[gobrdgo]

Rainy afternoon here trying to figure out my piston situation. Building a street strip 428, 455 crank and stock length rods. Block has been decked to 10.227. 428 block will clean to a stock 4.151 455 piston. 96 heads ccd @ 96cc. Give or take the valve job, new fererras we’re used just sitting in the unmachined heads. So in order to get a safe 9.25 to 1 comp ratio going the custom piston route it’s easy just spend the 775 dollars and wait 12 weeks. So my question.. the stock pistons sit below deck height ( not zero deck from factory) right? So with my actual 10.227 deck height and stock being 10.250 but being below deck a bit can I get away with 4 valve relief forged off the shelf pistons and maybe a thicker head gasket? Increasing valve reliefs or even skimming the top of an off the shelf flat top isn’t beyond my capabilities if need be. I wonder if off the shelf forged pistons sit .020 down in the bore? If so we cut .023 off the block deck so we be close to zero deck with off the shelf pistons or am I way off?

[steve25]

A big question is what cam are you running, as the cylinder pressure it will produce can make a need for more or less compression?

I would not be afraid of running a 9.5 compression , but if you are and your block has no intake valve chamfers then putting them in will take out close to 3 CCs yet provide better power.

[hurryinhoosier62]

Question: is your crank on stroke and what is the center to center length of your rods? Those are definitely going to affect the compression height of the pistons you are going to use.

[gobrdgo]

Stroke is stock 4.21 and rods are stock 6.625. I didn’t think about the chamfers making power on the 455. Thought that was just for the smaller bore blocks.

[gobrdgo]

Here you go

[67Fbird]

The only number you are missing in your math is the compression height of the piston.
Having just done this EXACT thing.... Deck hieght is 10.240 (stock)
so YOUR case is 10.227
- (4.210/2) = 2.105
- 6.625 (rods)
- 1.480 (compression height of DSS pistons)
___________
= .017

in order to get to a 9.6 ish compression using a basic head gasket of .039. You are looking for an off the shelf 4 valve relief piston (probably -7 cc) then 3cc valve scallops you are looking to deck the block -.015-.017 then deck your heads (clean-up only ..maybe -.006 typical) that puts you at 9.4 : 1 pending your head cc at finish (est 93 cc)

[steve25]

For all you folks using just a plain number of .039” compressed height in your formulas for Head gasket CCs please note this.

Many of the common gaskets are not round, due to the valve releaf notches they are oval and a bigger CC number then what using just .039” with a round bore gives you!

A common Fel Pro gasket Is 9.4 CCs!

Also for example the Fel Pro gasket bore size is 4.300”!

[steve25]

With that Bullet Cam with non veri duration lifters I would be shooting for a 9.5 to 9.7 compression ratio and then being sure to polish over/ break all the sharp edges in the chambers, especially around the plug hole and the deck to chamber transitions.

['ol Pinion head]

Originally Posted by steve25

For all you folks using just a plain number of .039” compressed height in your formulas for Head gasket CCs please note this.

Many of the common gaskets are not round, due to the valve releaf notches they are oval and a bigger CC number then what using just .039” with a round bore gives you!

A common Fel Pro gasket Is 9.4 CCs!

Also for example the Fel Pro gasket bore size is 4.300”!

Butler offers several thickness of Cometic 4.225 diam head gaskets, just have to open your wallet a little more versus common Fel Pros. .036's here on one 455HO build, need to buy more

[Jay S]

ICON pistons for 455s have a 1.49” compression height, should be .007” below the deck with the 10.227” deck height. ICON has a flat top that would put the compression close to 9.5, 4.5 cc valve reliefs and they have the FHR series with 10.8 cc valve reliefs and would be down close to 9:0.

DSS pistons have a little shorter 1.48” comp distance and can get in a 4340 alloy like the icon FHR, irc they are a 5cc valve relief, maybe slightly more. Depending on the head gasket the DSS would put to SCR somewhere in between the 2 icon pistons. I can’t really see why that combo would need a custom piston.

Personally I would not add the block chamfer on the 455 with 96 heads. The 455s I have seen that had a block chamfer they were just over 1 cc, never seen any that were 3 cc, but I have not seen all of the chamfer combinations on PMD blocks. I have a similar 455 and 96 head combo out in the shop right now. It is really close to 10:0 SCR with a bigger SFT cam and 280 cfm heads 96 heads with out the bore chamfers.

Looks like you have one of Bullets new computerized Ultradyne grinds, appears to have a good amount of lift and duration at .2” and above. Should make good power even at 9.0 if you don’t mind loosing a little idle quality. I think it would work well in the 9.0 to 9.5 compression range on 91 pump gas. You could go a little higher on compression but it likely would not see to much gain with stock head flow on pump gas . 9.0 SCR is getting close to working as a 87 octane combo, all depends on your goals.

[android 211]

I have a similar engine with much less cam (068). The consensus out here is that the 4 valve relief pistons sit way down in the hole. If you use a thicker head gasket you'll have even less quench. It might ping on pump premium - mine does. Your cam is so much bigger may inhibit that.
Still the experts here counsel go for tightest quench you can get - like .030". I've got TRW flat top pistons, zero deck and a .035" Felpro gasket.

[hurryinhoosier62]

Originally Posted by gobrdgo

Stroke is stock 4.21 and rods are stock 6.625. I didn’t think about the chamfers making power on the 455. Thought that was just for the smaller bore blocks.

Have you checked the stroke with a stroke gauge? Have you checked the actual c-to-c length of the rods? If you are assuming the crank is on stroke because it has never been ground, guess again. I’ve seen OE cranks be as much as .020 or more out of stroke. Nearly ALL piston manufacturers aim for a 10.235-10.240 deck height for Pontiacs. A normal machining variance for most internal engine components is .0015-.003. ALL these measurement are critical when choosing a piston. This is why I never order a piston until the deck height is set, the crank strokes are equalized and the rod c-to-c is confirmed.