General question about boost
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From: Fresno, CA
General question about boost
This is the first turbocharged car I've ever owned. Lately, though, I've been wondering specifically how the turbo functions with regard to RPM. I don't have any guages on my Evo and so I'd like to know the answer to the following question: If the boost kicks in at about 3000 RPM during WOT acceleration, is one technically in boost if they are driving down the freeway at 80 mph (~3500 RPM)? I know that the harder you step on the gas during acceleration, the more spool you get. But if I did have a boost guage and were driving steadily at 80 mph, would I be showing zero psi? Or would it be a negligible amount?
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well, i think the answers here don't really explain anything though. there is a lot more to a turbo then being at 3500 rpm and assuming you have boost.
i would read this: http://auto.howstuffworks.com/turbo.htm and then start asking more detailed questions.
i would read this: http://auto.howstuffworks.com/turbo.htm and then start asking more detailed questions.
Thread Starter
Evolved Member
Joined: Dec 2004
Posts: 660
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From: Fresno, CA
well, i think the answers here don't really explain anything though. there is a lot more to a turbo then being at 3500 rpm and assuming you have boost.
i would read this: http://auto.howstuffworks.com/turbo.htm and then start asking more detailed questions.
i would read this: http://auto.howstuffworks.com/turbo.htm and then start asking more detailed questions.
The original purpose of my question was to find out whether boost is a function of throttle or RPM. Clearly, as stated in the above responses, it is solely a funtion of throttle. I confess that I was concerned about gas consumption at highway speeds. In other words, if cruising steady at 80 mph meant some boost (and hence more gas consumption) then it would be better to stay at or below 3K RPM while on the highway in order to save a little gas - that's all.
Fuel consumption at highway speed in a car with a relatively small turbo (like an EVO) is purely a function of your foot. The more gentle you are with go-pedal movement, the better your economy will be.
The original purpose of my question was to find out whether boost is a function of throttle or RPM. Clearly, as stated in the above responses, it is solely a funtion of throttle. I confess that I was concerned about gas consumption at highway speeds. In other words, if cruising steady at 80 mph meant some boost (and hence more gas consumption) then it would be better to stay at or below 3K RPM while on the highway in order to save a little gas - that's all.
you should be taking 3 things into account, engine load, throttle position, and mass air flow rate (sort of RPM, but not).
here are a couple examples:
say you are sitting at a stop light, and you floor it. you can make some boost (with the stock 2 step on a 9, maybe 5-8psi). in this case, throttle is wide open, and RPMs are relatively high (5000 is the stock 2 step). the problem is there is no load, thus the engine cannot maintain the mass air flow rate needed to keep the turbo spooled. the 2 step artificially creates a load allowing you to keep the throttle plate open. without the 2 step the only load is the inertia of the engine and the friction of all the bearings and belts.
case two, you are rolling along at 45 mph in 5th gear and floor it. in this case you have a very high load. the load is so high the car is very slow to accelerate. the load is created from poor gear ratios at that speed and the weight of the vehicle accelerating. lets say at 45 mph it is 2400 rpm (don't know off the top of my head). so now you have a wide open throttle, a high load, but very low mass air flow because your down on the RPM's. thus you have no boost (it would prolly read 0 gauge psi on a boost gauge if you floored it.
case three, you are rolling along at 80 mph in 4th gear and floor it. in this case you have a high load, wide open throttle, and a decent mass air flow rate. i would imagine it will be 5000ish rpm. thus the turbo will spool quickly and you'll be at max psi in no time at all.
case four, you are rolling along at 80 in 4th gear and lift off the throttle. in this case you have closed the throttle plate and basically eliminated the mass airflow rate, thus the turbo is not spinning very fast. even with the engine at 5000 rpm, the throttle plate controls the MAF rate. the boost gauge will read vacuum in this case.
it should be noted that with low loads (in early gears like 1-3) boost takes more RPMs before it is there even with wide open throttle. in the later gears, there is a much higher engine load because of the rear ratios. thus you'll build boost earlier. think of it like this:
in 1st you'll see max boost at 5000 rpm
in 2nd @ 4300
in 3rd @ 3600
in 4th @ 3200
in 5th @ 2900
these are purely example numbers but you get the picture. if you have a ton of weight in your car or are pulling a trailer you can reduce all those numbers. additionally, "Brake boosting" the process of modulating the brake to control speed while holding the throttle open will help you spool the turbo faster as well.
i hope this makes some sense... when you have a boost gauge you can watch it and see what it does and all of this will make more sense.
Thread Starter
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Joined: Dec 2004
Posts: 660
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From: Fresno, CA
its not entirely a function of throttle position either though.
you should be taking 3 things into account, engine load, throttle position, and mass air flow rate (sort of RPM, but not).
here are a couple examples:
say you are sitting at a stop light, and you floor it. you can make some boost (with the stock 2 step on a 9, maybe 5-8psi). in this case, throttle is wide open, and RPMs are relatively high (5000 is the stock 2 step). the problem is there is no load, thus the engine cannot maintain the mass air flow rate needed to keep the turbo spooled. the 2 step artificially creates a load allowing you to keep the throttle plate open. without the 2 step the only load is the inertia of the engine and the friction of all the bearings and belts.
case two, you are rolling along at 45 mph in 5th gear and floor it. in this case you have a very high load. the load is so high the car is very slow to accelerate. the load is created from poor gear ratios at that speed and the weight of the vehicle accelerating. lets say at 45 mph it is 2400 rpm (don't know off the top of my head). so now you have a wide open throttle, a high load, but very low mass air flow because your down on the RPM's. thus you have no boost (it would prolly read 0 gauge psi on a boost gauge if you floored it.
case three, you are rolling along at 80 mph in 4th gear and floor it. in this case you have a high load, wide open throttle, and a decent mass air flow rate. i would imagine it will be 5000ish rpm. thus the turbo will spool quickly and you'll be at max psi in no time at all.
case four, you are rolling along at 80 in 4th gear and lift off the throttle. in this case you have closed the throttle plate and basically eliminated the mass airflow rate, thus the turbo is not spinning very fast. even with the engine at 5000 rpm, the throttle plate controls the MAF rate. the boost gauge will read vacuum in this case.
it should be noted that with low loads (in early gears like 1-3) boost takes more RPMs before it is there even with wide open throttle. in the later gears, there is a much higher engine load because of the rear ratios. thus you'll build boost earlier. think of it like this:
in 1st you'll see max boost at 5000 rpm
in 2nd @ 4300
in 3rd @ 3600
in 4th @ 3200
in 5th @ 2900
these are purely example numbers but you get the picture. if you have a ton of weight in your car or are pulling a trailer you can reduce all those numbers. additionally, "Brake boosting" the process of modulating the brake to control speed while holding the throttle open will help you spool the turbo faster as well.
i hope this makes some sense... when you have a boost gauge you can watch it and see what it does and all of this will make more sense.
you should be taking 3 things into account, engine load, throttle position, and mass air flow rate (sort of RPM, but not).
here are a couple examples:
say you are sitting at a stop light, and you floor it. you can make some boost (with the stock 2 step on a 9, maybe 5-8psi). in this case, throttle is wide open, and RPMs are relatively high (5000 is the stock 2 step). the problem is there is no load, thus the engine cannot maintain the mass air flow rate needed to keep the turbo spooled. the 2 step artificially creates a load allowing you to keep the throttle plate open. without the 2 step the only load is the inertia of the engine and the friction of all the bearings and belts.
case two, you are rolling along at 45 mph in 5th gear and floor it. in this case you have a very high load. the load is so high the car is very slow to accelerate. the load is created from poor gear ratios at that speed and the weight of the vehicle accelerating. lets say at 45 mph it is 2400 rpm (don't know off the top of my head). so now you have a wide open throttle, a high load, but very low mass air flow because your down on the RPM's. thus you have no boost (it would prolly read 0 gauge psi on a boost gauge if you floored it.
case three, you are rolling along at 80 mph in 4th gear and floor it. in this case you have a high load, wide open throttle, and a decent mass air flow rate. i would imagine it will be 5000ish rpm. thus the turbo will spool quickly and you'll be at max psi in no time at all.
case four, you are rolling along at 80 in 4th gear and lift off the throttle. in this case you have closed the throttle plate and basically eliminated the mass airflow rate, thus the turbo is not spinning very fast. even with the engine at 5000 rpm, the throttle plate controls the MAF rate. the boost gauge will read vacuum in this case.
it should be noted that with low loads (in early gears like 1-3) boost takes more RPMs before it is there even with wide open throttle. in the later gears, there is a much higher engine load because of the rear ratios. thus you'll build boost earlier. think of it like this:
in 1st you'll see max boost at 5000 rpm
in 2nd @ 4300
in 3rd @ 3600
in 4th @ 3200
in 5th @ 2900
these are purely example numbers but you get the picture. if you have a ton of weight in your car or are pulling a trailer you can reduce all those numbers. additionally, "Brake boosting" the process of modulating the brake to control speed while holding the throttle open will help you spool the turbo faster as well.
i hope this makes some sense... when you have a boost gauge you can watch it and see what it does and all of this will make more sense.
its not entirely a function of throttle position either though.
you should be taking 3 things into account, engine load, throttle position, and mass air flow rate (sort of RPM, but not).
here are a couple examples:
say you are sitting at a stop light, and you floor it. you can make some boost (with the stock 2 step on a 9, maybe 5-8psi). in this case, throttle is wide open, and RPMs are relatively high (5000 is the stock 2 step). the problem is there is no load, thus the engine cannot maintain the mass air flow rate needed to keep the turbo spooled. the 2 step artificially creates a load allowing you to keep the throttle plate open. without the 2 step the only load is the inertia of the engine and the friction of all the bearings and belts.
case two, you are rolling along at 45 mph in 5th gear and floor it. in this case you have a very high load. the load is so high the car is very slow to accelerate. the load is created from poor gear ratios at that speed and the weight of the vehicle accelerating. lets say at 45 mph it is 2400 rpm (don't know off the top of my head). so now you have a wide open throttle, a high load, but very low mass air flow because your down on the RPM's. thus you have no boost (it would prolly read 0 gauge psi on a boost gauge if you floored it.
case three, you are rolling along at 80 mph in 4th gear and floor it. in this case you have a high load, wide open throttle, and a decent mass air flow rate. i would imagine it will be 5000ish rpm. thus the turbo will spool quickly and you'll be at max psi in no time at all.
case four, you are rolling along at 80 in 4th gear and lift off the throttle. in this case you have closed the throttle plate and basically eliminated the mass airflow rate, thus the turbo is not spinning very fast. even with the engine at 5000 rpm, the throttle plate controls the MAF rate. the boost gauge will read vacuum in this case.
it should be noted that with low loads (in early gears like 1-3) boost takes more RPMs before it is there even with wide open throttle. in the later gears, there is a much higher engine load because of the rear ratios. thus you'll build boost earlier. think of it like this:
in 1st you'll see max boost at 5000 rpm
in 2nd @ 4300
in 3rd @ 3600
in 4th @ 3200
in 5th @ 2900
these are purely example numbers but you get the picture. if you have a ton of weight in your car or are pulling a trailer you can reduce all those numbers. additionally, "Brake boosting" the process of modulating the brake to control speed while holding the throttle open will help you spool the turbo faster as well.
i hope this makes some sense... when you have a boost gauge you can watch it and see what it does and all of this will make more sense.
you should be taking 3 things into account, engine load, throttle position, and mass air flow rate (sort of RPM, but not).
here are a couple examples:
say you are sitting at a stop light, and you floor it. you can make some boost (with the stock 2 step on a 9, maybe 5-8psi). in this case, throttle is wide open, and RPMs are relatively high (5000 is the stock 2 step). the problem is there is no load, thus the engine cannot maintain the mass air flow rate needed to keep the turbo spooled. the 2 step artificially creates a load allowing you to keep the throttle plate open. without the 2 step the only load is the inertia of the engine and the friction of all the bearings and belts.
case two, you are rolling along at 45 mph in 5th gear and floor it. in this case you have a very high load. the load is so high the car is very slow to accelerate. the load is created from poor gear ratios at that speed and the weight of the vehicle accelerating. lets say at 45 mph it is 2400 rpm (don't know off the top of my head). so now you have a wide open throttle, a high load, but very low mass air flow because your down on the RPM's. thus you have no boost (it would prolly read 0 gauge psi on a boost gauge if you floored it.
case three, you are rolling along at 80 mph in 4th gear and floor it. in this case you have a high load, wide open throttle, and a decent mass air flow rate. i would imagine it will be 5000ish rpm. thus the turbo will spool quickly and you'll be at max psi in no time at all.
case four, you are rolling along at 80 in 4th gear and lift off the throttle. in this case you have closed the throttle plate and basically eliminated the mass airflow rate, thus the turbo is not spinning very fast. even with the engine at 5000 rpm, the throttle plate controls the MAF rate. the boost gauge will read vacuum in this case.
it should be noted that with low loads (in early gears like 1-3) boost takes more RPMs before it is there even with wide open throttle. in the later gears, there is a much higher engine load because of the rear ratios. thus you'll build boost earlier. think of it like this:
in 1st you'll see max boost at 5000 rpm
in 2nd @ 4300
in 3rd @ 3600
in 4th @ 3200
in 5th @ 2900
these are purely example numbers but you get the picture. if you have a ton of weight in your car or are pulling a trailer you can reduce all those numbers. additionally, "Brake boosting" the process of modulating the brake to control speed while holding the throttle open will help you spool the turbo faster as well.
i hope this makes some sense... when you have a boost gauge you can watch it and see what it does and all of this will make more sense.
get a boost gauge OP, u need it. if u don't have the money right now get a cheap one like Autometers, $40 or something...monitoring boost on a turbocharged is very important.