Which route for more power?
 

I am finally nearing the point where I'll have the time to focus on my 928 and do a few of the things that I've put off for years. Like replacing the felt window guides, installing a bigger wing and increasing its power.

I do not want to build a forced induction kit from scratch or build my own stroker motor. Those would be fun, but I don't have the time for that depth of research. My preference would be a kit that I can install myself. I don't have a sharktuner, but I would be fine with getting one and learning to use it.

To the best of my knowledge, the three options available are:

1) Stage 3 supercharger kit with engine management from 928 Motorsports
2) Twin turbo kit from Kuhn Performance
3) Have a stroker motor built by Greg Brown

I would absolutely love to have a stroker motor built by Greg, but my understanding is that the cost is in the general vicinity of $40k. I'm sure that it's worth it, but it's more than I want to put into this particular car right now. If I've grossly overstated the cost, please correct me.

The twin turbo kit and the 928 supercharger kit are both roughly in the $15k ballpark. So the cost is a wash. It really comes down to things like:

• Ease of installation
• Ease of removal and returning to stock
• Longevity of the system
• Quality of after purchase support
• How long the vendor will likely remain in business for long term support
• Suitability for the 928 in general and my car specifically

I'm sure that I've missed a lot of important things to consider, but those are just off of the top of my head.

I don't want to start a turbo vs. supercharger vs. stroker antagonistic debate. But, I do want to consider valid benefits and detriments.

My car is a 5 speed 1987 928 S4 with an LSD and 2.20 final drive ratio. It has S3 profile cams in it, a cross pipe and rear muffler bypass. It is in very good mechanical condition. The motor was rebuilt and installed roughly 10k miles and 12 years ago. I already have on the shelf a "Stage 2" Spec Clutch ready for installation. Supposedly it's good for up to 525 ft lbs of torque. I'm not opposed to going with a different clutch if necessary.

My intended uses are mostly street driving with brisk and legal accelerations to the speed limit, and occasional auto-crossing and maybe some driver's education style track days. Nothing too intense, but I'll surely be using the power at least a few times every time that I take the car out.

If I'm not supposed to mention who supplies the various options for some reason, let me know. Or, if I've missed an option that should be considered, please let me know.

Thank you in advance for any input that you have.

VCB Engineering, has a supercharger kit that's complete, for way less than 15K. I have like 5K into mine, and it's a stage 1, with plenty of power.

Thanks for the recommendation. Their web page is down and their Facebook page hasn't had any new posts in over a year. Are they still active? I'm looking for something in the neighborhood of 600 HP at the crank. They had an old dyno sheet showing something in the 400's. Do you know if they have something stronger?

Be careful sending that much power through the manual trans. The autos can easily take it. The manuals hate it with a passion and tend to destroy themselves and wear out quickly.

I would just start by sharktuning your car as it sits and see where you wind up.

If anything it will get you more familiar with the software and what is needed to make those huge hp gains on the engine management side.

I would go with kuhn, doesnt his start making torque around 2500 rmp.....and his gets 600 rwhp , my stroke has 600 at the crank both awesome rides, more show of force with the kuhn product.....wish ptuomov would chime in how his build is going !!!

Or Hans's kit when it becomes available.

https://rennlist.com/forums/928-foru...ew-thread.html

Hans' kit looks very nice visually, but it's been a very long process and the latest posts indicate that it's at, or just barely leaving, the prototype phase. Which begs the question of...when would it actually be available and would I end up being one of the guinea pigs as an early adopter?

With respect to a 5 speed being less tolerant of increased power vis-a-vis an automatic, what are the perceived and tested limits? Kuhn's site indicates that an 87-88 S4 5 speed is the preferred car for their kit and they have examples of 5 speed cars making about 600whp. Given, that's their marketing material and may or may not represent a configuration that can hold up over the long term.

Honestly, I would rather not have to mess with tuning the motor after installation. But, it sounds like that is going to be necessary no matter what direction I go.

For every $1000 you spend on HP, put aside $200 for transmissions.

Mine is a '79 so the 16v motor, but I have the 928MS Powerdyne Stage 2 supercharger on my car. I created my own closed cold air intake drawing through the driver's side fender through a moisture barrier. Stock power from a '79 was 222 and on the dyno after my build, I'm seeing about 300 HP and 300 TQ, so a significant increase. With my car being a convertible conversion, I don't think I can put down much more power or torque without worrying about the longevity and structural integrity of the chassis.

My Powerdyne actually just came back from Carl this week where I had him install his upgraded billet Vortex impeller into it, some upgraded bearings and belt, and had him vent the case to keep it cooler and increase the boost a bit for a little more power. I'm not installing it until this weekend as I'm doing a Silicone coolant hose replacement at the same time from a blown hose that happened last week, so I can't speak to the improvements yet.

I also have his C5 Z06 6-speed transmission conversion kit in my car so am not worried about the power damaging the trans, as my original trans synchros were not in good shape and the reason I did the upgrade. Having to pause in neutral during both upshifts and downshift, and double-clutch each downshift with my original 5-speed, made the car much slower than the power numbers should have made it. The transmission upgrade was the single best upgrade I made to the car from a drivability and enjoyment perspective, yes, even more so than the supercharger, though they work extremely well together as the Z06 gear ratios are very well suited to the increased power from the supercharged motor. I can now run 85 MPH on the highway in 6th gear at 2,000 RPM. My only complaint with the whole drivetrain is the upgraded Spec clutch. It's very grabby and doesn't spin smoothly against the flywheel and I think it's because it doesn't have either Marsailles springs between the friction discs nor a spring center hub, like the OEM versions do. My clutch setup is upgraded to the dual-disc with intermediate plate version used on the 83 on cars so I have a pair or OEM friction discs in my parts cabinet to replace the spec ones when I get a chance or when I can't take it anymore. Launches have to be super slow with no throttle or very aggressive - there is no middle ground.

Service and support from Carl has always been prompt and informative, even when I bombard him with multiple questions in an e-mail string, because I like to understand how stuff works.

Anyway, hope those insights might be helpful.

Here are some pics


https://cimg7.ibsrv.net/gimg/rennlis...4e013bf02d.jpg
4" air intake in Driver's Side Fender & Stage 2 Heat exchanger for Supercharger



https://cimg3.ibsrv.net/gimg/rennlis...94004e8bc4.jpg
Heat Exchanger fluid circulation pump and secondary fuse box for HID lights, push/pull fan controller, etc.



https://cimg4.ibsrv.net/gimg/rennlis...2a4f5a0255.jpg
CF CAI and supercharger



https://cimg7.ibsrv.net/gimg/rennlis...48865f8c2a.jpg
Engine Bay



https://cimg0.ibsrv.net/gimg/rennlis...eaa78afa95.jpg
Upgraded supercharger will reinstall with a few of the bracketry components powdercoated



https://cimg8.ibsrv.net/gimg/rennlis...f598bc5f7f.jpg
Dyno Graph



https://cimg6.ibsrv.net/gimg/rennlis...0368432a37.jpg
Original 5 speed transaxle and new Z06 6-speed setup



https://cimg7.ibsrv.net/gimg/rennlis...e51ef2cc5e.jpg
6-speed assembled and ready for installation
https://cimg5.ibsrv.net/gimg/rennlis...77a5d05227.jpg
Modified C6 shift knob with SSK



https://cimg6.ibsrv.net/gimg/rennlis...a076376c4f.jpg
The rest of the car

The attention to detail in your cars has always been impressive, Pete! Beautiful installs!

Here's Victor Edgar's number 616-293-2582, he has an electric Go-Kart business, and it consumes a lot of his time. Call Victor, and leave him a msg, explaining what you want. The supercharger that he uses is the Rotrex, same system is on the Koenigsegg, cars. I'll be upgrading to his intercooler, not for more power, just to cool the air entering the engine. It's furnace hot here in Las Vegas, in the summer. My car never over heats, and I want to keep it that way.

FYI, Victor's set up is very streetable! You really don't know it's there till you step on it, and it's quiet. I have an X-pipe, no cats, just stock resonators and an RMB exhaust. The Pantera in my avatar, has 1100 HP out of a modular Coyote supercharged engine, it's crazy fast, weighs about 800 lbs less than my 89 S-4. You can hear it coming a long ways off. We don't hardly take it out anymore, it's worth to much. Drinks gas like it's still 1974, @ .32 a gallon for premium.

The software/tuning is EQUALLY important to the hardware itself, and you'll need 15 psi to hit 600 chp.

If you plan on skimping here you can plan on paying the ultimate price a little way down the road.

" longevity and structural integrity of the chassis."

Being a torque tube, and not just a floating rear end, that's not a significant issue anymore is it?

Not for all you normal 928 guys, but since my car no longer has the structure of a roof and instead has added reinforcements, the forces that could be generated by a lot of power or torque either directly from the powertrain or from cornering or acceleration to the chassis could at some point overpower the structural add-ons. My convertible has what I lovingly refer to as a "secondary suspension" with a bit more chassis flex and twist than a regular 928, but I love the uniqueness and open top enjoyment it brings as a fast-enough cruiser and GT car.

I have a different car to use when it's about total performance, handling, and speed - the supercharged 997 with complete RSS Stage 2 suspension setup, fully active PASM Bilstein Damptronics, forged wheels and PCCB brakes for as little unsprung weight as possible.

Well..thats just it.

With a torque tube, the whole car is the torque 'member' so to speak.

Its not a front -vs- rear twist of the chassis where the trans mounts and rear links are fighting the front of the car.

Its tied together front to rear...the forces are in the mounts and rear links directly pushing the chassis forward.

There is no rear twist of the chassis...and no Hp/Torque practical limit the way you believe there is.