Turbo 101 question

[hobieboy]

Why does "bigger" turbo makes more power than "smaller" turbo at the same boost level ?

I know I have gone down the wrong path somewhere trying to explain to myself why but not sure exactly where...

A bigger turbo will push more air into the engine/intake plenum in same amount of time.

A bigger turbo will push cooler air in assuming we are close to efficiency limit of smaller turbo.

But, for the same boost (pressure) level, the volume of air in the same closed environment (cylinders / intake plenum) will be the same regardless of how (which turbo) it was pushed in.

Granted, cooler air will have more density (more O2 so better combustion), but if the smaller turbo has no problem reaching the desired boost level, this factor alone certainly cannot account for the big HP difference we typically hear (in the tune of 100hp)?

Where have my thinking gone wrong?

[e170drvr]

I think the bigger turbo will maintain the set boost level higher in the rpm range. At higher rpm you are moving more air through the motor, thus producing more horsepower. I think that is why folks upgrade to a Hf or gt35 series, because the stock or standard k27 pooch out around 4500 to 5500 rpm.

[Thierry25]

Originally Posted by hobieboy

Why does "bigger" turbo makes more power than "smaller" turbo at the same boost level ?

I know I have gone down the wrong path somewhere trying to explain to myself why but not sure exactly where...

A bigger turbo will push more air into the engine/intake plenum in same amount of time.

A bigger turbo will push cooler air in assuming we are close to efficiency limit of smaller turbo.

But, for the same boost (pressure) level, the volume of air in the same closed environment (cylinders / intake plenum) will be the same regardless of how (which turbo) it was pushed in.

Granted, cooler air will have more density (more O2 so better combustion), but if the smaller turbo has no problem reaching the desired boost level, this factor alone certainly cannot account for the big HP difference we typically hear (in the tune of 100hp)?

Where have my thinking gone wrong?

Actually it 's a bit more complex and question of flow.

You include "intake side" only into your thinking and you forget the "hot side" . Turbine side has also it own efficiency.

For a given flow at intake , the small turbo will need higher rotation speed.

Basically, you can reach the pressure you speak about with a small turbo. But, past a certain amount of CFM, the turbine ( hot side) is flow limited and can not follow. The flow speed becomes very high. Because the flow speed and the restriction, efficiency becomes poor and the back pressure increase.

[hobieboy]

Thanks for the reply guys... but a few more follow on questions:

If one does not have a boost level issue at high rpm (meaning I can sustain the boost level all the way to redline), then a GT35/K27HF is same as stock K27?

Thierry, you lost me on flow speed/back pressure - other than increasing intake air temp, what more does it harm?

thanks!

[spuggy]

Originally Posted by hobieboy

other than increasing intake air temp, what more does it harm?

I think Thierry was saying that if the hot side can't keep up, not only does it fail to rotate fast enough to keep up with the exhaust flow, but it also becomes a restriction in the exhaust flow.

For the cold side, my understanding is that if the cold side compressor is sized smaller then it is more efficient at lower RPM (boosts sooner) - but the trade-off (without variable-vane turbos) is that as it gets outside the efficiency map at higher RPM it can't flow as much air as needed/heats it more than a turbo better sized for that much airflow would.

Which is why there used to be a number of different K27 hybrid turbos - none of which were the same as a stock K27.

The K27/S was a quick-spooling turbo intended for CIS, ran out of flow at the top end. The K27/HF was a quicker-spooling (than stock) turbo that made more power at the top, and the K27/HFS is intended for use with EFI, because it needs so much fuel at the top.

In comparison to the K27/HF, the 3DLZ is just a boat anchor.

[e170drvr]

What is the difference between the HF and the HFS? And does it have anything in common with my K27s?

[Thierry25]

Originally Posted by hobieboy

Thierry, you lost me on flow speed/back pressure - other than increasing intake air temp, what more does it harm?

thanks!

when the back pressure increase, the exhaust gaz is not correctly extracted from the combustion chamber.

As to the "flow speed" I mean flow velocity. For a given CFM flow at intake side , with a smaller turbine casting ( smaller casting = smaller section ) , the flow velocity (from exhaust input to the turbo exit ) will have to be higher and turbin will have to reach higher rotation speed .

[RarlyL8]

The K27 based turbos all use the K27 (7200) bearing housing and retain the exterior hot/cold housings. Specific components are modified to achieve the goal of that particular version. Here are the basics:
K27S - goal is quicker spool than 7200. Cold housing is machined and impeller design optimized.
K27HF - goal is to increase flow at top end. Hot housing machined and impeller design optimized.
K27HFS - goal is to broaden entire boost range. Engineering of both K27S and K27HF combined.
K27HF2 - goal is ultra-high output as needed for big power EFI engines. Complete redesign of just about everything.

Yes the bigger the turbo the more flow capable at higher rpm before loosing efficiency resulting in back pressure and no further increase in power.
It's pretty much that simple.

[fredmeister]

I have a k27s and am building a 3.4 liter with SC cams and 36mm intake ports.
Will I expect to exceed the limit of the k27S to flow at higher rpm now? Am I venturing beyond the flow capacity now with these changes?
I dont really know where I fit in the compressor map for the turbo, let alone where can i find a compressor map for the k27S?....like searching for the holy grail it seems.
Maybe someone has found this info already?

For reference:
It never had an issue with the stock 3.3liter and stock cams when i switched to the k27S and B&B intercooler and headers. It always held 0.7bar boost all the way to 6000rpm on track days without noticing any boost loss.

thanks
Fred

[fredmeister]

Did some fast calculations on flow rate for my 3.4 build.
Assuming 0.9 (90%) engine efficiency at 6000rpm:

Flow should be approx. 3400*6000*.9*0.5/60 = 156000 cm^3 per sec or
156mm^3 per sec
Where does this fit in the map for both the k27S and the next larger k27hf turbo?

[spuggy]

Originally Posted by fredmeister

I have a k27s and am building a 3.4 liter with SC cams and 36mm intake ports.
Will I expect to exceed the limit of the k27S to flow at higher rpm now? Am I venturing beyond the flow capacity now with these changes?
I dont really know where I fit in the compressor map for the turbo, let alone where can i find a compressor map for the k27S?....like searching for the holy grail it seems.
Maybe someone has found this info already?

For reference:
It never had an issue with the stock 3.3liter and stock cams when i switched to the k27S and B&B intercooler and headers. It always held 0.7bar boost all the way to 6000rpm on track days without noticing any boost loss.

thanks
Fred

Fred, do you have dyno charts? Where do you make peak power? This can help highlight if the turbo is running out of flow.

My 3.3 when stock-ish with SC cams and a 3DLZ made peak power quite low (330 FWHP @ 5300 RPM, IIRC), but with a K27/HF, Kokelyn, GHLs - and still with stock ports - made peak HP (and somewhere between 70-95 more of them) @ 5700 RPM.

My guess is that this is limited by the stock ports/ pancake manifold at that point, rather than the K27/HF, and I'd bet that with bigger ports (38mm or bigger) and a better flowing intake manifold (e.g. a 3,2), you could make still more & higher - and maybe want a turbo with more top-end flow still. Or just keep the K27/HF because it spools up nicely for the street, I dunno...

Both my 3DLZ and the K27/HF maintained boost just fine (12.5PSI on a factory wastegate, according to the mech VDO) all the way until power was clearly falling off (e.g. over 6,000 RPM); although it's a red flag if boost falls off, sustaining boost pressure is not an indicator that you're not out of the efficiency island on the compressor map - the 3DLZ was certainly out of puff, I'd think.

You probably need to talk to someone knowledgeable about turbo sizing about what you want/expect. It's all a compromise.

[fredmeister]

Originally Posted by spuggy

Fred, do you have dyno charts? Where do you make peak power? This can help highlight if the turbo is running out of flow.

My 3.3 when stock-ish with SC cams and a 3DLZ made peak power quite low (330 FWHP @ 5300 RPM, IIRC), but with a K27/HF, Kokelyn, GHLs - and still with stock ports - made peak HP (and somewhere between 70-95 more of them) @ 5700 RPM.

My guess is that this is limited by the stock ports/ pancake manifold at that point, rather than the K27/HF, and I'd bet that with bigger ports (38mm or bigger) and a better flowing intake manifold (e.g. a 3,2), you could make still more & higher - and maybe want a turbo with more top-end flow still. Or just keep the K27/HF because it spools up nicely for the street, I dunno...

Both my 3DLZ and the K27/HF maintained boost just fine (12.5PSI on a factory wastegate, according to the mech VDO) all the way until power was clearly falling off (e.g. over 6,000 RPM); although it's a red flag if boost falls off, sustaining boost pressure is not an indicator that you're not out of the efficiency island on the compressor map - the 3DLZ was certainly out of puff, I'd think.

You probably need to talk to someone knowledgeable about turbo sizing about what you want/expect. It's all a compromise.

I tend to agree with you from everyting I have read on the forums. Simplifying things, now if I increase the port size to 36mm and match port the manifold and insulator blocks, I could effectively increase the engine efficiency to say 95-100% and therefore flow even more air under boost.

I REALLY WAS HOPING FOR SOMEONE TO POST THE COMPRESSOR MAPS FOR THE K27S AND K27HF TURBOS TO SEE WHERE I LIE.
ANYONE WANT TO SHARE THE INFO?

[Thierry25]

Originally Posted by fredmeister

I tend to agree with you from everyting I have read on the forums. Simplifying things, now if I increase the port size to 36mm and match port the manifold and insulator blocks, I could effectively increase the engine efficiency to say 95-100% and therefore flow even more air under boost.

I REALLY WAS HOPING FOR SOMEONE TO POST THE COMPRESSOR MAPS FOR THE K27S AND K27HF TURBOS TO SEE WHERE I LIE.
ANYONE WANT TO SHARE THE INFO?

Maps for K27S and HF are not available ....

K27S map should really close to the K27-7200 map

K27 T3 Singlescroll *30R Equivalent (340-500hp) | BorgWarner AirWerks 2010 Catalog

[spuggy]

Originally Posted by fredmeister

I REALLY WAS HOPING FOR SOMEONE TO POST THE COMPRESSOR MAPS FOR THE K27S AND K27HF TURBOS TO SEE WHERE I LIE.
ANYONE WANT TO SHARE THE INFO?

Unless anyone else went to the trouble to produce them, probably the only person who actually has those - which may be proprietary, if they even exist - for the K27 hybrids is Kevin at Ultimate Motorwerks, who builds them.

I don't think 36mm ports would behave that differently to the stock 34mm ports - as the factory made at least some models with a K27/7200 and 38mm ports, I would expect a K27/HF to still be pretty useful there.

[911st]

Originally Posted by hobieboy

Why does "bigger" turbo makes more power than "smaller" turbo at the same boost level ?...

On most street cars the turbo is sized to be most efficient at TQ peak. Not HP peak.

Put on a turbo that is more efficient at HP peak instead of at TQ peak and it will make more HP as the intake air will not be as heated.

FWIW, the difference in air flow between a stock USA 930 at TQ peak (223chp) and a stock 91/2 3.3 turbo at HP peak (320chp) is over 40%.

Same principle as fitting an intercooler. Fit a good intercooler to a stock 930 and it should be good for up to 5-10% more HP.

For most street cars we usually want the turbo to be as efficient as possible at TQ peak. This makes a car more drivable as it lines up the natural efficiency of the motor with that of the turbo. It also makes the motor less sensitive to detonation where detonation is most likely to occur.

For many race cars we MAY want the turbo to be as efficient as possible at HP peak instead. The higher the average HP the faster a car is going to be. However, this usually comes at a cost at lower rpm operation.

For what it is worth, there is a very, very significant difference in air flow between a stock USA 930 at TQ peak and a modified 400whp 930 at HP peak.

In fact we are talking 118% more air flow in this case (223chp at TQ peak v 470chp).

From this we should be able to see how quickly and 930 might outflow the potential of a stock turbo.

Further, we might also conclude that almost ANY turbo mod or change on a stock and especially on a built 930 is probably going to give the builder bragging rights to an increase in HP.

[911st]

What do we mean by a bigger turbo?

For Porsche with the k27-7200 it was a smaller hot side and a larger cold side over the original stock 930 turbo.

The K27-7006 is considered a bigger turbo than a 7200. However, for the K27-7006 it basically uses a hair smaller compressor wheel than the 7200 but fitted a larger hot side.

Thus, we can get a 'bigger' turbo by making the hot or cold side bigger.

From what I remember the 7200/7006 compressor wheels are most efficient at an air flow up to about the 330chp range. Thus on a "street motor" where the turbo's efficiency peak is best fit to TQ peak they could work on a up to about a 400chp/340whp street motor ok. They will not make the as high of HP as would be possible with a larger turbo with the right hot side but they could still be a solid fit.

Again, there is two ways to make the 7200 a bigger turbo. The way Porsche and BW did it was to fit a larger hot side. The way some builder's have done it is to fit a larger cold side. Within limits and with tradeoffs.


It is my belief that Porsche/BW sized the 7200's hot side to choke off before its compressor wheel reaches its full potential / capacity. Maybe to keep from exceeding the fuel deliver capacity available or just to have a smaller hot side for better low end response. With a larger compessor wheel the turbine is still going to stall at about the same point however the compressor will pump a larger quanity of air.


It is just my opinion but I do not think one can mod a 7200's hot side enough to deal with say up to a 100% increase in capacity in a balanced way. Again, only my opinion as there is a lot of high HP 930's doing so but no one seems to be measuring the exhaust pressure on these cars to be sure.

As to HF compressor map availability:

I suspect one can interpolate from other maps.

Thus, if a HF has the same size compressor inducer size as a GT35 (and I am not saying it is but if I remember they are close), it probably is going to operate efficiently at about the same CFM range. Or less because of the smaller housing.

That is, if a compressor wheel has about a 61mm inducer, at 1 bar it is going to run most effecently in about the 350-425chp range.

Even if we could get the HF compressor wheel maps, they probably would not be equal's when put in a 7200's compressor housing. Turbonetic's puts a 60-1 compressor wheel in two different housing's and I think they note that in the smaller housing to expect about 10% less potental.

Yes, there could be a small difference in peak absolute efficiency like one wheel achiving 77% and another 75% but I suspect this would not be seen by most on the dyno or street by most. It is probably much more important to fit the hot and cold side best to one's build and use.

As to the hot sides, the wheel's exducer seems to be the bigger controlling variable when comparing turbos.

[hobieboy]

991st, thanks for the insights... you brought up a number of good points.
But please bear with me being a little slow -
The size of the hot side will directly impact the onset time (spool up of turbo) and will also impact the top end (when it chokes hence back pressure).
The size of the side will affect volume of air being pushed into the engine given same turbo spinning speed.
Got these...

But, given same boost level & same engine/intake plenum, why would a bigger cold side (ok, if we don't care about lag let's make the hot side big too) makes more power than a smaller one? It takes same volume of air (with some density different of course due to air temp) to produce same boost?

[the_preacher197]

Originally Posted by fredmeister

I REALLY WAS HOPING FOR SOMEONE TO POST THE COMPRESSOR MAPS FOR THE K27S AND K27HF TURBOS TO SEE WHERE I LIE.
ANYONE WANT TO SHARE THE INFO?

As mentioned above, the information you want is not widely available. The K27S should have similar flow characterstics as the stock K27.

Without your dyno chart and power expectations we have to make a lot of assumptions about your engine.

I've got dyno runs for a stock 930 cam and 964 cams, so assuming the SC makes peak power at 5900 rpm, 1.0 bar boost and about 450 FWHP, you end up with:



This is putting you right out near the choke line of the K27 with an efficiency of about 65%. Looking at that graph I'd say you'd definitely benefit from going up to an HFS.

[the_preacher197]

Originally Posted by hobieboy

991st, thanks for the insights... you brought up a number of good points.
But please bear with me being a little slow -
The size of the hot side will directly impact the onset time (spool up of turbo) and will also impact the top end (when it chokes hence back pressure).
The size of the side will affect volume of air being pushed into the engine given same turbo spinning speed.
Got these...

But, given same boost level & same engine/intake plenum, why would a bigger cold side (ok, if we don't care about lag let's make the hot side big too) makes more power than a smaller one? It takes same volume of air (with some density different of course due to air temp) to produce same boost?

I think you just answered your own question? Same volume of air but greater density = greater mass = more O2 molecules to mix with your C and H molecules and therfore produce more power?

[hobieboy]

Originally Posted by the_preacher197

I think you just answered your own question? Same volume of air but greater density = greater mass = more O2 molecules to mix with your C and H molecules and therfore produce more power?

But... 100+hp more?

Or in a "reverse" way - if the turbo can hold desired boost level all the way to red line, how much gain one will get to swap to one that can supply colder intake air (a.k.a. "bigger" turbo), say by 15 degF (post intercooler)?