2.5'' on procharged LS

[spf_lexus]

Hi all, finished my procharger 6psi and am loving the power and the low end power. I have a cat back JIC mandrel 2.5'' from mid cat back and I didnt loose much low end from it but here's my question:

-If I went thru and replaced the first "Y" and went 2.5 from header back utilizing the stock "y" locations would there be a significant drop in low end? I have a lot of low rpm air being pumped and i figure it will help the flow better.


I plan on going to Mufflers West here in Temecula CA and have them replace the stock y w/ a race merge collector and bore out the mid cat ID to accomodate the 2.5''. Hoping for opinions/suggestions

thnaks all

 

[JBrady]

Hi all, finished my procharger 6psi and am loving the power and the low end power. I have a cat back JIC mandrel 2.5'' from mid cat back and I didnt loose much low end from it but here's my question:

-If I went thru and replaced the first "Y" and went 2.5 from header back utilizing the stock "y" locations would there be a significant drop in low end? I have a lot of low rpm air being pumped and i figure it will help the flow better.


I plan on going to Mufflers West here in Temecula CA and have them replace the stock y w/ a race merge collector and bore out the mid cat ID to accomodate the 2.5''. Hoping for opinions/suggestions

thnaks all

Shawn, first of all congratulations on supercharging your LS400. You have already learned much. Now, keep in mind your BOV experiments and costs with what I am going to say on exhaust. Your instinct will be to go big thinking it will work the best. Let me STRONGLY recommend you keep your stock parts so you can put them back ON the car if you aren't happy with any changes you make.

The stock Y pipe works much better than it would look and is NOT a restriction below 250hp and very little up to and over 300hp. So, installing larger pipes will only allow more flow ABOVE the stock restriction power levels. Below those levels how could a bigger pipe help?

Now, you have a centrifugal supercharger. These do not make much low RPM boost but are moving more air than no supercharger at low RPMs. That is why you get more power even without noticing any actual boost. At the intake valves there is MORE pressure than without the supercharger compared to stock.

You also have a torque converter which allows the engine to rev more quickly than stock. Both the TC and SC give better low RPM power. Still, the power levels are below the stock Y pipes ability to flow. Therefore increasing the pipe size will reduce velocity and therefore make the engine work harder to PUSH out the exhaust at low RPMs. How much difference will you notice? Hard to say because each design and construction is different. What you will probably notice is that the engines FEELS softer, somewhat sluggish and less sharp that with the stock Y pipe.

So, how much could you gain in the higher RPMs with bigger pipes? Again, depends on many variables. May be very little.

My recommendation is if you want to experiment (R&D, research and development) would be to get 2 bolt flanges that match the catalyst outlets and have whatever system you have made created from there. This leaves the stock Y pipe intact and easy to re-install for back to back testing. You may find that the stock pipe makes the car more fun to drive. Of course it is possible to create a better pipe than stock but it is NOT easy and takes a bunch of testing an solid understanding and experience.

The stock pipes use twin 1.97" mandrel bent stainless pipe that Y's into a single 2.36" center pipe. This as I said above is pretty darn efficient at 300hp. Trying a racing merge pipe using 2.25" pipes into a single 2.5" pipe on a GS400 with S&S headers showed 1 rwhp gain (ONE horsepower) over the stock Y pipe. Now, this bigger Y pipe really made the car feel LAZY. Going back to stock makes the car MUCH more fun to drive.

So, spend some money and try some mods. It is a good way to learn. Exhaust systems are similar to musical instruments and small differences and changes can make big differences in performance. Pipe size is only ONE variable out of MANY. Racing designed parts are for high RPM power and a street car needs power from idle up. I always recommend the SMALLEST pipe sizes that will support the desired peak power. I would rather make 5 or even 10hp LESS peak power for a sharper crisper feeling engine. In fact this will usually make a FASTER car!!!

 

[Thermactor]

so you're saying that the gains to be had by aftermarket exhaust up to 300ish hp are very limited?

 

[spf_lexus]

Jbrady

Thanks for the response, appreciate the info from an exhaust expert (If i had an exhaust question you would be the 1st on my list for answers). I was curious after reading the con of 2.5'' "Y" pipes but sicne i have a mid cat back 2.5'' JIC setup i have a half/half exhaust. half is 2.5'' and half is stock, my instincts 1st told me to match, but thats going to rob the streetable power so my next idea was 2.25'' for the front "Y" and leave the mandrel 2.5'' mid cat back as is. To be honest i like the low end SLIGHTLY sluggish because i tend to get myself in trouble w/ low end daily driving so i opted for the freeway/cruising specs. I read your thread on merge collectors and strongly believe that JUST adding one to the front "Y" could open up some HP. Any ideas about adding .25'' to the front "Y"?


Thanks for the tip on adding flanges and keeping the stock "Y" for future re-install. I'm leaning towards upping to a 2.25'' mandrel w/ merge. What are your feelings on 2.25'' if i upped from 6psi to 9psi? That was my original goal of under 10psi but i dont want to do this mod more than I have to.

 

[JBrady]

so you're saying that the gains to be had by aftermarket exhaust up to 300ish hp are very limited?

IT is tough to improve on factory Y pipe at these power levels with the goal of having a wide power band and response that a street car needs. There are some gains after the Y pipe. Baffled mufflers flow much less than pipe.

With enough time, money and experimentation I am sure a better part than stock can be made. Making a specific recommendation that someone can easily copy is not realistic.

 

[spf_lexus]

Alright I decided to move the money into my electrical/ignition instead. the stock front y is not changing but do you have an idea of gain if i added a merge collector and kept everything OEM... just cut and re-weld. I like the concept of exxhaust velocity, just recenty started learning about it. you stated that faster velocity means more efficiency, for a system thats already efficient, just allowing more gas to exit at a faster rate and in theory more hp.

 

[JBrady]

Jbrady

Thanks for the response, appreciate the info from an exhaust expert (If i had an exhaust question you would be the 1st on my list for answers). I was curious after reading the con of 2.5'' "Y" pipes but sicne i have a mid cat back 2.5'' JIC setup i have a half/half exhaust. half is 2.5'' and half is stock, my instincts 1st told me to match, but thats going to rob the streetable power so my next idea was 2.25'' for the front "Y" and leave the mandrel 2.5'' mid cat back as is. To be honest i like the low end SLIGHTLY sluggish because i tend to get myself in trouble w/ low end daily driving so i opted for the freeway/cruising specs. I read your thread on merge collectors and strongly believe that JUST adding one to the front "Y" could open up some HP. Any ideas about adding .25'' to the front "Y"?


Thanks for the tip on adding flanges and keeping the stock "Y" for future re-install. I'm leaning towards upping to a 2.25'' mandrel w/ merge. What are your feelings on 2.25'' if i upped from 6psi to 9psi? That was my original goal of under 10psi but i dont want to do this mod more than I have to.

Well, this is how you learn. Instincts are not easily over-ruled by someone telling you otherwise. Instincts can be expensive :cool2:

Exhaust design handles flow WELL below capacity and at some point deals with restriction as capacity is approached.

Below capacity design is HARD to visualize... let me try to explain.

Air has PRESSURE and density. It takes energy to move air. When exhaust leaves the cylinder and enters the exhaust pipe system it must FORCE the existing air AWAY from the engine. Once this happens the air speed and therefore has enertia. As the exhaust cycle finishes the force on the MASS in the pipes is reduces dramatically and the air MASS slows, stops and reverses like a rubber band.

Now, the higher the VELOCITY in the pipes the less reversion can happen. Going to larger pipes reduces velocity and allows MORE reversion to happen a LOW FLOW conditions I.E. during low and mid RPM engine operation. The higher the reversion the harded it is for the engine to work against this reversion created BACKPRESSURE. This is why larger pipes make less low and mid RPM power... they actually INCREASE backpressure at THESE RPMs.

 

[JBrady]

Alright I decided to move the money into my electrical/ignition instead. the stock front y is not changing but do you have an idea of gain if i added a merge collector and kept everything OEM... just cut and re-weld. I like the concept of exxhaust velocity, just recenty started learning about it. you stated that faster velocity means more efficiency, for a system thats already efficient, just allowing more gas to exit at a faster rate and in theory more hp.

Actually, the stock Y pipe already has a merge collector. Each 50mm pipe is expanded and shaped into a half circle that is half of a 60mm pipe. Then the single 60mm center pipe is stretched (swaged) to slip OVER the two half circles.
The cross section flow area of each half 60mm circle is roughly 2.2 square inches.
Each 50mm pipe BEFORE the Y has 2.65 square inches cross section flow area.
So, as the gas flows from the round 50mm pipe to the half round formed 60mm section the gasses accelerate. This is desireable and is a trade off between higher velocity and reduced total capacity. Now, we are WELL below total capacity at low and mid RPMs so no restriction to flow is significant. Now, the SHAPE of this section flows fine away from the engine but flows poorly back toward the engine. This is not the case with a racing Y merge which flows very well in both directions. This better reverse flow HURTS the merge collectors performance at low and mid RPMs. The racing merge collector does accelerate the gasses but offers no one way flow action like the stock Y does.

Continue this thinking and you may start understanding why catalytic converters help low and mid RPM power. The work as one way check valves.

If you REALLY think about the exhaust cycle you will visualize that pressure and rate of flow is highest during blowdown. Blowdown is the time that the exhaust valves open during the end of the powerstroke BEFORE bottom dead center (BBDC). After BDC (ABDC) the crankshaft must PUSH out the remaining gases. As the piston approaches top dead center (TDC) is slows way down and exhaust flow is slow.

So, during blowdown the gases get forced through the passages, manifolds, cats, pipes ect. Towards the end of the exhaust cycle reversion can pressurize the cylinder, reduce power, are trapped in the chamber as the exhaust valves close and reduce how much new charge can be ingested during the intake stroke. This extra pressure and burnt gas residue will also expand into the intake manifold as the intake valves open and the piston is near TDC. This causes the black buildup inside intake manifolds. REVERSION.

 

[spf_lexus]

Thanks, sometimes more is less. My ignition is going to thank me now lol.

 

[edz]

Correct size for dual pipe tt

JBrady

I'm wondering what size dual pipes you recommend for a dual system on my car, 1uz with twin turbos. Firstly I definitely want twin pipes as I love the sound of them and have to run cats. Aiming for 400 - 450 rwhp.

Not a lot of room from the turbos, do I run 3" or 2.5" from there to the cats, then dual 2.25 in, or should I go 2in or 2.5in? Looking at a X pipe or balance pipe in between? I'm building the system myself from mandrel bends and straight pipe.

Edz

Actually, the stock Y pipe already has a merge collector. Each 50mm pipe is expanded and shaped into a half circle that is half of a 60mm pipe. Then the single 60mm center pipe is stretched (swaged) to slip OVER the two half circles.
The cross section flow area of each half 60mm circle is roughly 2.2 square inches.
Each 50mm pipe BEFORE the Y has 2.65 square inches cross section flow area.
So, as the gas flows from the round 50mm pipe to the half round formed 60mm section the gasses accelerate. This is desireable and is a trade off between higher velocity and reduced total capacity. Now, we are WELL below total capacity at low and mid RPMs so no restriction to flow is significant. Now, the SHAPE of this section flows fine away from the engine but flows poorly back toward the engine. This is not the case with a racing Y merge which flows very well in both directions. This better reverse flow HURTS the merge collectors performance at low and mid RPMs. The racing merge collector does accelerate the gasses but offers no one way flow action like the stock Y does.

Continue this thinking and you may start understanding why catalytic converters help low and mid RPM power. The work as one way check valves.

 

[JBrady]

JBrady

I'm wondering what size dual pipes you recommend for a dual system on my car, 1uz with twin turbos. Firstly I definitely want twin pipes as I love the sound of them and have to run cats. Aiming for 400 - 450 rwhp.

Not a lot of room from the turbos, do I run 3" or 2.5" from there to the cats, then dual 2.25 in, or should I go 2in or 2.5in? Looking at a X pipe or balance pipe in between? I'm building the system myself from mandrel bends and straight pipe.

Edz

Edz,

Nice project.

Turbos have completely different exhaust requirements. The way the turbine extracts energy is the heat and pressure differential from inlet to outlet. Turbine is the highest restriction but is also the ultimate ONE way valve. Since the turbo is such a restriction you have a very hard time making the primary pipes too small and can easily make the too large. Yours are on the large size. Also the outlet flow from the turbine likes a big area to change from spinning and turbulant to smooth directional flow. A large outlet helps here as well.

For outlets I recommend 3" if you can fit them. If not maybe a separate WG pipe and that reduces the turbine outlet requirements and 2.5" would be ok. 2.5 will work either way but again bigger outlet and smaller inlets are the rule for turbos. No crossover is required as the turbines negate any help they create. Some sound reduction is probably available though with an X pipe.

 

[edz]

Exhaust sizing for turbos

Thanks for the quick response J Brady. Can I clarify a couple of things please as ther are a number of posts from different people on this forum that contradict each other, whereas after reading a lot of your posts I tend to trust what you say.

When you say the primarys are a bit large are you talking about my steampipe manifold or the sizes I'm talking about for the pipes after the turbo? My primaries are 35mm id (1 3/8"), the next size down was a lot smaller.

I'm definitely separating the wastegate from the mainpipe so I'll try to use 3" if I can, otherwise 2.5". Should I run 2.5" for the rest of the system or smaller, or step it down? Just finished reading a good article on exhausts by a well known exhaust building company here in Aus and they say they always drop the size down as the pipe goes over the diff to the rear as it improves the sound without any measurable loss in power?

Definitely fit an X pipe, as I am running a live axle and cats, so it may be tricky fitting an xpipe in? There are a couple of posts on here that say don't fit an x pipe on a turbo as it 'disrupts the flow from the turbos'? The prementioned article shows a good x pipe as being one where the hole between them is only about 40mm in diameter rather than a large scollaped section, but others are saying this is very poor in their testing - check this link with photos http://www.surefloexhaust.com.au/xpipe.htm as the 'bad x-pipe' shown is manufactured by the people I mentioned from the article above.

 

[JBrady]

Edz, my apologies. I was actually creating a couple of answers to threads both here and on another forum...

1.375" ID primaries should work very well.

You do not need an X pipe. The link you provided is a shop that creates parts but does not give credit to other designs and does not seem to comprehend what a true X pipe really does. See Burns Stainless for the current state with my nozzle idea possibly better for street driving without turbos.

With turbos the X pipe may make it quieter but not likely to effect performance unless it creates a flow restriction.

As for reducing pipe size past the rear axle... basically the same as X pipe answer above. You may wish to build it to the rear axle then have a crush bent pair of pipes made to test different sizes to determine power and sound changes. I like quiet but of course that is subjective.

Keep us posted!

 

[edz]

Pipe design for turbos dual pipe

No worries, that makes sense now. So what size pipe do I use for the dual system with the twin turbos?

Edz, my apologies. I was actually creating a couple of answers to threads both here and on another forum...

1.375" ID primaries should work very well.

You do not need an X pipe. The link you provided is a shop that creates parts but does not give credit to other designs and does not seem to comprehend what a true X pipe really does. See Burns Stainless for the current state with my nozzle idea possibly better for street driving without turbos.

With turbos the X pipe may make it quieter but not likely to effect performance unless it creates a flow restriction.

As for reducing pipe size past the rear axle... basically the same as X pipe answer above. You may wish to build it to the rear axle then have a crush bent pair of pipes made to test different sizes to determine power and sound changes. I like quiet but of course that is subjective.

Keep us posted!

 

[JBrady]

No worries, that makes sense now. So what size pipe do I use for the dual system with the twin turbos?

My previous recommendations above are valid.

 

[edz]

Exhaust size

Just re-read it, all makes sense. Cheers for the info.

My previous recommendations above are valid.