Turbo Back Pressure

[ns158sl]

I read a few threads on this on this forum and quite a few on other forums like yellowbullet and turbotech. I am trying to figure out if I should keep running this turbine housing or should I put on another? Another question is where is the ideal place to check the 2 pressures. I am comparing against from the Intake Manifold and inbetween the two cylinder banks at the crossover pipe. At 15 PSI boost at the intake I am seeing around 27 PSI exhaust pressure.

383 GM 5.3
Stock 317 Heads
212/212 112 LSA straight up
GT5018 .96 Turbine A/R housing

I have a 1.41 A/R housing laying around, but before I go through with all the work to put it on, I was curious if anyone had some insight on maybe do some more testing or data points before I jump the gun.

The truck is mostly street driven but occasionally sees the 1/4 mile track. I get full Boost around 4500ish RPM and wouldn't really want to experience a whole lot more lag, but if that is the only option, then that's what its going to be

Thanks
Eric

[JodyB]

Getting the exhaust pressure down leads to a more broad and forgiving tune window. Fewer melted piston ashtrays.

[ptuomov]

At peak power, what fraction of the exhaust mass flow do you estimate bypasses the turbine thru the wastegate?

[Orr89rocz]

Check the pressure at the turbo flange as close as possible. Thats the pressure that matters imo

If you are seeing full boost by 4500, you likely will be upset by the 1.41 ar

[BLSTIC]

Have you tried measuring restrictions elsewhere? If you've got 5psi in the downpipe, 5psi between the compressor and the manifold, and a 20"H2O restriction on the compressor inlet, fixing those to acceptable levels could yield a 5-7psi drop in backpressure for the same boost, along with a noticable response improvement.

I would expect a turbo that only came on in the top half of the rpm band to be capable of nearly 1:1 backpressure:boost ratio, unless the turbine and compressor are a drastic mismatch (I'm lookin at you, generic t3t4 Chinachargers)

[Orr89rocz]

Thats an 88 mm with a 90/98 mm turbine wheel. Similar to s400 t6 96 mm turbine stuff which is very popular in that cube range and smaller. I wouldnt have expected 27 psi backpressure on 15 psi or so but if the motor is making good power i wouldnt worry to much. You will max out the turbine side eventually tho probably not too much more boost than that but hard to say. Some guys push things really hard. Still think a reading at the flange would be better

[David Redszus]

Another question is where is the ideal place to check the 2 pressures. I am comparing against from the Intake Manifold and inbetween the two cylinder banks at the crossover pipe. At 15 PSI boost at the intake I am seeing around 27 PSI exhaust pressure.

Measurement of boost is fairly obvious; its the exaust pressure that is tricky.

Exhaust pressure should be measured in a single header pipe close to the head, using a high speed electronic pressure sensor. The data should be logged and its high (peak) and low (residual) values compared to the boost pressure.

If residual back pressure is higher than boost, you'll get flow reversion. If much lower, you'll get intake charge shortstopping right into the exhaust.

[BLSTIC]

David Redszus - Is there a way you could read that mechanically? And does the peak pressure actually matter?

[David Redszus]

David Redszus - Is there a way you could read that mechanically? And does the peak pressure actually matter?

Turbo exhaust pressure peaks occur very rapidly; at 6000 rpm they pulse 100 times a second. Mechanical gauges dampen the pulses into oblivion. The pressure peaks are what drives the turbine and are not as important as the residual or low pressure periods. The low pressures affect flow during the overlap period.

If a high speed data logger is not available, an electronic pressure sensor fed into a USB scope will allow measurement and observation. USB scopes are very cheap and every tuner should have one.

[cjperformance]

David Redszus - Is there a way you could read that mechanically? And does the peak pressure actually matter?

Turbo exhaust pressure peaks occur very rapidly; at 6000 rpm they pulse 100 times a second. Mechanical gauges dampen the pulses into oblivion. The pressure peaks are what drives the turbine and are not as important as the residual or low pressure periods. The low pressures affect flow during the overlap period.

If a high speed data logger is not available, an electronic pressure sensor fed into a USB scope will allow measurement and observation. USB scopes are very cheap and every tuner should have one.

On the topic of peak pressure pulses driving the turbine, take 2 engines , say a 4 Liter 6 cylinder and a 5 Liter 8 cylinder both single turbo, the 6 cylinder has more capacity per cylinder but the v8 has more pulses per given time period at the same rpm. Assuming they are both at the same hp level at the given rpm the exhaust gas mass should be practically equal but the 8 cylinder will see lower peak pressure pulses and therefore require a tighter turbine a/r to achieve the same spool/boost.