The compression in my 96 Volvo 850 Turbo is 8.5 to 1. When the turbo kicks in I would think the compression would go up. Is this correct? Would I benefit from using high octane fuel?
Turbo Compression Question.
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Your theoretical, mechanical compression is 8.5:1, i.e. the cylinder volume is 7.5 times the combustion chamber (8.5 volumes being squeezed into 1).
Running compression will be less than that depending on whether it completely fills the cylinder or not, or overfills it due to tuning etc.
With a turbo, you are making the engine think that it's operating in an environment that's at a higher atmospheric temperature.
So if you have 14.5lb boost at sea level, you are (basically) getting 2 lots of air in, and compressing it 8.5 times, (theoretically 17:1).
However, turbos heat air, and the cumulative total is never the simplistic additive of the two.
Unless there's very low boost, higher octane fuel is probably a good thing.
Running compression will be less than that depending on whether it completely fills the cylinder or not, or overfills it due to tuning etc.
With a turbo, you are making the engine think that it's operating in an environment that's at a higher atmospheric temperature.
So if you have 14.5lb boost at sea level, you are (basically) getting 2 lots of air in, and compressing it 8.5 times, (theoretically 17:1).
However, turbos heat air, and the cumulative total is never the simplistic additive of the two.
Unless there's very low boost, higher octane fuel is probably a good thing.
14.7 static @sea level?ya? How many PSI does the guage read "on the turbo"in boost?
The book says 87 minimum, but use premium for best performance.
The gauge doesn't show psi. It just moves further into the boost side.
I'm not sure if the book shows max boost or not.
I'm not sure if the book shows max boost or not.
Yes, higher octane fuel if the engine is in a good state of tune. And some 5w-40w LL A3/B3-4 oil too. Compression ratio always stays the same - the cyl pressure will vary. It does on a normally aspirated engine and moreso on a turbo where the boost can raise air pressure above 1 bar. There is no such thing as a vacuum perse - its the air pressure around you "pushing" the air into the engine. Take a small lidded jar with nothing in it, and open it under a bath of water and see the water run in. This is how your engine is filled.
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Originally Posted By: ARCOgraphite
There is no such thing as a vacuum perse - its the air pressure around you "pushing" the air into the engine.
Why do boost gauges that also measure vacuum exist then? I have seen a lot of dyno videos with the camera focused on the boost gauge and it is always in vacuum until the vehicle acclerates (if the vehicle in question is super or turbocharged). Also, I have heard the analogy that engines are just big vacuum pumps. Also, a lot of braking systems use vacuum to operate. There are ignition systems that have vacuum advance which change the timing of the spark based on the amount of vacuum.
There is no such thing as a vacuum perse - its the air pressure around you "pushing" the air into the engine.
Why do boost gauges that also measure vacuum exist then? I have seen a lot of dyno videos with the camera focused on the boost gauge and it is always in vacuum until the vehicle acclerates (if the vehicle in question is super or turbocharged). Also, I have heard the analogy that engines are just big vacuum pumps. Also, a lot of braking systems use vacuum to operate. There are ignition systems that have vacuum advance which change the timing of the spark based on the amount of vacuum.
all those gauges work relative to normal atmospheric pressure!
We need to go back to physics class.
Engines really are big pumps. They create a region of lesser pressure and atmospheric pressure pushes air in.
The word vacuum is relative, as it is virtually impossible to create a real vacuum.
We need to go back to physics class.
Engines really are big pumps. They create a region of lesser pressure and atmospheric pressure pushes air in.
The word vacuum is relative, as it is virtually impossible to create a real vacuum.
So a turbo would have the same added affect as higher compression when dealing with octane in fuel. My inlaws, who gave us the car, always ran 93 octane and i've been running 87 just because of its 8.5 to 1 compression. The more I thought about it the more i thought the 93 may give better power and mileage.
I wonder if the car as a sensor to determine octane in the current fill.
I wonder if the car as a sensor to determine octane in the current fill.
In a gasoline engine, vacuum is being pulled when the throttle is not fully open while the turbo hasn't produced boost yet.
A sensor to detect octane? Most modern cars have a knock sensor which retards timing, and/or reduces turbo boost once the engine produces knock.
A sensor to detect octane? Most modern cars have a knock sensor which retards timing, and/or reduces turbo boost once the engine produces knock.
93 octance will not give better fuel mileage. Octane rating is an anti-knock rating and 93 octane fuel contains marginally higher if any BTU/gallon.
My car runs a 8.5:1 compression ratio and recommends 91 octane because of the supercharger.
Also the knock sensor will advance timing, not retard it.
My car runs a 8.5:1 compression ratio and recommends 91 octane because of the supercharger.
Also the knock sensor will advance timing, not retard it.
Originally Posted By: iunderpressure
So a turbo would have the same added affect as higher compression when dealing with octane in fuel.....
Similar effect of higher comp ratio - but we are talking about cylinder pressure not static compression ratio that is fixed, mechanical and non-variable.
So a turbo would have the same added affect as higher compression when dealing with octane in fuel.....
Similar effect of higher comp ratio - but we are talking about cylinder pressure not static compression ratio that is fixed, mechanical and non-variable.
I would use the 89 octane. I allway`s have good results with that. (on cars that use 87)
Originally Posted By: SVTCobra
93 octance will not give better fuel mileage. ...
Also the knock sensor will advance timing, not retard it.
These assumptions are incorrect on a boosted engine in which the KS is retarding timing. Most k sensor applications do indeed retard timing.
93 octance will not give better fuel mileage. ...
Also the knock sensor will advance timing, not retard it.
These assumptions are incorrect on a boosted engine in which the KS is retarding timing. Most k sensor applications do indeed retard timing.
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Originally Posted By: artificialist
In a gasoline engine, vacuum is being pulled when the throttle is not fully open while the turbo hasn't produced boost yet.
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IN a physics sense (which is the real mechanism) The Engine isnt "pulling a vacuum", the cylinder pressure is lower than atmospheric, which is a system not in stasis, and the air molecules will fill the cylinders when the gate is open (throttle) like bestbuy shoppers on black friday. Again look to the jar under water example. Think of Air as fluid like water and the systems "under the air blanket" are tending toward a non-differential. All the nooks an crannies that are not a 1 bar (@sealevel) will tend to be filled with molecules until they are the same. Opening the throttyle just lets the air rush in - air that was previously "pushed' out during the exhaust stroke - An empty jar if you will. When the throttle is "wide open" the engine will have no air pressure delta and the vacuum gauge will read zero.
In a gasoline engine, vacuum is being pulled when the throttle is not fully open while the turbo hasn't produced boost yet.
...
IN a physics sense (which is the real mechanism) The Engine isnt "pulling a vacuum", the cylinder pressure is lower than atmospheric, which is a system not in stasis, and the air molecules will fill the cylinders when the gate is open (throttle) like bestbuy shoppers on black friday. Again look to the jar under water example. Think of Air as fluid like water and the systems "under the air blanket" are tending toward a non-differential. All the nooks an crannies that are not a 1 bar (@sealevel) will tend to be filled with molecules until they are the same. Opening the throttyle just lets the air rush in - air that was previously "pushed' out during the exhaust stroke - An empty jar if you will. When the throttle is "wide open" the engine will have no air pressure delta and the vacuum gauge will read zero.
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This reminds me of the guys who always wanted me to add a larger throttle body or carburetor to their stock "sportscar" for "more power" and to "get more air in". I always said NO and tried to explain that the engine was getting all the air it needed. If you put a "vacuum" gauge on it, as long as it reads zero at RPMS up to say 4500 (on a 6000rpm redline) when the throttle is wide open, you are getting all the air in you can get - naturally aspirated. Then they go off and do it anyhow
Talking with a person I work with that use to be a mechanic, he says that if the knock is because of low octance fuel, it will advance the timing. If the knock is because the timing is too far advanced to begin with, then obviously the timing will be retarded. With an electronically controlled engine, I seriously doubt that the timing is too far advanced to begin with. If this doesnt jive then give a detailed explanation as to why it is incorrect.
Knock sensors retard timing. They actually 'listen' to the noises.
the engine control software tries to advance timing to its designed limits, and the timing is pulled back if knock is detected.
And higher octane fuel is simply harder to ignite. No more power at all. If your car is stock boost then run the suggested octane. If you raise the boost then you can data log and monitor the KS values. This will tell you if you need more octane.
the engine control software tries to advance timing to its designed limits, and the timing is pulled back if knock is detected.
And higher octane fuel is simply harder to ignite. No more power at all. If your car is stock boost then run the suggested octane. If you raise the boost then you can data log and monitor the KS values. This will tell you if you need more octane.
Why would it retard the timing? As the compression stroke is occuring and the piston is getting closer to TDC and the fuel ignites without the spark, then wouldn't the timing needed to be advanced so the spark plug shoots a spark before the air/fuel mixture ignites on its own? I would think that if the timing was retarded then it would continue to knock unless the timing was the cause of the knock. This is the same for NA or forced induction engines.
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