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Vintage79
Vintage79 GRM+ Memberand New Reader
9/23/23 11:04 a.m.

I'm planning to change to a 5-speed, probably the new Tremec TKX, to allow running at tracks with longer straights than Portland International Raceway (Oregon) and Pacific Raceways (Kent WA).  My hope is to run at Road America someday, so I'm seeking information on how much taller my gearing must be for the Kettle Bottoms straight leading to Canada Corner.  I'm just about topped out on Pacific Raceways' long front straight with my 1:1 4th gear now.  I prefer to keep similar 1st thru 4th [1:1] gears for my usual tracks and just add a suitable 5th OD gear.

What overdrive ratio for 5th gear would be needed for Road America?  The Tremec offers a 0.81 OD gear which would allow about 25% higher speed.  The car probably has the HP to use it (Fox Mustang, about 3100 lb, about 430 hp peak), so I want to assume it will accelerate to the corner to be safe.

Advice from someone experienced at RA who's done the gearing for themselves?  Thanks, all.

Vintage79
Vintage79 GRM+ Memberand New Reader
9/24/23 11:55 a.m.

I keep reading, and now see folks reporting their highest speed at the downhill approach to turn 5 at RA, so the gearing need would be there it looks like.  Is that speed 25% or more higher than typical max at tracks like PR in Kent Wa or Portland without the chicane?

stafford1500
stafford1500 GRM+ Memberand Dork
9/24/23 12:32 p.m.

Your answer is going to involve some estimates on the drag coefficient of the car, which requires giving some details on the bodywork/aero configuration. The short answer is the math is pretty straightforward if you have a good estimate of the variables. For what its worth, ride height has a reasonable impact on drag in addition to any body work modifications. Estimates can be made given enough information...

Vintage79
Vintage79 GRM+ Memberand New Reader
12/23/23 10:04 a.m.

Late reply, thanks for your thoughts, SteveO.  The 1979 Fox Mustang at stock ride height had a drag coefficient of .43-.44, somewhere closer to a brick than a teardrop shape :-)

I've got the TKX 5 speed now, with .81 5th gear.   We are "committed." With 245-45/16 Hoosier R7 tires, it will touch 6000 rpm in 5th at about 172 mph.  Given my top end of about 145 mph at Portland OR PIR without the chicane, and a bit less on Kent's (Pacific Raceways) front straight, it seems unlikely I would exceed 172 at RA.  I could take one set of 275-45's (exceed 180 mph top end with them ) for insurance, I guess.  They wouldn't go to waste.  Have been on them with 4 speed for years now, but they're wide for the 16x8 wheels.  RPM-limited on 245-45's with 1:1 4th and 3.08 rear ratio.

red_stapler
red_stapler SuperDork
12/23/23 11:08 a.m.
Vintage79 said:

folks reporting their highest speed at the downhill approach to turn 5 at RA

GT4 racecars have 430hp and typically top out at 150 here.  You probably wouldn't even need 5th gear with the 275s.

wvumtnbkr
wvumtnbkr GRM+ Memberand PowerDork
12/23/23 12:33 p.m.

Dang, how much power are you making?

In a 200 hp bmw 3 series, we were touching 130 on the front straight and back straight.  We did the bend though, not the kink.

Pmacinmn
Pmacinmn New Reader
1/1/24 7:05 p.m.

This is a lap from a couple years ago before the re-pave, to give you some reference on Road America speeds.  This is a 2001 Z06 track car on Michelin scrub slicks, similar power to your Mustang.  I never shift to 5th gear, 6800 red line, 3.42 gears

collinskl1
collinskl1 GRM+ Memberand Reader
1/3/24 3:33 p.m.

Can confirm that the highest speeds are into T5.

Running with lemons, the layout using the bend chicane vs the kink, we were fastest into T5, then T1, then Canada.

DeadSkunk  (Warren)
DeadSkunk (Warren) UltimaDork
1/4/24 2:55 p.m.

So here's where I get a surprise. I've never been to the track, but from years of seeing it on TV I'd have bet the back straight was slightly downhill all the way to Canada Corner. It's actually dead flat, where turn 3 to 5 drops 45 feet . I really need to get there before I die.laugh

No description available.

wvumtnbkr
wvumtnbkr GRM+ Memberand PowerDork
1/4/24 3:14 p.m.
DeadSkunk (Warren) said:

So here's where I get a surprise. I've never been to the track, but from years of seeing it on TV I'd have bet the back straight was slightly downhill all the way to Canada Corner. It's actually dead flat, where turn 3 to 5 drops 45 feet . I really need to get there before I die.laugh

No description available.

Yes.  Yes you do.  It is epic.  Some of the corner speeds are literally "hold your breath and hope she sticks".

Amazing track.  Even for something with less than 200hp.

akylekoz
akylekoz UltraDork
3/5/24 11:31 a.m.

This is funny.  We built a special trans for RA, a T5 with a road race fifth gear.  In testing, driving into town and back it turned out to be a 3.5th gear instead of a 4.5th gear.  It was handy in turns 3 and 14 but not worth the trouble shifting from 3rd to fifth and back to fourth.

You see the problem is the road race fifth gear is only for V8 T5's, a 4 cylinder T5 is already a close ratio with short overdrive.

Bottom line is you need some overdrive for Road America, definitely not a third and a half gear.

Now that we are going over 90 mph we can get back to aero.  

Hard to answer your question without all the variables, tire dia, final drive ratio, HP, size and material of balls.  That last one is a real factor at RA.

Vintage79
Vintage79 GRM+ Memberand New Reader
5/10/24 4:50 p.m.

Was traveling all of April and lost track of the thread. Thanks for all the comments. That bit about getting to RA while still on the "right side of the grass".......yes.

theruleslawyer
theruleslawyer Reader
5/14/24 12:13 a.m.

My f82 m4 (444hp 3600lb) did 149 GPS confirmed into T1 and T5. Probably a couple more mph in there if I was braver.

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