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Quote:
Originally Posted by Flojo View Post
my sound 3.0 SC with overhauled K-Jet, fresh 915 and SSI, Permatune runs 150mph/240 kmh as per GPS no problem
Cool,

How does one explain their way out of a ticket or handcuffs?

just wonderin
Old 11-12-2018, 10:18 AM
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A German in the Fatherland is allowed to do that.
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Old 11-12-2018, 10:21 AM
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Formerly known as Syzygy
 
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Quote:
Originally Posted by 911SauCy View Post
I've been to about 115 with a lot left...

I'm sure wheel/tire combinations would cause a considerable difference in top speed application to application. My last w/t setup was 17" euromeisters with 255/40 rears. Current is 16" BBS with 245/45 rears... This has made a very notable difference in highway speed cruising RPM. I was about 72mph at 3k, not it's about 68...

Probably going to upsize again because that small difference sure makes the car feel old when trying to keep up w modern traffic
Shouldn't make all that much difference as they are almost the same outside diameter. The 255s are about 25" and the 245s just under at around 24.6 or 7, so maybe 3/8" less? I can't see that making a 4mph difference at 70.

Bill V in a couple posts above (#35) suggested that a two inch difference in diameter wouldn't make a huge difference and a 245/45-16 and 225/50-16 being the same diameter as a 255/40-17 would only be about 1mph different, and that was at double your normal highway speed.

When I changed my 245 turbo wheels out to 255 and 17" wheels, I didn't notice any real difference. I wasn't looking for any, but nothing jumped out at me.

It's pretty insignificant - I think there could be that much difference in tire brands. But maybe I'm out to lunch.
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Old 11-12-2018, 10:27 AM
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Quote:
Originally Posted by Bill Verburg View Post
Here are the actual acceleration curves for a friend '78 SC, The motor was blueprinted and runs headers, tires are 275/35 x15, aero is SC-lip + whale, car is lowered, rear wheel hp from dyno 191.4, top speed is 142mph@6750



Bill,
That's a great graph. Shift when the acc. in the higher gear is higher than the lower gear.
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Old 11-12-2018, 11:15 AM
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Quote:
Originally Posted by Canada Kev View Post
Shouldn't make all that much difference as they are almost the same outside diameter. The 255s are about 25" and the 245s just under at around 24.6 or 7, so maybe 3/8" less? I can't see that making a 4mph difference at 70.

Bill V in a couple posts above (#35) suggested that a two inch difference in diameter wouldn't make a huge difference and a 245/45-16 and 225/50-16 being the same diameter as a 255/40-17 would only be about 1mph different, and that was at double your normal highway speed.

When I changed my 245 turbo wheels out to 255 and 17" wheels, I didn't notice any real difference. I wasn't looking for any, but nothing jumped out at me.

It's pretty insignificant - I think there could be that much difference in tire brands. But maybe I'm out to lunch.
Yea I don't know...

Did the circumference calculations and they're within an inch of each other.

Scientific approach aside, logic says the photos are telling. Fender gap is completely different, all else equal and untouched

It's apparent the 17/255 set-up was a much taller one



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Old 11-13-2018, 05:23 AM
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I bet nearing 150 mph the tire circumference changes somewhat...
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Old 11-13-2018, 05:38 AM
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I can't even imagine doing 150 MPH in my SC. It was scary enough in my modern TT and there was plenty more to go.

I can't even imagine why I would want to do 150 MPH in my SC, either. Getting old I guess, but straight line speed never did it for me unless it was from a stoplight in a muscle car.
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Old 11-13-2018, 06:50 AM
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you guys have balls. 50mph corners are terrifying in my SC
Old 11-13-2018, 06:59 AM
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Quote:
Originally Posted by Clark Griswold View Post
you guys have balls. 50mph corners are terrifying in my SC
I'll bet it is more your imagination...they really have much more than that when you get the feel.
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Old 11-13-2018, 07:22 AM
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Sketch is a great description

With my stock 87 the fastest I've gone indicated is in the low 130's at a tracknight event at Charlotte Motor Speedways Roval. Legitimately into 5 gear (which never happens at any other track) on the back straight, feeling floaty and I chickened out heading into the very rough turn 3 on the banked oval. Just not something that feels familiar. Those kind of speeds on a turn are gut clenchers. Harry's lap timer did not show that high of a speed but it seemed to behave erratically in there. The speedo always seems accurate at highway speeds up to 80 or so per various mapping apps.
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Old 11-13-2018, 08:02 AM
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An inch in tire diameter doesn’t make much difference day to day around town but it’s huge on the race track. In 2012 when Grand Am changed the tire size on the GT cars everyone was scrambling around to find and change their ratios prior to Daytona because they were geared too tall.

Fastest I’ve taken my 3.2 is 120mph. That was on the track and it felt fine to me. Only car I’ve taken over 150 is a 996 Cup.
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Old 11-13-2018, 08:07 AM
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Quote:
Originally Posted by Bill Verburg View Post
Here are the actual acceleration curves for a friend '78 SC, The motor was blueprinted and runs headers, tires are 275/35 x15, aero is SC-lip + whale, car is lowered, rear wheel hp from dyno 191.4, top speed is 142mph@6750


That is a good top speed for a dyno. Pushing through the atmosphere, it will never get there on the road or track. Wind resistance is major at those speeds.
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Old 11-13-2018, 10:21 AM
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Quote:
Originally Posted by GH85Carrera View Post
That is a good top speed for a dyno. Pushing through the atmosphere, it will never get there on the road or track. Wind resistance is major at those speeds.
That is w/ aero factored in
Here are the thrust curves, which don't take the mass of the car into account, but do show the effect of aero. The car becomes aero limited @ 142mph


There are lots of thing happening, here that don't show up in the graphs
for one the inertial cost of the different tires, the gearing cost does show but the inertial cost doesn't.

Here are the theoretical data just changing tires, for better accuracy this uses the tires rolling radius not the OD, rolling radius accounts for the flatening of the tire where is meets the road and also any distortion due to forces acting through the tire


lastly 911 speedo are notoriously optimistic w/ stock tires, change the tires and it can be better or worse, but bottom line unless you have GPS data acquisition and can generate a plot like this you will never know.

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Old 11-13-2018, 12:04 PM
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Quote:
Originally Posted by Clark Griswold View Post
you guys have balls. 50mph corners are terrifying in my SC
Not all of us.

Told this before but drove a pal's 2008 GT3 RS for eight 25 minute sessions at Motorsport Ranch one weekend. Floor it to 60, maybe up to 90 before some corners, and brake for the turn. Over and over.

Instructor is barking "You don't have to brake that much".
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Old 11-14-2018, 08:48 AM
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A important factor that might not be weighed in here is the output of the engines in question and the weight of the cars. I'm guessing very few are stock. My SC was a widebody with front spoiler and wing so very stable at speed but a lot of drag as well. Weight was 2400lb at that time. The engine was mildly modified with headers, muffler and a few other goodies which put down about 180whp. The trans gearing was just perfect for that configuration, the engine would pull nicely to redline in 5th but I could tell that was all she had. Calculated speed was 145mph with tires and gearing. Not sure a bone stock SC could do that.
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Old 11-14-2018, 09:38 AM
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And these SCs with big heavy wheels and tires; they look great but the rotating mass is a lot of work for a lil' ole SC motor. The stock wheels and tyres are much more suitable for impressive top speed attempts.
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Old 11-14-2018, 09:46 AM
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Quote:
Originally Posted by Flojo View Post
my sound 3.0 SC with overhauled K-Jet, fresh 915 and SSI, Permatune runs 150mph/240 kmh as per GPS no problem
I must add to my statement, that a) I lightened it by close to 90kg (*), b) run it with the front lip and c) lowered it to 63/62 cm.
So weight and aerodynamics sum it up.

(* ac total delete, battery only 9kg, lightweight damping, RS carpet, rear seat deleted, center console deleted, alloy floor boards, alloy spare wheel, no frunk carpet, Carrera cooler, deleted water reservoir, deleted gas tank expander reservoir, no junk in car)
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Last edited by Flojo; 11-14-2018 at 11:05 AM..
Old 11-14-2018, 11:02 AM
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