Iridium

I currently have a 26/8 in the car (and I hate it). Obviously huge transient lag, and the boost onset point is higher than I'd like. Car is a pure racecar: 2.5 liter, 3.5 open exhaust, racegate with controller, etc.

Past friends have had their 26/6 hot side bored out and run a 54 trim wheel, ball bearing, and the transient response was fantastic.

I'm looking primarily to reduce transient response, as when I'm wheel-to-wheel that kills me (if it's just hot-lapping I can drive around transient lag, but when someone does something and you have to lift, waiting for the power to come back just loses two lengths and that always stinks).

Power is somewhat secondary. Car makes 275 square right now at 16PSI. Would love to make 335ish at 17PSI or so, but it's not a straight goal.

So, directly, anyone have direct experience with the transient response difference between a 26/8 and a 27/8, or with a 27/8 and a KKK hotside and a 54-trim bb setup?

I poured through some posts like this https://rennlist.com/forums/944-turb...-on-951-a.html but haven't found direct discussion about how transient lag and boost onset compares to a 26/8 (which is horrible at both).

xschop

LS1,2,3, or 6 fixes those pesky lag issues.

pole position

Lol if you think a 26/8 is laggy than you will be in a world of hurting with your mentioned turbos....you need a Garrett Gt 28xx for instant spool at the expense of a limited top end , lots of custom fab work involved

Dave W.

How about a GTX 2863 or 2867?

Paulyy

If you want to go straight to Garrett. a gtx 3071R is what you'd want to go for with the .63 rear housing.

or this turbo
http://www.lindseyracing.com/LR/Pors...R-SUPER53.html

Apparently there isn't much or any difference between the #8 #10 housings, just slightly more back pressure in the #8 but Shawn (refresh951) used something similar or same and got a similar (IICR better) torque curve to stock but better top end.

a GTX2867 will get you super quick spool, but you'd need to match up the right exhaust housing.
No experience with the 28 on a 2.5L so i'm not sure there.

t3rra

There one 3063 that's smaller then 3071.

Iv got 3071 0.63 it's matches the 26/8 for spool but will hold more boost then the engine can to redline

Not dyno yet but I can honestly say 60/100 4.5/5 secs

28 range will spool faster but not sure if the engine ve would hold it back?

TurboTommy

Using a recirculation valve with a stiffer spring can help the transient response you're talking about. The stock valve takes a little time to close again even after the slightest throttle lift. On the street one wouldn't even notice it but, when racing, you feel every split second. When modulating the throttle, a stiff spring might not even open at all with slight throttle lifts, rendering good response. The downside it might be a little harder on the turbo.
What you're describing doesn't really seem like turbo lag per say.
Even if you already have an aftermarket bypass valve, it still could be on the weak side.
You can buy them with assorted spring strengths or some have a spring adjuster. You can then bench tested and set it so it only opens at full deceleration (or lift for shifts) let's say; or maybe when intake manifold has 15 in.hg vacuum, let's say. You gotta play with it.

Iridium

This was my dyno with the old chips; need to re-dyno with the A-Tune (just installed). Will switch over to M-Tune if I swap out the turbo.

TT, already have a pretty aggressive blow off valve. And the Blitz is cranked on its build curve. The turbo is just laggy when you're fighting wheel to wheel.



This was a friend's car. Bored out 26 hotside with a 54 cold side if I remember correctly, ball bearing turbo. It built boost fast both in terms of the onset point and transient response.

TurboTommy

Yeah, it could be your AFM; I wrongly assumed you had upgraded to MAF.
A MAF is much more responsive.
I still think a k26/8 with a free flowing intake and exhaust should have very little lag.
I had that combination at one time and made full boost at just over 3,000 RPM. I kinda miss that part of it.
You didn't mention your boost onset RPM

Dave W.

Have you checked for boost and exhaust leaks at the header recently? Those two things will hurt boost response.

pole position

I don't think your Web cams are helping with spool, I am certain they move the powerband to the right

I am not aware of any ball bearing KKK center cartridges that can be used for a hybrid build, so your friend had a ball bearing Garrett with a KKK hot side machined to it. Specs ? There are endless possibilities with compressor and turbine wheels, clipping etc

shortyboy

i have a k27/8 and i start to build boost at 3k and have full boost by 3.4-5k. Vitesse MAF/SMT6, Tial WG with 3" exhaust and electronic boost controller.

Dougs951S

As stated above, the k27 isnt a huge turbo and really doesnt deserve its laggy reputation. I have a tial runninng off spring pressure, a basically stock exhaust, a huge front mount with long 3" piping and a totally free intake track due to stand alone, and I see positive pressure at 2.6k in 4th gear. 1 bar comes at 3.2k.

Paulyy

I think some of you guys might be missing the point.

Have a look at the new cars. They have almost no lag. New technology... New turbos.

the k26 is about 30 years old in technology of turbos.

My turbo (gtx3076 / .83) which is rated at 700 hp has 2psi of boost in 3rd gear at 2000 rpm.
5psi @ 3000
10psi @ 3500
18psi @ 3700

in relation to a 27/8. mine produces 80% more airflow (maximum), quite a lot larger and spools quicker than a k27.
For a 2.5L, most (including me) will say that the turbo i'm running is the largest you'd want to go. any bigger will be a waste of money and more lag for nothing.

Having said that, in todays world the k27 is a very laggy turbo for it's power output.
You can get much better turbos that will spool sooner than a k26/6 and produce more power than a k26/8

Spool times play a lot when you're racing on a small tight circuit or even doing autocross or gymkhana which involves 1st and 2nd gear only. getting your engine to spool quickly in 1st & 2nd gear is a huge challenge.

fejjj

Paulyy,

How much modification did you have to do to mount up that turbo?