Turbocharger exhaust outlet flow question

[jdc0589]

Im new on this forum but I have been on various others for a few years now. Just joined here in the hopes of finding a larger group of people well educated in forced induction design and concept.

So, I would love to get some detailed answers to the follow questions.

***Question #1 and #2 refer to the same engine and turbo setup, the only difference being the lack of a wastegate in question #1. I dont think engine and turbo specifics are really necessary to yield the broad answer im looking for on a conceptual level.

Question 1: If no wastegate was connected to an otherwise traditional single turbo setup, and the turbo was allowed to boost without restriction (at any engine rpm), when the turbine reached its maximum speed (governed only by the exhaust flow and exhaust housing a/r), how large of a CFM drop would there be between the turbine exhaust inlet and outlet?

Question 2: With a completely traditional single turbo setup operating at full boost -'a' (with a functioning wastegate), if the turbine exhaust outlet flow (cfm-x) was combined with the wastegate outlet flow (cfm-y) would the total post turbine outlet flow (cfm-x + cfm-y) experiance the same CFM drop relative to the total exhaust flow produced by the engine as present in question #1?

Thanks a bunch, any info is greatly appreciated.

-Davis Clark

[UncleBob]

your questions are very hard to answer because back pressure is directly connected to the size of the turbo. It will completely depend on the turbo and engine in question. If a turbo is too big for the application, it will be limited in psi regardless of a wastegate.

Pressure drop over a turbo is the same thing. The turbo is basically a restriction. A motor is an air pump. A bigger air pump flowing into the same restriction, will see a higher pressure drop over the restriction. Vice versa a less restrictive restriction for the same air pump, will see a smaller pressure drop

One of the hardest parts of custom turbo'ing is sizing a turbo properly for an application.

edit: I should add, the reason I kept saying "pressure drop" instead of your reference to "CFM drop" is because CFM doesn't consider air density. This is very important since the pressure drops greatly change the density. The temp drop over the turbo would effect that also, which is not only controlled by the restriction-level of the turbo, but also the tuning of the engine. But to put it into perspective, if you have 30psi in the exhaust manifold, pre-turbo, and had 0 psi post turbo, your CFM would actually INCREASE after the turbo due to the density difference....and thats before temps were added into the mix. A much better measuring stick on flow would be lb/min

[KiwiBacon]

What exactly do you mean by CFM drop?
CFM implies you're talking about flow, but by definition the engine spits out the same flow as it ingests (apart from temp differences).

In which case the answer to your first question is:
Depends entirely on the exhaust temperature (which is also related to backpressure).

[2.2 Straight six]

Moved to appropriate forum.

[jdc0589]

I realized my mistake about the flow measurement late last night, just didnt feel like firing the computer back up.

I guess I can go into a little detail.

The engine is an un-modified 2jzgte (3.0L)
As for the turbo, a garrett GT2860RS is the right size, specs are here http://www.turbobygarrett.com/turbob...S_739548_1.htm
Id like to get data for both the .86 and .64 a/r.

[UncleBob]

ah! The disco-potato!

That is a relatively small turbo for your engine. It will probably spool very early with the larger housing. Its only good to 350ish hp.

The easiest way to find a good discription of what you'll get with what AR, is to find someone with your engine and the same turbo. The complexity with AR's can be rather vague. Its not just the AR that factors into, but also the volume of the housing, which isn't something they tell you. Different turbo's have different effects for a given AR

[jdc0589]

Originally Posted by UncleBob

ah! The disco-potato!

That is a relatively small turbo for your engine. It will probably spool very early with the larger housing. Its only good to 350ish hp.

The easiest way to find a good discription of what you'll get with what AR, is to find someone with your engine and the same turbo. The complexity with AR's can be rather vague. Its not just the AR that factors into, but also the volume of the housing, which isn't something they tell you. Different turbo's have different effects for a given AR

I know its way too small for the 2j, I have never considered running only one.... I just need some data on the turbine exhaust outlet flow before I can tell if my concept holds any water....just havent figured out how im going to get the data...

it doesnt really even matter what engine, or turbo is used to get the data, I just need a pre and post exhaust housing lb/hr measurement for any old random single turbo setup at a couple of random boost levels.

[UncleBob]

Originally Posted by jdc0589

I know its way too small for the 2j, I have never considered running only one.... I just need some data on the turbine exhaust outlet flow before I can tell if my concept holds any water....just havent figured out how im going to get the data...

it doesnt really even matter what engine, or turbo is used to get the data, I just need a pre and post exhaust housing lb/hr measurement for any old random single turbo setup at a couple of random boost levels.

as kiwi said, the flow entering and exiting the motor will be identical, as well as all points along the way.

physical matter can't suddenly be there, then disappear and reappear somewhere else. That would be anti-matter

if you're doing twins, I'd advise the smaller AR...but again, its a question of performance vs drivability. The peak capability will always be the larger housing.....ASSUMING your engine is big enough/efficient enough to spool it properly

[KiwiBacon]

Here's how A/R works.

It takes your exhaust flow and accelerates it to the high speeds necessary to spin the turbine wheel.

Garrett's A/R ratios are measured in inches, if you multiply that by the radius of the turbine wheel, you get the effective flow area at the turbine tip.
That effective flow area can be used (within reason) to compare turbos of different size from different manufacturers.

As far as you're concerned though, the difference between a .86 and a .64 A/R housing is about 1/3.
The .86 will take roughly 30% more engine speed to spool, the .64 will take about 25% less engine speed.

If you're at an engine rpm where both are boosting, then the .86 housing will give you less exhaust restriction which will free up a few hp more.

Since a 3l engine is on the high side for a disco potato, I'd definitely suggest the .86 housing. But AFAIK they are completely swappable, so finding a turbo shop that'll lend you different housings to try would be a good idea.


Do you require flow data for your engine or for the GT2860 turbo?

[UncleBob]

Its a question (mostly) of the inlet hole area between the two housings. This is where the trouble happens with AR's. If they are both identical, then the will be straight up as kiwi says.

If they are not, then things don't spell out that simply. I have a T28 (T3-55 compressor) off of some australian car, that only has a 1.75"x1.75" inlet. It has a .62 AR, and spools waaaay too fast on my particular engine, for a .62 (compared to other, larger inlet housing turbo's)....thats just one example where both examples were garrett's, but they were not a direct corrilation between the two.

I would assume both housings have simular inlets.....but you still need to translate it to spool time for your engine build.

[KiwiBacon]

Originally Posted by UncleBob

Its a question (mostly) of the inlet hole area between the two housings. This is where the trouble happens with AR's. If they are both identical, then the will be straight up as kiwi says.

Of course by inlet, you mean the area where the scroll meets it's tail. The actual inlet area at the flange means nothing.

[UncleBob]

well, I guess we could debate where the scroll starts. The point was, ratio's don't mean much if you don't have a point of reference

Just offering my experiences....the main point is, regardless, you need a baseline before the AR means anything (assuming simular parameters anyway)

[KiwiBacon]

Originally Posted by UncleBob

well, I guess we could debate where the scroll starts. The point was, ratio's don't mean much if you don't have a point of reference

Just offering my experiences....the main point is, regardless, you need a baseline before the AR means anything (assuming simular parameters anyway)

IMO, there are two baselines that fit with the A/R. The first is the radius of the turbine wheel (gives you tip speed), the second is the rpm that that turbine/compressor combo will start to compress air.
For example, comparing turbine A/R's across turbos with different wheel diameters is pointless.

The area means nothing without the radius, as that defines the start of the free vortex that accelerates the exhaust gas into the turbine.

[jdc0589]

Originally Posted by KiwiBacon

Do you require flow data for your engine or for the GT2860 turbo?

Both, kind of...

I know the basics of how a/r, though I do start to get a little hazy on some of the info the the posts below the one that I have quoted.

Looks like im just going to have to get out the trusty book and start doing some math.

First i need to get the total exhaust output in lbs/min per rpm from the engine (assuming full load). Then I will need some help from you guys figuring out the turbine exhaust outlet lbs/min value (im just going to use the 28rs specs with the .86 housing, can go back later if I need to). At that point I will have all the info I need to figure out exactly how im going to plumb the setup (assuming the concept checks out).

Im not going to reveal everything yet, but the final setup will use two turbochargers. Before you say it, I am not talking about a compound setup. Obviously im not going to test it with the $$$ disco's. I havent decided what the guinea pig turbos will be, anything that can efficiently flow 28-31lbs/min will work, maybe a standard td04h-16g.

[UncleBob]

what exactly are you trying to figure out? Is this for figuring out the exhaust outlets from the turbo's?

Doesnt seem like you're worried about spool