defiant1

Looking to get feedback on a +6cc volume domed piston from wiseco for my 425. The pistons have a 1" compression height. Are these strictly used for all out racing with high compression applications?

I am guessing it will not be practical for a street/strip car, but was curious why I have only seen flattop pistons utilized in performance builds

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rustyroger

Domed and forged pistons are heavier than stock cast ones, more reciprocating weight and faster bore wear.
Not a problem with a race engine that sees little real time running and will likely be stripped and rebuilt often.
If you have a car you want to cruise along the highways in for lots of miles stock cast pistons are the best option imo.

defiant1

I thought maybe with the 425 having a shorter stroke it would mitigate the additional reciprocating weight the domed piston has...

Thanks for the reply.

380 Racer

I'll bet there are a few engine builders that will disagree with you.

Oldsmaniac

I always thought domed pistons were used to give higher than stock compression ratio's. If true they surely would not be needed on a street engine running on the pee we call gasolene these days.

Lee_A

It is ONLY a 6cc "dome"! That is not really a "dome", it is just enough to give you a little more compression. Most regular flat tops have about 6 to 7cc's of negative volume from the valve reliefs, these piston have just enough dome to compensate and swing the volume the other way.

Just do the math to see if these will give you an acceptable compression ratio.

GOOD forged pistons are NOT heavier than stock pistons, either.

rustyroger

[QUOTE
GOOD forged pistons are NOT heavier than stock pistons, either.[/QUOTE]
True, but to keep weight down they are very carefully made and very expensive.
By volume forged aluminium is heavier than cast, the cost comes from making light AND strong pistons.
I still contend cast pistons are fine for normal highway use, other engine builders may disagree. I'm ok with that - it would be a dull world if we all agreed with each other.

Roger.

cutlassefi

Shaun, what's the Wiseco part number on that piston?
To get back to your point, the availability of dome or flat top pistons has to a lot do with what head volumes are available as well.
Case in point, a 396-454 BBC has stock chamber volumes as high as 120cc. Guess what, you'll need a dome piston to achieve any high compression ratio.
Now take a 425-455 Olds. The largest heads available are the stock ones at about 80cc. You can achieve a pretty high ratio just buy milling them to around 70cc and running a flat top piston. You don't need a domed piston, therefore no demand for one, simple as that.

You really don't want to run a big dome piston if possible anyway. It effects the flame front moving across the chamber. Flat is better, Nascar now runs soup bowl type dish pistons with small chambers and very little squish. They claim more hp across the board because the combustion process now actually takes place in the piston so to speak. Seems to be working for them.

And by the way, there are many hollow dome pistons out there for various applications, most of which actually weigh less than a flat top counterpart. Check it out for yourself.

MDchanic

Cast pistons are obviously okay for regular highway use.
All of us have seen factory engines go well over 100,000 miles with cast pistons, as well as engines that failed spectacularly with very expensive forged pistons (not to say that the pistons were the cause...).

- Eric

defiant1

Mark,

The pistons I saw were on eBay (I can't access the ad at work). I believe they were custom made.

Thanks for the info.

Shaun

joe_padavano

Not according to MIL-HDBK-5.

Density for cast aluminum ranges from 0.098-0.102 lb/in^3
Density for forged aluminum ranges from 0.097-0.101 lb/in^3

The exact density depends on the specific alloy. Forging aligns the grains in the metal. It does not measurably increase density.

And if you don't believe MIL-HDBK-5, you had better not ever fly in an airplane.

captjim

If you look at the cost of quality pistons vs cheap ones in the overall scheme of the cost of a performance build, it is really not very much and worth the extra cost, IMHO. Especially on less popular engines where stock replacement pistons are pricey. Good pistons also usually come with or require a better ring pack, less friction = more power.

defiant1

Here is the link. If I did my math right, I would be pushing 11:1 with these pistons.


http://cgi.ebay.com/ebaymotors/NEW-W...item19c65bce5e

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Oldsmaniac

I did not see them labeled as 425 Pistons. Even though the 425 and 455 have the same basic bore the pistons are different.

Warhead

Shawn,
Everyone also missed the 1" compression height (stock is 1.625").
You"d better get a 7.625 rod to make it fit, or you will be lucky to have a 4:1 compression ratio, because with that 1" piston, it will be over 1/2 inch short the top of the bore at TDC.
Needless to say, I think these bad boys are a no go.
Figure out the compression you want, then go looking for that piston.
Jim

coltsneckbob

I always thought a forged piston was lighter then a cast piston. Primarily because I thought a forged piston HAD to be made mostly of aluminum.

cutlassefi

Exactly Jim, that's why I asked him for a part number, I thought they may be for a SBC.

Plus with .043 rings that ain't exactly the perfect setup for the street.

defiant1

Fair enough, Jim. How is compression height measured anyway?

I'm not pursuing these BTW, just wanted to get an idea why the dome type pistons were not popular with Olds engines. Mark explained it well.

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Warhead

Glad to hear that.
Center of the wristpin to the top of the piston, use the quench area if it has a dome. For our stuff, the whole outside edge is flat and that will suffice.
All of the small block pistons use this same height also, as did the early big blocks, one reason a standard 350 piston (4.057" bore) is a good .060 over piston for the early 400 (4.00" bore). Same wrist pin, same compression distance, and a multitude of dish depths to tailor the compression to whatever the owner wants.
A 455 Oldsmobile piston has the same wrist pin, and the same bore as a 425, but a 1.735 compression distance, so it will not just bolt in.

425 slugs are out there, you may just have to look around.

Good luck with the build.
Jim

defiant1

Jim, thanks for the explanation and teaching me a few things.

Gutted the engine today. Had to kill a couple of pistons today due to the fact they were rusted in pretty bad. But I got them removed. Marked the connecting rods, got the crank removed (without jacking it up) and cam bearings out. Just got to clean up the outside of the block a little more and remove the freeze plugs. Need to do a little research on what I can do to the block to help with the potential oil starvation issue. I like to do any mods before I take it to the machine shop.

Thanks again

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