Turbo engines

[Tony]

A given power level infers a certain mass airflow to achieve that power level.

But the engine itself could be a small high rpm, high boost screamer, or a very large capacity low boost low rpm workhorse to produce the same "X" horsepower.
The turbo rotating assembly may actually be the same or very similar for both, but the a/r on both compressor and turbine side would be radically different.
This brings us to pressure ratios and flow areas, and the throat size of the exhaust housing is a pretty good starting point for sizing both the exhaust runner and the wastegate flow areas, because it takes everything else into account.

[tt911er]

Hi,

Back again. Bench is nearly ready, all I need is the power contactor and after that it's ready to fired up

Tony, when you mentioned exhaust duration 240, did you mean 0,05 (my guess) or seat time?

What is your opinion about head/cam intake/exhaust ratio?

When summing up my first post after gone through pipemax/speedtalk and here forums I'm still lost with amount of exhaust flow. At what flowrate the exh side should be ported? Can the massflow calculated via exhaust (back) pressure ratio and temp?

Really looking for the pipemax 4 with turbo update, have you heard anything about the releasing?

Juhani

[Tony]

Hi,
Tony, when you mentioned exhaust duration 240, did you mean 0,05 (my guess) or seat time?

Seat to seat Juhani.
You definitely want one exhaust valve shut before the next one opens if you run compact minimum volume exhaust manifolds, and if you want good flexibility and response from fairly small turbos.
Nissan GTR factory RB26 twin turbo engine exhaust duration is 236 degrees seat to seat, and it peaks about 8,000 rpm.
Aftermarket cams for that engine start at around 260 seat to seat (232 @ .050)
http://www.camtechcams.com.au/niss_6cyl_rb26.html

Head/cam intake/exhaust ratio is debatable. Some people argue greater inlet duration than exhaust duration, others say the exact opposite. Both seem to work ???
I suppose it depends on pressure differential across the engine and intended use, I don't really know.
But a high dollar race engine with higher boost than turbine inlet pressure is a vastly different thing to a very mild street turbo.

I had more success with increasing the exhaust duration and lift on my four cylinder DOHC engine, earlier exhaust opening seems to help everywhere, right from off idle to absolutely flat out.
Increasing the intake duration much beyond stock definitely effects the engine flexibility.
Later inlet closing destroys low end torque and spooling. Earlier inlet opening increases overlap and exhaust reversion. The engine just became more temperamental and very unhappy without being any faster.

Power gains on the exhaust side with more lift and duration were far more noticeable, without any similar disadvantages. At least with a DOHC engine it is pretty easy to change things independently.

[tt911er]

Hi Tony,

What was your opinion about early exhaust opening. Was the improvement duo the less overlap or earlier energy to the turbo. Could be both as well.
Engine that I'm building is aircooled 3,2 l boxer six from 81 911. it's single overhead cam engine and that's why I'll like to get more info before make decisions.
If the exhaust duration is limited to 240 would you compensate it by increasing the exhaust flow closer to intake flow rate?
That leads to the amount of overlap which usage (in my understanding) is dominated by boost/back pressure ratio.
Do you know overlap and pressure rations in GT-R?
In my case with sohc cams I don't know is it possible to gain with overlap. With adjustable twin cam engine it's totally different situation.

Juhani

[Tony]

I don't know the pressure ratio of the GTR, but these were fairly primitive sleeve bearing turbos on the original R32 GTR, so I suspect the pressure ratio was not wonderful.
Valve overlap is essentially zero to achieve the strict idle emission standards required for a Japanese factory car.
Through the years, up to and including the R34 GTR the turbos improved, but the cams remained pretty much the same, basically 230 to 240 seat to seat duration with zero overlap.

I had three pairs of cams for my engine, the standard 230 degree seat to seat, 245 degrees, and 254 degrees seat to seat.
This was all about twenty years ago, and my turbo and pressure ratio were not great either at that time, so this information is now probably becoming very dated.
With the latest turbos now available, much better pressure ratios are much easier to obtain, and that is a real game changer.

But what I discovered was that when turbine back pressure exceeds boost pressure, anything more than minimal valve overlap is an absolute disaster.
For me, about 20 degrees of overlap was optimum, 25 was bad, and 30 degrees the engine would simply die at full throttle (from exhaust reversion).

Inlet valve closing point greatly influenced low end torque, and hence spooling.
No cylinder filling, no torque, no spooling if inlet closing is significantly delayed.
In theory, later inlet closing should help power, but plenty of top end boost pressure more than compensates for early inlet closing.

Early exhaust opening seems to be beneficial as it seems to both reduce pumping losses and kick the turbo harder. The improvement of early exhaust opening made the engine far more torquey and responsive absolutely everywhere, even right off idle, and gave about three quarters of the final total improvement from valve timing changes.

What I finally ended up with after trying many many different combinations was the 245 inlet cam set to 4/61, and the 254 exhaust cam set to 57/17.
Quite likely an even longer duration exhaust cam may have been even better strill, but I never tried it.

The critical area seems to be choosing total overlap, which will be determined by the boost to exhaust pressure ratio.
I cannot now remember what pressure ratio I ended up with back then, it's a long time ago.
There were many cam and turbo changes, many different numbers to remember.....

But I would say that if you have a lot of turbine back pressure on a street engine, keep the overlap low, and have most of the overlap after TDC. Factory n/a cams with very restrictive factory exhaust systems are like that. That is what worked best for me, 21 degrees of overlap split 4/17

If turbine pressure and boost end up being about the same, you can run more overlap, and split it even both sides of TDC. A standard "sensible" aftermarket n/a cam should work fine in a turbo engine.

With a serious turbo race engine, with monster turbo, where boost is much higher than turbine inlet pressure, the required overlap requirement becomes more like a mechanically supercharged engine.
Even more overlap should be workable, and have most of the overlap before TDC.
But it is going to be a very peaky engine !!

I suppose first choose your turbo and get some idea of what the pressure ratio across the engine is likely to end up.
Then from that start thinking about total valve overlap, and how it will be split either side of TDC.

Be a bit conservative with inlet closing, it will have a major impact on spool and low end flexibility, especially when running a low compression ratio.

Exhaust opening can be reasonably early.
Only disadvantage of early opening will be small throttle fuel economy will reduce.

I stopped playing around with all this about 20 years ago, and then spent a couple of years fiddling with a variable vane turbo, which was an absolutely huge step backwards.
After that I spent a few years more developing twincharging, using a turbo in series with a roots blower,
which is absolutely the very best thing out there !!!

These days I would not consider doing anything other than twincharging, it is that good.

[Hotz]

Mr. Tony, once again a great explanation, thank you for sharing your knowledge.

[tt911er]

Thank you very much!
Hopefully someday I can give something back for the information I have found from here.
Juhani

[tt911er]

Tony,

I don't know should I thank or curse your inputs from twin charging in different forums but I'm about to be converted.
On earlier post about exhaust flow you mentioned rule of thumb 2.2 cfm/hp. Do you still think it'll be valid? And if so how that cfm figure relates to bench cfm at 28"?
Cylinder heads still on the table so if the engine will be twin charged exhaust port flow can be made to support that.
The story about the Mitsu evo in engine-tips was the last nail to my coffin and I'm really clad that I read that story.

Juhani

[Tony]

Haha, yes, twincharging beats anything else hands down.
there are now some very fast record holding vehicles in many classes of racing out there that are twin charged.
It's particularly good for road racing, hill climbing and street with a small capacity engine.

Its not just the horsepower, but the engine flexibility and throttle response that makes it so fast and easy to drive.
No bang....wait...woosh of a turbo, instant throttle response, and massive torque at any rpm.

In some ways it's not as dramatic and heart stopping to drive as a turbo, none of that glorious running away feeling as the boost climbs at wide open throttle.
It just GOES, pulls like a Kenworth in overdrive highest high gear at silly low road speeds, and is just explosive in the lower gears at any rpm.
Difficult to describe, you really need to drive one to fully appreciate it.

That 2.2 CFM per Hp exhaust flow is tied to mass air (and fuel) flow pretty closely, and is handy for guestimating the probable final back pressure contribution of whole exhaust systems and mufflers.

It's derived from the 1.5 CFM per Hp induction air flow, expanded by EGT in roughly absolute temperature terms.

If you blow test a muffler and it flows 220 CFM at 28 inches (= 1psi) and you fit it to a 100 Hp engine, expect to see about a 1psi pressure drop across that muffler in the car flat out.
If you have a 200 Hp engine, pressure drop with that same muffler might be about 4psi, and 9psi with a 300 Hp engine.
All this won't be exact, but it will get you pretty close.
Close enough to be very useful.
Its a lot easier to bodge up some pipe and a few bends plus muffler with duct tape and flow test it on the bench stone cold, than to weld it all up on the car and then road test it.

[tt911er]

Received today Eaton from 2012 Audi 3,0 V6, looks that there is no turning back. It fits nicely top of the boxer 6 and between intake manifolds. I couldn't verify that for shure but info that I found from Audi forum it should squeeze 1,3 lit/rev. So for my 3,2 litr engine maybe little less than 1 to 1 ratio to start with? Now need to search turbos that flow around 550 cfm with 1,8 pressure ratio to end up 700+ hp at round 20 psi total?
For exhaust porting, if planned to support 800 hp it need to flow around 300 cfm/port which mean 1,6 inch inside diameter tubing for headers, and port to keep velocity below 350 ft/sec at 28"?? My intake flows now round 260cfm at 28" 320 ft/sec with that diam?
I think that I have missed something
What about intake manifolds, what kind of you have noticed to work? Resonance tuning, sizing? I have played an idea that make manifold for 3rd or 4th harmonic and build the plenums that air is coming in to it from bottom, to the root of the bell mouths inside to try to get even distribution. Individual throttles.
If the exhaust will be made to flow close 1 to 1 ratio then overlap need to be quite conservative? Does the other specks; early exhaust (round 50 to 60) opening and intake closing still apply? With those it'll end up round 250 deg. openings maybe closer to 240? With intake closing 40 abdc my engine will make 7,8 dynamic compression. My apologies about the megaload of questions but I don't remember when I was this exited