The new turbo is one that I've had for several years.  It's actually the one I wanted to use originally, but didn't because of the work involved with making it fit.  With the new header though, I could make it fit whatever I wanted.

This is a Garrett VNT-25 off a late 80's - early 90's Chrysler.  It's a NOS turbo, so it was new and unused when I got it.  It uses vanes to control boost and does not use a wastegate, although an external one could be used.  

The vanes act to "size" the turbine housing dynamically based on need.  At low rpm, low boost, the vanes close down.  This increases the velocity of the exhaust going through the turbine and gives it a kick to spool up quickly.  As boost comes up, the vanes open to limit boost and reduce back pressure.  The benefit of all this is that it has the quick response of a smaller turbo which is great at low rpm (especially on a heavy truck), but can still flow like a bigger turbo at higher rpms with much less back pressure. 

I've had the turbo on for several weeks now and it's really working great.  I left on a 1200 mile trip about a week after the first test drive.  Needless to say, I was a bit nervous about it.  It's been trouble free the whole time though.  The links on the left detail the VNT and the header construction.

Performance results?  Well, the VNT is still considered a small turbo for the 2.6.  I think a T3, 60 trim is about the ideal size.  Compared to the Impulse IHI turbo, the VNT turbine is substantially larger, but the compressor is only larger by a little bit.  From some of the Dodge/Chrysler turbo lists, and from talking to a few re-builders, the VNT-25 will flow enough for about 200 - 210 hp.  After that, it's getting too far out of it's efficiency range.  There are places that'll upgrade it with a larger (T3-60, or even a TO4) compressor, but it's fairly expensive.  I'd like to do this, but it's low on the budget priorities.

Boost definitely comes on much quicker and it's *nearly* lag-free.  If I floor it, it'll jump to 6 - 9 psi with little delay.  From there, it holds steady as the vanes are opening along with the boost coming up.  Around 3100 rpm, the boost will then jump again.  This, I think, is the point the vanes open fully so boost shoots up.  How high it goes depends on the gear and load.  I keep in under 15 psi with my foot, but early on, I did make a couple 30 second blasts down the freeway at just under 20 psi.

Currently, I'm not running an intercooler.  My next step is to get a fully programmable EFI computer installed.  Initially, it'll be fuel only, but by year end, I think a distributor-less ignition will be available too.  It's the MegaSquirt if you're interested.  Currently, I'm still running the 2nd fuel pump and a rising rate fuel pressure regulator, but this is less than ideal.  There are still lags in performance in certain situations.  When flooring it for instance, there is still a moment of lag due to fuel issues, even though boost is coming up.  There's other issues too, all relatively minor, but I expect throttle response to improve immensely once I get the programmable 'Squirt installed.

Right now, the combo is putting out about 150 hp and 200 lb-ft torque at the wheels.  This is a bit low, but I think it's due to a lack of a good intercooler and the still somewhat small VNT turbo.  That's still about 195 hp at the crank which is right about the max the VNT is good for.  Torque drops off rather quickly after 3900 rpm, which means it's not breathing well up there.  Not sure if this is due to turbo size, cam specs, fueling or what.  I'll be working on them though, one at a time.  For comparison, the old turbo was putting out around 150 hp & 170 lb-ft of torque, but they both peeked several hundred rpm higher.   In the 1500 - 2500 rpm range though, the VNT makes substantially more torque.  At 2500 rpm for instance, VNT torque is about 167 lb-ft, and the Impulse turbo torque was about 118 lb-ft.  Being a heavy, approx 4400 lb truck, this is a driveability difference that I can definitely feel.

Created by: Dan Houlton
This page was last updated on 14 Apr 2004
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