Ruminants and low-end torque: Is your turbo a cow?

[tdonohue@xnet.com (Terry Donohue)]

STEADIRIC@aol.com said:
>And with 2.2 bar on tap coupled with low ambient air temp's it can
>sometimes be downright annoying!!
     Ah, the sacrifices we must make.  Trying to keep on the topic, Eric's
higher boost will mean that the low end (low boost) will be that much worse
in comparison.

"Linus D. Toy" <linust@interramp.com> then said:
>if you want quicker response (boost), try swapping your turbo for the one
>used in the 20v motors.  it's a smaller (K24 vs K26) wheel, so it builds up
>quicker.  i think a few folks out here have done this w/ either their 5kcstq
>or ur-q.  also check w/ PDQSHIP@aol.com for more details--he's been doing a
>playing with a bunch of stuff on the 10v motors and believes there's more hp
>potential out of the 10v, though i'm thinking that's 'cause there's more
>experience out there with the 10v
    See below.

Glen Powell <gpowell@acacianet.com> comments:
>Is this due to different turbos and not the 2V Vs 4V/cyl? Boost pressure
>should be more a function of the turbo Vs the number of valves, especially
>at low RPM where there is not a lot of flow anyhows.....
    A good 4V usually wins at the low end too, which is why they typically
have about 15% more peak torque than a comparable 2V.

>My 87 5KTQW has much less low-end Vs my 88 5KTQ. What year did the 5KTQs
>switch from the too-big K26 to the more responsive K24 turbos? Could my 88
>have the K24 and the 87 have the too-big K26? Sure feels like it........
     Audi made a number of "running changes" in mid-'89 to the 200T(Q).
These included higher C.R. (7.8 to 8.4); dual knock sensors; smaller turbo.
(They also made some non-performance-related changes, including dual
inside temperature sensors and more wood.)  The rated HP stayed the same
(157 @ 5500), but the torque increased (from 166 to 177 @ 3000).  I was
unaware of these changes when shopping for a 200TQ 2.5 years ago.  On
consecutive days, I drove an early '89 then a '90.  The difference in
responsiveness was quite noticeable-I bought the '90 for this reason, and
only found out later that the higher C.R. and smaller turbo were the
reason.  The smaller turbo limits the peak HP of a modified car (I've
measured 200 HP at 1.8 bar in mine), but sure works better in traffic.

"Bob D'Amato" <Bob.Damato@snetel.com> then noted:
>Is this true just for the turbos? My 20V atmo has squat low end grunt,
>where my old 2V 4kQ had plenty.
     When 4v engines first became available to the masses (for example, I
seem to remember the Jensen Healey in '74), the object was mainly to
increase low end performance, while getting a little more at the high end.
But manufacturers, engineers, and tuners got greedy, and the balance has
shifted to even more high end with resulting loss in the low end.  Some
have found ways around this; they include variable intake tract length
(relatively cheap-many are using it); variable cam timing (this works
nicely and transparently in my M3), and even varying the cam itself (Honda
has made good use of this in its high end machines, but it's expensive).
Ferrari even has variable exhaust primary pipe length (now illegal in
Formula 1), and variable backpressure, to further enhance their low end
torque performance.

Terry Donohue
tdonohue@xnet.com
'71 BMW 2002Ti
'90 Audi 200TQ
'93 Honda Accord
'95 BMW M3