Piston and Bore Choices Help - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 03-13-2016, 06:44 PM Thread Starter
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Piston and Bore Choices Help

Hey.. I'm rebuilding my 400. I have edelbrock 72cc aluminum heads that I don't want to spend the money to replace, and I'm buying a new rotating assembly anyway, so I have two options for bore and pistons if I want to run 93 octane pump gas. Option 1: Bore it to .060 over making it a 412 CI and use flat top pistons. Option 2: Bore it to a 461 and use dish pistons. The cost is almost the same to do either. Which will give me better performance? I've read that dish pistons don't have very efficient combustion so I'm not sure what makes the most sense to do.
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post #2 of 10 (permalink) Old 03-13-2016, 09:20 PM
 
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Hey.. I'm rebuilding my 400. I have edelbrock 72cc aluminum heads that I don't want to spend the money to replace, and I'm buying a new rotating assembly anyway, so I have two options for bore and pistons if I want to run 93 octane pump gas. Option 1: Bore it to .060 over making it a 412 CI and use flat top pistons. Option 2: Bore it to a 461 and use dish pistons. The cost is almost the same to do either. Which will give me better performance? I've read that dish pistons don't have very efficient combustion so I'm not sure what makes the most sense to do.

Given that you have already done a price comparison, bigger cubes are always better with regards to torque & power in a Pontiac engine.

Do you have to bore it to .060 over? Would .040 work? You don't want to go any more than necessary -leaves a little meat for another rebuild. Even though pistons are generally listed at .030 & .060 over, you can typically get them at .040 over if you inquire.

I assume you are asking about the 461 CI stroker kit which includes the 4.25" stroker crank, 6.8" rods, and forged pistons? From my understanding, the dish portion of the piston is "D" shaped to match the heads, so they work well. The important thing to consider when building the engine is the "squish" - the area between the top of the piston and the head where no combustion takes place. Your machinist can get the correct clearances on this.

Another option, and I can't verify this, but can't the Edelbrock heads be machined for larger cc combustion chambers to get the compression you are looking for with flat tops? Again, maybe your machinist/builder can perform this operation and open the heads up a little more in the combustion chamber.

Pontiac's are torque engines, so I would go for Option 2 all things considered.
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post #3 of 10 (permalink) Old 03-14-2016, 07:14 AM Thread Starter
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Thank you Jim. It is the 461 stroker kit that I'm looking at from Butler Butler Performance - Pontiac Engine & Rotating Assembly Combinations - Featuring Eagle Pontiac Kits

Good point about the heads possibly being worked and going .040 over instead of .060. Right now I'm at .030 so splitting difference would be good thing. I'll ask them if that's an option.
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post #4 of 10 (permalink) Old 03-16-2016, 04:36 PM
 
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What is your compression issue? The 461 stroker with flat pistons and and E-heads at 72cc will give you around 10.25:1 which is fine with those heads and 93 octane. Zero deck it to get the squish down and let er rip.
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post #5 of 10 (permalink) Old 03-17-2016, 02:55 PM
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X2 on boring the block no more than needed to clean it up. That way, it can be rebuilt again someday and it will run cooler. No sense going to maximum oversize out of the gate. If you want to rebuild it again, it'll need sleeving. What do you do with your car? Stokers are great for torque and power. Stock-ish sized 400's are great all around engines for street driven cars. The 406 (.030") over original 400 in my '67 GTO sees a lot of use and drives great. Not a world-beater, but it does get over 20mpg at 75-80 mph all day long with my tall rear gears.....pretty nice for long weekend trips!
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post #6 of 10 (permalink) Old 03-17-2016, 06:44 PM
 
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Originally Posted by BierManVA View Post
What is your compression issue? The 461 stroker with flat pistons and and E-heads at 72cc will give you around 10.25:1 which is fine with those heads and 93 octane. Zero deck it to get the squish down and let er rip.
Typical flattop stroker combo pistons will yield near 11.5-1 C/R on a 4.25 stroke 461, when 72 cc heads are used. With zero deck, will need close to 22 cc piston dish on piston top, to get to 10.0-1 CR with same combo. 10.0-1 with proper cam selection will run fine on 91...not a fan of box stock 87cc E heads and the Edelbrock RA4 clone cam in a 455-467.
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post #7 of 10 (permalink) Old 03-17-2016, 06:52 PM
 
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Originally Posted by Pinion head View Post
Typical flattop stroker combo pistons will yield near 11.5-1 C/R on a 4.25 stroke 461, when 72 cc heads are used. With zero deck, will need close to 22 cc piston dish on piston top, to get to 10.0-1 CR with same combo. 10.0-1 with proper cam selection will run fine on 91...not a fan of box stock 87cc E heads and the Edelbrock RA4 clone cam in a 455-467.

You are right! My brain was thinking 87cc when I read the post initially. Sorry for that. Glad you corrected my bad info.
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post #8 of 10 (permalink) Old 03-18-2016, 03:51 PM Thread Starter
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I ended up going .040 over and got a 461 kit from Butler. Dish pistons.. not sure how many cc, but they said I'm looking at 10.2 compression, 480 HP, but biggest increase will be in torque. Also going with roller cam since from what I've heard will give me better performance across the RPM range. 4-8 weeks for delivery on the rotating assembly. Went with forged crank, as they said it could be 6 months for a cast crank. Whats another $500 at this point? Can't wait to put this thing together. Would love to get it over 500 HP.. not that 20 HP is a big deal, but just for bragging rights :-). Might have to port the heads :-).
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post #9 of 10 (permalink) Old 03-18-2016, 04:16 PM
 
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I ended up going .040 over and got a 461 kit from Butler. Dish pistons.. not sure how many cc, but they said I'm looking at 10.2 compression, 480 HP, but biggest increase will be in torque. Also going with roller cam since from what I've heard will give me better performance across the RPM range. 4-8 weeks for delivery on the rotating assembly. Went with forged crank, as they said it could be 6 months for a cast crank. Whats another $500 at this point? Can't wait to put this thing together. Would love to get it over 500 HP.. not that 20 HP is a big deal, but just for bragging rights :-). Might have to port the heads :-).

Good choices. The bragging rights is not in the HP with a Pontiac, its in the torque and that will be over 500 ft lbs.
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post #10 of 10 (permalink) Old 03-18-2016, 04:53 PM Thread Starter
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Good choices. The bragging rights is not in the HP with a Pontiac, its in the torque and that will be over 500 ft lbs.
Works for me
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