would you try to shoot for the same dcr on a boosted motor as you would an n/a motor?

 

No same reason you don't shoot for the same static compression ratio....

The air/fuel charge is much denser in a blown motor so the limit of octane to DCR is now much lower.

Reference this thread first.....

http://web.camaross.com/forums/showt...47#post2581947

Just like NA motors can run more compression at higher altitude, air density applies to how high of compression you can run in a blower motor, except this time the environment doesn't dicate it the blower does. Air density in the motor is directly linked to the DCR, the denser the air the lower the DCR.

I'll prolly have some math to back this up later but it's going to take a little bit.

Bret

 

ok thanks...i read that thread and didnt really see anything about DCR and boost?

 

Still thinking about it. You gotta fully grasp the density and volume of the NA vs. FI motors first though.

Bret

 

i got it now...volume was really messing me up for a little while. i was thinking you would have more volume of air in the cylinder but mass was the real answer!


so, just like static, dynamic would be different depending on your goal of boost levels and fuel to run it on...right?

 

Exactly, nope you are not confused on that.

it's hard to come up with a set CR for the motor, but normally 8-9:1 SCR is what you shoot for and then play with the cam to get the right DCR.

Bret

 

so...what are some typical DCR's for boosted motors?

i always thought that you were supposed to pick the cam first and then play with the SCR to get the DCR where you wanted it?

 

like everything else, it depends on your budget. we practically dont have one at work so we pick a cam and then design our own piston to give us what we want.

you might be limited by what is available off the shelf, although that shouldnt be a big deal with a SBC

you shouldn't really worry about 0.25 points of compression and get a custon made piston, $$$ better spend in valvetrain and heads.

I suggest picking the correct cam (maybe letting someone else even do this if you are really hard core) and with that going for the desired compression ratio

 

My street/strip setup is ~8:1 SCR and ~6:1 DCR with 17-18psi of boost and seems to work well on 100 octane unleaded. It will run on 93-94 octane without any audible pinging, but frankly that makes me nervous with that much boost. I am somewhat skeptical of the people claiming that they run 9-10:1 with 15+psi on 93 octane and have no problems. Either they never get into the boost, are very lucky, have some secrets I don't, or else they are "exaggerating" a little.

Rich

 

maybe by "pump gas" they mean they dont use a bucket and scoop it out of a well or something

 

"Pump gas" can be blended to either favor the Motor Octane number or the Research Octane number. Final octane number is (R + M)/2.

Motor Octane is more resistant to WOT detonation, and Research to part throttle cruising conditions. We had some experience with some legally "92" octane that was very detonation resistant. Unfortunately what you get at the pump is really a crap shoot on how it is blended.