Compression

[Hawkeye]

Just a few questions about compression
What's the highest compression you can run with pump gas?

What's the highest compression you can run for a SC?
I know it depends on the boost but I welcome any opinions.

[HidalgoGT]

Just a few questions about compression
What's the highest compression you can run with pump gas?

What's the highest compression you can run for a SC?
I know it depends on the boost but I welcome any opinions.

I believe pump gas is in the 11.1 area
Heard of 9-9.5 with a sc but if you go a lil lower like 8.5-8 you can run more boost with less issues andmake more power.

[somethingclever]

About 200psi cranking pressure for 93 octane and alum heads.
About 185psi cranking pressure for 93 octane and iron heads.

For a centrifugal application 9:1 is pretty common. If you have a lot of cubes/less wt/lots of gear and are willing to sacrifice some bottom end power, you can use less compression and make up for it with more boost, which will equal more top end power. (meaning you can make more hp with reduced comp. ratio and increased boost pressure than higher comp. ratio and less boost...assuming same fuel, motor (aside from comp. ratio of course), car...etc)

I cannot approximate cranking compression for a given static ratio. I would talk to a cam designer.

[Rodeheaver's]

your always safe to stay away from going any higher thatn 10.5:1, i have however ran up to 11.5:1 with 93 octane, but you have to be very careful with a small block becuase the burn time is very fast, so timing plays a huge role in this...

[biminiLX]

John has the right answer there---you want more info on the cranking compression over the static (what's quoted above) and the cam design.
As for boost, it depends how you want to set it up but as always its in the tune and I choose to go with 9.0:1 with a turbo.
-J

[Hawkeye]

Thanks guys....
I am rebuilding my 408....and I think I am going to make it 10.5 to 1
The thought went through my mind about adding a SC.
It's a NA carbed engine right now

[HidalgoGT]

Could do a blow threw carb turbo setup for around 5k. Thats a 76 mm. Or go N/a with a lil go go juice.

[map351]

A 2.3 Turbo with a 8:1 @ 25 psi is 21.32 : 1 on pump gas with Torco accelerator.



Mike

[torqueboxer]

my car is 11 to 1 with aluminum heads and 93 octane garbage fuel. you really just need to watch your quench when setting up the motor i set mine at around .037" and get no detonation. Any more than that and you will get pre ignition.

[Martin0660]

The "rules of thumb provided are very basic paramaters, but the answer is no where as simple as 10.5-1 on 93 pump gas. There are so many issues with fuel octane, compression, and timing that all come into play. It can be a lifetime work to find and optimum combination of the best compression, boost pressure, and timing (or compression and timing in NA form). EVERY combo is going to like different things. I have wasted 100's of passes trying combos of boost pressure versus ignition timing versus cam timing (of course my comresssion is fixed in my case). I have a combo that I really like for the 2.3 stuff, and as we have applied it to other cars they seem to like it too, but then we find one that doesent every once in a while. I was stunned what we found with 4 degrees of cam timing, and what it let me get away with.

Even altitude (density altitude more precisely) is a factor, and is even part of the calculations for boosted effective compression.

My 2.3 at 25 psi and 1400 feet DA is 21.33:1 and runs on 93 pump gas with a quart of Torco for insurance (will run on pump gas alone).

Change the DA on that combo to 3500' and I loose 1/2 a point effective compression to 20.91:1. This isnt unheard of at PRP in the summer.

Point is, pick your combo based on some experience, and tune it for what you end up with. No hard and fast rules for sure.