Low end torque
10-03-2016, 06:56 PM
#16
Burning Brakes
10-04-2016, 03:05 PM
#18
Three Wheelin'
You still don't want backpressure in an NA car... You want to take full advantage of the scavenging effect and high velocity. Having too large of an exhaust can reduce velocity which in turn hurts scavenging.
Turbo cars is simple, bigger always better. No scavenging effect in the exhaust system because turbo gets in the way of that.
NA cars is more nuanced.. bigger is better, until it isn't, but then it can still be better if something else changes upstream. You want to reach zero back pressure without losing flow velocity... NA cars is best to tune the exhaust system as a whole so the scavenging is at it's highest in the part of the operating range that you want to spend most of your time. Pick a camshaft, know where it makes power, design exhaust to take advantage of that range.
Turbo cars is simple, bigger always better. No scavenging effect in the exhaust system because turbo gets in the way of that.
NA cars is more nuanced.. bigger is better, until it isn't, but then it can still be better if something else changes upstream. You want to reach zero back pressure without losing flow velocity... NA cars is best to tune the exhaust system as a whole so the scavenging is at it's highest in the part of the operating range that you want to spend most of your time. Pick a camshaft, know where it makes power, design exhaust to take advantage of that range.
10-04-2016, 05:29 PM
#19
Pro
No, it really doesn't. This is not the case in any turbocharged vehicle.
Back pressure causes a slow down in exhaust flow at the point of the bottleneck and this pressure builds up in the exhaust pipe and into the turbine housing; to overcome this restriction will take more energy to spin the turbine wheel.. Less restrictions, less energy required to spin the turbine wheel, turbo builds boost faster, makes more torque/power at a lower rpm.
Back pressure causes a slow down in exhaust flow at the point of the bottleneck and this pressure builds up in the exhaust pipe and into the turbine housing; to overcome this restriction will take more energy to spin the turbine wheel.. Less restrictions, less energy required to spin the turbine wheel, turbo builds boost faster, makes more torque/power at a lower rpm.
10-04-2016, 06:33 PM
#20
Three Wheelin'
This as a general (full powerband) rule yes, but not when looking only at low-end torque... although when comparing a stock exhaust system designed for that specific engine. So the appropriate 'pulse' is followed through the entire system. When you change the exhaust, by removing catalysts or the diameter, it will alter the pulse and minimize the scavenging effect. The scavenging effect, allows better flow patterns. Even though it's not as prevalent on Turbo engines, as it is on NA, it still affects the low-end torque (for the better). It's all about the best gas velocity, and to achieve low-end torque, you would want a smaller (or stock in this case) exhaust. Top end power is completely different, you want it to breath nearly free. If you take out the cats on the stock exhaust, you will gain top-end power, but likely loose low-end torque. EDIT: why is the message not including my paragraph spacing? it makes it into one large paragraph instead of how I typed it.
If you were to make a dyno pull from low rpm with stock exhaust and again with open exhaust on a turbo car, the open will make more torque at every rpm.
You're on the right track for how things work in NA applications, but it doesn't translate over to turbo cars which have something disrupting the velocity nearly immediately upon exhaust exiting the cylinders.
I've found that I have to double space the paragraphs to make it not block together.
10-05-2016, 11:28 AM
#21
Pro
There is little to no scavenging effect in turbo cars because of the turbine wheel, as I mentioned in another post right before this one. There might be a small amount of scavenging in the collector pre-turbo, but there is definitely zero scavenging happening post turbo in the catalyst or in the exhaust piping. It's just the way it is.
If you were to make a dyno pull from low rpm with stock exhaust and again with open exhaust on a turbo car, the open will make more torque at every rpm.
You're on the right track for how things work in NA applications, but it doesn't translate over to turbo cars which have something disrupting the velocity nearly immediately upon exhaust exiting the cylinders.
I've found that I have to double space the paragraphs to make it not block together.
If you were to make a dyno pull from low rpm with stock exhaust and again with open exhaust on a turbo car, the open will make more torque at every rpm.
You're on the right track for how things work in NA applications, but it doesn't translate over to turbo cars which have something disrupting the velocity nearly immediately upon exhaust exiting the cylinders.
I've found that I have to double space the paragraphs to make it not block together.
In addition, with the particular exhaust, which came with a 'silencer' (don't flame too bad, I was 19) and the low-end / high-end were completely different animals with/without it.
Anyway, not trying to argue, but I guess it depends on how you look at it, as backpressure and scavenging are two different things, and I shouldn't have associated them.
If someone has a dyno and it shows the LOW-END torque higher due ONLY to swapping a free-er breathing exhaust, I would be surprised. And will admit my wrongness!
and buy you a beer... or seven!
10-05-2016, 12:11 PM
#22
Race Director
the low end torque (say from rev 1500 to 2200) of my 2004 turbo is a little bit mild than what I have expected.
The factory figure is about 415 lb ft at 2700 rev which is quite a great deal of power even in nowaday standard
not yet really tested as i am busy in cleaning my new ride
May be the first thing to do is to add techron to clean the injectors and the second things to do are to change spark plugs /an afe air filter.....(not mobile 1 as the seller has changed it recently)
https://www.caltex.com/hk/zh/motoris...rate-plus.html
The factory figure is about 415 lb ft at 2700 rev which is quite a great deal of power even in nowaday standard
not yet really tested as i am busy in cleaning my new ride
May be the first thing to do is to add techron to clean the injectors and the second things to do are to change spark plugs /an afe air filter.....(not mobile 1 as the seller has changed it recently)
https://www.caltex.com/hk/zh/motoris...rate-plus.html
In the case of my Turbo I used a bottle of Techron to address cold misfires -- suspected of being due to dirty injectors -- and one bottle didn't eliminate the misfires nor did I notice any improvement in the engine's performance. But I used a 2nd bottle anyhow. Still no improvement, but I feel the two bottles were not wasted.
Oh, the recommendation is after the last of the fuel with the Techron is used up, or at least the tank level is down to 1/4 tank to have the engine oil/filter service done. Techron can increase oil contamination and you don't want to run that extra contaminated oil if you can avoid doing so.
Even if the Techron helps it won't change the engine's low end that much. The engine is not intended to be a low RPM torque monster. As you note peak torque doesn't come on until 2700 RPMs. While one doesn't have to drive around town in 1st at 2700 RPMs the engine needs/likes a bit more RPMs than say even a N/A engine.
Briefly, with my Turbo I avoid operating the engine below 1K in any gear except very rarely and then only in 1st gear. In 2nd gear the low RPM limit is 2K and higher gears require 2.5K RPMs.
I've have the plugs changed in my Turbo several times and never really noticed an improvement in the engine. The last time I had the plugs changed was when I had the coils replaced -- at the time thinking it was the coils that were at the root cause of cold start misfires -- and at the same time the tech recommended even though the plugs weren't due for another 10K miles to change them "while he was there". I agreed.
After I got the car back the engine was a bit perkier. Not lots but it was noticeable.
The only time I've noticed more improvement from the engine after a parts replacement was when I had all 4 O2 sensors replaced at around 132K miles. There was an O2 sensor error and I decided it was time to just replace all 4. I am not in favor of just replacing one sensor or even the sensors on just one bank anyhow.
I have no experience with any Turbo engine tune.
My advice is before you do a tune, if that is what you decide to do, you first ensure the engine is in proper tune. We've already talked about Techron. You are going to change the plugs. Give the coils an close inspection and replace all if any one looks "bad". Or you could just replace them anyhow.
Change the fuel filter. The engine air filter. (And while not engine performance related the cabin air filter, too.)
I don't the miles on the car but consider new O2 sensors. I've never had to replace a MAF in my Turbo so I can't really recommend that.
After you get the car back due an E-Gas calibration. The steps are in the owners manual.
You want the engine at its factory best so you can then establish a baseline of performance and behavior so if you then decide to get a tune you will have something to judge the effectiveness of the tune against.
10-05-2016, 12:34 PM
#23
Thx for the detailed sharing
it feels better after adding techron
i really don't realize the need to change engine oil after using it. Although the owner changed in July 2016, to play safe,
i will do it again
it feels better after adding techron
i really don't realize the need to change engine oil after using it. Although the owner changed in July 2016, to play safe,
i will do it again
10-05-2016, 01:52 PM
#25
Three Wheelin'
pre-turbo there is definitely a scavenging effect, no question. Same as with an NA car, but post-turbo, the effects are still there. Albeit, very, VERY subdued, but are still present.I had a dyno that clearly showed this as well, with my first car (S13 240sx with SR20det).
In addition, with the particular exhaust, which came with a 'silencer' (don't flame too bad, I was 19) and the low-end / high-end were completely different animals with/without it.
Anyway, not trying to argue, but I guess it depends on how you look at it, as backpressure and scavenging are two different things, and I shouldn't have associated them.
If someone has a dyno and it shows the LOW-END torque higher due ONLY to swapping a free-er breathing exhaust, I would be surprised. And will admit my wrongness! and buy you a beer... or seven!
In addition, with the particular exhaust, which came with a 'silencer' (don't flame too bad, I was 19) and the low-end / high-end were completely different animals with/without it.
Anyway, not trying to argue, but I guess it depends on how you look at it, as backpressure and scavenging are two different things, and I shouldn't have associated them.
If someone has a dyno and it shows the LOW-END torque higher due ONLY to swapping a free-er breathing exhaust, I would be surprised. And will admit my wrongness!
and buy you a beer... or seven!I love discussions and going back and forth with people from time to time. This is how to learn and share knowledge, it is the purpose of the forum. I have had opposite experience with previous cars, granted moving a lot more air than SR20, so maybe the effect is more pronounced on smaller displacement platforms? I have no idea as far as that goes, but seems plausible, less displacement moves less exhaust so there could be some sort of issue with velocity.
If I had easy access to a dyno, I still have stock exhaust on my car, and I have a 3" catless no muffler exhaust (doesn't get much more free-flowing than this
) and would love to do a same day test. Not so easy for me to do though. Right or wrong, I'd be down to go for beers haha. I am always happy to be shown an error in my thinking
10-05-2016, 06:35 PM
#27
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10-05-2016, 07:24 PM
#28
The OP said
Not sure how a tune will help that much since there is basically no boost then. It might make the lack of low end even more obvious since one you hit full boost there is a lot more torque on a tuned car vs more gradual build up on a stock setup.
the low end torque (say from rev 1500 to 2200)
