wave tuning & turbo

[ptuomov]

You cannot usually "tune" a turbo exhaust system in the usual sense, because the reflection from the turbo is a positive pressure wave. You only get the negative pressure reflected wave from an open ended pipe.

[...]

Most street turbos have exhaust pressure greater than boost pressure. Only an extreme sized turbo on a real race engine will boost be significantly higher than exhaust back pressure. Then, and only then might the theory of negative return wave pipe tuning be realistic. For most of us, that is not going to be the case.

Doesn't the inertia of the exhaust gas column in the exhaust port still suck the combustion chamber empty and intake charge in during the overlap? I understand that this is about exhaust resonance tuning, but just wanted to confirm that inertia tuning works the usual way even with the turbos.

[ptuomov]

I would never connect more than 3 pipes to one turbo inlet. Use a twin entry turbo if you have 4 cylinders per turbo. I would also use the shortest tubes possible to eliminate the possibility of a compression pulse to arrive as the exhaust valve is open. Never use a termination box or a large collector before the turbo. Use the pulses and kinetic energy to drive the turbo. A collector will make it pressure driven instead and you will loose a lot of efficiency. Erland.

That basically makes sense to me. A book by Nicholas Baines makes it sound less cut and dry, though. In pulse turbocharging, the variations caused by the pulse requires a turbocharger that works well in a wide variety of conditions. In constant pressure turbocharging, the conditions cn sometimes be made very stable which allows for turbocharger slection/design that is very efficient. He leaves the reader with an impression that pulse turbocharging is superior for say car engines that have to present the turbo with a wide variety of conditions anyway.

Can you explain to me what you mean by "use the shortest tubes possible to eliminate the possibility of a compression pulse to arrive as the exhaust valve is open"? The turbocharger behaves like restriction / closed pipe end, so it is going to reflect a compression wave back, right? When do you want to time the compression back to the cylinder? Do you want it sooner rather than later? During the time when the exhaust port is sonic?

Thanks for all you informative posts here. I have virtually no experience with engines and have just tried to study the topic over the last year or so armed mostly with a library card.

[ptuomov]

If you make the exhaust short enough you can have the exhaust pulse to reflect of the turbine, back to the closed exhaust valve and then back to the turbine again. That way you use the pulse twice. The second reflection back at the exhaust valve must happen before exhaust opens. if you don't have a twin entry turbo you can always build a pulse converter. This way you get enough energy to drive the turbos from lower rpm:s and you can use a camshaft suited for your highest rpm. Erland.

This is very interesting. I am going to ask you to explin this to me like to a three year old. In the process, I will undoubtedly expose my total ignorance.

Exhaust valve opens, and compression wave is sent to the turbo. It's reflected from the turbo as a compression wave. It hits the exhaust port. If the exhaust valve is open, it will reflected as a low-pressure wave back to turbo. If the exhaust valve is closed, it will be reflected as a compression wave back to turbo. Is this correct?

If it is correct, two questions:
(1) Why is it desirable to have the second pressure wave hit the turbo? Simply to harvest the energy that was not harvested initially during the first pulse?
(2) Doesn't longer pipe make the pulse travel longer and make it more likely that the pulse will be reflected to a closed exhaust valve? A short pipe will return the pulse faster, which means that the exhaust valve is more likely to be open.

Thanks for spending time educating me (and hopefully some others).

[ptuomov]

While I agree that a 4-1 collector is a bad idea on a turbo 4-cylinder motor, there's no harm in two GOOD 2-1 collectors going into a twin-scroll turbine housing, which still generates scavenging pulses to help reduce back pressure during overlap. It's the best of both ideas, if you ask me.

According to the books I've read (and full admission of very little practical experience is in order here), the choice between 4-to-1 single scroll and 4-to-2 twin scroll (or divided housing or whatever) is not clear cut. For a six cylinder engine, choice is easy, twin scroll or parallel twin turbos is better than a single turbo. But for a four cylinder engine, is it really as clear?

An additional question related to this. What is one supposed to do with 90-degree crank V8 engines and twin turbos? Two twin scrolls or two single scrolls? Assume that it's not practical to route the header pipes across banks.

[Warpspeed]

Doesn't the inertia of the exhaust gas column in the exhaust port still suck the combustion chamber empty and intake charge in during the overlap? I understand that this is about exhaust resonance tuning, but just wanted to confirm that inertia tuning works the usual way even with the turbos.

Suppose you have a normally aspirated engine, with the normal tuned headers.
Flat out on the dyno, with a correctly tuned exhaust system, you might be ble to see a -4Psi reflected suction wave during valve overlap. That can really assist exhaust scavenging, and starting the induction process, and is extremely beneficial to power.

Now consider a turbocharged engine running flat out on the dyno with red hot exhaust pipes. Boost pressure is 30 psi, and exhaust manifold pressure is 45 psi,
If you could design an exhaust system that dropped exhaust back pressure by 4 psi during valve overlap, It will not have any significant effect during valve overlap, There will still be a bunch of exhaust back pressure higher than boost pressure.

If you really want more top end power, a far more productive approach to reducing exhaust back pressure is to fit a higher flowing turbine.
As we all know, that has it's own set of disadvantages and limitations.

[Warpspeed]

In constant pressure turbocharging, the conditions cn sometimes be made very stable which allows for turbocharger slection/design that is very efficient. He leaves the reader with an impression that pulse turbocharging is superior for say car engines that have to present the turbo with a wide variety of conditions anyway..

There are two conflicting design approaches to turbo exhaust systems, depending upon the application.

The first assumes a fully muffled exhaust, with some unavoidable back pressure, and a reasonably tractable engine, as in a road car. The exhaust turbine will be small, and exhaust manifold pressure will always be considerably higher than boost pressure. Typically twice the gauge pressure.
The exhaust manifold will be made minimal size and volume to get best turbo response. There will be almost zero valve overlap.
All production turbo road cars are like this, and it works well for what it is.

The second assumes a professional race engine with an absolutely huge turbo, and considerable valve overlap. The power band will be minimal, and very high in the rpm range. Boost pressure will be slightly higher than exhaust manifold pressure.
Here IT WILL be possible to run tuned exhaust lengths that work in the traditional manner. The exhaust turbine being so large, it has minimal back pressure. Have a look at Indy and Formula One turbo race engines.

You cannot mix these design philosophies.

Fitting long tuned header pipes to your stock factory turbo car will do nothing except reduce turbine response, and increase under hood temperatures..

[ptuomov]

In constant pressure turbocharging, the conditions cn sometimes be made very stable which allows for turbocharger slection/design that is very efficient. He leaves the reader with an impression that pulse turbocharging is superior for say car engines that have to present the turbo with a wide variety of conditions anyway..

There are two conflicting design approaches to turbo exhaust systems, depending upon the application.

The first assumes a fully muffled exhaust, with some unavoidable back pressure, and a reasonably tractable engine, as in a road car. The exhaust turbine will be small, and exhaust manifold pressure will always be considerably higher than boost pressure. Typically twice the gauge pressure.

The exhaust manifold will be made minimal size and volume to get best turbo response. There will be almost zero valve overlap.
All production turbo road cars are like this, and it works well for what it is.

The second assumes a professional race engine with an absolutely huge turbo, and considerable valve overlap. The power band will be minimal, and very high in the rpm range. Boost pressure will be slightly higher than exhaust manifold pressure.

Here IT WILL be possible to run tuned exhaust lengths that work in the traditional manner. The exhaust turbine being so large, it has minimal back pressure. Have a look at Indy and Formula One turbo race engines.

You cannot mix these design philosophies.

Fitting long tuned header pipes to your stock factory turbo car will do nothing except reduce turbine response, and increase under hood temperatures..

Are you saying that there are two modes that work and something in the middle is not continuous and therefore a middle ground doesn't work? If so, please tell me what do you think about the project that we're working on:

Street 5.0L 32v V8, 90 degree crank, CR about 9:1, 260/246 adv. duration, 10.5mm/9.5mm lift, 114 LSA, heads flow 315/270 CFM at max lift @ 28", intercoolers, a pulse-converter style exhaust manifold with about 6" individual runner lengths, two GT3071R-20 90 trim .86 A/R turbos, dual 3" pipes from the the turbos, probably Dynomax #17220 mufflers, rpm specific boost limits with eboost2.

To me, it seems that what we've got here is smack right in the middle between the two approaches. I don't think that's a problem, but should I?

Also, there's still the one question that has left me puzzled reading these posts is that how is the compression wave reflected from the turbo going to be arriving when the exhaust valve is closed if the runners are short and arriving when the exhaust valve is open if the runners are long? It doesn't make sense to me.

What might make sense is that with short runners, the compression wave hits the cylinder thru the open exhaust valve so early that intake valve is not open yet. Then, the low-pressure wave is reflected back to the turbo, which will reflect it back to the cylinder as a low-pressure wave. This low pressure wave may then hit the cylinder and exhaust port at the time when the intake is open. Is the mechanism that people have in mind when talking about the short exhaust runners for turbo motors?

Then one more question. It's my understanding that the turbine, even a large one, will always reflect a wave of same kind back. In other words, the turbine will work like a reduction in the pipe CSA and not like expansion in the CSA. Is this true, irrespective of the two approaches you listed?

[Horsewidower]

I believe that its an "order of magnitude" situation. Which has more of an impact, 2:1 backpressure to inlet pressure, or an amplitude wave?

What is the use of the engine?

[ptuomov]

I believe that its an "order of magnitude" situation. Which has more of an impact, 2:1 backpressure to inlet pressure, or an amplitude wave?

What is the use of the engine?

In Porsche 928, all around street / strip / maybe some road course thing every now and then. Possibly some open road. The car will end up being probably around 3500 lbs with driver. It has a five speed manual which will become unhappy with more than 600 ft-lbf torque from the engine side. The car has to do everything well.

The EAP 3.5 simulation gives me exhaust back pressure that is about 1:1 with boost. I think that's too optimistic, in other words, I don't believe the simulation. What it does tell me though is that the exhaust back pressure will probably be much less of an issue than for a typical production turbo car. We are planning to put in sensors for exhaust back pressure just to be get the data to better simulate the system.

To answer your question about the order of magnitude: with 15 psi boost and 30 psi exhaust back pressure, the 3 psi or so suction wave is going to not flip the sign on the pressure differential. But everything helps in the margin, even that engine is going to run better with some cam overlap and then that 3 psi is going to help on the margin. Right?

My posts have really two purposes. First, what to do with this engine. Second, understanding more generally the theory. So I am total novice armed with a library card, but I have some time and energy to learn more about this stuff.

[Warpspeed]

Are you saying that there are two modes that work and something in the middle is not continuous and therefore a middle ground doesn't work?

Basically yes.

Not two modes, but two totally different from scratch design philosophies.

Can you think of any mass produced production road car that runs an engine with cams and turbos anything like those you plan to run ?

Can you see why ?

it is just that they have very different priorities, so they build very different engines.

Their approach works, and so will yours.
Two different modes of operation, well maybe. It depends on your point of view I suppose.

But will yours reach full boost at 1,800 rpm, and pull smoothly in fifth gear at 30 mph ? Probably not.

[KnightEngines]

As Erland states there is a debate concerning the "brute force" pressure driven method and the pulse "low pressure" method. It is an interesting debate, at least to me.

This is the best summary of the debate that I've found, over at gofastnews.

"There are two Basic Approaches to Turbocharging

What you describe with log manifolds or manifolds with short, stubby runners are prime examples of the traditional Approach to boosting: Brute Force Turbocharging.

With Brute Force, the exhaust gases are forced into restrictive and constrictive exhaust manifolds. And from there, those compressed gases are forced to pass through small turbine housings.

The rationale behind Brute Force is: in order the make the turbine spin, the exhaust gases HAVE to be compressed and forced through. Those exhaust gases need to be as hot as possible also. And the turbocharger turbine housing needs to be small enough so as to force the turbine wheel to spin up faster. [that's the only way to reduce lag, don't cha know!]

Well, that Approach does work. It has been working for years. It is the Approach used on nearly all OEM boosted motor cars for decades.

Brute Force has its drawbacks: greatly elevated operating temps for things like exhaust valves; tremendously increased exhaust reversion back up past the intake valve during overlap; and substantially increased levels of negative work that the pistons have to deal with.

To address those drawbacks, a 'new' Approach to Turbocharging has been showing up. First, in competition motors. Now, a lot more on the street.

Low EBR Turbocharging

With Low EBR Boosting, the exhaust gases are NOT constrained and constricted in tiny little manifolds. Headers are used to let the gases FLOW. Turbine housings are not tiny little snail shells either.

With Brute Force boosting, the EBR....Exhaust Backpressure Ratio...the ratio of exhaust pressure to the intake boost pressure....can exceed 3:1.
[i.e., for 10 lbs of boost on the boost gauge, there is possibly 30 PSI (or more) of exhaust pressure resisting that boost pressure.]

With Low EBR Boosting, the exhaust backpressure may remain less than the boost pressure reading.

[there are some guys in Sweden (the savarturbo boys) running EBRs less than 1:1 on motors that are running over 30 PSI of boost. And the output is in the neighborhood of 6 hp per cubic inch of displacement.]

The rationale of Low EBR Boosting is: the energy to drive the turbine comes from the velocity of the exhaust gases. Let the exhaust flow its best. Gather and direct those gases into a turbo that can take the velocity and use it to spin the turbine wheel. Keep the exhaust flow flowing well, so that the motor itself does not have to fight the exhaust gases during the exhaust stroke, or during overlap.

Or, to put it another way: Low EBR Turbocharging is an Approach that uses free exhaust gases to achieve, or exceed, the power output levels of motors with mechanically driven superchargers, but without the parasitic drag of the mechanically driven huffer.

In my own experience, every time I have done something that lowers the EBR, the power output has gone up without raising the boost level.

good luck with your further research."

I like this, it gives me a better way of explaining how I set up turbo engines to guys.
I've been setting up motors to run significantly less ex man pressure than boost pressure for a while - despite what people tried to tell me about larger turbos, free flowing ex ports & manifolds etc being laggy I have found the opposite - often faster boost response, faster boost ramp up, more midrange torque & a lot more top end power.
A good example would be a street 2J I build a while back, well ported head, 268/272 cams, nice steam pipe ex manifold, GT4094 turbo, ported stock intake lower with Greddy plenum & 90mm TB.
The thing makes 24psi boost with only 17psi in the exhaust manifold, gets into positive boost around 2800rpm, hits full boost around 4000rpm & makes 720hp at the tyres through an auto trans & loosish converter on a pump fuel/toluene mix.

On subaru engines I've been able to get a GT35r turbo on full boost around 3700rpm with power around the 480hp ath the wheels on pump fuel. With those engines I'm only using 264 cams, just letting the heads do the work.
I'm doing one atm with a new design exhaust manifolds, real nice corsa veloce intake & GT4088 turbo to be run on C16 - aim is full boost before 4000rpm & 650odd wheel hp under 8000rpm, hoping to see significantly less ex manifold pressure with this one.

[SWR]

The EAP 3.5 simulation gives me exhaust back pressure that is about 1:1 with boost. I think that's too optimistic, in other words, I don't believe the simulation.

...but you should. That "you cannot have less than 1:1 boost /exhaust pressure even if you're lucky" might be a valid point. In 1992 or thereabouts. 30psi boost, 22 psi backpressure in a 3 liter daily driver with 4000+ rpm wide on-boost powerband on pump gas 93 octane has been seen, and not by me only. They're all over these days.

What it does tell me though is that the exhaust back pressure will probably be much less of an issue than for a typical production turbo car.

No wonder, you have to take into consideration that a "typical production turbo car" has a TON of different things to take into consideration than just drivability. Emissions are one, a low fuel consumption and CO release is another. Skipping having to do that changes the parameters of "what is possible" by a whole lot.

To answer your question about the order of magnitude: with 15 psi boost and 30 psi exhaust back pressure, the 3 psi or so suction wave is going to not flip the sign on the pressure differential. But everything helps in the margin, even that engine is going to run better with some cam overlap and then that 3 psi is going to help on the margin. Right?

As said, skipping the original low-emissions demand you can open it up and use a tad more overlap for a slight performance increase. But if you have that abysmal ratio, i.e. 2:1 you better not open it much, and atleast keep the IVO quite close to TDC..

Edit: Knight beat me to it... I type too slow. Better boost the fingers...

[Horsewidower]

Knight, that's one of my favorite posts. Excellent explaination.

Ptuomov: I don't think that a 27:15 vs 30:15 backpressure ratio provides much in the way of improvement. When the IV opens in overlap you'll still have reversion. You need to pick a theory and go with it, all the way.

You may, in the end, be constrained by the packaging of the system.

[crazyman]

My post is nowhere near as technical as some, however I feel I must chime in.

The RPM at which the turbo spins will change the wave reflection frequency.

Talk into a window fan on low, medium, and high. The soundwaves change speed.

It would be much faster on a turbo, but the principle is still there. There is much to be speculated on an engine, things we haven't learned yet.

Scott

(Edit) As an afterthought, what effect would a manifold with a 1/4" or so step up accomplish in the fight against reversion?

[ptuomov]

Ptuomov: I don't think that a 27:15 vs 30:15 backpressure ratio provides much in the way of improvement. When the IV opens in overlap you'll still have reversion. You need to pick a theory and go with it, all the way.

You may, in the end, be constrained by the packaging of the system.

In practice, the packaging is of course one of the main constraints, and the person I am working with has really thought thru what constraints can be relaxed and what can't be relaxed.

I am just trying to understand the theory here. Three questions:

First, even if the average back pressure over the cylce is twice the boost over the cycle, during the valve overlap the exhaust back pressure must be much smaller. This is because the inertia in the exhaust gas column. At the margin, that 3 psi could make a big difference, bigger than what the averages make it seem. Is this correct?

Second, and this may be somwhat unrelated, what's the optimal intake port CSA and velocity for a boosted engine? On the one hand, denser charge will have more inertia even at lower speeds. On the other hand, I am guessing that frictions may be larger for denser, higher pressure charge. Do the two effects combine to a larger optimal CSA and lower optimal velocity for the intake port for boosted engines relative to NA engines?

Third, what is the reason why (from the wave tuning perspective) the turbo exhaust runners should be much shorter than the NA exhaust runners? Is it that the turbo reflects a pressure wave as a pressure wave and NA merge collector reflects a pressure wave as a low-pressure wave? Then the turbo exhaust tuning will have to hit the cylinder when the exhaust valve is open but the intake valve is closed on the first round to convert the pressure wave to a low-pressure wave and get that reflected the second time from the turbo? Twice the distance to travel?