How do you pick out which piston????

[Coyote Jack]

I have been trying to figure out which piston I would need for a build. Is there a formula I can use to know what compression height, dish or dome or any other variables needed? Please post it if there is one.

Jack

 

[rumblefish360]

Hey Jack;

I would decide which piston to use based on intended purpose of the car. Each piston manufacture should have the piston height listed on there site/ad.

For general rebuilding a replacement piston can be purchased almost anywhere without concern.

For performance street, street/strip engines, theres a slight consideration before purchase. That would be the deck height of the block. How tall is it? Has it been machined?

An example of similar slugs for the 360 would be the KB-107 and the Federal Mougal equal. (I forget the numbers, *I think cp114H)
The Fed=Moug slug has slightly less height. This makes it easier to use on a milled deck, or a heavy milling on the deck. Both pistons are otherwise extremely similar. And designed to achieve a zero deck height when installed while looking for a 10-1-ish ratio with an open chambered iron head, 11-1-ish with a closed chambered head, like an Edelbrock.

These two ratios with a decent typical high performance cam are pump gas freindly and meet the requirments for most camshafts required operating perameters.

Domed pistons are used for higher ratios. Though, this is engine family/manufacturer dependent. Sometimes domes are needed to achieve a 9.5-1 ratio in big bore engines.

Dished pistons are also like domed slugs, but in reverse. An example would be a Magnum engine. The head chamber is small, so a dished piston helps keep the ratio pump gas friendley. A bigger dish in a Magnum engine is used to lower the ratio further. In an open chambered head a big dish slug could be used for superchargeing.

 

[rumblefish360]

Now, with that read, what is your intended purpose of the engine?

If you can, add in info;

engine size
tranny
rear gear and tire size
stall converter or 4spd
intended purpose of the car
car weight

Horse power sought is not really needed to add, but is helpful of what you want. While many people like to say, it made 430 hp or I want a 430 HP engine etc......

This is mostly bench race fodder and mostly worthless in the final application of the package. The real boast is the time slip.

 

[Coyote Jack]

You read my mind Rumble. Here is what I am trying to do. I want to take a stock 5.9 Magnum engine and get the compression down to 7.5 - 8.0 to 1. This would be a 6-71 blower type application. The rest of the setup would be a 727 r/m 3.23 gears in the rear with 26 to 28 inch tires. Right now the car weight is 3102 with a half tank of gas and the spare tire. I am another 220 lbs. (I gotta lose some weight)

Jack

 

[Burntorange70]

There are calculators around. Try a search you will find them.
http://www.wiseco.com/Calculators.aspx

Going to need a dished piston. You should talk with a motor builder that has expereance with blower motors. Any one try talking you in to a twin screw set up? They are better than the roots stuff and you can still see where your going.

 

[Coyote Jack]

There are calculators around. Try a search you will find them.
http://www.wiseco.com/Calculators.aspx

Going to need a dished piston. You should talk with a motor builder that has expereance with blower motors. Any one try talking you in to a twin screw set up? They are better than the roots stuff and you can still see where your going.

Yup, the twin screws have been broached. But you don't have the eye candy you get with a roots blower and that is just as important as performance in this case.

Jack

 

[rumblefish360]

Jack, this is a high performance application then, right?

I'd look at aforged piston. Off hand, I do not remember the head cc amount of the Magnum head. 65? Sound right? Well, what ever it is, use that and the head gasket diamentions to figure out the ratio at the piston sites calculators. KB has one on hand for this.
If the ratio is a ever so slight high, a grinding of the sharp edges of the pistons and head chamber can help, in a ever so slight manor. Removal of the sharp edges help prevent detionation at high engine temps.

(Coating piston tops and head chambers also help)

 

[70aarcuda]

just for your info...

1/2 of crankshaft plus the length of the rod plus the compression height of the piston equals your deck height for your block.

1/2(3.58) + 6.123 + 1.67 (compression height of KB107 pistons) = 9.583 deck height for zero deck...

and If you stayed awake during Algebra...you can use that equation to solve for any unknown as long as you have 3 of the 4 knowns...

 

[1wild&crazyguy]

cc the heads 1st jmo

or buy pistons 'in the realm' comp wise and just mill a lil later, or buy a thicker head gasket.

1st what do you expect power wise and at what peak rpm?
choose a cam [dur/CL=o lap] based off that, then look at pistons of that comp requirement.

 

[PanGasket]

some good learning here!

 

[Coyote Jack]

O.K. Here are the numbers I know right now.

Stroke = 3.58
Rod length = 6.123
Deck height = 9.600
Comp height = 1.427
Head chamber = 62 cc
Comp ratio = 9 to 1

These are the numbers for a stock 5.9 Magnum.

Jack

 

[RustyRatRod]

Jack, I think I might have a solution. The 383 Chevy stroker piston for the 6" rod seems to come close physically. I've run out of brain power to do the compression calculations......but here's how they measure up in terms if actually fitting....

STD chevy bore-4.00"
Comp height -1.130"
Chrysler 6.123" rods

So, with all that, 1/2 X 3.58= 1.79. 1.79+ 6.123"= 7.913" 7.913"+ 1.130" comp height = 9.043

That means whichever piston you choose would sit around .557" in the hole, because 9.600-9.043=.557. This would be an off the shefl piston and be on the cheap because the 383 chevy is a popular build. also, the valve reliefs should be in the right places because the chevy shares the same valve arrangement with the chrysler. You will just need to do some math to figure out which piston would come up with the right compression ratio. I feel pretty certain with that -.557 deck clearance you'd have no trouble getting the compression low enough. The pin diameter difference between the two is only .057" difference. Well within the parameters of the chevy piston being bored out to accept the chrysler pin.

 

[rumblefish360]

Thinking out loud here.

FWIW valve angles are 23* vs 18*.

Bore out the pin in a piston? While I think it possible and not a problem, the added cost equals what vs proper piston for the Dodge engine. Costs?

The 383 vs 360 stroke should be a consideration.

Deck height between the two are different.

Actual deck height of the 360 engine should be a known, not what is listed.

If purchasing a Chevy 383 piston with the additinal work needed in the pin bore is still cheaper than a slug made for the Magnum, great, even more so if it works well.

 

[dgc333]

O.K. Here are the numbers I know right now.

Stroke = 3.58
Rod length = 6.123
Deck height = 9.600
Comp height = 1.427
Head chamber = 62 cc
Comp ratio = 9 to 1

These are the numbers for a stock 5.9 Magnum.

Jack

Jack,
That compresion hieght you list is awful small, with the rest of the numbers you list the compression ratio is 6.4:1 not 9:1. I know the 5.2 magnum block I had sitting in the garage the pistons were about 0.050" in the hole, with a 1.427 compresion height they will be 0.260" in the hole. To have a 9:1 ratio the compression height would need to be 1.597".

Not sure what the stock engine is but the KB107 compression height is 1.675" which gives you 0.012" below the block deck on a 9.600" deck heght. That is about right since I had to mill my decks 0.020" to get the KB107s to zero.

Also, I have cc'd two sets of stock magnnum heads and they both had 64 cc chambers.

The piston set up StrokerScamp mentioned will give you a compression ratio of 4.9:1, I would think that is too low for even a blower motor.

 

[RustyRatRod]

Jack,
That compresion hieght you list is awful small, with the rest of the numbers you list the compression ratio is 6.4:1 not 9:1. I know the 5.2 magnum block I had sitting in the garage the pistons were about 0.050" in the hole, with a 1.427 compresion height they will be 0.260" in the hole. To have a 9:1 ratio the compression height would need to be 1.597".

Not sure what the stock engine is but the KB107 compression height is 1.675" which gives you 0.012" below the block deck on a 9.600" deck heght. That is about right since I had to mill my decks 0.020" to get the KB107s to zero.

Also, I have cc'd two sets of stock magnnum heads and they both had 64 cc chambers.

The piston set up StrokerScamp mentioned will give you a compression ratio of 4.9:1, I would think that is too low for even a blower motor.

Impossible to calculate compression with the info i gave. i gave no piston cc volume. did you just make one up?

also, as far as boring pin holes out it's a very common practice. Nuthin to it. just tryin to throw out some ideas is all. usin somethin off the shelf would save jack some bucks. just because an idea is unorthodox, doesn't mean it's not good or wont work. People used to mix and match all kinda stuff before these stroker kits came out. I've build a stroke 460 based ford with chevy pistons and pontiac rods. Windsor fords with cleveland cranks....and on and on.

 

[fstfish66]

call BIG ALS TOY BOX, he build blower motors,,is a great guy,,he does um all gm ford mopar,, he will tell you the best piston choice for reliabilty,,

 

[dgc333]

Impossible to calculate compression with the info i gave. i gave no piston cc volume. did you just make one up?

also, as far as boring pin holes out it's a very common practice. Nuthin to it. just tryin to throw out some ideas is all. usin somethin off the shelf would save jack some bucks. just because an idea is unorthodox, doesn't mean it's not good or wont work. People used to mix and match all kinda stuff before these stroker kits came out. I've build a stroke 460 based ford with chevy pistons and pontiac rods. Windsor fords with cleveland cranks....and on and on.

I assumed flat tops with no valve reliefs. If there are reliefs or a dish the ratio will be even lower, if they have a dome the ratio will be a little higher. In anycase that piston won't work unless you go with a much longer rod. Much better idea to get a piston that is appropriate for the application.

 

[RustyRatRod]

I assumed flat tops with no valve reliefs. If there are reliefs or a dish the ratio will be even lower, if they have a dome the ratio will be a little higher. In anycase that piston won't work unless you go with a much longer rod. Much better idea to get a piston that is appropriate for the application.

...and i agree. however, we were doing stroker builds on all kinds of engines long before stroker kits were readily available. I was merely pointing out that he could have a affordable choice. I mean really now, as long as it all fits togerher and balances out good, how the heck will those parts know what application they are for? A slug is a slug. my example was just given as a strating point.......but i think now you understood that.

 

[Coyote Jack]

Dave, thanks for jumping in. I typed out the wrong comp. height for the Magnum. I had 2 written down. The actual comp. height for a stock Magnum piston is 1.667.

Jack

 

[Coyote Jack]

Rob, I had not even dreamed of looking at pistons for other makes. It makes a lot of sense to do so. Like you said, a slug has no idea what brand of block it is in.

Jack

 

[dgc333]

Jack,

Absoultly look at pistons for other applications but keep in mind that there is a cost associated with drilling out pins to the mopar size or bushing rods to match the piston size. Check with the psiton manufacturers and see what the cost of a custom set are, it may be the same or could be a little cheaper just to have them made for what you need.

FWIW, to come up with less than an 8:1 compression ratio you need to be looking at a flat top pistons with typical 5 cc valve reliefs that has a compression height of around 1.537". Rememmber the block deck height of 9.600" is a nominal number and most all un-touched blocks willl be larger, this will lower the compression. Also, dish pistons will allow the compression height to be larger too.

The fist thing to do is have the block measured so you know the deck height and have the machine shop calculate what they will need to remove to square it up, also cc heads you plan to use. From there you can use the calculators mentioned to figure the compression ratio for different heads. I like this calculator http://www.csgnetwork.com/compcalc.html

 

[Coyote Jack]

Although this is a cast piston, I believe it has the proper size requirements to get me down to about 8 to 1 comp. What do you think?

Jack

 

[oldkimmer]

..........Have u though of useing cometec head gaskets, they will make any thickness................kim............

 

[Coyote Jack]

I will definitely use cometic gaskets, but even those have their limits when you are going with forced induction.

Jack