Does Compressor Size Radically Effect Lag/ Performance?

[Bennis]

I need some help and info to understand a bit more about turbos. So I understand that a bigger turbine will spool slower on an engine compared to a smaller one.

Most of the info out there comparing turbos focuses on different turbine wheels and housings, but what difference will a different compressor size make between two turbos where the turbine is the same.

Items in question are two garretts.

GTX3067R : http://www.atpturbo.com/mm5/merchant.mvc?Screen=PROD&Store_Code=tp&Product_Code=GRT-TBO-260&Category_Code=GTX

GTX3076R : http://www.atpturbo.com/mm5/merchant.mvc?Screen=PROD&Product_Code=GRT-TBO-101&Category_Code=

Both have the same hotside, just one has a considerably smaller compressor. But if both turbos have the same turbine and the same turbine housing will they produce say e.g. 15psi at the same RPM on the same motor?

[zebra]

Yes, bigger compressor = more weight to spin = more lag

and depends on the design of the comp blades and the size of the housing

[Lithium]

GTX3067R : http://www.atpturbo.com/mm5/merchant.mvc?Screen=PROD&Store_Code=tp&Product_Code=GRT-TBO-260&Category_Code=GTX

GTX3076R : http://www.atpturbo.com/mm5/merchant.mvc?Screen=PROD&Product_Code=GRT-TBO-101&Category_Code=

Both have the same hotside, just one has a considerably smaller compressor. But if both turbos have the same turbine and the same turbine housing will they produce say e.g. 15psi at the same RPM on the same motor?

No, they won't - the GTX3067R and GTX3076R will be really different to drive in terms of spool and especially response. Way more inertia on the 76mm, and there are also aerodynamic effects which will come into play. There is a possibility that at steady state on a dyno (ie, find the minimum rpm you can build 15psi by holding WOT against the dyno) there may not actually be as huge a difference as one might expect - but that means next to nothing in the real world.

[discopotato03]

My take only .

I believe the thing that separates a good system from an average one is what goes on at part throttle - on a roadie anyway .

I will state that if a compressor makes it easy for the turbine to spin up readily it will .

Something I used to mention is that you're not just spinning up a hunk of aluminium when your turbo "spools up" . It's attempting to pump air and the mass of the air being moved adds to the wheels weight/inertia - turbine drive power ask .

As has been mentioned aero tech in the compressors makes a difference and the modern equivalents tend to be lighter and smaller in diameter . Depending on what the designer wanted wheel designs can be smaller whilst pumping more air but the spool priority comes back to the designers goals .

There are obvious differences between a GTX3067R/3076R and while they use the same turbine - both the compressor and its housing is larger on the 76mm version . Also for the sort of power goal you'd expect from a GTX3076 user a larger turbine housing than a 3067 usually gets would generally be fitted so no longer apples to apples .

For a given turbine size the variables are everything on the compressor side vs the turbine housing AR size and type .

A .

Edited May 6, 2014 by discopotato03

[533 RYC]

This link should help, its a bit technical, but answers your questions on airflow.

1st link in the thread is the one you want:

http://www.skylinesaustralia.com/forums/topic/442555-building-the-right-combo/

[Lithium]

Those links appear to contain nothing at all to do with what he is asking? And also posting a link to more links seems a messy way of doing things, could have just posted a specific link and explaining what it is.

Anyway, he never mentioned airflow, he was asking about spool/response

[533 RYC]

Ok Mr Lith, you seem to know what your talking about!

Did you actually READ the link? It is COMPLETELY RELEVENT to spool/response!

Understanding AIRFLOW characteristics of your engine determines which turbocharger is RIGHT - not necessarily the biggest - for your setup!

"Turbocharger Compressor Calculations"

OP's first line: I need some help and info to understand a bit more about turbos.

[XKLABA]

I'm sure the question at the top of the topic states

Does Compressor Size Radically Effect Lag/Performance "

And the last line in the second paragraph says

" but what difference will a different compressor size make between two turbos where the turbine is the same."

Which I take as he wants info about the compressor side of the turbo, I didn't read anywhere him asking about engine air flow

Lith is right, it makes a sizeable difference in both lag and performance, the 67mm will spool faster but the 76mm will make more power

In my view it is to do with leverage and inertia and even though the weight difference between the two is not a great deal you are trying to change their rpm from say about 20,000rpm to upwards of 100,000rpm ( depending on boost level ) whilst trying to pump pressurised air, so naturally the smaller lighter wheel will spin up faster and the turbine wheel has better leverage against the outlet side of the wheel where the bigger wheel has more volume allowing it to pump more air volume

Hope that helps

Lith, its not his fault he is trying to drink vintage grange and read/reply to forum posts while riding his pushy on a busy back road

[533 RYC]

LOL, you guys don't have a clue!

*sigh* I guess there are some you can't educate......

I'll stick to what I do everyday, building engines - all turbocharged.

Mick, stick to driving trucks & looking out for cyclists!

[XKLABA]

I'm not saying engine air flow doesn't effect the turbo response, I know better then most it does, I'm just saying that isn't what he asked

I would probable build engines for a living but it doesn't pay enough , so hobby it is

The 26/30 I built MYSELF pumped out 623hp at ALL 4 using HKS GT-RSs ( -10s )and push a 1661kg car plus 100+kgs for me to 10.88 at 132.23mph, not bad for a excavator operator/truck driver

[discopotato03]

Bennis I re read your original post and I'll have one shot at your questions .

Your asked specifically about a GTX3067R and a GTX3076R and having the same identical turbine , which they do , and the same turbine housing which is optional .

Direct comparison is difficult because these two use different compressors and different compressor housings to suit the airflows they can generate . Off the top of my head the flow rates are around 47 pounds/min vs 57 pounds/min so not insignificant .

These compressors are advertised as 67 and 71mm diameters but because of their extended tip blades are actually bigger than this . Also the blade count is different so 10 for the GTX67 and 11 for the GTX76 .

Dimensions aside it was obviously Garretts intention to make the smallest GTX30 turbo their most responsive and a large part of that is making the compressor smaller in diameter meaning less weight or inertia for the turbine to accelerate . The compressor housing on the GTX67 is also a smaller family one so you'd think for a given wheel speed the air speed would be higher .

My experience with turbochargers is that once you reach a certain point boost and airflow wise the whole engine turbo system becomes self feeding and takes off like a rocket at wide open throttle .

Below this point it's quite possible to generate positive pressure at the turbo but not enough to create significant acceleration .

You may think this is getting long winded but the point is I think a GTX3067R is going to reach the varooommm point a bit earlier and get into it's stride a bit sooner than a GTX3076R . I reckon the 67 would get to 15 pounds earlier engine rev wise on say an RB25 because it doesn't have the extra diameter and mass of the X76 wheel and would be spinning turbine and compressor faster as well .

Feedback is slowly getting out there with the GTX3067R after what looked like little market interest in them . I think people doubted a pissy looking 67mm compressor was worth worrying about but in the right app they are shaping up fine .

I'm interested to know what specific engine/car you have in mind and what sort of performance you'd like .

It's difficult to make hard and fast rules with turbocharging because there are different ways to solve different issues and the fixes aren't universal .

IMO with these specific two turbos the GTX3067R is a more modern and efficient GT3071R and the GTX3076R a more compact almost GT3582R with out the bigger heavier turbine .

I think there is enough experience in these matters with the people here not to have to bother with thermal dynamics formulas and ditto polar moment of inertia ones . Different people try different things and eventually you have to go with what you think will suit you best . Getting a ride in someones car that has what you're considering is best provided you know every mod it has and it's tuned properly .

I'm out cheers A .

[XKLABA]

What I found interesting about this is the GTX3076 had an option for RBTwin comp housings, doesn't that mean they can be a Bolton option for the GTRs

The GTX3067 would be a nice turbo for the 26/30 in twin form

[Lithium]

LOL, you guys don't have a clue!

*sigh* I guess there are some you can't educate......

You are quite right, we don't. Want to help fix that issue by telling me where in those links it shows what affect on lag and response there is between the two compressors?