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I pull 8,000 lbs with 3.55's and 33x12 10 plys, and while I am tuned, I've not lost any low end with the 4" and it pulls like a mule. As far as a DD, I'm not sure how you could make it any better. Is it on the loud side? Yes, but I'm just one of those that prefers it that way.
 
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the cause is at low RPM there is insufficient exhaust velocity.
Please explain the effect of exhaust velocity on a turbo charged engine once the exhaust has passed through the hot side... Then I'll tell you how wrong you are.
 

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Please explain the effect of exhaust velocity on a turbo charged engine once the exhaust has passed through the hot side... Then I'll tell you how wrong you are.
Just got frustrated and skiped alot. But im pretty shure velocity still plays a small role. Even with a 3"down pipe dont they kneck to one 3" y? Or worse with stock mid pipe. This to me would be a much better subject.

Just an oppen turbo outlet is not ideal. Look at jets. They have things that shrink or grow based on needed thrust at whatever speed. Now comepair thrust to back pressure. At low boost or no boost this could be a way to achieve torque. Also on the open dumping turbo turbulence can happen and make it harder for the exhoust to get out. Imo this is the limmit of velocitys need. Keep in mind tuning and manipulating the waste gate plays a role.

Its to my understanding the ability to get in boost sooner is a much more affective way to power then scavenging. To do that with turbo back exhaust being the control subject. the idea is mape the lowest post turbo pressure possible. Its also my understanding that a few degree cone or trumpet is the best thing to do that. To put that in a practical application says to me a progressively growing exhoust is best thing you can have. So if we are talking about what follows a 2 1/2" choking point i would think any thing you do is neerly in vain?
 

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Please explain the effect of exhaust velocity on a turbo charged engine once the exhaust has passed through the hot side... Then I'll tell you how wrong you are.
Its really no different from a naturally aspirated setup. It still has to go somewhere. Also, Steven hits it. But please, since I'm so wrong, tell me why.
 

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whoa whoa whoa. Water jet velocity is a used to create thrust to move the jet ski forward. If we were making thrust with our turbo's the exhaust would have to point straight back.....

#checkyourfactsyo
 

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whoa whoa whoa. Water jet velocity is a used to create thrust to move the jet ski forward. If we were making thrust with our turbo's the exhaust would have to point straight back.....

#checkyourfactsyo
Not shure who directed at. But for my example i was leaving transferring thrust to pressure on the back of exhoust valve to the imagination.
 

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Not shure who directed at. But for my example i was leaving transferring thrust to pressure on the back of exhoust valve to the imagination.
It was directed at you, and I agree... your analogy is way off base. If you're ever in Houston I'll be happy to sit down with a dry erase board and a 12 pack and walk you through the explanation of why. I don't think I'm intelligent enough to clearly explain it on a forum space, but I'll try.

The difference is that by the time the gas exits the turbo it's job is done and it's just waste. Any backpressure past that point only decreases the Delta P across the turbine and lowers it's efficiency.

In the jet application the work is not done until the point of exit from the tapered exhaust. To maximize that Delta P you taper it down at the exit. It's a trade off in that case between reducing mass flow rate and increasing velocity. For turbos you ALWAYS benefit from mass flow rate increases.

To your previous point about gradual tapers flowing best. If I recall the optimum range is typically 7-11 degrees for most gasses.
 

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It was directed at you, and I agree... your analogy is way off base. If you're ever in Houston I'll be happy to sit down with a dry erase board and a 12 pack and walk you through the explanation of why. I don't think I'm intelligent enough to clearly explain it on a forum space, but I'll try.

The difference is that by the time the gas exits the turbo it's job is done and it's just waste. Any backpressure past that point only decreases the Delta P across the turbine and lowers it's efficiency.

In the jet application the work is not done until the point of exit from the tapered exhaust. To maximize that Delta P you taper it down at the exit. It's a trade off in that case between reducing mass flow rate and increasing velocity. For turbos you ALWAYS benefit from mass flow rate increases.

To your previous point about gradual tapers flowing best. If I recall the optimum range is typically 7-11 degrees for most gasses.
Relly think we are mostly on the same page but i think the possibility of post turbo back pursuer affecting pre turbo pressure exsitets aspetay out of boost. I have an understanding that low rpm low throttle input you want so back pressure maybe even more then the turbo offers. Yes my analogy sucks but thinking of thrust as a force helping bring up dynamic compression.
 

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It was directed at you, and I agree... your analogy is way off base. If you're ever in Houston I'll be happy to sit down with a dry erase board and a 12 pack and walk you through the explanation of why. I don't think I'm intelligent enough to clearly explain it on a forum space, but I'll try.

The difference is that by the time the gas exits the turbo it's job is done and it's just waste. Any backpressure past that point only decreases the Delta P across the turbine and lowers it's efficiency.

In the jet application the work is not done until the point of exit from the tapered exhaust. To maximize that Delta P you taper it down at the exit. It's a trade off in that case between reducing mass flow rate and increasing velocity. For turbos you ALWAYS benefit from mass flow rate increases.

To your previous point about gradual tapers flowing best. If I recall the optimum range is typically 7-11 degrees for most gasses.
all the correct answers are already in the thread..for those that read it all..

AS far dry erase marker thks but, I personally spent enough time teaching performance among other stuff here in Houston with a dry erase....
but I get your point about trying to get the correct information across on a forum ,it doesn't always come out correctly,which means you can say one thing and mean another....the only other option is to write out long drawn out technical explanations where it's just too easy to make a single mistake,which derails the whole thread....I get in a rush and make mistakes all the freaking time...
in a class room,you can just say opps,wipe it clean,and start over,and everyone gets it at the same time...

but you know it would not be a bad idea to have a classroom teaching event ,food,fun etc at someone's shop covering a different subject every month....

I know the Buick guys used to do it..
 

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so a LET cutout is worse than a 4" catback hey. Hmm.

Cause if people are losing on 4". Then what are they losing on a no exhaust. ie CUTOUT.


Sent from my non using Tapatalk in ??
 

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Lots of wrong information.
This subject has me so confused! I have now read through the whole thing.

I feel like in one breath guys are saying post turbo exhoust tuning is irelavent to scavenging, then explaining the effects of it in the next breath!

If the anwser is in here, it lacks the "why" explanations, and and or the disproof of opposing theorys.

One thing i tend to coclued when opposing theory our fought to death is, both sides are right or wrong, and theres more to it.

I like that aboosted brings up the point of load as an important variable. But why? Is exhoust velocity less or more? Is it safe to say waste gate, cam timing, spark timing , oporating rpm, power made, and ...(aka tuning) all play a role in this discussion? Would it be possible to manipulate a 4" cat back to make lowend torque also?

Its a bit frustrating when these topics turn in to i know more then you debates. Here is an important fact. largely in part by way of header tuning the Mazda "sky active" is making race engine compression on pump gas. Why i bring this up is to make the point that if you asked the most brilliant mind in the field to accomplish that on a mass produced application in the year 2000 it would have been deemed impossible.

My point is no one know every thing or even close to it. When at the track i have seen many turbo exhoust setups varying across the subjected spectrum. Same for exhaust design on production turbo cars.

If i was trying to play the know it all role my posts would lack "?" Marks and "to my understanding" just here to contribute and learn.

Now for facts i have that could start some good talking points:

My first mod was a 4" cat back. I felt allmost nothing. But i used the stock mid pipe to cat back flange. (2 1/4 +/-) after cuting the flange back as far as i could (3"+/-) the but dyno felt it as well as mpg. (Stock tune)

I think hart throb clamed 50/30 ish with just there cat back.

To my understanding one of the site vendors noticed a small torque loss with a 4" vrs 3" tuned right?

Catless downpipes lost power almost everwere with superchips tune, and is amazing every were with one custom tune and is amazing mid and up top with another. (4" cat back my truck)

Hoping we can spend more time debating the subject, and less proving who know more then the other. Im guilty my self at times i know its tough, but at the end of the day its the internet and we should all be taken as simple opinions that in conjunction may come to a conclusion or help a viewer, and or our selves come to an extremely educated conclusion. Sizing up each other dos nothing for any of us.
 

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so a LET cutout is worse than a 4" catback hey. Hmm.

Cause if people are losing on 4". Then what are they losing on a no exhaust. ie CUTOUT.


Sent from my non using Tapatalk in ??
Imput from someone with cut outs would be helpful. Calling scap! Lol
 

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Imput from someone with cut outs would be helpful. Calling scap! Lol
Cut outs and large exhaust from what I understand are definitely not the same thing. An exhaust which is too large can actually increase backpressure, like the 4" to 3" debate. When an exhaust is too large you are slowing down the velocity of the exhaust, and allowing it to cool, this cool exhaust requires a push to get through to the tail pipe, rather than flow properly due to heat, volume, and velocity. A cut out would allow this exhaust out right away, thus negating the effect.
 

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I just can't imagine running a cutout. I started up mine with just the catless downpipes and it sounded like a hopped up dragstrip car. Lol loud as ****. I don't know if its louder or quieter when driving because I put the cat back on. But the neighbor on the next street over came over and was wondering what I did to it when I revved it up. Haha.
 

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so a LET cutout is worse than a 4" catback hey. Hmm.

Cause if people are losing on 4". Then what are they losing on a no exhaust. ie CUTOUT.


Sent from my non using Tapatalk in 
Not sure how anyone got that idea...keep saying it..the best exhuast for power on any motor is no exhuast !!!!!
 

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anytime you install bigger turbos it equals less low end flow with more total exhaust gas flow normally,and sometimes less low end power , much more power on top normally..
the 58 mm turbos are a little different ,because your not increasing the size of the exhuast impeller on the turbo,so your not dealing with any loose of low end exhaust speed which equal,no increased lag...
plus your increasing the amount of intake flow which increases power all around the rpm band it's the whole idea built around batmo wheel,and the Larger intake wheels on any turbo....

so scaps truck might see even more low end power , with dumps than a turbo set up that just increased both sides of the turbo..as like i said there is a difference between talking about totally stock truck with tiny turbos ,and a truck with 58mm turbos..other than the best thing for a turbo is no exhaust period...

the point is the factory made a well built match system for low end power,towing,and drivability ... If anyone has a low numeric geared truck that's torally stock,then goes and adds a 4 inch set of DPs off road...with a 4 inch exhuast...I will bet money that there will be a loss of low end drivability that feeling like there is a little lag off throttle ,off the line with no boosting period ,I would bet money...I am not saying the truck will even be slower in the 1/4 mile because you can bring up the boost off the line to make up for the little lag you may get...
i am not saying that it will show a loss of power on the dyno,there is a difference,ask any tuner worth a grain of salt..period...

anyway its why most DPS are 3 inch ,and not 4 inch for exhuast speed,flow,drivability other wise they would all be 4 inch or larger if they fit...everything has to be matched, for what your trying to accomplish....

anyway i hope this helps,if I didn't explain something correct..I tried..lol
 
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