How do you build a motor to handle 12-15 compression ratio?

 

Kinda confused on what you want to know. 12: CR is a lot. 15:1 is a ton. Even 12:1 would have to run on race gas. Are you asking how you get that high of a compression ratio? Why do you want a compression ratio that high? You really wouldnt be able to drive it on the street at all.

 

I have a motor with 9.5:1 ratio and like to run a supercharger about 6-8 psi. Holley compression chart says it would be 13:1.

 

Not sure how you were looking at a Holley supercharger compression chart without access to the rest of this info.......it lays it out in detail.

http://www.holley.com/data/TechServi...ech%20Info.pdf

 

what would be a safe psi to run? Im new to the supercharger world?

 

Your questions are not making a lot of sense without more background of what you are wanting to do or know..........try doing a bunch of reading/research with the link I provided and info found at BDS and Dyers sites, they pretty much cover all the basics and will give you a better understanding of how it all works on a roots blown build.

 

Super chargers & turbos use 9, 9.5:1 at the most, & forged pistons, (forged rotating assembly for all out racing//hi rpm use.)

7.5:1 is a starting point. Great heads & right cam would be beneficial.

flirtn1 makes perfect sense.

I'm thinking you are asking what your total compression would be with a blower/supercharger.
In short, the engine must be assembled for blower/super charger if you want it to last.

 

If you want to run over 12:1 compression with forced induction, you're gonna need to go with E85 at the bare minimum. You may even be pushing it with E85.

E85 behaves similar (note the wordage, similar) to C16 race gas. E85 is about the equivalent to 105 Octane, and the cooling effect it has on the cylinder helps it out a bit more. The only catch is, you're going to need about 50 percent more fuel at WOT, and less at part throttle and cruise.

Also, 12:1 is only your static compression ratio. You'll lose some of that cylinder pressure with a big cam, which is referred to as a dynamic compression ratio, and the more important of the two. Most engines running higher static ratio compression do so because they have a lot of valve overlap, and need to compensate for the air that's pushed out of the exhaust valve at the beginning of the compression stroke. Once you learn a little more about engines, you'll know that forced induction engines don't like a lot of valve overlap.

 

What the???

i thought you want low compression with forced induction... doesnt higher compression create higher vacuum pressure??? boost and vacuum press. are on the oposite sides of the spectrum.

12 to 15 compression... thats insane especially with iron heads. ive heard of 10.5 to 11.5 on an engine with aluminum heads.

everyone i know in the real world that runs a forced induction v8 stays around 8.

 

Generally, yes. Most people run less than 9:1 compression with forced induction. Higher compression does not create more vacuum. Valve overlap, throttle angle, and engine RPM do.

 

good point... compression has to do with how much the combustable gases are compressed. hence the term compression ratio.
why then, does everyone stick to a low c/r for forced induction?

just interested...

 

I have a 12.1: 1 383 alum heads
The 236 / 242 cam brings the DCR back to the approved range. (8.6 )

 

Usually to run more boost. More air = more fuel = more bang. Higher compression doesn't add more fuel and air into the engine, it only compresses the fuel/air mix more, which creates more bang, but not as much bang as more fuel and air.