Don Carter

Tuomo,

That makes sense, my only previous turbo experience was with 4 cylinder engines.

Mike,

Forgot to mention this car was LOUD, too loud for me. There's just not much room to add mufflers between the back of the car and the turbo, even less room with your setup I would imagine.

Lizard928

Funny Don,
I made a mid mount setup on my car and it is quiet at cruise speeds, but really loud once the waste gate opens as I'm basically dumping to atmo

dprantl

Boost lag is a very simple concept, and it can happen just as easily on a 5.0 liter V8 as on a 2.0 liter inline-4 (depending on relative turbo size of course). Keep RPM constant with the brake (or on a dyno that keeps RPM constant), then floor the accelerator pedal and see how fast it takes to get to the maximum boost level for that RPM point. Lag is the time it takes to build this full boost. On a supercharged car, the time in which maximum boost in a specific RPM point is reached is almost instantaneous, whereas on a turbo car there is a delay. The lower the RPM point, the more the delay with turbos. Now, if you have enough power without boost to spin the wheels, then lag is completely irrelevant because you will spin the wheels regardless. But if you are in 5th gear at 2,000RPM using a 2.2 rear pinion, I can guarantee you that such a turbo'ed 928 will have lag unless the engine displacement to turbo size ratio is ridiculously large, in which case you are choking upper end HP. A twin-screw 928 will have instantaneous ~5psi in those same conditions. There are specific tests designed to measure this, such as the time taken to get from 50 - 70mph in highest gear test.

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

Roy928tt

Whilst I agree with your method of determining, Lag, I would argue that it is all but eliminated in a larger capacity twin turbo installation.

my own experience is, that if I plant the throttle at 1800 rpms in 4th by the time my eyes have shifted from the tach. to the boost gauge (in the ashtray) it is showing positive boost, and when it pegs at 7 psi I shift my eyes back to the tach. as it passes 2500, and I'm sure it takes less time than it does to read that !

Which is why I love a good twin turbo installation...

ptuomov

The concepts are indeed simple.

There are two concepts that people often confuse: Boost threshold and lag. Boost threshold is the rpm at which engine makes a certain level of boost. Lag is the time it takes the engine to make the desired level of boost at a given rpm, starting from the initial condition of slowest turbine speed possible at that rpm.

A general question to people on this thread: Have you ever driven a twin turbo S4-GT-GTS with a short exhaust manifold?

My experience is that the boost threshold to reach a knock-limited level of boost is about 2900 rpm for two gt2871rs and 3200 rpm for two gt3071rs. My experience is that if you "cruise" at low load above 3200 rpm, the turbocharger spools so fast that it's not perceptible. It's not in practice possible to drive my car above 3200 rpm on a flat surface and not have the turbochargers spinning over 20,000 rpm. Starting from 20,000 rpm turbo speed and 3200 rpm engine speed, the turbocharger speed sensor needle moves by human perception as fast as one can press the gas pedal down. Not literally, but a human can't tell the difference. By my perception, the wastegate opens immediately after I've floored the gas pedal.

The reason why this is so is that when you floor the pedal at 3200 rpm, on the next gulp without any boost the engine ingest enough air to produce exhaust at mass flow rate or about 400 kg/h. At that mass flow rate and, say, 800 K exhaust temperature, the exhaust manifold will be pressurized instantly to about pressure ratio of 1.5. The turbine will instantly produce about 3 hp, basically the next cycle. That 3 hp is enough to drive my compressors at 2x200 kg/h flow rate against a pressure ratio of 1.4 or about 7 psi boost in steady state. So by this computation it takes the engine about two revolutions to be on boost, or 37.5 milliseconds. The three effects that are ignored in this computation are turbo rotating assembly intertia (minimized by using two relatively small ball-bearing turbos), exhaust manifold volume (minimized by installing the turbos as close to the exhaust ports as possible), and intake manifold volume (minimized by not oversizing the cold side piping). All things considered, it's easy to see from these numbers why there is no perceptible lag at rpms above the boost threshold.

Lag boy: Do not try and anticipate the lag. That's impossible. Instead... only try to realize the truth.
Neo: What truth?
Lag boy: There is no lag.
Neo: There is no lag?
Lag boy: Then you'll see, that it is not the lag that you anticipate, it is only yourself.

Imo000

Ok but why some older cars have such a horrible turbo lag? Like the early 930s and for example the Evo on Top Gear (I think it was an Evo)?

Roy928tt

The 930 being a single turbo flat 6 had a relatively long exhaust from cyl head to turbo, contributing to lag.

The Evo would have had a relatively larger turbo to achieve a higher headline output figure, and what Top Gear were really experiencing was the slow crawl from a low engine speed to the boost threshhold, which would have been at a higher rpms with the oversized turbo.

All of which, reminds me of why I consider John Kuhn's twin turbo installation just brilliant. He understands the principles behind turbocharging and has applied them to a 928 superbly.

Actually in my own cut price, minor league, manner my own installation is very good also, even if I do say so myself.

jetrainor

Mention of a wastegate 'muffler' caught my eye, as i was thinking along the same lines for my MurfSC....essentially muffling the whistle/howl from the wastegate when your off the gas....i would envision a sort of baffled attachment which deadens sound but allows the pressure to escape.

ptuomov

I think that 930 had a lag/high boost threshold because of the following factors. It's an air cooled engine, and therefore the desingers didn't want to have high exhaust manifold pressure and gas temperature. They picked a free flowing turbine and long manifold to manage the heat.

hacker-pschorr

I'm not even sure where to begin, or to even take this as a serious post. The highest HP strokers don't come anywhere close to the power levels of the top boosted cars. How is that even a relevant comparison?

Complete nonsense. You just need to contact Tim Murphy:

www.murf928.com
info@murf928.com

With over 80 kits sold, the question of reliable performance at these levels is beyond question. Todd's 800+rwhp Twin Turbo knocked that debate out of the park.

It many not be simple to achieve that kind of power with the setup on your car, that doesn't mean the same limitations apply to other kits.

Some would argue less.......
With the 951 guys cranking out 350 - 400rwhp left and right, it's rather ridiculous to still see 928 owners claiming such HP limitations.

Exactly


Here is the kind of power you can expect from a Murf928 Stage 3 Supercharger Installation:

-

rgs944

If I went to 10 or 12lbs what I mean by a lot of screwing around is updating my clutch, maf, fuel pump and likely a few other things. Don't get me wrong I would love to have that much power but just those things alone are at least 3K just in parts. For a street car I feel like I have very good power to use. I know the 928 internals are very strong but I just want to be as safe as possible with my engine. I know I also just worry too much.

polecat702

Let me ask a stupid question. Is all this hp possible, without changing pistons, head work, rods, etc?

This is just bolt on a blower, change injectors, intercool it, tune, and go?

What everyone here is saying is everything I've ever learned about building a blower motor doesn't apply to a Porsche V8 engine. And it'll stay together?

To be honest, you could buy a new Vette for what the engine in my Pantera cost.

Hilton

The 928 engine internals are insanely over-engineered for the factory power output. There are a lot of cars running on original factory head gaskets with >500rwhp, with a variety of supercharger types, turbos etc. There are a few cars running north of 600rwhp on unopened factory engines.

The trick is making sure your 928 is running right (all sensors working, good compression/leakdown) and has been well maintained (regular coolant changes being the important one), before boosting.

From memory the Murf stage 3 kit includes everything - supercharger, piping, clamps, injectors, IC, and of course pre-tuned ECU chips to match the kit.

I think the people who're worrying about tweaking their systems don't realise the time and effort that's gone into the Murf kit to make it a "bolt on" solution. He (and others) have already spent hundreds of hours refining it, so his customers don't have to (unless they have the compulsion ).

Rob Edwards

All I can say is that if I didn't live in California and have to deal with biennial visual inspections and smog, I would have a Murf stage III on my car. Proven track record and the best bang for the buck there is in 928powerland....

Still looking forward to seeing how badly beaten I'd get in a little side-by-side with Tim and AO. Need to figure out how to make that happen someday....

rgs944

I know the Murph kit is a nice kit but I know the VCB kit is pretty strong in the bang for the buck rating as well. As far as Ca smog, I know I would be getting a kit that is an easy install and take off for smog if I was out there. Is it every two years? What about finding a buddy in Nevada to register the car at his house.