Sorry if I'm getting annoying with all these question posts, I'm just trying to learn as much as I can and I always get great responses here.
It is to my understanding that when you turbo a car, the more PSI you're running, the more unreliable it'll be. To help reliability, the solution is to lower the compression, which also lowers the power output, right?
If I'm right so far, than if you were making 220 horsepower on 10:1 compression at 9 PSI, and you lowered compression to 9:1, wouldn't you have to turn up the PSI to say 12 to make the same 220 horsepower? If so, doesn't this make the car just as unreliable as if you just stayed at 9 PSI with 10:1 compression?
I know this question is silly, but I'm just trying to understand and learn a few things. Keep in mind all these numbers are just for demonstration purposes, and I'm not basing them off of anything at all. Thanks.
lowering compression is to prevent detonation under boost. yes it lowers HP but only a few....like 3 or 4. so the HP gains of raising boost with lower compression outweigh the 3 to 4 HP loss.
the unreliable part depends on the bulid quality of the motor.
Lowering the compresion does not make it more reliable. It just allows for higher boost before you need race gas. It makes it less likely to detonate. There are many other things you can do to keep detonation at bay other than lowering compression.
FORGET GIRLS GONE WILD WE HAVE GOVERNMENT SPENDING GONE WILD!
Ron Carvalho wrote:Sorry if I'm getting annoying with all these question posts, I'm just trying to learn as much as I can and I always get great responses here.
It is to my understanding that when you turbo a car, the more PSI you're running, the more unreliable it'll be. To help reliability, the solution is to lower the compression, which also lowers the power output, right?
If I'm right so far, than if you were making 220 horsepower on 10:1 compression at 9 PSI, and you lowered compression to 9:1, wouldn't you have to turn up the PSI to say 12 to make the same 220 horsepower? If so, doesn't this make the car just as unreliable as if you just stayed at 9 PSI with 10:1 compression?
I know this question is silly, but I'm just trying to understand and learn a few things. Keep in mind all these numbers are just for demonstration purposes, and I'm not basing them off of anything at all. Thanks.
<cracks knuckles>
to understand why we lower compression for boost, you have to understand some basic principals of how gases behave under pressure.
ideal gas law states that all other variables constant, if you increase the pressure of a gas, its temperature also rises (due to friction between atoms being forced closer together... like rubbing your hands together. the harder you push your hands together, the hotter they get)
an enine working on its own squeezes the incoming air at whatever its set at.. usually referred to as compression ratio. When you put a compressor in front of it (in this case, a turbocharger) the air the engine is injesting is already compressed... which is why power output is increased so dramatically... it's squeezing already compressed air. You're essentially fitting (in essence.. no real calculation here) 20lbs of air in a 5lbs bag. Get it?
now... whats the difference between a 10:1 engine on 9psi making 220hp and a 9:1 engine on 12psi making 220hp?
not much.. other than detonation resistance. and as you push PSI higher, you'll quickly see what the real reason for it is.
at lower boost levels, the advantages aren't as obvious, but once you start to push higher it quickly comes into focus. air compressed by the turbo can be intercooled, which even though the air is pressurized, it gets cooled off back to almost normal intake temperature. This cooler air charge, once compressed by the engine has a much lower probability of causing detonation.
thus, you can run lower grades of fuel (premium, 92-93 octane is the benchmark) without detonating.
the other engine however, will hit a limit on pump gas, and require a highergrade of fuel in order to avoid detonation.
assuming you're sticking to pump gas, an 8.5:1 compression engine needs more boost to make the same power yes, but has the potential to make a LOT more power than a 9:1 or 10:1 engine without using a higher octane.
and its not very convenient to fill up with C16 or 102octane from the racetrack on a daily basis.. especially if you're trying to build a car to be driven everyday.
also, compression ratio doesn't effect reliability... cutting corners and pushing an engine beyond its limits is what makes it unreliable. You can have a perfectly reliable 500hp or more car so long as you build it to withstand the rigors of such.
Thank you whiteboyz24, Wade, and especially PJ. You could write a book with what you explain to me
I appreciate all of you responses very much, thank you.
so i'm dusting this one off to hopefully get my question answered by the hands of PJ, the ecogod....
Typically how much does 9.5-10 psi raise your compression ratio?
I'm going to be running a GT3071R on my 2.4 hybrid (086 head) before the turbo goes on the final compression will be in the area of 9.9-10.1:1
running 9.5 psi, how much should that raise my copmression by, and would i be able to get away with 91 - 94 octane?
Thrice . wrote:so i'm dusting this one off to hopefully get my question answered by the hands of PJ, the ecogod....
Typically how much does 9.5-10 psi raise your compression ratio?
I'm going to be running a GT3071R on my 2.4 hybrid (086 head) before the turbo goes on the final compression will be in the area of 9.9-10.1:1
running 9.5 psi, how much should that raise my copmression by, and would i be able to get away with 91 - 94 octane?
You could get by with it. but that turbo is going to want to be in the 15-25 psi range on your LD9. I saw it happen on Speedracers car with the gt 35. It was fast but it wanted to go up a bit more into its efficency range.
I would suggest a turbo that matches the motor better if you are planning to run the 086 head and a High CR.
any suggestions of what turbo would more suitable match my build?
I was having a chat with datwhitecav a while back and he used gt3071r and was running around the same psi with no troubles. I'm aiming for 400whp but i'll be happy with 350
