When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
In-between the sniping, I'm trying to grasp the intended takeaways from the exchanges above...
So, with both the 928 Specialists Twin Screw kit, as well as the Murf928 Stage 3 kit, I have to run an intercooler AND double the boost, for roughly "only" 100 to 120 more horsepower? Stated (or asked) differently; those are much larger, heavier blowers, utilized in much more expensive and much more complicated installation configurations (and which require much longer installation times - I've watched some of the videos..) Something doesn't sound right.
In the examples provided above, those two kits (which are no longer available for sale as I understand it), run anywhere from 6psi to 7psi MORE boost (almost 12psi), or 120% to 140% more boost, than the base ex-VCB Rotrex kit being revived here in this thread (which ran just 5 to 6 psi.) But those installations "only" produced an extra 100 to 120-horses...? That's only about 27% more power, on 120% to 140% more boost! Those chargers sound horribly inefficient.
To the moderator's point above - we can probably add the intercooler, change the pulley for slightly more boost (say, around 8-9-10psi on the current C38-91 setup) and hit the same 475 to 495 horses (with less boost), while making more power and torque along the mid-ranges of the entire curve...
We're going to install one of these base-base ex-Victor kits without an intercooler, with the pulley configured for a low-low 5-6psi of boost, 36-pound injectors and tuned with the SharkTuner to get as close to perfect AFRs as we can get across the range. We'll keep the timing conservative and see where we are!
Note: the Rotrex does not produce the heat that turbos do, or the heat produced by other less efficient superchargers for that matter), so you can run more timing advance and you don't have to run as much injector duty-cycle to cool things off!
It's such a simple kit to install; I think folks are going to love it. We'll try to keep the price down as much as possible, while providing 928 owners access to a fun kickoff, to their forced-induction journey! Over this winter, we'll work on refining the kit (which would include an intercooler, more boost and remapping - still on the SharkTuner for the moment), followed by a smaller pulley for more boost and re-tuning with a standalone ECU.
Next year, we'll start on a different kit configuration (utilizing the Rotrex C38R-112 - flipped around in a CCW rotation and with the larger 3.5" inlet and even more boost. At some point we'll have to replace the head gaskets, I'm sure and even further down the road, we'll do a forged internals build and run even more boost.
Let's just start simple and see. Fun times ahead (and I really hope that folks will set their own personal biases and self-interests aside, get behind development of the kit and participate in building something here.) There's so much amazing knowledge and experience here, but the sniping and bickering is exhausting!
Last edited by Rasant Products; 12-01-2023 at 03:54 AM.
So, with both the 928 Specialists Twin Screw kit, as well as the Murf928 Stage 3 kit, I have to run an intercooler AND double the boost, for roughly "only" 100 to 120 more horsepower? Stated (or asked) differently; those are much larger, heavier blowers, utilized in much more expensive and much more complicated installation configurations (and which require much longer installation times - I've watched some of the videos..) Something doesn't sound right.
Stage 1 Murf928 was offered without an intercooler. I was never a fan of this setup as I don't like boost without an IC ever, but that's just me. The 87+ ECU's appear to be good enough to handle any knocking that may occur from higher intake temps. Again, not a fan of doing things this way, especially on a pre-87 without knock sensors.
When I supercharged my 81 we designed the intercooler first, and built three different designs before I was happy with it. The goal was 17+psi though. The first two would have worked with a 12psi or less setup.
But again, I wouldn't be so concerned with peak HP with this crowd. That nice flat torque curve you get with the twin screw will be more appealing IMO and is clearly possible with the Rotrex.
Originally Posted by Rasant Products
In the examples provided above, those two kits (which are no longer available for sale as I understand it),
Kind of. I have enough parts to put together a few Murf kits and I'm asked occasionally to do so, I don't have the time or the desire to dive into that any time soon except for my own cars. My next build will be with a Procharger as I'm not happy with the current quality of Vortech units. The power goal is 600+rwhp though, but not until I have Motec installed and fully operating first.
Originally Posted by Rasant Products
run anywhere from 6psi to 7psi MORE boost (almost 12psi), or 120% to 140% more boost, than the base ex-VCB Rotrex kit being revived here in this thread (which ran just 5 to 6 psi.) But those installations "only" produced an extra 100 to 120-horses...? That's only about 27% more power, on 120% to 140% more boost! Those chargers sound horribly inefficient.
The data is what it is, just sharing as a data point. We were looking at the Rotrex blowers years ago since we liked the obvious advantages they offer. We didn't get very far then 2008 happened and nobody wanted to spend any money.
Here is another data point for the 85/86 cars. One thing to note, the 85/86 cars have lower fuel pressure than the 87+ cars. If we could do this all over again, we'd make the 87+ FPR part of the kit. https://rennlist.com/forums/928-foru...s-new-car.html
FWIW all the Murf928 kits used Bosch 42lb injectors. Today I'd lean more towards 650cc injectors from Fuel Injector Clinic.
IMO the #1 disadvantage the twin screw setup has is the intake manifold. Contrary to what a lot of people think, intake tuning and design is just as important under boost as it is N/A. IMO that's what kept the kit on this GT from making more power up top. The best part of any centrifugal setup is retaining the stock intake manifold and flappy (we did test runs with the flappy open, closed, and operating as they do stock. It has the same affect as a N/A engine, so again, be a good idea to keep that functional with your kit).
Originally Posted by Rasant Products
Let's just start simple and see. Fun times ahead (and I really hope that folks will set their own personal biases and self-interests aside, get behind development of the kit and participate in building something here.) There's so much amazing knowledge and experience here, but the sniping and bickering is exhausting!
If I came off wrong in the beginning that's on me. I love boosted 928's, I don't care how it's made as long as it's done well, doesn't look like a hack job and is tuned properly. I first dove into the world of supercharged 928's back in the mid-90's and I've seen a lot of folks come and go in this arena, and often times a big part of the downfall was BS data and put-offing attitudes. One of the big players in this arena was still selling supercharger kits for 85+ cars using an FMU for fueling and stock ignition timing until they more or less went away a few years ago. It was appalling to see folks use such archaic tech on these cars when at least the Shark Tuner is available (which does work very well, only thing that needs to be adjusted is the MAF). On that note, if you're interested in what we did with our kits to make the stock MAF work with even the stage 3 kits, let me know. I'd rather keep that offline for now.
If it were me, I'd figure out an economical way to boost the 80-84 US cars since there are a lot of those running around. No good way to tune the L-Jet brain IMO so some kind of a simple PnP stand-alone would be ideal. I converted my Supercharged 81 to LH / EZF from an 85 EuroS just so I could shark tune it. Now I'm switching it to Motec since Shark Tuning the 85/86 brains is kind of a PIA (you cannot tune both fuel and ignition at the same time, unless you have two ST-1's and two laptops).
In my testing from 6 to 12, then 17psi, I managed to put down 400rwhp but it was sloppy (using an FMU back then). Dialing it back to 350rwhp the car ran very well and drivable. I think if a kit like that existed, it would sell very well.
My next build will be with a Procharger as I'm not happy with the current quality of Vortech units. The power goal is 600+rwhp though, but not until I have Motec installed and fully operating first.
Friends don't let friends buy MoTeC - Bosch acquired them last year. Lots of changes (for the worse...) There's a reason why even Singer just booted them out as well and are now going with Emtron systems - it's the future of high-end stand-alone. Rasant was doing 40-50 MoTeCs a year, but the games that MoTeC plays with "firmware" charges is absurd!
You can't for example take your map and share it with your buddy. Nope, HE has to buy a $1250 "firmware package" from MoTeC (on top of their crazy cost), in order for that map (a map that YOU paid the dyno time for), to be shared with him. Emtron is OPEN architecture - share away - and a feature-rich platform. Rasant is moving to all-Emtron for the air-cooled 911 ITB and custom GT3 intake plenum setups.
As far as the rest of the supercharger field; my experience with supercharging - and specifically Vortec - goes back as far as my Fox-chassis Mustang back in the early-90s. The ProCharger noise is also unbearable, it's heavy and inefficient. You'd think that with changes in technology, you'd have something better available than the bulky, gearbox-off-to-the-side POS that came through here last week strapped onto a Nismo 370Z.
Nah, get with the program there moderator and piggy-back your personal development off of the modern technology and efficiency of the Rotrex. Install an Emtron. Unify the efforts. Makes MUCH more sense (at least from where I'm standing..) From what you guys are telling me, 600 would be a walk in the park and the Rotrex can handle it. (The C38R-112 is rated to 1000 horsepower in a single configuration...) Duals, well that would be more then.
Last edited by Rasant Products; 11-01-2023 at 02:06 PM.
Friends don't let friends buy MoTeC - Bosch acquired them last year. Lots of changes (for the worse...) There's a reason why even Singer just booted them out as well and are now going with Emtron systems - it's the future of high-end stand-alone. Rasant was doing 40-50 MoTeCs a year, but the games that MoTeC plays with "firmware" charges is absurd!
I'm aware, we're a Tier-1 Motec shop. We have a number of teams and race series sticking with Motec for the foreseeable future, so that's going to be our main ECU so I'm going to stick with what most of our volume is in. We've used Emtron on a few projects, it's a great option.
Originally Posted by Rasant Products
The ProCharger noise is also unbearable, it's heavy and inefficient. You'd think that with changes in technology, you'd have something better available than the bulky, gearbox-off-to-the-side POS that came through here last week strapped onto a Nismo 370Z.
We do quite a bit with Procharger here, it's the "budget" kit for most of the domestic vehicles we get through here. Most of the trucks / SUV's go with Whipple. Todd had a Procharger on his first test car, that was crazy loud. The new ones are not really any louder than the Vortechs we had on the Murf kits. Both still louder than a Rotrex LOL
All of my bracketry, pullies etc.... are made for the vortec and will require minimal modification for the Procharger. I don't feel like staring over with something else right now on this car. One thing to note I forgot to mention with the charts above, the inlet box we had made to feed the Vortech was a bit restrictive, designed for packaging not performance, which hurt the breathing in the upper RPM's. Since our goal was 500rwhp and we were able to easily reach that, a redesign was never considered. Except with my car we plan to address this.
My 79 track car is most likely going to be getting a pair of turbos, power goal again in the 450-500rwhp range so it won't need very big ones.
The last Rotrex we had in here was on a Honda S2000. Awesome setup.
Here's a dyno chart on my 16v supercharged setup running K-Jet CIS. This engine was a 79 4.5L that had like 219 or 229 HP. I've driven the car with the supercharger belt removed and it's not nearly as much fun.
Since this run, the engine is quite different, but I've not dyno'd it again.'
custom cold air intake pulling air from the fender instead of from directly behind the radiator
dual chamber AOS - one for head ventilation and the other for crankcase ventilation
Engine is now a US 4.7L with slightly higher compression but with the heads and cams from the '79 for a bit better lift and duration (not Euro versions)
Vortex Supercharger now has ceramic bearings, vented case, vortex instead of flat blade impeller (supposedly moves 30% more air volume), grip pulley and different tensioner pulley to control belt slippage
Fueling is a little richer now getting down to around 12 AFR at higher RPM with full boost
Also using a later vacuum advance and vacuum retard distributor using positive pressure into the vacuum advance port to retard the timing under boost.
Why build with forged internals? I'd say for obvious reasons, but perhaps I misunderstood the question. The stock internals will only handle "so much" power. Not being certain where the limits are for the stock internals on these motors, why risk it?
For anything this side of say, 500 horses perhaps, it's okay - what's the consensus here? I would just imagine that at some point - increasing boost - I'd hit that limit. Also, the test mule has 130K miles on it and at some point, she's going to need a rebuild anyway.
I'd go larger bore and forged pistons / forged rods at that point. That would be my reasoning.
Originally Posted by hacker-pschorr
My 79 track car is most likely going to be getting a pair of turbos, power goal again in the 450-500rwhp range so it won't need very big ones.
Interesting choice - given the packaging constraints of the chassis and the relatively low horsepower goal (from my perspective anyway.)
I'd like to know more - why not run a single supercharger (or even a single turbo), for anything this side of say 600 horsepower? Especially with a track car - I wouldn't want the heat of the turbos and quite prefer the linear power curve of the supercharger.
Last edited by Rasant Products; 11-02-2023 at 03:23 AM.
Yes, and this kit is soooo far away from that value.
Re: Todd's TT monster.
Now I see why you asked that...
I think that you missed the part where I wrote "Over this winter, we'll work on refining the kit (which would include an intercooler, more boost and remapping - still on the SharkTuner for the moment), followed by a smaller pulley for more boost and re-tuning with a standalone ECU. Next year, we'll start on a different kit configuration (utilizing the Rotrex C38R-112 - flipped around in a CCW rotation and with the larger 3.5" inlet and even more boost...."
Lots of examples of different variation rods / pistons (and in different 928 models, years and versions - some with rods and pistons that last and some that don't...) The Rotrex can handle close to 3-BAR boost. Long before that point, the head gaskets pop, I'm sure... Still I would want to rebuild and use forged rods and pistons on ANY 40-year old production car / production engine, for anything much over a BAR of boost.
Last edited by Rasant Products; 12-01-2023 at 03:58 AM.
Well I got past the brutal negativity but just barely.
Good on you and I hope this product comes to market. Its ounds like your development plan is sound and methodical.
I had a Rotrex supercharger kit on my 968 from Design1Racing and it performed well at 5 lbs.
My, new to me, '91 S4 goes well but it could use a bit of Oooooph when the wife isn't in the car
One (should not) assumes the 4 speed auto box is up to the challenge? It's my first auto P-Car so I don't know what to expect from Mercedes
My, new to me, '91 S4 goes well but it could use a bit of Oooooph when the wife isn't in the car
One (should not) assumes the 4 speed auto box is up to the challenge?
The auto can handle much more power than the manual and the auto can be upgraded for even more. (Installing a spring kit to up the pressures in the auto trans is a good start.)
The GTS manuals have a built in pump to cool the transmissions, but they are still relatively fragile. Major parts are NLA for the manuals.
My Raptor supercharger is hopefully on its way back from Australia after being rebuilt to new specs. I considered putting it in my '88 manual, but I think it is a better fit for the '89 auto from whence it came.
One thing of note as Porsche tends to reach into the parts bin to make a new car. Hopefully not the same rods as my 93 968.
Design1Racing ran 0.5 Bar and bent the rods on the 968 test car. The 1R rings a bell as a warning but not sure. I would have to get into the 968forums archives.
The production kit ran 5 lbs safely.
Well I got past the brutal negativity but just barely.
Good on you and I hope this product comes to market. Its ounds like your development plan is sound and methodical.
I had a Rotrex supercharger kit on my 968 from Design1Racing and it performed well at 5 lbs.
My, new to me, '91 S4 goes well but it could use a bit of Oooooph when the wife isn't in the car
One (should not) assumes the 4 speed auto box is up to the challenge? It's my first auto P-Car so I don't know what to expect from Mercedes
Thanks for sticking with it there DaveN - look forward to working with you on an installation (should it come to pass.)
Originally Posted by hacker-pschorr
This thread was a popular subject at 3rd coast. Many folks excited to see such a kit back on the market.
Did Victor ever fit one to an 85/86? That question came up a few times.
Common opinion was a kit making use of the Shark Tuner and stock ECU's will be far more popular.
Thanks for the supportive feedback. (It is nice to know that this effort will not have been in vain.) The shop is pretty slammed right now and we have revival of other core Rasant offerings (air-cooled 911 ITB and custom GT3 intake plenum products), running in parallel as well, so the Rotrex installation is lingering a bit - sorry folks. Parts are still trickling in anyway (some 36-pound injectors showed up today - and I see that a SharkTuner box from Jim just arrived as well.)
As far as 16-valve kits; as you guys know, Victor's development of the 928 kit stopped with the 32-valve cars, because I think that prevailing thinking at the time was "great results and the kit remains affordable, easy to install and tuning is simple and accessible with with the SharkTuner..." I can't say that I disagree with that philosophy entirely and then again, there's always room for improvement with everything in life.
We want to offer higher boost kits and adaptation to larger displacement builds down the line as well, so at some point, the stock EZK and LH chip-based tuning will likely only take us "so far"...
We'll try to stay the current course as much as possible, but the planned development path will eventually include a programmable stand-alone engine management system with a custom wiring harness - that much is already a forgone conclusion (because the shop already does so much with stand-alone installations on other applications.) This means that the leap to a 16-valve kit is not far behind, because once the bracket is done and the majority of the intake and boost plumbing is sorted, it's down to fuel and ignition management.
While there are some "workaround" solutions to the 16-valve engine management systems available, I hate the restrictive and antiquated flapper-crapper barn-door airflow meters with a passion. Since we already offer a nice, modern little stand-alone engine management system with fast processors - and a harness - for under $2K (and have been installing and tuning those successfully for years), that final piece of the 16-valve kit puzzle falls into place naturally.
This would eliminate the AFM and stock engine management system completely and give one full laptop-based tuning control over both fuel and ignition.
We may even install one system on an NA 16v car and tune it - just to make the point (or should I say, to validate the path.) I passed on a rough-ish, but cheap local 16-valve 5-speed car last year. Effen hell - I should have seen this coming. It would have been perfect for this development.
Last edited by Rasant Products; 11-07-2023 at 10:36 AM.
11-01-2023, 05:41 AM
