4in exhaust.....

[klh6686]

3 inches is fine for single turbo setup,most supras that have 4 inch exhaust that moves in to a 6 inch opening have a twin set up, just throwin that out there

Huh?? i've seen plenty of singled supras AND skylines w/ 4"

A FINE example is here:

http://kismo.dk/sider/kismo%20exhaust.htm

[002-M-P]

Yes, Kismo is a sweet car. None of his video links work on that sight, but I believe they still do from Exvitermini.

[Sil240]

3 inches is fine for single turbo setup,most supras that have 4 inch exhaust that moves in to a 6 inch opening have a twin set up, just throwin that out there

I've been around a whole bunch of BIG HP supras and I've never seen such a thing in my life. My boss had 700 hp in his Mark 4 and he had a off the shelf HKS or Greddy exhaust.

A 6" exhaust has got to look rediculous!!!!

Reminds me of that picture of the Gray 87 Civic with the bucket in the middle of the bumper. LOL hahaaha

[klh6686]

Yes, Kismo is a sweet car. None of his video links work on that sight, but I believe they still do from Exvitermini.

I've never had his videos not work, Right Click>Save As

[002-M-P]

I've never had his videos not work, Right Click>Save As

Yeah...maybe its just my work computer that won't let me download them? I can download them from exvi though.

[Swedish Mike]

I normally use 3" up to 700bhp, tried bigger but no gain. Not worse either.

My buddy got a 800bhp Supra and an "off the shelf" 4" exhaust with 6" tip, possible to find but can´t remember the brand.

The exhaust system can´t be too big on a turbo car but on the other hand, no idea to mount a huge, heavy and hard to fit exhaust if you don´t need it.And yes, I´ve tested different sizes and length in the dyno, can´t go too big.The restriction you need is built in in turbo cars, the turbine gives you more than enough.N/A´s on the other hand need some love and thoughts.

Will go 3.5" DP and 3" catback aluminum exhaust on my new 600whp engine, wish me luck and pray it wont melt.

[BoostFab]

When in doubt, go by Corky Bell's quote "The best exhaust for a turbo, is no exhaust at all"

But just incase that doesn't do it for you, I found this on another forum...

thanks for the follow up; i didn't have time to search for supporting information in my point; as many are doubting what i've said. but again, how many are running 600hp here. lol.

Will go 3.5" DP and 3" catback aluminum exhaust on my new 600whp engine, wish me luck and pray it wont melt.

it's better to do a 3" down pipe and a 3.5" all the way back. i've been tossing around that aluminum catback idea for a while now, i might weld one up for laugh and giggles.

[Cpt_Impossible]

Wow this argument comes up all the time... There is always someone quoting their teacher with no real world experience. Even on the N/A KA24DE it was dynod that 2.5<3<3.5 with no losses only gains. No one has done a comprehensive test bigger than that though. As far as I have seen on turbo cars you can go as big as you want but after a certain point you wont see any more gains. If you would see gains with every size increase then running open turbo would be infinitely powerful.

[Swedish Mike]

thanks for the follow up; i didn't have time to search for supporting information in my point; as many are doubting what i've said. but again, how many are running 600hp here. lol.

it's better to do a 3" down pipe and a 3.5" all the way back. i've been tossing around that aluminum catback idea for a while now, i might weld one up for laugh and giggles.

The exhaust gases expand like crazy when they leave the turbo, that´s why you want a big DP.When it cools down it need less space and you can probably build a great system going 3.5" DP -> 3" catback -> 2.5" tip and get the same result as straight 3.5".

According to some US program for exhaust calculation the best way to go is a cone right after the turbo, huge DP and then smaller.This construction is also used in WRC and other crazy "by the book" race cars like Audi IMSA/GTO (720hp at 2.1 litre with restrictor).

This is what I´ve learned and worked so far.

About the alu exhaust, never measured EGT at the end of the DP, any idea how hot it can get?I´ll run Ethanol and that will help a bit but don´t know how much yet.

[002-M-P]

Yea, I'm looking for 550-600whp or so on my RB26 on 93 octane...probably going to be running a TO4Z...my preliminary plans as of right now are 4" downpipe and 3.5" all the way out if I run an open wastegate or probably 4" downpipe to 4" all the way out if I plumb the wastegate gases back in. Like I said, not positive yet, still a couple of months till the motor goes into the car.

[DriftingisLame]

I believe it was in the same book by Corky Bell, Maximum Boost, that explained that the ideal exhaust on a turbo car is a cone that is the size of the turbine housing outlet that tapers into a larger diameter.. Not for backpressure reasons but for directing heat away from the turbo.

I think the turbo provides a ton of backpressure to begin with, therefore do what you want after the turbo..

I made my own 3.5" turbo back exhaust, and noticed a huge gain on the as$ dyno. The problem is, everywhere in town had either 3" or 4" mandrel bends, I found it extremely hard to track down mufflers, flex pipes, bends, and flanges for 3.5". Go 3 or go 4!

I doubt you'd loose any power going with the bigger piping, but if you're not going to be around 500 whp where the extra diameter is going to make huge gains, then dont bother with the more expensive, harder to fit, larger piping..

Good luck!

[mello88]

I think the turbo provides a ton of backpressure to begin with, therefore do what you want after the turbo..

Yes the turbocharger creates all the backpressure you could ever need. It's referred to as Turbine Inlet Pressure aka TIP, which is directly influenced by turbine housing A/R:

(Clicky click for some more reading on the topic)

The only reason a turbo works at all is due to the pressure differential before and after the turbine. There is a very high pressure contained in the exhaust manifold trying to escape by pushing through the housing/turbine, to the very low pressure zone on the other size (downpipe, exhaust, atmosphere). If the TIP is 20psi and the exhaust backpressure 6psi, you have a 14psi force that is spooling your turbine. By lowering the exhaust backpressure, you have just increased the efficiency of the turbocharger without adding more TIP (more boost and/or fuel). It's "free" power, up to a certain point, when you'll cease to see any benefit... But even so, you'll never see the turbine shaft speed drop, you'll only get closer and closer to it's maximum speed with a 0psi exhaust system (which probably resembles Corky's description: a cone that exists only to direct heat away from the turbo). When you look at it this way you see it's ridiculous to argue that a more restrictive exhaust (smaller/longer/bendy-er) could possibly be more beneficial than a less restrictive one (larger/shorter/straighter) on a turbo engine.

LOL exhausts are one of the few things I know about simply due to my fluid dynamics class. The overly large exhaust theory can be demonstrated by buying a milkshake. You ever notice why they don't give you the regular straws for drinking the shake? Because it requires more energy to move the fluid through the straw. Likewise they don't give you a straw a foot in diameter because it will be equally difficult.

Your analogy is great -- for an NA motor. But what if we change it a little to apply to turbo engine..... Take your milk shake cup, seal it up with RTV, then pipe in 20psi of pressure from the bottom of the cup. Now which straw, 1/4", 3/8" or 12" is going to offer the fastest way to empty the cup? Obviously the 12" straw, it will just take slightly more time for each smaller straw size you use. On a turbo engine the exhaust after the turbine is being forced out, and on an NA engine it's being sucked out by the pulse ahead of it. The physics are different because of the turbocharger and the NA exhaust logic doesn't apply as readily.


Modified by mello88 at 1:57 PM 2/22/2007

[jobestudios]

what about off boost in the lower RPM's though?

[Shocker]

i

what about off boost in the lower RPM's though?

its a 2-2.6 liter off boost with lower compression than an N/A motor its gonna be a dogg anyways.

[rbsileighty]

NIc

Yes the turbocharger creates all the backpressure you could ever need. It's referred to as Turbine Inlet Pressure aka TIP, which is directly influenced by turbine housing A/R:

(Clicky click for some more reading on the topic)

The only reason a turbo works at all is due to the pressure differential before and after the turbine. There is a very high pressure contained in the exhaust manifold trying to escape by pushing through the housing/turbine, to the very low pressure zone on the other size (downpipe, exhaust, atmosphere). If the TIP is 20psi and the exhaust backpressure 6psi, you have a 14psi force that is spooling your turbine. By lowering the exhaust backpressure, you have just increased the efficiency of the turbocharger without adding more TIP (more boost and/or fuel). It's "free" power, up to a certain point, when you'll cease to see any benefit... But even so, you'll never see the turbine shaft speed drop, you'll only get closer and closer to it's maximum speed with a 0psi exhaust system (which probably resembles Corky's description: a cone that exists only to direct heat away from the turbo). When you look at it this way you see it's ridiculous to argue that a more restrictive exhaust (smaller/longer/bendy-er) could possibly be more beneficial than a less restrictive one (larger/shorter/straighter) on a turbo engine.

Your analogy is great -- for an NA motor. But what if we change it a little to apply to turbo engine..... Take your milk shake cup, seal it up with RTV, then pipe in 20psi of pressure from the bottom of the cup. Now which straw, 1/4", 3/8" or 12" is going to offer the fastest way to empty the cup? Obviously the 12" straw, it will just take slightly more time for each smaller straw size you use. On a turbo engine the exhaust after the turbine is being forced out, and on an NA engine it's being sucked out by the pulse ahead of it. The physics are different because of the turbocharger and the NA exhaust logic doesn't apply as readily.



Modified by mello88 at 1:57 PM 2/22/2007

Nice to see someone who actually knows what they are talking about...

I'd say the biggest diff on the whole "milkshake" thing is this however...

The milkshake scenario is sucking, so it would apply to intake and this is exhaust... exhaust diameter/length in NA applications helps more in scavenging I believe than anything else... which is important before the turbo for us... and mello is exactly right on the pressure diff at the turbo... the higher change in pressure and change in temp between either side of the turbine... the higher the efficiency and the more energy

Unfortunately fluid flow is just as important, so a turbine that will exit at say 2.5"/3" is less efficient to dump directly into a 4" exhaust vs tapering smoothly from the turbine exducer to the main exhaust in a gradual transition to the desired size system... also remember that as the cross section area goes up... velocity goes down... and the more bends... and... and........ exhaust tuning is not simple

[mello88]

Definitely. Another issue with the large sized exhaust and poor velocity has to do with the exhaust cooling off as it expands, slowing and "blocking" the next pulse behind it. More apparent on N/A engines but should still applies to off-boost turbo engine as well.

Anyone have any recommendations for some books that go into better detail on this subject? I already have "Corky Bell/Turbochargers", "Scientific design of intake & exhaust systems", and "Engines, an Introduction".. Always looking for more info for my library.

[redtop91]

The milkshake scenario is sucking, so it would apply to intake and this is exhaust...

You are actually quite incorrect. The properties of fluids are the same whether pulling or pushing. Try the same scenario blowing air into the shake. Same results. Although I agree with mello88.