Compression Ratio???
09-03-2006, 10:51 AM
#1
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Compression Ratio???
I went back through old posts and got a lot of different info depending on how the topic came up. Thought I'd post the specific question so that everyone's opinions are in one place:
How much compression can you run with various Octanes on a BBC with bigger than stock cam that still has low enough overlap to idle around the docks?:
87
89-91 (mid grade)
93
Does running Aluminum heads really allow you to run a full extra point of compression?
The HP525 runs 8.7 on 87 octane but the 496 mag runs 9.1. I assume they both run iron heads, but would think the 525 would have higher overlap, but at least it's a place to start the discussion.
How much compression can you run with various Octanes on a BBC with bigger than stock cam that still has low enough overlap to idle around the docks?:
87
89-91 (mid grade)
93
Does running Aluminum heads really allow you to run a full extra point of compression?
The HP525 runs 8.7 on 87 octane but the 496 mag runs 9.1. I assume they both run iron heads, but would think the 525 would have higher overlap, but at least it's a place to start the discussion.
09-03-2006, 08:27 PM
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Re: Compression Ratio???
the problem is always going to be, how do you know what you are buying...
10 :1 on the best pump gas you can buy if you believe thats what they are selling you... or 100 LL from the airport
anything in the low 9's is safe
anything less than low 9's and you are giving away good power for no reason
and the cr you decide on will define the cam... don't forget... more cam means less dynamic compression ratio... big cams need lots of mechanical compression to work....
if i was building a 502 right now, ide build it in the low 9's
with a cam very similar to what the mag mpis came with , some decent al heads with ports no bigger than what the mag came w/ originally... just better...
as for the al heads... yes... al heads will traditionally allow you some leeway because they can bleed the heat quicker... but that should be considered a safety margin for when you screw up... not a design element... and the fact is that an al head with a bad combustion chamber is worse than an iron head with a GOOD combustion chamber...
and if you inject it, as opposed to a carb, you will have better everything... power , longevity, oil life... everything...
i would imagine that 490 to 525 hp should easily be doable in a bulletproof configuration done that way... and it will run for 700 years
10 :1 on the best pump gas you can buy if you believe thats what they are selling you... or 100 LL from the airport
anything in the low 9's is safe
anything less than low 9's and you are giving away good power for no reason
and the cr you decide on will define the cam... don't forget... more cam means less dynamic compression ratio... big cams need lots of mechanical compression to work....
if i was building a 502 right now, ide build it in the low 9's
with a cam very similar to what the mag mpis came with , some decent al heads with ports no bigger than what the mag came w/ originally... just better...
as for the al heads... yes... al heads will traditionally allow you some leeway because they can bleed the heat quicker... but that should be considered a safety margin for when you screw up... not a design element... and the fact is that an al head with a bad combustion chamber is worse than an iron head with a GOOD combustion chamber...
and if you inject it, as opposed to a carb, you will have better everything... power , longevity, oil life... everything...
i would imagine that 490 to 525 hp should easily be doable in a bulletproof configuration done that way... and it will run for 700 years
09-03-2006, 08:59 PM
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Re: Compression Ratio???
Here is the best article I've read on the subject - The Power Squeeze
End answer is it depends on a lot of things including but not limited to:
fuel octane
head material
compbustion chamber shape
cam
valves
piston coatings
timing
ignition
knock sensing on ignition
etc, etc, etc....
End answer is it depends on a lot of things including but not limited to:
fuel octane
head material
compbustion chamber shape
cam
valves
piston coatings
timing
ignition
knock sensing on ignition
etc, etc, etc....
