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-   -   K16 v K24 the "lag" issue (https://rennlist.com/forums/993-turbo-forum/382500-k16-v-k24-the-lag-issue.html)

TB993tt 10-04-2007 09:02 AM

K16 v K24 the "lag" issue
 
With the 996tt forum begining to look like an offshoot of 6speed with lots of talk and little substance I thought I would treat 993tters with some data I found.

This data comes from Flat 6 magazine, the cars are 993tt stock 408PS and 993ttS Euro spec which means K24 and 450PS the gearing is the same for both cars, the temperatures were similar for all the tests

The time between when one hits the throttle pedal and when the turbos spool up and start boosting is one element of "lag" which is never tested for however the other element is using WOT from low revs and timing how long the car takes to accelerate between two times giving us a picture of the boost curve of the turbos.

I have extrapolated approximate rpm's from the before and after speeds to give the following numbers:
K16 K24
3rd gear 3650rpm - 5470rpm 2.8s 3.2s
4th gear 2820rpm-4235rpm 4.1s 4.3s
5th gear 2380rpm-3570rpm 6.2s 6.6s
6th gear 1760rpm-2640rpm 8.6s 10.4s

Just because I have the numbers here are the timings for the same speed increment for the K16 420hp 996tt versus the X50 K24 450hp 996tt

K16 K24
3rd gear 3.77s 3.80s
4th gear 4.4s 5.0s
5th gear 5.48s 6.4s
6th gear 7.2s 9.0s

What we need now is some hard data from a K16/24 hybrid car - Phelix :) ?

GIA 10-04-2007 09:49 AM

TB,
This is really interesting subject at least for me.
I suffer from K24 Lag, and searching for additional info.

Hope to see the hot discussions and diverse opinions on our forum too, but more specific than on 996TT forum.

LT1 10-04-2007 10:22 AM

I have real world experience with this. My brother (996TT X50) and I did some 3-4th gear pulls on the highway. Both cars stock (I have muff bypasses). Each time I pulled him from the beginning and we ran out of room before he could get to his higher horsepower. We were on our way to drop off his car for a upsolute flash and exhaust (probably good for nearly 100hp). Needless to say, the next run left me in his dust from the start. His car hits boost like a rocketship now-very intoxicating.

eclou 10-04-2007 10:29 AM

what is the size differential for the K16 vs K24 turbine housings?

Rassel 10-04-2007 06:05 PM

K24's really benefit from muffler bypasses and a ECU upgrade

Jean 10-06-2007 04:21 PM

Very interesting thread. Lag is always seen as evil in turbo circles :) Measuring it can be tricky business, especially when things such different engine setups are concerned...

The way TB is looking at it might be one of the best ways, when comparing two similar cars (lbs/HP ratio). But how does one differentiate "lag" from torque on a dyno chart?

Notice (from another TB993TT thread) how the HP and torque factory curves are somewhat higher at low RPMs in the K16 setup, and then the lines cross each other around 3K RPMs, where the K24 takes off and never looks back. THIS is "Lag" (Boost onset), whereas a curve that remains higher than the other throughout the RPM range has little to do with lag if anything, but rather has more to do with higher boost used and programming.

http://forums.rennlist.com/upload/dy..._vs408_mod.jpg

I have seen my torque vary by more than 25% at the same RPM point by simply using two different dynos and loads with the same exact engine....

This is where the comparison in this thread is interesting! Real life comparable data.

From Flat 6 magazine also, I am providing some additional data to this topic. Listed by better Lbs/HP ratio to worse. Time it took to achieve a 80-120kph (50-75mph) in 3rd, 4th, 5th, 6th gears.

Observations:

- 996TT vs. 996 TTS, very close in 3rd and 4th, from 3200RPMs or so upwards....Bigger difference in 5th and 6th, where they start at around 2K RPMs.

- 993TT and 993TTS, big difference in 5th and 6th whereas not so much when starting from 3K RPMs+ in 3rd and 4th....

- 993GT2 favourable lbs/HP ratio, therefore it offsets the lag of its K24s somewhat and beats the stock 993TT, except when it starts in 6th gear, at less than 2K RPMs, where even the better lbs/HP does not help it.

- 993TT vs 996TT, same lbs/HP ratio, the 993TT out accelerates the 996TT in 3rd because it starts at 3600 RPM (different gearing), but in 5th and 6th, the 996TT eclipses the 993TT as a result of more efficient intake and exhaust, which reduce lag even with the same turbo in both cars.

-993TTS vs. 997TT, same lbs/Hp ratio, very close numbers in 3rd and 4th, however the 997TT shows its fantastic VTG technology quickly from 5th gear and then 6th gear is way ahead.

- Lag differences are very limited with runs starting at around 3K RPMs+, starting at 1.5-2K RPM in higher gears, they are substantial.

Also interesting to note that despite larger turbos having more lag, they accelerate faster from a standstill to 200kph (125mph), or even in a 100-200kph run going through the gears, where RPMs don't drop below 4K RPMs. The 993TTS puts over a second vs. the 993TT over a 100-200kph run despite the big disadvantage seen on the table below for the TTS running within the gears. :eek: The same thing is true between the 996TTS vs. the 996TT.

Morale of the story, decide what your needs are before making a decision based solely on lag. For the track I would choose somewhat bigger and laggy turbos as I will not suffer from it, if you like cruising in highways in 5th. at 50mph and nail it, go for smaller ones for sure. Torque throughout the RPM range is a totally different story, good mapping and *mainly* boost levels makes the difference, not lag.

http://forums.rennlist.com/upload/80120kph_runs.jpg
Thanks TB, interesting angle to the lag question. :cheers:

Acropora 10-06-2007 09:36 PM

Some bosch engineer at the track today was saying one can reduce lag by retarding the ignition at lower rpm's? This causes the combustion to occur even in the exhaust, is harder on the turbos, but is a way of reducing lag? Never heard of this before but was wondering. I'd love to be able to go around the track at 4K and always be on boost.

eclou 10-07-2007 12:31 AM


Originally Posted by Acropora (Post 4648209)
Some bosch engineer at the track today was saying one can reduce lag by retarding the ignition at lower rpm's? This causes the combustion to occur even in the exhaust, is harder on the turbos, but is a way of reducing lag? Never heard of this before but was wondering. I'd love to be able to go around the track at 4K and always be on boost.

You can do 2 things to help improve spool up
1)retard timing
2)run a leaner fuel mixture
both the above lead to increased exhaust gas temps which in turn increases exhaust gas velocity and thus quickens spool. It is a common trick that some tuners implement in their programming though it can be dangerous if done carelessly

Jean 10-07-2007 07:35 AM


Originally Posted by Acropora (Post 4648209)
I'd love to be able to go around the track at 4K and always be on boost.

Acropora, if you don't have boost at 4K RPMs, I would consider a thorough check up on your setup. :confused: Also, try getting off 6th gear every now and then :p..j/k

Felix 10-07-2007 12:21 PM

Are these runs done starting at that speed/rpm or from a lower rpm and the clock "starts" when you hit the entry speed? And please post the road speeds as well. I'll see what I can do ;)

Basal Skull 10-07-2007 12:33 PM

TB and Jean, this is all great info. I'm always amazed about the amount of knowledge/time/energy/effort you guys put in to get us this type of info! Now I know why my car seems so fast compared to others starting in 3rd or 4th at reasonable speeds.

I (and I'm sure many others) really appreciate this kind of stuff.
Thank you,

Ryojo

eclou 10-07-2007 01:26 PM

looking at the bar graph nicely illustrates available torque under the curve. The graphs with the widest discrepancy between 3rd and 6th are peakier/narrower power bands such as the 993TTS which has a 7.2s spread between 3rd and 6th times. The VTG of the 997tt seems to offer no real advantage over the non VTG 996tt - both have about a 3.4/3.5s spread between 3rd and 6th times. Granted, the run length is very short.

Jean 10-07-2007 01:52 PM

Ryojo

Thanks for the kind words. Glad to see some people like this stuff. :cheers:

Eclou,

IMO these numbers are a great testament to how well the VTG techonology works! One should not look at third gear looking for lag effect as much as the higher gears.

These VTG turbos are about the size of the K24s, yet see how much faster they take the 997TT in 5th and 6th gear when compared to the 996GT2 even with its better lbs/HP ratio, it absolutely destroys it and is as fast as the K16 based 996TT.

What we don't see on these graphs is how well they pull at the higher end of the RPMs as well, this is what makes them unique IMO. Small turbos will spool faster for sure, but they will not deliver at the top end, whereas the VTGs do.
:cheers:

Basal Skull 10-07-2007 02:55 PM

It would be interesting to see what the numbers are for the K16/24 and other hybrid turbos - to see how much of an advantage the VTG is in comparison...

Looking at the numbers again, I'm surprised that the 993tt does so well in 3rd and 4th when the rpms are higher, and not as well in the higher gears where the rpms are lower, where you'd think the smaller turbos should build boost faster. I guess the hp differential overcomes the lag at lower rpms in these cars? I would have thought the larger turbo cars would do better in the lower gears where the rpms start off higher - almost counter intuitive... I guess in the lower gears, the larger turbo cars still take longer to get on boost (rpms still not high enough) - despite starting at a higher rpm. Is this just testing off boost engine torque in the higer gears?

eclou 10-07-2007 03:54 PM


Originally Posted by Jean (Post 4649766)
IMO these numbers are a great testament to how well the VTG techonology works! One should not look at third gear looking for lag effect as much as the higher gears.

These VTG turbos are about the size of the K24s, yet see how much faster they take the 997TT in 5th and 6th gear when compared to the 996GT2 even with its better lbs/HP ratio, it absolutely destroys it and is as fast as the K16 based 996TT.

What we don't see on these graphs is how well they pull at the higher end of the RPMs as well, this is what makes them unique IMO. Small turbos will spool faster for sure, but they will not deliver at the top end, whereas the VTGs do.
:cheers:

Good point Jean. BTW the EVOMS GT700 package 997tt (bigger VTG turbos) pulled 185mph at the Texas Mile. IIRC this is about the same as the RT12 did last year


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