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Turbo comparison #4 vs #6 hotside

 
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MikeDanger  



Joined: 21 Nov 2002
Posts: 663
Location: Denver

PostPosted: Fri Jun 04, 2021 6:22 pm    Post subject: Turbo comparison #4 vs #6 hotside Reply with quote

Has anyone here driven Both? and or a carrera GT vs. a 931 with intercooler and #4 turbo?

Im debating what size turbo to go with for my T3 mount headers.

The #4 housing is about the same size as the garret style .48 A/R housing. ( I have both in my hands)

But I can't imagine any more lag.

I had a 951 with a #6 and that was laggy enough,

the #6 is about the same size as a .63 A/R

So Im curious if there is a noticeable difference in lag between the 2?


On interesting note is that the Turbine on the #4 is about the same size as the turbine on the .63 A/R, T3. So despite the housing size difference, what would be the best size A/R to go with on the T3 style?

Untill I can Get a dual BB turbo.
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2315
Location: MI

PostPosted: Thu Jun 10, 2021 5:53 am    Post subject: Reply with quote

It depends on how much power your goal is to make..
I think you should probably go with the smallest turbo that will reasonably make your power goal to keep lag as minimal as possible..

No sense in running a 500hp turbo to make 220hp..
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Mike9311  



Joined: 14 Dec 2004
Posts: 1147
Location: Chicago-ish

PostPosted: Thu Jun 10, 2021 6:05 am    Post subject: Reply with quote

I've driven both but there are other modifications along side the housings so it can't really compare exactly

I also did a study on both. End of third page are some videos. Super easy to see how the 4.10 spools so much faster with identical input...obvious really

http://www.924board.org/viewtopic.php?t=44094&postdays=0&postorder=asc&start=0&sid=1cfc5aee4add9fc29d1a4fa3627813ec

Edit: Dan's Club Sport had a 6.10 and a US / ROW S1 Compressor and it did not seem laggy to me. Laggy maybe in comparison to a 4.10 / 2660 which almost drives around like it doesn't have a turbo (next to zero lag). Both intercooled. The CS was at 10PSI and the '82 at stock 7-ish
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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Cedric  



Joined: 27 Aug 2004
Posts: 2160
Location: Sweden

PostPosted: Thu Jun 10, 2021 7:42 am    Post subject: Reply with quote

The 951 have a horrible set up with the long tube exhaust manifold, its a bad design for boost response, dont drag it in here

Anyway, there are more system parameters to take into account here. I had the #4 with 2664 compressor before (#4/2660 before that), now with the #6/2670 billet and intercooler the boost comes on much earlier than it did without intercooler and smaller turbo according to the logs, its very sharp. So the total system performance depends on many parameters. IF you had identical setups and only swapped the turbine housing you would probably lower the boost threshold a couple of hundred rpms with that step in turbine housing.

AR is kind of partly useless for turbo matching,, since it relates to the mean diameter of the croll, its a hassle to calculate the actual critical area of the turbine housing (which is the interesting part, its what the KKK or holset states in their turbine specs). And depending on the turbine wheel design and compressor match the low end boost and maximum flow capacity can go all possible ways compared to the original setup. As eddie says, first of all set your power goals, and then find a suitable turbo
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Mike9311  



Joined: 14 Dec 2004
Posts: 1147
Location: Chicago-ish

PostPosted: Fri Jun 11, 2021 3:08 am    Post subject: Reply with quote

Cedric wrote:
The 951 have a horrible set up with the long tube exhaust manifold, its a bad design for boost response, dont drag it in here

Anyway, there are more system parameters to take into account here. I had the #4 with 2664 compressor before (#4/2660 before that), now with the #6/2670 billet and intercooler the boost comes on much earlier than it did without intercooler and smaller turbo according to the logs, its very sharp. So the total system performance depends on many parameters. IF you had identical setups and only swapped the turbine housing you would probably lower the boost threshold a couple of hundred rpms with that step in turbine housing.

AR is kind of partly useless for turbo matching,, since it relates to the mean diameter of the croll, its a hassle to calculate the actual critical area of the turbine housing (which is the interesting part, its what the KKK or holset states in their turbine specs). And depending on the turbine wheel design and compressor match the low end boost and maximum flow capacity can go all possible ways compared to the original setup. As eddie says, first of all set your power goals, and then find a suitable turbo


That is mega interesting Cedric. You had a counter intuitive result. I have had direct experience with going from a stock #4 2664 (which I liked actually) to a stock #4 2660 (which I kinda don't like). the 2664 had lag, or at least "boost onset" feeling while the 2660 seemed instant or always there with no really rush. #4 housings are all we have over here so it took decades for me to experience a #6 2664 which, like you point out, was part of a greater system. Then there is noise, no insulation, stones coming off sticky tires, etc etc .. just a lot going on in the CS. I do remember commenting how I did not feel a massive rush like I did with my 80 but the CS was a stroker at the time which can change a lot. She is getting another motor right now for "a time" and that will be mated to the #6 2667 which is now balanced and 95% assembled. Will be interesting. Bottom end is '82 spec but if I can get the head done, that will be the 'variable'. Waiting on a Cometic 0.05 inch right now. If they keep dragging their feet in producing that gasket, I'll use the 0.07 inch I have. Technically I have to wait another month.
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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Cedric  



Joined: 27 Aug 2004
Posts: 2160
Location: Sweden

PostPosted: Fri Jun 11, 2021 5:36 am    Post subject: Reply with quote

Mike9311 wrote:
Cedric wrote:
The 951 have a horrible set up with the long tube exhaust manifold, its a bad design for boost response, dont drag it in here

Anyway, there are more system parameters to take into account here. I had the #4 with 2664 compressor before (#4/2660 before that), now with the #6/2670 billet and intercooler the boost comes on much earlier than it did without intercooler and smaller turbo according to the logs, its very sharp. So the total system performance depends on many parameters. IF you had identical setups and only swapped the turbine housing you would probably lower the boost threshold a couple of hundred rpms with that step in turbine housing.

AR is kind of partly useless for turbo matching,, since it relates to the mean diameter of the croll, its a hassle to calculate the actual critical area of the turbine housing (which is the interesting part, its what the KKK or holset states in their turbine specs). And depending on the turbine wheel design and compressor match the low end boost and maximum flow capacity can go all possible ways compared to the original setup. As eddie says, first of all set your power goals, and then find a suitable turbo


That is mega interesting Cedric. You had a counter intuitive result. I have had direct experience with going from a stock #4 2664 (which I liked actually) to a stock #4 2660 (which I kinda don't like). the 2664 had lag, or at least "boost onset" feeling while the 2660 seemed instant or always there with no really rush. #4 housings are all we have over here so it took decades for me to experience a #6 2664 which, like you point out, was part of a greater system. Then there is noise, no insulation, stones coming off sticky tires, etc etc .. just a lot going on in the CS. I do remember commenting how I did not feel a massive rush like I did with my 80 but the CS was a stroker at the time which can change a lot. She is getting another motor right now for "a time" and that will be mated to the #6 2667 which is now balanced and 95% assembled. Will be interesting. Bottom end is '82 spec but if I can get the head done, that will be the 'variable'. Waiting on a Cometic 0.05 inch right now. If they keep dragging their feet in producing that gasket, I'll use the 0.07 inch I have. Technically I have to wait another month.


Yes i fully agree, with the 2660 the boost response at low revs were really surprisingly good, especially if you only had experience with s1 cars before. But it got quite soft at the top end, especially after a session on the track. The 2664 the boost onset was so much more pronounced, less practical, more fun. But with that turbo at 0,7 bar it made 168hp at the rear hubs, so obviously pushed a bit more than the standard engine.

The IC and ignition timing will change the airflow and power through the engine even at part load, low boost, so my data isnt an apple to apple comparison with the old turbos, since the whole system changed at the same time. on a high gear i have about 0,5bar at 2600rpm ish and 1 bar before 3500rpm i i check some logs from last year. I guess it wont run worse with a sealing head gasket this year

The s2 (or if you run full EFI or good programmable ignition) makes any turbo change making a bigger difference, since it adjust ignition based on boost temp, and lets not talk about IC which makes it advance the ignition alot. So the power at the same boost will be very very different.

I think the #6/2667 will be a great turbo for around 240-250hp, good IC is the key for fun with our fairly detonation sensitive ol 2 valvers.
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2315
Location: MI

PostPosted: Sun Jun 13, 2021 11:07 pm    Post subject: Reply with quote

Just started driving this 941 car just now...
(Drove it around a bunch just to drive it, still In Illinois)

It has light rods, pistons, “high” 8.7 compression, cam, and 6.10 2664 turbocharger..

Below the boost threshold this engine revs faster and decels faster than my 7.5:1 engine, and has more NA torque..
Feels something like maybe it has 90HP @ 2500 RPM where my 7.5:1 engine had 75-80HP..
Little more guts to jump down low..

But the 6.10..
Hmmm yes.. It doesn’t build boost in the mid to high 2k rpm range when “lugged” at these RPMs as low as my 4.10 would..
I’d kinda lugs/bogs a little longer before it spools up when crossing into boost RPMs..

But, say if it’s already at 3500RPM, it picks up boost seemingly just as fast..
So far I’d say not really any more “lag” to get boost once you are already in the RPMs for it, but the required RPMs to get to that instantaneous boost response zone are a little higher..

Not really sure where yet, but somewhere around 3,000 and up it feels exactly the same..

Top end gains?
I can’t tell.. I doubt it matters much up to the 10-11 psi I’ve gotten it to..
I don’t really know, but I think it’s advantages would be seen somewhere in the 15-18 psi range where above that I know my 4.10 starts feeling flatter as it approaches redline..


One more thing..
This 941 as set up kinda feels like the WG “flutters” at WOT.. Like it can’t decide to open or close..
Not sure.. Just an early observation..

About to drive it like 6 hours home..
Runs good, doesn’t seem to want to blow up, AFRs aren’t as perfect as I’m used to especially while part throttle/cruising.. Not bad but not spot on.. Not on the lean side though so that’s good..

TTYL
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Mike9311  



Joined: 14 Dec 2004
Posts: 1147
Location: Chicago-ish

PostPosted: Mon Jun 14, 2021 12:52 am    Post subject: Reply with quote

Also the 941 is a long rod motor too as far as I remember which will cause your pistons to dwell more at TDC. Something to think about at higher boost levels but in the end likely a non-issue since it probably isnt that much of a difference. Might be more det prone but easier on engine wear with less side load and piston acceleration characteristics. The long rod is just another variable to be added to how it feels

You probably passed my house a few minutes ago!
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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924-76  



Joined: 05 Jun 2006
Posts: 1434
Location: Ontario, Canada

PostPosted: Sat Jun 19, 2021 1:05 am    Post subject: Reply with quote

How much boost are planning on running?
What RPM range is ideal for application/driving style?

The #4 spool faster, but run out of breath between 5500 and 6000 RPM. It will still go, but doesn't push as hard. This is based on my observation with my 2664 4.10.
The #6 will keep pushing hard pass 6000 RPM.

There is more than just the turbine housing side. There is different options for turbine wheel, compressor size and compressor wheels.
A 2660 4.10 won't drive the same as a 2664 4.10.
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2226
Location: Somerset, UK

PostPosted: Sat Jun 19, 2021 4:12 am    Post subject: Reply with quote

If your concerned about lag in a 924 turbo, then you're in the wrong car......
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Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2315
Location: MI

PostPosted: Sun Jun 20, 2021 7:59 am    Post subject: Reply with quote

Carrera RSR wrote:
If your concerned about lag in a 924 turbo, then you're in the wrong car......


https://www.nitrousexpress.com/instaboost-digital-efi-nitrous-system-10lb-bottle-20928.asp

I dream about running something like this..
Tell the MS: (If TPS >97% & MAP <15psi & RPM >3,000 = spray 30-50HP wet shot)

Just need the bottle and wet shot solinoids/nozzle I think.. (MS as brain)
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