Forced Induction - Who runs the lowest compression ratio?
01-04-2003, 10:36 AM
Mine is 8.75:1 on stock heads.
01-04-2003, 11:57 AM
i believe Harlan has his at 8.5:1 with 6.0 heads.
a few others have theirs at 8.7ish:1
mine of coarse is 9.0:1
01-04-2003, 06:10 PM
I am in the early stages of trying to decide which compression to have my motor set to but what is the advantage of 8.5:1 over say, 9:1. May be a stupid question but I don't understand so here it is. Thanks in advance
01-04-2003, 06:29 PM
mines 8.75 dynoed 301rw with headers.. its sloooow NA!
01-04-2003, 06:30 PM
i am in the same boat, trying to figure out the best compresion ratio.
from what people are telling me with lower compresion you will be able to run more boost and make more power up top but at the expence of some pre boost power.
seems to me if you have a centrfigual blower than this might be a concern since they dont make much boost down low. with a turbo you can get boost down low if you load it a little so lower might be ok especialy for a drag car. with my blower(twin screw) i have instant boost, as soon as i touch the gas i have boost so it seems to me keeping it on the low side might be especialy good for me. i think i am going to shoot for 8.5-1
01-04-2003, 06:41 PM
I think people go too low on the C/R to be on the safe side. What is the Porsche Turbo, like 9.4:1 ? What is the LPE TT setup?
I wouldn't go below 9:1 myself. We aren't running 20psi.
01-04-2003, 06:42 PM
parish8 is right on, I like 8.5 with early boosting power adders(twin screw and turbo) and around 9.0 with cent. blowers
01-04-2003, 06:54 PM
I am seriously considering the new CAS stage II system speed inc has combined with a ARE 370 or 382 cid. I will be making most of my purchases around May. I have considered going ahead and doing the motor first then adding the turbos in a couple of months but I still don't know what CR to have ARE set the motor to. My goal is to be on that top ten list.
Did you ever get my email?
01-04-2003, 07:14 PM
i really have no idea what the best ratio is and am reading everything i can find on the subject. just for reference
new 911 turbo 9.4-1
new lightning 8.4-1
new cobra 8.5-1
87 grand national 8.0-1
oh yea, harlen 8.5-1 <<< thats nearly reason enuff to go with 8.5 <img border="0" alt="[worship]" title="" src="graemlins/gr_hail.gif" />
01-04-2003, 07:29 PM
Reason enough for who, not me. A measly 700/715 <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" /> , (Yota) heeh.
I would imagine that the twins could spool fast enough with some low timing to have low end grunt on 8.5:1
Did Harlan ever post a dyno mpeg of that pull? Anyone have it if he did?
01-04-2003, 11:49 PM
I think with Intercooling you don't have to go down to the lower 8.5:1 C/R anymore unless you are planning on 15psi+.
9:1 should be enough for 8-12psi Intercooled. Actually 9.5:1 is probably still ok. 9:1 is extra safe.
01-05-2003, 12:33 AM
all of those cars listed above are intercooled however i am not.
i think more important than the amount of boost is the type of blower/turbo and when you will get that boost. i see as much as 5psi while using my cruze control in overdrive at a low 2000rpm, with that kind of boost so earily i think i am a good canadate for lower compresion.
you(in the future) from what i understand wont see hardly any boost unless you prety much floor it and from what i have been able to tell these engines are ok with alot of boost in the upper rpm's even with higher compresion. but since you wont see any boost most of the time it would be a shame to give up alot of performance where you do most of your driving just to get a little more up top.
i have been reading up on this subject and am just begining to form some oponions of my own, it would be nice to see some other people get in here and throw out some more info on the subject.
01-05-2003, 05:04 PM
low to mid boost(under 12psi) I think 9:1 is good for all types, I'm going 8.5:1 with my 400 cu in. Incon Twin 37's, over 12 or so I think 8.5:1 is safer, especially with turbo's if you can get them to spool to a high boost level in the 3,000 rpm range, before peak torque occurs/max. cylinder pressure you'll be more safe.
01-08-2003, 08:37 PM
parish8, sent you pm