exhaust velocity question for turbo guys....

[WS6]

This is what's routing around in my brain: The engine is a pump, as the exhaust flow exits the cylinder (being pushed out by way of piston like a reciprocating compressor), the first restriction this flow encounters is in the head (valve and exhaust port). So I figure anything smaller than this area on the primary exhaust tube will be considered added restriction. (Typical right?) If we have an appropriate free flowing manifold then this flow will be backed up at the next restriction. In this case, the turbine. This is ultimately the bottleneck. As this back up occurs, it will start building pressure in the exhaust manifold to a point that it will equate its flow rate thru the turbine that is being pushed out by the engine. At idle this pressure will/should be low. At peak rpm, this pressure I'd suspect will be much/much higher.

Now we add heat energy to the equation.. When you go from a low volume, high heat area (before turbine) to a larger volumearea, (after turbine) you cool the exhaust gases rapidly because of the rapid expansion of the gas (this is how an AC system works to remove heat from the air). This heat change across the turbine is what I'm thinking spins the turbine faster than just the exhaust flow by itself, hence creating a pulling affect reducing backpressure. Not sure about the pulling affect. I have never heard that and would have to read up on it. However, that aside you're correct. Turbos spin at extremely high RPMs and you need a lot of heat to do that. The example by Engr Mike about a diesel under load or not is a great example.

Reading previous posts about wrapping to keep heat in or he only had 2 psi boost—if you have no heat energy at all in the manifold (just flow) , this kills that extra turbine spin to a point that it’s like perpetual motion on both blades.



or at least this is how I'm interrupting it..

Ok back from training. Really the only thing else I wanted to add was that the restrictions add up in the system. It's not a matter of the worst one dictating the pressure build up. It's all the restrictions that are causing pressure to build. Restrictions can include bends, merges, forks, cross sectional area, and even material surface properties. The reason higher temps lower back pressure is the energy needed to spin the turbine is present there by removing a restriction in the exhaust path ie the turbine that's not spinning fast enough to be less of a restriction.

Using too small of a down pipe could cause the volume of the gases to remain unnecessarily low there by keeping temperatures on the backside of the turbine high. This would lower the temp differential across the blades and there by restrict the amount of energy available to spin the turbine as quickly as it could. This is why race cars use large down pipes or use open down pipes. The exhaust temps are lowered on the outlet side of the turbo by increasing the volume into which the exhaust gases dump. The result is a quicker spool time which equates to more power available sooner. The same as if you wrapped or did whatever necessary to retain the heat in the system. Only you're increasing the heat going to the turbine while leaving the down side the same. The temperature differential is still increased which is what's needed.

[Nothingface5384]

good to know, i'll be using a 3in pypes exchange setup ..electric cutouts are at the end of the x. maybe i wont waste my time with 3.5 dp off the manifolds to the pypes system..
everyone I talked to said the X is a waste on a turbo cause the turbo takes away the scavenging pulses ..but apparently you proved them wrong lol

[overZealous1]

thats exactly the arguement i was battling with everyone. it just so happened they were shops also making exhausts for the turbo vq35 crowd though.
the pulses will get muddled a little bit, but you are not talking extreme psi of backpressure. you have to look at it like an air compressor tank. you can create pressure in the tank (exhaust manifolds in this situation) but at every push in of more volume, will create a spike. that spike will still travel through the turbine. the vq35 engine though had a perfect firing order of bank 1 then bank 2 through the sequence. the ls1 or older small blocks are not as effecient for the use of an x-pipe. i also used aeroturbine mufflers. if you don't know about them, look them up.

[RSX302]

see bold

At least I’m in the ball park..not too many bold corrections. lol

When you say Low Volume, I assume you are referring to the area in the exhaust manifold?
You say like an A/C system…now you’re talking my language as that’s my background… You removed Pressure? Pressure must be there to overcome the resistance of the turbine as previously stated. Example: as the wastegate opens, it releases “pressure/flow” around the turbine to slow it down—path of least resistance.

Here's a hypothetical:
Let's say both turbine and compressor impellers are the same flow (1:1) and just flow was present at the exhaust turbine (no heat), the flow on the compressor side will be the same—right. So if we do nothing but add high heat to the exhaust turbine flow, this would spin the shaft faster due to the heat energy. By spinning faster, this will create increased flow on the compressor side? So theoretically if the exhaust turbine flow remains the same and the turbine impeller is spinning faster, it will take less input pressure to turn...That’s kinda where I came up with the pulling/sucking theory to lower backpressure...

[WS6]

Ron I was editing my post while you replied. You'll want to read what I wrote again as I added more info.

[WS6]

At least I’m in the ball park..not too many bold corrections. lol

When you say Low Volume, I assume you are referring to the area in the exhaust manifold? Yes or even the area inside the turbine housing since it's tight and the tolerance with the turbine blades in minimal
You say like an A/C system…now you’re talking my language as that’s my background… You removed Pressure? Pressure must be there to overcome the resistance of the turbine as previously stated. Example: as the wastegate opens, it releases “pressure/flow” around the turbine to slow it down—path of least resistance. The waste gate doesn't release pressure. It increases available volume for the gases to flow through which results in a pressure decrease as well as a temperature decrease. It's a very important distinction even if it sounds like I am splitting hairs.

Here's a hypothetical:
Let's say both turbine and compressor impellers are the same flow (1:1) and just flow was present at the exhaust turbine (no heat), the flow on the compressor side will be the same—right. So if we do nothing but add high heat to the exhaust turbine flow, this would spin the shaft faster due to the heat energy. By spinning faster, this will create increased flow on the compressor side? So theoretically if the exhaust turbine flow remains the same and the turbine impeller is spinning faster, it will take less input pressure to turn...That’s kinda where I came up with the pulling/sucking theory to lower backpressure...

That sounds like it could be correct. I say could be because I question if the design on the impeller blades could create a low pressure are in front of the blades. This low pressure area would be needed to suck the air forward into the blade. I actually posed a similar question to my fluid dynamics teacher in reference to wind turbines and oscillating fans. Does a lower pressure or vacuum area exist in front of a fan at a certain point or does it only push what it comes into contact with? He couldn't tell me but believed that it could only push what it comes into contact with.

Now I realize a fan or wind turbine blade and a turbo's impeller blades are different. To further question the possibility of your thoughts, it is possible for a submerged water pump to move water with such speed and volume that it does create pockets of air in front of the impeller because there is not enough water volume and there by weight above the pump to keep the water forced against the blades. This is an example of how cavitation can occur. Cavitation is very bad. When this occurs, is a vacuum area created right in front of the blades? If so, is it just not strong enough to pull the water down into the blades fast enough? I honestly don't know.

[WS6]

can't forget about the forces generated by the compressor. that is the main reason for the back pressure. if the turbine was not connected to anything, it would simply catch up to the speed of the exhaust and cause very little resistance or bottleneck. more boost, needs more exhaust pressure to create it. the compressor wheel takes more power to make more boost. it is trying to slow down the turbine. powering the compressor is the work being done by the turbine.
this is the reason for blow of valves or recirculating vents in a turbo system. lift to shift, and you cut out your exhaust flow/power. pressure in the intake would try to stop the compressor from spinning, hence, the turbine also. no bov and you will have to wait for the turbo to spool up again. it can make a car fall flat on it's face with every shift. vent the compressor side for that instant and it allows the turbo to free spool, so once you get into it again, the bov shuts and you have a turbo that lost hardly any rpm and will be making boost. To further explain since this does get a little complicated for anyone new that maybe trying to learn. A gas motor has a throttle plate either in the throttle body or the carb. The pressure present in the intake pipe would slam and hold the throttle plate shut if you removed your foot from the pedal which is what was keeping it open in the first place. Because of inertia and the exhaust flowing through the turbine on the other side of the shaft from the compressor impeller, the impeller doesn't just stop instantly. You can do serious damage to a turbo if this pressure is trapped inside the intake pipe hence the need to relieve the pressure by increasing the volume available to the charged/compressed air.

on the 3.5" to 3" down pipe thing. it has been explained to me that the purpose is to allow the circular exhaust flow into linear again. in my opinion, i think it would need to be like 10" back to 3" to make a real difference.
one thing to note- i was the first person to actually test putting an x pipe (not the cheap ones either, handmade like burns stainless) behind a twin turboed vq35 (350z) and test its effectiveness on that engine (has a perfect bank to bank firing order) with turbos. i argued with every ****ing shop in country about the idea and argued it for months. finally came dyno day with my first prototype, and the testing which was done by a 3rd party (jim wolf, he had his prototype twin turbo set up on it) and low and behold the car picked up 70lb ft and 45rwhp at the same boost setting, no changes and no retune even. the down pipes were 3" and stayed 3" all the way out the back of the car. point i am making here, is that adding or keeping your x-pipe can make a difference in turbo spool up. i never tested the exhaust without the xpipe in it to make a solid comparison, but the numbers surely speak for themselves of the effectiveness.
btw- my n/a and turbo versions of that exhaust still holds the highest bolt on power rating out of even all the dam copy cats trying to recreate it even 6 years later.

To help someone that may still question if the pulses are still there which would be the only reason an xpipe could scavenge, simply ask them if they have ever been to a place with turnstiles. If a whole mass of people are slowly going through a turn style, you obviously see the pulses. Each person is one pulse. Now say Godzilla is attacking and that same mass of people is now forcing its way through those turnstiles. Even if they are getting two people through it at one time and everyone is moving fast enough to make it look like it's one continuous line, is the turnstile not still spinning?(obviously assuming no one is jumping over the turnstile) The air's still pulsing and with the firing order of that engine, it's pulsing in step with respect to each bank. You proved that. I honestly don't see why you caught such resistance with the idea

[WS6]

Nothingface

In general you should be fine with the cutouts after the x-pipe or right off the down pipes. The problem is the question is too vague to be able to fully answer. It's like asking do I need a 2.5" dual exhaust or 3" dual exhaust for my NA motor without listing specs.

You could experiment with the location of the cutouts though and see which works best. I think that would be interesting to see.

[overZealous1]

To help someone that may still question if the pulses are still there which would be the only reason an xpipe could scavenge, simply ask them if they have ever been to a place with turnstiles. If a whole mass of people are slowly going through a turn style, you obviously see the pulses. Each person is one pulse. Now say Godzilla is attacking and that same mass of people is now forcing its way through those turnstiles. Even if they are getting two people through it at one time and everyone is moving fast enough to make it look like it's one continuous line, is the turnstile not still spinning?(obviously assuming no one is jumping over the turnstile) The air's still pulsing and with the firing order of that engine, it's pulsing in step with respect to each bank. You proved that. I honestly don't see why you caught such resistance with the idea

i like the fact you got a godzilla reference into a turbo conversation, hahha.
i caught resistance because nobody had tried it. i do stuff in both import and domestic worlds and have for years. i don't consider myself an import guy or a domestic guy, just a car guy. so i feel it gives me a slight advantage to be able to combine the knowledge from both worlds that don't necessarily communicate that well together.

the propeller cavitation thing is basiclly a relationship between pitch, rpm, efficiency. look at the pitch on a frieghters prop, then a hydroplane. pockets of air takes away area you are pushing against.

to figure out the pushing/pulling thing, i like to think of it this way. a fan, where the fan itself is stationary, and you are moving something through it, you are creating a low pressure that is needing to get filled in. now, think of a boat prop, where the water is stationary (lets say no current for the sake of the example) and you are moving the boat. then you are pushing against it. ideas the same, but depends on which one is so called stationary.
plane=pushing against
pump=drawing in

[RSX302]

That sounds like it could be correct. I say could be because I question if the design on the impeller blades could create a low pressure are in front of the blades. This low pressure area would be needed to suck the air forward into the blade. I actually posed a similar question to my fluid dynamics teacher in reference to wind turbines and oscillating fans. Does a lower pressure or vacuum area exist in front of a fan at a certain point or does it only push what it comes into contact with? He couldn't tell me but believed that it could only push what it comes into contact with.

Now I realize a fan or wind turbine blade and a turbo's impeller blades are different. To further question the possibility of your thoughts, it is possible for a submerged water pump to move water with such speed and volume that it does create pockets of air in front of the impeller because there is not enough water volume and there by weight above the pump to keep the water forced against the blades. This is an example of how cavitation can occur. Cavitation is very bad. When this occurs, is a vacuum area created right in front of the blades? If so, is it just not strong enough to pull the water down into the blades fast enough? I honestly don't know.

I don't know what the pressure is in a turbo manifold or what the ideal target should be, but I'd imagine that there needs to be a target when sized properly. That would be like putting a 115mm turbo on a stock little 4 banger. It would blow right through the blade without spinning it—or you would need to be revving 8000rpm. Talk about turbo lag..lol

I wouldn't say low pressure in terms of vacuum. If you had the capability of spinning the turbine at a given point a little faster with no change in exhaust flow, then you will drop the overall pressure in the manifold.

I've dealt with cavitation on water pumps for cooling towers. This was a classic engineering failure by someone sizing the pump inlet distribution line too small (14" vs 16") and not having the appropriate NPSH. In this case it was reading 0 psig on the 14" line. Inlet to the pumps reduce to the impeller about a foot out. If the proper NPSH isn’t present at that point, you will go into a negative pressure situation entering the pump impeller. Aka; gas bubbles, big marble sounds, etc….as you said..not good.

[Nothingface5384]

Nothingface

In general you should be fine with the cutouts after the x-pipe or right off the down pipes. The problem is the question is too vague to be able to fully answer. It's like asking do I need a 2.5" dual exhaust or 3" dual exhaust for my NA motor without listing specs.

You could experiment with the location of the cutouts though and see which works best. I think that would be interesting to see.

yea i Understand
truthfully it'll just be a stock 73 buick 350...so a measily 180-90 hp or so and around 245 ft lbs and 8 lbs of boost on top of tthat with a sheetmetal s/p intake
once it blows up goes the built 355 or perhaps 370(6.4" honda rods) with lots of head porting, chevy 1.7 roller rockers and maybe a roller cam with 4-7 swap(ta212 cam profile)
aiming for 750 hp and simlar torque..but atleast 650 min

[alfatwin]

seems to me that the size of the restriction (turbine tunnel) in relation with the amount of exhaust flow has more effect on the speed of the turbo shaft than simply heat. The surface area of the turbine vanes and the size of the inside of the turbine vs the amount of exhaust gas trying to get by them has a direct effect on how fast said turbine accelerates to an effective speed to pressurize the intake or, as we say, spools up (the smaller the hole the faster the velocity of the air going through it). I can't say the hot gas doesn't effect the spool up time as Ive never tried to power a turbo with cold air, but as the exhaust gas is the remains of fuel/air mixture that has ALREADY EXPANDED TO MOVE THE PISTONS, unless there is still burning fuel in your exhaust under throttle (check your timing) this "expansion" would be minimal.. can we get Mr. Garrett to solve this?

[RSX302]

Found this read to help explain..

http://www.thedodgegarage.com/turbo_fun.html

[overZealous1]

So, in real world terms, what does this tell us? All else being equal, The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in english, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

^^ most relevent sentence quoted from your link. if you check back to my post back on #7, it pretty much sums up his paragraph, in different words. common denominator is pressure. heat does make a difference, but to quote him "a measureable difference". heated gases is kind of the icing on the pressure cake, lol.

[RSX302]

This was a great thread...I learned a bunch about turbo operation...Thx everyone..

[supernatural]

I have been worked with turbo systems for years, and I have to say that this tread http://www.ls1tech.com/forums/forced...-inside-3.html has a lot of misinformation.

overZealous1 put some quality info.


Alex

[RSX302]

Found this in garretts site..good read

http://www.turbobygarrett.com/turbob...tech102.html#b

"Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds."
_________________
Also, I got to tune (FAST EFI) the STS system..Worked well, and yes there is a little more lag than if it were at the engine. Thing still moved out nicely..just keep the rpm up.

[alfatwin]

It looks like I may have a lot to learn about temperature effects on gasses with expansion, where could I go to get info? I mean besides "back to school!" can I find a text relating to this? my knowledge is based on average temp gas with restrictions and pressures/ volume etc. how much do the thermodynamics effect them?

[MonzaRacer]

I've just been thinking about this and was wondering what experience anybody here has with respect to the best place to mount the exhaust housing.

Now, I know you want to place the turbo on a V8 for example where the exhaust gas velocity is the highest which on a header would be at the collector closest to the four runners as possible (assuming TT here.) I wonder why kind of lag issues anybody may be having when mountign the turbos further down the exhaust route? For example I've seen some setups where the runners are pretty long and either a goofy pipe is plumbed inline with an exhaust housing pad welded on or worse-on a single turbo, I've seen a long crossover exhaust pipe connect to two headers facing towards the front of the car.
What I want to know is, wouldn't that contribute to a significant amount of lag and even worse once an intercooler is plumbed in? I'm working on my own turbo system now fo rmy Cutlass and am working out details between a single and TT setup. Nothing is available in a kit form that looks reliable imo so I am making my own but a lot of what's out there I am not sure about. I am building for maximum exhaust velocity (and heat retention) and zero turbo lag (or as absolute as little as possible). It doesn't matter that this is all in an 80 Cutlass...moreover this is just a question of exhaust velocity relating to turbo lag. Thoughts?

OK as for the info I learned from some discussion with Gale Banks a long time ago, is that in turbo systems, turbo placement is not as critical as proper selection AND keeping the heat and velocity UP. After reading what the guy on l1tech did, basically his switch from long tube headers to cast manifolds had a two fold effect, one was it upped his exhaust velocity enough to help his spool issue, same as having three sets of turbo headers with different tubing sizes will change the lag effect, and cast iron would allow the heat to stay in the exhaust a little more where as his long tube headers would radiate more heat out effectively losing the thermal dynamic and slowing the exhaust gas velocity.
I designed a 3 in exhaust for a guy many moons ago when working for a Meineke shop while in school. He had a stroked 305 (400 crank)
and his car would only run hard if he let it sit and get good and hot in the exhaust, then it would pull hard till he drove a bit then get little doggy. The car would hammer most guys if he ran short runs and had let car sit. So I pulled him back in and we wrapped it when turbo wrap was like uber expensive Indy car stuff(lucky this was in Indy where the 500 is ran and I had a friend at Patrick Racing, we double wrapped his complete exhaust to the mufflers and tada he could run that little 305 soooo hard. All from the exhaust being large enough to let it breath for the stroke, but if it wasnt hot , it was laggy , like a turbo.
He built the car back up after a minor wreck that ripped exhaust off and we went back with an odd 2 3/4 pipe we had in shop,,,,and dies to bend at the time,,,,never seen any since. The car NEVER ran as hard since going to the 2 3/4 pipes, even wrapped or with out mufflers, I cant remember the muffler brand but the came with cool paint job on them, said sonic turbos.
After telling Gale about this as he was telling me to think turbos back when I was thinking about blowers and NOS was becoming in vogue. Guess I should have listened more huh?
Everything I have been told in turbos is to not restrict the engine enough to hurt power but to not make turbo headers too large so to as effect exhaust pulse velocity,,, oh and this was many moons ago I talk to Gale and he was saying that even if you pulled a muffler and put a turbo on the car back there it would still work and even have some benefits as the piping going up front would allow the air charge to cool down. But you would have to keep proper size on the exhaust pipe to turbo and keep it warm enough, but then the STS setup has one benefit as to packaging but it also has limitations on how quick it will spool up. But its not set up as a hammer you back from a dead stop type system.
I always thought of proper turbo setups for driving should run like a properly set up Qjet, you should just feel it start pulling like gangbusters NOT bog (lag) then slam you into the seat.
Some means of design might be done on paper but a lot will need to be done on car ,in person.