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Tractor Pulling Discussion Forum

do dome pistons...less cubic inches?

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new to engines

03-18-2008 17:31:13




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I am a little new to this so here goes...

Does a dome piston create less cubic inches? (probably very minor, but i am curious) If i am correct you will get more compresion with a domed piston right? Cant you get the same effect by just shaving the head? shaving the head you are leaving the big area alone (volume of the cylinder) and making the smaller area smaller (head)

if a domed piston is the same as shaving the head how does that work out?

you are taking the big area (volume of the cylinder and making it smaller) and leaving the little area the same (the head, assuming you only add a dome piston and not shave the head)

please correct me if i am wrong in my theory!
thanks

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mEl

03-20-2008 19:01:55




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
I"m gonna stick my neck into this one, seems to me we are making the mistake of looking at the cylinder seperately from the whole combustion chamber/cylinder combination, the popup reduces the size of the combustion chamber as stated by others, but the instant the piston starts down the lowered pressure pulls fuel/air mix into the chamber, even if the popup may be partially out of the bore. The Swept volume does not change, the amount of fuel/air mix will be equal to the volume of the cylinder with or without the popup less whatever losses for inefficiency in the intake run. You have to look at it as a total, not just a simple cylinder. mEl

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mlpankey

03-19-2008 17:54:08




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
flat top ,pop up or dished pistons all should that is if the combustion chamber is in the head be at tdc flush with the deck. so all the pistons in the bore have the same effect its just the space from the combustion chamber and the head gasket plus ccs or piston dish or dome that makes the change in the compression ratio.



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ChadS

03-20-2008 05:09:47




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 Re: do dome pistons...less cubic inches? in reply to mlpankey, 03-19-2008 17:54:08  
My cyl has 1075.55CC's I have a dome/dish piston,, a 25 cc dome, and a 14 cc dish, and a 112.5 cc head chamber, 13 cc head gasket.

Those are some numbers you can play with if you need em. I came up with 13.70 to 1, estimated cyl pressure 273-275 psi at test speeds. Have fun! Let me know if I made a mistake. LOL! ChadS



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Lamont

03-19-2008 08:45:54




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
Think about this. Does the dome on the piston change the stroke length? NO. THE DISTANCE OF PISTON TRAVEL REMAINS THE SAME. Does the dome on the piston really affect the cylinder displacement? NO. THE DISPLACEMENT OF THE CYLINDER REMAINS CONSTANT BECAUSE THE AMOUNT OF CYLINDER CHANGED BY THE DOME ON THE PISTON IS THE SAME AT TDC AS IT IS AT BDC. IN OTHER WORDS, THE DOME IS DOME AT THE TOP AND BOTTOM. What is really changed by the dome on a piston? THE COMBUSTION VOLUME. JUST TO MAKE IT EASY, USE THE IDEA OF ADDING A PISTON WITH A 1 CUBIC INCH DOME AND LEAVING THE HEAD ALONE. THAT WOULD HAVE THE SAME EFFECT, COMPRESSION WISE, AS A HEAD SHAVE THAT REDUCES THE COMBUSTION CHAMBER BY 1 CUBIC INCH AND LEAVING THE PISTON ALONE. Doing one or the other or both will not change the displacement.

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Rwolfejr

03-19-2008 06:41:39




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 Re: do dome pistons...less cubic inches? in reply to NONO, 03-18-2008 17:31:13  
That's right... The dome effects the volume of each cylinder by whatever amount of space it takes up because it's there taking up... space or volume... or cubic inches.
If you're going to play with your compression ratio be sure to look into this heavy before you run out and start buying parts that are usually pretty expensive. There are a bunch of motor builders out there that will get you on the right track depending on what you're looking for. You'll want to do that before you start buying things that will require other changes to make them work.

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Lamont

03-19-2008 19:49:21




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 Re: do dome pistons...less cubic inches? in reply to Rwolfejr, 03-19-2008 06:41:39  
It affects ONLY the combustion volume; not displacement of the cylinder. Displacement is the amount of volume swept by the stroke of the piston. If an engine has 100 cubic inches per cylinder with flat-top pistons, the shape of the top of the piston, dished, domed, or flat, will not affect the volume that the pistons sweeps. The volume of a cylinder is effectively this; area of the base x the height of the cylinder. Now, changing the shape of the base does change the area of the base, but you have to remember, when that piston moves to the top of the stroke, whatever volume was removed at the bottom by the dome is added to the top by the dome; no change in displacement.

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toolz

03-18-2008 21:15:41




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
Piston dome will affect compression ratio, and , depending on the shape, affect efficiency, but will not alter displacement. The definition of displacement (cubic inches) is "swept volume", meaning the area of the circle made by the cylinder times the "sweep", or stroke, of the piston. Compression ratio, on the other hand, is the total volume of the cylinder with piston at bottom dead center (bdc) compared to the volume with the piston at top dead center(tdc). Any change in chamber volume (milling the head), or piston dome will alter compression ratio, not displacement.

But, changing displacement (boring or stroking the cylinder) will change compression ratio, if all else stays the same, since more cubic inches (displacement) will be compressed into the same head chamber.

Clear as mud?

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mlpankey

03-18-2008 18:51:12




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
bore times bore times stroke times no. of cylinders times .7854 = cubic inches .the distance above the piston pin known as pin height plus the volume of combustion chamber in cylinder head gives you youre static compression positve dome gives a negative combustion chamber value.



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beau5278

03-18-2008 17:53:13




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
Cubic inches is the measurement of the area of the cylinder bore,it doesn"t include the area inside the head.There is another term for the measurement inside the head but it escapes me right now.Shaving the head and adding dome pistons does the same thing,it cuts down the area inside the head and increases the compression,more compression,more horsepower.Dome pistons don"t change the cubic inches at all.

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JDGnut

03-18-2008 18:02:09




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 Re: do dome pistons...less cubic inches? in reply to beau5278, 03-18-2008 17:53:13  
Like the first postered said.. Don't over think it... Cubic inches are calculated using the bore and stroke.. easiest formula is to take the bore diameter X bore diameter X .7854 X Stroke X number of cylinders = CID.
JD G

6.125 Bore

7.00 Stroke

2 Cylinders

6.125 X 6.125 X 7 X 2 = 412.5

Compression height will change HP.. but not CID..
Good Luck,

JDGnut

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Jim Johnson

03-20-2008 20:10:54




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 Re: do dome pistons...less cubic inches? in reply to JDGnut, 03-18-2008 18:02:09  
JDGnut your calculation came out right but you forgot to put in the .7854 in your figures to show how you arrived at that number.

Bore X bore X stroke X # of CYL. X .7854

But who's counting anyhow. HA HA

See you in Lincoln

Jim



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new to engines

03-18-2008 18:08:10




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 Re: do dome pistons...less cubic inches? in reply to JDGnut, 03-18-2008 18:02:09  
arent you taking away the volume of the cylinder with the dome?

I am new this so please bear with me.



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superih

03-19-2008 05:28:39




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 18:08:10  
Yes you are taking away from the volume of the cylinder with piston dome, effectively lowering the displacement a little. However, the lowered displacement is very minimal and the raise in compression is alot higher.



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beau5278

03-18-2008 18:14:06




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 18:08:10  
No,the only way you can change that is to change the size of the bore or length of the stroke,more cubic inches.Your confusing two different things,you'll get more horsepower by increasing the cylinder area or by cutting down the area inside the head.The piston has no bearing on cubic inches.



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superih

03-19-2008 09:38:21




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 Re: do dome pistons...less cubic inches? in reply to beau5278, 03-18-2008 18:14:06  
I agree & disagree. Nominal cubic inches is bore x stroke. He is trying ot calculate compression, this is where you have to calculate actual cylinder volume and piston displacement. For example lets use a piston that comes flush to the block deck at the top of its stroke. Think of it as water being displaced by the piston, with the piston at the bottom of the cylinder fill the cylinder with water, then raise the piston to the top of the cylinder and measure the volume of water that was displaced (pushed out of) the cylinder. Now if you do the same thing with a piston that has a dome that sticks above the block deck at top of its stroke, there will be less water displaced, because the actual volume of the cylinder is less because of the volume the dome took up. The volume displaced is then divided into the volume of the head plus head gasket, if you are using a domed piston the dome volume must be subtracted from the head volume. If really becomes fun when you have an automotive domed piston with valve releif cut into the piston.

This should be as clear as mud.

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superih

03-19-2008 09:43:27




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 Re: do dome pistons...less cubic inches? in reply to superih, 03-19-2008 09:38:21  
Compression calculation I left out that the cylinder diplacement plus head and gasket volume is all divided into head and gasket volume.



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don't over think it!

03-18-2008 17:46:27




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 Re: do dome pistons...less cubic inches? in reply to new to engines, 03-18-2008 17:31:13  
the shape of the piston has no effect on the displacement. it lessens the small area more than the large area. thus the term ratio.



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B-maniac

03-19-2008 16:13:31




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 Re: do dome pistons...less cubic inches? in reply to don't over think it!, 03-18-2008 17:46:27  
The answer to the simple question of cu. in. in a cylinder is YES. If you pour a cylinder with the piston at bdc full of water , it will take LESS water to fill it if there is a domed piston as apposed to a flat top piston. YES you raise compression ratio if you mill the head. Yes you raise comp. ratio if you install domed pistons. If you are asking if the minute displacement decrease caused by the dome will offset any gains because of increased compression/ no it won't. If the dome is 1 cu. in. then you lost 1 hp. per cyl. assuming a car engine that makes 1 hp/cu. in. , that's 8 hp. If that dome raised comp. ratio from 10:1 to 11:1 and you add the correct octane fuel to take advantage of it , you would gain around 25 hp. Only you could decide whether it's worth the time and money. If you are talking tractors you might better use a "taller" piston and/or rod combo with a flat top piston to go from maybe a 6:1 to a 9 or 10:1 and you will see big rewards.

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don't over think it

03-19-2008 17:46:02




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 Re: do dome pistons...less cubic inches? in reply to B-maniac, 03-19-2008 16:13:31  
the dome piston does decrease the volume of the bore. but it does not effect the displacement.



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B-maniac

03-20-2008 19:21:53




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 Re: do dome pistons...less cubic inches? in reply to don't over think it, 03-19-2008 17:46:02  
You are correct to the extent that the term "displacement" assumes that there is a cylinder head installed and the chamber factored in as apposed to the terms "cubic inches" which is the product of the bore to stroke dimension of a bare block and would describe the "volume" of the cylinder above the piston which WOULD change with a domed piston. The area or swept volume that a piston displaces during it's stroke is the same no matter what shape it is. It's not a case of overthinking , it's a case of how the question was worded and ones personal experiences with the words "cubic inches" , "displacement" and other phrases describing the relative internal "size" of a particular engine. Hopefully he can take something positive away from all these answers and clear it in his mind. Your last statement is correct , they are two different words with two different meanings. I am just not sure which one he was asking about.

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