When is a Turbo too Big?

Basically, once you're above the boost threshold, when you WOT, you'll spool as fast as that particular turbo will allow. This has a lot to do with the mass of the CHRA and its resulting inertia. Bigger turbine/compressor wheels have more inertia, everything else being equal, so the spool isn't as quick. Also, twinscroll setups lower the boost threshold by pairing the appropriate cylinders and effectively channel/synchronize sufficient exhaust gasses to reach boost threshold at a lower rpm.

Now if you WOT below the boost threshold, you'll see very little boost level climbing until you reach the point that the combustion gasses are sufficient to spool the turbo. That's what sometimes throws people off. The interplay between boost threshold and spool has a significant impact on driving experience. That's why I love my BNR S3. I never WOT below its boost threshold because stock block. And it'll spool just as quickly at 3k as it will at any engine speed above 3k because the engine is pumping enough exhaust gasses to have eclipsed the boost threshold.

 

Yeah I get the physics of it. What I dont get is how people can compare turbos of varying sizes if you arent starting either A: at the same RPM, or B: at the boost threshold point. In my mind its just an arbitrary number at that point. I would want to know apples to apples I guess.

 

This is true. It's because even beyond boost threshold, the inertia of the CHRA needs to be overcome so that it's spinning fast enough to make appreciable boost. With my BNR, this takes ~250 rpms or so in 4th. It's even quicker in higher gears because the longer gearing increases the load on the engine and flows more gasses through in a shorter period of time, rpm-wise.
[doublepost=1471028167][/doublepost]

I think the only way to really compare spool between various turbos (assuming the tunes are setup for optimal spool) is to look at the boost onset angle. If it angles up sharply = faster spool; this will take less of the poweband to accomplish too. If it has a shallow angle of boost building initially = slower spool.

But ultimately, comparing the angles of boost building AFTER the boost threshold of both turbos would give the most direct comparison.

 

So long and short of all of this: Every number everyone says for "spool" means nothing and we arent comparing the right data between turbos? Thats pretty much what I had thought.

 

Not sure if you're being facetious

I'm certainly no expert, but I've been very interested in the topics of boost threshold, spool/turbo lag, transient response, etc., for a while and have researched them a lot. It's a very wise thing to do when you're considering turbo selection. From what I gather, most folks that are talking about X turbo spooling faster than Y turbo are usually speaking about a phenomenon that's actually a combination of boost threshold AND spool time, yet they group it all together under the term "spool." In other words, X turbo will start building significant boost more quickly than Y turbo. But in truth, those types of statements are usually a function of both boost threshold and spool, because the sooner the threshold is reached, the sooner the turbo can start spooling to really pump out that psi, you know? I hope I'm not making it too complicated (I sometimes have a talent for that). And on top of that, boost threshold pretty much goes completely out the window when you're above it in your powerband. At that point, only spool and transient response matter.

Maybe a tuner can come in here and make my statements much more succinct and relevant, LOL

 

Sorry, I wasnt being facetious. And I followed everything you said. Really what Im getting at is that most people just say where they are hitting X psi when someone post a thread asking about turbos. But that number is pointless unless they are starting at the boost threshold or at a specified RPM.

Now if people were comparing the delta pressure over the delta RPM, that would be a better comparison IMO. But when someone tells me their PTE-AAAA spools faster than a GTX-BBBB because they can hit 20 psi at XXXX RPM where as their buddy gets to it 200 rpm later. Well I just cant take any stock in that.

 

Very well said. And statements like your hypothetical GTX vs. PTE comment is an example of how some people completely disregard influential factors, such as how the tune is setup. Some of the best tuners won't calibrate for the fastest spool possible in pursuit of traction management, for instance. So it really is hard to do an exact comparison between two different turbos on two different cars with two different tunes. At best, such comparisons give us trends, which are illuminating, but not absolutely definitive.

And we haven't even begun to discuss the differences in spool between gears. If you do a 3rd gear WOT pull and nail the throttle at X rpms, you'll build boost more slowly than if you do an identical pull in every way, but in 4th gear due to engine loading and its influence on building boost. There really is a lot to turbocharging.

As a very input/learning-oriented individual, I could study/talk about this stuff until the cows come home, LOL

 

Ok, just curious if anyone else can post a comparison for me. Im running a frankenstined 3071 (Very similar to an HTA3076 with a ATP hotside)

Looking at a log and trying to pull out delta RPM over delta pressure. Using the initial line in the log as the first place I see 99 on the APP, and the final as the one that is closest to 20psi.

RPM (initial) = 2464

RPM (final) = 3215

Pressure (initial) = 1.4 psi

Pressure (final) = 19.87 psi

(3215-2464) / (19.87-1.4) = 751 / 18.47 = 40.66 as the slope of my line.

Can someone else post their numbers for their turbo to see what type of comparison we get?

Looking at a second log, doing the exact same calculation I get 33.00. So maybe this isnt the comparison I was hoping it would be.

 

You can post my slow ass 35R numbers since you haz the final log.

 

Yeah I dont think this works quite the way I was hoping. With the 2 logs you sent me I get 54.89 and 45.25. So it looks like its not much of a usable number. Really was hoping to see more consistency in them than that

 

I'd participate, but there's no way I'm going WOT @ 2,500, LOL. You can see how steep my boost ramp up is in here, though:


This was with 97-degree IATs. Did I mention I love methanol?

RPM initial: 3,051; RPM final: 3,453
Boost Initial: 1.5psi; Boost final: 20.3psi (figures from the actual log - the very bottom isn't on the graph)
(3,453 - 3,051) / (20.3 - 1.5) = 402/ 18.8 = 21.38 as the slope of my line

Interesting results. You can see by my graph above that as soon as VDyno registered WOT, there's a steep climb to ~20psi (which I used since that what you used too). Do you have a graph as well?

It appears that my slope is about twice the rate, but something about how we crunched the numbers is odd because I have half the figure. And an HTA3071 wouldn't spool 2x as fast as a BNR. That'd be faster than a K04.

Another way to look at it is it took you 750 rpms to gain 18.5psi
It took me 400 rpms to gain 19 psi.
Since the boost amount gained is almost identical, we can compare just rpms:
750 / 400 = 1.875
So according that that way of crunching numbers, an S3 spools at 1.875x the rate of a HTA3071; in other words, 87.5% faster. So it appears that in the RPM/Boost Level figure you came up with, lower is faster. In other words, it's basically a measure of time. Lower number = less time.

But all of this really is spitballing unless I could WOT @ 2,500 too. We used virtually the same starting and ending boost levels. But you WOT'd 500 rpms sooner than me. Who knows what the BNR would've done if I went WOT at 2.5k?

 

I will post up a graph in a bit, but I think there are some issues with the resolution of the data that make this not really work. Comparing multiple logs I get wildly varying results

Sent from my SM-G900V using Tapatalk

 

Can I play? GT3071 on built motor, Freektuned. Not sure on the spool math that y'all speak of but car hauls ass. Maybe someone can interpret for me.

 

I don't have much in the way of good data anymore (that I know of, if there's interest I can look around for some) but I did remember about this exact subject that was posted on in one of the Facebook groups:

 

To really do an accurate comparison, you'll have to pull your data from the logs from those graphs, looking at the time and rpm change amount between 0psi and 20psi. The pictorial representation of VDyno just isn't nearly accurate enough. BTW, I went from ~0psi to ~20psi in 1.3 seconds on the BNR. Seems very close to that 6758, but I'm sure the EFR is so much better in other ways.

 

Second pic has logged data in it. -1 vacuum to 23 psi in a little over 1 second and 450 ish RPM.

 

Data log of above dyno.

 

Crunch yo numbers, fool! LOL
On cursory glance, it looks like you went from 0.5 - 20psi in 1.52 seconds and 563 rpms. Pretty quick, indeed!
[doublepost=1471063927][/doublepost]

Very nice. What about 0 - 20psi? How many rpms? How many seconds? Sounds comparable to the BNR S3, but I'm sure your next-level tech will reign supreme. It does, however, make me even more pleased with my BNR's performance. It really is an awesome turbo overall.

 

VT logging wasn't the greatest back then. In this video, if you listen close, you can hear the turbo fully spool, despool slightly due to overboost compensation (from not changing tune and running no intercooler instead of TMIC) and then respool again. Listen to the exhaust and the whistle.

 

Yeah, it's hard to tell clearly, but after playing a few times I could definitely hear it. BTW, didn't you say that the EFR7163 would spool even faster? That kinda boggles my mind since, you know, it's a bigger turbo that flows 60 lbs/min over the 6758's 49 lbs/min.