Pounds vs Horsepower
What I don't understand is why people come on and say I want to make X amount of pounds of boost instead of a horsepower goal. Who cares if you are making 5lbs but have 600 HP!!, My car which is a 95 LT made 648 RWHP on Pump with only 12lbs and 722 RWHP on 14Lbs with the Fast Gas, This was with the D1...and all by 6000 RPM
Aint I a stinker
Somebody who asks for 15lbs probably has some sense of that being _roughly_ double the horsepower they have now. They may not care that it's 450 or 500 or 550 or 622.81 hp. Double the power is good enough... 
10 lbs of boost measured in the intake of a given engine combo, operating at a given rpm and load, is the same airflow no matter what turbo it happened to come out of.
(as long as the turbo can make 10 lbs throughout the operating range of the engine, and without superheating the air while producing that boost)
There is a wastegate which is throttling the turbo. The turbo is allowed to spin just fast enough to make 10 lbs of boost. It doesn't matter if it's a teeny turbo spinning faster, or a bigger turbo spinning slower, it's moving the _exact_ amount of air that it takes to build up a 10psi backlog in your intake.
10 lbs of boost measured in the intake of a given engine combo, operating at a given rpm and load, is the same airflow no matter what turbo it happened to come out of.
(as long as the turbo can make 10 lbs throughout the operating range of the engine, and without superheating the air while producing that boost)
There is a wastegate which is throttling the turbo. The turbo is allowed to spin just fast enough to make 10 lbs of boost. It doesn't matter if it's a teeny turbo spinning faster, or a bigger turbo spinning slower, it's moving the _exact_ amount of air that it takes to build up a 10psi backlog in your intake.
Staging Lane
Joined: Jul 2008
Posts: 51
Likes: 0
It doesn't matter if it's a teeny turbo spinning faster, or a bigger turbo spinning slower, it's moving the _exact_ amount of air that it takes to build up a 10psi backlog in your intake.
if some of you were asleep during high school physics, the ideal gas law states pv=nrt. p (pressure) remaining constant while t (temperature) of the air coming from the turbo can STILL fluctuate and affect mass of air
The volume is the same, the n (mass) of the air changes with temperature though.
I don't understand why people come on and say I want to make X amount of HP on a dyno instead of having an ET or MPH goal. Who cares if you are making 500 rwhp but only run 12s!!, My car which is a 00 LS1 ran 10.5 with only 470 RWHP and 10.1 with 510 RWHP with race gas, This was with the T76... and all by 6000 RPM
Aint I a stinker
Aint I a stinker
are you serious? you're telling me that if i strapped a stock JB buick GN turbo to my motor an ran it at 15psi working that turbo to death putting out 180* air AFTER the intercooler is the same AMOUNT of air coming in if I got a GT35R and ran it at 15psi and the air was only 140* coming out after the intercooler? 15psi of HOT air is NOT 15psi of COLD air.
That previous sentence was
"(as long as the turbo can make 10 lbs throughout the operating range of the engine, and without superheating the air while producing that boost)"
The point I was making, is that putting an oversized turbo on an engine, but maintaining the same boost level, is not going to magically shove more air through the engine.
Last edited by John_D.; Jul 30, 2008 at 10:52 AM.
Staging Lane
Joined: Jul 2008
Posts: 51
Likes: 0
Then what, exactly, was the purpose of your statement other than reiterating the obvious?
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
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Yes, mass flow rate is what amounts to the hp you make. If I remember correctly I think turbo engines make about 10 hp for each lb/min of airflow.
Then what, exactly, was the purpose of your statement other than reiterating the obvious?
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
There's a lot of things that get said over and over again, some with a grain of truth, some not really based on truth. One of those things I see come up on here is that a bigger turbo always pushes more air at the same psi, and it begins to be interpreted as a truth just because of the repetition.
I have a turbo making 12 psi at practically ambient temp after the intercooler. Putting a bigger turbo on my setup, and capping it at 12 psi would not push more air into my engine.
Sometimes I will take a moment to pipe up, to clarify things like that. It may be obvious to me and to you, but it's not obvious to 100% of the other posters on here. And what they're posting may lead others astray too.
Then my statement is to give the other side and some clarification, and people can make up their own mind about what makes sense instead of just hearing one side and without clarification.
In my original post I took the time to make a completely separate parenthetical point to make sure nobody got sidetracked on the heat issue that occurs if starting with a clearly undersized turbo. It would be a pointless comparison to start there, and I made an effort to be sure people didn't start there. You blew right past that and did exactly what I made an effort to head off. I assumed it was intentional in order to be able to say my point was wrong. Perhaps you just overlooked it.
A big enough turbo and a bigger turbo are the basis for my statement.
TECH Fanatic
Joined: Nov 2004
Posts: 1,251
Likes: 0
From: Tuscaloosa, AL.
Then what, exactly, was the purpose of your statement other than reiterating the obvious?
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
And conveniently overlooking insinuates intent, there was none.
I think it's just ambiguous to make a statement that a little turbo and big turbo move the same amount of air and not address the air temperature issue. That's the entire reason they produce different amounts of power. Scientifically speaking it doesn't make sense (to me) to compare two things and hold constant the significant reason they're different.
I believe you were refering to a smaller turbo has the capacity to produce the desired boost but in order to do so must spin at higher than optimum RPM. In doing so it creates excess heat due to the friction which results in higher iat's. This causes the air to heat up which causes it to expand (become less dense) which results in a decreased amount of oxygen forced into the motor which results in less power.
Is this correct? Still trying to wrap my head around this whole FI game.
Actually it could. Youu are ignoring the turbine side which can have a very large effect on over all cylinder filling due to having less pumping losses / cylinder filling dilution.
It's hard to compare two highly different combos when talking about boost and HP. 15 psi with a PT106 and a 408ci would be very different than a GT47-88 and a 348ci.
But if you are looking at two highly similar combos, you can generalize to some extent though if you have two engines with two different turbos the power output at a given boost level will be different.
I'm comfortable with the idea that 15 lbs of boost is double the atmosphere, so if you have an engine putting out 400 crank hp, 15 psi could be 800hp... +/- due to variables like exhaust, converter, turbo size, how good the cylinder heads are, intake manifold...etc.
But if you are looking at two highly similar combos, you can generalize to some extent though if you have two engines with two different turbos the power output at a given boost level will be different.
I'm comfortable with the idea that 15 lbs of boost is double the atmosphere, so if you have an engine putting out 400 crank hp, 15 psi could be 800hp... +/- due to variables like exhaust, converter, turbo size, how good the cylinder heads are, intake manifold...etc.
A side effect of the bigger turbo (but only if the exhaust side is bigger too, not just a bigger wheel on the cold side) is it can improve the overall VE of the engine. In that case, the engine does need more incoming airflow in order to still be presented with a 10 psi backlog.
And a bigger turbo allows you to extend the usable rpm range of the engine. Which comes back around to my "limiting" statement that was meant to clarify the same engine combo (and operating range) and not dealing with an undersized turbo to start with.
Of course, this whole thought process just leads us right smack into all those factors (and compromises) in choosing a turbo. Will it take too long to spool? Will it choke off the exhaust on the top end? Will the intake temps go through the roof? Will it fit? All those specifics that come into play when choosing the ideal turbo for your application.
Just because a turbo is 50% bigger doesn't mean you'll get 50% more airflow/power/whatever -- at the same psi. If the turbo was too small and choked off the exhaust, or generated 180+ temps, sure. If the turbo was already a good match for your setup, then going bigger provides little or no gain and just causes packaging, spooling, and budget issues. But going bigger _definitely_ gives you overhead if you want to be able bump the "psi" later, or extend the rpm operating range of the engine.
No, that is not correct. All the head units I know of have a know CFM flow rate. You can easily calculate the amount of air generated at a specific RPM based on the head unit, blower and crank pulley, and engine RPM. A boost gauge is just a simple indicator of the air that is not making it into the engine.
Keith
Keith
