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Turbo exhaust sizing: primary vs collector vs merge vs turbo
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96Mustang460cid
New Member
Joined: 26 Apr 2007
Posts: 12
Location: Oklahoma
Posted: Thu Nov 15, 2007 9:36 pm Post subject:
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Post deleted due to misplaced logic.
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Last edited by 96Mustang460cid on Mon Nov 19, 2007 8:40 am; edited 1 time in total
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Dynoroom
Member
Joined: 01 Aug 2007
Posts: 73
Location: So.Cal.
Posted: Thu Nov 15, 2007 11:48 pm Post subject:
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96Mustang460cid wrote:
blown265 wrote:
I certainly don't want more pressure in the exhaust than the inlet manifold, and am happy to sacrifice turbo response [via pipe size] for a clean cylinder blown down on overlap.
Paul,
You don't have a choice about this. That's how a turbo works -- i.e. a pressure ratio of 2.0 means your exhaust has twice as much pressure as your intake tract. By minimizing your cam's overlap, you can minimize any reversion/flowback into the cylinder.
Let us 'ass'ume we have a 2.0 pressure ratio and are running 14.7 psig (29.4 psia) boost. This means the exhaust system (up to the turbo) has 29.4 psig (44.1 psia) pressure.
Density = (absolute pressure) / (gas constant * absolute temperature)
Gas contant = constant
Temperature = constant (not exactly, but close enough)
Therefore, Density is directly proportional to pressure. If the exhaust pressure is three times that of atmospheric pressure (29.4 psig or 44.1 psia), the exhaust gases will also be three times more dense. With that being the case, your 1.52" exhaust is equivalent to 2.63"
3.14*.25*1.52^2 = 1.81 in^2
Above, I concluded that your exhaust is three times more dense ---->
1.81 *3 = 5.44 in^2
(5.44/(.25*3.14))^(0.5) = 2.63"
All of these calculations also assume the exhaust is an ideal gas (which it's not).
So, Paul, this is how it works in my head. That's why guys can make so much hp using such small exhaust systems. I'm not saying I'm right...(though I think I am)...I do invite people to dispute my reasoning .
Have a good day!
Michael
Edit: Paul, one more thing regarding cam overlap. Though people disagree, I am finding that many 'smart' people recommend less than 3* overlap @ 0.050". Some even have negative overlap -- i.e. -0.5*, ect.
Have a good day! #2
Michael
Hi Michael,
I'm not going to disagree with your ideas but I will add a different twist for others to think about.
1st I know what you mean when you talk about pressure ratio but you are using the term incorrectly. Not a slam just clarification. Pressure ratio in used with the turbocharger compressor map. The pressure ratio is the ratio of inlet pressure + boost pressure divided by inlet pressure. Example is 14.7 inlet + 18 lbs boost / 14.7 = 2.22 pressure ratio.
When back pressure goes past or higher than boost pressure we call it "crossover" less exhaust pressure than boost = positive crossover and goes to negative crossover when the backpressure goes higher than boost. This is the beginning of the end of the hp battle. As backpressure continues to climb hp output will start to level out generally speaking. I look at turbo systems in terms of output. If a guy wants 800 hp out of a 355" v-8 it's really no big deal what type of exhaust system you run as long as the turbo(s) (say T-04's) required to make the power are used. Now if you want to make 1400 + hp with the same engine you need different turbos (IMO) but the mass air flow increases so the exhaust system can now become a problem. The very high output turbo system in simple terms needs room in the exhaust system to allow the engine to move the air through it. Yes I use some formulas to design turbo exhaust systems but I'm not locked into "tuned" exhaust due to backpressure. I've made well over 1700 hp with "log type" systems.
As for pulse tuning, unless you have a flat crank v-8 or a inline 6 or 4 & run under 6000 rpm don't waste your time, JMHO.
As far as camshaft overlap goes it was a huge problem in the past when we ran constant flow fuel injection. Today with EFI I don’t design my cams worrying about overlap. Cams are chosen with considerations to the type of use, hp wanted, etc. Turbo cams are like cylinder heads in that what works to make a normally aspirated engine run better will also help a boosted engine run better. Many of my cams have over 20 degrees of overlap @ .050.
Many of these ideas are in fact running in engines I’ve built and are racing & setting records. In this world people are always coming up with ways to build a better mouse trap, these examples above only show some of what I think I’ve learned in 25 years of building high output turbocharged engines. Your mileage my vary….
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Lifetime Member of the Bonneville 200 MPH Club
Frist Stock Bodied Car to Exceed 300 MPH
"Aerodynamics are for people who can't build engines" Enzo Ferrari
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blown265
Member
Joined: 18 May 2006
Posts: 110
Location: Western Australia
Posted: Fri Nov 16, 2007 12:21 am Post subject:
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Quote:
a pressure ratio of 2.0 means your exhaust has twice as much pressure as your intake tract
I've usually used the term 'pressure ratio' in reference to boost in the intake, not as a relationship between intake and exhaust pressures. Most compressor maps use this definition as well. Working in absolute pressures, the pressure ratio of the inlet is:
boost + atmospheric / atmospheric,
so with, say, 20psi from the compressor, the pressure ratio of the inlet becomes
20 + 14.7 / 14.7 = 2.36.
This 2.36 figure is only a reference to conditions in the intake, not necessarily a measure of the backpressure pre-turbine. I dont believe there is a set relationship between the pressure in the inlet and that in the exhaust [ie 20psi of inlet boost will result in x pressure in the exhaust], but rather exhaust pressure is determined by the turbine/ exhaust pipe size/ cam /hp /etc.
Perhaps we're just using the term 'pressure ratio' to explain different things- my point/aim is to have as little pressure before the turbo as possible.
Regards
Paul
EDIT Mr LeFevers beat me to it and explained it much better as well. Thank you sir.
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blown265
Member
Joined: 18 May 2006
Posts: 110
Location: Western Australia
Posted: Fri Nov 16, 2007 12:47 am Post subject:
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Quote:
As for pulse tuning, unless you have a flat crank v-8 or a inline 6 or 4 & run under 6000 rpm don't waste your time, JMHO.
Mr LeFevers,
We have an inline six and a conservative redline. Could you expand on the pulse tuning mentioned [and how to achieve it]?
Cheers
Paul
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blown265
Member
Joined: 18 May 2006
Posts: 110
Location: Western Australia
Posted: Sun Nov 18, 2007 8:12 pm Post subject:
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The mention earlier of a tuned turbo system has me thinking further about our design.
It was advised in this thread to keep the tubing small and short, so a traditionally long primary tube and large collector [as per Pipemax NA specs] may not be ideal.
Pipemax does however, give a tuned 'shorty header' option, and for our combo it suggests a 15-18 inch primary [merged 3 into 1] and an 8 inch collector [x 2 into the turbine]. Diameters will be 1 5/8 and 2 1/4.
These lengths are workable for the engine bay we have, and while it may be a little more effort, is possible to build.
Any thoughts on whether this will be better/worse/ same for hp/ boost threshold/ etc ?
Thanks
Paul
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Warpspeed
Pro
Joined: 17 Nov 2006
Posts: 448
Posted: Sun Nov 18, 2007 8:48 pm Post subject:
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Just remember that conventional tuned headers/collectors are usually open to atmosphere at the far end, and it is the sudden large pressure drop at the open end that creates the negative return wave that is "tuned".
Turbo pipes operate into a turbo housing that is going to be relatively restrictive. Any return wave will therefore be positive, because what the primary pipes are feeding into looks more like a closed end than an open end at the turbo.
Forget conventional header tuning theory with a turbo. It does not work and is just not appropriate.
There is only one reason to run individual pipes right to a turbo, and that is to help prevent reversion where there are many cylinders with considerably overlapping exhaust pulses.
When an exhaust valve first opens, the flow is somewhat explosive in nature. If that port is closely coupled to another adjacent port, where the exhaust valve is at the very last stage of closing, you do not want to blast hot exhaust gas right into the second cylinder during valve overlap.
By running individual pipes right to the turbo, each pulse has to travel right to the turbo, then back along another runner to reach the vulnerable cylinder. By making the primary pipes long enough, interference can be reduced down to much lower Rpm. This can improve mid range torque and turbo response where long duration exhaust cams are used, and more than one exhaust valve is open simultaneously.
All this has nothing at all to do with conventional header tuning theory, where a negative reflected exhaust pulse is used to improve cylinder scavenging. It would be difficult to imagine a reflected pulse from a turbo header tube powerful enough to overcome say 30 psi of static turbine inlet pressure. But a couple of psi of exhaust depression during valve overlap on a normally aspirated engine can be well worth having.
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Cheers, Tony.
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Dynoroom
Member
Joined: 01 Aug 2007
Posts: 73
Location: So.Cal.
Posted: Sun Nov 18, 2007 9:32 pm Post subject:
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Mr LeFevers,
We have an inline six and a conservative redline. Could you expand on the pulse tuning mentioned [and how to achieve it]?
Cheers
Paul
Hi Paul,
My dad is Mr. LeFevers, I'm Mike.
What are you running for a red line?
What hp are you tring to make?
What do you want from your engine, what type of work/racing do you want it to do?
Pulse tunning requires a divided turbine housing, do you have one or is do they make one for your turbo?
Pulse tunning will not make more power, as a matter of fact it can hurt peak power sometimes but it CAN spool the turbine sooner in some applications. Lets find out what you really want to do.
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Michael LeFevers
www.mitechengines.com
Lifetime Member of the Bonneville 200 MPH Club
Frist Stock Bodied Car to Exceed 300 MPH
"Aerodynamics are for people who can't build engines" Enzo Ferrari
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blow-thru
Member
Joined: 21 Feb 2007
Posts: 83
Posted: Mon Nov 19, 2007 7:55 am Post subject:
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I've always had a problem with people who are not concerned with ecessively high exhaust back pressures on turbo engine for many many years.
This is not to single out anyone in particular as they have there own ideas and obviously work but I can not see the benefit of a higher pressure in the exhaust than in the inlet tract ....
I personally would try to reduce the backpressure to an absolute minimum...
Are turbo exhausts possibly one of those trade offs where which one is the lesser of two evils ???? or as was previously mentioned are satisfactury to a particular level say X hp then they become more scientific when you are pushing the limits of turbo sizing/engine limits etc ????
Dynoroom also reminded me of a little rule that I picked up and that is to build a turbo engine much like an N/A engine, as efficient as possible and then add the turbo ....
Cheers Carl...
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SWR
Guru
Joined: 29 Jan 2006
Posts: 1355
Location: Norway
Posted: Mon Nov 19, 2007 7:34 pm Post subject:
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blow-thru wrote:
I've always had a problem with people who are not concerned with ecessively high exhaust back pressures on turbo engine for many many years.
Me too... The first thing to do with a new setup is to try to find the specs that you need to have the lowest backpressure possible...
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Warpspeed
Pro
Joined: 17 Nov 2006
Posts: 448
Posted: Mon Nov 19, 2007 7:45 pm Post subject:
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It fascinates me when you ask a hard core turbo freak how much boost his engine is running, he puffs out his chest and will tell you to within 1/10 of a psi. He may even have a very large boost gauge permanently installed right in front of his face, which he constantly watches while driving.
Ask the same guy what his total exhaust back pressure is, and he will probably go, "Huh??" and roll his eyes, What is that ???? I have never measured it. Hehehehehe..
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Cheers, Tony.
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Unkl Ian
Expert
Joined: 06 Oct 2005
Posts: 878
Location: Just outside Toronto
Posted: Mon Nov 19, 2007 9:29 pm Post subject:
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And the same guy will tell you it takes "no power" to drive a turbo.
Some will even tell you that turbos don't create any back pressure.
But for some reason,the headers glow red.....
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ThomasL
Member
Joined: 14 Sep 2007
Posts: 89
Posted: Mon Nov 19, 2007 11:37 pm Post subject:
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One look at our data logger will explain the Red header Scared the crap out of me when we started to log EGT's years ago. I've used a guide of about 5psi backpressure over boost (30 to 40 in our case)as the point of diminishing returns (power) for our stuff, and it seems to work out. Look at the EGT's once you go over 5psid.
"All this has nothing at all to do with conventional header tuning theory, where a negative reflected exhaust pulse is used to improve cylinder scavenging. It would be difficult to imagine a reflected pulse from a turbo header tube powerful enough to overcome say 30 psi of static turbine inlet pressure. "
While I would think the gains of a true tuned header may be diminished at boost, I still think there should be some gains. The way I look at it, the reflective wave is not trying to overcome the 30psi, it is merely traveling with in it, or superimposed on top of it. Wouldn't traveling within an elevated pressure atmosphere would be more conducive to transmitting the sound and pressure pulses? Amplified if you will. I think that conventional header theory should be modified for turbo applications not disregarded. Yes they will run great without header tuning, but if there are possible gains to be had, and packaging doesn't require throwing theory out the window, why not try?
"But a couple of psi of exhaust depression during valve overlap on a normally aspirated engine can be well worth having"
Agreed, but in a turbo application, if the pressure at overlap can be depressed a couple of psi from whatever elevated pressure it's at, I could see potential gains. Could it also somewhat alter what the motor would like for lobe center on the cam / cams? Because of the greatly elevated EGT's and the increased backpressure, the speed of sound in that environment is increased, resulting in a different tuned length for the primary. I'm sure that's something that could be calculated by a brainiac like Mr Meaux. I'm not saying this is the way it is, just my opinion.
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Stef
Pro
Joined: 06 Oct 2006
Posts: 258
Location: UK
Posted: Tue Nov 20, 2007 8:27 am Post subject:
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This is how I now design turbo manifolds now: two tuned lengths are used.
1. From valve to collector - tuned as normal.
2. from valve to a couple of inches into the turbine housing - tuned as a two-stroke engine exhaust.
Lengths are selected to have two pressure waves. One a negative, scavaging wave from 1 arriving at overlap. The other, a positive wave from 2 arriving just after EVC point.
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vincer77
New Member
Joined: 19 Nov 2007
Posts: 15
Location: Tennessee
Posted: Tue Nov 20, 2007 1:26 pm Post subject: Crucified Cranks
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96Mustang460cid wrote:
Guys,
Does the following example demonstrate the correlation between the camshaft's exhaust duration (@ 0.050") and exhaust diameter:
225* (exhaust duration @ 0.050") x 4 cylinders = 900* ---> 900 (total exhaust flow duration per bank) divided by 720 (duration per cycle) = 900/720 = 1.25. Now, multiply my 1.75" primaries by the calculated 1.25 ---> 1.75 * 1.25 = 2.125" ----> ~2.25". The slightly larger size will help reduce frictional losses.
This is an interesting methodology. Setting aside for a moment the fact that this is a turbo (I for one do not want to discount Mike Lefevers experience) , there is one more thing to consider. Since this is a "crucified" crankshaft motor, the outlet size calculated is too small for NA motor in my experience, as two cylinders fire sequentially or 90 degrees apart and leads to a "crowded" condition in the collector. 90 degree V8s typically need larger collectors than flat crank V-8s or 4 cylinder engines. Also, you should be working with the areas of pipes, not the diameters.
So, following this logic, and to be on the safe side, assume that you will need the equivalent of two pipe areas to handle the 90 degree firing impulse. Therfore, D = sqrt(2 * (1.75^2)) = 2.47". This is closer to what we find works for an NA engine. The actual sizing will vary with other considerations such as cam timing valve sizing, lift etc...
Vince
Burns Stainless LLC
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96Mustang460cid
New Member
Joined: 26 Apr 2007
Posts: 12
Location: Oklahoma
Posted: Tue Nov 20, 2007 1:50 pm Post subject: Re: Crucified Cranks
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vincer77 wrote:
So, following this logic, and to be on the safe side, assume that you will need the equivalent of two pipe areas to handle the 90 degree firing impulse. Therfore, D = sqrt(2 * (1.75^2)) = 2.47". This is closer to what we find works for an NA engine. The actual sizing will vary with other considerations such as cam timing valve sizing, lift etc...
Vince
Burns Stainless LLC
Vince,
I'm not following your mathematics there. How do you come up with, "two pipe areas?"
D = sqrt(2 * (1.75^2)) = 2.47"
Does the (2) represent the doubled pipe size? Does the (1.75) represent the primary size?
Can you further explain it please.
Have a good day!
Michael
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