Heh...my 10.72:1 domed pistons have finally seen the end of the line.

My buddy took an old piston he had lying around and threw it on his 3 axis mill/lathe/drill press and the mill ate right through the piston. I mean, it's aluminum, theres no doubt that the domes on my pistons will be no problem to shave off, then just sand the piston flat.

My pistons are by Sealed Power and have a .125 Dome and 2 valve reliefs.

My buddy has Sealed Powers 9.72:1 pistons, 1 point down. His is a flat top piston with 4 reliefs.

My first concern is that even if I shave the dome off flush to the rest of the piston the compression will still be too high since I have 2 less reliefs.

The next concern is the fact that I will have no real idea what my compression actually is. I mean, yes, the compression will drop, but I won't have any way to know by how much.

The question becomes, will shaving the dome off drop me at least 1/2 point of compression. I would love to get down to right around 10:1.

If not I can just buy the 9.72:1 Sealed Power pistons and be done with it. Thing is, I would like to have 10:1 rather than 9.72:1, but again, if I shave the dome, I would have no real way to know my new compression, the only thing I could know is that it would be lower, but I wouldnt' know if it would be low enough.

PS: On a happy note, during this tear down every single one of my bearings was PERFECT. Other than the fact that my oil pan was leaking out the front and getting 9MPG, I've at least got down the mechanical aspect down. Hopefully I can get the compression down during this rebuild so that it will run better, stop my leaks, and finally I can say I'm DONE, hell, it only took 3 tries.

 

What is your reasoning to not wanting the 9.72:1 pistons? you really wont notice that great of a power difference between 10:1 and 9.72:1

 

Sure you will... after the piston is installed you can measure how far down in the hole, your gasket thicknes, chamber size et al and figure it out. If you need to calculate before you cut, throw a piston back in and measure the depth to where you will machine the piston. Then just figure out (or call the mnfg.) the volume of each relief. No one said it would be easy

 

you'd want to make sure the dome was a solid design and there would be enough material left after cutting the piston. i tihnk a lathe would be the way to do it. i have, or had, a speed pro ( i think) catalog that shows how to build a fixture to cuck a pistion in a lath. it might even tell a bit more about doing it.

 

Edit: The plan was to make a jig to hold the piston so the top of it is level, then just pass the bit horizontally across the top of the dome and mill it right of with a few passes. The machine is big enough that it can clamp the piston w/o any adjustment, then just bring the mill bit across the top a few times.



I'm not too worried about screwing the pistons up, because if I cant mill them down to drop the CR, I'm going to buy new ones anyway.

9.72:1 Pistons -My buddies pistons.

10.72:1 Pistons -My Pistons

His valve reliefs are smaller, but I think the height of my piston is higher overall, not just counting the dome. I mean, if I knew for sure I was going to drop at least a 1/2 point of compression I would be happy, but I don't know how much compression the dome is giving. I want to get down to at least 10.2:1, and would prefer to stay around 10:1 exactly, give or take a 1/10.

 

I noticed that those are Sealed Power pistons, but I didn't see whether they're hyper or forged.. (they look like hypers)
Reason for asking, Sealed Power hyper flattops for a 350 are some pretty inexpensive pistons. Unless you're doing the work yourself, or someone's doing you a favor, the cost of machining them might exceed replacing them.

Other than that... Exactly what ede said.
Be sure the dome has enough thickness, and use a lathe.

 

They are hypers. My buddies are forged, both by sealed power.

My buddy has a Grizzly combination 3 axis mill/lathe/drill press in his garage.

From the picture, if I mill the dome off, underneath should be like the rest of the piston shouldn't it? Its not like it would be hollow under the dome while the rest of the piston is solid would it?

Why the lathe? I mean, the mill is easier, simply lock the piston in place, then a few passes horizontally across the dome to take it off. I mean, using the lathe would take longer, and then the problem of holding the piston in place.



Yea, it's not that messy in the garage anymore.

The problem with the lathe is that the rod is still pressed into the piston. If I use the lathe, wouldn't the piston be spinning as the cutter gets near the dome, meaning I'd have to separate the piston from the rod? With the Mill, the piston can be stationary, and the bit simply passes over the dome and takes it off.

 

With a lathe, especially one that has the right size chuck, which by the looks of it yours doesn't have, you could clamp the piston around the ring land area.
Clamping the skirt of the piston could easily distort the skirt. And with hypers the skirt to wall clearance is very tight, so .002" distortion could push the piston out of round to the point where it's actually an interference fit in the bore.
It isn't impossible, but be careful if you do it.

 

Turn a piston over and look at the underside of the piston dome.
Is there a step cast in the under side?
Or is it flat all the way accross?

You could also split the difference in volume increase needed to achieve your needed compression ratio , between the heads combustion chamber ( enlargeing it) and the the pistons dome (milling it)
I believe those piston have a +3.5cc dome. If you were to enlargen each combustion chamber by 3cc's to 67cc's and mill off 3.5cc off each piston dome you would get a true compression ratio of 9.98:1. You would likely be still left with a very slight dome on the piston top while maintaining sufficiant piston dome thickness on a hollow dome piston.
Taking 3cc's out of each combustion chamber is not a big deal on a vortec head.
These calcs are assuming a assembled piston to deck clearance of .025"

Keep in mind if you mill a signfiicant amout of meterial off a piston, it will be lighter now and may require rebalancing.

Another way is to get new flat top pistons and deck the block to achieve the cr you want. decking to .015" clearance would get you 9.99:1 with flat tops and 64cc heads Felpro .039" gasket.

First, what octane rated fuel will you be useing?

 

Sunoco 94

The piston has a stamp on the underside and is shallow in the very center, I'm assuming thats what you mean by "hollow", with a depth micrometer we found that the dome side of the piston is indeed not as thick as the flattop side. At this point the plan is to mill .1" of the dome off, which we roughly estimated to be about 2.4cc worth of dome. This would leave about .015" of dome remaining on top, my buddy (the one with the mill) decided that there would be more than enough piston thickness even if we did mill the entire thing off, though I'm sure thats up for debate.

I'm wondering what 2.4cc of dome equates to as far as compression loss.

Someone told me that at a certain CR, cylinder pressure begins to jump. I'm wondering what the "sweet" spot of compression I should be aiming for with my setup. It was always explained to me that compression equates to mechanical advantage, thus more compression is more power within a certain reason. That was the whole reason behind going with 10.7:1 CR to begin with, unfortunately that was a little too much. I'm basically looking to run the highest CR that will yeild the most power without hurting how it runs. So right now I'm aiming for something between 10:1 and 10.3:1, though I'm not exactly sure where I need to be.

Fbird88: What method do you use for measuring combustion chamber volume? Assuming I take off about 2.4cc of dome, how much should I take out of the heads to get me around 10.2:1? How can I accurately know how much I've taken out, I mean, when your talking 1 cc, thats not much material is it?

I'm going to mill the pistons, not lathe. I spend about 4 hours with my buddy fabricating a very nice jig to hold the piston in place while I mill. It won't let the piston move at all, but at the same times allows for quick turnover and consistency while switching from piston to piston, so it'll take exactly the same amount off of each piston.

 

i wouldn't chuck up on the piston. i looked and can't find my catalog with the jig i nit, but basically it is two pads or up rights that the bottom flats of the piston would sit on and a eye bolt deal that pulled on the wrist pin to hold the piston tight aginst the two pads and then a pin that the eye bolt went throught to chuck on

 

if you were to take 2.4cc off the dome
and 2cc's out of each chamber, you would be at 10.22:1 assuming you're at 10.77 now and deck height is .025 and the chambers are 64cc's now.

The only way to tell how much you've removed from the chamber is to cc the chamber after going all around and smoothing and deshrouding the chamber with a die grinder.



You should be able to take all the required volume from the just the domes. They are 8.5cc's the valve reliefs are -5cc's net dome volume is 3.5cc's

Removeing a little too much (lowering the compression more) is better than removing too little.

First you'll have to find out exactly what your compression ratio is now.
You'll need to cc your heads and measure assembled piston deck clearance. You can also fine tune the finished cr with different thickness head gaskets .015",.028", .039" .041" .046" and .051" are common compressed thicknessess available

No, the power increase is linear and predictable with each increase in compression ratio.

More compression makes more power and torque, true, but only up to the point where the motor will detonate at WOT.
Then power output nose dives and engine damage soon follows.

You'll make more power with the right amount of compression ratio (or a little less) and full max power ignition timing than you would with a little too much cr and retarded timing to avoid detonation.

Cam timing and overlap does effect the "dynamic compression" of an engine but the actual octane tolerance effect is unpredictable, not rpm linear and not near what some claim. Overcamming the motor to gain octane tolerance is a sure way to get less power output overall. The power you could be making will be going right out the tail pipe.
{Over scavenging}

I built my street motor at 9.8:1 for Sunoco 94
Allows for some carbon build up and easy moderate nitrous power adder.
Also allows me to fill up with 92 octane if the is no Sunoco gas station near by.

Can't tell you the exact cr limit for Sunoco 94 but 10.2 sounds reasonable.
I had a motor with 10.34:1 (not vortec heads) and it was just ok on 94 and real touchy on 92 octane.

I can tell you this: There is way way more power in small improvements in totol engine airlfow and carb jetting tuning and ignition timing and cam timing experimentation than there ever will be with +/-.5 of a compression ratio. Also, a motor with just a little too much compression ratio (for the fuel octane) that runs in detonation some of the the time or a lot of the time will be soon down on power from damaged piston ring seal. *Short engine life*

 

I'm using a .041" Gasket Fel Pro #1003.

The pistons have 618 stamped on the dome, that is the part number. So if I shave off 90% of the dome I should be able to drop how many ccs? About 3ccs?

The block was never decked, it should be stock height, whatever that is.

Do I have to worry about unbalancing the assembly by taking the dome off?

We measured the piston and even if I take off the entire dome and make it flush with the rest of the piston, there is well over .25" of piston top still left under the dome, and it's only a .75"x.75" area that is that thin. I can't imagine (assuming no ping/deton) the piston would break in that spot.

-5cc from reliefs, +8.5cc from domes. So if I shave 5ccs off the dome , that would leave me with -1.5cc, which should drop me 1/2 point of compression?

I'm not sure what forumula your using to calc CR.

How many cc total should I look to get rid of to get around 10.2:1? If I can drop the CR to 10.2:1 through milling the domes alone that would be nice.

Also, is your 94 octane different than mine? I'm in NJ, so we have RFG or something. I dont know if that means anything since it's still 94 octane.

 

"stock deck height" it actually varies a llot. you know what happens when you assume something? Measure it.

was the motor "balanced" to begin with?
making the piston lighter will affect the balance. it will move you towardes a "overbalance" condition not nessessarily a bad thing. Don;t know enough about engine balancing to tell you for sure, Consult a crank balancer tech. compare the weight before and after milling. .25" is plenty thick

I use this compression ratio calc

I use 4.166 for the gasket bore and .025 for the deck clearance

if you make the pistons -1.5cc with a .041" gasket, I get 10.10:1 Looks good to me.

 

woops

 

Why does Sealed Power advertise the dome as 3.5cc, when it's actually 8.5cc? (Just wondering where you got 8.5cc from, because the Sealed Power website is *** awful) When I read that the dome was 3.5cc on Summit it made me worried, because 3.5cc wasn't going to be enough of a drop in CR.

As far as milling the dome, like you said, I just have to "error on the side of caution". I plan on taking off about 80% of the dome, which should get rid of 5ccs minimum, putting me in the CR range I want to be.

As far as the balancing....I mean, 5cc off each piston, thats gotta be like 5grams per piston....I guess I could weigh them all then just have the crank rebalanced...I'll have to look into that.

I'll check the deck height, as I recall, with "stock" deck height, the piston should be EVER so slightly below the deck.

 

Mathematically the dome works out to 8.5, and then you subtract 5cc for the valve reliefs.

...I'll Bet

 

"as I recall, with "stock" deck height, the piston should be EVER so slightly below the deck."
The nominal spec is .025" with a 1.56" piston pin height. Trouble is the stock blocks vary a bit.

"Why does Sealed Power advertise the dome as 3.5cc?"

Because that is the effective piston dome volume combined with the valve relief volume.
The valve relief volume is typicaly 5cc's
+or- a cc. (the valve reliefs are the same as on a same/simular FM flattop piston)

8.5 -5cc =3.5cc's

If you'd like to calculate how much you'd have to remove from the dome to get 1cc volume you'd want to "map" the surface area of the dome with 1centemeter squares and count how may whole 1x1cm squares and how many half squares there is to get the area of the dome top in sq/cm's then calcualte in the depth (height of the dome) to get the volume of the dome in cubic/cm's (cc's)

I don't have a FM catalog any more. I lent it out to someone and never got it back.. What is the listed piston weight of your piston and the simular FM flat top piston w/two valve reliefs? Give you an idea.

 

if you'd like to measure the actual piston dome volume:

Is popping a new set of flat tops looking better and better yet?

simply switching to -5cc flat tops with a .041" gasket gets you to 9.72:1
if you want 10:1 even use a .028" gasket
(GM performance part) 10.03:1

 

I took .1" off of the dome. There is probably .02 of the dome left. I estimated the volume I milled off to be right about 4.5cc, which is pretty much where I want to be. I'll have to take pictures of the jig me and my engineer buddy made. I thought about using some playdoh to make an impression of the dome and just use a graduate to pour water into the impression, but I just broke the dome up into 1cmx1cm squares, times the .1" (.256cm) I was taking off, gave me a rough number of about 4.5cc which was fine.

I got 4 pistons done tonight, everything went perfectly. It was time consuming, but I wanted them to be set up perfectly so they would all be exactly the same. *** bless the autofeeder, not as nice as a CNC machine, but good enough that I didnt' have to stand there and handcrank the thing....allowed me to just walk away and go clean up the block and cylinder heads with some acetone.

I ordered a 2400 stall converter today as well....hopefully that will help me out...since alot of people were telling me that my stock converter was probably dragging my motor down.

I'm gonna talk to my machine shop about the balancing. The rods and pistons are supposed to be matched to the weight of the crank correct? If I wanted could I just weight the pistons/rods after I mill and have my machine shop readjust the crank? I don't know how critical it is, but if it needs to be done, I'm not gonna skip out on it.

Lol, I'm doing all my work in my buddy's garage (nice guy, lets me use his heated garage, and all his nice tools). I dont' know how many times he's explaining something to me and goes "I know I have a book that has xxxxx in it.....oh wait, I can't find it, I must have lent it out and never got it back."

 

Based on your info, assuming a .025" deck clearance and a .041" gasket, your new cr will be 10.16:1

Yes, if the pistons are now lighter than before, material would then be removed from the crank to rebalance it.

Weight a machined and unmachined piston. If the difference is less than 4 or 5 grams I'd not loose too much sleep over it. More important that they all weight the same when finished.

Weigh the pistons before and after cutting the dome and see that your machinist says.