Turbo engines

[Farrell]

I built a pressurized flow bench and for the first time in 30 years of porting I've been able to see what's going on under boosted conditions.With the boosted stuff,I've ported intuitivley for years and actually seen significant gains by reasoning out the air flow patern as opposed to being able to test it. With my pressurized bench I've tested as high as 300".Very seldomly did the flow equal the calculated flow, but on some rare occurances a port would flow better than it should have.For anyone interested,this bench is for sale.I've had it with the business and people not paying their bills.It's less than a year old and repeats within .3 of 1%.I have about $8K in parts in it and will let it go for that.Go to my facebook page and have a look.Farrell Vaughan

[200cfm]

Never heard of a pressurized flowbench. Can you explain the design?

[SWAirflowServices]

Very cool and welcome Farrell! Lets see some pictures. There are a few folks on around here that work wih boosted applications.

Shane

[Farrell]

There's pictures of my positive pressure bench on a different thread called "Positive pressure flow bench",I think.

[artigas]

I what would happen if we applied our vacuum source in blow configuration into the plenum, rather than drawing it through the port if it would closer mimic a positive pressure situation? One would probably have to set the manifolds, and throttle to the equation to be accurate, but then also would have to mimic the positive pressure and any cam overlap to the mix since the exhaust pressure is so great in a turbo system.

Until we experiment, we are just going with what we know works in my little world.

-John

I'm not too sure what you are saying there ^^.

**If you suck through an average shaped port then the air will favour the short side, if you blow through it will favour the long side**

Do you think this is true? If so then does it have any complications, or would it change the way we shape a turbo port or measure it with our benches?

I think maybe it depends on your overall boost pressure, you may see +14.7psi in the plenum, but will you still be seeing that in the inlet port? Here you still might be seeing vacuum until (throws a number in) say, 28psi.

For sure inlet plenums must be designed with some 'equal port share' in mind, but I think they too are to be questioned and could work differently at different boost pressures. The low boost OE style ones don't seem to have anything which guides the air into the ports, so I'm guessing that at a certain boost pressure or air speed we need to be thinking of doing something about this.

It's very difficult to find some empirical evidence on this subject so I am either;

Wrong

People are keeping it to themselves

It's too advanced (hardly likely!)

I'm just theorizing, but...

We all know that airflow is what's sought after.

I agree about the SSR (love the handle BTW).

But here's the catch. It's NOT really about the boost pressure, but rather the airflow - I generally use pressure numbers for 2 reasons: 1. Place the compressor map to desired power levels 2. To map the fueling. Let's use a simple example of twin vs single turbo. At the same boost levels using identical turbos, do you think the single turbo will flow as much as the twins?

Another case in point. A local is talking about his 21psi setup where I know the turbo is past it's efficiency and he wonders why he's not making any more power. Why? Not because of the amount of boost, but rather the ability for the compressor wheel to move the volume needed to make more. Boost levels go up but power output becomes stagnant only increasing inlet temps.

My thought is if you are using a turbo that can outflow the engine (positive pressure), you would think that the air IS actually being "blown" into the engine, the only way any vacuum would be seen is if the piston was already on it's way down before the valve was opened.

Am I in left field with this thinking?

Artigas,

There is a fine line when deciding on the application of a specific turbo between being "too big". You have to remember, the closer you are to 1:1 ratio on exhaust to intake manifold pressure, the better. The same reason that causes the compressor to be too small as I mentioned above, the turbine can become a restriction giving you the same outcome (high EGT's, choke points). This causes undesirable intake/exhaust pressure ratios.

As far as spooling time, there are many ways to correct it that are rather simple for someone who builds turbo systems. One is a ball bearing journal. You can have on in your hand a literally blow into it and watch the wheels move. Truly amazing, but is also very expensive.

Divided pulse manifold/turbine housing. This is also great. I have recently worked on 2 different cars. A BMW inline six and a Honda 4cyl. Both using the same turbo from a Deisel truck (Holset HX35), both have divided turbines/manifolds. The BMW sees full boost at about 3500rpm under light load, the Honda sees it at 4000 under light load. The BMW redlines at 6800RPM the Honda at 7200RPM. You would think that because of the two different sizes/styles of engines there would be a big difference in the spool time, but it's not the case in real life. This same turbo runs a large diesel engine to 30psi.

Finally the quick spooling device. This allows you to have a large turbine, large compressor and not choke and not lag, either. You feed all cylinders to one half of the twin scroll turbine until a desired pressure level is reached, then open the other half to prevent choke.

Well, enough rambling. Got to get back to welding these headers. Sorry to get off topic.

-John

Hi,
thanks for your opinion and for the point in the twinscroll, I like very much this system, The twinscroll with the exhaust manifold for this housing is great; but one of the problems for know the diferent pressures between the inlet and exhaust is usually we wave a gauge in the inlet and not in the exhaust... is harder ( for me ) know the pressure between the exhaust valve and the turbine for know the max boost we can use for guard the 1:1.

I think step by step we can learn a lot! I want and I hope !
Thanks for your coments.

[tt911er]

Hello everybody,

One of the new ones in the forum and trying to figure out turbo engine specs. You can see it from my questions that I'm pretty novice when it comes to engine design.

Anyhow trying to get out of chasing me tale;)

All comments are welcome for my turbo engine dialemma;D

Even if we can lock the intake/ exhaust pressure ratio to be better than 1, what turbo header specs need to be followed?
1. Short primaries connected just before turbo? And keep egt high etc.
2. Pulse tuned primaries with secondary to also support rpm area under boost level with more overlap?
No matter how good intake/ echaust rate with turbo is possible, it'still more restrictive than without?
Until what intake/ exhaust rate the the exhaust can produce scavenging?
With more tune lenght ( longer total lenght ) we also loose some egt but I believe that can be compensated with turbine sizing.
What about exhaust flow rate! If the boost pressure will mainly increase massflow more than CFM rate, how that will increase amount of exhaust flow?
-> port and header design ?? Multiplied by pressure ratio?
As long the turbo header scavenging isn't solved it is difficult to figure out needed/ max overlap -> cam?
Still behind my tale;O

Juhani

[Tony]

Its quite a few years since I messed about with turbos, but here is my take on the subject.

Exhaust manifold design depends entirely on the intended use, there is no "best" design, but one approach may give much better results on a particular engine and application than something different.

The main aim should be to get as much exhaust volume out of the engine as possible and minimize exhaust back pressure as much as possible right at the end of the exhaust stroke, especially if there is any significant valve overlap.
You cannot achieve that if one cylinder is blasting high pressure exhaust straight back into the exhaust port of another cylinder, while the second cylinder is at the final stage of its exhaust stroke.

You need to think about how many cylinders are driving one turbine, and if two exhaust valves feeding that turbine are going to both be open simultaneously, and the time intervals involved if two valves are open together.

Think about three cylinders of a six cylinder engine (inline six or V6) driving one turbine. The firing interval between cylinders will be 240 degrees, and if the exhaust open duration is 240 degrees or less, only one exhaust valve will be open at any time. You can run very short exhaust stubs into your turbo with very little manifold internal volume, and it will have the best possible turbo response at any rpm.

As soon as you start adding more cylinders, or increasing exhaust duration it spells trouble.
One exhaust valve will open before the previous one shuts, and that is very bad.
You can get over that by running longer exhaust branches to the turbine.
These are not resonant tuned lengths in the normal sense, but create a delay so that the exhaust pulse from one cylinder has to travel down one pipe to the turbo, then back up all the other pipes, and hopefully by then the closing valve will have closed.

Depending on valve timing and rpm, and the length of the branches, this can be made to work above a certain rpm, but the increased total exhaust volume of longer branches is definitely not an advantage for turbo response, however the results will be good if is all done correctly.

This is why you see "tuned" pipes on more highly tuned turbo engines. The lengths are pretty short compared to properly tuned headers on a normally aspirated engine, the pipes are not cut to any resonant length, just made long enough to start working where you want the turbo to begin spooling.

Very often a one foot runner on a turbo engine will be quite sufficient, compared to two, three, or even four feet for tuned street headers on a normally aspirated engine.

[tt911er]

Thanks Tony for your info.
The engine that I'm working now is aircooled 3.2 litr boxer 6 with twin setup. Now I have rather short 3 to 1 header where the turbo flange is right after 3to1 collector. It worked ok, 1 bar boost there was 450 hp/5400 rpm and 600Nm/3800. Engine was then 3.0 lit. Backpressure was well in control up to 6000 rpm (max 7000) where the ratio was 1/1. I didn't push it further. New 3.2 engine with stronger bottom hp target is round 600 (compression lowered to 8.5 and little more boost)
A lot is depending on amount of exhaust flow. Is it possible to estimate the exhaust flow based on target Hp rating as produced by N/A engine? Is it any easier??With correctly estimated flow, the head and header size could be easier designed?
Of course turbine house/turbine selection have to right for the target.
Looking forward the pipemax 4 that should give better support to engines with boost.
thank again all the info

Juhani

[Tony]

It is possible to over complicate things, and the critical exhaust velocities required for optimum tuning of n/a tube headers really has no application for designing a turbo exhaust. The situation is entirely different.

The way I would approach this problem would be first select an appropriate turbine and turbine housing suitable for the application.
Then eyeball the throat size of that turbine housing, its usually not exactly round, and difficult to see and tricky to measure. That would be A2 in the following picture.

I would then look at the effective exhaust valve and exhaust port flow area. I would probably choose a pipe size with a flow area no larger than the smaller of the two.
This works, because with three cylinders powering one turbine, only one exhaust valve will be flowing into the turbine housing at any one time. We want to sustain exhaust velocity without adding any unnecessary extra pipe volume or back pressure. So just matching the flow areas in the complete flow path should get you close.

A further consideration would be exhaust open duration. Anything much in excess of 240 degrees (with three cylinders) may benefit from having longer exhaust branches, but its difficult to be specific.
The more hairy the cam and turbo, the more experimentation might be required with the runner lengths, struggling up onto boost is where the gains will be, rather than flat out.

In my own limited experience, there is some real power to be gained on the exhaust side, more so than the induction side. The trick is not making more boost, but to reduce the total back pressure on the exhaust side as much as possible and get the turbine working as efficiently as possible.

[tt911er]

"There is some real power to be gained on the exhaust side, more so than the induction side.."

Agreed! Also make the head/engine to flow better and try to get your target amount of airflow a less amount of boost!
That leaded me to this forum and building bench to be able to find better flow. (not only bigger cfm figures;)
Bigger exhaust housing/turbine doesn't always reduce the backpressure, could be other way around.
If you look at F1 turbos from "few years back" you can see the intake/exhaust housings ratio.
With programmable EFI system you could control the wastegate not only according intake pressure but also with backpressure. On upper rpm range there is usually more that needed exhaust energy available.

Juhani