I've come to understand that manufacturers have their turbo charged engines CR at like 8.5 and then run like 20psi of boost. For people with NA engines that weren't originally turbo'ed, they'll take an engine with a 9.5:1 CR and reduce it to like 8.5 and from what I heard, as low as 7:1! One thing some people do is lower the boost in order to maintain the stock compression ratio. So I have to wonder, wouldn't the decrease in CR negate the benefit of running more boost? If not, why doesn't someone run like 5:1 CR and then 100psi of boost or something like that? It just seems like using a turbo and then decreasing the compression ratio is just trading one thing for another but maybe I'm wrong cause I really don't know.

 

reduces knock limit on small bore engines, aka they can make more power as there is more room in the combustion chamber for boost.

that's what I'm reading on the low comp pistons.

 

basically knock limit.

the lower the compression, the shittier the gas you can use

 

LMFAO!!!!!!!

 

mmmm, i love boosting on ARCO gas

 

it all depends on what kind of power you want to make on what kind of gas. there are guys with 11 to 1 turbo motors but I would not wanna push one past 350whp on pump, and there are guys with high 400whp motors on 9 to 1 comp that do just fine. I think about 9 to 9.5 is a good median between too low and to high of comp.

 

also depends on the fuel type.... (as alluded to above)

there are plenty of ppl with high CR that run lots of boost. many recipes for big power.

 

I'm kind of new to forced induction, but it appears that the only reason to reduce your compression ratio is if your tuner stinks or you're trying to make more power than what you can. A lower compression ratio would let you ram more air and fuel in without compressing it to the point it preignites. With a given turbo, intake, etc, let's say you can make 250hp at 5lbs of boost on a 11:1 motor without getting preignition. Reducing it to 10:1 may let you run 10lbs and make 300, but you'll have to wait for the turbo. These are made up numbers and meant only for explaining the concept.

The reason you don't just run like 5:1 with tons of boost is it will be a total dog out of boost, and it will take forever to spool the turbo enough to get enough boost to make power.

 

Your not getting the concept fully, go read a book on turbocharging. oem's reduce compression for a reason when they turbo cars.

compression plays a smaller roll in turbo spool up then your giving it credit for. Yes it helps but i would say the engine pumping away at 3k/4k/5k rpm would be more help then the minor bump in hp out of boost. (see hp increases when n/a, they help but don't raise hp by leaps and bounds)

but it is almost 100% due to knock limit. think about it the s2000 running 11:1 comp is more prone to bad gas then say a 8:1 engine. so there is the benefits, plus when a turbo is at 70,000 ish rpm (making 5 psi) how hard is it to spin up another few thousand rpm to make 10 psi. not that hard.

 

No magical tune will raise the octane of a fuel. A good tuner just skirts closer to the limit ALL OVER the ignition map. In other words, more average timing for the same amount of total knock counts.

93 octane usually can't support much over 20:1 dynamic compression, IE static compression multiplied with boost. You'd get ~20:1 with a 10:1 engine using 14.5psi of boost [29psi absolute], so (10*(29/14.5)=20), for a very rough example. With FI, boost is the major factor in finding dynamic compression, but there are other factors.

All that a higher compression ratio will do is increase the production of power from the same amount of air. More boost just adds more air, so it's more of a brute-force attempt. For almost all turbo engines, gains from a point of compression is usually much less than a few more PSI of boost. At the boost levels many Honda people run, a few more PSI may actually increase their turbo's efficiency. One little example would be a 57trim and an LS/vtec. At 7psi (85% VE, 8k rpm), the calc below gets you near 73% at redline; 16psi gets you on the 78% "efficiency island". As far as spooling faster, the only test I've ever seen (stock 10:1 bottom end swapped for 9:1 built) showed the exact same spool up, but the higher-CR block made a few % more power at the same boost level. The calculator shows 310cfm at 7psi, while 16psi flows 420cfm, or 35% more. Do you think 1-2 points of compression can bump power up by 35%??



BTW this is an older calc I know has seen use in the past with some NA people:
http://www.rbracing-rsr.com/comprAdvHD.htm
This is the turbo map calc:
http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

 

good post.