My Mid-Mount Turbo System - After One Year of Use
 

It's been a little over a year since I finished building, installing and tuning a mid-mount turbo system on my 1987 928 S4 that has a 5 speed and LSD. This post is for posterity for those who come along in the future and want to do something similar or learn from what I did. My hope is that someone else that wants information on this kind of thing will glean something useful from my experiences.

A lot of credit is due Colin (aka @Lizard928 ). When I posted about power upgrades in 2019, he suggested a turbo, and then provided me with a lot of information on building one for the 928. It was a tremendous help and pointed me in the right direction to learn a lot more about forced induction than I'd ever even dreamed of before.

Aside from Colin's advice, he also welded the exhaust pipe for me, and recommended the specific turbo/compressor/AR that I installed.

Everything else was done by me. Fabrication, parts selection, installation, tuning, etc.

Now, for some of the technical details. Not a complete parts list, but the major components:

Turbo: Comp Turbo Oil-Less 6767 with v-band inlet/outlet and an A/R of 0.82. The "Oil-Less" line of turbos is lubricated with grease via a zerk fitting and has no oil connections. It is cooled with a loop from the HVAC heater supply. Supposedly, this turbo is good for 1,000hp.
Wastegate: Tial 44mm MVR. Springs set to deliver 11.6psi of boost.
Blow off Valve (aka BOV): TIal 50mm Q. Configured with a 10psi spring, which is for vehicles that produce between 16in/hg to 19in/hg vacuum.
Intercooler: A generic water to air intercooler with a 9.25"x3.5"x3.5" core. Inlet and outlets are 180 degrees from each other, providing a "U" turn air flow arrangement.
Heat exchanger: A generic 30 row radiator, 10"x13"x2" overall.
Water pump for intercooler: Bosch 0392022002. 264gph (4.4 gpm) at 4.4 psi (0.3bar)
ECU tuning software/hardware: SharkTuner MK2 with PEMs
AFR monitoring: Innovate Motorsports LC-2 Wideband O2 Sensor Controller
AFR Gauge: Speedhut GR-AFWB-02. Reads 8.5 to 18.0 AFR via a low voltage signal from the LC-2 O2 controller.
Boost/Vacuum Gauge: Innovate Motorsports MTX-A 20psi Vac/Boost Gauge Kit w/ MAP sensor
Fuel Injectors: Siemens Deka IV 60# Long Style with EV1 Connectors
Fuel Pump: Bosch 044
MAF Upgrade: SuperMAF with lips machines into both ends for clamp/boot/hose retention
Coolant reservoir: Generic 1L aluminum reservoir
Air straightener: 96mm OD, 8:1 ratio
Air filter: Spectre HPR0882 (estimated 1,000 CFM)
Pre-filter water sock: Filterwears HPR9884


All of the above can be looked up online, but if you would like my explanation of any of the terms or components, just ask.

A couple of minor notes on the system:

The turbo compressor outlet has a bung for connecting directly to the wastegate's signal port. That makes for a very short run and is very easy to plumb. However, there will be pressure drop between the compressor outlet and the intake manifold. Between the pipe diameter expansion from 2" to 3.75" right before the MAF, and the cooling of the air between the long run under the car and the intercooler, the air pressure at the intake manifold will not be the spring rate in the wastegate. Worse, it won't even be a consistent drop. It will vary based on ambient temperature, heat soak, and vehicle speed. And that will have a big impact on tuning and overall power. That's why I pulled the wastegate signal directly from the MAF boot right before the throttle plate. That gives me a very consistent maximum pressure at the intake manifold. It's more work to make it happen, but I think that it's worth it.

I used PTFE steel braided AN hoses everywhere that I could. And stainless steel t-bolt hose clamps everywhere else.

The exhaust is ceramic coated and I added heat wrap and a turbo blanket. That keeps the heat in, which helps the turbo to spool faster.

There are a lot of other aspects beyond what I addressed in this post. A new fan shroud with fans and fan controller. Oil breather system. Fans on my external oil cooler. Venturi delete. I won't go into all of those in this post, but if you have questions, feel free to ask.

Installation:

Installing the system was really pretty straight forward. Once the stock exhaust and intake is out of the way, there's quite a bit of room to work with. Initially, I had planned to use 3-bolt to v-band adapters to connect the new exhaust to the existing stock exhaust manifolds. However, since I had Colin weld the exhaust, I also let him talk me into welding v-bands directly to the stock exhaust manifolds. I'm glad that he did, it's a much cleaner setup this way. And replacing the exhaust manifolds is really pretty easy. Colin's fabrication skills made the exhaust side a relatively simple matter of hanging a new exhaust.

The turbo itself is just in front of the transmission. It has a brace that uses one of the torque tube mounting bolts, so it's very solidly mounted.

On the intake side, using mandrel bent aluminum pieces made it very easy. Just cut them to length, use silicone elbows to connect everything, and it's done. I used my hydraulic drop bandsaw to make the cuts precise and clean. I used solid pipe as much as possible and only used the silicone elbows where necessary. Welding it all together would make things cleaner, but it would also remove all flex from the system. Given the length of the intake tract, I preferred to have the flex.

The air filter is under the car. I agonized over the placement. Partly from debris, but mostly from the risk of water ingestion. There is no room anywhere else for an air filter that is sized properly for the CFM that I need. A smaller filter would fit, but would also choke the engine. In the last year, I've driven the car in rain a few times with zero issues. After researching other rear/mid-mount systems on other cars, I have no more concerns. As long as I don't submerge the filter, there will never be an issue.

The heat exchanger is mounted in front of the AC condenser. I fabricated some brackets with feet that slip over the retaining bolts for the power steering cooling loop, and some brackets on the top that share bolts with the condenser mounting brackets. This makes it rock solid, precisely distanced, and no modifications to the car required.

I have AFR and boost/vacuum gauges mounted just below windshield level so that they are very easy to see without losing sight of the road. It's the only sign from inside the cabin that the car isn't stock.

With the hood closed, there was no indication at all from the outside that the car isn't stock. After doing so much work, I wanted to put a little something on the car to show that I had modified it. There is now a "Turbo" vinyl sticker on the back, right below the 928 S4 lettering. I think that it's done well enough that a non-928 person would think that it's a stock badge. A little corny, but I like it.



Anyway, enough with the background. On to the impressions and results.

To put the new driving experience into perspective, I'll share what I've read about other supercharged and turbo charged cars. In general, I've read that the supercharged 928's are like beasts. Always on. Always making boost. Brutal from the moment you step on it and just getting more and more powerful as the RPM's increase. I've heard similar statements from both the centrifugal owners as well as the twin screw owners. On the turbo side, I've often heard statements like "it drives just like a stock car, until you don't want it to". That below 3k RPM it's a sedate stock car, and above that it turns into a monster.

My car doesn't really drive like either of those. There is always more power available at any RPM and any load level. To me, it's like driving a modern super car. The power is always there. And it is smooth and manageable. I do 80% of my driving in the under 2,500 RPM range. Because 80% of my driving is on the street and just getting from point A to point B. 2k RPM is my target for casual driving when maintaining speed. I don't want to go much below that because I don't want to lug the engine, and not much above it because I'm not preparing to launch out of a corner. And at those RPM's, the throttle is responsive, and the power is significantly over stock. I never feel like I need to plan to accelerate. If I want to go faster, I go faster. Before the turbo, at those RPM's, I would need to plan a little bit. Either downshift, or give it gas early to build up the RPMs to get into a better torque range. Now, I just drive.

During that 80% of my driving, I'm never past half throttle. Usually below 1/4. Not because I'm going slow, but because there is so MUCH power that I don't need to use more than that to accelerate faster than most traffic. If I'm cruising in 4th gear at 2k RPM and about 100 feet behind the car in front of me, and then go to half throttle, I'll be bump drafting them in a couple of seconds.

What about that other 20% of the time? Those time when I'm able to actually use the power are very short. Because the car gets to extra legal speeds so fast. When you're the driver, you don't feel the acceleration as much. You're braced with your left foot on the dead pedal, you're holding onto the steering wheel, and you're focused on the driving. My best gauge of what it feels like is the reactions of my passengers. And they assure me, each in their own way, that it's ridiculously fast and pushes you back in your seat harder than they've ever felt or could have imagined.

So, yes. Acceleration is there. But, it's not all or nothing. It's not there later. It's always there, always smooth, always more than you need.

I've only had the opportunity to autocross the car once since I finished the build, but it did incredibly well. I had boost right at the launch with zero lag, and since I kept it in gear and above 3k RPM for the entire run, there was zero lag. It performed flawlessly. Very smooth, very powerful, more than I could use most of the time on the course.

It's also gotten better since the initial installation. The Oil-Less Comp Turbo's have grease in them. And it takes some use before the grease level reaches an equilibrium with the specific use of the turbo. Mine has done so in the last few months. The result isn't more maximum power. The result is that the turbo spools faster. The primary effect is reduced lag.



Speaking of lag and when the turbo comes on...

First...boost threshold. I've never tried to develop boost below 1,500 RPM because I really don't want to lug the car. However, I have given it enough gas to develop boost at 1,500 RPM. So, I know that the threshold is below 1,500. How far below, I have no idea.

Which leads to lag. When I first installed the system, if I were in 4th gear and coasting with my foot off of the gas at about 1,400 RPM and floored it, I would have boost by 1,500. I only did that a couple of times. Then, a few more times from 1,700. Same thing. 100 RPM later I had positive boost. So, about 1/2 second, give or take. However, when maintaining speed or with any load at all, the lag was much shorter. Less than a quarter second, but still noticeable. Most people's threshold for noticing delay when speaking on the phone is about 0.2 seconds. I would equate the original lag to that.

Now, however, things have changed. With the turbo broken in, I really have to concentrate to notice the lag. And I can only pick up on it if I'm in the 2k or lower range. If I'm closer to 3k, I can't detect it at all. Any "lag" that exists is how long it takes my foot to move the throttle once I've decided to accelerate.

Here's an example...I'm cruising at about 2k RPM in 4th gear. Mostly level ground, foot barely into the throttle at all. I decide to accelerate gently. Before the pedal hits 1/4, I've got 2 psi of boost. It's fast enough that it's almost simultaneously occurring with RPM increase.

I had assumed that I would have some lag and I was resolved to integrate it into my driving style. Instead, I've ended up with no lag and the only thing that I've had to change about my driving style is using less throttle.

For reference, I hit 11.6 psi boost by 2,700 RPM and it holds that all the way to redline.

Response from non-928 owners has mostly been "nice stock turbo 928". I kid you not, most people think that it's stock. Probably due to the sticker on the back. Porsche owners too.

I've driven the car to Florida and back, all over the region, with the PCA on a tour, in an autocross, and generally as much as I can whenever I need to go anywhere. Not a single hiccup of any sort. It's rock solid.

The only problem that I've had is that my daughter keeps asking me NOT to shift. She's three, so her understanding of my shifting was that I do that to stop accelerating. And she wants to keep accelerating. Forever. She also whoops and screams with delight when I do any sort of spirited driving. I explained to her the limits of legality with respect to accelerating, and she countered with the suggestion that I just find a place where it's legal so that she can experience the full acceleration for as long as possible. We found a place to do that on our way to church, and her thirst for speed is temporarily sated.


Another significant aspect, at least to me, is wheel spin. Invariably, every thread that I ever read about a 928 with forced induction has someone commenting about how it easily spins the tires at X speed or in Y gear. For that reason, I began my tuning with great care. Fully expecting my 11.6 psi boosted car to spin the tires all over the place. Not the case. Probably partly because the turbo doesn't slam on the power, and partly because I have a custom 50% LSD, and partly because I started with 285 R compound tires and moved to 295 track tires. When it breaks loose in a straight line, it's not dramatic. In fact, I didn't even notice it at first because the acceleration is still extreme. It's the comparison of the ECU and GPS logs that reveal the times when I spin the tires. Now that I know how little drama there is with it, I can detect it when it happens.

I'm extremely pleased with the traction. It makes the power usable instead of just a number.


Hard numbers...

Everyone wants to know the bottom line, right? How much horsepower. Or what's the 0-60. I prefer to know how much torque, when it hits that, and how long it holds it. I don't have any 0-60 runs since I don't want to abuse my driveline hard enough to produce respectable numbers. What I do have are dyno graphs produced from ECU and GPS logs.

Keep in mind that dyno numbers, no matter where you get them done, are subject to a lot of variables. The one that I'm posting below is equivalent to a SAE corrected Dynojet, but the correction is for about 15 degrees cooler than the actual temperature. I collected so much data from so many runs and tuning, that I gave up plugging in temperatures and pressures for SAE correction.

The important thing to me, is what the torque curve looks like. Regardless of the absolute numbers.

I believe that I have approximately 530whp and around 550 wtq. Or somewhere in that general ball park.

I also tuned it fairly conservatively. I tuned for zero knocks, even on a 95 degree day with back to back runs. I could definitely get a little more out of this with some more aggressive timing and leaner fuel mixture.

This dyno graph is from before the turbo was "broken in". Now that the turbo spools faster, I believe that the power before 2,800 RPM would be higher earlier, but the overall maximum won't change. It doesn't matter enough to me to setup the measuring equipment for more runs, so this is it :)


The before:
https://cimg0.ibsrv.net/gimg/rennlis...2d3845a82e.jpg



The after:

https://cimg6.ibsrv.net/gimg/rennlis...edf47069f6.jpg



The major pieces on the ground before installation:

https://cimg9.ibsrv.net/gimg/rennlis...2ba47ef095.jpg



Installed underneath:

https://cimg1.ibsrv.net/gimg/rennlis...b236b6735e.jpg



Under the hood:

https://cimg3.ibsrv.net/gimg/rennlis...80872e4cf4.jpg



The "Turbo" decal:

https://cimg5.ibsrv.net/gimg/rennlis...cca53742be.jpg

Excellent work- well done!

Awesome. Great job and thanks for posting so much detail.

:)

I had followed this earlier and thanks for the update. Have you managed to make any videos of what its like driving?

Thanks

Very cool

When you were cool enough to make me 2 battery disconnects-I thought you were just a nice guy-
When I met you at Hershey with your dad-
(not even knowing you made me the battery disconnects)- you became my 928 idol!
Now I need to give you my 89 automatic so you can design a system with the turbo in the spare tire wheel well.
Plexiglass please so it looks awesome with the hatch open.
Very impressed with you sir!

I don't have any videos that do a good job of showing what it's like to drive it. After looking around a little, there don't seem to be any mounts that would work on the 928 style seats to give a view of the dash and windshield. If you have any suggestions or links to something, I'll see if I can work something out.

Thank you, but you're too kind. I'm just another 928 owner doing my thing :)

If you really wanted the turbo visible from the hatch area, I bet that you could move the battery to the spare tire compartment, closer to the side of the car. If the battery is too tall, you can use a different battery combination to make it fit. Then, cut out the battery box itself so that the turbo can sit there and be open to the air flow below. Put in a view port where the battery box lid is so that it can be seen from above. You'd have to use something other than plexiglass, since I think that it softens at 200 degrees Fahrenheit. You would probably need to use some kind of Pyrex or Quartz glass? I don't know a lot about the different types of heat resistant glasses. It gets a lot easier if you don't care to see the turbo from above. You could probably just leave the battery box lid in place with additional insulation after cutting out the box itself.

You can see in the below photo how the battery box is right next to the muffler. The exhaust pipe can go straight into where the battery box is, and the compressor outlet pipe can go back to the front of the car on the opposite side of the transmission, mirroring the exhaust pipe path. And the turbo would be in a really good spot to run the compressor intake pipe to the rear bumper and place the air filter up there. That would keep it safe from road debris and well away from any water.

https://cimg0.ibsrv.net/gimg/rennlis...78f12c2795.jpg

I can skip the plexiglass-

Love the setup! I'd like to make something similar for my euro s2. How do you like the comp oil-less turbo? Don't they also make a turbo that has no oil or coolant lines? Just grease? I haven't found much info/reviews on them.

Let me know when to drop it off!!!

That definitely opens up the mounting options :)


I really like the turbo. I've only had it a year, but it's been flawless. I don't have a one-to-one to compare it against, but I have zero complaints and would buy it again without hesitation. It does have a break in period, though. So, you have to make sure that you do your initial tuning in at least 3rd gear or higher so that spool time isn't a factor. Once it breaks in, the lower gears are phenomenal.

They do have an air cooled version that doesn't require water or oil lines. They had just come out with it when I ordered mine, and I didn't want to be in the first generation. However, I haven't heard anything negative about it, so it might be worth a shot. I still prefer the extra cooling cushion of the water cooled, personally.

The Comp Turbos are used in a lot of kits for the Nissan Z's, particularly the 370Z's. You can find a good bit of comments in the Z forums about them, and I'm sure that if you contact the kit sellers they will give you their impressions. And maybe even get you a better deal on one versus buying direct. Keeping in mind that people are more likely to complain than they are to praise, it's pretty much what you would expect. A great product with imperfect people making/selling them. They've acquired a couple of dedicated naysayers. My experience with their customer service has been so-so. I would absolutely buy another one, but if there are problems you need to be gently aggressive in following up on it. Not much different than anything else, really.

Absolutely amazing build!! Kudos and really nice fab. work \ skills.
Is the Air to Water IC supported with brackets above the right valve cover?

This is just my preference, I would have welded a flange for the Tial on the actual exhaust pipe upstream of the Turbo and vented it to atmosphere (just for the more oral \ visceral sound) and also simplify the fab vs. routing it back into the exhaust, but certainly understand why you did that.

I bet your car pulls like a freight train with 550 to the tire!

Beautiful work and beautifully written post. I am looking forward to your future Parachute installation. LOL I am sure you will need it to stop whenever you push things to max on a tight run.

The intercooler is mostly supported by the intake pipes, but also resting lightly on the oil breather hoses. It would be possible to use a bracket to secure it to the shock tower bolts or even the cross brace, but the current arrangement seems to be working well. I have had it off a few times recently and there is no issue with contacting other parts, rubbing, etc.

There are a few reasons for the wastegate configuration. One is space. There is more vertical space available where it is, and more horizontal space for the exhaust pipe. Another is flow. Technically, a 90 degree connection will alleviate pressure. Theoretically, an angle gives faster response. The last reason is volume. I do not want the car to be louder when the wastegate opens. I like hearing the turbo whistle, and being able to hear myself think. A non-muffled wastegate is too loud for me.


Thank you :)

I have upgraded "Big Red" calipers on the front. From a 1993-1995 928 GTS or 1994 Porsche 911 Turbo (993). 322mm cross-drilled rotors.

The rears are from a 1996+ 993 Turbo with 322mm cross-drilled rotors.

I also have a 50% brake proportioning valve.

The pads are Hawk HPS 5.0. I've found that the first VERY hard and long braking from high speed when the brakes are cool is a little soft, but I can still get the ABS to activate with enough foot pressure and there is no problem slowing down. After that initial use, they are warmed up enough to be much grippier under very heavy use. For normal street driving, though, they feel very firm and grippy.

Combined with the Yokohama A052 tires, stopping hasn't been an issue yet.

Very clean and amazing numbers behind it. :thumbup: