928mac

mmmm twin screw, morning and night

White Lightnin'

...or sisters?

EspritS4s

Turbo's can be more efficient, but they are also usually more complicated and costly to retrofit.

Shane

Murf Kit!

Start with a sound engine with any kind of boost it will save you grief later in life. If the car smokes find out why and fix it first, if it is down on power, fix it first.

My last boost project cost me the price of the car to get all the engine systems (including AC) fixed and in serviceable condition prior to boost. As it turned out, though, 25 year old headgaskets don't hold too well under 12psi.

dprantl

I've driven all sorts of supercharged and turbo cars. From the sound of your priorities, you should look hard for a twin screw or roots kit. My car with an old Eaton M112 makes 5psi of boost instantly at 1,500RPM and goes up from there to about 9psi peak at 3,700RPM. That's 310lb.ft rear wheel torque @ 1,500RPM. Low-end torque doesn't usually win races, but it sure is fun . A newer twin-screw can take the peak over 12psi and 500rwhp.

Killav, if you don't make any boost until 2,300RPM and it is all in at 2,700RPM, then your boost lag is 400RPM there. Of course it doesn't feel like a 4 cylinder's boost lag, but when comparing different FI options, it is a big minus.

Dan
'91 928GT S/C 475hp/460lb.ft

killav

AO,

I have not done this yet and I really need to. I will post on my original RMT thread as soon as I can get my butt down there.

Mv

SQLGuy

I've seen a few postings saying this (that the twin screw setup is best overall, especially for low-end). What I don't understand is why there don't seem to be any currently available twin screw kits for the 928. There's the one listed by 928 Specialists (which is a bit out of my price range), but I'm hearing that this is not actually shipping...?

Was yours a "from scratch" project, or was there once an Eaton based 928 kit?

I'd like to see an RPM-based dyno chart for Carl's kit. From what I can guestimate of the graph, it looks like it's giving about 20% more HP at 2700 RPM. I'd really like to see at what RPM the "before" and "after" lines cross....

http://www.928motorsports.com/parts/...1_Baseline.jpg

soupcan

My no low end torque centrifiical supercharged 928 has shown its taillights to a **** load of big torque making Nitrous, Turbo and Roots blown cars. Torque is great for the dragstrip where you can hook, I'll take the top end HP for the street. This is just speculation cause I never street race.

bd0nalds0n

I have an Andy Keel twinscrew kit that was the precursor to the 928 specialist's kit. It's an autorotor unit, which I understand are used on the Kenne Bell Cobras.

I have had the car retrofit to be a 5speed but it started forced induction as an auto. I chose the twin screw because I wanted the low end grunt. I have put on a smaller pulley and make 11psi boost. Haven't had the car dynoe'd since it was properly tuned, but I don't get third-gear chirps. Still, the car pulls and pulls from idle. I'm sort of a lazy driver, so I like not having to row through the gears constantly. The auto was kind of nice that way--on the highway, when I wanted to pass, I'd just flick the shifter from 4 to 3 and mash on it. It really had a lot of get up and go. It's a bit more time consuming to downshift from 5 to 4 with the manual. OT, but the 5 speed gearbox in the 928 really isn't a very fast shifting or precise unit.

If I was going to do another car, I'd go with the Murf kit. I agree with AO that it's not as pretty, but there are a lot out there and it's pretty much plug and play. It has sort of been a pain in the *** to get my system sorted over the years. Fine if you're into that kind of thing. I like my car a lot more with the hood down than up. But it has been an interesting journey, and you learn something about the car and something about yourself for having gone on it.

dprantl

Yes, there used to be a kit called the Supermodel by Andy Keel. It was based on the Eaton M112, then later on the Autorotor Twinscrew. DR from 928 Specialists bought the stuff when Andy lost interest in the kit-making business. All the history is on this list if you want to do a search for user GoRideSno.

Dan
'91 928GT S/C 475hp/460lb.ft

928mac

There are no prices of the Murf kits on his site.
it is $6000 and $7600 at Carl's site

EspritS4s

It doesn't seem to me that this should be much of a decision for most people considering the forced induction options these days. The Murf kit has been installed on a relatively large number of 928s and has been improved over time. Unless the 928SP kit becomes generally available, I don't know why anyone would choose another option unless they really enjoy the DIY experience or being on the leading edge with the more recent turbo kits.

hacker-pschorr

The kits on Carl's site are NOT Murf kits.
At one time Carl was selling the Murf928 supercharger kit, not anymore.
Your post is a bit confusing.

That's because people like to confuse "boost" with "power". Just because you are making boost doesn't mean the car is making any power.

As for the claim "no power under 4k".......the person claiming this did not buy a Murf928 kit, and the setup he did buy also had him buying a SharkTuner to tune it himself.
I'll stop there to avoid saying something I might regret later. It's just a perfect example showing there is a lot more to making power than simply shoving some boost at the throttle plate.


How about actual performance? Isn't that what we are really after here? To actually go faster?.....what a silly concept.
I don't putz around town in 5th gear all day.

Here is a good comparison.
Same day, same road, one right behind the other. Here is a Murf Stage III S4 Automatic versus AO's twin screw GT:

The red line / box is the Murf928 Stage III

Landseer

Ditto, for 32V it looks like 1 very clean supercharger option and 1 pretty interesting single turbo option (that will require some level of detailed tuning).

EspritS4s

It might be confusing because this may not be general knowledge. I must have missed the announcement, but I thought the 87+ kits that 928 Motorsports sells are the Murf kits. So, are the Murf kits still being actively sold then? The website doesn't have any recent updates and also doesn't include pricing information. Are the 87+ 928 Motorsports kits then related to the Murf kits, a completely original design, or are they NLA?