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 #12 Heads
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Chavez
Buffalo

USA
44 Posts

Posted - 01 Mar 2016 :  1:55:57 PM  Show Profile  Reply with Quote
Just bought a set of #12 heads. I'm aware of the high compressions these pigs will be out putting out, however, I want to get down to 9:5 compression for pump gas. What do I need to modify on my 400 to get to that compression? I have .30 over cast speed pro flat pistons and a stock cam. 3.08 gears, 2000 stall and edelbrock performer intake. My engine builder tells me I'm fine with those pistons but I'm hearing I need dish pistons. What will I need to get down to 9:5 cr?

tjs44
Cochise

USA
412 Posts

Posted - 01 Mar 2016 :  6:14:06 PM  Show Profile  Reply with Quote
you should have him calc all the numbers,know what your deck height is,know CCs in the done heads,CCs in valve reliefs,crush thickness of the head gaskets used.If he cant do that look for another engine builder.Tell him YOU want 9.5 to 9.3 when done with no guessing.Tom
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Chavez
Buffalo

USA
44 Posts

Posted - 01 Mar 2016 :  8:53:37 PM  Show Profile  Reply with Quote
Heads haven't been done yet. It's been 3 years since engine has been built. Engine builder isn't around anymore so I'm flying solo on this. Basically need to know if my cast pistons will be enough to handle those 12s or do I really need dish?
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Admin
Ye Olde Webmaster

847 Posts

Posted - 02 Mar 2016 :  06:49:50 AM  Show Profile  Reply with Quote
Work the heads to reduce chamber size and keep your pistons as is. Jim Lehart at CVMS did it for me; he can do it for you as well.

"Dedicated to keeping the classic Pontiac engine alive."

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Tub
Sitting Bull

172 Posts

Posted - 02 Mar 2016 :  2:33:25 PM  Show Profile  Reply with Quote
If you really have cast pistons, as you say, that means they are the dreaded 8-eyebrow pistons.

http://www.summitracing.com/parts/slp-411np30/overview/make/pontiac

If this is the case, I'd go with forged dish pistons. Many suggest that the 8-eyebrow pistons increase the chance of detonation.

Or, you can go with the SP L2262F forged pistons, and have the chambers enlarged, as mentioned.

http://www.ebay.com/itm/Speed-Pro-TRW-Pontiac-400-Forged-Coated-Skirt-Flat-Top-4VR-Pistons-Set-8-030-/190855076806?hash=item2c6fd95bc6:g:ZnwAAOSwDuJWvhoN&vxp=mtr

But, I suppose there have been thousands of engines survive with those cast pistons. So I reckin it is possible that they'll work just fine for you--who knows.

Edited by - Tub on 02 Mar 2016 2:46:33 PM
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tjs44
Cochise

USA
412 Posts

Posted - 02 Mar 2016 :  2:41:40 PM  Show Profile  Reply with Quote
You can have cast pistons without the 8 valve reliefs.Tom
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Tub
Sitting Bull

172 Posts

Posted - 02 Mar 2016 :  2:53:50 PM  Show Profile  Reply with Quote
quote:
Originally posted by tjs44

You can have cast pistons without the 8 valve reliefs.Tom



He said Speed-Pro Cast. So, I just assumed he meant Sealed Power cast, which are the 8-eyebrow kind.

And if you decide to change pistons, why would you buy cast pistons, when the SP L2262F.030 forged pistons are only $315 ?

I mean, you COULD. But I wouldn't. I'd choose forged over cast every time, when the price difference is no more than that, especially if the CR is right on the detonation borderline.

Edited by - Tub on 02 Mar 2016 2:59:44 PM
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Steve C.
Crazy Horse

1675 Posts

Posted - 02 Mar 2016 :  4:57:29 PM  Show Profile  Reply with Quote
If desired and your budjet will allow skip the catalog 'shelf' pistons if their specs are not compatable with your combustion chamber volume. Ross can custom design a forged piston to bring the static compression ratio right where you want it in three weeks. Other piston companies can do so as well. Obvious, you will need to measure the actual combustion chambers to varify their volume. And at the same time determine your actual varified block deck height and they can alter the pin location to suit your needs.
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Tub
Sitting Bull

172 Posts

Posted - 02 Mar 2016 :  5:15:53 PM  Show Profile  Reply with Quote
Yeah, Ross are some of the best, and highest.

If price is important, I've read that the Race-Tec customs are about $300 cheaper than Ross. Can't verify, since I never bought any.
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Chavez
Buffalo

USA
44 Posts

Posted - 02 Mar 2016 :  6:43:59 PM  Show Profile  Reply with Quote
Great input guys. Thanks. It didn't cross my mind to have the heads reworked. The pistons I have are actually 4 valve reliefs. Now you guys got my gears turnin. Ok so now the questions for me are of I rework those heads, will the cast pistons hold up? Is it better to go dish and not rework the heads or go flat with reworked heads? I suppose it's all the same just as long as desired compression ratio is achieved right!?
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Steve C.
Crazy Horse

1675 Posts

Posted - 02 Mar 2016 :  7:30:25 PM  Show Profile  Reply with Quote
Not Pontiac specific but for conversation around the coffee table...

Before buying pistons you need to understand that flat-top pistons and small, compact combustion chambers are the racer's most user-friendly choice--by a big margin. Always get all the compression conveniently possible by minimizing the quench clearance and cylinder head chamber volumes before considering domed pistons.

The domed piston may look like an easy short cut to more compression but it does not always deliver the hoped-for power increase. Although intended CR ratio may be achieved, the combustion process may be severely compromised by the presence of the dome. I have seen a piston dome that was a tad too high and cost well over 100 hp. Unless you or your engine builder are prepared to spend time developing a piston crown form that works, stick to domes no more than about .001-inch high.

Source / General info regarding compression:

http://www.hotrod.com/events/coverage/0311em-power-squeeze/

And a comment from Mr P-body in conversation regarding a small dome or flat top with proper valve relief's:
"ANY engine will benefit from a flat top piston. Logic. Straight, flat path for the fire to travel across"

Edited by - Steve C. on 02 Mar 2016 7:34:37 PM
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Steve C.
Crazy Horse

1675 Posts

Posted - 02 Mar 2016 :  7:38:07 PM  Show Profile  Reply with Quote
Source SD Performance website:

"SD Performances uses Ross Racing Pistons exclusively in all our engines and kits, there's lots of cheaper pistons out there, but the old saying you get what you pay for applies. Pistons are a critical internal engine component and saving a few bucks on a cheaper product shouldn't be considered when building a quality engine. Ross is the leader in producing the most precise pistons on the market today and for that reason SD Performance only sells and uses Ross Racing Pistons."

Comments from Jim Butler:

After many years of working closely together, Butler Performance and ROSS Racing Pistons have developed a new line of Performance Forged Pistons for traditional Pontiac engines base on a new Butler exclusive forging. For years piston forging designs used by performance piston manufacturers have been based off Small or Big Chevy, Ford, etc. When making Pontiac pistons, these companies would typically use a raw forging originally designed for a small block Chevy, being the closest thing to Pontiac bore size, valve spacing and order, etc. This has worked for many years but sometimes resulted in pistons being heavier than needed because the Pontiac designs didn't "fit" the Chevy style forgings perfectly. Now ROSS Racing Pistons has designed a true "Pontiac" forging exclusively for Butler Performance.These pistons are some of the lightest and strongest available, with no special internal milling required to get the reduced weight.
These are the latest "strut-style" design, which lends to the reduced weight as well as increased strength. These pistons can also be run at tighter piston to wall clearance so they are great for performance street applications as well as all-out racing.



Edited by - Steve C. on 02 Mar 2016 7:39:06 PM
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Blued and Painted
Chief PONTIAC

USA
3406 Posts

Posted - 04 Mar 2016 :  9:47:44 PM  Show Profile  Reply with Quote
Just reshape the shrouding of the combustion chamber around the intake valve until the desired cc's are reached.

Remember its not easy to get that compression back once cut away.

I would select a camshaft that complements 10 to 1 and Pump Gas...

One with Longer advertised duration.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 04 Mar 2016 9:51:15 PM
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