Hey guys, for the last..forever:confused: ive been looking for the answer to the question, "why is it better to run a lower compressino with turbo?" Betwwen the USDM 9.5:1 to the JDM 10:1 and the BB 8.5:1. well the answer has to do with the (FCR)Final Compression ratio. there is a forumula to figure out FCR, it is: FCR = [(Boost/ 14.7) +1] x CR
Note: 14.7 is the atmospheric PSI at sea level.

So if you take a stock BB det with 8.5 compression running at 6psi stock: FCR =[6/14.7)+1]x8.5 Your FCR will be 11.96, or 12.0. This is the compression in the combustion chamber. but if you take the same PSI on a 10:1 ening your FCR will be about 14.
Now, the more compression you have = better compustion = more power. BUT* with higher compression you risk detonation (see bottum of post about detonation). So, even though you can make more power out of less boost in a higher compression engine, it is ideal to have a lower compression engine and run higher boost to compinsate. Also, with the higher compression engines there is a compression ceiling (no real set number) once oyu exceed this ceeling you risk heavy damage to your engine. So in the lower compression 8.5BB engine with NO ceeling you can exceed the compression far great then you can in a high compression engine with turbo....ami making any sense, am i on the right track?
----
DETONATION
BAsically detonation is when your engine gets realy hot and it no longer needs the spark plugs to combust, and you will get a "ping" sound, coming from the actualy engine combusting with itslef (internal combustion) and the spark plugs trying to cumbust. now there is good detonatoin, this is in diesel engines...but thats a differnt story right now :) our detonation is bad...very bad you dont want your engine dong the spark plugs work, right? again..correct me if im wrong.

The worst part of detonation is because the pre ignition happens when the piston is on its way up. This puts ungodly ammounts of stress on the motor.. Plus the heat with it.


Danny

Originally posted by xTurbo Classicx
Well the answer has to do with the (FCR)Final Compression ratio. there is a forumula to figure out FCR, it is: FCR = [(Boost/ 14.7) +1] x CR
Note: 14.7 is the atmospheric PSI at sea level.

So if you take a stock BB det with 8.5 compression running at 6psi stock: FCR =[6/14.7)+1]x8.5 Your FCR will be 11.96, or 12.0. This is the compression in the combustion chamber. but if you take the same PSI on a 10:1 ening your FCR will be about 14.
Now, the more compression you have = better compustion = more power. BUT* with higher compression you risk detonation (see bottum of post about detonation). So, even though you can make more power out of less boost in a higher compression engine, it is ideal to have a lower compression engine and run higher boost to compinsate. Also, with the higher compression engines there is a compression ceiling (no real set number) once oyu exceed this ceeling you risk heavy damage to your engine. So in the lower compression 8.5BB engine with NO ceeling you can exceed the compression far great then you can in a high compression engine with turbo....ami making any sense, am i on the right track?


Well, mathematically it looks right. But I think the compression ceiling is measured against the FCR NOT the engine compression ratio itself. Whether you run hight boost on a low engine compression ratio OR low boost on a high engine compression ratio, either way the FCR is the final figure that resulting stress on the engine and subsequently, possible detonation. For instance, I could run 10 psi on the 9.5:1 CR and result in a FCR of 16 or 12.9psi on the 8.5:1 CR for the same FCR. The compression ceiling is the FCR. If what you are saying is correct, the more FCR the better hp output, then it is this figure that everyone is putting it to the limit, if not compression ceiling (prior to detonation). The turbo is to vary the boost level, as already known.

i dont think you can actually determine the compression ceiling, you have yo kinda....guestimate:confused: , but i do understand that on a higher compression engine, the same boost ex, 6psi will have a better effect then on a lower compression engine. but higher compression engines have much lower celings then low compression engines :-)

Yeah. In really basic terms, the reason lower CR is better on turbo motors is because it allows you to run more boost without detonating and therefore more than makes up for the loss of HP from the reduced compression. Going from 9.5:1 to 8.5:1, you lose maybe 10 HP. But being able to run 16 psi vs. 12 psi on the street using the same pump gas you always have... well there's another 50 wheel HP for you. :)

And to be clear, preignition and detonation are two different things, but preignition can and will lead to detonation. The hotter the cylinder gets, the closer to the detonation line you get. Preignition is caused when the combustion chamber gets so hot that the fuel ignites before the spark fires. This causes stress because when it fires early like that, the piston tries to turn the crank in the opposite direction. Really bad for your motor.

Detonation is caused when the air/fuel mix autoignite (again, from too much heat) and produces multiple flame fronts in the chamber. When they collide, it produces a massive pressure front that's moving at the local speed of sound. The "pinging" sound (usually called pinging, or knocking) you hear when you're detonating is actually that pressure front slamming into the cylinder walls, the head, the pistons or the rings at the speed of sound. The impact is so intense that just hitting the cylinder wall causes a metallic pinging sound loud enough for you to hear in the cabin of the car, with the engine running (obviously). That sort of impact will actually take chunks out of whatever it hits, and if allowed to continue will eat through your piston rings, eat into the cylinder walls and perforate your head.

In other words, detonation is very bad. If you ever hear pinging, immediately do something to fix it. You can retard your timing and lower your boost level right off the bat, and in the long run you can lower the CR, do something to cool your intake temps even more (such as a better intercooler and water injection - both of those will lower intake temps, which is critical for keeping the combustion chamber temps down), run higher octane gas. And actually if you're just barely detonating, keeping the car at higher RPMs will probably prevent detonation as well, at least until you can get to a safe spot to pull over. The reason for that is because at higher RPM, the ignition sequence happens faster and there is not enough time for autoignition to occur before it cycles over again.

For that reason, you're more likely to get detonation at lower RPMs and loading the engine (lugging up a hill in 5th) than you are at higher RPMs (less load and faster ignition).

well spoken.....

finally im edjumackated on this topic! i feels so good to understand things! :-)