing more!) can help a tiny bit, too. Do not mess w/the rest of the piston unless you know exactly what you're doing.
You polish the pistons with a bench grinder that has longer shafts for a large polishing cloth wheel. Heads can be done with a diegrinder/dremel but it will take forever.

Polishing on the street is not needed. But is not effected by carbon for two reasons. One the carbon does not stick to the polished surfaces, two the polished surface still reflects heat even if it cant be seen. This is way home insulation has reflective foil backing and is never going to see any light because its buried in the ceiling and under the roof of the house but still reflects heat back into the house.

This will help on any engine street or race but usally time consuming and if your not doing the work your self you will have to pay way to much to have it done. If you got the time it doesnt hurt.

Only forged pistons can be machined with little impact on the strength. This is only becasue they are way stronger than most apps can use. But if they are borderline in strength and they are cut they will fail for sure. Also cutting the piston head tends to allow the head to collapse and expand at different rates from heat and casues issues that buying pistons cast in the correct shape is always the way to go.

Hyper, and cast iron pistons can not be but they will fail. They are sometimes cut to reduce compression but this is done to engines that make 200hp not 500. So once you think about your goals and you will be buying the correct pistons and go no need to cut them at all.

Last edited by hcompton; 11-25-2012 at 09:51 AM.
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Old 11-25-2012, 11:26 AM
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Originally Posted by hcompton View Post
You polish the pistons with a bench grinder that has longer shafts for a large polishing cloth wheel. Heads can be done with a diegrinder/dremel but it will take forever.

Polishing on the street is not needed. But is not effected by carbon for two reasons. One the carbon does not stick to the polished surfaces, two the polished surface still reflects heat even if it cant be seen. This is way home insulation has reflective foil backing and is never going to see any light because its buried in the ceiling and under the roof of the house but still reflects heat back into the house.

This will help on any engine street or race but usally time consuming and if your not doing the work your self you will have to pay way to much to have it done. If you got the time it doesnt hurt.

Only forged pistons can be machined with little impact on the strength. This is only becasue they are way stronger than most apps can use. But if they are borderline in strength and they are cut they will fail for sure. Also cutting the piston head tends to allow the head to collapse and expand at different rates from heat and casues issues that buying pistons cast in the correct shape is always the way to go.

Hyper, and cast iron pistons can not be but they will fail. They are sometimes cut to reduce compression but this is done to engines that make 200hp not 500. So once you think about your goals and you will be buying the correct pistons and go no need to cut them at all.
I'm getting a bit tired of correcting you. Have you ever even opened up an engine?? There is carbon almost the second you start the engine the first time- polishing chambers is a total waste of time for him.

Maybe you don't believe me- but how about Larry Freaking Widmer??

It does NOT take "forever" to use sanding rolls, etc. to do the chambers w/a die grinder- that's how it's done by everyone who does this!

Son, I have hand machined cast pistons to work w/a set of Trick Flow Twisted Wedge heads to work w/a 0.640" lift solid roller that we ran on nitrous, I hogged the HELL outta those pistons- guess what? No failures of the pistons. Period. Do I recommend it? Of course not- that's why I added the comment above. But it's not like it cannot be done as easily as it can w/forged pistons. What do you think was done in the early days of turbocharging, before there were "turbo" pistons?

I have the distinct feeling you're shooting totally from the hip and have no actual experience w/ANY of this.

Last edited by cobalt327; 11-25-2012 at 11:36 AM.
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Old 11-26-2012, 09:44 AM
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A shiny finish reflects heat...huh is this maybe because a piston is somewhat porous before polishing?
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Old 11-26-2012, 11:33 AM
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Cobalt327 you might want to go scroll on some other forums, cause I believe there is some schooling needed to take place. Haha a few others are under the impression that a polish will resist carbon aswell.
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Old 11-26-2012, 12:57 PM
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Originally Posted by cobalt327 View Post
I'm getting a bit tired of correcting you. Have you ever even opened up an engine?? There is carbon almost the second you start the engine the first time- polishing chambers is a total waste of time for him.

Maybe you don't believe me- but how about Larry Freaking Widmer??

It does NOT take "forever" to use sanding rolls, etc. to do the chambers w/a die grinder- that's how it's done by everyone who does this!

Son, I have hand machined cast pistons to work w/a set of Trick Flow Twisted Wedge heads to work w/a 0.640" lift solid roller that we ran on nitrous, I hogged the HELL outta those pistons- guess what? No failures of the pistons. Period. Do I recommend it? Of course not- that's why I added the comment above. But it's not like it cannot be done as easily as it can w/forged pistons. What do you think was done in the early days of turbocharging, before there were "turbo" pistons?

I have the distinct feeling you're shooting totally from the hip and have no actual experience w/ANY of this.
Carbon or not the heat is still reflected when it reaches the mirror surface. You are wrong on how heat is reflected you are thinking that if it is covered it will not reflect and that is just not the case. It still works just as well with or with a layer of carbon on it. Yes it is not needed on normal engines. But carbon does not stick to it as bad. you can take the engine apart after running it and wipe the pistons with a rag and the carbon wipes right off. It also does not build up as much over time it comes loose on its own much easier than the stock finish.

Sorry to say you are just wrong engines do not run better dirty. Now a layer of carbon build up that contribute to compression may help. But the polished pistons can still make more power. a test done in 1969 with one engine has not swayed the public that it is standard practise to polish the piston tops and combustion chambers on race engines. I know i have taken the heads off a few actual race engines and seen what they look like when they come apart.

I also have not seen this mythical thick carbon you think is inside your engine. You take your pistons out right now they should not be coverd across the top with heavy thick carbon if they are something is not running right. Carbon build up should be very light and thin only really building up on ring lands and top of the cylinder walls where the piston does not reach. Everything else is either polished or coated from the factory not to let the carbon stick. You never noticed even cast pistons have a smoothed finish. Polishing just makes this a little better.

Now if you take the pistons out of any engine and put it back together its not going to seal up as well as it did once its been run a few times. Maybe Larry whoever should have though about that. Everytime i have removed pistons and cleaned the carbon and re-assembled an engine it always ran worse just be cause it needs to seat the rings in again. Usally it will never make the power it did before you removed the pistons unless you install new rings and hone when you do.

Machining cast pistons is possible I have a lathe and a mill in my garage but its not needed nore is it recommended by anyone involed with your engines many parts. I have made pistons from scratch but would not cut on a sbc piston for any reason. I can always send them back and get the ones that fit.

Only forged piston manufactures allow the pistons to be cut by the machine shop. even then they have strict spec to follow. Its a matter of doing a job right or just shadtree machanic work. Anybody can do the job wrong.

You dont use a die grinder or a dremal for polishing because they both spin to fast and burn up the clay in the compound and scrtch up the surface. You can never get a mirror finish with 36K rpm tools. More like 2000 rpm is what is needed with lots of torque. Using an angle buffer with small 4 inch disc is best for the larger areas. then finish up with a little low rpm electric polishing wheel 2 inch works good for the small spaces any smaller and it takes for ever.
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Old 11-26-2012, 12:59 PM
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A shiny finish reflects heat...huh is this maybe because a piston is somewhat porous before polishing?
They are smoothed over from factory but a strong polish can help a little more. the effect is small only good if you need every bit of power.
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Old 11-26-2012, 01:10 PM
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Cobalt327 you might want to go scroll on some other forums, cause I believe there is some schooling needed to take place. Haha a few others are under the impression that a polish will resist carbon aswell.
I agree with cobalt!!

Jester

Last edited by painted jester; 11-26-2012 at 01:15 PM.
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Old 11-26-2012, 01:33 PM
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When I pit crewed we polished pistons and used water injection on the supercharged gas engines and some normally aspirated among some of the benefits was keeping carbon from building up on the polished pistons (no carbon build up to hold heat) But even on some tear downs there would be some carbon here and there!

We didn't polish pistons to stop carbon build up we polished them so there would be no hot spots that could start the aluminum pistons melting!

Jester

Last edited by painted jester; 11-26-2012 at 01:53 PM.
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