More HP but less Torque
12-16-2002, 02:13 AM
#1
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Location: Reston VA (not dead, just reston)
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Car Info: '02 WRX TurboXS Stage 4
More HP but less Torque
People have told me that a loss of Torque my result from some products that increase hp, such as a turbo up pipe (or maybe it was down pipe). I know that free lunches do not exist. Added hp always comes at the expense of something, but Torque is one thing I would rather not sacrifice. Is it true about certain exhaust modifications? I.e. that they can cost in the torque department. What else does this apply too? how do I walk the line and get increases in both areas? Are there some specific upgrades I should watch out for or is it going to be different depending on design or manufacturer?
12-16-2002, 04:05 AM
#2
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Unspoken assumptions
Your making some unspoken assumptions that make an answer kind of difficult.
I assume you're refering to low rpm torque when you refer to loss of torque.
Torque and horsepower are directly related, at a specific rpm, you must increase torque to increase hp.
That said what many folks are talking about is that some modifications that improve high rpm horsepower (and torque), reduce low rpm torque -- that throw you back in the seat effect as you leave a light.
What usually happens is you've just moved the torque peak up the rpm scale, so it gives better high rpm power.
Examples of this would be putting a big honkin turbo on the engine that won't spool until you get to 5500 rpm. It will feel like crap as a daily driver because it will destroy your "drivability". The car will feel like it has no pep when you drive normally. On the other hand it will feel like you got kicked into warp drive at the top end if you can ever find a long enough road to really spin up the big turbo.
The opposite extreme is the stock turbo on the WRX, it sacrifices top end performance to give a good mid range torque that will set you back in the seat when your passing etc.
Most performance modifications have some sort of trade off, so you need to understand that bigger is not always better. Big heavy wheels reduce your low speed acceleration, but may improve cornering if you have the right tires. Very big exhaust systems favor high rpm power where engine breathing is the most important factor, but can hurt low rpm throttle response if they are too big.
First you need to define what aspects of performance are important to you.
Do you want the highest Dyno numbers on the planet ? --- get the biggest turbo you can stuff under the hood, and an exhaust system that flows like a storm sewer.
Do you want good around town street light performance? Keep the turbo size within reason, so it will spool up before you get to the next street light. Have an effecient but not overly large exhaust system, and keep rotating mass down with the lightest tires and wheels you can find that fit your appearence and cornering goals.
If you want enough low end grunt to be able to smoke the tires from a stop light? Increase the engine displacement if you can, use a smallish turbo, small diameter exhaust headers to keep exhaust velocity up, and a big shot of N2O. ( and we will ignore the issue of whether you can put the power down without breaking drive train pieces)
Larry
Yes poorly chosen mods can hurt performance outright, or just make the car no fun to drive.
I assume you're refering to low rpm torque when you refer to loss of torque.
Torque and horsepower are directly related, at a specific rpm, you must increase torque to increase hp.
That said what many folks are talking about is that some modifications that improve high rpm horsepower (and torque), reduce low rpm torque -- that throw you back in the seat effect as you leave a light.
What usually happens is you've just moved the torque peak up the rpm scale, so it gives better high rpm power.
Examples of this would be putting a big honkin turbo on the engine that won't spool until you get to 5500 rpm. It will feel like crap as a daily driver because it will destroy your "drivability". The car will feel like it has no pep when you drive normally. On the other hand it will feel like you got kicked into warp drive at the top end if you can ever find a long enough road to really spin up the big turbo.
The opposite extreme is the stock turbo on the WRX, it sacrifices top end performance to give a good mid range torque that will set you back in the seat when your passing etc.
Most performance modifications have some sort of trade off, so you need to understand that bigger is not always better. Big heavy wheels reduce your low speed acceleration, but may improve cornering if you have the right tires. Very big exhaust systems favor high rpm power where engine breathing is the most important factor, but can hurt low rpm throttle response if they are too big.
First you need to define what aspects of performance are important to you.
Do you want the highest Dyno numbers on the planet ? --- get the biggest turbo you can stuff under the hood, and an exhaust system that flows like a storm sewer.
Do you want good around town street light performance? Keep the turbo size within reason, so it will spool up before you get to the next street light. Have an effecient but not overly large exhaust system, and keep rotating mass down with the lightest tires and wheels you can find that fit your appearence and cornering goals.
If you want enough low end grunt to be able to smoke the tires from a stop light? Increase the engine displacement if you can, use a smallish turbo, small diameter exhaust headers to keep exhaust velocity up, and a big shot of N2O. ( and we will ignore the issue of whether you can put the power down without breaking drive train pieces)
Larry
Yes poorly chosen mods can hurt performance outright, or just make the car no fun to drive.
Last edited by hotrod; 12-16-2002 at 04:16 AM.
12-16-2002, 08:27 PM
#4
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hotrod,
{Your making some unspoken assumptions that make an answer kind of difficult.}
Yes I absolutely am but that is because of my admittedly feeble understanding of the issues.
I have put allot of thought into what I want from my car and drive-ability is very important to me. I am unsure how to go about researching the issues. I have sort of been under the impression that I can offset some of the lost low end torque with a more efficient turbo like a ballbering unite as long as I stay at or close to the stock size. This may be an unrealistic assumption but I would love some feed back on it. Basically I want to build the quickest car I can while keeping lag low and retaining some decent midrange torque. I know this is not necessarily easy, that my be an understatement, but I am willing to take my time and spend the kind of money I have to to make it happen. I am not trying to build a drag car but a faster and more enjoyable "sports car". If I wanted to be a stoplight king the road there is well mapped and it starts with a big turbo and lots of lag. I would really like to hear some thoughts on this and I know the question is somewhat ambiguous. what I guess I am asking is how can I make my car quicker, and more powerful throughout the rpm band? And can I do it at all with out internal engine work? This is the first turbo I have owned and though I am reading all I can get my hands on about forced induction and turbo cars I still know very little. My Trans-Am, Porsche and Corvair tuner projects of the past were completely different beasts form the WRX I now own.
Thank you for the initial feed back and I am hoping for more.
{Your making some unspoken assumptions that make an answer kind of difficult.}
Yes I absolutely am but that is because of my admittedly feeble understanding of the issues.
I have put allot of thought into what I want from my car and drive-ability is very important to me. I am unsure how to go about researching the issues. I have sort of been under the impression that I can offset some of the lost low end torque with a more efficient turbo like a ballbering unite as long as I stay at or close to the stock size. This may be an unrealistic assumption but I would love some feed back on it. Basically I want to build the quickest car I can while keeping lag low and retaining some decent midrange torque. I know this is not necessarily easy, that my be an understatement, but I am willing to take my time and spend the kind of money I have to to make it happen. I am not trying to build a drag car but a faster and more enjoyable "sports car". If I wanted to be a stoplight king the road there is well mapped and it starts with a big turbo and lots of lag. I would really like to hear some thoughts on this and I know the question is somewhat ambiguous. what I guess I am asking is how can I make my car quicker, and more powerful throughout the rpm band? And can I do it at all with out internal engine work? This is the first turbo I have owned and though I am reading all I can get my hands on about forced induction and turbo cars I still know very little. My Trans-Am, Porsche and Corvair tuner projects of the past were completely different beasts form the WRX I now own.
Thank you for the initial feed back and I am hoping for more.
12-17-2002, 02:01 AM
#6
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On a turbo engine, it's generally a good idea to remove as much back pressure from the exhaust as possible. Proper backpressure is essential to maintain a nice torque curve. However, on a turbocharged engine, the turbo itself creates more than enough backpressure to maintain the torque curve. The ideal way to run a turbo engine is with no exhaust at all. However, this would be VERY loud and probably dangerous. This is why most people run a 3" exhaust.
To answer your question... Most people will say that a big (ie 3") exhaust will increase top end power but reduce low end torque. This is true on a normally aspirated engine, but not really true on a turbo engine. Actually more toward the opposite. On a turbo engine, less back pressure post-turbo (ie bigger exhaust) is going to allow the turbo to spool quicker, thereby creating more torque and power at lower RPMs. This is why replacing the Cats on the WRX makes it so much more punchy around town. You're reducing the backpressure and allowing the turbo to spool sooner.
Hope this helps.
To answer your question... Most people will say that a big (ie 3") exhaust will increase top end power but reduce low end torque. This is true on a normally aspirated engine, but not really true on a turbo engine. Actually more toward the opposite. On a turbo engine, less back pressure post-turbo (ie bigger exhaust) is going to allow the turbo to spool quicker, thereby creating more torque and power at lower RPMs. This is why replacing the Cats on the WRX makes it so much more punchy around town. You're reducing the backpressure and allowing the turbo to spool sooner.
Hope this helps.
12-17-2002, 10:03 AM
#7
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Originally posted by djphrenzy
On a turbo engine, it's generally a good idea to remove as much back pressure from the exhaust as possible. Proper backpressure is essential to maintain a nice torque curve. However, on a turbocharged engine, the turbo itself creates more than enough backpressure to maintain the torque curve. The ideal way to run a turbo engine is with no exhaust at all. However, this would be VERY loud and probably dangerous. This is why most people run a 3" exhaust.
To answer your question... Most people will say that a big (ie 3") exhaust will increase top end power but reduce low end torque. This is true on a normally aspirated engine, but not really true on a turbo engine. Actually more toward the opposite. On a turbo engine, less back pressure post-turbo (ie bigger exhaust) is going to allow the turbo to spool quicker, thereby creating more torque and power at lower RPMs. This is why replacing the Cats on the WRX makes it so much more punchy around town. You're reducing the backpressure and allowing the turbo to spool sooner.
Hope this helps.
On a turbo engine, it's generally a good idea to remove as much back pressure from the exhaust as possible. Proper backpressure is essential to maintain a nice torque curve. However, on a turbocharged engine, the turbo itself creates more than enough backpressure to maintain the torque curve. The ideal way to run a turbo engine is with no exhaust at all. However, this would be VERY loud and probably dangerous. This is why most people run a 3" exhaust.
To answer your question... Most people will say that a big (ie 3") exhaust will increase top end power but reduce low end torque. This is true on a normally aspirated engine, but not really true on a turbo engine. Actually more toward the opposite. On a turbo engine, less back pressure post-turbo (ie bigger exhaust) is going to allow the turbo to spool quicker, thereby creating more torque and power at lower RPMs. This is why replacing the Cats on the WRX makes it so much more punchy around town. You're reducing the backpressure and allowing the turbo to spool sooner.
Hope this helps.
backpressure is the enemy of a turbo charger,
ideal is to have none,
however exhaust velocity is more the key here than backpressure, and many people sometimes confuse these two
run too big of an exhaust and your exhaust velocity will decrease, thus causing less power
12-17-2002, 04:31 PM
#8
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The velocity only matters before the turbo, i.e. - the header and up pipe. After the turbo, as stated by djphrenzy, the most efficient exhaust would be no exhaust at all. Of course, this is not practical. So a 3" exhaust will offer the least restriction while keeping you out of jail (if you live in CA). If you increase the header and up pipe diameter, the velocity of the exhaust gases coming out of the engine (and hence into the turbo) decreases as a square function of the pipe radius (A = pi*r^2, v=flow/A). So...if you increase the header diameter from only 1.5 inches to 2 inches (don't know of anybody recommending this), you effectively reduce the velocity by a factor of over 70% (0.75^2 = .56, 1^2 = 1). This would be incredibly detrimental to turbo spool up time, not to mention the simple fact that you would not really gain anything until spinning insane rpms. Flow is a direct relationship to the displacement of the engine, the engine rpm, and the atmospheric conditions you place the moving gas under (i.e., absolute pressure, temperature). This is from basic thermodynamics. If you need to flow more through the engine, you have to look at everything from the intake to the tailpipe. Then design and choose the system based around the engines design rpm operating range. Granted, an engine will flow more if you port the head, match the intake and exhaust manifolds, yada yada yada, but where does all this really get you more power? Mostly when the engine itself becomes the restriction - only at higher rpms. A 2.0 L engine displaces 780 m3/h or 460 cfm at 6500 rpm. There is some difference here from the normally aspirated world, in that you must apply some common sense to the reality that you can stuff a little more through there to get the flow up with the turbo. But, what is the cost? Running race-like boost pressures will surely toast a stock internals engine in no time. The best way is incremental upgrades. You mentioned you have lots of money to make a performer out of the suby, now you have to decide what that means. If you just want to get the most out of the stock engine, start with the exhaust from the uppipe through the tailpipe. You will find after reading a zillion posts and lots of surfing time that most people agree that about 300 hp is the limit on stock engine internals/drivetrain for the US spec WRX. If you don't mind the possibility of shooting a connecting rod out your oilpan going down the highway after 30,000 miles, you can probably make 450 hp without sinking too much money in it. Then you can buy a replacement STi EJ206 and start over. It all depends on how fast you want to go.
12-17-2002, 06:02 PM
#9
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Location: colorado
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maximum drivability with performance
Andersonwrx:
OKay now you've made the first important determination.
If you goal is maximum drivability with good performance then I would look at the following.
1) use no more turbo than you need. By that I mean you want the smallest turbo that will "just" support your maximum hp goal.
Figure it takes about 1.5 CFM to support each hp. Lets say your goal is 350 hp, then you want the most effecient turbo you can find that will max out at about 525 CFM (give or take a bit). You can even sacrifice a bit of top end flow to get good midrange which is where you spend most of your time during daily driving.
2) keep the stock exhaust manifolds, Nathan at TXs and several others have said that they lost low end when they went to most of the headers out there.
A good small uppipe to keep velocity up, (jet hot coated or header wrapped to keep heat in)
3) best high flow tuboback/cat back system you can find (depending on your priorities regarding emissions)
4) minimize intake losses, so sampco hoses or similar is good. As a daily driver you won't really stress the IC much. The only upgrade I would make there would be to look at low pressure drops across the IC. with in its capacity the stock IC actually works pretty good. Folks have gotten into the high 11's with it.
5) get a good EBC and or after market engine management. A LOT of turbo spool character depends on how you program waste gate /boost limits and curves.
6) get enough engine management so you can keep as much ignition advance as possible. Low speed off idle performance is strongly influenced by ignition timing.
7) lose some weight off the vehicle if you can, its a mod that pays off in all aspects of performance and requires no tuning. ( it also makes life easier on the tranny)
8) get a light weight flywheel, and run light weight wheels
Hope that helps
Larry
OKay now you've made the first important determination.
If you goal is maximum drivability with good performance then I would look at the following.
1) use no more turbo than you need. By that I mean you want the smallest turbo that will "just" support your maximum hp goal.
Figure it takes about 1.5 CFM to support each hp. Lets say your goal is 350 hp, then you want the most effecient turbo you can find that will max out at about 525 CFM (give or take a bit). You can even sacrifice a bit of top end flow to get good midrange which is where you spend most of your time during daily driving.
2) keep the stock exhaust manifolds, Nathan at TXs and several others have said that they lost low end when they went to most of the headers out there.
A good small uppipe to keep velocity up, (jet hot coated or header wrapped to keep heat in)
3) best high flow tuboback/cat back system you can find (depending on your priorities regarding emissions)
4) minimize intake losses, so sampco hoses or similar is good. As a daily driver you won't really stress the IC much. The only upgrade I would make there would be to look at low pressure drops across the IC. with in its capacity the stock IC actually works pretty good. Folks have gotten into the high 11's with it.
5) get a good EBC and or after market engine management. A LOT of turbo spool character depends on how you program waste gate /boost limits and curves.
6) get enough engine management so you can keep as much ignition advance as possible. Low speed off idle performance is strongly influenced by ignition timing.
7) lose some weight off the vehicle if you can, its a mod that pays off in all aspects of performance and requires no tuning. ( it also makes life easier on the tranny)
8) get a light weight flywheel, and run light weight wheels
Hope that helps
Larry
Last edited by hotrod; 12-17-2002 at 06:05 PM.
12-17-2002, 09:45 PM
#10
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Join Date: Dec 2002
Location: Reston VA (not dead, just reston)
Posts: 162
Car Info: '02 WRX TurboXS Stage 4
I TRIED TO DELETE THIS POST AND WAS TOLD I DID NOT HAVE PERMISSION SO I EDITED IT AND WROTE THIS INSTEAD. I WROTE IT AND IT WAS NOT THE FIRST ONE SO IT SHOULD NOT EFFECT THE OTHERS???
(EXCEPT FOR THE OTHER ONE I POSTED AFTER IT)
SO NOW THIS USELESS SPOT IS HERE. SORRY.
FEEL FREE TO REMOVE THIS IF ANY ADMINS. SEE THIS.
(EXCEPT FOR THE OTHER ONE I POSTED AFTER IT)
SO NOW THIS USELESS SPOT IS HERE. SORRY.
FEEL FREE TO REMOVE THIS IF ANY ADMINS. SEE THIS.
Last edited by Andersonwrx; 12-17-2002 at 09:56 PM.
12-17-2002, 09:46 PM
#11
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I appreciate the input very much guys, but HOTROD, you suggested a lightened fly wheel. while this offers quicker throttle response what is the negative effect. Meaning will it allow my rpms to drop to quickly? How will this effect the power after shifting and is it enough to matter as long as I shift fairly quickly? I'm talking about the need to keep the rpm's high so that the car is still in it's power band. What are your thoughts?
likestogofast. thanks for responding, but I didn't say I was rich. I simply have the patience to take the time to save whats nessesary. If I was rich I would have Carrera 4 and not a WRX. I love my WRX but I love it for what it offers at the price, and the price was not $70,000.
likestogofast. thanks for responding, but I didn't say I was rich. I simply have the patience to take the time to save whats nessesary. If I was rich I would have Carrera 4 and not a WRX. I love my WRX but I love it for what it offers at the price, and the price was not $70,000.
12-18-2002, 01:48 PM
#12
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Location: colorado
Posts: 79
flywheel and revs
The lightened flywheel will help the engine rev quickly especially during lower gears. Much better low speed performance. It is the safe way to accomplish what folks are trying to do with a light weight crank pulley. It will make the car act like it is about 150 lbs lighter, due to the reduction in rotating weight.
Yes your engine will drop in rpm quicker during shifts, but it will also rev faster when trying to rev match and down shift.
There will be a slightly higher tendency to low speed stall until you get used to it, as it will be less forgiving of lugging the engine as you start to roll.
Your increased power output will partially compensate for that though.
The only way to keep engine revs up during shifts is to shift quickly. Regardless your engine will end up going the appropriate rpm for you wheel speed anyway as soon as the clutch is released.
Larry
Yes your engine will drop in rpm quicker during shifts, but it will also rev faster when trying to rev match and down shift.
There will be a slightly higher tendency to low speed stall until you get used to it, as it will be less forgiving of lugging the engine as you start to roll.
Your increased power output will partially compensate for that though.
The only way to keep engine revs up during shifts is to shift quickly. Regardless your engine will end up going the appropriate rpm for you wheel speed anyway as soon as the clutch is released.
Larry
12-19-2002, 10:58 PM
#13
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Sorry if this drags a little off-topic, but I've been curious about this:
Looking at modding an automatic-WRX (no s******ing please ), but I've been reading up mostly on exaust systems and whatnot.
My question is, is a lightened flywheel something that is a safe mod to an auto-WRX, or will it totally screw up the shifting?
Thanks in advance for y'all's patience with a newbie!
Looking at modding an automatic-WRX (no s******ing please
), but I've been reading up mostly on exaust systems and whatnot. My question is, is a lightened flywheel something that is a safe mod to an auto-WRX, or will it totally screw up the shifting?
Thanks in advance for y'all's patience with a newbie!
12-20-2002, 01:51 PM
#14
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Car Info: 02 WRX wagon=dead; rollin' in a Craptastic Camry!
"Yes your engine will drop in rpm quicker during shifts, but it will also rev faster when trying to rev match and down shift."
I would think the quick dropping revs would be a very good thing. It seems like I have to wait a long time when shifting from 1st to 2nd on my stock WRX to get a smooth shift.
I would think the quick dropping revs would be a very good thing. It seems like I have to wait a long time when shifting from 1st to 2nd on my stock WRX to get a smooth shift.
12-21-2002, 07:11 AM
#15
good post guys...nice answer likestogofast..nail on the head!
Post turbo diamter does in fact make a difference, not in all out pweor but in the shift of the curve upwards or downwards. This is why IHI often plays with various exhaust housing confirgurations...shiifts the power curve to suit the motor it is going on, though pak power is left argely unchanged (as it is really determines by the compressor side)
Adam
Post turbo diamter does in fact make a difference, not in all out pweor but in the shift of the curve upwards or downwards. This is why IHI often plays with various exhaust housing confirgurations...shiifts the power curve to suit the motor it is going on, though pak power is left argely unchanged (as it is really determines by the compressor side)
Adam