Apogee89

Thanks Ted. That article you dug up is very useful reading. I'll bookmark it for my future reference.

I just wonder if there are differences/exceptions for NA and FI setups.

Ted B

iTrader: (6)

There are considerations that vary between NA and turbo setups, especially where overlap and the potential for reversion are concerned.

Apogee89

Hoping for some elaboration.

ItalianEvo

Hi Ted B,

I'd really appreciate your contribution on overlap in turbo engines...

speaking of valve opening at TDC, not of LC angles, and independently from cams duration ( btw i'm referring to 264 cams ), how many millimeters or inches do you look for ?

Thank you.

newimportowner

iTrader: (2)

Dont forced induction engines usually beifit from more exhaust than intake duration and lift? it does not have to suck air, its being crammed in, so more exhaust would help turbo spool and even out Idle on a super caharger..No? On my blown 355 small block I have 1.6 ratio rocker on intake and 1.7 on exhaust to even out the cam profile a bit.

mayabe I'm wrong

Ted B

iTrader: (6)

I am not quite sure of what you're asking, but valve lift for both the intake and exhaust sides will be low at TDC just after the exhaust stroke, simply because TDC lies right in the center of the overlap period - when the exhaust valve is closing and the intake is just starting to open.

On overlap . . .

With an NA engine, any residual exhaust backpressure in excess of 1-2 psi creates a pumping loss that reduces power. With a turbo engine, exhaust backpressure tends to be much greater. When the ratio of backpressure:intake pressure is enough to force exhaust gas back into the cylinder during overlap, increasing the boost pressure further isn't going to do any good. A primary cause of excessive backpressure in a turbo engine is a turbo that has a compressor:turbine efficiency ratio that is mismatched for the application.

For example, in his A/B testing of 20G9-6 vs. 20G9-5 turbos, D. Buschur and Robert of F.P. measured exhaust backpressure to compare the P.R. (pressure ratio) of the two turbos, which is the ratio of exhaust pressure to intake pressure. The results were as follows:

20G9-5 - 28 psi intake, 32 psi exhaust 420whp - 475 ft/lbs

20G9-6 - 28 psi intake, 48 psi exhaust 385whp - 397 ft/lbs

If these results are correct, it shows that the 20G9-5 has much better PR, and is better balanced where compressor:turbine efficiency is concerned. This being the case, a cam setup with large overlap (like mine) is not going to work well with the 20G9-6, because as the power levels rise, the high pressure exhaust gas will try to push itself back into the cylinder and compromise VE.


A PR of 2:1 is typical for a street setup where quick spool is preferred over peak hp numbers. A smaller hotside or larger compressor with no change in hotside will drive the PR upward.

Race engines and high rpm, high hp applications want to see a PR as close to 1:1 as possible. They achieve this with larger exhaust systems and hotsides that compromise spool, but allow large volumes of air flow at high rpm and boost pressures.

Obviously, a large overlap cam set works better with a setup that has a PR of closer to 1:1 than 2:1.


Don't think of the intake charge as being crammed in by the turbo, because that's not really what's happening. The air is still being sucked in, but the air charge has greater density. The exhaust gas also has greater density (a potentially offsetting factor), so keep that in mind.

Typically, the exhaust cam can apply a bit more lift and duration without advsersely affecting low speed performance. However, depending on the intake/exhaust flow characteristics, it may not be doing anything productive.

ItalianEvo

Hi Ted,

exactly what I meant, and thank you for your contribution !


Now I have, at TDC, 0.016" exhaust valve opened ( LC 115 ) and 0.07" intake valve opened ( LC 113 ):
I'd like to know your experiences/numbers on that.


Let me explain my scenario...

- GT3071 .63 - ext wg - 2.5" turbine exducer bore
- comp T04S ported shroud ( for surging problems I had @ 3500 rpms and 29 psi max boost )
- DP begins with a 2.5" 90 deg curve and then it becomes 3"
- 3" full exh system

- in track use I'm getting high EGTs, 1700F at the 3rd cyl exh valves, but I suspect that at high rpm/full boost I'm getting higher values where the 4 runners get into 1 just before the turbine flange

- black patina in intake plenum, I'm quite sure is reversion at high rpm/full boost caused by the small ar turbine

1. Reduce boost ?
I mean to have a 29 psi peak boost and then reduce it with an overboost option to a value that will give me lower EGT ?
I'd like to avoid this option...

2. Turn to a bigger ar turbine ?
I'd like to avoid cause I'd like not to loose some low to mid power and spool up characteristics

3. Find out a better cam timing ?
This is the option I'd like to develop, if possible, and if could take me to some results...




I even suppose that with -5 setup you're getting lower EGTs....

Would be very interesting to look at the power curves of those 2 options and see where they have peak torque/peak power/spool up...



I think this is the point to analyze...

Thank you !

Ted B

iTrader: (6)

Which cams are you using, and how do you have them timed?


These three things appear to be significant. What mods have you made? What method of tuning are you using? What do your fuel and timing curves look like?

ItalianEvo

Sorry...
I'm using Jun 264/264. I timed them with the LC method.
Factory spec was 110i 115e, I retarded the intake to 113i to decrease what I think is reversion..


Full forged internals, FMIC, 2.5" fmic pipes, 4-1 tubular manifold, 850 FIC, Walbro 255 hp.
It was tuned by Motec engineer, on street and then on rr:
I don't know how the fuel/timing curves look like, I know only it's 10.5 afr at WOT.

Could it be interesting that I have a very flat curve ? 425 4whp at 6000 and 410 4whp at 7000 ?

Thank you.

EFIxMR

iTrader: (10)

Ted was wondering if you had any details on how exactly buschur measured the exhaust back pressure, since the turbo/manifold are fully divided. does he have 1 pressure sensor per bank, or 1 pressure sensor hooking together the two banks?

Ted B

iTrader: (6)

So your cam gears are set at -1.5/0, yes?

That shouldn't contribute to the high EGTs, but it may contribute to the surging issues. With a 3071, 29 psi of boost at 3500rpm is more air than the engine can accept, which has the compressed air trying to force its way back through the intake. A diffferent cam setting may alleviate this somewhat, but I suspect there is another aggravating factor. The fact that you're getting 29psi out of a 3071 at only 3500rpm makes me wonder . . .

You quoted 425whp at 6000rpm. I assume this is with race fuel and recorded on a Dynojet?


Two things that contribute to high EGTs are an overly rich AFR and lazy ignition timing. Both cause fuel to be burned in the exhaust, and shorten turbo life.

As far as the black residue in the intake plenum, I suspect that is the result of oil vapor in the PCV system. Do you have a catch can in your PCV system?

Ted B

iTrader: (6)

I don't know specifically where he tapped it, but the divide shouldn't make much of a difference because the backpressure should be equalized everywhere between the exhaust ports and turbine housing.

ItalianEvo

exactly...


Probably, but I wonder if anyone else have the exhaust valve 0.016" opened at TDC !
Retarding a bit the exhaust cam could lower the EGTs a bit, but the reversion could increase...




You're talking about retarding the intake cam.. correct ? For example 115i ?
BTW now I ported the comp cover and I have no more the surging issue...



That wondered me and other people on the dynojet !
And it was with pump gas !

Maybe it's due for the "strange" GT3071 I have ?
T04S .70 comp cover and T31 .63 turbine ?
Probably I get less surging with its own comp cover, the T04E .50...



This is not possible... I threw PCV system to atmo when car was brand new



Thank you Ted.

Ted B

iTrader: (6)

No, I am talking about advancing the intake cam to 110 deg and tightening the LSA to 110 deg to make the engine more mechanically efficient at lower rpm.


No, you do not have a strange 3071. Your 3071 is normal.

You are running 29 psi on pump fuel, and this is what I suspected. This does not appear to be normal, and is almost certainly the cause of your problem.

I don't have any first-hand experience with a 3071 on an EVO, but 29psi is too much boost on pump fuel. In order to run this much boost without getting detonation, the ignition timing has to be retarded quite a bit. If the ignition timing is retarded far enough, the air/fuel charge is still burning when the exhaust valve opens. This creates high EGTs. Also, the mixture is burning inside the turbine housing, which is causing the turbo to spool quicker, and this is the cause of the surging issues. The high heat is not good for the turbo, and you aren't making any more power than you would with less boost and a more realistic ignition curve.

Also, your AFR is a very, very rich 10.5:1. This is only going to make things worse, because you're going to have unburned fuel igniting as soon as it contacts free oxygen in the exhaust system.

What I would do is bring the boost down to ~23-24psi, bring the AFR to ~11.2-11.5:1, and retune the ignition curve. You should make the same power, but you won't have the surging issues and your turbo will last longer.

ItalianEvo

Ah ok... so run 110/110...



Ok... very clear...





Thank you very much !



PS: I even have many other ideas in mind...
make and hand made divided exh manifold, and buy the new .78 divided exhaust turbine for the GT3071...
less backpressure than .63 and better spoolup...
But this is another story....