Volumetric Efficiency

[kdwebber]

Hey guys, I am currently searching for the properly sized turbo for my rb25det. I have been looking at various compressor maps but I need to know the volumetric efficiency of my engine. Does anybody have an idea of the volumetric efficiency of an rb25 engine with a stock displacement and stock cams. I'm assuming there are many more factors that could influence this but I thought I'd start the discussion here...

Sydneykid any ideas?

Cheers, Kyle

Edited September 29, 2008 by kdwebber

[Lithium]

Hey guys, I am currently searching for the properly sized turbo for my rb25det. I have been looking at various compressor maps but I need to know the volumetric efficiency of my engine. Does anybody have an idea of the volumetric efficiency of an rb25 engine with a stock displacement and stock cams. I'm assuming there are many more factors that could influence this but I thought I'd start the discussion here...

Sydneykid any ideas?

You'd really need a full rev range graph, I usually just use 95% at 1atmosphere as a thumb suck when perusing compressor charts.

[discopotato03]

Not to steal SK's fire but I try to work compressor capacity around desired power output - which can be measured in pounds (or g/Kg) of air mass per minute .

The general rule of thumb is that it takes 11 pounds mass of air to make 100 horsepower so if you look on a common compressor map like say a GT3037/GT3076R 56 trim the compressor wheel speed lines fall away at ~ 50-55 lbs/min airflow .

A common fopar people sometimes make is trying to use compressors that are too big and turbines/housings that are a bit small . They also try to find a map that will supply the correct amount of air to make the desired power - but right in the middle of the map at the area of highest compressor efficiency . By that I mean efficiency as in the least heat increase or density drop area/island .

I always though you'd want to have the area of highest compressor efficiency in the airflow range that the engine made its peak torque because this is where the engine is at maximum load and the lowest possible inlet charge temperarure would help with detonation suppression .

Gotta run , cheers A .

[Lithium]

A common fopar people sometimes make is trying to use compressors that are too big and turbines/housings that are a bit small . They also try to find a map that will supply the correct amount of air to make the desired power - but right in the middle of the map at the area of highest compressor efficiency . By that I mean efficiency as in the least heat increase or density drop area/island .

I always though you'd want to have the area of highest compressor efficiency in the airflow range that the engine made its peak torque because this is where the engine is at maximum load and the lowest possible inlet charge temperarure would help with detonation suppression .

He said volumetric efficiency so I would have thought he was not talking cross purposes with what you are talking about? The VE curve seems to usually be a bit like a flat bell-curve looking thing, and is well worth taking into account if you want to work out where on the compressor chart the engine at x.psi will be roughly. There is no point working out what the potential airflow of the compressor is if the engine is not able to breathe all that air at the pressure ratio it can do so with.

I remember arguing a mate of mine should get a I think it was a GT22 over a GT28 - was a while ago but for examples sake, for his stock head 4EFTE (1.3l Starlet GT motor) as an upgrade because while the GT28 could flow a higher peak lb/min it started dropping off at higher pressure ratios which the 4EFTE would need to run to be able to ingest that airflow with a relatively stock head. The GT22 however came into its own above 1.6bar of boost, and was not really any worse off up there than the GT28 (or whatever two compressors I was compraring) so he could have got the bonus of having the smaller compressor but at no cost of power for his application.

I tend to target full spool being just before the peak comp efficiency area, peak torque at peak compressor efficiency and peak power before the choke line, where possible I guess drawing my "line" from 1/3rd to 4/5ths of the way across the map at the target nozzle pressure.

[djr81]

...There is no point working out what the potential airflow of the compressor is if the engine is not able to breathe all that air at the pressure ratio it can do so with...

Absolutely, but it is about balance. Chose too small a compressor & whilst it may produce the airflow (just) it will only do so at a huge shaft rpm & with low efficiency. So as ever it is about balance

I tend to target full spool being just before the peak comp efficiency area, peak torque at peak compressor efficiency and peak power before the choke line, where possible I guess drawing my "line" from 1/3rd to 4/5ths of the way across the map at the target nozzle pressure.

By full spool I take it you mean reaching the target boost level?

Looking at the compressor maps a few point come to mind:

The y axis is pressure ratio, not boost so keep this in mind.

In the area above the peak efficiency island the shaft speed climbs markedly to deliver the requisite boost. Note also the fall off in efficiency. As an aside this is what annoys me about the "just add another 1lb boost" argument that alot of people use.

As mentioned you can plot the airflow versus pressure ratio for your engine by using the dyno chart & the compressor map. It can be an enlightening exercise. The plot can look markedly different depending on how much boost you are running.

Edited September 30, 2008 by djr81

[Lithium]

Absolutely, but it is about balance. Chose too small a compressor & whilst it may produce the airflow (just) it will only do so at a huge shaft rpm & with low efficiency. So as ever it is about balance

Looking at the compressor maps a few point come to mind:

The y axis is pressure ratio, not boost so keep this in mind.

In the area above the peak efficiency island the shaft speed climbs markedly to deliver the requisite boost. Note also the fall off in efficiency. As an aside this is what annoys me about the "just add another 1lb boost" argument that alot of people use.

As mentioned you can plot the airflow versus pressure ratio for your engine by using the dyno chart & the compressor map. It can be an enlightening exercise.

You are making a wild assumption that I haven't taken these things into account. Seeing as you are enlightened - have a look for yourself at the compressor efficiency of a 52trim GT22 compressor and a 60trim GT28R compressor and bare in mind its for a 1.3litre motor in a car the owner wants solid 12s with for cheap and easy without too much lag. Shaft speed will be heathy, but not as much more than the GT28 as you'd think and the compressor efficiency actually is a bit more in favour of the GT22. A 1.3litre making under 200kw probably doesn't need all the turbine flow of a GT28 and the GT22 isn't really exactly what I'd call lacking in relation - so while the GT28 woTell me which you'd go for for an 1.3litre aiming for up to around 190kw @ fly?

If you read my post properly you'd realise I had said "boost" where boost was relevant and "pressure ratio" where that was relevant, when having a discussion about such things I don't feel inclined to explain the theory behind every single thing I am talking about as I assume that others who participate in the conversation understand whats involved or will ask if they don't and assume the same of me.

I am actually a little more on the side of relying on 1psi more boost than extensive work on the engine to try and make more power on lower boost, especially when talking about a 1.3litre - bear in mind that 1psi more boost doesn't just raise peak power, but the entire torque curve gets raised. When dealing with a little motor (or any motor really) every little bit of torque helps - look at 1/4 mile times for people running 1bar on GT30Rs and 1.2+bar on GT2535s on RB25s, you'll find the GT30Rs making a bit more power than GT2535s often trail the GT2535s unless the GT30R car has the turbo really humming to capacity. Area beneath the curve is where its at...

And yes, full spool I mean where the target boost pressure is reached.

Edited September 30, 2008 by Lithium

[djr81]

You are making a wild assumption that I haven't taken these things into account. Seeing as you are enlightened - have a look for yourself at the compressor efficiency of a 52trim GT22 compressor and a 60trim GT28R compressor and bare in mind its for a 1.3litre motor in a car the owner wants solid 12s with for cheap and easy without too much lag. Shaft speed will be heathy, but not as much more than the GT28 as you'd think and the compressor efficiency actually is a bit more in favour of the GT22. A 1.3litre making under 200kw probably doesn't need all the turbine flow of a GT28 and the GT22 isn't really exactly what I'd call lacking in relation - so while the GT28 woTell me which you'd go for for an 1.3litre aiming for up to around 190kw @ fly?

If you read my post properly you'd realise I had said "boost" where boost was relevant and "pressure ratio" where that was relevant, when having a discussion about such things I don't feel inclined to explain the theory behind every single thing I am talking about as I assume that others who participate in the conversation understand whats involved or will ask if they don't and assume the same of me.

I am actually a little more on the side of relying on 1psi more boost than extensive work on the engine to try and make more power on lower boost, especially when talking about a 1.3litre - bear in mind that 1psi more boost doesn't just raise peak power, but the entire torque curve gets raised. When dealing with a little motor (or any motor really) every little bit of torque helps - look at 1/4 mile times for people running 1bar on GT30Rs and 1.2+bar on GT2535s on RB25s, you'll find the GT30Rs making a bit more power than GT2535s often trail the GT2535s unless the GT30R car has the turbo really humming to capacity. Area beneath the curve is where its at...

And yes, full spool I mean where the target boost pressure is reached.

Look I wasn't making any assumptions. I was just trying to help the bloke who started the thread. Sorry if whatever I said offended you. I didn't mean to.

The lb/min to horsepower relationship is a useful one & one that I point people towards when looking at turbos. Not sure why you so rarely see it discussed. The shame of it is how bloody difficult it is to find maps for alot of brands of turbo. Garrett is the notable exception to this.

The pressure ratio thing is important simply because people rarely speak of it in terms related to the turbo performance. They just talk of restrictions without quite realising what the restriction most affects & how cheap an upgrade fixing it is. The compressor map doesn't tell the whole story in this regard anyway as its x axis is in lb/min rather than the more common volume flow you see in industrial stuff. This coupled with the hp & boost obsession of many people puts too much emphasis on one side of the turbo only.

You are right in that performance is all about the area under the torque curve. Or to put it another way the average power output over an rpm range. A graph of rear wheel torque versus roadspeed (ie for each gear) can help enormously when optimising motor/turbo setups.

Edited September 30, 2008 by djr81

[Lithium]

Look I wasn't making any assumptions. I was just trying to help the bloke who started the thread. Sorry if whatever I said offended you. I didn't mean to.

Sorry, none was taken - I was having a non-tactful moment while having a stress break, sorry about that

[2rismo]

A common fopar people sometimes make

Is that anything like 'faux pas' or is it a Chrysler-specific indiscretion?

[Abo Bob]

For what it's worth I've always heard 95% VE used in relation to RB25's.

[The Dan]

For what it's worth I've always heard 95% VE used in relation to RB25's.

It really depends on what method of calculation you are using.

If you are just trying to plot compressor maps and need something that will give you a good close guide. Stay with 90%.

I have found that it is closer to the mark with the below equation

A good way to work out air flow in lb/min for a given engine is:

(Displacement X RPM X VE X Pressure Ratio) / 5660 = CFM.... / 14.27 = lb/min

This is only a guide mind you. There are many other factors involved to accurately calculate Air Flow etc.

Displacement = 2.0, 2.5, 2.6 etc. etc.

RPM = You can use this to calculate air flow at different RPM values so you can see what RPM the turbo will come on boost etc

VE = Stick with 90% for arguments sake and for this excercise

Pressure Ratio = well....pressure ratio

Some good reading:

http://www.epi-eng.com/piston_engine_techn..._efficiency.htm

[discopotato03]

I think I can see where this is going but I'd go with airflow potential because you can easily get buried under reams of paper crunching volumetric efficiency numbers .

VE will never be the same in atmo charged and then supercharged states so I'm not sure of the value in crunching estimated VE figures .

I think the formulas intended for engine airflow potential are aimed at NA or at least pre boost supercharged engines and it all changes when the pressure goes positive .

I'll stay with engine and compressor airflow potential because at least the state of the air going into an NA engine or a compressor is easily measured and the compressor maps accurate enough to have an idea of the airs volume and density coming out of it .

It's also the reason I think measuring airflow into a turbocharger upstream of the compressor with a Mass air Sensor is way to go , mainly because it's before its temperature and density states are altered .

Personally I think there is enough data about various turbos on mechanically std RB25DET's on this site to get a bit of an idea , going for a ride in few would give the real world experience .

A .

[djr81]

This isn't terribly accurate & doesn't purport to show anything much beyond some general principles, but...

There are three attachments

1. Dyno chart of an RB26 (Roughish tune due to time constraints)

2. A plot of the air flow versus pressure ratio on the compressor chart for the turbo actually used - a GT2860 707160-5.

3. A plot of the equivalent airflow on a GT28-60 707160-7.

Now obviously the two wont & cant work the same but as I said - general principle.

Some things are noteable.

1. The dash 5 can flow more air & more boost.

2. The -7 efficiency is always lower than the -5.

3. The compressor efficiency falls away markedly on the -7.

Question I have is how far to the right ie how much more airflow can be had from these things?

Edited October 2, 2008 by djr81

[Lithium]

Some things are noteable.

1. The dash 5 can flow more air & more boost.

2. The -7 efficiency is always lower than the -5.

3. The compressor efficiency falls away markedly on the -7.

Question I have is how far to the right ie how much more airflow can be had from these things?

Sorry I'm not 100% sure what you mean by that question - do you mean how much more to the right can the RB be made to go, or how much more airflow can you get from one of those turbos?

You should check out Garrett's GT2560R (466541-9) compressor map, quite interesting reading.

[discopotato03]

You wouldn't by any chance mean this one would you ?

http://www.turbobygarrett.com/turbobygarre...0R_707160_9.htm

Some of those RB26 specific turbos use a "T3" sized compressor housing where others are T4 sized internally .

The 707160-9 (GT-SS)'s comp cover is T3 sized internally .

A .

Edited October 2, 2008 by discopotato03

[Lithium]

You wouldn't by any chance mean this one would you ?

The 707160-9 (GT-SS)'s comp cover is T3 sized internally .

Thats exactly what I meant, I must have had a brain fart and wrote the CHRA # instead of the whole unit's part number - my bad

[djr81]

Sorry I'm not 100% sure what you mean by that question - do you mean how much more to the right can the RB be made to go, or how much more airflow can you get from one of those turbos?

You should check out Garrett's GT2560R (466541-9) compressor map, quite interesting reading.

I meant the turbos. It is just that a number of motors I have seen sheets for run more air flow that is shown on the compressor maps.

That other turbo is interesting & look well sized for that 435 rwhp- but the -5 & -7 were a straight bolt up.

[Lithium]

I meant the turbos. It is just that a number of motors I have seen sheets for run more air flow that is shown on the compressor maps.

That other turbo is interesting & look well sized for that 435 rwhp- but the -5 & -7 were a straight bolt up.

The -9 is a bolt up too as far as I understand. The airflow thing, there is no magic conversion - its all basically thumb sucks and the fact that Dynamics dynos read stupidly low probably mean the estimates go all up the wazoo. For the dynos that I am used to the calculations all seem to match up quite nicely and everything has worked out well so far - within reasonable margains of course.

[djr81]

Well the housings look to be different?

http://www.turbobygarrett.com/turbobygarre...0R_707160_9.htm

http://www.turbobygarrett.com/turbobygarre...0R_707160_5.htm

[djr81]

You wouldn't by any chance mean this one would you ?

http://www.turbobygarrett.com/turbobygarre...0R_707160_9.htm

Some of those RB26 specific turbos use a "T3" sized compressor housing where others are T4 sized internally .

The 707160-9 (GT-SS)'s comp cover is T3 sized internally .

A .

I am a little confused. The -7 has been tagged as the GTSS equivalent but now a few people have moved over to the -9 as being more closely aligned.

Can the -9 be bought with the right flanges etc to bolt straight up?