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Old 01-10-2012, 10:54 AM
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Dome Pistons

Currently i have a stock 1994 SBC 350 block and rotating assm. The pistons a Dish pistons with 4 valve relief cut into them. I think that gives me a compression of 8.75:1.

Since my goal is to get 300hp at the wheels would it be a good idea to switch out the pistons with a set of Dome 0.125 pistons which would bring my compression to somewhere between 10.5 to 11:1. The cost of the new pistons is only $170 and seem like they would provide a good jump in HP.

I will be running a set of Vortec heads, and Holly 600cfm carb, and a Comp cam Xtreme Energy 268 cam. (Duration @ 0.050":224 / 230; Max Lift w/ 1.5RR: .477" / .480")


The pistons i want to buy:

http://www.ebay.com/itm/STD-125-Dome...21386079550418

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Old 01-10-2012, 01:20 PM
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Don't buy domed pistons. Just get a pair of Felpro steel-shim head gaskets. That'll get you over 9:1. If you want more, have the machine shop shave 0.030" off each head. That'll get you over 9.5:1. That's plenty for 300 RWHP on pump gas, especially with that cam.
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Old 01-10-2012, 01:22 PM
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Going from 9.5:1 to 10.5:1 is worth maybe 4% more power, or maybe 15 HP for you. But at the cost of always having to use premium unleaded plus pour-in bottles of octane booster every tank? Retarding the timing is not a solution. It's just a Band-aid at best.
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Old 01-10-2012, 02:33 PM
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Quote:
Originally Posted by whyholdback
Going from 9.5:1 to 10.5:1 is worth maybe 4% more power, or maybe 15 HP for you. But at the cost of always having to use premium unleaded plus pour-in bottles of octane booster every tank? Retarding the timing is not a solution. It's just a Band-aid at best.
wow i would only get around 4% hp gain from dome pistons? currently the engine has dish pistons.

In any case the hassle of running octane booster seems like too much trouble for a 15 or so HP gain.

What compression would i get with a set of flat tops and would i still need to run premium fuel with an octane booster?
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Old 01-10-2012, 03:05 PM
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With a standard bore, assuming the pistons are .025 in the hole, 64cc heads, .041 gasket, using these pistons with a .125 dome, which are 3.5cc, your c.r. would be approximately 10.69:1.

Without knowing the volume of your current piston dish, you can't even guess what your c.r. is now, I doubt you have valve reliefs unless the dish is very small. With a flat piston, 4 valve reliefs you should be at about 9.55:1, good numbers for a pump gas motor build.
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Old 01-10-2012, 03:22 PM
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Biggest problem with your plan is that a domed piston will not work with Vortec heads. The dome will hit directly on the spark plug boss..
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Old 01-10-2012, 03:26 PM
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Stock Vortec 350s are supposed to be 9.1:1. The pistons are dished, plus valve reliefs. The tops of the pistons are 0.025" below the deck. To get proper quench you need the felpro steel-shim head gaskets, at 0.015"-thick.
If we do a 350 with flat-tops, using Speed-Pro hypereutectics, these gaskets, the 0.025" deck volume, and true 64 cc heads, that puts you at 10.266:1, and you don't need that. Really, you don't want that, either. So keep the stock pistons, which have a dish volume, including the VRs, of around 10 cc. If they have the perimeter chamfer, then they're more like 13 cc.
Adding a 10 cc dish to the combo I just figured, you're at 9.21:1. Mill the heads 0.030" if you want 9.70:1. I would.
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Old 01-10-2012, 03:45 PM
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Quote:
Originally Posted by whyholdback
Stock Vortec 350s are supposed to be 9.1:1. The pistons are dished, plus valve reliefs. The tops of the pistons are 0.025" below the deck. To get proper quench you need the felpro steel-shim head gaskets, at 0.015"-thick.
If we do a 350 with flat-tops, using Speed-Pro hypereutectics, these gaskets, the 0.025" deck volume, and true 64 cc heads, that puts you at 10.266:1, and you don't need that. Really, you don't want that, either. So keep the stock pistons, which have a dish volume, including the VRs, of around 10 cc. If they have the perimeter chamfer, then they're more like 13 cc.
Adding a 10 cc dish to the combo I just figured, you're at 9.21:1. Mill the heads 0.030" if you want 9.70:1. I would.
I'm showing 9.7 without cutting the heads, with a 10cc dish, using that .015 steel gasket, 9.16 with an .041 gasket. Flat pistons with 6cc of reliefs and that gasket I show 10.15. Neither calcs are that far apart, but you should find out what you have and want before you start buying parts. I suggest you find an online calculator, find out exactly what you have and do your own calculations.
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Old 01-10-2012, 03:55 PM
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below is a picture of my current pistons.





Since i pulled my motor from 1994 chevy shortwide i looked up the spec for the 5.7L 350 that came with the 94 truck found that it had a compression 8.75:1 and was rated for 190hp and 300ft.lb of tq.

Last edited by logan1; 01-10-2012 at 04:15 PM.
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Old 01-10-2012, 04:04 PM
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Looking at those pistons, if you have 64cc heads, an .041 gasket and it looks like maybe 15cc's of dish and reliefs (only a guess), it would put you in that ball park, at around 8.72 using my own calculator. There's also a few cc's to consider in the angle around the circumference.
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Old 01-10-2012, 09:03 PM
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on the bottom of the piston there will be a number. Check it against this site... http://webpages.charter.net/beckracing/slvpg29.htm Great resource for stock specs of factory pistons.
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Old 01-11-2012, 09:54 PM
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ok so i made an excel sheet to see how much it would cost to build a sbc 350 that would give me 300hp and the wheels and i'm shocked at the price. I was heading into this think it was going to be a budget build. This so called budget build is gonna cost me a fortune. I wrote down all the numbers and it looks like its going to be way more expensive than i had expected. The expected cost is price of parts off ebay and other internet sites and the actual cost is what i have bought so far.

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Old 01-11-2012, 11:26 PM
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Quote:
Originally Posted by logan1
Currently i have a stock 1994 SBC 350 block and rotating assm. The pistons a Dish pistons with 4 valve relief cut into them. I think that gives me a compression of 8.75:1.

Since my goal is to get 300hp at the wheels would it be a good idea to switch out the pistons with a set of Dome 0.125 pistons which would bring my compression to somewhere between 10.5 to 11:1. The cost of the new pistons is only $170 and seem like they would provide a good jump in HP.

I will be running a set of Vortec heads, and Holly 600cfm carb, and a Comp cam Xtreme Energy 268 cam. (Duration @ 0.050":224 / 230; Max Lift w/ 1.5RR: .477" / .480")


The pistons i want to buy:

http://www.ebay.com/itm/STD-125-Dome...21386079550418
Use this calcuator http://www.kb-silvolite.com/calc.php to help you find a more accruate SCR or {static compression ratio } You can make good power with dished piston with the right cam and head setup. Since you have your engine disasembled, find a local machine shop or ask a few buddies where one is and have them zero deck the block make it a 9" block, then buy a .039 or .041 compressed head gasket with a 4.060 or 4.1 compression ring depending on your bore size.. you want to set your {squish} which is the distance between the top land of your piston not including dish or dome area with cylinder at true top dead center and the bottom side of your cyclinder head.. with a zero deck and a .041 gasket, that will give you a .041 squish which would be perfect. a squish setting .035-.045 is the best mark to get to make a more effeiciant combustion which helps make more power..
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Old 01-13-2012, 11:10 AM
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3100 to build a performance car is NOT a fortune. There are guys who pay half that for a T56 core and a set of synchro rings runs upwards of 400 bucks, and an overhaul kit (incl the rings) is almost a grand

I like the JTR stuff, aside from the advertisements for V8 jaguars and etc every couple pages. Ive been out of the game for a while on these (my best friend built one about 10yrs ago) but I heard there's a better engine placement than the JTR kit. This may have been for the LS series of engines. I don't know.
Good things take time and good deals are out there, just ask questions before you buy.
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Old 01-13-2012, 12:12 PM
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I agree with AutoGear. I shake my head at those who think a budget build is possible for $1,500, they're dreamers unless they have a lot of useable parts on the shelf. It looks like you did some homework, but I wonder which of those parts on the list will be used/new. Headers for $150? You bought heads for a bargain price of $65, so I assume they need a rebuild or refresh. Your allotted $200 for the machine shop, around here, might get the heads and block tanked, no cylinder hone, line hone, truing the decks, etc. What I don't see on your list is shipping for those parts. Where is "T-town" in Oklahoma, and what resources are available locally?
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