Been looking for the answer but haven't found it, so at the risk of starting something, I'll ask it here. Assuming an efficient air intake system and a good free-flowing exhaust, on the 3800 series II engine, how much turbo boost pressure is needed to make a given HP number? Yes there will be an intercooler. Is there a formula somewhere, or is it more of a real-world experience thing? I'm really just looking for something general, I know every engine/tune/setup is different and will make different amounts of power, but there has to ba a starting point somewhere.
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11:25 AM
PFF
System Bot
BV MotorSports Member
Posts: 4821 From: Oak Hill, WV Registered: May 2001
Only part of the exhaust that needs to be free flowing is after the turbo. Dump it as fast as you can.
mostly stock setup, 12psi ~400hp
I am running 11 psi of intercooled boost on 93 octane gasoline. I presume that your set up must also be intercooled . It was hell trying to reprogram the PCM to control the KR's without one. Now the timing is back, the fueling is in a more reasonable 11.9 WOT range and everything seems in order.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
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02:14 PM
nosrac Member
Posts: 3520 From: Euless, TX, US Registered: Jan 2005
Boost pressure is kind of "cheating" to get power. Don't get me wrong, I LOVE boost! I really do, however it takes a lot more than just PSI to make power. You can increase the PSI all you want, but if you don't have the flow in your intake system to support the amount of air needed for a given HP, you will just stress the turbo without really any gains.
Basically, when you turbo mod, look into both the PSI, and free flowing intake and exhaust setups. It should flow freely pre and post turbo, and the turbo itself acts as somewhat of a muffler. The real thing you want to look for is lbs of air per given HP amount. If you can get more air squeezed into the cylinders by combining scavenging effects with higher boost pressures and smooth flow patterns... you can get much more HP per PSI of boost. I plan on making ~600 crank HP with twin intercooled turbos running @ 10psi on a 3.8L, however it will take special heads and fully custom exhaust setups. Also with the twin setup, I would run integral BOVs and wastegates to cut down on engine bay clutter.
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05:55 PM
Dennis LaGrua Member
Posts: 15727 From: Hillsborough, NJ U.S.A. Registered: May 2000
Boost pressure is kind of "cheating" to get power. Don't get me wrong, I LOVE boost! I really do, however it takes a lot more than just PSI to make power. You can increase the PSI all you want, but if you don't have the flow in your intake system to support the amount of air needed for a given HP, you will just stress the turbo without really any gains.
Basically, when you turbo mod, look into both the PSI, and free flowing intake and exhaust setups. It should flow freely pre and post turbo, and the turbo itself acts as somewhat of a muffler. The real thing you want to look for is lbs of air per given HP amount. If you can get more air squeezed into the cylinders by combining scavenging effects with higher boost pressures and smooth flow patterns... you can get much more HP per PSI of boost. I plan on making ~600 crank HP with twin intercooled turbos running @ 10psi on a 3.8L, however it will take special heads and fully custom exhaust setups. Also with the twin setup, I would run integral BOVs and wastegates to cut down on engine bay clutter.
It is correct that you cannot continue to add boost and expect horsepower to rise at the same rate as the boost goes up. Once you reach the max practical flow rate of the engine the efficiency drops. At this point adding more boost can serve to create only super heated air. The 3800SC Series III swaps have a slight advantage in that the heads flow a bit better than on the series two engines. With slightly larger valves , an improved port design and a more efficient supercharger, even with intercooling, still there is a point of diminishing return. Therefore, there is no linear correlation between boost and horsepower.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
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06:30 PM
Will Member
Posts: 14274 From: Where you least expect me Registered: Jun 2000
A basic guesstimate is to double the N/A horsepower at 15psi. This is because standard atmospheric pressure at sea level is 15 pounds of pressure. If you add 15 pounds of MORE pressure then you would get twice the power. This does not reflect reality 100% because of various losses. It is a good starting point though.
[This message has been edited by Hudini (edited 04-22-2012).]
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07:54 PM
PFF
System Bot
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
a kid in town makes "18 psi" on his small pulley m90 car... And I made significantly more power than he does with a simple turbo setup on 11psi....
Its just really really hard to apples to apples compare "boost pressure" directly inrelation to horsepower... More boost is not always more power....
Correct. Boost != power. Its just something that aids power output in proper scenarios to increase your volumetric efficiency to > 100%. That also being said, it takes MORE to make a Supercharged car to make the same power as a turbocharged car. Superchargers take from the crankshaft to compress thus take engine power to make engine power, however turbos take exhaust energy to make engine power. Turbos will always be more efficient in their output, when designed properly, when running the same boost as a supercharger.
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09:27 PM
seajai Member
Posts: 1544 From: Linwood Township, Minnesota Registered: Feb 2012
So with all that being said, how do you select a turbo? There are so many sizes and specs. Is it a proper assumption that the smaller turbos can only make so much boost before they become inefficient? But too big can be bad too, right? And what about a twin turbo setup? Like I said, I really want to learn this stuff and not just rely on somebody telling me "well just use this and that and the other thing". I have a HP number in mind and want to make sure I get the proper turbo, so that's why I was asking about boost for the 3800. I suppose it would have been better to ask what range of turbochargers work well on a 3800 series 2 engine. I found This turbo forum which seems to have some good info. There was a link to a PDF about turbocharging history. theory, and sizing formulas. The problem is formulas require VE numbers, etc, but that requires dyno runs and flow data.
What is a good way for a turbo newbie to get to know this stuff?
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10:17 PM
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
The right turbo is simply the turbo that is as small as you can get with still meeting your power goals.
The BEST turbo would have its peak efficiency, aka highest flow areas, around the boost level you would be running which is basically a function of displacement and overall engine efficiency.
All of the turbos I have ran on my 3800s suck in the ranges I run them at... but we are talking about 1-2% numbers here.... So its hard to go that wrong.
400 rwp..... with the option to go bigger when I bored with that.
you know your RWHP goals, but what is your flywheel horsepower goal? Knowing that is moreso what you want to look for when sizing a turbo. You need to map the turbos, and for twin turbos you have to map for half of your engine if a true twin turbo, or if its sequential, you have to map twice.
You do not need dyno runs to size a turbo for your car, you need to know how much air it will flow. This can be determined with a LOT of math. Here is a breakdown, I will include a link and copied text:
This equation is for finding the volume of air going into the engine. The displacement on our cars is 231 cu.in. We have a four stroke engine; the intake valve on a cylinder opens once every 2 revolutions of the engine. So, for every 2 revs the engine takes in 231 cu.in. of air. How many pounds of air is that? That depends on the pressure and temperature of the air in the intake manifold. But the volume is always 231 cu.in. every 2 rpm.
volume of air (cu ft/min)= engine rpm x engine cid/(1728 x 2)
... (Too much to copy here)
Read the link in its full to be able to mathmatically map the correct turbo for your application.
[This message has been edited by Racing_Master (edited 04-22-2012).]
Garrett has an app for the iPad and iPhone that allows you to plug in your engine info and it will suggest an appropriate Garrett turbo. The beauty of this app is it will show you compressor maps with your needed airflow to make your desired horsepower. Once you have the required airflow you can compare other brand turbo compressor maps to find a suitable match. It's just another tool in the learning process.
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08:48 AM
RotrexFiero Member
Posts: 3692 From: Pittsburgh, PA Registered: Jul 2002
It is typically the same point, which is why I put the AKA in there... They not particularly the same thing but if you take the pressure ratio of peak flow on a turbo and follow it to the left on a compressor map, 99/100 times you will find its dead in the center of peak efficiency island.
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04:59 PM
Will Member
Posts: 14274 From: Where you least expect me Registered: Jun 2000
When you match a turbo to an engine, the points you plot on the compressor map are expected flow at spool RPM and expected flow at redline at a constant pressure ratio. For a street system, the peak efficiency island should be right between these two points, as this will give the best "area under the curve" for a wide range of operating speeds and loads.
If you're sizing your turbo to put your max flow point right in the middle of your efficiency island, you've built a Bonneville car.
[This message has been edited by Will (edited 04-23-2012).]
When you match a turbo to an engine, the points you plot on the compressor map are expected flow at spool RPM and expected flow at redline at a constant pressure ratio. For a street system, the peak efficiency island should be right between these two points, as this will give the best "area under the curve" for a wide range of operating speeds and loads.
If you're sizing your turbo to put your max flow point right in the middle of your efficiency island, you've built a Bonneville car.
Wouldn't you want it to be the most efficient when the engine is under the most stress?
I'd want it to be at its most efficient for the greatest fraction of time under boost as I could make it.
A Bonneville car is going to run near redline RPM and maximum boost for 6 miles. In that case, I'd go for maximum efficiency under those operating conditions.
In a street car, it should be at its most efficient for the largest fraction of the time it spends under boost. That happens when the path from spool RPM to redline RPM goes right over the efficiency island with peak efficiency right in the middle.
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09:46 PM
Apr 24th, 2012
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
I'd want it to be at its most efficient for the greatest fraction of time under boost as I could make it.
A Bonneville car is going to run near redline RPM and maximum boost for 6 miles. In that case, I'd go for maximum efficiency under those operating conditions.
In a street car, it should be at its most efficient for the largest fraction of the time it spends under boost. That happens when the path from spool RPM to redline RPM goes right over the efficiency island with peak efficiency right in the middle.
Flow in normal turbo motors is fairly flat vs rpm. If you have a motor that makes some seriously pimp VE maybe, but typically your VE gain per RPM is VERY low in a turbo motor, limited by head flow over anything else.
So yes, if you have a bonneville engine with bonneville heads, get a bonneville turbo? I am not even sure how you spec one out?
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12:13 AM
seajai Member
Posts: 1544 From: Linwood Township, Minnesota Registered: Feb 2012
Ok guys, another question....... The turbo compressor calculations article that RM linked me to suggests:
quote
Summary So, how do tie all this together? Well, suppose you are in the market for a new turbo. Which one to buy?
First, I would pick about 4 different operating scenarios. Highway cruise, part throttle acceleration (say 2/3 @ 2700 rpm), full throttle acceleration at 3500 or 4000 rpm, and full throttle acceleration at 5500 or 6000 rpm sound like 4 good points to me. Second, calculate the volumetric flow for each one of those cases. Then, making estimates of the intercooler outlet temperature (or turbo outlet temperature if nonintercooled), turbo discharge pressure, volumetric efficiency, manifold pressure, etc.. calculate the mass air flow for each case. You may also want to check the difference between summer and winter, ie air temps at maybe 90 deg F and 40 deg F. This will affect the manifold temperature and so the air flow. Note that when cruising and at idle, even though the manifold pressure is at a vacuum the turbo discharge pressure is not. It has to pump up the air some, even if it is only to 0.5 psig or so. You can check it out by moving your boost gauge to some point upstream of the throttle body. Besides the mass air flow, calculate the Pout/Pin for each case. Third, and this is the hard part, find the compressor maps for the turbos you are interested in. Turbonetics has maps for their Cheetah, 60-1, and 62-1 in their catalog. The other vendors may not want to let you have the maps for theirs. Plot the points from the 4 cases on the compressor map. Fourth, evaluate the proposed compressors performance. Are the idle/cruise operating points to the left of the surge line? Then this turbo will surge and isn't a good choice. Is the 5500 rpm point so far out to the right that it is off the map? Then this turbo doesn't flow enough for your application. You want all the operating points within the map, and preferably at as high an efficiency as you can get.
If you are trying to choose between 2 turbos, pick the one with the better efficiency where you do most of your driving. Good luck!
Part of the calculations require plugging in a boost psi number. Lets say I use 12psi at high rpm wot, what boost psi numbers would be a good estimation for the other scenarios the author suggests? Highway cruise, part throttle, full throttle at lower rpm's? It's amazing how much science goes into getting it right.
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12:57 AM
Will Member
Posts: 14274 From: Where you least expect me Registered: Jun 2000
Flow in normal turbo motors is fairly flat vs rpm. If you have a motor that makes some seriously pimp VE maybe, but typically your VE gain per RPM is VERY low in a turbo motor, limited by head flow over anything else.
So yes, if you have a bonneville engine with bonneville heads, get a bonneville turbo? I am not even sure how you spec one out?
Flow is flat with RPM? So you pull (example numbers...) 250 g/sec at 21 psi from 2000 RPM to 6000 RPM?
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02:09 PM
PFF
System Bot
E.Furgal Member
Posts: 11708 From: LAND OF CONFUSION Registered: Mar 2012
So this thread is very confusing. All i need is boost to make serious hp. What have i been doing wrong with cams and Rockers. There seems to be too many variables to just get a certain answer..
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10:35 PM
GraterFang Member
Posts: 1387 From: Grants Pass, OR Registered: Feb 2008
lol.. why.. if I have a set up that makes 9psi and change to ported heads that flow better, and now the same set up is at 7.5psi.. but making more power.. well.........................
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10:48 PM
seajai Member
Posts: 1544 From: Linwood Township, Minnesota Registered: Feb 2012
Opinions are like bellybuttons.......everybody has one.
It would seem to me that horsepower is kind of a moving target. I suppose the best way would be to have a test motor on a dyno, a pile of parts to swap in and out, and a really big checkbook to find which combination of parts makes the most power.
BUT, at any rate, seeing as how I started this thread, could somebody answer my air flow calculation question from 7 posts back?
Ok guys, another question....... The turbo compressor calculations article that RM linked me to suggests: <snip>
Part of the calculations require plugging in a boost psi number. Lets say I use 12psi at high rpm wot, what boost psi numbers would be a good estimation for the other scenarios the author suggests? Highway cruise, part throttle, full throttle at lower rpm's? It's amazing how much science goes into getting it right.
Seriously, run the garrett app and find the required airflow that will make your desired horsepower. The app will give you the estimated boost made for your variables. That method you have above is far too complicated and bass-ackwards. You find the airflow needed for your engine and setup first which then tells you pressure ratio at your altitude above sea level which translates to the boost you see on the gauge. "There's an app for that" is very appropriate here.
[This message has been edited by Hudini (edited 04-24-2012).]
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11:38 PM
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
Opinions are like bellybuttons.......everybody has one.
It would seem to me that horsepower is kind of a moving target. I suppose the best way would be to have a test motor on a dyno, a pile of parts to swap in and out, and a really big checkbook to find which combination of parts makes the most power.
BUT, at any rate, seeing as how I started this thread, could somebody answer my air flow calculation question from 7 posts back?
that was just some mindless theory rambling... It really has no place in this thread.
As I have said a billion times... The heads limit flow potential... Just get a turbo and if it doesnt work get a new one... I really dont think you can go that wrong if you get a turbo rated for the power you want to make... Exhaust housing characteristics are probably more important to you at this point if you really want to tune a turbo for your needs... but other than that get a turbo that makes the power you want...
I have a T76 going on a special project I am building right now.. I have a PT6776 on my drag car making 600+whp...I have random truck turbos all over making good power in the 400-500 range.. do whatever you want to do.
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11:51 PM
Apr 25th, 2012
seajai Member
Posts: 1544 From: Linwood Township, Minnesota Registered: Feb 2012
that was just some mindless theory rambling... It really has no place in this thread.
As I have said a billion times... The heads limit flow potential... Just get a turbo and if it doesnt work get a new one... I really dont think you can go that wrong if you get a turbo rated for the power you want to make... Exhaust housing characteristics are probably more important to you at this point if you really want to tune a turbo for your needs... but other than that get a turbo that makes the power you want...
I have a T76 going on a special project I am building right now.. I have a PT6776 on my drag car making 600+whp...I have random truck turbos all over making good power in the 400-500 range.. do whatever you want to do.
Fair enough. I appreciate the input from everybody, it's all very good in helping me learn this turbo stuff.
Where is a good source for specs on factory turbos if I wanted to get one from a salvage yard?
