Why lower compression?

[got_boost702]

Can anyone explain why when running forced induction it is best to run lower compression???

[BadMojo]

Can anyone explain why when running forced induction it is best to run lower compression???

Less chance of detonation with more boost...gooooood.

[Hijacker]

when you compress air, it gets hotter. That's why turbos run intercoolers. The same happens within the engine. So, when the piston begins the compression stroke, it compresses the air, heating it up. Deisel engines don't run with spark plugs, because they use high compression to ignite the fuel.

In short, a high comp piston compressing compressed air from a turbo will lead to detonation because it will get too hot.

[BadMojo]

In short, a high comp piston compressing compressed air from a turbo will lead to detonation because it will get too hot.

Hey, that was a hell of a lot more elegant than my explanation. I'm waiting to see how long before SCC blows up their turbo SVT Focus. That thing's got a static compression ration of 10.2:1 and you *know* they're gonna boost it 'til it blows.

[GlacierFreeze]

It's not best to lower compression with forced induction. You lose power.

It's only best if you're running pump gas on your daily driver. Now if you had race gas or alcohol/water injection then you wouldn't want to lower compression.

[Hijacker]

not many people add alcohol or water injection to their cars. much like how most stock ECU fuel maps run rich, it's a safety design. Lowering the compression for a street turbo car keeps every tom d!ck and harry out there from blowing their cars up before they get out the driveway

[BadMojo]

not many people add alcohol or water injection to their cars. much like how most stock ECU fuel maps run rich,

Not to mention, race gas is expensive and I don't relish repeated exposure to Toluene.

[Hijacker]

i could see pouring some race gas in my tank for a track day, but that's it. not on the street.

[Checkered-Member]

Why lower compression. The simplest answer will be to avoid pre ignition

pre ignition is when the air/fuel mixture ignites before its intended to…the worse will be while the piston is rising in the compression stroke. You end with blown head gaskets, blown pistons rings, bent rods, the works.

why would the a air/fuel mixer ignites before its intended to. If you took chemistry you will know this, if not…the simple example…peace of paper + magnifying glass + sunny day. You concentrate the rays of the sun, which causes the surface of the paper to heat up, and at one point it will ignite, same happens within the cylinder, if you heat up the fuel mixture to much (aka compress too much air & fuel) it will ignite…not good…So when you lower the compression you compress the air less, and pre-ignition is less likely to accrue

[GlacierFreeze]

I was just answering the question asked. I never assumed anything about a street car.

[TrunkMonkey]

lowering your compression ratio to run boost is old school. most people don't push enough power to justify having to go through the trouble of rebuilding for a lower compression ratio.

10-10.5:1 is the average compression ratio of most vehicles. that would be considered high back in the day, but not anymore. there are plenty of boosted cars running these compression ratios and living healthy lives.

-demetrius

[TrunkMonkey]

I'm waiting to see how long before SCC blows up their turbo SVT Focus. That thing's got a static compression ration of 10.2:1 and you *know* they're gonna boost it 'til it blows.

then it's gonna blow not because it's running 10.2:1 compression ratio, but because they're gonna push it to it's limits.

-demetrius

[got_boost702]

so if i was to raise the compresion on the sr20 to say 9.5-10.0 to 1 it would increase power and as long as i dont race around the streets everyday it would be a reliable street/track car.

[BadMojo]

then it's gonna blow not because it's running 10.2:1 compression ratio, but because they're gonna push it to it's limits.

-demetrius

I'm just curious, but what would happen if they get one tank of really crappy pump gas? I know that the WRX computer can switch to a conservative map when you've got some ****ty fuel, but is that really something that can be done on a custom turbo setup? Is there something out there for the SR folks or the KA-T folks that's advanced enough to do that?

Also, I guess it's just a margin of safety thing, but the Dodge SRT-4 has 8.1:1 and the Subaru WRX has static compression of something like 8.0:1.

And yeah, I'm sure SCC is gonna push that poor Focus to its limits and beyond.

Edit: To go one step further - what happens if you live in Southern CA, get a really bad tank of pump gas AND the ambient temperature is somewhere approaching the "fires of hell" level?

[Hijacker]

lowering your compression ratio to run boost is old school. most people don't push enough power to justify having to go through the trouble of rebuilding for a lower compression ratio.

10-10.5:1 is the average compression ratio of most vehicles. that would be considered high back in the day, but not anymore. there are plenty of boosted cars running these compression ratios and living healthy lives.

-demetrius

with EFI I would say 10-10.5:1 is on the very high side of normal. Most inline four cars I see today run somewhere in the 9.5-10:1 range.

and back in the old school days, 12:1 could be seen in some cars. But that was only because of the leaded gasoline.

[TrunkMonkey]

so if i was to raise the compresion on the sr20 to say 9.5-10.0 to 1 it would increase power and as long as i dont race around the streets everyday it would be a reliable street/track car.

i'm not telling anyone to raise their compression ratio. it's just that there's no need to lower it to have a reliable street car.

-demetrius

[TrunkMonkey]

I'm just curious, but what would happen if they get one tank of really crappy pump gas? I know that the WRX computer can switch to a conservative map when you've got some ****ty fuel, but is that really something that can be done on a custom turbo setup? Is there something out there for the SR folks or the KA-T folks that's advanced enough to do that?

crappy gas is crappy gas. think about it. what happens when you put bad gas in a completely stock engine...it doesn't blow up. it just runs like crap. why wouldn't the same thing happen in a boosted engine? even though it is a factor, the quality of the fuel is not as important as the amount (air/fuel ratio).

a bad tank of gas is not going to destroy an street car. if this was the case, then no one would bother to drive high perfromance cars since there's no way to tell the quality of fuel you're getting beforehand.

Also, I guess it's just a margin of safety thing, but the Dodge SRT-4 has 8.1:1 and the Subaru WRX has static compression of something like 8.0:1.

all factory turbocharged vehicles have low compression ratio, and yes it's a emissions/safety thing.

Edit: To go one step further - what happens if you live in Southern CA, get a really bad tank of pump gas AND the ambient temperature is somewhere approaching the "fires of hell" level?

i live in south carolina where the ambient temperatures in the summertime easily hit 100 degrees, i run 87 octane instead of the recommended 93 in my 240sx, and i drive 90+ miles an hour on the stretch of interstate between my house and work everyday....i did the same thing with my wife's oldsmobile aurora which had a compression ratio of 10:1 (or 10.5:1, i can't remember which).

-demetrius

[TrunkMonkey]

with EFI I would say 10-10.5:1 is on the very high side of normal. Most inline four cars I see today run somewhere in the 9.5-10:1 range.

inline fours aren't the only things on the street .

-demetrius