[GuidoK]

Originally Posted by JamesNoBrakes

What lag? These engines make full torque from around 1500rpm, which means lag is pretty much impossible.

In theory lag has nothing to do with rpm. It has to do with spool times. That an engine can make full torque on low rpm doesnt mean it still can have a long spool up time. On low rpm's where there is the potential to make full torque the turbo hasn't really spooled up as long as you dont step on the throttle. And spooling up always takes some time.

Supercharged engines gradually build up torque and are happiest in the high RPM ranges, think that V6 from before, but magnified many times. A boatload of torque at lower RPMs they are not.

Thats not correct. There are -in fact- basically 2 types of superchargers: volumetric superchargers like I mentioned before, which make boost from very low rpm and the centrifugal supercharger (non volumetric). Only the last ones have increasing boost with rpms and has poor torque at low rpm.
But if you take a roots or twinscrew (both volumetric), it will make a lot of torque from very low rpm's. Maybe even lower than turbos. But there is of course no lag (no lag possible)

If the TS wants zero lag and a lot of torque on low rpms, it's either large displacement or a volumetric supercharger. (or a tesla roadster as someone cleverly opted)

[aloksatoor]

After two caymans you probably should try the 991.1 c2s it's geared better than the taller Cayman and has more torque.

Another $$$ option is to have bgb regear the Cayman manual box for closer ratios. Won't cure the lack of torque but will have lesser rpm drop at least.

[RM7]

Originally Posted by GuidoK

In theory lag has nothing to do with rpm. It has to do with spool times. That an engine can make full torque on low rpm doesnt mean it still can have a long spool up time. On low rpm's where there is the potential to make full torque the turbo hasn't really spooled up as long as you dont step on the throttle. And spooling up always takes some time.

And why can't it spool? Because not enough exhaust pressure is there, hence, the lag until you get to the RPM where it can quickly spool. Ever drop a turbo car from 5th to 3rd? The turbo spools almost instantly, but maybe you are talking about where tuners attach huge turbos to small engines, like the ones that get you 500+hp out of 2 and 3 liter engines, those of course are big turbos that don't spool unless there's a lot of pressure, so you got to get up to 3.5K rpm or sometimes more.

Thats not correct. There are -in fact- basically 2 types of superchargers: volumetric superchargers like I mentioned before, which make boost from very low rpm and the centrifugal supercharger (non volumetric). Only the last ones have increasing boost with rpms and has poor torque at low rpm.
But if you take a roots or twinscrew (both volumetric), it will make a lot of torque from very low rpm's. Maybe even lower than turbos. But there is of course no lag (no lag possible)

If the TS wants zero lag and a lot of torque on low rpms, it's either large displacement or a volumetric supercharger. (or a tesla roadster as someone cleverly opted)

It's still correct, even roots superchargers behave in that same basic way. They'll pull, but it's not the drop-in-your-pants massive torque at relatively low RPM that can be had with turbos. Even the roots isn't massive torque at low RPM, it takes harnessing exhaust gasses for that I looked long and hard at getting a roots vs. turbo for my LS3.

[Em135eye]

Ultima GTRs aren't too expensive but they are light, torquey and VERY fast.

[David70]

I think everything except newer/warranty but at $10k you should have money left over for repairs.

http://nashville.craigslist.org/cto/5424923964.html

More -http://monstermiata.webs.com/classifieds.htm

[Kidscollege$]

or this...http://autoweek.com/article/car-news...firmed-america

[GuidoK]

Originally Posted by JamesNoBrakes

And why can't it spool? Because not enough exhaust pressure is there, hence, the lag until you get to the RPM where it can quickly spool. Ever drop a turbo car from 5th to 3rd? The turbo spools almost instantly, but maybe you are talking about where tuners attach huge turbos to small engines, like the ones that get you 500+hp out of 2 and 3 liter engines, those of course are big turbos that don't spool unless there's a lot of pressure, so you got to get up to 3.5K rpm or sometimes more.

You didn't read my answer correctly. If an engine can make full torque at 1500rpm, it will still have lag. THAT was your initial statement, and it is not correct.
Actually you're correcting yourself with your statement here.

It's still correct, even roots superchargers behave in that same basic way. They'll pull, but it's not the drop-in-your-pants massive torque at relatively low RPM that can be had with turbos. Even the roots isn't massive torque at low RPM, it takes harnessing exhaust gasses for that I looked long and hard at getting a roots vs. turbo for my LS3.

A twinscrew or roots can make full boost at 2000rpm or so easily.
I call that relatively low rpm.
Lots of turbos still have trouble with these low rpms. They usually solve that with variable geometry turbo's, a relative new and complicated technique.
If you think a volumetric supercharger cant do that, you're incorrectly informed. It's pretty much in the definition of volumetric supercharger that it can do that. In practice they perform better at low to midrange rpm than in the higher rpms, although that also varies with the size of supercharger you're designing with.

[imserious]

They don't make the car you and every other enthusiast is looking for.

Corvette and 911 are the closest out of the box solutions.

[ASAP]

You want a light, nimble car, with low end torque but no turbos...

GL buddy... cuz you will need it.

It sounds to me like you want a Miata with an LS swap.

only other option is a supercharged lotus or Alfa 4c.

[zer0cool]

Wait for an electric roadster.

[fcman]

Rolls Royce Phantom Drophead

[chris82]

BMW 128i M-Sport

[auf Deutsch]

http://www.bing.com/images/search?q=...20ba9db00533H0

[sygazelle]

Originally Posted by baege

I drove an m235i about a year ago. I felt lag. Otherwise a pretty sweet car. the m2 is on my radar but the turbo thing dulls my enthusiasm.

Seems like BMW offerings are not going to do it for you.

One last idea before we give up. Take that engine out of your Cayman and drop this bad boy in its place. Problem solved.

[Red Bread]

Why not do a 3.8L or 4.0L swap like the Gemballa one?

[imserious]

Originally Posted by Red Bread

Why not do a 3.8L or 4.0L swap like the Gemballa one?

Yes or a GT4. The Cayman S is already so close to the best apart from seriously focused track cars like the Atom, Catamaran and others.

If you need nimble, you should also add mid-engine to your list of requirements.

I think my ideal might be a supercharged NSX.

[Never_Enough]

No TQ, but a Miata is what you need to buy. You'll thank me later.

[RM7]

Originally Posted by GuidoK

A twinscrew or roots can make full boost at 2000rpm or so easily.

Except "full boost" for a roots twin screw type is somewhere like 5-7psi, which does add a lot of power to an LS3, but in reality, that's nowhere near the PSI that turbos can provide.

[GuidoK]

Originally Posted by JamesNoBrakes

Except "full boost" for a roots twin screw type is somewhere like 5-7psi,

?!?

You are not correctly informed.
Here is the performance map of the twinscrew I use (lysholm ax1600):
http://www.lysholm.us/pdf/diagram_lys1600ax.pdf

You can clearly see that the peak efficiency is at 1.8 pressure ratio, that is around 11 psi, and that the widest flow range is about between 1.6 and 2.0 ratio, so between 8.5psi and 15psi.
That is it's ideal operating range for this twinscrew.
Nowhere near the 5-7psi you think of.
It will work on that low range if selected carefully when the cfm requirements of the engine fall within it's range (or you'll loose boost/gain heat in ranges), but it's not designed for it.
Maybe if you want to go in those low boost ranges the little eaton m45 is an option. That's used in a lot of miata sc conversions (mostly sources from small mercedes engines I think), doing it on the cheap so to say. But that is a very small and low yeald supercharger (although I still suspect that it'll still perform at 8psi, but with low cfm/for small engines)
But these are all facts/data which is easily available on the interwebs....

Eaton V series sits between about 5 and 12psi and eaton R series sits even between 7 and 17psi.
So you can pretty much find a roots or twinscrew supercharger for most applications.
Only if you want to go above 17psi (in fact that eaton R will go up to 21...), you have to go turbo, but that is even for gas turbo engines pretty high. For comparison: the n54 runs stock on 8.8psi.
Boost pressures above 17psi are normally only seen in turbodiesels or very very high performance builds where the stock turbo probably also has run out of air (well....gains too much heat) (335i's with 600hp+ and such)

That concludes this weeks' turbo and supercharger lesson

[RM7]

Originally Posted by GuidoK

?!?

You are not correctly informed.
Here is the performance map of the twinscrew I use (lysholm ax1600):
http://www.lysholm.us/pdf/diagram_lys1600ax.pdf

You can clearly see that the peak efficiency is at 1.8 pressure ratio, that is around 11 psi, and that the widest flow range is about between 1.6 and 2.0 ratio, so between 8.5psi and 15psi.
That is it's ideal operating range for this twinscrew.
Nowhere near the 5-7psi you think of.
It will work on that low range if selected carefully when the cfm requirements of the engine fall within it's range (or you'll loose boost/gain heat in ranges), but it's not designed for it.
Maybe if you want to go in those low boost ranges the little eaton m45 is an option. That's used in a lot of miata sc conversions (mostly sources from small mercedes engines I think), doing it on the cheap so to say. But that is a very small and low yeald supercharger (although I still suspect that it'll still perform at 8psi, but with low cfm/for small engines)
But these are all facts/data which is easily available on the interwebs....

Eaton V series sits between about 5 and 12psi and eaton R series sits even between 7 and 17psi.
So you can pretty much find a roots or twinscrew supercharger for most applications.
Only if you want to go above 17psi (in fact that eaton R will go up to 21...), you have to go turbo, but that is even for gas turbo engines pretty high. For comparison: the n54 runs stock on 8.8psi.
Boost pressures above 17psi are normally only seen in turbodiesels or very very high performance builds where the stock turbo probably also has run out of air (well....gains too much heat) (335i's with 600hp+ and such)

That concludes this weeks' turbo and supercharger lesson

And you do realize that turbos can be kicking out around 20psi under "full boost", right? 15psi is more the starting point, rather than the upper limit.

That's the point I made originally, yes, your supercharger may be operating under "full boost", but that "full boost" for a super charger is usually less than that of a turbo, although a turbo will drop off more above around 5.5K due to going past it's volumetric efficiency usually, which can be solved with a larger turbo, but that creates more lag issues.

You spent all that energy to basically say the same thing I did?

[GuidoK]

Originally Posted by JamesNoBrakes

And you do realize that turbos can be kicking out around 20psi under "full boost", right? 15psi is more the starting point, rather than the upper limit.

As I said, the stock turbo pressure on a N54 is 8.8psi. And that is pretty typical for a stock turbocharged gasoline engine
If you'd make 15psi on that engine, you'd have a 335i with approx. 520HP.
I'd say that that's already pretty extreme for a 3 liter turbo and by no means a 'starting point'
If you make 20psi, you'd be pushing 700HP theoretically.
By that time, the stock turbo would have probably melted in a solid lump of steel.
So the 'starting point' is like 8-10psi on most petrol cars (mind you, 8.8psi is full boost on a stock n54!, and that is a good high performance engine considering all the awards it got).
If you look at how boost is set on various tunes, for example that n54 used in so many bmw's, 8.8psi is stock, 12psi is a stong tune on a stock engine and 15psi is only possible with mods (intercooler, downpipe etc), but still on stock internals. Go to 20 psi, you need high performance turbos, maybe different internals, the lot (although the n54 has proven to be a very good engine with a strong crank). I also doubt that that's possible on pumpgas.
The only cars that run 15-20psi stock are turbodiesels. Thats one of the reasons why a dieselblock is much stronger built. But I dont think the TS is considering a diesel car.

That's the point I made originally, yes, your supercharger may be operating under "full boost", but that "full boost" for a super charger is usually less than that of a turbo, although a turbo will drop off more above around 5.5K due to going past it's volumetric efficiency usually, which can be solved with a larger turbo, but that creates more lag issues.

You spent all that energy to basically say the same thing I did?

No, I spent all that energy to basically correcting all the mistakes in your dialoge:

First you say that a turbo can make full torque at 1500rpm, and that it has therefore no lag. That is not true and after my post you correct that yourself by saying the thing about shifting down...

Also you say that 'Supercharged engines gradually build up torque and are happiest in the high RPM ranges', which is also not true. That only goes for centrifugal superchargers (non volumetric).
Volumetric superchargers (roots, twinscrew, G-lader) make boost pretty much instantly down low.

Then you say that '"full boost" for a roots twin screw type is somewhere like 5-7psi', which is also not true.

and then you say "You spent all that energy to basically say the same thing I did?"
WTF? cant you read?
I'm constantly correcting all the incorrect information you give.

I think your opinion was formed by all the cheap superchargerkits (or should I say entrylevel kits, cheap is very relative) that are on the market; small centrifugal sc, no cooling, no plenum, sometimes even no injectors etc that give a certain NA engine maybe 25-40% extra HP.
Those have only extra torque on high revs (centrifugal sc, small cfm bandwith) and operate generally at 5psi or so etc.
Or cheap miata conversions where an eaton m45 is harvested from a crashed mercedes and bolted to the NA block.

The better SC kits operate at 8psi or little above, before you have to lower the compresion ratio. Once you do that, kits and installs go up to 15psi or more if desired.
If you go for top whp on a high performance build, turbo is more efficient (because a turbo only creates pumplosses, no parasidic crank loss), and therefore a turbo will consume a little bit less fuel when cruising (comparable to for example having the ac on or off), but from a drivability pov a twinscrew is a nice alternative, as there is per definition zero lag. Especially aftermarket turbokits or upgraded turbo's create extra lag compared to stock turbo installs (a manufacturer spends lots of money to minimize the lag, but that also always has influence on the efficiency of the turbo)

[Jamesons Viggen]

For purity, an s54 01-02 M Coupe or more modern Z4M would do the trick.