Still No Power

[apsaauga]

Hi, changed the exhaust sytem, removed pre-cats and my car still is slooooooow!

Now it is only one problem, no power on low revs like: going 20km/h in 2nd gear and floor the pedal, and it moves like a turtle with some jumping, also the same with 3rd in 40 km/h ans so on. Besides exhaust tip is very dirty (i mean black) it means that not all fuel burns right, so, what the hell? Lexus can tell me nothing, because there is no error!

Please, help me

[TheDon]

potentialy you have lost some critical back pressure?

turbos need straight thru exhausts but N/A cars suffer if the exhaust is incorrectly designed

[apsaauga]

I don't know what kind of pressure, but i am sure that there is no power in low revs. You think i should try to take the back box? Because i changed the back box, sorry for not the right term

[Gord]

Hi, changed the exhaust sytem, removed pre-cats and my car still is slooooooow!

Now it is only one problem, no power on low revs like: going 20km/h in 2nd gear and floor the pedal, and it moves like a turtle with some jumping, also the same with 3rd in 40 km/h ans so on. Besides exhaust tip is very dirty (i mean black) it means that not all fuel burns right, so, what the hell? Lexus can tell me nothing, because there is no error!

Please, help me

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The pre-cats that you took out, was thay ok

[TheDon]

After driving UKAZN's car with a powerflow exhaust i know what you mean about it having no power till 4K + revs.

As soon as he changed it for the HKS hiper it worked perfectly.

[apsaauga]

Gord>> pre-cats were dirty, but not too dirty. It feels better at high revs and during the gear change.

TheDon>>> but i already felt the loose of power with stock exhaust. I changed the back box, they made shorter the pipe to the back box from main cat, could that do some changes to air flow?

[ColinBarber]

There could be something wrong with the fueling then. Possibly a lazy O2 sensor or one of the other sensors.

Have you checked the plugs?

[apsaauga]

Colin, where are they?

[Gord]

Colin, where are they?

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2 on your manifold, i have 2 if you want them.

[apsaauga]

Sorry gord, but didn't understand:)

[Gord]

Sorry gord, but didn't understand:)

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there are 2 sensors on your manifold (you can see them from the engine bay) i have 2 if you want, you can have them.

[Dayu]

On the pre-cats:

I've been advised by a very reliable workshop (over here in Singapore) that it is not advisible to just cut out the pre-cats and weld in 2 metal pipes. There's a reason why people pay very high sums for on-car custom construction of manifolds (headers). While not being able to fully appreciate the mechanics behind the advantages of equal length manifolds vs. unequal length manifolds, etc etc, it does make sense that the flow of exhaust gasses would be negatively impacted by a crude hack and weld job. For example, the workshop's recommended solution would be to cut out the pre-cats, replace the missing link with carefully machined and appropriately curved pipes with various features to help equal out the 2 uneven length halves of the manifold.

Back Pressure:

On the topic of 'back pressure', there's no such thing as good back pressure really. Back pressure is always bad, but the idea is that too big a pipe will reduce the scavenging effect (urm.. i'm sure there's a more techy sounding name for this.. but I don't know it).

Scavenging effect = exhaust gas leaving help pull the exhaust gas behind it out, which in turn helps draw fresh air/fuel in thru the intake valves. Sort of like how one siphons liquids thru a tube. In a small exhaust, exhaust gasses achieve their max velocity (and hence max scavenging effect) sooner. That is why it is commonly said that having too large an exhaust on a comparatively weak engine may have a detrimental effect on power (ie. a more powerful engine is able to generate more exhaust = earlier max exhaust velocity = scavenging effect). Of course, having too small an exhaust would physically limit the amount of exhaust that can pass thru, which is bad also.

Perhaps an analogy would help. Lets use.. drinking a (large) cup of water using a straw. Sure, using 2 straws would be faster than 1, as would 3 straws, and so on. But imagine if you were to use 20 straws. Apart from the difficulty in fitting that in your mouth, you'd probably find it immensely difficult to suck the water up. Now, equate the straws to the exhaust and the "sucker" :) to the engine. That is why, without a serious increase in engine power (be it via turbo, bore/stroke, etc), fitting an enormous exhaust would probably do more harm than good, whereas a mild increase (given the conservative nature of car manufacturers and the need to fit noise, emission regs, etc) would probably be beneficial.

Oh, a final note. The reason why everyone likes to remove cats are because as a air pollution filter, they retard the flow of exhaust gasses, both impeding the scavenging effect and reducing the overall flow of exhausts out of the system.

Urm.. ok.. that's pretty much all I know. As always, this is based on my limited knowledge, and I would appreciate it very much if the more experienced folks would correct the bits that I got wrong! :)

[Gord]

On the pre-cats:

I've been advised by a very reliable workshop (over here in Singapore) that it is not advisible to just cut out the pre-cats and weld in 2 metal pipes. There's a reason why people pay very high sums for on-car custom construction of manifolds (headers). While not being able to fully appreciate the mechanics behind the advantages of equal length manifolds vs. unequal length manifolds, etc etc, it does make sense that the flow of exhaust gasses would be negatively impacted by a crude hack and weld job. For example, the workshop's recommended solution would be to cut out the pre-cats, replace the missing link with carefully machined and appropriately curved pipes with various features to help equal out the 2 uneven length halves of the manifold.

Back Pressure:

On the topic of 'back pressure', there's no such thing as good back pressure really. Back pressure is always bad, but the idea is that too big a pipe will reduce the scavenging effect (urm.. i'm sure there's a more techy sounding name for this.. but I don't know it).

Scavenging effect = exhaust gas leaving help pull the exhaust gas behind it out, which in turn helps draw fresh air/fuel in thru the intake valves. Sort of like how one siphons liquids thru a tube. In a small exhaust, exhaust gasses achieve their max velocity (and hence max scavenging effect) sooner. That is why it is commonly said that having too large an exhaust on a comparatively weak engine may have a detrimental effect on power (ie. a more powerful engine is able to generate more exhaust = earlier max exhaust velocity = scavenging effect). Of course, having too small an exhaust would physically limit the amount of exhaust that can pass thru, which is bad also.

Perhaps an analogy would help. Lets use.. drinking a (large) cup of water using a straw. Sure, using 2 straws would be faster than 1, as would 3 straws, and so on. But imagine if you were to use 20 straws. Apart from the difficulty in fitting that in your mouth, you'd probably find it immensely difficult to suck the water up. Now, equate the straws to the exhaust and the "sucker" :) to the engine. That is why, without a serious increase in engine power (be it via turbo, bore/stroke, etc), fitting an enormous exhaust would probably do more harm than good, whereas a mild increase (given the conservative nature of car manufacturers and the need to fit noise, emission regs, etc) would probably be beneficial.

Oh, a final note. The reason why everyone likes to remove cats are because as a air pollution filter, they retard the flow of exhaust gasses, both impeding the scavenging effect and reducing the overall flow of exhausts out of the system.

Urm.. ok.. that's pretty much all I know. As always, this is based on my limited knowledge, and I would appreciate it very much if the more experienced folks would correct the bits that I got wrong! :)

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Hi mate, the reason that most take pre-cats out over in the UK is coz of the supercharger will melt them :duh: when i take the pre-cats out i just cut into the pipe take the cats out, then weld together again, not adding any pipes at all, so still as standard, but without the pre-cats. :D

[Mr Morse]

I also know a little about fluid dynamics and would point the finger at a back pressure related problem.

If it helps any, look at the shape of power station cooling towers - the reason they are not pure cylindrical is because steam would not be encouraged to flow upwards. Tapering the tower in the mid section increases pressure which increases the fluid (steam) to flow faster through that section and so on and so on.

Hope it helps some

[apsaauga]

Thanks for your advices:)

gord>>>that's how i did it!

Mr. Morse, could you explain a bit more detailed? because my english is still not perfect

[Gord]

Thanks for your advices:)

gord>>>that's how i did it!

Mr. Morse, could you explain a bit more detailed? because my english is still not perfect

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But, was the pre-cat in one or was it in bits , did you take all the exhaust off, or just the manifold

[Dayu]

Mr. Morse, could you explain a bit more detailed? because my english is still not perfect

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I think the general point of Mr. Morse's post is that sometimes having too big an exhaust will result in the exhaust gasses coming out slower.

That said, what exactly did you do to the exhaust? Did you replace everything after the manifold/headers (ie. straight-thru all the way)? Or did you just cut out the pre-cats and change to an after-market back-box?

(sorry for asking so many questions, I'm trying to decide what to do to my exhaust after having just bolted on a new back-box :))

[Gord]

thats it, we need to know what you have done to help you out :D

[star a]

http://www.sportcompactcarweb.com/tech/9912scc_ssbbpart2/

some nice simplified intake and exhaust tuning advice

[Dayu]

http://www.sportcompactcarweb.com/tech/9912scc_ssbbpart2/

some nice simplified intake and exhaust tuning advice

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Great stuff! :)

[legendswraith]

this touches on something i was looking at and probably will try again to try and give you a better idea its best to think of your exhaust as a duct and the way fluid (for the sake of ease gas is refered to as a fluid ) in pre supersonic speeds in a convergent duct (picture funnel) this causes an increase in exhaust gas exit velocity a drop in temp and a drop in pressure the reverse is tru in a divergent duct (picture girlfriends hairdrier diffuser) velocity drops pressure increases and temperature increases

Now if all the pipes on a header are different lengths and bends this will cause a variation in the pressure and exit velocity of the gasses which will cause either a boosting action or a choking action (if the last of the pipes is at a higher velocity than the rest it will suck the other gasses out) if you do get this sucking action that will cause you to lose backpressure on the engine and lose low end torque

I would imagine ideally you would want to produce a plus 1psi difference across the engine in the correct phase to keep the minimum backpressure on however when i find what the ideal psi for the engine to run at is i may sit down and do the maths

I could go more in depth but i'm losing the will to live :duh:

[apsaauga]

It is nice to have You all here! :)

What i have done: first i changed the stock back box, which was cut off and i put the new one (with better air flow as they said), but the hole of back box was the same as stock one, so there were no changes. The original back box tip was boiled on ( no huge tip). The thing that was changed a bit, that they cut a little peace of pipe to back box, because the structure of it was a little different than stock. It didn't help so...today i took the pre-cats just like Gord did! (no pipe was putted)

It looks like power is ok on high revs and low gears, but when i try floor it in low speed it waits, then slowly with noticable jumping forward till it goes fast. Everytime i do that it is the same more or less... Oh, in warm engine it looks a little better, but when it is hot, problem is back.

Besides pre-cats were in one peace, no falling parts:( However the front was dirty from smoke, so choking was in progress.

i hope it is clear now, maybe You can tell me more, because Lexus computer shows no error!!

[Gord]

It is nice to have You all here! :)

What i have done: first i changed the stock back box, which was cut off and i put the new one (with better air flow as they said), but the hole of back box was the same as stock one, so there were no changes. The original back box tip was boiled on ( no huge tip). The thing that was changed a bit, that they cut a little peace of pipe to back box, because the structure of it was a little different than stock. It didn't help so...today i took the pre-cats just like Gord did! (no pipe was putted)

It looks like power is ok on high revs and low gears, but when i try floor it in low speed it waits, then slowly with noticable jumping forward till it goes fast. Everytime i do that it is the same more or less... Oh, in warm engine it looks a little better, but when it is hot, problem is back.

Besides pre-cats were in one peace, no falling parts:( However the front was dirty from smoke, so choking was in progress.

i hope it is clear now, maybe You can tell me more, because Lexus computer shows no error!!

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Have you got the sensors in the manifold do you want to try changing them with the ones that i have if you do pm me your address and i will send them to you ( you pay the post :D )

[Dayu]

Well, hopefully it's just the sensors that are not plugged into the manifold. If that's the case, then the car is just confused and not giving the correct air/fuel mix. Easy enough job to have the sensors put in. (though I would be wary of a workshop that didn't know to put the sensors in! )

apsaagua, you mentioned that the back box was cut off. Does that mean you only changed the muffler (as opposed to a typical aftermarket back-box change where the after-market part includes the muffler and the pipe all the way to the cat, and is a simple bolt on job)?

Here's a pic to show what I mean

Did your new back box include the pipe or was it just the 'box' itself?

[apsaauga]

Ok, no i am confused, the lexus director drove my car today, and he said that Is200 has no power in low revs, and only from 3000RPM it gets fast, so i have to trash it. Hmz... i tried it, it looks like it is true, but i don't remember that it was all the time, really...

Gord>>>thanks, but before buying something i want to be 100% sure that thisd is the problem. Besides my sensors were plugged in, so i think this is not the problem and if it was, i think there were some errors?

Dayu>>>if your sensors were unplugged, did computer show any errors?

And guys, could anyone of You try to drive slowly in second(<20 km/h) or third gear(40 km/h), and tell me how was your acceleration, i would appreciate.

Oh on monday i am testing new GS300, let You know mu opinion:) One thing i didn't like is the clastrophobia(or how it is written) feeling inside:(