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Turbo Lag?

 
Old 05-15-2013, 06:48 PM
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Waffles944
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Default Turbo Lag?

Lets start off with the basic question:

When should I be seeing Full boost on my '89 951 assuming it is completely stock? in 3rd 4th or 5th gear that is.

In 5th if I roll onto the throttle at 2000 rpm the car seems to make max boost at about 3300 rpm, but in 3rd and 4th it seems to take till almost 4100rpm to do this. Is that normal? The car just felt faster before i decide to mess with the rats nest of vacuum lines under the intake...

I recently had to change the distributor cap and while I was at it fixed a minor intake leak, and replaced those old hard small vacuum lines with some nice silicon ones, but didn't change the way they were hooked up at all.

After doing this it seemed like my car was down on power or the turbo was spooling later.

I double checked all the vacuum lines and have run multiple pressure tests on the intake system and found some minor leaks near the throttle body, but don't think these would cause what i think to be is unusually large amounts of turbo lag.

I went ahead an installed an electronic AEM TruBoost boost controller, as i wanted to, and i wanted to make sure it wasn't my Circ Valve causing issues, but i still think i am feeling more turbo lag than i should.

I also just shimmed my wastegate to see if the spring was weak, and that seems to have helped a tiny bit with the max boost i see, but not really the turbo lag!

Thanks for any info or help!
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Old 05-15-2013, 06:56 PM
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CyCloNe!
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that seems to be late on full boost. A standard k26/6 in 4th typically is around 3000 RPM for full boost. Not sure if your car has a k26/6 or k26/8 but even with a k26/8 should only be a few hundred RPM later. Now if you don't have a stock turbo that will play a major part in the lag.

I'd double check vac lines, boost couplers and for exhaust leaks.
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Old 05-20-2013, 12:17 PM
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Well, no more intake leaks, sealed up a few small ones and no exhaust leaks.

No boost coupler links, i upgraded to t-clamp style and they are all on tight.
I can roll onto the throttle in 4th and it seems to get max boost at ~3300 RPM, but I am also boosting higher than stock.

I am curious is the 951 S Turbo, the K26/8 just takes a bit longer to get to boost?

Has anyone ever done the intake pressure test and watched how fast the boost drops when you charge the system and remove the pressure supply?
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Old 05-20-2013, 02:25 PM
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A typical k26/8 will boost slightly later than a k26/6 your current spool time of 3300RPM should be fairly right. The K26/8 takes longer spool and reach full boost due to the turbine wheel is a tad larger than on the K26/6. The good news is that the K26/8 can build a good amount more power any where from 280-309 at the wheels depending on supporting mods. A K26/6 generally would be around 240-270whp. I don't know if anyone has run that test maybe some one else will chime in.
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Old 05-20-2013, 05:54 PM
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Thanks for your help Cyclone, I guess I am just a rookie with the Turbo systems, even if I understand how they work I am still learning the ins and outs of each one.

Looks like I will go for a dual port waste-gate and a good tune to soon to see if i can get a little quicker spooling out of the turbo.

Just getting ready for the track so i want to make sure the car is top notch!
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Old 05-20-2013, 07:40 PM
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Anytime... if lag is your concern if you can do a cat delete you usually gain some spool with that. A new wastegate will help hold boost better and longer.
Exhaust turbines when larger produce higher peak, less back pressure under boost but produce more lag time.
Coldsides when larger will product more peak horsepower but also can product a bit more lag. The larger the base of the wheel the faster it shoots the air out. The larger the inlet of the wheel the more power she can give.
If you decide you want a different turbo but equal or less lag than you have currently a K27/6 is a good choice as well as a K27/8 (little more lag but close to what you see now). If you go garret then a 50trim or 57trim with a stage III wheel are good (what I have). If you don't mind custom work a GT3076R seems to be the best combo for good power and reasonable lag. Which is simular to a 57trim with a stage III but the bottom of the coldside wheel is 1.2mm larger.
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Old 05-21-2013, 06:02 AM
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check your catalyst converter. If that's blocked, it causes major lag
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Old 05-22-2013, 11:54 AM
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Great, thanks for the tips. I don't think I'll be getting a different turbo anytime soon, unless mine goes out of course.

I do plan to get 3" full exhaust sometime within the year so i know that will help with flow and I plan to get the high-flow cat for the road and a cat delete pipe for track days.

As for it possibly being a bad catalytic converter, i don't have any major loss of power, don't see any unusual vacuum readings and the car idles, drives, and revs normally, just seems slow on the boost.

I think i am just faking myself out, thinking it should be much better. I'll get an exhaust eventually and dual boost controller.

Thanks!
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