Author Topic: Trying to figure compression ratio for Pontiac 455 build  (Read 1286 times)

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Offline TAFEVER

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Trying to figure compression ratio for Pontiac 455 build
« on: November 10, 2012, 07:51:44 PM »
Here is what I will be working with. Does anybody know how far in the bore the pistons were on a stock 1975 Pontiac 455 engine?

1975 Pontiac 455 .030
Bore = 4.181
Stroke = 4.21

Sealed Power Forged pistons
Piston compression height = 1.497
4valve relief Piston head volume = 6.70cc

Edelbrock heads = 87cc
Felpro head gasket = .039 compressed

If somebody could calculate the compression ratio I would sure appreciate it.
I would like it to be able to run on 91 Octane

Offline 72blackbird

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Re: Trying to figure compression ratio for Pontiac 455 build
« Reply #1 on: November 10, 2012, 09:42:51 PM »
You left out a few specs that are critical to calculating static compression ratio (SCR)- your piston-to deck clearance and gasket bore diameter. Most Pontiac V-8's leave the production line with the pistons .020 down in the hole- this will decrease SCR by 0.4 points.

I'm running SD Performance ported 87cc e-heads on my .020 over 455-  my block has a piston-to deck clearance of .0015 and I'm also using the fel-Pro no.1016 head gaskets. I have .020 Probe flattops w 5.5cc valve reliefs. My SCR specs out to 10.23:1- perfect for running on 91 octane.

Geno

http://www.wallaceracing.com/cr_test2.php

Offline TAFEVER

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Re: Trying to figure compression ratio for Pontiac 455 build
« Reply #2 on: November 11, 2012, 12:24:28 AM »
You left out a few specs that are critical to calculating static compression ratio (SCR)- your piston-to deck clearance and gasket bore diameter. Most Pontiac V-8's leave the production line with the pistons .020 down in the hole- this will decrease SCR by 0.4 points.

I'm running SD Performance ported 87cc e-heads on my .020 over 455-  my block has a piston-to deck clearance of .0015 and I'm also using the fel-Pro no.1016 head gaskets. I have .020 Probe flattops w 5.5cc valve reliefs. My SCR specs out to 10.23:1- perfect for running on 91 octane.

Geno

http://www.wallaceracing.com/cr_test2.php


Given the stock deck height of a Pontiac 455 is 10.25 I was able to calculate how far down in the bore my forged piston will be and that is .023

With that known my compression ratio will be 9.76 with 87cc heads.
« Last Edit: November 11, 2012, 01:05:32 AM by TAFEVER »

Offline 72blackbird

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Re: Trying to figure compression ratio for Pontiac 455 build
« Reply #3 on: November 11, 2012, 01:29:04 AM »
I would consider zero decking your block to raise compression and maximize the power-making potential of your heads. 9.78:1SCR with aluminum heads is like running 8.78:1 with iron heads.

Geno

Offline TAFEVER

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Re: Trying to figure compression ratio for Pontiac 455 build
« Reply #4 on: November 11, 2012, 09:00:27 AM »
I would consider zero decking your block to raise compression and maximize the power-making potential of your heads. 9.78:1SCR with aluminum heads is like running 8.78:1 with iron heads.

Geno

I thought about that as I would be sacrificing some power but then I could even run the lower cost 81 octane with the lower  9.76 compression ratio.  My plan is to be driving this TA alot.

Offline 72blackbird

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Re: Trying to figure compression ratio for Pontiac 455 build
« Reply #5 on: November 11, 2012, 01:08:38 PM »
The ability to use 87-89 octane would definitely be a plus if you plan to drive your TA alot. Run a PPR TC-01 HF with stock e-heads and you'll be in the neighborhood of 500 hp, even with the lower octane and slightly lower compression. You would probably be able to run 87 octane in cooler weather, but put 89 in it during the the hotter months to protect your engine.

I built my 455 with a different purpose in mind- I was shooting for max hp w/ the SD e-heads, around 580-600 hp with their "Road Paver" HR cam. Dave Bisschop over at SD Performance recommended around a 10.25:1 SCR to run on 91- great for hp, but not so economical due to the price of 91. My 72 Esprit will become my demonstrator car for my business, so it's not going to get driven alot after it gets rebuilt.

Geno