NoPistons -Mazda Rx7 & Rx8 Rotary Forum

This is going to be an ongoing thread as I tune my new turbo setup so I will have updates as things progress. I wanted to do a bit of writeup on the installation because there are several things I feel people should know before attempting to swap this turbo in place of another turbo. For the record, this is not the same turbo that A-Spec uses in their ball bearing GT40 kit. It's a plain bearing GT40 with an 82mm compressor wheel exducer (58mm inducer), 77mm turbine major, 70 trim. The housing is a .94. It's replacing a modified GReddy T78 which I thought had the 88mm GT40 wheel mated to the front by Majestic Turbo who swore it was a Garrett GT series compressor. Well, I took measurements again and the exducer diameter is 92mm and the inducer, if I remember was 65 or 66 mm. No wheels in the GT catalog match those specs. The turbine is 74mm major in diameter and has more turbine blades. The inducer size is quite a bit larger than my GT4082 and I was initially worried it would be too small. I don't believe that now.

I must thank a few individuals for their help in picking, acquiring, and installing the turbo. This is an untried installation with the GReddy manifold/downpipe and I knew there would be a bit of fab work (little did I know!). In short, you will save yourself lots of time and trouble if you just purchase the entire kit from A-Spec. It's a great turbo but if you're simply replacing your existing T04, T78, IHI, whatever, there is no guarantee it'll work "easily" with your manifold and it may require extensive modifications. So...first I have to thank Sean. My initial plan was to purchase a GT40 ball bearing version from A-Spec. Due to supply/demand problems, we were unable to get a turbo in the short amount of time that I needed to acqure a new turbo. Even though Sean did not make a big sale off of me, he spent an inordinate amount of time with me via phone and e-mail, calling long distance on a regular basis to keep me informed and up to date. He even supplied an adapter when honestly, it was probably too much trouble for him. Great guy and highly recommended shop. Second, I have to thank Nick Riefner at N-Tech Engineering who stayed up for essentially two days installing this turbo, putting his own fully-built 350Z project car on the back burner, not to mention various other things and his pregnant wife! He really sacrificed his time and did not, in the end, sacrifice my bank account. He also attempted a few things he had never done before and workmanship was first quality. His technical, mathematical and fab skills were simply awesome and without his help, this installation never would have happened. And he's a first-class guy to boot. The RX7 community is lucky to have both these shops. Enough brown nosing and......

...on to the installation. The major problem we encountered was the long discharge with the 4" ID V-band. The OD of the V-band flange surface is actually about 4.5". Due to the GReddy manifold, the turbine placement caused the V-band to hit the lower intake manifold. No if ands or buts, it will not fit without serious mods to the turbine housing. Additionally, although the turbine inlet is a T4, the bolt pattern is ever so slightly different. We had to lengthen the bolt holes a bit to work with the GReddy manifold. It was checked against an XS manifold as well. It's definitely the turbo.

We decided to cut off the rear of the turbine discharge and weld on a 3" ID V-band adapter. This decision cost us about 15 hours in footwork and labor, running around central Florida having the turbine housing modified and locating a Turbonetics V-band turbine adapter. The adapter Sean sent from A-spec (CNC milled mild steel) was thick enough to "whittle" down on a CNC machine and make it work with a 3" ID, 3.51" OD (if I remember correctly) flange for the V-band. Problem is, the CNC operator took a large initial cut, spinning the adapter out of the chuck and shattering it. Crap. So...sprouting a hydraulic leak, the CNC machine was abandoned and a new adapter spun on a metal lathe. The long turbine discharge was cut off and we managed to locate a Turbonetics V-band adapter through Bill Hartzell of Florida Atlantic Turbo. He had one at home and allowed us to stop by at about 9pm to pick it up. His number is 321-231-5988. They're a Garrett, Mitsu and IHI dealer. He had many interesting observations about the new GT series garrett stuff. More on that later.

With all the necessary parts and fully drained cell phones, we headed back to the shop and Nick used his TIG welder to weld on the Turbonetics adapter. The turbine housing is cast iron and the adapter was mild steel. Stainless rod was used for the welding. Nickel 99 is not needed as confirmed by several sources. It was Nick's first time welding these two metals together and he did an amazing job. He was even impressed :-)

We cut the end off the original downpipe and squeezed the machined V-band adapter on the end. After the correct alignment was achieved, he tack welded it in one spot and finished the welding with the downpipe off the car. Most everything else was straightforward, slightly difficult in some regards. The overall size of both turbos is surprisingly similar and minus the GT40's long discharge, the GT40 is a bit shorter, especially through the CHRA. The overall alignment with the downpipe was about identical too, making things much easier.

While the compressor cover rotates 360 degrees, the hot side does not. It uses 4 bolts to secure the CHRA to the turbine housing and you basically have TWO positions. This causes the drain to cant slightly inboard and not straight down but it's not too much of an issue. For drain hose, I would recommend something that has an internal spring so as to keep the hose from collapsing when it's bent. Aeroquip and several others manufacturers make such a hose.

The oil drain on the CHRA is threaded. Not sure what the thread is. I had already purchased an milled aluminum adapter from ATP Turbo (www.atpturbo.com) which uses a thread-in 5/8" barbed adapter to slide the oil drain hose on. I had to drill/tap the CHRA to use two 8mmx1.5 bolts to secure it to the center housing. Cast iron is easy to drill tap, so it's not a big issue.

On the oil feed side, ATP also has a T4 milled aluminum adapter. Use the T4 style, not the T3. They also have the thread-in adapter that allows you to use a 4AN fitting/hose. The T78 and the GT40 plain bearing are both oil lubed/cooled. No water jacket. So what you have is an aluminum flange and then, if I remember correctly, a 1/4" NPT to 4AN male/male adapter that threads in. I acquired two swivel 90 degree 4AN to 4AN fittings from www.techna-fit.com along with a 14mm to 4AN steel adapter which threads into the top of the engine block. ATP supplied a pre-crimped swivel end 4AN braided hose.

We finished the installation at about 7:30am the next day. I drove the 2.5+ hours back to Naples to work a full shift at work. Not fun. The installation was a total PITA. Again, just purchase the whole damn kit from A-spec and save yourself lots of headaches and skinned knuckles.

Impressions:

I gave the turbo a bit of time to break in. I use an Apexi AVC-R as a boost controller and forgot to turn the duty cycle down. Once I had put some mileage on the turbo, I promptly hit about 1.3kg at 4K rpm. Yikes. Boost response is worlds better than the T78 and again, this is a plain bearing turbo on an undivided manifold. I feel it's comparable to the stock sequential twins on a completely stock car (bottom end response) as I am able to achieve about 8-10psi by just over 3K rpm on an untuned map. I am able to hit 17psi and greater thanks to boost spikes before 4K rpm. In contrast, the old turbo, with it's much larger inducer and 10mm larger exducer, would produce 16psi by 5400rpm or so. I figure I have gained about 2000 rpm earlier boost response. I credit this not only to the smaller wheel but to the turbine as well as the matched components of the Garrett turbo. Even though the turbine is 3mm larger in diameter vs the GReddy/Mitsu turbine, it features 4 fewer blades, more curature, and according to Bill Hartzell, carries it's weight closer to the turbine shaft. It's a very agressive wheel. It actually made the exhaust note louder and changed it's sound from idle through the rpm range. I personally feel I have a "weak" engine. I ported it myself (first time) and I may have overdone the secondaries and exhaust. I think this turbo, esp. in BB form will be even better than what I am experiencing in my car. I need that Judge Ito Cosmo motor!

I am still trying to dial in my boost controller as 3rd through 5th gear pulls are giving me boost creep issues. Overall the engine does rev noticeably quicker and I would say there is a top end loss in comparison. The modified T78 would pull without falling off up to and past 8500rpm. While I haven't redlined it in the three upper gears, common sense would dictate that top end power is not going to be the same. Regardless, it's a very very nice turbo and driving the car daily, esp. in traffic, is now a pleasure instead of a burden. Overall transient response is amazing. The turbo spools when I am in vacuum, something that did not happen with the old modified T78. Compared to say a T04S, I would say the GT4082 is about 20-30hp stronger throughout the rev range. Part of that may be responsiveness, but a lot of it comes down to the more efficient turbine and compressor over the old T-series stuff. Besides the even better boost response of the BB version, the A-spec kit offers a full V-band setup and the increased durability of the ball bearing center section. I can only imagine how much better that turbo kit is.

How about a few pictures! Here's the old (left) vs. the GT4082 (right). Look how much bigger the inducer is on the old turbo. It's HUGE in comparison. Again, the GT40's probably get away with smaller inducers with increased efficiency. The 4082 seems to be very efficient looking at the compressor map at higher boost levels while the 4088 is more efficient at lower boost levels. Both have the same hp rating.